A recently developed micro-commanding rotational position control system offers advantages of less mechanical complexity, less susceptibility to mechanical resonances, less power demand, less bulk, less weight, and lower cost, relative to prior rotational position control systems based on stepping motors and gear drives. This system includes a digital signal processor (DSP)-based electronic controller, plus a shaft-angle resolver and a servomotor mounted on the same shaft. Heretofore, micro-stepping has usually been associated with stepping motors, but in this system, the servomotor is micro-commanded in response to rotational-position feedback from the shaft-angle resolver.
The shaft-angle resolver is of a four-speed type chosen because it affords four times the resolution of a single-speed resolver. A key innovative aspect of this system is its position-feedback signal- conditioning circuits, which condition the resolver output signal for multiple ranges of rotational speed. In the preferred version of the system, two rotational-speed ranges are included, but any number of ranges could be added to expand the speed range or increase resolution in particular ranges. In the preferred version, the resolver output is conditioned with two resolver to digital converters (RDCs). One RDC is used for speeds from 0.00012 to 2.5 rpm; the other RDC is used for speeds of 2.5 to 6,000 rpm. For the lower speed range, the number of discrete steps of RDC output per revolution was set at 262,144 (4 quadrants at 16 bits per quadrant). For the higher speed range, the number of discrete steps per revolution was set at 4,096 (4 quadrants at 10 bits per quadrant).
In the preferred version, there are position-feedback signal-conditioning circuits that generate two separate outputs. The electronic controller is used to select the rotational-speed range along with whichever of the two position-feedback outputs is appropriate to that range. The controller also receives a speed command through an RS232 serial interface. This rate command is converted to a position command updated at a set frequency — that is, the position is commanded at so many steps per unit time to obtain rotation at a desired speed. Finally, the controller takes the position command and the selected position feedback and implements a proportional + integral + derivative (PID) control law with a current- command output. The current-command output is fed as input to a current amplifier that provides power to the motor. The motor can be a brushless or a standard brush-type DC motor.
The main innovative aspect of this system is the use of the multiple signal-conditioning circuits with the single resolver in generating micro-rotation commands for the motor. The use of the multiple signal-conditioning circuits increases the rotational resolution and the dynamic range for speed control. The speed-control dynamic range of this system is 5 × 107; a greater dynamic range could be obtained by adding signal- conditioning circuits for additional speed and position ranges.
This work was done by Dean C. Alhorn, David E. Howard, and Dennis A. Smith of Marshall Space Flight Center, Ken Dutton of Madison Research Corp., and M. Scott Paulson of Mevatech. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category.