Artificial Muscle Spindle

Abstract

Goals

The artificial muscle spindle project goal was to produce a physical model of the muscle spindle. The framework of the spindle design was built around the Schaafsma model concept of a sensor unit in series with an intrafusal muscle fiber simulation

Modeling Analogs

The physical analogs between the artificial and biological spindle are as follows:

1. Intrafusal Muscle Contractile Elements: In the biological spindle, these elements are used to adjust spindle length, stiffness and damping. This feature is reproduced in the artificial spindle using a lead screw mechanism. The motor causes a long translation screw to rotate, driving the transducer element back and forth. The various features of intrafusal muscle dynamics such as length and velocity dependencies are reproduced by the software controlling the motor.

2. Mechanotransducer: In the biological spindle, the sensory endings of the Ia nerve fiber wrap around the intrafusal fibers. Stretching of the stretch sensitive ion channels in the Ia nerve membranes depolarizes the cell in proportion to the amount of stretch. In the artificial spindle, this role is performed by strain gages mounted on thin cantilever beams on the transducer unit. Cables are used to apply force from the muscle to the end of the cantilevers. The amount of strain across the strain gages is then dependent on the compliance of the sensor unit to which the cantilever is mounted, which is determined by the software model of intrafusal muscle dynamics.

3. Action Potential: The task of firing action potentials is accomplished by a Wheatstone bridge whose output controls a voltage controlled oscillator. Hence, the Ia signal to the CPU is indeed in the form of a frequency modulated spike train.

Pierre-Henry Marbot designed the first generation of artificial muscle spindles as his dissertation project. Its performance was evaluated by testing it under experimental conditions identical to the classic muscle spindle behavior experiments. The results show that the spindle is both capable of detecting the minute movements seen by real muscle spindles, as well as able to reproduce spindle output as well as the best numerical model available, the Schaafsma model.

Second Generation Muscle Spindle Design

The mechanical system was redesigned in 1997 by Kristen Jaax. The goal of the new design is to improve the robustness of the system as well as tune the balance of static vs. dynamic Ia output.

Anthroform Arm Project

Powered Prosthetics Project

(*) Note: Most of the BRL publications are available on-line in a PDF format. You may used the publication's reference number as a link to the individual manuscript.

[061]
P.H. Marbot, B. Hannaford, 'Mini Direct Drive Arm for Biomedical Applications,' Proceedings of ICAR 91, pp. 859-864, Pisa Italy, June 1991.

[067]
P.H. Marbot, B. Hannaford, 'The Mechanical Spindle: a Replica of the Mammalian Muscle Spindle,' Proceedings, IEEE Conference on Engineering in Medicine and Biology, San Diego, CA, October, 1993.

[Th013]
P.H. Marbot, 'Ia Response of a Mechanical Spindle Replica,' Ph.D. Dissertation, University of Washington, Department of Electrical Engineering, March 1995.

[141]
K.N. Jaax, P.H. Marbot, B. Hannaford, 'Development of a Biomimetic Position Sensor for Robotic Kinesthesia,' Proceedings, IROS 2000, Takamatsu, Japan, November, 2000.

[149]
K. Jaax, B. Hannaford, 'A Biorobotic Structural Model of the Mammalian Muscle Spindle Primary Afferent Response,' Annals of Biomedical Engineering, vol. 30, pp. 84-96, January 2002.

[150]
B. Hannaford, G. Klute, K. Jaax, 'Bio-inspired Actuation and Sensing,' Autonomous Robots (Special Issue, Papers from the JPL workshop on Biomorphic Robotics, August 2000), vol. 11, pp. 267-272, Kluwer Academic Publishers, Boston/Dordrecht/London, November 2001.

[Th024]
K.N. Jaax, 'A Robotic Muscle Spindle: Neuromechanics of Individual and Ensemble Response,' Ph.D. Thesis, University of Washington, Department of Bioengineering, June 2001.