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Servo-Controlled High-Accuracy and Low-Cost Electronic Scale

IP.com Disclosure Number: IPCOM000040760D
Original Publication Date: 1987-Dec-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 39K

Publishing Venue

IBM

Related People

Rand, RA: AUTHOR [+2]

Abstract

This article describes a design for a laboratory scale that allows a high degree of accuracy by use of a closed-loop servo system. This causes certain kinds of errors to be self-cancelling, thereby allowing the use of lower cost parts, while still maintaining accuracy. Referring to Fig. 1, the scale works on the principle that if the beam is brought back to the same position whenever the weight on the scale pan is changed, the force necessary to do this can be measured. In this design, force is provided by a solenoid pulling down on the beam. To detect the scale 'balanced' position, an optical position sensor is used. Optical position sensors (OPSs), while not perfectly linear, exhibit very good zero repeatability, which is all that is necessary in this application.

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Servo-Controlled High-Accuracy and Low-Cost Electronic Scale

This article describes a design for a laboratory scale that allows a high degree of accuracy by use of a closed-loop servo system. This causes certain kinds of errors to be self-cancelling, thereby allowing the use of lower cost parts, while still maintaining accuracy. Referring to Fig. 1, the scale works on the principle that if the beam is brought back to the same position whenever the weight on the scale pan is changed, the force necessary to do this can be measured. In this design, force is provided by a solenoid pulling down on the beam. To detect the scale 'balanced' position, an optical position sensor is used. Optical position sensors (OPSs), while not perfectly linear, exhibit very good zero repeatability, which is all that is necessary in this application. In addition, OPSs use two pairs of photo transmitter/receiver sensors, so the temperature and drift effects can offset and cancel. This also aids zero repeatability. The signal from the OPS is amplified and used to drive the solenoid thereby closing the servo loop. Each time the weight on the scale pan is changed, the OPS senses the change in position. When this occurs, the drive current to the solenoid is altered to bring the system back to the balance point. This current is directly proportional to the force

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exerted by the solenoid. Since the solenoid always returns the beam to the same position, spatial relationships are maintained in the various assemblies, so parasitic effects are constant (such as fringe effects in the solenoid). This results in good linearity over the entire range. To make the system practically frictionless, the beam is balanced on a k...