At2 H 19 Transducer

DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY TRANSDUCERS

1.

INTRODUCTION

Sensors, transducers, and transmitters are most important for automation as they provide feedback information about how well the system is doing. A sensor is defined as a device that is sensitive to motion, heat, light, pressure, electrical, magnetic and other types of energy. A transducer is defined as a device that can receive one type of energy and convert it to another type of energy. A transmitter is a device that can convert a very small signal to a more useable signal.

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AT2-222 – Sensors & Fiber Optic Technology Chap.19 – Transducer

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY

Type of Sensor or Transducer That Produces Voltage or Current

Type of Energy Source

Type of Sensor or Transducer That Produces a Change of Resistance or Impedance

Motion

Generator

Linear potentiometer

Light

Solar cell, photovoltaic

Cds Cell

Force (pressure)

Piezoelectric

Strain gauge

Temperature

Thermocouple

RTD

Magnetic

Transformer hall effects

Magneto resistive

Chemical

Batteries, fuel cells

Different concentrations

2.

SIGNAL TYPE

The change of energy that the sensor detects must be converted into an electrical signal. Typical electrical signals are: 

Voltage



Current



Resistance



Frequency



Capacitance



Inductance

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AT2-222 – Sensors & Fiber Optic Technology Chap.19 – Transducer

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY In some cases a transmitter will provide additional signal conditioning, such as: 

from voltage to current



from current to voltage



frequency to voltage

Standard Types of Electrical Signals 0 - 1000 millivolts 0 - 20 milliamps 4 - 20 milliamps 0 - 10 volts 2 - 10 volts -10 to +10 volts Another important conversions of signals are: 

analog signals to digital



digital signals to analog

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AT2-222 – Sensors & Fiber Optic Technology Chap.19 – Transducer

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY

3.

TRANSMITTER

A transmitter accept signals from sensors in millivolt, milliamp, voltage, or frequency pulses, and convert it to: 

4-20 mA



1-5 volt



0-5 volt, or



0-10 volt

Electrical diagram of a typical signal transmitter that converts a voltage signal to 4-20mA signal

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AT2-222 – Sensors & Fiber Optic Technology Chap.19 – Transducer

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY

A diagram that shows an ammeter, recorder, and a controller connected in series so that they all receive the same current.

The two-wire transmitter receives a small voltage input signal and the transmitter converts the signal to a 4-20mA signal that is useable as the current input signal for recorders or controllers. The advantage of a 4-20mA signal compared to 0-20mA signal is that the 4mA represents a live zero. This means that if the signal from the transmitter ever drops to 0mA, the circuit wiring or components have caused an open circuit to occur.

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AT2-222 – Sensors & Fiber Optic Technology Chap.19 – Transducer

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY

A diagram of a sensor connected to a filter, amplifier, and display.

Transmitters mounted on DIN rails.

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AT2-222 – Sensors & Fiber Optic Technology Chap.19 – Transducer

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY

Transmitters that can be mounted directly into any cabinet.

3.1

Calibration of Transmitter and Sensors

A transmitter provides a means of calibrating a sensor with the environment in which it is used. For example, a level sensor may be placed in a treated water reservoir where the level is artificially controlled so that the reservoir is considered empty when it has reaches 1 meter above the bottom and it may be considered full when it reaches 1 meter from the top of the reservoir. A transmitter provides a means of adjusting the zero (bottom measurement) and the span (distance from bottom to top). Different types of calibration methods: 

Set the minimum level first



Set the maximum level first



Set the mid point first and then work to the maximum and minimum.

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AT2-222 – Sensors & Fiber Optic Technology Chap.19 – Transducer

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY



Example of calibrations procedures (level sensor with transmitter 4-20mA signals) 

Fill the tank at maximum level.



Adjust the span potentiometer so that the signal is 20mA.



Empty the tank to its minimum level.



Adjust the zero potentiometer so that the signal is 4mA.



The above procedures should be repeated several times until the zero and span are well adjusted.



Check the linearity and sensitivity of the transmitter and sensor:



Check at 0% (min) – 4mA



Check at 25% (1/4 full) – 8mA



Check at 50% (1/2 full) – 12mA



Check at 75% (3/4 full) – 16mA



Check at 100% (full) – 20mA

If the sensor and transmitter send the correct mA values at each point; the sensor is linear. Some sensors require that you check in increments of 10% or 5% to ensure that the sensitivity of the sensor is consistent across its entire span. If the sensor and transmitter are not linear, you must change one or both until you get the system calibrated.

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AT2-222 – Sensors & Fiber Optic Technology Chap.19 – Transducer

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY 4.

RECORDERS

The recorder may be a circular or strip chart type with the paper showing the readings over a specific period of time such as 24 hours. The recorder can accept 4-20mA or 0-10volt signals as input.

Example of a recorder used with sensors and transmitters.

Example of a circular chart recorder.

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