A large amount of hemiplegic survivors are suffering from motor impairment. Ankle rehabilitation exercises act an important role in recovering patients’ walking ability after stroke. Currently, patients mainly perform ankle exercise to reobtain range of motion (ROM) and strength of the ankle joint under a therapist’s assistance by manual operation. However, therapists suffer from high work intensity, and most of the existed rehabilitation devices focus on ankle functional training and ignore the importance of neurological rehabilitation in the early hemiplegic stage. In this paper, a new robotic ankle rehabilitation platform (RARP) is proposed to assist patients in executing ankle exercise. The robotic platform consists of two three-DOF symmetric layer-stacking mechanisms, which can execute ankle internal/external rotation, dorsiflexion/plantarflexion, and inversion/eversion exercise while the rotation center of the distal zone of the robotic platform always coincides with patients’ ankle pivot center. Three exercise modes including constant-speed exercise, constant torque-impedance exercise, and awareness exercise are developed to execute ankle training corresponding to different rehabilitation stages. Experiments corresponding to these three ankle exercise modes are performed, the result demonstrated that the RARP is capable of executing ankle rehabilitation, and the novel awareness exercise mode motivates patients to proactively participate in ankle training.
Recently, a large number of stroke survivors are suffering from motor impairment. The recovery of motor loss function is difficult conducted only by biomedical treatment [
In the traditional ankle exercise, physiotherapy (PT) manually holds patients’ affected ankle to carry out internal/external rotation, dorsiflexion/plantarflexion, and inversion/eversion motion during ankle rehabilitation. This manual training method makes PT exhausted, and the rehabilitation performance highly relies on a physiotherapist’s experience. Furthermore, patients may act confrontation on this passive training method which reduces the rehabilitation efficiency. In order to address the manipulation challenges in ankle rehabilitation, many studies have considered the development of robotic systems to reduce a physiotherapist’s workload and enhance patients’ rehabilitation performance. A parallel structure robotic system and an exoskeletal structure robotic system are the two main research interests in the ankle rehabilitation assistance.
Saglia et al. reported a two-DOF foot pedal using parallel structure for ankle rehabilitation [
Besides the parallel structure robotic ankle rehabilitation robot, a number of exoskeletal structure ankle rehabilitation robots were also developed. Jeffrey et al. presented a powered ankle-foot orthosis. The device owns only one DOF to realize ankle dorsiflexion/plantarflexion training [
The summary of the existing state-of-the-art parallel structure ankle rehabilitation robots and exoskeletal ankle rehabilitation robots can be found in Table
Existing state-of-the-art ankle rehabilitation robots.
Catalog | System or developer | DOFs | Payload |
---|---|---|---|
Parallel structure | Saglia et al. [ |
2 | ≤120 Nm |
Rutgers University [ |
6 | ≤35 Nm | |
Liu et al. [ |
3 | ||
Meng et al. [ |
6 | — | |
Muhammad and Shafriza [ |
3 | — | |
Yu et al. [ |
3 | — | |
Exoskeletal structure | Jeffrey et al. [ |
1 | ≤30 Nm |
Delaware University [ |
2 | — | |
Rahman and Ikeura [ |
1 | ≤60 Nm | |
Hong et al. [ |
3 | — |
Through examining the existing robots for ankle rehabilitation, a few improvements could still be identified as follows: (1) the rotation center of the foot pedal should easily point to the rotation center of the ankle joint via mechanical configuration; (2) the robot should be friendly and stimulate the patient to participate in ankle training, encouraging the patient to self-balance both their ankle joints during exercise; and (3) maintainability and modularity might be improved. Trying to achieve these improved specifications, we developed a novel robotic ankle rehabilitation platform (RARP) for hemiplegic stroke survivors, as shown in Figure
Constructed ankle rehabilitation robotic platform.
This paper is organized as follows. The design specification of the RARP is presented in Section
Figure
The anatomical planes and terms of location and orientation; (a) projection planes; (b) rotational axes.
Ankle physiological data.
Axis | Motion | Angle range (degree) | Torque (Nm) | Angular velocity (degrees/s) |
---|---|---|---|---|
Inversion | 0~30 | 10 | ≤100 | |
Eversion | −20~0 | |||
Dorsiflexion | 0~30 | 45 | ≤80 | |
Plantarflexion | −40~0 | |||
Internal rot. | 0~20 | 20 | ≤80 | |
External rot. | −30~0 |
The final design of the RARP mechanism was obtained after several rounds of design iterations. The following considerations led to the convergence of the final design.
Corresponding to the different sources of tractive force for ankle movement, ankle rehabilitation can be sorted into patient-passive exercise and patient-active exercise. In the early stage of ankle therapy, the patient can hardly self-move his/her foot; therefore, a passive exercise with constant-speed movement to avoid muscle atrophy is necessary. This kind of task can be accomplished after the patient has carried out delicate movement by himself/herself. In order to allow the patient to fully reobtain his/her motor function, constant torque-impedance exercises can be executed with the device to enhance the strength of the patient’s affected muscles.
To the above exercise modes, researchers pay most attentions to recover the range of motion and force strength of the patient’s affected ankle; however, ankle exercise content should consider the balance and the coordination between the sound side ankle and the affected side ankle due to the individual differences. Hence, in this paper, we propose a new ankle exercise mode, named as awareness exercise that allows the patient to train his/her affected side ankle using the motor parameters obtained from his/her sound side ankle. With this exercise mode, the motor capability of the affected side ankle will approach that of the sound side ankle; therefore, it allows the patient to regain symmetric balance capability for walking after ankle exercise.
A 3-DOF robotic platform is proposed to assist the ankle to achieve inversion/eversion, dorsiflexion/plantarflexion, and internal/external rotation. As shown in Figure
Three design solutions of the 3-DOF ankle rehabilitation robotic platform: (a) mechanical diagram of solution I; (b) mechanical diagram of solution II; (c) mechanical diagram of solution III; (d) 3D model of solution I; (e) 3D model of solution II; (f) 3D model of solution III.
In the first design solution (Figure
In the second design solution (Figure
In the third design solution (Figure
All the three design solutions can accomplish ankle exercises with motion of inversion/eversion, dorsiflexion/plantarflexion, and internal/external rotation. In the first solution, the use of a screw pair mechanism can increase systematic rigidity; however, the volume is hard to be compacted. In the second solution, although the serial mechanism is easy for a compact design, the payload on the pedal will generate a large torque at the serial structure robotic joint; hence, high-torque motor is needed to execute ankle exercise and the cantilever needs to be strengthened. In the third solution, the rotation of the motor passes through the combination of gear pairs to drive the circular plate rotating along its central axis. This layer-stacking mechanism design takes use of the advantage that the high-torque transmission of worm gear pair and the payload on the pedal can be transmitted to the ground through the circular plates. Finally, the third solution is used in our robotic platform.
As shown in Figure
The mechanism that activates ankle exercise of internal/external rotation (Figure
Components of the power transmission mechanism for internal/external rotation exercise.
The input spur gear a1 is actuated by a brushless DC motor (MT8N42P06V2E), which is a 60 W motor coupled with the 5.2 : 1 gearhead, and the rated speed of the motor is 3000 RPM, while the rated torque is 0.2 Nm. By substituting the rated parameters into (
The mechanism for dorsiflexion/plantarflexion exercise, corresponding to J2 in Figure
Components of the power transmission mechanism for dorsiflexion/plantarflexion exercise.
As presented previously, the mechanism for inversion/eversion exercise is located at the top layer and fitted a foot pedal at the distal zone, as shown in Figure
Components of the power transmission mechanism for inversion/eversion exercise.
Gear c1, actuated by a brushless DC motor (MT8N42P06V2E), bites with gear c2 in gear pair III, and the similar configuration was constructed in gear pair IV and gear pair V. The equation bridging joint movement
Two force sensors (FL25-100 kg, Forsentek Co., Shenzhen, China) are mounted under the foot pedal, based on the lever principle; the torque generated for dorsiflexion/plantarflexion and inversion/eversion movements can be calculated by
For the ankle rehabilitation robotic platform, six IBL3605A motor drivers (made by Techservo Co. Ltd., Shenzhen, China) are used to drive the two 3-DOF layer-stacking robotic platforms. Encodes embedded in the motors provide the current relative angular position of each motor shaft, thus enabling a semiclosed loop position control. All the drivers communicate via CAN bus with a PC-104-based personal computer (PC). The C++ software that runs on the PC obtains control instructions from the peripheral equipment (mouse, keyboard, and sensors) and sends the desired velocity of the motors to the controllers through a CAN-PC104 card (PEAK-System Technik, Germany). The relations between each submodule of the control system are described in Figure
Control architecture of the ankle rehabilitation robotic system.
As shown in Figure
Based on the designed ankle rehabilitation robotic platform, a healthy subject with no experience of using the robot was selected to complete three trajectories on each exercise mode mentioned in Section
As introduced in Section
In the early stage of ankle therapy, the patient is unable to move his/her foot; hence, a patient-passive training which can delicately move the patient’s foot is needed. In this stage, the patient’s foot is unable to provide force to move the foot pedal. Therefore, the objective of this rehabilitation stage is to exercise the affected muscles to avoid muscle atrophy.
In order to avoid exercise injury, the robot should provide moderate exercise with a constant speed in the early stage. The experimental scenario is shown in Figure
Experimental scenarios under patient-passive exercise mode: (a) initial posture; (b) dorsiflexion/plantarflexion exercise; (c) inversion/eversion exercise; (d) internal/external rotation exercise.
Parameters for constant-speed ankle exercise.
Parameter joint | Angular velocity (degrees/s) | Range (degrees) |
---|---|---|
Dor./plantar. | 60 | −30~30 |
Inv./ev. | 36 | −20~20 |
Int./ex. rot. | 60 | −20~20 |
The trail of joint velocity was recorded during exercise, and the relation between the experimental result and the target parameter is shown in Figure
Experimental tracking result and setting parameters of patient-passive exercise: (a) dorsiflexion/plantarflexion movement; (b) inversion/eversion movement; (c) internal/external rotation.
In this exercise mode, the patient lays his/her affected side foot on the foot pedal and executes force to push the foot pedal to enhance strength of the affected side ankle. A force threshold is set through GUI, and the foot pedal will move after the detected force on the foot pedal has exceeded the threshold value. The parameters under constant torque-impedance ankle exercise are noted in Table
The setting parameters under constant torque-impedance ankle exercise.
Movement | Dorsiflexion/plantarflexion | Inversion/eversion | Internal/external rotation |
---|---|---|---|
Torque (Nm) | 5 | 2 | 10 |
The experimental result under constant torque-impedance ankle exercise is shown in Figure
Experimental data and the target threshold under patient-active ankle exercise mode: (a) dorsiflexion/plantarflexion movement; (b) inversion/eversion movement; (c) internal/external rotation.
In this training mode, the sound side of the patient’s foot will lie on the foot pedal and drive the foot pedal to move in the ankle range of motion under self-awareness control. Since the mechanism of the affected side robotic platform is symmetric to that of the sound side, the movement of the sound side will be directly mapped to drive the mechanism at the affected side under position control. The experimental scenarios are shown in Figure
Ankle exercise under fusion of awareness and passive exercise modes: (a) movement of dorsiflexion/plantarflexion; (b) movement of inversion/eversion; (c) internal/external rotation.
Corresponding to the ankle exercise shown in Figure
The ankle movement under awareness exercise mode: (a) dorsiflexion/plantarflexion; (b) inversion/eversion; (c) internal/external rotation.
The ankle exercises under constant-speed exercise mode are described in Figures
The mean error and standard deviations on the three exercise modes.
Exercise mode 1 | Trail | Mean error | Standard deviation | ||
1 | 2 | 3 | |||
Dorsiflexion/plantarflexion (°/s) | 6.6 | 6.3 | 5.1 | 6 | 0.79 |
Inversion/eversion (°/s) | 1.2 | 0.6 | 0.9 | 0.9 | 0.3 |
Internal/external rotation (°/s) | 1.2 | 0.6 | 0.9 | 0.9 | 0.3 |
Exercise mode 2 | Trail | Mean error | Standard deviation | ||
1 | 2 | 3 | |||
Dorsiflexion/plantarflexion (Nm) | 6.2 | 5.8 | 6.4 | 6.13 | 0.31 |
Inversion/eversion (Nm) | 3.6 | 3.2 | 3.5 | 3.43 | 0.21 |
Internal/external rotation (Nm) | 8.6 | 8.1 | 8.3 | 8.33 | 0.25 |
Exercise mode 3 | Trail | Mean error | Standard deviation | ||
1 | 2 | 3 | |||
Dorsiflexion/plantarflexion (°) | 6.94 | 6.82 | 5.89 | 6.55 | 0.57 |
Inversion/eversion (°) | 1.69 | 1.47 | 1.52 | 1.56 | 0.12 |
Internal/external rotation (°) | 1.65 | 1.47 | 1.62 | 1.58 | 0.10 |
The above-proposed three exercise modes correspond to different rehabilitation stages. In the constant-speed exercise mode, the patient cannot manage the affected muscle by himself/herself; therefore, the ankle rehabilitation robotic platform will move with a constant moderate speed with the consideration of individual difference, which aims to avoid patient muscle atrophy. The constant-speed exercise mode can be replaced by a torque resistance control scheme, which is used for enhancing the strength of the patient’s ankle. In this mode, the ankle rehabilitation robotic platform is controlled to provide a certain level of torque to the footplate while the patient tries to fully regain his/her range of motion.
In the awareness rehabilitation procedure, the patient can exercise his/her affected side ankle using the motion data captured from the sound side ankle under proprioceptive training. In order to reobtain the individual balance, the patient has to lay his/her sound side ankle and affected ankle on the corresponding robotic platform, respectively, and then drive the sound side to move within the range of motion under his/her awareness, while the affected side ankle robotic platform will track the same trail mapped from that of the sound side ankle robot to exercise the affected ankle.
The experimental results demonstrated that the ankle rehabilitation robotic platform is capable of executing the above three exercise modes. Based on the experimental results, several improvements are needed to be addressed as follows.
Currently, robot users input instructions to the control system via a mouse and keyboard. A touch panel, integrated into the control architecture, will enable the robot user to easily enter the parameters and instructions. Furthermore, the touch panel is portable, which allows the user to set the rehabilitation parameters at a comfortable location.
Although patients can perform ankle training under awareness exercise mode, a virtual training scenario integrated with patient rehabilitation information will stimulate the robot user to concentrate on training and make the rehabilitation become an attractive activity.
This paper presented a novel layer-stacking structure robotic platform used for robot-aided ankle rehabilitations. The ankle rehabilitation robot consists of two symmetric 3-DOF robotic platforms, where one is used to detect the movement of the sound side ankle and the other is used to exercise the affected side ankle. The unique mechanical configuration allows the patient to exercise his/her affected side ankle via the movement taught by the sound side ankle. The rehabilitation protocol has been considered the basis for design of the control architecture. Based on the designed robotic system, three exercise modes including constant-speed exercise, constant torque-impedance exercise, and awareness exercise modes have been developed to perform ankle training corresponding to different rehabilitation stages. The experimental results demonstrated that the promising performance of tracking trail between the two symmetric robotic platforms was obtained with a mean tracking error of 6°/s under constant-speed exercise mode. The robotic platform can move smoothly in the constant torque-impedance mode, and the robotic platform at the affected side can well track the movement of the sound side with the maximum mean error of 6.55°.
Future work will look at the development of a friendly human-machine interface and the integration of a virtual environment based on the robot’s vision to stimulate the patient during training [
The authors declare that there is no conflict of interest regarding the publication of this paper.
Quanquan Liu, Chunbao Wang, and JianJun Long contributed equally to this work.
The authors would like to thank C. Wei, A. Hou, P. Chen, and Z. Lu for the prototype manufacture and thank Dr. Q. Shi for the experimental advice. This work was supported in part by the Science and Technology Foundation of Guangdong, China (nos. 2016A020220001 and 2014A020225004), Technology Research Foundation of Basic Project of Shenzhen, China (nos. JCYJ20160428110354308 and JCYJ20170413095245139), Research Foundation of Health and Family Planning Commission of Shenzhen Municipality, China (nos. SZBC2017006 and 201601054), Returned Overseas Business Foundation of Shenzhen (no. 2016001), Research Foundation of International Cooperation of Shenzhen Municipality (no. GJHZ20170331105318685), and Guangxi Key Scientific and Technological Project (no. 1598013-12).