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# Servo Control of Stepping Motors

IP.com Disclosure Number: IPCOM000087253D
Original Publication Date: 1977-Jan-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 52K

IBM

## Related People

Bateson, JE: AUTHOR [+2]

## Abstract

Servo control techniques cannot ordinarily be applied to open- or closed-loop stepping motor systems which utilize on/off drivers. In order to apply servo-control techniques, the torque should be a linear function of some control input. Torque, which is a function of a control input, can be obtained from a stepping motor by properly controlling the motor phase currents.

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Servo Control of Stepping Motors

Servo control techniques cannot ordinarily be applied to open- or closed-loop stepping motor systems which utilize on/off drivers. In order to apply servo- control techniques, the torque should be a linear function of some control input. Torque, which is a function of a control input, can be obtained from a stepping motor by properly controlling the motor phase currents.

Fig. 1 is a sketch of phase torques in a typical three-phase variable reluctance stepping motor. The three phases are labeled A, B and C. Positive torque will be assumed to be clockwise. Consider the point labeled Theta0 in Fig. 1. If coil A is not energized and the current in coil B is twice that in coil C, then the total torque produced is Tm/2. At Theta1 the same current ratios will again produce a torque Tm/2. At Theta2 the same holds true. At Theta3, coil B is not energized and the current in coil C is twice that in coil A. Again the resultant torque is Tm/2. If the value of coil current is varied, the torque output will be varied. Fig. 2 shows the current ratios for a continuous resultant torque.

To accomplish the above, the system shown in Fig. 3 may be used. The torque command 1 is multiplied by the current control command 2, i.e., in this case 0, .5, or 1, in multiplier 3. The output of the multiplier is used to control a current source 4. Current is fed to coil 5. Position is sensed by position encoder 6 and is sent to the phase control circuit 7. As the...