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TRUE BACK EMF SERVO CONTROLLER

IP.com Disclosure Number: IPCOM000025979D
Original Publication Date: 1989-Jun-30
Included in the Prior Art Database: 2004-Apr-04
Document File: 4 page(s) / 127K

Publishing Venue

Xerox Disclosure Journal

Abstract

The idea of a Back EMF servo, or speed control system is well know. There are many different circuits for this type of controller, but all use the same general principal to keep a DC motor at constant speed, independent of the torque the motor has to deliver. A speed torque curve is familiar, and shows that the motor will indeed slow down with load.

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Page 1 of 4

XEROX DISCLOSURE JOURNAL

TRUE BACK EMF SERVO CONTROLLER Proposed Classification Thomas A. Reitze U.S. Cl.35513R

Int. C1. G03g 15/00

VOLTAGE HIGH , -

FIG. 1 I

PWM OUTM

FIG. 2

Volume 14 Number 3 May/June 1989 141

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Page 2 of 4

TRUE BACK EMF SERVO CONTROLLER (Cont'd)

The idea of a Back EMF servo, or speed control system is well know. There are many different circuits for this type of controller, but all use the same general principal to keep a DC motor at constant speed, independent of the torque the motor has to deliver. A speed torque curve is familiar, and shows that the motor will indeed slow down with load.

As the load on the motor increases, the current in the motor will increase as well, the relationship between the two being a constant Kt (Torque constant) for any single motor. This constant is in units of ounce-inches per amp. So, if the load increases 10 oz-in, and Kt is 20 oz-idamp, the current in the motor will increase .5 amp.

The motor slows down when the IR drop (current times resistance increases, and this increased voltage subtracts from the applied voltage on the motor, reducing the voltage the enerator affect of the motor operates against. The

effect must reach.

The existing back EMF circuits measure the current in the motor, usually through a sense resistor, converting the current to a voltage and then multiply the voltage by a value such that it is equal to the IR loss of the motor plus sense resistor. Then this voltage is added to the motor drive voltage, keeping the motor speed constant.

The problem with this concept is that the multiplyin factor is a function of the

the same lot), and is also a function of temperature. To work correctly, the gain needs to be set up for each unit, and some form of temperature compensation should be used.

The proposed circuit as shown in Fig. 1 and Fig. 2 solves the motor resistance problem by eliminating the effect of t...