Elsevier

Mechatronics

Volume 23, Issue 8, December 2013, Pages 1072-1083
Mechatronics

Control design of a novel compliant actuator for rehabilitation robots

https://doi.org/10.1016/j.mechatronics.2013.08.004Get rights and content

Abstract

Rehabilitation robots have direct physical interaction with human body. Ideally, actuators for rehabilitation robots should be compliant, force controllable, and back drivable due to safety and control considerations. Series Elastic Actuators (SEA) offers many advantages for these applications and various designs have been developed. However, current SEA designs face a common performance limitation due to the compromise on the spring stiffness selection. This paper presents a novel compact compliant force control actuator design for portable rehabilitation robots to overcome the performance limitations of current SEAs. Our design consists of a servomotor, a ball screw, a torsional spring between the motor and the ball screw, and a set of translational springs between the ball screw nut and the external load. The soft translational springs are used to handle the low force operation, while the torsional spring with high effective stiffness is used to deal with the large force operation. It is a challenging task to design the controller for such a novel design as the control system needs to handle both the force ranges. In this paper, we develop the force control strategy for this actuator. First, two dynamical models of the actuator are established based on different force ranges. Second, we propose an optimal control with friction compensation and disturbance rejection which is enhanced by a feedforward control for the low force range. The proposed optimal control with feedforward term is also extended to the high force range. Third, a switching control strategy is proposed to handle a transition between low force and high force control. The mathematical proof is given to ensure the stability of the closed-loop system under the proposed switching control. Finally, the proposed method is validated with experimental results on a prototype of the actuator system and is also verified with an ankle robot in walking experiments.

Introduction

In classic industrial applications such as the autonomous welding systems for the automobile industry, robots are always designed with stiff actuators for precise and rapid position control with good repeatability. Stiff actuators are good for handling external disturbance forces and internal frictions, but cannot handle external impacts and shocks. Therefore, they are most suitable for working environments that are well defined with no direct physical interactions with humans. In recent years, due to the rapidly aging populations in most developed nations, there is a strong need for service robots, assistive and rehabilitation robots in both domestic [1], [2] and hospital settings [3], [4]. In these applications, the robots need to adjust with versatile unstructured environments as well as human demands [5]. Stiff actuators may be unsuitable for unstructured environments and the environments where human’s safety is a vital issue. This motivates the need of research on compliant actuators (CAs) because of their ability to safely interact with the user, and their ability to store and release energy in passive elastic elements. The most well-known CA is the series elastic actuator (SEA) [6], [7] where a spring is placed between the motor and the load. Compared to a stiff actuator, series elastic actuators have the following benefits [8]:

  • The actuators exhibit lower output impedance and back-driveability.

  • Shock tolerance is greatly improved by the springs.

  • The force transmission is smooth.

  • Energy can be stored and released in the elastic element, thereby improving efficiency in applications.

Many different SEAs have been developed for rehabilitation robots to capitalize the advantages of the SEA concept. In [9], [10], the SEA is designed based on a linear spring coupled to a ballscrew which is connected to a dc motor. In [11], [12], a Bowden cable is connected to linear spring to achieve a rotary SEA. In [13], [14], rotary SEAs are designed in which torsional spring is used to transmit the output force. Although current SEAs have achieved reasonable performance, they still face a common fundamental limitation, which is the fixed spring constant of the elastic element as discussed by Pratt et al. in [9], [15]. The performance of the SEAs largely depends on spring constant [15]. Soft spring produces high fidelity of force control, low output impedance, and reduces stiction, but also limits the force range and the force control bandwidth at high force range. On the other hand, stiff spring increases large force bandwidth, but reduces force fidelity. In order to achieve the desired output force/torque, most current SEAs are designed with very stiff springs, leading to compromised force control performance, low intrinsic compliance and back-drivability, and bulky and heavy systems. The novel design presented in this paper aims to overcome the above limitations of the conventional SEAs while improve the performance. This design concept was first proposed in [16], [17] and supported with simulation results and simple experiments.

Apart from the mechanical development of SEAs, the control design of SEAs is also gaining attention in recent years. Many controllers have been presented for SEA and their performance have also been analyzed. The following literature will give a brief insight in this area. In [10], [11], [15], [18], pure PID control is used to produce a desired output force. In [9], PD plus feedforward control is used to improve the dynamical performance for a class of SEAs. In [13], [19], [20], [21], a type of cascaded control is presented to ensure stability in human interacting devices where a PI torque control is used in the outer loop, while a PI velocity control is used in the inner loop. In [6], a modified PID with feedforward term and human joint compensator is designed to generate desired force and low impedance. In [14], based on the model of a SEA, the authors further improve the human joint compensator used in [6] by adding a low pass filter. A disturbance observer is also designed to compensate the modeling error. In [22], the authors use a feedback plus feedforward force control which is enhanced by a disturbance observer to compensate for plant variations, where feedback and feedforward controls are optimally designed. However, all these current controllers are designed based on the actuators with fixed stiffness spring, either linear spring or torsional spring. For an actuator working with both types of springs in different force ranges, existing results are not available.

In this paper, we present a novel compliant actuator system and its force control design. Unlike existing SEAs, our system uses two types of springs (torsional and translational) at different force ranges. In order for this SEA to work well, there are challenges to be addressed in the control system. An adequate control strategy should be designed to deal with system dynamics and make a transition between high force and low force. The goal of the paper is to address these issues. First, two dynamical models are established with respect to low force range and high force range. Second, an optimal control is designed for low force range. Since there is a frictional behavior at the low force, we design a compensator in our controller to deal with the friction. As sliding mode control has been used as general tools for handling unknown nonlinear uncertainties [23], we incorporate it into our controller to deal with the unknown disturbance. The proposed optimal control plus feedforward term is also extended to high force range. Third, a switching control is proposed to make a smooth transition between low force and high force. The mathematical analysis is given to prove the stability of the closed-loop system. Finally, experimental results are provided to verify the effectiveness of the proposed method.

Section snippets

Compliant actuator design for gait rehabilitation robot

Gait disorders are common for patients post stroke and in most cases cannot be treated medically or surgically. Therefore, treatment often relies on rehabilitation service. Rehabilitation robotics has shown promise in providing patients with intensive therapy leading to functional gains. This involves the use of a robot exoskeleton device or end-effector device to help the patient retrain motor coordination by performing gait movement. Here, we are developing a portable wearable knee ankle

Control system and controller design

Controlling a compliant force actuator can be defined as achieving output forces according to certain requirements. In this section, we describe a detailed control design for our novel actuator, including hardware configuration and model-based control algorithms.

Experimental results

In this section, the proposed control is applied to a real compliant actuator. The experimental setup is shown in Fig. 5. The system consists of the linear compliant actuator, two encoders, a motor driver and a PC with the controller. The compliant actuator is actuated by one brushless dc motor (MAXON 311537), capable of generating 1130mNm stall torque. The first encoder (it is Encoder 1 in Fig. 5) is used to check the low force, while this encoder with other encoder (it is Encoder 2 in Fig. 5)

Conclusion

We have presented a novel compliant actuator design and the control system for this actuator. By introducing one extra torsional spring at the high force range, a truly compliant actuator with excellent force control fidelity. Theoretical analysis for the controller design has been given to guarantee the stability of the closed-loop system, especially for the switching control law. Experimental results have confirmed that the proposed controller can achieve good performance for the low, high

References (23)

  • S. Jezernik et al.

    Robotic orthosis lokomat: a rehabilitation and research tool

    Neuromodulation

    (2003)
  • T. Sugar

    A novel selective compliant actuator

    Mechatronics

    (2002)
  • H. Yu et al.

    An adaptive shared control system for an intelligent mobility aid for the elderly

    Autonomous Robots

    (2003)
  • C. Zhu, M. Oda, H. Yu, H. Watanabe, Y. Yan, Walking support and power assistance of a wheelchair typed omnidirectional...
  • T. Nef, M. Mihelj, G. Colombo, R. Riener, ARMin-robot for rehabilitation of the upper extremities, in: Proceedings of...
  • M. Wassink, R. Carloni, S. Stramigioli, Port–Hamiltonian analysis of a novel robotic finger concept for minimal...
  • G.A. Pratt, M.M. Williamson, Series elastic actuators, in: IEEE International Workshop on Intelligent Robots and...
  • J. Pratt et al.

    Series elastic actuators for high fidelity force control

    Industrial Robot

    (2002)
  • D.W. Robinson, J.E. Pratt, D.J. Paluska, G.A. Pratt, Series elastic actuator development for a biomimetic walking...
  • J.E. Pratt, B.T. Krupp, C.J. Morse, S.H. Collins, The roboknee: an exoskeleton for enhancing strength and endurance...
  • J.F. Veneman et al.

    A series elastic- and bowden-cable-based actuation system for use as torque actuator in exoskeleton-type robots

    The International Journal of Robotics Research

    (2006)
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