Exoskeleton control using active impedance
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Active Impedance:
A Novel Concept in Assistive Exoskeleton Control


This work is a collaboration with Prof. Ed Colgate, Prof. Michael Peshkin and Gabriel-Aguirre Ollinger of Northwestern University, Evanston, Illinois.
We present a novel form of lower-limb assist, consisting of making the human limb interact with an exoskeleton that displays active mechanical impedance. Our overarching goal is to develop a control method for exoskeletons that provides sufficient flexibility to assist a wide variety of lower-limb motions, such as can be encountered in activities of daily living. Our approach to human assist is based on enhancing the kinematic response of the human limbs. Exoskeleton designs can be classified in terms of their assistive capabilities as either passive or active devices. Exoskeletons that display passive behavior assist human users mainly by helping them employ their own muscle power more effectively, but do not actually supply energy to the user. In passive gravity support, an unactuated orthotic device can provide partial support of the userís weight by forming a mechanical path to the ground. Gravity balance of the freely-moving leg using springs has been implemented by other. Load-carrying assist is a special case of gravity support, typified by the BLEEX system, in which the exoskeleton supports a load carried by the user. The exoskeletonís controller uses positive kinematic feedback to scale up the deviceís mechanical admittance, however, the device remains passive in its interaction with the user.

Our work focuses on active exoskeleton assist. An active device is one that can behave as a continuous energy source. Probably the most common approach to active assist is using the musclesí electromyographical (EMG) activity to control the actuators of the exoskeleton or orthosis.

The above figure shows a model of a 1-DOF exoskeleton designed to assist the motion of the knee joint. For the case of linear behavior, this device can be modeled as a linear time invariant second-order rotational system.

The above figure shows the coupled system formed by the exoskeletonís virtual impedance and the human limb. For simplicity we are assuming the coupling between the exoskeleton and the human to be rigid.

We propose using the virtual modification of the lower limbís impedance as the primary source of human assist. The present study focuses on the use of negative exoskeleton damping, which is a particular case of active-impedance control. Because natural damping is an energy dissipation term, it makes sense to consider negative damping as a way to source energy from the exoskeleton to the user. Furthermore, since damping is a velocity-dependent effect, the exoskeleton-human interaction forces generated by negative damping become nearly zero when the leg is at rest or in quasi-static motion.

The general admittance controller used for our system.


For the actual implementation of the exoskeleton we have chosen an admittance control scheme : a torque sensor measures the interaction torque between the exoskeleton and the user, and uses it to issue a trajectory command. Our experimental platform is a 1-DOF exoskeleton mounted on a rigid base(see figure below), designed to assist a person performing knee extensions and flexions. A custom-built ankle brace couples the userís leg to the exoskeleton arm. The armís construction has been made as lightweight as possible in order to minimize its inertial effects. The ankle brace is mounted on a sliding bracket in order to accommodate any possible radial displacement of the ankle relative to the deviceís center of rotation.

The 1-DOF exoskeleton is designed for high backdriveability; to that purpose we have chosen to employ an AC servomotor with a large torque capability, and a low-ratio cable-drive transmission. The main advantage of the cable drive is the elimination of transmission backlash and friction, both of which can be a hindrance to impedance control, especially in the active region. The cable drive is similar in concept to that of the PHANToM haptic device. A potential disadvantage of this type of transmission is axial cable compliance, which limits the bandwidth of the mechanism. However, since typical lower-limb motions occur at low frequencies, the bandwidth requirements for this application are not particularly demanding.The figure below shows a detail of the exoskeletonís main assembly, consisting of the servomotor, the drive transmission and the exoskeleton arm. The motor is a brushless direct-drive AC servo with a power rating of 0.99kW and a continuous torque rating of 2.0Nm; it features an emulated encoder output of upto 32,768 counts before quadrature. The transmission ratio of the cable drive is 10:1, thus allowing a continuous torque output of 20.0Nm.




A list of my papers on this topic:

  • Gabriel Aguirre-Ollinger, J. Edward Colgate, Michael A. Peshkin, and A. Goswami,
    Inertia Compensation Control of a One-Degree-of-Freedom Exoskeleton for Lower-Limb Assistance: Initial Experiments,
    IEEE Transactions on Neural Systems Rehabilitation Engineering, Vol. 20, No. 1, January 2012.

    (pdf).


    Abstract
    A new method of lower-limb exoskeleton control aimed at improving the agility of leg-swing motion is presented. In the absence of control, an exoskeleton's mechanism usually hinders agility by adding mechanical impedance to the legs. The uncompensated inertia of the exoskeleton will reduce the natural frequency of leg swing, probably leading to lower step frequency during walking as well as increased metabolic energy consumption. The proposed controller emulates inertia compensation by adding a feedback loop consisting of low-pass filtered angular acceleration multiplied by a negative gain. This gain simulates negative inertia in the low-frequency range. The resulting controller combines two assistive effects: increasing the natural frequency of the lower limbs and performing net work per swing cycle. The controller was tested on a statically mounted exoskeleton that assists knee flexion and extension. Subjects performed movement sequences, first unassisted and then using the exoskeleton, in the context of a computer-based task resembling a race. In the exoskeleton's baseline state, the frequency of leg swing and the mean angular velocity were consistently reduced. The addition of inertia compensation enabled subjects to recover their normal frequency and increase their selected angular velocity. The work performed by the exoskeleton was evidenced by catch trials in the protocol.

  • Gabriel Aguirre-Ollinger, J. Edward Colgate, Michael A. Peshkin, and A. Goswami,
    Design of an Active 1-DOF Lower-Limb Exoskeleton with Inertia Compensation,
    The International Journal of Robotics Research, vol. 30, no. 4, April 2011.

    (pdf).

    My affiliations in the paper are incorrect, please note the Corrigendum
    (pdf).

    Abstract
    Limited research has been done on exoskeletons to enable faster movements of the lower extremities. An exoskeleton's mechanism can actually hinder agility by adding weight, inertia and friction to the legs; compensating inertia through control is particularly dicult due to instability issues. The added inertia will reduce the natural frequency of the legs, probably leading to lower step frequency during walking. We present a control method that produces an approximate compensation of an exoskeleton's inertia. The aim is making the natural frequency of the exoskeleton-assisted leg larger than that of the unaided leg. The method uses admittance control to compensate the weight and friction of the exoskeleton. Inertia compensation is emulated by adding a feedback loop consisting of low-pass ltered acceleration multiplied by a negative gain. This gain simulates negative inertia in the low-frequency range. We tested the controller on a statically supported, single-DOF exoskeleton that assists swing movements of the leg. Subjects performed movement sequences, rst unassisted and then using the exoskeleton, in the context of a computer-based task resembling a race. With zero inertia compensation, the steady-state frequency of leg swing was consistently reduced. Adding inertia compensation enabled subjects to recover their normal frequency of swing.

  • Gabriel Aguirre-Ollinger, J. Edward Colgate, Michael A. Peshkin, and A. Goswami,
    A 1-DOF Assistive Exoskeleton with Inertia Compensation: Effects on the Agility of Leg Swing Motion,
    Proceedings of the Institution of Mechanical Engineers, Part H, Journal of Engineering in Medicine, vol. 225, no. 3, pp. 228-245, 2011.

    (pdf).

    Abstract
    Many of the current implementations of exoskeletons for the lower extremities are conceived to either augment the userís load-carrying capabilities or reduce muscle activation during walking. Comparatively little research has been conducted on enabling an exoskeleton to increase the agility of lower-limb movements. One obstacle in this regard is the inertia of the exoskeletonís mechanism, which tends to reduce the natural frequency of the human limbs. A control method is presented that produces an approximate compensation of the inertia of an exoskeletonís mechanism. The controller was tested on a statically mounted, single- DOF exoskeleton that assists knee flexion and extension. Test subjects performed multiple series of leg-swing movements in the context of a computer-based, sprint-like task. A large initial acceleration of the leg was needed for the subjects to track a virtual target on a computer screen. The uncompensated inertia of the exoskeleton mechanism slowed down the transient response of the subjectsí limb, in comparison with trials performed without the exoskeleton. The subsequent use of emulated inertia compensation on the exoskeleton allowed the subjects to improve their transient response for the same task.

  • Gabriel-Aguirre Ollinger, J. Edward Colgate, Michael Peshkin , and A. Goswami,
    A 1-DOF Assistive Exoskeleton with Virtual Negative Damping: Effects on the Kinematic Response of the Lower Limbs,
    IROS 2007, San Diego, CA.

    (pdf).

    Abstract: We propose a novel control method for lowerlimb assist that produces a virtual modification of the mechanical impedance of the human limbs. This effect is accomplished through the use of an exoskeleton that displays active impedance. The proposed method is aimed at improving the dynamic response of the human limbs, while preserving the userís control authority. Our goal is to use active-impedance exoskeleton control to improve the userís agility of motion, for example by reducing the average time needed to complete a movement.

    Our control method has been implemented in a 1-DOF exoskeleton designed to assist human subjects performing knee flexions and extensions. In this paper we discuss an initial study on the effect of negative exoskeleton damping (a particular case of active-impedance control) on the subjectís time to complete a target-reaching motion. Experimental results show this effect to be statistically significant. On average, subjects were able to reduce the time to complete the motion by 16%. 

  • Gabriel-Aguirre Ollinger, J. Edward Colgate, Michael Peshkin , and A. Goswami
    Active impedance control of a lower-limb assistive exoskeleton,
    10th Int. Conf. on Rehabilitation Robotics (ICORR'07), Noordwijk, the Netherlands, Jun 13-15 2007.

    (pdf).

    Abstract: We propose a novel control method for lower-limb assist that produces a virtual modification of the mechanical impedance of the human limbs. This effect is accomplished by making the exoskeleton display active impedance properties. Active impedance control emphasizes control of the exoskeletonís dynamics and regulation of the transfer of energy between the exoskeleton and the user. Its goal is improving the dynamic response of the human limbs without sacrificing the userís control authority. The proposed method is an alternative to myoelectrical exoskeleton control, which is based on estimating muscle torques from electromyographical (EMG) activity. Implementation of an EMG-based controller is a complex task that involves modeling the user's musculoskeletal system and requires recalibration. In contrast, active impedance control is less dependent on estimation of the user's attempted motion, thereby avoiding conflicts resulting from inaccurate estimation.

    In this paper we also introduce a new form of human assist based on improving the kinematic response of the limbs. Reduction of average muscle torques is a common goal of research in human assist. However, less emphasis has been placed so far on improving the userís agility of motion. We aim to use active impedance control to attain such effects as increasing the user's average speed of motion, and improving their acceleration capabilities in order to compensate for perturbations from the environment. 

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Page last updated September 24, 2012