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Feedback Control of Torque of a Stepper Motor

IP.com Disclosure Number: IPCOM000079355D
Original Publication Date: 1973-Jun-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 41K

Publishing Venue

IBM

Related People

Mako, J: AUTHOR

Abstract

Stepper motors are always operated either in the closed loop or open-loop mode. In the closed-loop mode of operation, feedback is necessary. The most often used device to obtain feedback is a position detector that is attached to the motor shaft; usually a magnetic or optical transducer. Motor speed for a given load will be determined by the time lag between phase switching and feedback pulse. Load variations will result in motor speed change.

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Feedback Control of Torque of a Stepper Motor

Stepper motors are always operated either in the closed loop or open-loop mode. In the closed-loop mode of operation, feedback is necessary. The most often used device to obtain feedback is a position detector that is attached to the motor shaft; usually a magnetic or optical transducer. Motor speed for a given load will be determined by the time lag between phase switching and feedback pulse. Load variations will result in motor speed change.

In the open-loop mode, the motor speed is fixed. There is no feed-back device, and the system parameters (speed, torque) are always tailored to the highest possible load.

This arrangement combines the advantages of both systems. The motor is running at constant speed, as in open-loop mode. Internal feedback is generated (no external transducer is necessary), and is used to control torque, as changes in the load demand. Thus, if the load is light, the motor will run cool, the phase resistors generate less heat, and current drained from the power supply will be less. But if the load increases, the motor will be able to supply the needed torque.

The principle of operation is as follows. As a stepper motor is advanced, each time a driver is turned off, the winding inductance will generate a back EMF or "Flyback" voltage. The magnitude of this pulse is frequently too great for the drive transistor to withstand, therefore, clamping diodes are employed to limit the voltage excursion, usually to the value of the supply voltage. If the flyback voltage can be allowed to rise unrestricted, it will have a typical wave shape, as in Fig. 1. There is a relationship between flyback voltage and torque. A motor running without load will have high-flyback voltage. As load is increased, flyback voltage will decrease, approaching the supply voltage. Fig. 2 shows flyback voltage vs torque for a given motor and speed.

The fact that flyback voltage is a direct indication of torque can be used to connect the motor in a feedback loop, where torque can be increased or decreased on demand. The circuit which accomplishes this is shown in Fig. 3. Operation is as follows:.

Flyback voltage is available on the anodes of diodes D3 and D4. Diodes D1 and D2 rectify these pulses. The DC...