Browse Prior Art Database

Motor Load Sensing by Time Measurement

IP.com Disclosure Number: IPCOM000062679D
Original Publication Date: 1986-Dec-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 59K

Publishing Venue

IBM

Related People

Goldrian, G: AUTHOR [+3]

Abstract

High-performance stepper motor drives require a feedback signal from a step encoder to permit the stepper motor to operate at maximum performance. This mode of operation is known as closed-loop control, the load of the motor being defined by the time that passes between the electrical step (phase change) and the performed step (encoder signal). This time must be related to the actual motor speed to determine the momentary motor load which is defined as a lead or load angle. There are many types of closed-loop control, but all of them require a step encoder which is built into the motor or takes the form of a separate assembly. (Image Omitted) The gist of the present disclosure is to analyze the dynamic response of the motor phase current to the EMF (electromotive force) for determining the momentary motor load. Figs.

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Motor Load Sensing by Time Measurement

High-performance stepper motor drives require a feedback signal from a step encoder to permit the stepper motor to operate at maximum performance. This mode of operation is known as closed-loop control, the load of the motor being defined by the time that passes between the electrical step (phase change) and the performed step (encoder signal). This time must be related to the actual motor speed to determine the momentary motor load which is defined as a lead or load angle. There are many types of closed-loop control, but all of them require a step encoder which is built into the motor or takes the form of a separate assembly.

(Image Omitted)

The gist of the present disclosure is to analyze the dynamic response of the motor phase current to the EMF (electromotive force) for determining the momentary motor load. Figs. 1A to 1C show how the EMF influences phase current i of a stepper motor. The phase current is driven by a switching motor driver (chopper). The voltage produced by the EMF (marked by a broken line) is not directly measurable and has merely been shown for explanatory purposes. In Figs. 1A to 1C, a 2-phase stepper motor is electrically stepped at 2 kc/s. Only one phase current i is shown. As the phases are alternately switched, each phase is switched at half the step frequency = 1 kc/s. Fig. 1A shows the phase current of a blocked stepper motor. As the rotor is not turning, no EMF is generated. The motor coil acts as a normal inductance. Phase switching at N + 2 causes a linear current discharge. The time tL, required for passing through a positive and a negative threshold of, say, +- 1 A, is small, as there is no EMF.

Fig. 1B shows phase current i and the course of the EMF of a stepper motor at maximum performance. The load angle (1.9 step) is the difference between the electrical step N (not shown) and the performed step S. At the time the phase crosses the two thresholds (+- 1 A), the EMF is small, so that tL is about the same as in Fig. 1A. Curves of the stepper motor at a low-torque operating point,
i.e., minimum load, are shown in Fig. 1C. The time tL is much longer, since when phase current i crosses the two thresholds, a substantial negative EMF impairs the coil discharge and the reverse charge. As previously explained and shown in Fig. 1, it is possible to determine the momentary load of a moving stepper motor by time measurement. The critical time tLC for the load limit can also be de...