An improved motor controller provides a circuit that will allow a motor to run at full speed while traveling from point A to point B, and still be able to obtain the desired damping coefficient when reaching point B. Previously, a motor would overshoot its desired stopping point when running at full speed, especially in high-inertia systems. Some form of rate feedback, or lead compensation, was required to stabilize a position loop, but the motor would not run at full speed while traveling from point A to point B.
The innovators recognized this problem while working on the Electromechanical Actuator Advanced Development Program at Marshall Space Flight Center. Consequently, they developed a nonlinear motor controller that will allow the motor to obtain both the desired motor speed rate and the desired damping coefficient when reaching point B.
This motor controller concentrates on the position error of the system. Buffers are used to tie in the position command and position feedback points of the system with a high impedance input. An adder circuit simply adds the position command to the negative position feedback. The output of the adder circuit is a signal proportional to the position error.
Depending on the amplitude of the position error, the controller will take the rate feedback of the system out of the system control loop, or it will sum it into the system control loop. If the position error is too large in the comparator block, the rate feedback will be removed from the control loop. If the position error is too small, the rate feedback will be added to the system control loop. In order to achieve this, the position error is fed into a minus absolute-value circuit. This will allow the invention to work whether the position error sign is plus or minus.
The minus absolute value of the position error is then compared, using the circuit in the comparator block, to a user definable set point. This set point allows the user to select at what point the rate will be summed into the system control loop. By selecting the set point, the user determines how long the motor can run at full speed as the position mechanism travels from a given point to another commanded point. Also, by selecting the set point, the user determines when the rate will be summed into the system controller to add damping to the system control loop.
A discrete signal from the comparator block toggles between a plus and minus voltage. The diode and resistor condition the output signal of the comparator block so that it will only go from zero to a plus voltage. This signal then controls the analog switch, which actually does the switching in and out of the rate feedback to the system controller.
The end result is a circuit that allows the motor to run at full speed until the final position gets close, as defined by the user. The rate feedback is then summed into the system controller to achieve the desired damping for the position loop.
This work was done by David E. Howard and Justino Montenegro of Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Circuits category, or circle no. 185 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Patent Counsel
Marshall Space Flight Center; (205) 544-0021.
Refer to MFS-31182.