Towards Optimal Platform-Based Robot Design for Ankle Rehabilitation: The State of the Art and Future Prospects

Table 1

Characteristics description of ten typical ankle rehabilitation robots.

Ankle robot	DOF	Type	Alignment	Posture adjustability	Supplementary information
Yu et al. [15]	1	Wearable	Yes	No	This robot system consists of an ankle-foot module and a knee module, specially designed for gait training.
Park et al. [5]	2	Wearable	Yes	Yes	The prototype generates an ankle range of motion (ROM) of 27° (14° dorsiflexion and 13° plantarflexion). This is acceptable for gait training rather than ankle stretching due to limited workspace.
Roy et al. [6]	2 (3)	Wearable	Yes	Yes	The MIT Anklebot allows normal ROM in all three DOFs of the foot during walking overground, on a treadmill, or while sitting. Two DOFs are actively actuated by motors.
Zhang et al. [7]	1	Platform	Yes	Yes	This device is portable and low cost, making it available to patients for frequent and long-term use at clinics or home.
OptiFlex Ankle CPM [20]	2	Platform	Yes	Yes	A commercial device for full ankle ROM exercises along dorsiflexion/plantarflexion and inversion/eversion.
Girone et al. [18]	6	Platform	Depending on control design	Yes	Limited robot workspace if controlled for aligned rotation center between the robot and the ankle joint.
Saglia et al. [3]	2	Platform	No	Yes	This robot used customized linear actuator to meet the required forces and torques for strengthening and balance exercises.
Tsoi et al. [16]	3 (6)	Platform	Yes	No	This robot itself has six DOFs and three rotational DOFs if considering the human ankle as a constraint. It can be only used in an upright posture.
Jamwal et al. [8]	3	Platform	Yes	Yes	Limited actuation torque for patient-robot interactive training.
Zhang [21]	3	Platform	Yes	Yes	Optimization techniques should be involved to enhance the usability and functionality of this robot.
Wang et al. [22]	2 or 3	Platform	Yes	Yes	This device has the flexibility of reconfiguring into either a 2-DOF rehabilitation device or a 3-DOF one depending on the specific exercise mode. But it has not been validated experimentally.

Note: the robot degrees of freedom are calculated based on the famous Chebyshev-Grubler-Kutzbach criterion [23]. Degrees of freedom of a mechanism , where refers to the degrees of freedom of the space, is the number of links including the base, is the number of binary joints, is the degrees of relative motion permitted by joint , and is denoted for the total number of passive degrees of freedom.