Proxy-based Sliding Mode Control Research Articles

Passive and Active Control Strategies of a Leg Rehabilitation Exoskeleton Powered by Pneumatic Artificial Muscles

Nerve injury can cause lower limb paralysis and gait disorder. Currently lower limb rehabilitation exoskeleton robots used in the hospitals need more power to correct abnormal motor patterns of stroke patients’ legs. These gait rehabilitation robots are powered by cumbersome and bulky electric motors, which provides a poor user experience. A newly developed gait rehabilitation exoskeleton robot actuated by low-cost and lightweight pneumatic artificial muscles (PAMs) is presented in this research. A model-free proxy-based sliding mode control (PSMC) strategy and a model-based chattering mitigation robust variable control (CRVC) strategy were developed and first applied in rehabilitation trainings, respectively. As the dynamic response of PAM due to the compressed air is low, an innovative intention identification control strategy was taken in active trainings by the use of the subject’s intention indirectly through the estimation of the interaction force between the subject’s leg and the exoskeleton. The proposed intention identification strategy was verified by treadmill-based gait training experiments.

An online gain tuning proxy-based sliding mode control using neural network for a gait training robotic orthosis

In the last decades, wearable powered orthoses have been introduced in the state of the art for lower-limb rehabilitation. Most of these applications are driven by electric motors. Comparing with electric motor actuators, pneumatic artificial muscle (PAM) actuators are compliant because of the elasticity of PAMs. Consequently, for more safety and comfort of lower-limb rehabilitation, a compliant robotic orthosis powered by PAMs is developed. Based on safe control, a new control method, proxy-based sliding mode control (PSMC), was introduced into rehabilitation robotics a few years ago. It combined safety and accuracy of tracking to make it suitable for the safe control of PAM actuators. As the reason of low frequency response of PAM actuators and variable loads caused by different human subjects, the fixed parameters of PSMC makes the tracking performance vary from subject to subject, and lacks robustness. This paper presents a modification of PSMC by using neural network to tune PSMC gains online, and implements both PSMC and modified PSMC control schemes in the robotic orthosis. Experimental results demonstrate that the improved PSMC method performs better on tracking with little degradation on safety for different loads and human subjects.

Proxy-based sliding mode control of a robotic ankle-foot system for post-stroke rehabilitation

Robotic platform-based ankle–foot rehabilitation systems have been proved effective in treating joint spasticity and/or contracture of stroke survivors. However, simple force or velocity limiters are not adequate, since they cannot explicitly guarantee slow and overdamped motions without overshoot. In this paper, we propose a proxy-based sliding mode control (PSMC)-based approach, to avoid unsafe behaviors of a robotic ankle–foot rehabilitation system. The proposed method has three advantages: (1) without deteriorating tracking performance during normal operation, it guarantees overdamped, slow, and safe recoveries after abnormal events; (2) it provides a simple and accurate way to confine the output torque exerted on the subject’s ankle; (3) though effective, the control law avoids the necessity to identify the specific system model or build state observer, which is usually difficult for human–robot interaction system. A 71-year-old stroke patient and 10 able-bodied subjects were recruited for the experiments. Preliminary studies comparing PSMC and PID are performed on trajectory tracking, controlled torque output, slow and safe response under disturbance. Additionally, by fulfilling the rehabilitation method and obtaining biomechanical indicators, the proposed controller is proved to be feasible for the system.

Proxy-Based Sliding-Mode Tracking Control of Piezoelectric-Actuated Nanopositioning Stages

In this paper, a proxy-based sliding-mode control (PBSMC) approach is proposed for robust tracking control of a piezoelectric-actuated nanopositioning stage composed of piezoelectric stack actuators and compliant flexure mechanisms. The essential feature of the PBSMC approach is the introduction of a virtual coupling proxy, which is controlled by the sliding-mode controller (SMC) to track the desired position. Simultaneously, due to the virtual coupling, a proportional-integral-derivative (PID) controller on the other side of the proxy ensures the position of the end-effector of the stage to follow the position of the proxy. Therefore, the PBSMC guarantees the end-effector to track the desired trajectory. The advantages of the developed PBSMC lie in the facts that 1) the discontinuous signum function in the traditional SMC is omitted without any approximation. Hence, the output of the PBSMC is continuous, which does not suffer from the chattering phenomenon; and 2) the PBSMC laws are developed without having the necessity to include the nominal system model, hysteresis model or the state observer. Hence, the PBSMC provides a novel effective yet simple control method, which permits to avoid the lack of performances from PID and the chattering from SMC, and permits to combine the advantages from them. The stability of the closed-loop control system is proved through Lyapunov analysis. Finally, comparative studies are performed on a custom-built piezo-actuated stage. Experimental results show that the tracking errors of the PBCM are reduced by 74.22%, as compared to the traditional PID controller, with the desired sinusoidal trajectory under the 50-Hz input frequency, which clearly demonstrates the superior tracking performance of the PBSMC.

Proxy-based sliding mode control on platform of 3 degree of freedom (3-DOF)

Proxy-based sliding mode control, created by Ryo Kikuuwe and Hideo Fujimoto, combines the advantages of sliding mode control and proportional integral derivative controller. This paper implements this control algorithm on a platform of 3 degrees of freedom (3-DOF) driven by electropneumatic actuators with proportional flow valves. This paper presents an extension of proxy-based sliding mode control. This time it is implemented on a parallel robot. Despite the robot being a coupled system, is reached a perfect decoupled control. The simulations in MSC-ADAMS and experimental results demonstrate good tracking.

Safe and Compliant Guidance by a Powered Knee Exoskeleton for Robot-Assisted Rehabilitation of Gait

In the research field of robot-assisted gait rehabilitation there is increased focus on the improvement of physical human–robot interaction by means of high-performance actuator technologies and dedicated control strategies. In this context we propose a combination of lightweight, intrinsically compliant, high-torque actuators (pleated pneumatic artificial muscles) with safe and adaptable guidance along a target trajectory by means of proxy-based sliding mode control. We developed a powered knee exoskeleton (KNEXO) to evaluate these concepts. In addition to the trajectory-based controller a torque controller was implemented with a view to minimizing the interaction during unassisted walking. First, various treadmill walking experiments were performed with unimpaired subjects wearing KNEXO to evaluate the performance of the proposed controllers. Test results confirm the ability of KNEXO to display low actuator torques in unassisted mode and to provide safe, adaptable guidance in assisted mode. Subsequently, a multiple sclerosis patient participated in a series of pilot experiments. Provided there was some patient-specific controller tuning KNEXO was found to effectively support and compliantly guide the subject's knee.

Proxy-Based Sliding Mode Control: A Safer Extension of PID Position Control

High-gain proportional-integral-derivative (PID) position control involves some risk of unsafe behaviors in cases of abnormal events, such as unexpected environment contacts and temporary power failures. This paper proposes a new position-control method that is as accurate as conventional PID control during normal operation, but is capable of slow, overdamped resuming motion without overshoots from large positional errors that result in actuator-force saturation. The proposed method, which we call proxy-based sliding mode control (PSMC), is an alternative approximation of a simplest type of sliding mode control (SMC), and also is an extension of the PID control. The validity of the proposed method is demonstrated through stability analysis and experimental results.

The Safety of a Robot Actuated by Pneumatic Muscles—A Case Study

In situations where robots share their workspace with humans, and where physical human-robot interaction is possible or even necessary, safety is of paramount importance. This paper presents a study of the safety of a lightweight robot actuated by pneumatic muscles. Due to its low weight, it has excellent hardware safety characteristics. In spite of this, it is shown that the system can be unsafe when under PID control. It is also shown that safety can be greatly increased by using Proxy-Based Sliding Mode Control (PSMC). The role of passive compliance in safety is also investigated. It is argued that passive compliance can have positive as well as negative effects on robot safety, depending on the situation.

Proxy-based Sliding Mode Control of a Planar Pneumatic Manipulator

For a robotic system that shares its workspace with humans and physically interacts with them, safety is of paramount importance. In order to build a safe system, safety has to be considered in both hardware and software (control). In this paper, we present the safe control of a two-degree-of-freedom planar manipulator actuated by Pleated Pneumatic Artificial Muscles. Owing to its low weight and inherent compliance, the system hardware has excellent safety characteristics. In traditional control methods, safety and good tracking are often impossible to combine. This is different in the case of Proxy-Based Sliding Mode Control (PSMC), a novel control method introduced by Kikuuwe and Fujimoto. PSMC combines responsive and accurate tracking during normal operation with smooth, slow and safe recovery from large position errors. It can also make the system behave compliantly to external disturbances. We present both task- and joint-space implementations of PSMC applied to the pneumatic manipulator, and compare their performance with PID control. Good tracking results are obtained, especially with the joint-space implementation. Safety is evaluated by means of the Head Injury Criterion and by the maximum interaction force in the case of collision. It is found that in spite of the hardware safety features, the system is unsafe when under PID control. PSMC, on the other hand, provides increased safety as well as good tracking.

A Guideline for Low-Force Robotic Guidance for Enhancing Human Performance of Positioning and Trajectory Tracking: It Should Be Stiff and Appropriately Slow

This paper considers the application of a low-force robotic manipulator to guide a human user's movements to place a tool (or the user's hand) at a predetermined position or move it along a predetermined trajectory. This application is potentially useful, e.g., skill training for humans, rehabilitation, and human-machine coordination in the manufacturing industry. A proportional-derivative (PD)-type position control can be used for this application, but the parameters for the controller should be appropriately chosen for enhancing the human performance of positioning and trajectory tracking. We hypothesize that the robot's position control should be stiff and appropriately slow, i.e., the proportional gain should be high and the time constant (the ratio of the derivative gain to the proportional gain) should be appropriately large. Such characteristic has been difficult to be realized in ordinary PD position control because it requires direct high-gain velocity feedback. However, our recent technique, which is proxy-based sliding mode control (PSMC), is capable of producing such a hypothetically preferred response and allows us to empirically validate the hypothesis. The results of experiments using two distinctly different robotic devices supported the hypothesis, showing that the time constant should be set around 0.1 s rather than 0.01 and 0.5 s.