Mina: A Sensorimotor Robotic Orthosis for Mobility Assistance

Anil K. Raj; Peter D. Neuhaus; Adrien M. Moucheboeuf; Jerryll H. Noorden; David V. Lecoutre

doi:10.1155/2011/284352

Mina: A Sensorimotor Robotic Orthosis for Mobility Assistance

Raj, Anil K.;Neuhaus, Peter D.;Moucheboeuf, Adrien M.;Noorden, Jerryll H.;Lecoutre, David V. 2011-12-14 00:00:00 Hindawi Publishing Corporation Journal of Robotics Volume 2011, Article ID 284352, 8 pages doi:10.1155/2011/284352 Research Article Anil K. Raj, Peter D. Neuhaus, Adrien M. Moucheboeuf, JerryllH.Noorden,and DavidV.Lecoutre Florida Institute for Human and Machine Cognition, 40 South Alcaniz Street, Pensacola, FL 32502, USA Correspondence should be addressed to Anil K. Raj, araj@ihmc.us Received 2 June 2011; Revised 10 September 2011; Accepted 15 October 2011 Academic Editor: Tetsuya Mouri Copyright © 2011 Anil K. Raj et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. While most mobility options for persons with paraplegia or paraparesis employ wheeled solutions, signiﬁcant adverse health, psychological, and social consequences result from wheelchair conﬁnement. Modern robotic exoskeleton devices for gait assistance and rehabilitation, however, can support legged locomotion systems for those with lower extremity weakness or paralysis. The Florida Institute for Human and Machine Cognition (IHMC) has developed the Mina, a prototype sensorimotor robotic orthosis for mobility assistance that provides mobility capability for paraplegic and paraparetic users. This paper describes the initial concept, design goals, and methods of this wearable overground robotic mobility device, which uses compliant actuation to power the hip and knee joints. Paralyzed users can balance and walk using the device over level terrain with the assistance of forearm crutches employing a quadrupedal gait. We have initiated sensory substitution feedback mechanisms to augment user sensory perception of his or her lower extremities. Using this sensory feedback, we hypothesize that users will ambulate with a more natural, upright gait and will be able to directly control the gait parameters and respond to perturbations. This may allow bipedal (with minimal support) gait in future prototypes. 1. Introduction individuals to regain access to areas that require legged locomotion and to restore the health beneﬁts associated with The limited mobility assistance options for those suﬀering an upright posture. In addition to improving quality of life from paraplegia or paraparesis typically utilize wheeled de- as orthotic devices, exoskeletons could also bridge the gap to vices, which require infrastructure (ramps, roads, smooth future regenerative medicine approaches for this population. surfaces, etc.), and 69.8% of spinal cord injured (SCI) par- For example, a paraplegic user of a robotic orthosis could aplegics use a manual wheelchair as their primary means of maintain healthy bone and muscle mass and range of joint locomotion, which limits range and terrain options [1]. motion that could reduce rehabilitation time following stem Wheeled conveyances allow access to only a small fraction cell therapy. of the locations accessible to pedestrians. Wheelchairs have trouble on curbs, stairs, irregular terrain such as hiking trails 1.1. Robotic Orthoses. Current robotic assistance devices and narrow corridors. Even with advances in powered wheel- such as the body-worn ReWalk from Argo Medical Tech- chairs, such as the iBot (http://www.ibotnow.com/), mobility nologies (http://www.argomedtec.com/) and the eLegs from remains limited to relatively smooth terrain, precluding Berkely Bionics (http://berkeleybionics.com/)havemotorsat access to much of the natural outdoors. Additionally, being the hips and knees to move the legs and provide powered gait. conﬁned to a wheelchair has signiﬁcant consequences on The user provides balance with the aid of forearm crutches physiological and psychological health, quality of life, and and uses torso motions, arm movements, and/or a push but- social interactions. Health-related issues include pressure ton interface. Both devices can operate untethered for several sores, poor circulation, loss of bone density muscle mass, hours on a single charge. Users have demonstrated stair and changes in body fat distribution [2–4]. Robotic lower extremity orthosis designs can oﬀer new mobility options climbing with the ReWalk; however, neither device has dem- for those currently limited to a wheelchair, enabling such onstrated operations over rough and irregular terrain. Both 2 Journal of Robotics devices target paraplegic users who cannot initiate any mo- tion of their legs and thus must operate in a rigid position control mode. Use by paraparetics, however, requires a more compliant mode of operation. Both devices are undergoing clinical trials, and neither device is currently available for personal use. The commercially available hybrid assistive limb (HAL) device, which has signiﬁcant operational experience with able-bodied users [5], augments user-initiated movement by detecting electromyographic (EMG) signals in the user’s lower extremity muscles. A new version of this device, HAL- 5 LB (Type C), speciﬁcally targets paraplegic users [6], but has only demonstrated transition from sitting to standing, not overground mobility. This design, however, does include an actuator at the ankle, a feature lacking from the ReWalk and the eLegs. The wearable power assist leg (WPAL) [7, 8], another paraplegic gait assist device, relies on a walker rather than crutches for the required balance stabilization. The walker provides a signiﬁcant support polygon for the user and requires a diﬀerent, less natural gait. Similarly, the EXoskeleton for Patients and the Old by Sogang University Figure 1: The IHMC Mina sensorimotor robotic orthosis for mobility assistance prototype. Mina adjusts to ﬁt users ranging from (EXPOS) [9], designed as a walking assist device for the ∼1.6 m to 1.9 m tall. elderly and for patients with muscle or nerve damage in the lower body, uses a wheeled-caster walker to carry the actuators and computer system. It transfers actuator forces to the exoskeleton joint via cables and employs position control mobility assist devices. SCI users lack body awareness below of the exoskeleton joints, but it lacks force sensing in the the level of injury, which makes user control of orthotic de- actuators. Force sensors on the leg braces are used to detect vices cumbersome. Reinstating sensory feedback should fa- the user intent, but the integral caster walker limits operation cilitate the integration of the orthosis into the user’s posture and utility of this device to smooth ﬂoors. Zabaleta et al. [10] and ambulation strategy and, potentially, restoration of bi- also propose to track EMG and utilize compliant actuation pedal gait for this population. for a robotic exoskeleton for rehabilitation. A number of robotic orthoses developed for treadmill- based operation face some of the same challenges, share some 1.2. The Sensory Substitution Paradigm. Because perception of the same technologies, and they are strictly limited to occurs in the brain and not at the sensory end organ [17], rehabilitation activities. The Powered Gait Orthosis (PGO) sensory substitution interfaces can provide an alternative [11] and LOwer-extremity Powered ExoSkeleton (LOPES) pathway for sensory perception. A sensory substitution sys- [12] utilize force sensors on each actuator, which allows for tem consists of three parts: a sensor, a coupling system, and torque control of the joints. One of the most utilized and a stimulator. Sensory substitution can occur across sensory studied treadmill-based robotic orthotic devices, the Loko- systems such as touch-to-sight or within a sensory system mat [13, 14], has demonstrated the advantages of compliant such as touch-to-touch. The human brain, in fact, can re- control strategies [15, 16]. interpret signals from speciﬁc nerves (e.g., from tactile recep- At the Florida Institute for Human and Machine Cogni- tors) given appropriate, veridical, and timely sensory feed- tion (IHMC), we have designed and built a robotic orthosis back. This forms the basis for sensory substitution interfaces called Mina (Figure 1) to provide overground mobility for that can noninvasively and unobtrusively use alternative, in- paraplegic and paraparetic users. Mina utilizes compliant tact sensory pathways. This plasticity inherent to the brain control actuators and can provide both rigid position control and nervous system supports both long-term and short- for paraplegic users and assistive force control for paraparetic term anatomical and functional remapping of sensory data users. In its current state of development, the prototype [18, 19] and will assist brain reorganization despite losses Mina oﬀers operates similarly to the ReWalk and eLegs for in muscle, bone, reﬂexes and will assist a user’s ability to paraplegic mobility with hip and knee actuation for powered perform activities of daily living [20]. Tactile and propriocep- execution of recorded gait. All three devices move the legs tive feedback sensory substitution technologies have been de- through predetermined joint trajectories with strict position veloped for use with lower limb prostheses [21–24]topro- control of the exoskeleton joint. However, the compliant con- vide foot sole pressure information, joint angle, and other trol actuators that Mina utilizes facilitate operation over forces. Because paralyzed individuals lack proprio- and exte- rough terrain. roception from the lower limbs, they must use their vision In addition, Mina provides the user with sensory feed- to monitor “what’s going on” below their level of injury. back from the exoskeleton. Sensory feedback provides a key Compensating for the loss of tactile information from the element for motor-control missing from other paraplegic soles, as well as proprioceptive information (i.e., muscle Journal of Robotics 3 stretch and joint position) visually requires signiﬁcant cog- area of the abdomen to improve spatial separation. While the nitive eﬀort that could be redistributed through other sen- density of torso sensory receptors is not as high as the sory modalities. Mina provides similar input for users with tongue, placing a high-resolution display (e.g., 24 × 32) on intact but paralyzed legs by providing ground reaction forces the abdomen allows for rapid perception of motion of objects and proprioceptive signal analogs from the insensate feet. [26] and can be worn discretely under the users clothing. The fundamental challenges for sensory augmentation in an The keratinized layer of dead skin cells of the epidermis on exoskeleton relate to identiﬁcation of the optimal informa- the torso requires the VideoTact to use higher voltage (15– tion for the user when walking and intuitive presentation of 40 VDC) and current (8–32 mA) than the BrainPort; how- that information without increasing cognitive workload or ever, it is battery driven and further electrically isolated by competing with vision or hearing. storing the stimulus charge in an array of capacitors that are disconnected from the power source prior to stimulus 1.3. Human Machine Interface (HMI) Considerations. The delivery. human motor control system relies on sensory information Integrating these two displays to provide sensory substi- (feedback) in order to respond to perturbations and stabilize tution of proprioception and somatosensation to Mina users errors. Sensory feedback, for example, enables the brain to should lead to shorter training requirements, improved sense maintain the body’s posture and helps it to determine the of balance, and sensorimotor reorganization that integrates both perception and control of the exoskeleton. Both devices positions of the limbs in space and the amount of force re- quired to execute a movement. Several sensory systems (i.e., have intensity control via software with user override for the vestibular, visual, and somatosensory systems) contribute intensity and shut-oﬀ. IHMC has used these general-purpose sensory substitution displays previously for augmentation of to the control of balance and oﬀer an important channel of information that help to coordinate human interaction individuals with balance disorders, as well as for vision and with the world. The vestibular system gives the sense of hearing substitution systems. IHMC is currently investigat- whole body orientation and motion in collaboration with ing the eﬀectiveness of various sensory substitution sym- the visual system. For both posture and gait, motor control bologies to provide the user with an eﬀective, intuitive un- derstanding of the state of the exoskeleton with low cognitive mechanisms seek to hold the body’s center of gravity (CG) over the polygon of support (deﬁned by the position of the demand. individual’s feet). While determining position of the center of mass under dynamic conditions is hard to compute, the 2. Exoskeleton Design central nervous system can infer its position by using the information provided by the muscle-tendon stretch receptors Mina is a second-generation [27] lower extremity robotic gait orthosis with two actuated degrees of freedom per leg, and the cutaneous pressure receptors of the foot sole. Paraplegia deprives the user of both the motor and sen- hip ﬂexion/extension, and knee ﬂexion/extension, for a total sory functions; restoring mobility requires reinstatement of of four actuators. Mina does not provide hip ab-/adduction or medial/lateral rotation of the leg and employs rigid ankle movement and sensation. The IHMC Mina system displays ground reaction forces and center of pressure, as well as joint joint with a compliant carbon ﬁber footplate. Mina users connect to a rigid back plate, which has a curvature to match positions and torque estimations, using noninvasive tactile that of the human spine, via shoulder and pelvic straps. The interfaces, speciﬁcally a BrainPort intra-oral display (Wicab, Inc., Middleton, WI) and a VideoTact abdominal display system can accommodate a range of body sizes by using nested aluminum tubing as the structural links to attach to (ForeThought Development, LLC, Blue Mounds, WI). These displays (Figure 2) interface to the relatively underutilized, the user’s thigh, leg, and foot. A tether provides the prototype with respect to hearing and vision, tactile channel and pro- with power for the computer and motors, as well as Ethernet communication; later versions will integrate battery power vide suﬃcient resolution for the data represented by Mina. The BrainPort electrotactile transducer array is held in and wireless communications technologies for untethered operations. A fall prevention tether connected to an overhead the mouth and connected to battery powered electronics trolley system supports the user and Mina only in the event that generate highly controlled electrical pulses that produce patterns of tactile sensations when the electrodes are in that the user loses balance or missteps. contact with the top surface of the tongue. The tongue’s sen- sitivity, excellent spatial resolution, mobility, and distance to 2.1. Actuators. Mina uses four identical rotary actuators the brainstem make it an ideal site for a practical electro- (Figure 3) capable of both position and torque control. Each tactile HMI. An electrolytic solution (saliva) assures good actuator consists of a DC brushless motor (Moog BN34- electrical contact. Perception with electrical stimulation of 25EU-02) and a 160 : 1 harmonic drive (SHD-20 from HD the tongue appears to be better than with ﬁngertip electro- Systems) gear reduction. The actuators are instrumented tactile stimulation, and the tongue requires only about 3% with two incremental encoders. One encoder measures the (5–15 V) of the voltage, and much less current (0.4–2.0 mA) relative position between the motor shaft and the base of than the ﬁngertip for electrotactile stimulation [25]. Current the actuator. This encoder (HEDL-5640#A13, Avago Tech- BrainPort arrays can provide a 100 to 600 pixel resolution via nologies, Inc. San Jose, CA) has a resolution of 2000 counts the intraoral display (IOD) tongue array. per revolution, resolving to 1.96e−5 rad/count at the output. The VideoTact is also an electrotactile interface; however, The second encoder (RGH-24, Renishaw, PLC, Glouces- it is placed on the abdomen. It can exploit the larger surface tershire, England) measures the relative position between 4 Journal of Robotics (a) (b) Figure 2: Tactile sensory substitution electrotactile displays used by Mina. (a) Wicab BrainPort 600 pixel tongue array; (b) a 768 titanium electrode abdominal array (ForeThought VideoTact). deﬂection sensor, the maximum speed of the motor, the amount of impact isolation allowed to the gear train, the acceptable reﬂected inertia at the output, the bandwidth re- quirements on position and torque control, and complexity of the design. Our application can tolerate a stiﬀ spring due to the inherent impact protection from the connection to the user and requires a stiﬀ spring due to tight positioning requirements. By utilizing high-resolution encoders (approx- imately 2.0e − 5 radians/count) the design is able to function with a very stiﬀ series spring. We determined that the inher- ent compliance of the harmonic drive was suﬃcient for this Figure 3: Actuator showing two encoders, golden read strip that is application and would result in a compact, low part count wrapped around the base of the harmonic drive gear reduction. design. For torque control, Mina uses a simple proportional plus derivative controller (see Figure 4) where the error signal equals desired torque minus the applied torque and the output of the actuator and the base of the actuator using a is used to determine the input current to the motor. The linear encoder with the read-head mounted onto the output feedback gains were tuned empirically. The value for K was of the actuator and the linear encoding strip wrapped around 2.0, and the value for K was 0.0002. the surface of the harmonic drive input (which is securely ﬁxed onto the base of the actuator). This encoder has a 2.2. Computer and Electronics. An embedded PC-104 com- resolution of 1 mm at a radius of 0.45 m, which resolves puter system mounted on the back plate, running a Real- 2.22e − 5rad/count. Time Java under Solaris (Oracle, Corp., Redwood Shores, The motion of the output encoder matches the motion CA) and the control software, written in Real-Time Java pro- of the motor encoder, minus any elastic deformation of the vides closed-loop control of the actuators via Accelnet digital harmonic drive due to torque applied to the output shaft. By servo modules (ACM-180-20, Copley Controls, Peabody, applying a known torque and measuring the deﬂection using MA) and communicates with a desktop host computer via the diﬀerence between the two encoders, we characterized a tethered Ethernet cable. The embedded computer runs the the elasticity of each actuator. With a peak torque of about control code, stores the trajectories used for the paraplegic 60 Nm, the elastic deﬂection is about 0.0025 rad, indicating walking-mode and transmits relevant state variables to a host astiﬀness of approximately 24 kNm/rad. In operation, an computer in real time (50 Hz) for display and monitoring. empirically determined look-up table is used to indicate the Mina uses F-Scan (Tekscan, Inc., Boston, MA) insoles torque of the actuator based on harmonic drive deﬂection. placed between the footplate and the shoe with up to 960 The Mina operates in position, or high impedance, pressure sensors to detect ground reaction forces and deter- control using only the motor shaft encoder (Avago HEDL- mine center of pressure on each foot (smaller insoles are 5640#A13) instead of the output encoder (Renishaw RGH- cut form the standard size, resulting in fewer total sensors). 24) due to occasional loss of counts of the output encoder. Figure 5 shows the normalized pressure map from the insoles Because the deﬂection of the harmonic drive is considered (black = zero pressure, white = normalized maximum re- negligible with regard to the tolerance required on the output corded during calibration). This map is resampled to match position, the simple proportional plus derivative feedback the 600 pixel array of the BrainPort IOD and presented as control algorithm only needs the output position to control intensity (a tingling sensation) on the tongue. With a few the motor input current. minutes of training, a user can learn to interpret this signal A series elastic actuator (SEA) was used in order to as pressure on his or her feet. achieve torque control. In designing the SEA, the major de- Similarly, joint position from the actuator encoders and sign element to select is the spring rate, which is dependent torque estimated from actuator current draw can be used on a number of factors, including the resolution of the spring to estimate the position of the Mina-user system CG over Journal of Robotics 5 Table 1 θ, τ (K + SK )e I τ e p d des Actuator Leg length Actual step Step Walking (dist. from hip joint to ankle) size period speed 0.840 m 0.24 m 1.4 s 0.18 m/s measured 0.785 m 0.28 m 1.4 s 0.20 m/s Figure 4: Diagram showing the feedback loop used to control the output torque of the actuator. feedback loop between terrain sensing, joint position, and body position. Replicating this complex loop, especially the terrain sensing, will be studied in future work. the stability polygon deﬁned by the current stance as well as After the recording phase, the trajectories were played the relative “eﬀort” exerted by Mina to maintain the current back in paraplegic assistance mode with an able-bodied user posture or execute a step. When presented on the VideoTact with relaxed lower limb muscles. From this playback, the best as a moving CG icon and a dynamic stability polygon, we single gait cycle (stance and swing phase) was selected to use believe that users will be able to eﬀectively maintain aware- as a basis for the ﬁnal walk. The joint angles at the end of this ness of their limits of stability during ambulation. gait cycle were adjusted to match the starting joint angles, allowing the step to be played back in a smooth, endless loop. The joint angles were then copied to the other leg with a half 3. Gait Generation and Operation cycle phase shift. This ensured that the left leg and the right Mina operates as a motion capture system that records tra- leg executed the exact same step with the appropriate phase shift. jectories from an able-bodied individual, which can then be “played back” in the paraplegic assistance mode. This meth- Three diﬀerent walks were recorded, with step sizes od allows for generation of a natural gait with a quick devel- ranging from zero (stepping in place) to what will be referred to as a large step. The precise value of the step size for a given opment cycle. walk depends on the leg length of the user. The quickest step period we have used to date with Mina is 1.4 seconds per step. 3.1. Generating Walking Trajectories. Walking trajectories The resulting walking speeds are presented in Table 1.Note were generated from joint position recordings made while an that the recorded gait consists of a sequence of desired joint able-bodied person wearing Mina walked over level terrain angles. The resulting walking speed is a function of how fast in the laboratory. This method of gait trajectory generation this sequence of desired joint angles is played and of the leg was selected because it allows for relatively natural gaits and length of the user. The longer the user leg length the larger the the ability to develop new gaits in a short period of time. actual step, and thus the faster the resulting walking speed. During this process, the actuators were set to torque control The fastest walk speed recorded was 0.2 m/s (see Table 1), mode. For the hip joints, the desired torque was set to zero so which was limited by actuator performance rather than user that Mina would follow the user’s motions without aﬀecting capability. them. Compliance in the user’s ﬂesh and the braces of Mina The joint angles at the end of the best single recorded gait can result in a few degrees of oﬀset between the user’s joints cycle (stance and swing phase) were adjusted to match the and the device’s joints. For example, full knee extension may starting joint angles, allowing the step to be played back in a occur before the knee joint of Mina extends fully. However, smooth, endless loop, and the joint angles were then copied in paraplegic assistance mode, stable stance requires that the to the other leg with a half cycle phase shift. Mina knee joint extend fully. In order to assist the knee joint to the fully extended position while recording the gait, the 3.2. Operation. TheMina currentlyrequiresanexternalcon- desired torque was set to be 10 Nm in extension. This torque, trol operator in paraplegic assistance mode to activate/deac- generated by the actuators, ensured that the knee was fully tivate the system, trigger a single step or continuous steps, extended during stance phase while allowing the able-bodied stop walking, and change gait speed between 50% and 130% user to overcome it during swing. of the recorded speed. In addition, the operator can adjust Toe-oﬀ provides a signiﬁcant component of natural gait the time the controller pauses between left and right steps [28] and people minimize the ground clearance of the foot and responds to verbal and gesture cues from the user. For as part of a muscle energy conservation strategy. However, eﬀective real-world mobility assistance, the user must have for a robotic orthosis, electrical energy conservation does not full control of the exoskeleton, which requires sensory per- equal muscle energy conservation. Because Mina lacks an ception of the orthosis dynamics. Sensory substitution inter- ankle actuator, the able-bodied user walked with an exagger- faces provide this functionality in the updated Mina device. ated ground clearance in swing phase during gait recording to guarantee that the toe does not stub on the ground. The resulting gait mimicked walking on a slippery surface (i.e., 4. Results from Initial Evaluations with minimal the ground reaction shear forces). Because Mina does not have the same actuated degrees of freedom Following IHMC Institutional Review Board (IRB) approval, as a healthy person, the resultant gait cannot match that two evaluators tested the initial Mina prototype. We required of a healthy person. In human walking, there is complex the evaluators to have an American Spinal Injury Association 6 Journal of Robotics (a) (b) (c) Figure 5: (a) Insole pressure sensory arrays (F-Scan); (b) visual representation of ground reaction force; (c) Mina representation of contact forces on Wicab BrainPort IOD. (ASIA) Impairment Scale [29]. A (Complete) and a Walking Index for Spinal Cord Injury (WISCI) level 9 (Ambulates with walker, with braces and no physical assistance, 10 m) or higher [30]. Although the evaluators were able to walk prior to their SCI, walking in Mina diﬀers signiﬁcantly from able-bodied walking. As mentioned before, complete paraplegics lack feedback of the ground reaction force and center of pressure on their feet. Additionally, they do not use their remaining proprioception feedback loop for balance as frequently as able-bodied persons because they spend most of their awake hours seated. Finally, when walking in Mina, their arms become an integral part of balance and ambula- tion as they ambulate with a quadrupedal crawl consisting of a hind foot, ipsilateral front crutch, contralateral hind foot, and contralateral front crutch sequence. Because of the lack of integrated sensory feedback in the initial prototype, we placed a video monitor in front of the evaluator during initial training, which provided a real-time side view but forced the user to choose between watching the monitor and watching his or her legs directly. The user must learn how to position Figure 6: The Mina during evaluation. his or her body at the point of heel strike. If the user leans too far backward, then the upcoming swing leg will still be loaded at the time of swing, causing a backward fall. If the trajectory-tracking mode, able-bodied users tend to actively user leans too far forward, the foot will contact the ground try to walk and balance using sensory feedback, such as before the swing completes, resulting in signiﬁcantly reduced ground reaction forces. The addition of sensory substitution step size. Large step sizes with this prototype often caused the interfaces to Mina will allow paraplegic users to receive sim- evaluator’s center of mass to remain between the two feet ilar information and should allow similar control behaviors. during double support, leaving the trailing leg loaded as it In evaluating Mina, we observed that all users required some initiated the next swing phase and triggering a fall. Using amount of training and practice and that more training and smaller steps mitigates this problem; however, this accentu- practice was required for paraplegic users than able-bodied ates the need to provide appropriate sensory feedback for a users. As with any new activity that requires coordinated more dynamic gait that could control a passively (spring motion, proﬁciency requires practice. The addition of pro- loaded) or actively actuated ankle for toe-oﬀ. While both prioception analogs for the lower extremities in paraplegic evaluators could easily walk with forearm crutches (Figure users should reduce the cognitive eﬀort and time to learn the 6) as a quadruped with low cognitive eﬀort [31], we believe task of coordinating arm motion with leg motion. that the next iterations with sensory augmentation will result in a more upright gait. 6. Future Work 5. Discussion Feedback systems integrated to Mina will seek to convey We evaluated Mina with two paraplegic evaluators and dem- sensory information related to these characteristics of human onstrated that Mina is currently capable of providing mobil- balance during stance and dynamic gait. IHMC is evaluating ity for paraplegic users on ﬂat ground at slow walking speed. the eﬀects on balance of various tactile display symbologies Even though Mina currently operates in a high impedance by determining the user’s control stability (maintenance of his Journal of Robotics 7 or her balance and the deviation of his/her center of mass) Institute for Human and Machine Cognition is a not-for- as well as the user’s perception accuracy by asking him or her proﬁt research institute of the State of Florida University to estimate how much he/she deviates from a desired body System. This material is based on research sponsored by the posture. It will also be interesting to measure the partici- Oﬃce of Naval Research under agreement numbers N00014- pants’ accuracy to estimate their body deviation when using 07-1-0790, N00014-09-1-0800 and N00014-10-1-0847. the tactile feedback. 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Kagawa and Y. Uno, “A human interface for stride control al control of the device using gestures provided by upper on a wearable robot,” in IEEE/RSJ International Conference on body motion. We will also investigate tracking user gaze as a Intelligent Robots and Systems (IROS ’09), pp. 4067–4072, St. control mechanism when navigating complex environments Louis, Mo, USA, October 2009. [8] T. Kagawa and Y. Uno, “Gait pattern generation for a power- to, for example, adjust step height and step plant when walk- assist device of paraplegic gait,” in Proceedings of IEEE Interna- ing on rough ground. Further development will continue tional Workshop on Robot and Human Interactive Communica- to reestablish the sensorimotor loop by restoring sensory tion, pp. 633–638, Toyama, Japan, September 2009. feedback and will improve user/orthosis coupling to form [9] K. Kong and D. Jeon, “Design and control of an exoskeleton an integral sociotechnical team. This should enable increas- for the elderly and patients,” IEEE/ASME Transactions on Me- ing the speed of walking, walking over rough terrain, on chatronics, vol. 11, no. 4, pp. 428–432, 2006. hiking trails, and in urban environments with stairs and nar- [10] H. Zabaleta, M. Bureau, G. Eizmendi, E. Olaiz, J. Medina, and row passageways as well as a more natural upright, assisted M. Perez, “Exoskeleton design for functional rehabilitation bipedal gait. in patients with neurological disorders and stroke,” in the 10th IEEE International Conference on Rehabilitation Robotics (ICORR ’07), pp. 112–118, Noordwijk, The Netherlands, June Conﬂict of Interests [11] Z. Feng, J. Qian, Y. Zhang, L. Shen, Z. Zhang, and Q. The authors have no ﬁnancial conﬂicts of interest with re- Wang, “Biomechanical design of the powered gait orthosis,” spect to the work described. Any opinions, ﬁndings, and con- in IEEE International Conference on Robotics and Biomimetics clusions or recommendations expressed in this material are (ROBIO ’07), pp. 1698–1702, Sanya, China, December 2007. those of the authors and do not necessarily reﬂect the views [12] R. Ekkelenkamp, J. Veneman, and H. Van Der Kooij, “LOPES: of the Oﬃce of Naval Research. selective control of gait functions during the gait rehabilitation of CVA patients,” in the 9th IEEE International Conference on Acknowledgments Rehabilitation Robotics (ICORR ’05), pp. 361–364, Chicago, Ill, USA, July 2005. The authors would like to thank their two evaluators for their [13] G. Colombo, M. Joerg, R. Schreier, and V. Dietz, “Treadmill invaluable feedback in testing Mina. They would also like training of paraplegic patients using a robotic orthosis,” Jour- to thank the team of medical professionals that volunteered nal of Rehabilitation Research and Development,vol. 37, no.6, their time to monitor the evaluation sessions. The Florida pp. 693–700, 2000. 8 Journal of Robotics [14] G. Colombo, M. Jorg ¨ , and V. Dietz, “Driven gait orthosis to do [31] P. D. Neuhaus et al., “Design and evaluation of Mina, a robotic locomotor training of paraplegic patients,” in the 22nd Annual orthosis for paraplegics,” in Proceedings of the International International Conference of the IEEE Engineering in Medicine Conference on Rehabilitation Robotics, Zurich, Switzerland, and Biology Society, vol. 4, pp. 3159–3163, Chicago, Ill, USA, 2011. July 2000. [32] P. Bach-y-Rita, S. Wood, R. Leder et al., “Computer-assisted [15] R. Riener, L. Lunenburger ¨ , S. Jezernik, M. Anderschitz, G. motivating rehabilitation (CAMR) for institutional, home, Colombo, and V. 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Publisher: Hindawi Publishing Corporation
Copyright: Copyright © 2011 Anil K. Raj et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ISSN: 1687-9600
eISSN: 1687-9619
DOI: 10.1155/2011/284352
Publisher site: See Article on Publisher Site

Abstract

Hindawi Publishing Corporation Journal of Robotics Volume 2011, Article ID 284352, 8 pages doi:10.1155/2011/284352 Research Article Anil K. Raj, Peter D. Neuhaus, Adrien M. Moucheboeuf, JerryllH.Noorden,and DavidV.Lecoutre Florida Institute for Human and Machine Cognition, 40 South Alcaniz Street, Pensacola, FL 32502, USA Correspondence should be addressed to Anil K. Raj, araj@ihmc.us Received 2 June 2011; Revised 10 September 2011; Accepted 15 October 2011 Academic Editor: Tetsuya Mouri Copyright © 2011 Anil K. Raj et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. While most mobility options for persons with paraplegia or paraparesis employ wheeled solutions, signiﬁcant adverse health, psychological, and social consequences result from wheelchair conﬁnement. Modern robotic exoskeleton devices for gait assistance and rehabilitation, however, can support legged locomotion systems for those with lower extremity weakness or paralysis. The Florida Institute for Human and Machine Cognition (IHMC) has developed the Mina, a prototype sensorimotor robotic orthosis for mobility assistance that provides mobility capability for paraplegic and paraparetic users. This paper describes the initial concept, design goals, and methods of this wearable overground robotic mobility device, which uses compliant actuation to power the hip and knee joints. Paralyzed users can balance and walk using the device over level terrain with the assistance of forearm crutches employing a quadrupedal gait. We have initiated sensory substitution feedback mechanisms to augment user sensory perception of his or her lower extremities. Using this sensory feedback, we hypothesize that users will ambulate with a more natural, upright gait and will be able to directly control the gait parameters and respond to perturbations. This may allow bipedal (with minimal support) gait in future prototypes. 1. Introduction individuals to regain access to areas that require legged locomotion and to restore the health beneﬁts associated with The limited mobility assistance options for those suﬀering an upright posture. In addition to improving quality of life from paraplegia or paraparesis typically utilize wheeled de- as orthotic devices, exoskeletons could also bridge the gap to vices, which require infrastructure (ramps, roads, smooth future regenerative medicine approaches for this population. surfaces, etc.), and 69.8% of spinal cord injured (SCI) par- For example, a paraplegic user of a robotic orthosis could aplegics use a manual wheelchair as their primary means of maintain healthy bone and muscle mass and range of joint locomotion, which limits range and terrain options [1]. motion that could reduce rehabilitation time following stem Wheeled conveyances allow access to only a small fraction cell therapy. of the locations accessible to pedestrians. Wheelchairs have trouble on curbs, stairs, irregular terrain such as hiking trails 1.1. Robotic Orthoses. Current robotic assistance devices and narrow corridors. Even with advances in powered wheel- such as the body-worn ReWalk from Argo Medical Tech- chairs, such as the iBot (http://www.ibotnow.com/), mobility nologies (http://www.argomedtec.com/) and the eLegs from remains limited to relatively smooth terrain, precluding Berkely Bionics (http://berkeleybionics.com/)havemotorsat access to much of the natural outdoors. Additionally, being the hips and knees to move the legs and provide powered gait. conﬁned to a wheelchair has signiﬁcant consequences on The user provides balance with the aid of forearm crutches physiological and psychological health, quality of life, and and uses torso motions, arm movements, and/or a push but- social interactions. Health-related issues include pressure ton interface. Both devices can operate untethered for several sores, poor circulation, loss of bone density muscle mass, hours on a single charge. Users have demonstrated stair and changes in body fat distribution [2–4]. Robotic lower extremity orthosis designs can oﬀer new mobility options climbing with the ReWalk; however, neither device has dem- for those currently limited to a wheelchair, enabling such onstrated operations over rough and irregular terrain. Both 2 Journal of Robotics devices target paraplegic users who cannot initiate any mo- tion of their legs and thus must operate in a rigid position control mode. Use by paraparetics, however, requires a more compliant mode of operation. Both devices are undergoing clinical trials, and neither device is currently available for personal use. The commercially available hybrid assistive limb (HAL) device, which has signiﬁcant operational experience with able-bodied users [5], augments user-initiated movement by detecting electromyographic (EMG) signals in the user’s lower extremity muscles. A new version of this device, HAL- 5 LB (Type C), speciﬁcally targets paraplegic users [6], but has only demonstrated transition from sitting to standing, not overground mobility. This design, however, does include an actuator at the ankle, a feature lacking from the ReWalk and the eLegs. The wearable power assist leg (WPAL) [7, 8], another paraplegic gait assist device, relies on a walker rather than crutches for the required balance stabilization. The walker provides a signiﬁcant support polygon for the user and requires a diﬀerent, less natural gait. Similarly, the EXoskeleton for Patients and the Old by Sogang University Figure 1: The IHMC Mina sensorimotor robotic orthosis for mobility assistance prototype. Mina adjusts to ﬁt users ranging from (EXPOS) [9], designed as a walking assist device for the ∼1.6 m to 1.9 m tall. elderly and for patients with muscle or nerve damage in the lower body, uses a wheeled-caster walker to carry the actuators and computer system. It transfers actuator forces to the exoskeleton joint via cables and employs position control mobility assist devices. SCI users lack body awareness below of the exoskeleton joints, but it lacks force sensing in the the level of injury, which makes user control of orthotic de- actuators. Force sensors on the leg braces are used to detect vices cumbersome. Reinstating sensory feedback should fa- the user intent, but the integral caster walker limits operation cilitate the integration of the orthosis into the user’s posture and utility of this device to smooth ﬂoors. Zabaleta et al. [10] and ambulation strategy and, potentially, restoration of bi- also propose to track EMG and utilize compliant actuation pedal gait for this population. for a robotic exoskeleton for rehabilitation. A number of robotic orthoses developed for treadmill- based operation face some of the same challenges, share some 1.2. The Sensory Substitution Paradigm. Because perception of the same technologies, and they are strictly limited to occurs in the brain and not at the sensory end organ [17], rehabilitation activities. The Powered Gait Orthosis (PGO) sensory substitution interfaces can provide an alternative [11] and LOwer-extremity Powered ExoSkeleton (LOPES) pathway for sensory perception. A sensory substitution sys- [12] utilize force sensors on each actuator, which allows for tem consists of three parts: a sensor, a coupling system, and torque control of the joints. One of the most utilized and a stimulator. Sensory substitution can occur across sensory studied treadmill-based robotic orthotic devices, the Loko- systems such as touch-to-sight or within a sensory system mat [13, 14], has demonstrated the advantages of compliant such as touch-to-touch. The human brain, in fact, can re- control strategies [15, 16]. interpret signals from speciﬁc nerves (e.g., from tactile recep- At the Florida Institute for Human and Machine Cogni- tors) given appropriate, veridical, and timely sensory feed- tion (IHMC), we have designed and built a robotic orthosis back. This forms the basis for sensory substitution interfaces called Mina (Figure 1) to provide overground mobility for that can noninvasively and unobtrusively use alternative, in- paraplegic and paraparetic users. Mina utilizes compliant tact sensory pathways. This plasticity inherent to the brain control actuators and can provide both rigid position control and nervous system supports both long-term and short- for paraplegic users and assistive force control for paraparetic term anatomical and functional remapping of sensory data users. In its current state of development, the prototype [18, 19] and will assist brain reorganization despite losses Mina oﬀers operates similarly to the ReWalk and eLegs for in muscle, bone, reﬂexes and will assist a user’s ability to paraplegic mobility with hip and knee actuation for powered perform activities of daily living [20]. Tactile and propriocep- execution of recorded gait. All three devices move the legs tive feedback sensory substitution technologies have been de- through predetermined joint trajectories with strict position veloped for use with lower limb prostheses [21–24]topro- control of the exoskeleton joint. However, the compliant con- vide foot sole pressure information, joint angle, and other trol actuators that Mina utilizes facilitate operation over forces. Because paralyzed individuals lack proprio- and exte- rough terrain. roception from the lower limbs, they must use their vision In addition, Mina provides the user with sensory feed- to monitor “what’s going on” below their level of injury. back from the exoskeleton. Sensory feedback provides a key Compensating for the loss of tactile information from the element for motor-control missing from other paraplegic soles, as well as proprioceptive information (i.e., muscle Journal of Robotics 3 stretch and joint position) visually requires signiﬁcant cog- area of the abdomen to improve spatial separation. While the nitive eﬀort that could be redistributed through other sen- density of torso sensory receptors is not as high as the sory modalities. Mina provides similar input for users with tongue, placing a high-resolution display (e.g., 24 × 32) on intact but paralyzed legs by providing ground reaction forces the abdomen allows for rapid perception of motion of objects and proprioceptive signal analogs from the insensate feet. [26] and can be worn discretely under the users clothing. The fundamental challenges for sensory augmentation in an The keratinized layer of dead skin cells of the epidermis on exoskeleton relate to identiﬁcation of the optimal informa- the torso requires the VideoTact to use higher voltage (15– tion for the user when walking and intuitive presentation of 40 VDC) and current (8–32 mA) than the BrainPort; how- that information without increasing cognitive workload or ever, it is battery driven and further electrically isolated by competing with vision or hearing. storing the stimulus charge in an array of capacitors that are disconnected from the power source prior to stimulus 1.3. Human Machine Interface (HMI) Considerations. The delivery. human motor control system relies on sensory information Integrating these two displays to provide sensory substi- (feedback) in order to respond to perturbations and stabilize tution of proprioception and somatosensation to Mina users errors. Sensory feedback, for example, enables the brain to should lead to shorter training requirements, improved sense maintain the body’s posture and helps it to determine the of balance, and sensorimotor reorganization that integrates both perception and control of the exoskeleton. Both devices positions of the limbs in space and the amount of force re- quired to execute a movement. Several sensory systems (i.e., have intensity control via software with user override for the vestibular, visual, and somatosensory systems) contribute intensity and shut-oﬀ. IHMC has used these general-purpose sensory substitution displays previously for augmentation of to the control of balance and oﬀer an important channel of information that help to coordinate human interaction individuals with balance disorders, as well as for vision and with the world. The vestibular system gives the sense of hearing substitution systems. IHMC is currently investigat- whole body orientation and motion in collaboration with ing the eﬀectiveness of various sensory substitution sym- the visual system. For both posture and gait, motor control bologies to provide the user with an eﬀective, intuitive un- derstanding of the state of the exoskeleton with low cognitive mechanisms seek to hold the body’s center of gravity (CG) over the polygon of support (deﬁned by the position of the demand. individual’s feet). While determining position of the center of mass under dynamic conditions is hard to compute, the 2. Exoskeleton Design central nervous system can infer its position by using the information provided by the muscle-tendon stretch receptors Mina is a second-generation [27] lower extremity robotic gait orthosis with two actuated degrees of freedom per leg, and the cutaneous pressure receptors of the foot sole. Paraplegia deprives the user of both the motor and sen- hip ﬂexion/extension, and knee ﬂexion/extension, for a total sory functions; restoring mobility requires reinstatement of of four actuators. Mina does not provide hip ab-/adduction or medial/lateral rotation of the leg and employs rigid ankle movement and sensation. The IHMC Mina system displays ground reaction forces and center of pressure, as well as joint joint with a compliant carbon ﬁber footplate. Mina users connect to a rigid back plate, which has a curvature to match positions and torque estimations, using noninvasive tactile that of the human spine, via shoulder and pelvic straps. The interfaces, speciﬁcally a BrainPort intra-oral display (Wicab, Inc., Middleton, WI) and a VideoTact abdominal display system can accommodate a range of body sizes by using nested aluminum tubing as the structural links to attach to (ForeThought Development, LLC, Blue Mounds, WI). These displays (Figure 2) interface to the relatively underutilized, the user’s thigh, leg, and foot. A tether provides the prototype with respect to hearing and vision, tactile channel and pro- with power for the computer and motors, as well as Ethernet communication; later versions will integrate battery power vide suﬃcient resolution for the data represented by Mina. The BrainPort electrotactile transducer array is held in and wireless communications technologies for untethered operations. A fall prevention tether connected to an overhead the mouth and connected to battery powered electronics trolley system supports the user and Mina only in the event that generate highly controlled electrical pulses that produce patterns of tactile sensations when the electrodes are in that the user loses balance or missteps. contact with the top surface of the tongue. The tongue’s sen- sitivity, excellent spatial resolution, mobility, and distance to 2.1. Actuators. Mina uses four identical rotary actuators the brainstem make it an ideal site for a practical electro- (Figure 3) capable of both position and torque control. Each tactile HMI. An electrolytic solution (saliva) assures good actuator consists of a DC brushless motor (Moog BN34- electrical contact. Perception with electrical stimulation of 25EU-02) and a 160 : 1 harmonic drive (SHD-20 from HD the tongue appears to be better than with ﬁngertip electro- Systems) gear reduction. The actuators are instrumented tactile stimulation, and the tongue requires only about 3% with two incremental encoders. One encoder measures the (5–15 V) of the voltage, and much less current (0.4–2.0 mA) relative position between the motor shaft and the base of than the ﬁngertip for electrotactile stimulation [25]. Current the actuator. This encoder (HEDL-5640#A13, Avago Tech- BrainPort arrays can provide a 100 to 600 pixel resolution via nologies, Inc. San Jose, CA) has a resolution of 2000 counts the intraoral display (IOD) tongue array. per revolution, resolving to 1.96e−5 rad/count at the output. The VideoTact is also an electrotactile interface; however, The second encoder (RGH-24, Renishaw, PLC, Glouces- it is placed on the abdomen. It can exploit the larger surface tershire, England) measures the relative position between 4 Journal of Robotics (a) (b) Figure 2: Tactile sensory substitution electrotactile displays used by Mina. (a) Wicab BrainPort 600 pixel tongue array; (b) a 768 titanium electrode abdominal array (ForeThought VideoTact). deﬂection sensor, the maximum speed of the motor, the amount of impact isolation allowed to the gear train, the acceptable reﬂected inertia at the output, the bandwidth re- quirements on position and torque control, and complexity of the design. Our application can tolerate a stiﬀ spring due to the inherent impact protection from the connection to the user and requires a stiﬀ spring due to tight positioning requirements. By utilizing high-resolution encoders (approx- imately 2.0e − 5 radians/count) the design is able to function with a very stiﬀ series spring. We determined that the inher- ent compliance of the harmonic drive was suﬃcient for this Figure 3: Actuator showing two encoders, golden read strip that is application and would result in a compact, low part count wrapped around the base of the harmonic drive gear reduction. design. For torque control, Mina uses a simple proportional plus derivative controller (see Figure 4) where the error signal equals desired torque minus the applied torque and the output of the actuator and the base of the actuator using a is used to determine the input current to the motor. The linear encoder with the read-head mounted onto the output feedback gains were tuned empirically. The value for K was of the actuator and the linear encoding strip wrapped around 2.0, and the value for K was 0.0002. the surface of the harmonic drive input (which is securely ﬁxed onto the base of the actuator). This encoder has a 2.2. Computer and Electronics. An embedded PC-104 com- resolution of 1 mm at a radius of 0.45 m, which resolves puter system mounted on the back plate, running a Real- 2.22e − 5rad/count. Time Java under Solaris (Oracle, Corp., Redwood Shores, The motion of the output encoder matches the motion CA) and the control software, written in Real-Time Java pro- of the motor encoder, minus any elastic deformation of the vides closed-loop control of the actuators via Accelnet digital harmonic drive due to torque applied to the output shaft. By servo modules (ACM-180-20, Copley Controls, Peabody, applying a known torque and measuring the deﬂection using MA) and communicates with a desktop host computer via the diﬀerence between the two encoders, we characterized a tethered Ethernet cable. The embedded computer runs the the elasticity of each actuator. With a peak torque of about control code, stores the trajectories used for the paraplegic 60 Nm, the elastic deﬂection is about 0.0025 rad, indicating walking-mode and transmits relevant state variables to a host astiﬀness of approximately 24 kNm/rad. In operation, an computer in real time (50 Hz) for display and monitoring. empirically determined look-up table is used to indicate the Mina uses F-Scan (Tekscan, Inc., Boston, MA) insoles torque of the actuator based on harmonic drive deﬂection. placed between the footplate and the shoe with up to 960 The Mina operates in position, or high impedance, pressure sensors to detect ground reaction forces and deter- control using only the motor shaft encoder (Avago HEDL- mine center of pressure on each foot (smaller insoles are 5640#A13) instead of the output encoder (Renishaw RGH- cut form the standard size, resulting in fewer total sensors). 24) due to occasional loss of counts of the output encoder. Figure 5 shows the normalized pressure map from the insoles Because the deﬂection of the harmonic drive is considered (black = zero pressure, white = normalized maximum re- negligible with regard to the tolerance required on the output corded during calibration). This map is resampled to match position, the simple proportional plus derivative feedback the 600 pixel array of the BrainPort IOD and presented as control algorithm only needs the output position to control intensity (a tingling sensation) on the tongue. With a few the motor input current. minutes of training, a user can learn to interpret this signal A series elastic actuator (SEA) was used in order to as pressure on his or her feet. achieve torque control. In designing the SEA, the major de- Similarly, joint position from the actuator encoders and sign element to select is the spring rate, which is dependent torque estimated from actuator current draw can be used on a number of factors, including the resolution of the spring to estimate the position of the Mina-user system CG over Journal of Robotics 5 Table 1 θ, τ (K + SK )e I τ e p d des Actuator Leg length Actual step Step Walking (dist. from hip joint to ankle) size period speed 0.840 m 0.24 m 1.4 s 0.18 m/s measured 0.785 m 0.28 m 1.4 s 0.20 m/s Figure 4: Diagram showing the feedback loop used to control the output torque of the actuator. feedback loop between terrain sensing, joint position, and body position. Replicating this complex loop, especially the terrain sensing, will be studied in future work. the stability polygon deﬁned by the current stance as well as After the recording phase, the trajectories were played the relative “eﬀort” exerted by Mina to maintain the current back in paraplegic assistance mode with an able-bodied user posture or execute a step. When presented on the VideoTact with relaxed lower limb muscles. From this playback, the best as a moving CG icon and a dynamic stability polygon, we single gait cycle (stance and swing phase) was selected to use believe that users will be able to eﬀectively maintain aware- as a basis for the ﬁnal walk. The joint angles at the end of this ness of their limits of stability during ambulation. gait cycle were adjusted to match the starting joint angles, allowing the step to be played back in a smooth, endless loop. The joint angles were then copied to the other leg with a half 3. Gait Generation and Operation cycle phase shift. This ensured that the left leg and the right Mina operates as a motion capture system that records tra- leg executed the exact same step with the appropriate phase shift. jectories from an able-bodied individual, which can then be “played back” in the paraplegic assistance mode. This meth- Three diﬀerent walks were recorded, with step sizes od allows for generation of a natural gait with a quick devel- ranging from zero (stepping in place) to what will be referred to as a large step. The precise value of the step size for a given opment cycle. walk depends on the leg length of the user. The quickest step period we have used to date with Mina is 1.4 seconds per step. 3.1. Generating Walking Trajectories. Walking trajectories The resulting walking speeds are presented in Table 1.Note were generated from joint position recordings made while an that the recorded gait consists of a sequence of desired joint able-bodied person wearing Mina walked over level terrain angles. The resulting walking speed is a function of how fast in the laboratory. This method of gait trajectory generation this sequence of desired joint angles is played and of the leg was selected because it allows for relatively natural gaits and length of the user. The longer the user leg length the larger the the ability to develop new gaits in a short period of time. actual step, and thus the faster the resulting walking speed. During this process, the actuators were set to torque control The fastest walk speed recorded was 0.2 m/s (see Table 1), mode. For the hip joints, the desired torque was set to zero so which was limited by actuator performance rather than user that Mina would follow the user’s motions without aﬀecting capability. them. Compliance in the user’s ﬂesh and the braces of Mina The joint angles at the end of the best single recorded gait can result in a few degrees of oﬀset between the user’s joints cycle (stance and swing phase) were adjusted to match the and the device’s joints. For example, full knee extension may starting joint angles, allowing the step to be played back in a occur before the knee joint of Mina extends fully. However, smooth, endless loop, and the joint angles were then copied in paraplegic assistance mode, stable stance requires that the to the other leg with a half cycle phase shift. Mina knee joint extend fully. In order to assist the knee joint to the fully extended position while recording the gait, the 3.2. Operation. TheMina currentlyrequiresanexternalcon- desired torque was set to be 10 Nm in extension. This torque, trol operator in paraplegic assistance mode to activate/deac- generated by the actuators, ensured that the knee was fully tivate the system, trigger a single step or continuous steps, extended during stance phase while allowing the able-bodied stop walking, and change gait speed between 50% and 130% user to overcome it during swing. of the recorded speed. In addition, the operator can adjust Toe-oﬀ provides a signiﬁcant component of natural gait the time the controller pauses between left and right steps [28] and people minimize the ground clearance of the foot and responds to verbal and gesture cues from the user. For as part of a muscle energy conservation strategy. However, eﬀective real-world mobility assistance, the user must have for a robotic orthosis, electrical energy conservation does not full control of the exoskeleton, which requires sensory per- equal muscle energy conservation. Because Mina lacks an ception of the orthosis dynamics. Sensory substitution inter- ankle actuator, the able-bodied user walked with an exagger- faces provide this functionality in the updated Mina device. ated ground clearance in swing phase during gait recording to guarantee that the toe does not stub on the ground. The resulting gait mimicked walking on a slippery surface (i.e., 4. Results from Initial Evaluations with minimal the ground reaction shear forces). Because Mina does not have the same actuated degrees of freedom Following IHMC Institutional Review Board (IRB) approval, as a healthy person, the resultant gait cannot match that two evaluators tested the initial Mina prototype. We required of a healthy person. In human walking, there is complex the evaluators to have an American Spinal Injury Association 6 Journal of Robotics (a) (b) (c) Figure 5: (a) Insole pressure sensory arrays (F-Scan); (b) visual representation of ground reaction force; (c) Mina representation of contact forces on Wicab BrainPort IOD. (ASIA) Impairment Scale [29]. A (Complete) and a Walking Index for Spinal Cord Injury (WISCI) level 9 (Ambulates with walker, with braces and no physical assistance, 10 m) or higher [30]. Although the evaluators were able to walk prior to their SCI, walking in Mina diﬀers signiﬁcantly from able-bodied walking. As mentioned before, complete paraplegics lack feedback of the ground reaction force and center of pressure on their feet. Additionally, they do not use their remaining proprioception feedback loop for balance as frequently as able-bodied persons because they spend most of their awake hours seated. Finally, when walking in Mina, their arms become an integral part of balance and ambula- tion as they ambulate with a quadrupedal crawl consisting of a hind foot, ipsilateral front crutch, contralateral hind foot, and contralateral front crutch sequence. Because of the lack of integrated sensory feedback in the initial prototype, we placed a video monitor in front of the evaluator during initial training, which provided a real-time side view but forced the user to choose between watching the monitor and watching his or her legs directly. The user must learn how to position Figure 6: The Mina during evaluation. his or her body at the point of heel strike. If the user leans too far backward, then the upcoming swing leg will still be loaded at the time of swing, causing a backward fall. If the trajectory-tracking mode, able-bodied users tend to actively user leans too far forward, the foot will contact the ground try to walk and balance using sensory feedback, such as before the swing completes, resulting in signiﬁcantly reduced ground reaction forces. The addition of sensory substitution step size. Large step sizes with this prototype often caused the interfaces to Mina will allow paraplegic users to receive sim- evaluator’s center of mass to remain between the two feet ilar information and should allow similar control behaviors. during double support, leaving the trailing leg loaded as it In evaluating Mina, we observed that all users required some initiated the next swing phase and triggering a fall. Using amount of training and practice and that more training and smaller steps mitigates this problem; however, this accentu- practice was required for paraplegic users than able-bodied ates the need to provide appropriate sensory feedback for a users. As with any new activity that requires coordinated more dynamic gait that could control a passively (spring motion, proﬁciency requires practice. The addition of pro- loaded) or actively actuated ankle for toe-oﬀ. While both prioception analogs for the lower extremities in paraplegic evaluators could easily walk with forearm crutches (Figure users should reduce the cognitive eﬀort and time to learn the 6) as a quadruped with low cognitive eﬀort [31], we believe task of coordinating arm motion with leg motion. that the next iterations with sensory augmentation will result in a more upright gait. 6. Future Work 5. Discussion Feedback systems integrated to Mina will seek to convey We evaluated Mina with two paraplegic evaluators and dem- sensory information related to these characteristics of human onstrated that Mina is currently capable of providing mobil- balance during stance and dynamic gait. IHMC is evaluating ity for paraplegic users on ﬂat ground at slow walking speed. the eﬀects on balance of various tactile display symbologies Even though Mina currently operates in a high impedance by determining the user’s control stability (maintenance of his Journal of Robotics 7 or her balance and the deviation of his/her center of mass) Institute for Human and Machine Cognition is a not-for- as well as the user’s perception accuracy by asking him or her proﬁt research institute of the State of Florida University to estimate how much he/she deviates from a desired body System. This material is based on research sponsored by the posture. It will also be interesting to measure the partici- Oﬃce of Naval Research under agreement numbers N00014- pants’ accuracy to estimate their body deviation when using 07-1-0790, N00014-09-1-0800 and N00014-10-1-0847. the tactile feedback. 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Mina: A Sensorimotor Robotic Orthosis for Mobility Assistance

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