Newnes Interfacing Companion

Computers, Transducers, Instrumentation and Signal Processing

Tony Fischer-Cripps

• Publisher: Newnes
• Year: 2002
• ISBN:
  9780080497457
• Topic: Diseño
• Subtopic: Diseño UI/UX

Tony Fischer-Cripps is a Project Leader in the Division of Telecommunications and Industrial Physics of the CSIRO (Commonwealth Scientific & Industrial Research Organisation), Australia. He was previously lecturer, University of Technology, Sydney (UTS), Australia, and has also worked for the National Institute of Standards and Technology, USA (NIST, formerly National Bureau of Standards - NBS).*The essential pocket reference for engineers and students
*Interfacing in action: PCs, PLCs, transducers and instrumentation in one book
*Develop systems and applications that work with Newnes Interfacing Companion

Part 1

Transducers

Introduction to Transducers

1.0 Transducers

A measurement system is concerned with the representation of one physical phenomenon by another. The purpose of the measurement system is for the measurement and control of a physical system.

In Part 1 of this book, we are mainly interested in transducers.

• A sensor is a device which responds to a physical stimulus

• A transducer is a device which converts a physical stimulus to another form of energy (usually electrical)

1.1

Measurement systems

1.1.1 Transducers

Of most interest are the physical properties and performance characteristics of a transducer. Some examples are given below:

Property	Method of measurement
Strain	Strain gauge, a resistive transducer whose resistance changes with length.
Temperature	Resistance thermometer, thermocouple, thermister, thermopile.
Humidity	Resistance change of hygroscopic material.
Pressure	Movement of the end of a coiled tube under pressure.
Voltage	Moving coil in a magnetic field.
Radioactivity	Electrical pulses resulting from ionisation of gas at low pressure.
Magnetic field	Deflection of a current carrying wire.

Performance characteristics

Static

Sensitivity

Zero offset

Linearity

Range

Span

Resolution

Threshold

Hysteresis

Repeatability

Dynamic

Response time

Damping

Natural frequency

Frequency response

Environmental

Operating temperature range

Orientation

Vibration/shock

A consideration of these characteristics influences the choice of transducer for a particular application. Further characteristics which are often important are the operating life, storage life, power requirements and safety aspects of the device as well as cost and availability of service.

In industrial situations, the property being measured or controlled is called the controlled variable. Process control is the procedure used to measure the controlled variable and control it to within a tolerance level of a set point. The controlled variable is one of several process variables and is measured using a transducer and controlled using an actuator.

1.1.2 Methods of measurement

All measurements involve a comparison between a measured quantity and a reference standard. There are two fundamental methods of measurement:

Null method

• Direct comparison

• No loading

• Can be relatively slow

Deflection method

• Indirect comparison

• Deflection from zero until some balance condition achieved

• Limited in precision and accuracy

• Loading (transducer itself takes some energy from the system being measured)

• Relatively fast

Null method: Bridge circuit.

An unknown component is inserted into the bridge and the values of the others are altered to achieve...