Research Of Displacement Measuring System Based On Capacitive. Grating Sensor

Transcription

1 Applied Mechanics and Materials Vols (2010) pp Online available since 2010/Jan/12 at (2010) Trans Tech Publications, Switzerland doi: / Research Of Displacement Measuring System Based On Capacitive Grating Sensor Yu Guangping 1, a, Wang Shanhui 2,b and Zhou Yinggang 3,c 1,2,3 School of Information and Electronic Engineering,Shenyang University of Technology, China a ygp008@126.com, b Wangshanhui1984@sina,com Keywords: Capacitive grating, Capacitive grating sensor, Displacement measuring, Charge amplify. Abstract - Capacitive grating sensor can be used to measure the length, as its capacitance changes with the shift difference. There are several measuring systems consisting of capacitive sensor, but all their basic principles are capacitive differential sensing. The structure, principles and characteristics of capacitive sensors are introduced in this paper. Especially, we designed a high-resolution electronic length measuring system based on Capacitive Grating Sensor. And the measurement error of the system and the reason are analysed. For this system, the measuring range is from 0 to 500mm, the resolution is 1um, the indication error is 5um, the measuring speed is 1m/s. Accurate measurement on account of large displacement is achieved with the system. Introduction Electronic capacitive length measuring system is a novel displacement sensor developed in the late seventies and early eighties in the international community, while internally the capacitive devices are gradually used in assembly[1]. Capacitive displacement measuring system consists of a unique measurement circuit and the capacitive grating board, and the board has a certain shape and size. Capacitive measuring system not only overcome the traditional shortcomings to achieve a large displacement measurement, but also has the advantages that other large-displacement measurement systems do not have, such as measuring speed up to 1m/s,the simple structure easily used in mechanical design, anti-electromagnetic field jamming, not sensitive to temperature and humidity energy-less, low cost and so on. So it has broad prospects for development and application in the field of precision measurement. In this paper,a length measurement technology based on capacitive sensor is introduced. Principle Of Capacitive Sensors The basic principle block diagram of capacitive measurement system is shown in Figure 1. Vo is the output voltage of control system and V1, V2 are the DC reference voltage. Either can be zero, but V1 and V2 should not be equal. V1 and V2 are respectively the reference voltage which is added on the plate K1,K2. The reference voltage or V0 can be connected periodically by electronic switch which is controlled by the square-wave signal.the phases of incentive voltage Vo1 Vo2 are always opposite to the Vo in capacitive system,and waveforms of Vo1, Vo2 are shown in Figure 2.(Vo1 for solid line and Vo2 for the dotted line). The working principle of capacitive sensor is essentially balanced charge. It is said that the charge changes from the plate is zero before and after the grating board relative motion. This charge-balance process can be achieved by adjusting the amplitude of the output voltage Vo[2]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-04/06/14,01:55:10)

2 Applied Mechanics and Materials Vols V2 Control system Vo1 Amplifier VO Square-wave Generator Inverter Vo2 Capacitive Grating Board Fig. 1 Working Schematic diagram Of Capacitive Sensor V1 Vo1 V2 Vo V1 Vo2 Fig. 2 Incentive Voltage Of Capacitive System When grating board is at the initial condition as shown in Figure 3,Vo=( V1+V2 ) /2,and Vo1,Vo2 have the same magnitude and contrast phase compared with Vo. At this point, the charge of the receiving electrode is a fixed value, and it is said that the output charge is a fixed amount. When the relative movement happened, receiving electrode charge will change. The charge changes of receiving electrode is zero when the output voltage Vo is regulated by the control system. So the charge keeps balance[3].in this way, the relative position can be expressed by the value of Vo. Fig. 3 Initital Position of Capacitive Grating Board The width of every sending electrode isλ=0.5 mm, hile the width of receiving electrode is L=2.0mm. When the capacitive grating board have a relative movement x (-0.5 mm <x <0.5 mm), differential capacitance CK1, CK2 Satisfied[4]: CK 1 L x = CK L x 2 + (1) When Vo1 Skip from Vo to V1, the sensing charge changes of receiving electrode: ( ) Q = V V CK When Vo2 Skip from V2 to Vo, the sensing charge changes of sending electrode: (2)

3 1262 Information Technology for Manufacturing Systems ( ) Q = V V CK (3) Control system adjust Vo, finally have: Q1 + Q2 = 0 (4) From (1) (2) (3) and (4),get: V = + ( ) V + V V2 V1 x 2 2L (5) System Design 4 channel DC reference voltages exported by the single-chip were sent to level converter controlled by prescaler. The 8 produced square wave signal were added to 8 supplying electrodes, which provide a group of signals with the change of cycle. The signals are the reference DC voltage of the supplying electrode[5]. Capacitive sensor is controlled by 8 sending electrode which electronic switch. In half-cycle they are connected to regulation voltage Vo, and in the other cycle it is connected to reference DC voltage Vo1 and Vo2 which have equal size and oppsite direction. When the fixed grid and moving grid have a relative displacement, the receiving electrode receives a signal voltage Vx. After the signal is amplified by amplifier, it is added on the input of the regulator by electronic switch, and there will be a output regulator voltage. There are two branches of Vo, one is for converter and the other one is added on the 8 supplying electrodes with the electronic switch controlled by the prescaler. There are two branches of Vo, one is for converter and the other one is added on the 8 supplying electrodes with the electronic switch controlled by the prescaler. In this way,the voltage added on the various supplying electrode can be regulated continuously, so the signal change generated on the receiving electrode tends to zero. It is called zero-tracking. The obtained analog signal Vo is converted into digital signal by A/D converter and delivered into MCU for data processing. The processed displacement value is delivered from MCU to LCD for displaying. System block diagram is shown in Figure 4. sensor ampl regulator A/D ifier converte M L electronic prescaler C C U D level converter 8 electronic switch Figure 5 System Block Diagram

4 Applied Mechanics and Materials Vols Experiment In order to detect system accuracy, one light gating digital display instrumentation whose resolution is 1um is installed on block machine of capacitive sensor. While the capacitive gating has a relative displacement, the light gating has the same displacement. The system measurements are compared with light grating digital display express values. The light grating digital display express values and system measurements are recorded every 1mm in the range of 0-500mm. The difference of capacitive grating show value and light grating show value acts as the error of capacitive system. The total error of the capacitive system is the average of 10 measurement errors every recording point. the system error curve of 500 points is shown in Figure 6. Figure 6 Error Curve of the System Error Analysis It can be known that the measurement error is not more than 3um compared with the grating digital display instrumentation from above experiments. The main reason to cause error is: conversion principle error, indicate changing error, grating board geometry error, material and process error,environmental error and other reason. 1) Conversion principle error of the capacitive grating sensor:the error caused by the excitation voltage and transfer function of capacitive grating sensor, and the phase error due to high times space harmonic wave of the capacitance. 2) Indicate changing error: the phase of the received signal will have a fluctuation when there is interference outside. The indicate changing error is generated, if the interference can not be completely eliminated by the internal average circuit. 3) Grating board geometry error: Although the sending electrode in the shifting capacitive grate and the receiving electrode in the static capacitive grate are parallel, at the direction of the electrode length there is inclination. The inclination changed the coverage between sending electrode and reflecting electrode. And measurement error is generated as output charge is affected[6]. 4) Environmental error: With the ability of anti-electric and magnetic interference, the precision of the sensor can not be affected by oil and dust at job site. There is no change in the capacitive grating sensor when the humidity is in the range of 0-80%.But the error become large when the humidity exceed 80%[7].

5 1264 Information Technology for Manufacturing Systems Conclusion The principle of the capacitive sensors are introduced in this paper.one capacitive electronic length measurement system is designed based on charge-balance principle of the capacitive grating sensor. Through experiments,the measuring range is 0-500mm,the system resolution is 1um,the indication error is 5um,the measurement speed is 1m/s. The origins of measurement error are conversion principle error, indicate changing error, grating board geometry error, material and process error and environmental error. After a variety of environmental testing, the system works stably and reliably. The system can have a quick completion in the large accurate displacement measurement, so it is an ideal electronic length measurement system. References [1] WANG Xi wen, QI Xin, and SONG Yu quan, Capacitive transducer and its development prospect, Journal of Jilin University of Technology, vol. 33, no. 2, pp.89-94, Apr [2] G.A.Berton et al, Investigation of Capacitive Based Displacement Transducer, IEEE Trans. Instrum. Meas, vol.39. no. 2,pp.235,1990. [3] ZHANG Yin fang, Principle and characteristic of Capacitive Displacement Transducer, Aviation Precision Manufacturing Technology, vol. 4, no. 4, pp , Aug [4] XU Ke Jun et al, Research and Development Present Situation of Capacitive Transducer, Tool Engineering, vol. 28, no. 12, pp , Aug [5] XU Ke Jun, and TIAN Xiao Feng, The Vectorial Analysis for Capacitance Grid Type Transducer, Process Automation Instrumentation, vol. 17, no. 5, pp. 4-7, Aug [6] HAO Wei Dong, Capacitance Grating Transducer, JOURNAL OF GUILIN INSTITUTE OF ELECTRONIC TECHNOLOGY, vol. 17, no. 1, pp , Mar [7] HONG Tao, The Design of Electric Charge Amplification Circuit Inturbocharger Revolution Measurement, Journal of Gansu Sciences, vol. 15, no. 1, pp , Mar 2003.

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