Nikolaos G. Tsagarakis

Development and Control of a ‘Soft-Actuated’ Exoskeleton for Use in Physiotherapy and Training

Full or partial loss of function in the upper limb is an increasingly common due to sports injuries, occupational injuries, spinal cord injuries, and strokes. Typically treatment for these conditions relies on manipulative physiotherapy procedures which are extremely labour intensive. Although mechanical assistive device exist for limbs this is rare for the upper body.In this paper we describe the construction and testing of a seven degree of motion prototype upper arm training/rehabilitation (exoskeleton) system. The total weight of the uncompensated orthosis is less than 2 kg. This low mass is primarily due to the use of a new range of pneumatic Muscle Actuators (pMA) as power source for the system. This type of actuator, which has also an excellent power/weight ratio, meets the need for safety, simplicity and lightness. The work presented shows how the system takes advantage of the inherent controllable compliance to produce a unit that is extremely powerful, providing a wide range of functionality (motion and forces over an extended range) in a manner that has high safety integrity for the patient. A training control scheme is introduced which is used to control the orthosis when used as exercise facility. Results demonstrate the potential of the device as an upper limb training, rehabilitation and power assist (exoskeleton) system.

A compact soft actuator unit for small scale human friendly robots

This paper presents the development of a new compact soft actuation unit intended to be used in multi degree of freedom and small scale robotic systems such as the child humanoid robot “iCub” [1]. Compared to the other existing series elastic linear or rotary implementations the proposed design shows high integration density and wider passive deflection. The miniaturization of the newly developed high performance unit was achieved with a use of a new rotary spring module based on a novel arrangement of linear springs.

Enhanced modelling and performance in braided pneumatic muscle actuators

Pneumatic technology has been successfully applied for over two millennia. Even today, pneumatic cylinder based technology forms the keystone of many manufacturing processes where there is a need for simple, high-speed, low-cost, reliable motion. But when the system requires accurate control of position, velocity or acceleration profiles, these actuators form a far from satisfactory solution. Braided pneumatic muscle actuators (pMAs) form an interesting development of the pneumatic principle offering even higher power/weight performance, operation in a wide range of environments and accurate control of position, motion and force. This technology provides an interesting and potentially very successful alternative actuation source for robots as well as other applications. However, there are difficulties with this approach due to the following. (i) Modeling errors. Models of the force response are still nonoptimal and for good results these models are highly complex, which makes accurate design difficult. (ii) Low bandwidth—the bandwidth of the actuator—link assemblies are often considered to be too low for practical success in many applications, particularly robotics. In this paper we address these limitations and show how the performance in each area can be enhanced with overall improvements in the response and utility of the braided pMAs.

Improved modelling and assessment of pneumatic muscle actuators

Traditional robotic/mechatronic design has successfully exploited the attributes of heavy mechanical systems engineering, but future scientific trends suggest a need for technology that will emulate natural systems. Among the most pressing of the requirements are actuation systems that can interact in a safer and more natural way. Pneumatic technology has many of the compliance forms needed for this softer interaction and a number of new systems based on McKibben muscles have been developed in recent years. In this paper a new model of operation of pneumatic muscle systems is developed. In particular, the model considers the distortion effects at the termination nodes and the radial pressure loss due to rubber elasticity. The new model is compared experimentation on a very large actuator and shows how this new model improves the assessment of forces and displacement that can be achieved by the actuator. The new model is compared against previous systems models.

A novel actuator with adjustable stiffness (AwAS)

This paper describes the design and development of a new actuator with adjustable stiffness (AwAS) which can be used in robots which are necessary to work close to or physically interact with humans, e.g. humanoids and exoskeletons. The actuator presented in this work can independently control equilibrium position and stiffness by two motors. The first motor controls the equilibrium position while the second motor regulates the compliance. The novelty of the proposed design with respect to the existing systems is on the principle used to regulate the compliance. This is done not through the tuning of the pretension of the elastic element as in the majority of existing system but by controlling the fixation of the elastic elements (springs) using a linear drive. An important consequence of this approach is that the displacement needed to change the stiffness is perpendicular to the forces generated by the springs, thus this helps to minimize the energy/power required to change the stiffness. This permits the use of a small motor for the stiffness adjustment resulting in a lighter setup. Experimental results are presented to show the ability of AwAS to control position and regulate the stiffness independently.

An integrated tactile/shear feedback array for stimulation of finger mechanoreceptor

VR and telepresence applications have placed increasing demands on the need for effective user interfaces. To date most of the interfaces have emphasised the use of visual and audio effects but tactile feedback has been identified as a leading feature for future systems where there will be an increased desire to truly interact with the virtual/remote world rather than being observational. The paper focuses on the cutaneous aspects of tactile feedback describing the design and construction of pneumatically powered tactile and shear feedback modules. It is shown that by incorporating a range of novel features into this design it is possible to stimulate all the mechano-receptive nerves (SAI, SAII, RAI, and RAII) with localised signals from DC to 400 Hz. All this is shown in a fully integrated, ultra-light and comfortable package. The design control and performance results are all presented.

AwAS-II: A new Actuator with Adjustable Stiffness based on the novel principle of adaptable pivot point and variable lever ratio

The Actuator with Adjustable Stiffness (AwAS) is an actuator which can independently control equilibrium position and stiffness by two motors. The first motor controls the equilibrium position while the second motor regulates the compliance. This paper describes the design and development of AwAS-II which is an improved version of the original realization. AwAS tuned the stiffness by controlling the location of the springs and adjusting its arm, length. Instead AwAS-II regulates the compliance by implementing a force amplifier based on a lever mechanism on which a pivot point can adjust the force amplification ratio from zero to infinitive. As in the first implementation, the actuator which is responsible for adjusting the stiffness in AwAS II is not working against the spring forces. Its displacement is perpendicular to the force generated by springs which makes changing the stiffness energetically efficient. As the force amplification ratio can theoretically change from zero to infinitive consequently the level of stiffness can tune from very soft to completely rigid. Because this range does not depends on the springs rate and length of the lever, thus soft springs and small lever can be used which result in a lighter and more compact setup. Furthermore as the lever arm is shorter the time required for the stiffness regulation is smaller.

A new variable stiffness actuator (CompAct-VSA): Design and modelling

This paper describes the design and modelling of a new variable stiffness actuator (CompAct-VSA). The principle of operation of CompAct-VSA is based on a lever arm mechanism with a continuously regulated pivot point. The proposed concept allows for the development of an actuation unit with a wide range of stiffness and a fast stiffness regulation response. The implementation of the actuator makes use of a cam shaped lever arm with a variable pivot axis actuated by a rack and pinion transmission system. This realization results in a highly integrated and modular assembly. Size and weight are indeed an open issue in the VSAs design, which ultimately limit their implementation in multi-dof robotic systems. The paper introduces the mechanics, the principle of operation and the model of the actuator. Preliminary results are presented to demonstrate the fast stiffness regulation response and the wide range of stiffness achieved by the proposed CompAct-VSA design.

The design of the lower body of the compliant humanoid robot “cCub”

The “iCub ”is a robotic platform that was developed by the RobotCub [1] consortium to provide the cognition research community with an open “child-like ”humanoid platform for understanding and development of cognitive systems [1]. In this paper we present the mechanical realization of the lower body developed for the “cCub ”humanoid robot, a derivative of the original “iCub”, which has passive compliance in the major joints of the legs. It is hypothesized that this will give to the robot high versatility to cope with unpredictable disturbance ranging from small uneven terrain variations to unexpected collisions or even accidental falls. As part of the AMARSI European project, the passive compliance of this newly developed robot will be exploited for safer interaction, energy efficient and more aggressive damage-safe learning. The passive compliant actuation module used is a compact unit based on the series elastic actuator principle (SEA). In addition to the passive compliance the “cCub ”design includes other significant updates over the original prototype such as full joint state sensing including joint torque sensing and improved range of motion and torque capabilities. In this paper, the new leg mechanisms of the “cCub ”robot are introduced.

Tele-impedance: Teleoperation with impedance regulation using a body-machine interface

This work presents the concept of tele-impedance as a method for remotely controlling a robotic arm in interaction with uncertain environments. As an alternative to bilateral force-reflecting teleoperation control, in tele-impedance a compound reference command is sent to the slave robot including both the desired motion trajectory and impedance profile, which are then realized by the remote controller without explicit feedback to the operator. We derive the reference command from a novel body–machine interface (BMI) applied to the master operator’s arm, using only non-intrusive position and electromyography (EMG) measurements, and excluding any feedback from the remote site except for looking at the task. The proposed BMI exploits a novel algorithm to decouple the estimates of force and stiffness of the human arm while performing the task. The endpoint (wrist) position of the human arm is monitored by an optical tracking system and used for the closed-loop position control of the robot’s end-effector. The concept is demonstrated in two experiments, namely a peg-in-the-hole and a ball-catching task, which illustrate complementary aspects of the method. The performance of tele-impedance control is assessed by comparing the results obtained with the slave arm under either constantly low or high stiffness.