The McGill/MIT Direct Drive Motor Project

The aim of this project has been to solve three problems with direct drive arms: the large size and weight of these motors relative to their mass, the propensity to overheat, and innacurate torque production. Specific goals are to design a motor from scratch with: These specifications are several times greater than for existing commercial designs. The design is a wound-field synchronous motor in a pancake geometry with water cooling. A table-based microprocessor controller utilizes calibration information to shape the current waveforms to achieve the required torque accuracy.

This project was originally begun as a collaboration with Jeff Lang and Steve Umans (responsible for the electromagnetic design) and Inaki Garabieta and Erik Vaaler (responsible for mechanical design) from MIT and Ian Hunter from McGill (Hollerbach, Lang, Vaaler, Garabieta, Sepe, Umans, and Hunter, 1993). Through a series of 3 prototypes, the performance goals of the motor were validated; three students (Ray Sepe, Art Kalb, and Deron Jackson) completed their S.M. theses in contributing to this project.

A commercial prototype of the motor was produced by MPB Technologies (Pointe Claire, Quebec), under the PRECARN TDS (Telerobotic Development Systems) project. Significant improvements have been made in the underlying design. Farhad Aghili, a former Ph.D. student, has contributed substantially to the detailed joint design and dynamometer design. A complete joint structure, with dual armatures for balanced axial loading and integral water cooling, was completed.

Control strategies for this motor are described by the following publications:

  1. Aghili, F., Buehler, M., and Hollerbach, J.M., ``Motion control systems with H-infinity positive joint torque feedback,'' IEEE Trans. Control Systems Technology, 9 no. 5, 2001, pp. 685-694.

    Earlier versions:

    Aghili, F., Buehler, M., and Hollerbach, J.M., `` Disturbance attenuation and load decoupling with H-infinity positive joint torque feedback,'' Proc. IEEE Intl. Conf. Robotics & Automation, Leuven, Belgium, May 16-21, 1998, pp.3613-3618.

    Aghili, F., Buehler, M., and Hollerbach, J.M., `` Dynamics and control of direct-drive robots with positive joint torque feedback,'' Proc. IEEE Intl. Conf. Robotics and Automation, Albuquerque, April 20-25, 1997, pp. 1156-1161.

  2. Aghili, F., Buehler, M., and Hollerbach, J.M., ``Optimal commutation laws in the frequency domain for synchronous direct-drive motors,'' IEEE Trans. Power Electronics, 15 no. 6, 2000, pp. 1056-1064.

    Earlier version:

    Aghili, F.,, Buehler, M.,, and Hollerbach, J.M., "Optimal commutation laws for torque control of synchronous motors," Proc. American Control Conference, Albuquerque, NM, June 1997, pp. 1968-1973.

  3. Aghili, F., Buehler, M., and Hollerbach, J.M., ``Sensing the torque in a robot's joints,'' Mechanical Engineering, 120, no. 9, 1998, pp. 66-69.

    Earlier version:

    Aghili, F., Buehler, M., and Hollerbach, J.M., ``A new joint torque sensor for robots,'' Proc. ASME Int. Mech. Eng. Congress and Exhibition, IMECE`97, Dallas, Texas, Nov. 16-19, 1997, paper number 97-WA/DE-7.

  4. Aghili, F., Buehler, M., and Hollerbach, J.M., `` Torque ripple minimization in direct-drive systems,'' Proc. IEEE/RSJ Intl. Conf. Intelligent Robots and Systems (IROS'98), Victoria, British Columbia, October 12-16, 1998.

  5. Aghili, F., Buehler, M., and Hollerbach, J.M., ``Model matching solution for optimal positive joint torque feedback,'' Proc. IEEE/RSJ Int. Conf. Intelligent Robots and Systems, IROS`97,} Grenoble, France, Sept 1997.

  6. Aghili, F., Buehler, M., and Hollerbach, J.M., ``A new indirect adaptive control strategy for a synchronous direct drive motor,'' Proc. IEEE Intl. Conf. Robotics and Automation, Minneapolis, April 22-28, 1996, pp. 2865-2870.

  7. Hollerbach, J.M., Lang, J., Vaaler, E., Garabieta, I., Sepe, R., Umans, S., and Hunter, I.W., `` The McGill/MIT Direct Drive Motor project,'' Proc. IEEE Intl. Conf. Robotics and Automation, May 2-7, 1993, Atlanta, pp. 2:611-617.