Chapter 9 - Encoders, Transducers, and Advanced Sensors

Chapter 9 - Encoders, Transducers, and Advanced Sensors

9-1. Objectives

Upon completion of this chapter, you will know

”the difference between a sensor, transducer, and an encoder.

”various types of devices to sense and measure temperature, liquid level, force, pressure and vacuum, flow, inclination, acceleration, angular position, and linear displacement.

”how to select a sensor for an application.

”the limitations of each of the sensor types.

”how analog sensor outputs are scaled.

9-2. Introduction

In addition to simple discrete output proximity sensors discussed in the previous chapter, the controls system designer also has available a wide variety of sensors that can monitor parameters such as temperature, liquid level, force, pressure and vacuum, flow, inclination, acceleration, position, and others. These types of sensors are usually available with either discrete or analog outputs. If discrete output is available, in many cases the sensors will have a setpoint control so that the designer can adjust the discrete output to switch states at a prescribed value of the measured parameter. It should be noted that sensor technology is a rapidly evolving field. Therefore, controls system designers should have a good source of manufacturers’ data and always scan new manufacturers catalogs to keep abreast of the latest available developments.

This chapter deals with three types of devices which are the encoder, the transducer, and sensor.

1.The encoder is a device that senses a physical parameter and converts it to a digital code. In a strict sense, and analog to digital converter is an encoder since it converts a voltage or current to a binary coded value.

2.A transducer converts one physical parameter into another. The fuel level sending unit in an automobile fuel tank is a transducer because it converts a liquid level to a variable resistance, voltage, or current that can be indicated by the fuel gage.

3.As we saw in the previous chapter, a sensor is a device that senses a physical parameter and provides a discrete one-bit binary output that switches state whenever the parameter exceeds the setpoint.

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Chapter 9 - Encoders, Transducers, and Advanced Sensors

9-3. Temperature

There are a large variety of methods for sensing and measuring temperature, some as simple as a home heating/air conditioning thermostat up to some that require some rather sophisticated electronics signal conditioning. This text will not attempt to cover all of the types, but instead will focus on the most popular.

Bi-Metallic Switch

The bi-metallic switch is a discrete (on-off) sensor that takes advantage of the fact that as materials are heated they expand, and that for the same change in temperature, different types of material expand differently. As shown in Figure 9-1, the switch is constructed of a bi-metallic strip. The bi-metallic strip consists of two different metals that are bonded together. The metals are chosen so that their coefficient of temperature expansion is radically different. Since the two metals in the strip will be at the same temperature, as the temperature increases, the metal with the larger of the two coefficients of expansion will expand more and cause the strip to warp. If we use the strip as a conductor and arrange it with contacts as shown in Figure 9-1 we will have a bi-metallic switch. Therefore, the bi-metallic strip acts as a relay that is actuated by temperature instead of magnetism.

Figure 9-1 - Bi-metallic Temperature Switch

In most bi-metallic switches, a spring mechanism is added to give the switch a snap action. This forces the strip to quickly snap between it’s two positions which prevents arcing and pitting of the contacts as the bi-metallic strip begins to move between contacts. As illustrated in Figure 9-2, the snap action spring is positioned so that no matter which position the bi-metallic strip is in, the spring tends to apply pressure to the strip to hold it in that position. This gives the switch hysteresis (or deadband). Therefore, the temperature at which the bi-metallic strip switches in one direction is different from the temperature that causes it to return to it’s original position.

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Chapter 9 - Encoders, Transducers, and Advanced Sensors

Figure 9-2 - Bi-metallic Temperature Switch with Snap Action

The N/O and N/C electrical symbols for the temperature switch are shown in

Figure 9-3. Although the zig-zag line connected to the switch arm symbolizes a bi-metallic strip, this symbol is also used for any type of discrete output temperature switch, no matter how the temperature is sensed. Generally, temperature switches are drawn in the state they would take at room temperature. Therefore, a N/O temperature switch as shown on the left of Figure 9-3 would close at some temperature higher than room temperature, and the N/C switch on the right side of Figure 9-3 would open at high temperatures. Also, if the switch actuates at a fixed temperature (called the setpoint), we usually write the temperature next to the switch as shown next to the N/C switch in Figure 9-3. This switch would open at 255 degrees Fahrenheit.

255 F.

Figure 9-3 - Discrete Output

Temperature Switch Symbols

Thermocouple

Thermocouples provides analog temperature information. They are extremely simple, very rugged, repeatable, and very accurate. The operation of the thermocouple is based on the physical property that whenever two different (called dissimilar) metals are fused (usually welded) together, they produce a voltage. The magnitude of the voltage (called the Seebeck voltage) is directly proportional to the temperature of the junction. For certain pairs of dissimilar metals, the temperature-voltage relationship is linear over a small range, however, over the full range of the thermocouple, linearization requires a complex polynomial calculation.

The temperature range of a thermocouple depends only on the two types of dissimilar metals used to make the thermocouple junction. There are six types of thermocouples that are in commercial use, each designated by a letter. These are listed in the table below. Of these, the types J, K and T are the most popular.

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Chapter 9 - Encoders, Transducers, and Advanced Sensors

In the table above, some of the metals are alloys. For example, chromel is a chrome-nickel alloy, alumel is an aluminum-nickel alloy, and constantan is a copper-nickel alloy.

It is important to remember that each time two dissimilar metals are joined, a

Seebeck voltage is produced. This means that thermocouples must be wired using special wire that is of the same two metal types as the thermocouple junction to which they are connected. Wiring a thermocouple with off the shelf copper hookup wire will create additional junctions and accompanying voltage and temperature measurement errors. For example, if we wish to use a type-J thermocouple, we must also purchase type-J wire to use with it, connecting the iron wire to the iron side of the thermocouple and the constantan wire to the constantan side of the thermocouple.

It is not possible to connect a thermocouple directly to the analog input of a PLC or other controller. The reason for this is that the Seebeck voltage is extremely small

(generally less than 50 millivolts for all types). In addition, since the thermocouples are non-linear over their full range, compensation must be added to linearize their output. Therefore, most thermocouple manufacturers also market electronic devices to go with each type of thermocouple that will amplify, condition and linearize the thermocouple output. As an alternative, most PLC manufacturers offer analog input modules designed for the direct connection of thermocouples. These modules internally provide the signal conditioning needed for the thermocouple type being used.

As with all analog inputs, thermocouple inputs are sensitive to electromagnetic interference, especially since the voltages and currents are extremely low. Therefore, the control system designer must be careful not to route thermocouple wires near or with power conductors. Failure to do so will cause temperature readings to be inaccurate and erratic.

In addition, thermocouple wires are never grounded. Each wire pair is always routed all the way to the analog input module without any other intermediate connections.

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Chapter 9 - Encoders, Transducers, and Advanced Sensors

Resistance Temperature Device (RTD)

All metals exhibit a positive resistance temperature coefficient; that is, as the temperature of the metal rises, so does its ohmic resistance. The resistance temperature device (RTD) takes advantage of this characteristic. The most common metal used in RTDs is platinum because it exhibits better temperature coefficient characteristics and is more rugged than other metals for this type of application. Platinum has a temperature coefficient of "= +0.00385. Therefore, assuming an RTD nominal resistance of 100 ohms at zero degrees C (one of the typical values for RTDs), its resistance would change at a rate of +0.385 ohms/degree C. All RTD nominal resistances are specified at zero degrees C, and the most popular nominal resistance is 100 ohms.

Example Problem:

A 100 ohm platinum RTD exhibits a resistance of 123.0 ohms. What is its temperature?

Solution:

Since all RTD nominal resistances are specified at zero degrees C, the nominal resistance for this RTD is 100 ohms at zero degrees C, and its change in resistance due to temperature is +23 ohms. Therefore we divide +23 ohms by 0.385 ohms/degree C to get the solution, +59.7 degrees C.

There are two fundamental methods for measuring the resistance of RTDs. First, the

Wheatstone bridge can be used. However, keep in mind that since the RTD resistance change is typically large relative to the nominal resistance of the RTD, the voltage output of the Wheatstone bridge will not be a linear representation of the RTD resistance. Therefore, the full Wheatstone bridge equations must be used to calculate the RTD resistance (and corresponding temperature). These equations are available in any fundamental DC circuits text.

The second method for measuring the RTD resistance is the 4-wire ohms measurement. This can be done by connecting the RTD to a low current constant current source with one pair of wires, and measuring the voltage drop at the RTDs terminals with another pair of wires. In this case a simple ohms law calculation will yield the RTD resistance. With newer integrated circuit technology, very accurate and inexpensive constant current regulator integrated circuits are readily available for this application.

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