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EE 416 Fundementals of Biomedical Engineering Laboratoiy - Experiment 1

EXP #1: RESISTANCE TRANSDUCERS AND TRANSDUCERS FOR LINEAR POSITION OR FORCE APPLICATIONS

I.    INTRODUCTION

Objectives: To review the resistance transducers, to study tłie construction, operation pnnciple and characteristics of an LVDT (Lmear Vanable Differential Transformer), to study the constiuction and characteristics of a variable capacitor, to study the constiuction and characteristics of a strain gauge.

Equipment: Digiac 1750.

Average Time to Complete: 2 hours.

Prereąuisites: Fainihanty with resistance transducers and strain gauges.

II.    BACKGROUND INFORMATION A. Resistance Transducers

i) Basic construction of rotory and slider types ofvariable resistance:

The unit consists basically of a ‘track’ having a fixed resistance and a vaiiable contact which can be moved along and make continuous contact with the track. If the track resistance is proportional to the length along the track (i.e. lmear track), the output voltage will be proportional to the movement of the vaiiable contact and the unit is suitable for use as a positiontransducer. These units are referred as lmear types. There are also logarithmic types.

The track may comprise a film of caibon fonned on a substrate or may be a length of resistance wire wound on an insulator former. The unit may be constmcted m a rotary form or may be straight as shown m Figurę 1.

Yariable Carbon

Connections

Wire ccii

Ysriable

Contact

Track

act / TCn *

I ^

] Meta Connections

Rotary Types

Slider type

Figurę 1

ii) Resolution:

The resolution provided by a caibon track resistor is good, the value being mfimtely smali. The resolution for a wire wound resistor is worse, sińce the vaiiable contact will m effect be moving from contact with one tum to contact with the next. The output voltage will therefore increase in steps, these representmg the voltage per tum or a fraction of tlns.

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