CA2320581A1

CA2320581A1 - Automated, sensorless, torque control for dc motors and ac servomotors

Info

Publication number: CA2320581A1
Application number: CA 2320581
Authority: CA
Inventors: Burkhard Becker
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-09-15
Filing date: 2000-09-15
Publication date: 2002-03-15

Description

AUTOMATED, SENSORLESS, TORQUE CONTROL FOR
DC MOTORS AND AC SERVOMOTORS
FIELD OF THE INVENTION
The present invention relates to a method for precisely and automatically controlling torque in brush type DC motors and brushless AC and DC servomotors without the use of torque sensors.
SUMMARY OF THE INVENTION
This sensorless torque control of brush type DC motors and brushless DC or AC
servomotors relies on the relations:
z = klfl° (for brush type motors) z = kl (for brushless motors) V _ IAR.~
= kl f (for brush type motors) ~ = V ~ R° (for brushless motors) Where: z = torque k = a proportionality constant 1 - motor current (brushless motor types) I f = field current I° = armature current V = applied motor voltage z=kl.
~ = V ~ R° or V = kw + IR then k = ew , with I held constant , this equation can be used to estimate k at any operating point.

From a motor specific, current versus torque equation with k as the parameter one can then calculate the required current to produce a given torque. This is under the assumption that the magnetic circuit temperature distribution remains essentially constant during the procedure. See Fig 1 for an illustration.
From the first order approximation:
z=kl it is clear that z can be controlled by controlling 1. Since k is a function of motor temperature and operating point k must be recalculated periodically in the region of the opera~ng point so as to retain a high degree of accuracy. k is determined from k = w , with I held constant .
Ow A third order polynomial equa~on such as:
I = k(az+bz2 +cz3) can then be used to estimate I as a function of k and z . For each specific motor model the equation is developed using actual measurements of l,k,z over a wide operating range of motor load and temperature. Once this data is compiled the coefficients a, b, c, can be calculated.
Periodically updating k using the rela~on:
k = w , with I held constant Ow allows one to recalculate I for use as a setpoint in a torque control system.
w and ~~ can be periodically calculated in the control system and used to update I so as to permit correction of the operating torque.
The procedure for measuring DY and 0~, will depend on the operating mode for the motor.
Examples are:
1. For a motor under current control slight variations in load torque will result in a measurable ey and 0~, , using this result to calculate k and storing the new k in the controller memory makes it possible to periodically correct the motor torque.

2. For a motor with periods of speed control and periods of torque control in the same operating cycle the switch over from speed control to torque control can be used to initiate measurement of e~ and ~~7 One embodiment of the present invention is the situation where screws or nuts are driven onto a hard surfaced work surface. Fig. 2A shows the typical torque versus position profile for both screws and nuts. FIG. 2B shows a typical speed profile of an operating cycle for screw and nut fastening operations.
Initially the screw or nut is driven under speed control to a preselected motor current level.
The motor current level is of course a direct indication of the motor torque level. Still under speed control the motor is then rapidly slowed and subsequently switched to current control.
During the slowdown phase Dy and 0~, are measured and k is calculated then using:
I = k(aZ+br2 +cr3) the required current to produce the desired torque is calculafied. The calculated current then becomes the setpoint for the torque control phase.

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