Industrial Control Technology

A Handbook for Engineers and Researchers

Peng Zhang

• Publisher: William Andrew
• Year: 2008
• Print ISBN:
  9780815515715
• eBook ISBN:
  9780815519669
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

This handbook gives comprehensive coverage of all kinds of industrial control systems to help engineers and researchers correctly and efficiently implement their projects. It is an indispensable guide and references for anyone involved in control, automation, computer networks and robotics in industry and academia alike. Whether you are part of the manufacturing sector, large-scale infrastructure systems, or processing technologies, this book is the key to learning and implementing real time and distributed control applications. It covers working at the device and machine level as well as the wider environments of plant and enterprise. It includes information on sensors and actuators; computer hardware; system interfaces; digital controllers that perform programs and protocols; the embedded applications software; data communications in distributed control systems; and the system routines that make control systems more user-friendly and safe to operate. This handbook is a single source reference in an industry with highly disparate information from myriad sources. - Helps engineers and researchers correctly and efficiently implement their projects - An indispensable guide and references for anyone involved in control, automation, computer networks and robotics - Equally suitable for industry and academia

1

Sensors and Actuators for Industrial Control

1.1 Sensors

1.1.1 Bimetallic Switch

Bimetallic switches are electromechanical thermal sensors or limiters that are used for automatic temperature monitoring in industrial control. They limit the temperature of machines or devices by opening up the power load or electric circuit in the case of overheating or by shutting off a ventilator or activating an alarm in the case of overcooling.

Bimetal switches can also serve as time-delay devices. The usual technique is to pass current through a heater coil that eventually (10 s or so) warms the bimetal elements enough to actuate. This is the method employed on some controllers such as cold-start fuel valves found on automobile engines.

1.1.1.1 Operating Principle

A bimetallic switch essentially consists of two metal strips fixed together. If the two metals have different expansibilities, then as the temperature of the switch changes, one strip will expand more than the other, causing the device to bend out of the plane. This mechanical bending can then be used to actuate an electromechanical switch or be part of an electrical circuit itself, so that contact of the bimetallic device to an electrode causes a circuit to be made. Figure 1.1 is a diagrammatic representation of the typical operation of temperature switches. There are two directional processes given in this diagram, which cause the contacts to change from open to close and from close to open, respectively:

Figure 1.1 Typical operation of temperature switches.

(1) Event starts; the time is zero, the temperature of switch is T₁, and the contacts are open. As environment temperature increases, the switch is abruptly heated and reaches the temperature of T₂ at some moment causing the contacts to close.

(2) When the temperature of the switch is T₂ and the contacts are closed, if the environment temperature keeps decreasing, the switch is abruptly cooled, which takes the temperature to T₁ at some instance causing the contacts to open again.

1.1.1.2 Basic Types

Bimetallic switches and thermal controls basically fall into two broad categories: (1) Creep action devices with slow make and slow break switching action and (2) snap action devices with quick make and quick break switching action.

Creep action devices are excellent in either a temperature-control application or as a high-limit control. They have a narrow temperature differential between opening and closing, and generally have more rapid cycling characteristics than snap action devices.

Snap action devices are most often used for temperature-limiting applications, as their fairly wide differential between opening and closing temperature provides slower cycling characteristics.

Although in its simplest form a bimetallic switch can be constructed from two flat pieces of metal, in practical terms a whole range of shapes is used to provide maximum actuation or maximum force during thermal cycling. As given in Fig. 1.2, the bimetallic elements can be of three configurations in a bimetallic switch:

Figure 1.2 The operating principle for bimetallic switches: (a) basic bimetallic switch, (b) adjustable set-point switch, and (c) bimetallic disc switch.

(1) In Fig. 1.2(a), two metals make up the bimetallic strip (hence the name). In this diagram, the black metal would be chosen to expand faster than the white metal if the device were being used in an oven, so that as the temperature rises the black metal expands faster than the white metal. This causes the strip to bend downward, separating from contact so that current is cut off. In a refrigerator you would use the opposite setup, so that as the temperature rises the white metal expands faster than the black metal. This causes the strip to bend upward making contact so that current can flow. By adjusting the size of the gap between the strip and the contact, you can control the temperature.

(2) Another configuration uses a bimetallic element as a plunger or pushrod to force contacts open or closed. Here the bimetal does not twist or deflect, but instead is designed to lengthen or travel as a means of actuation as illustrated by Fig. 1.2(b). Bimetallic switches can be designed to switch at a wide range of temperatures. The simplest devices have a single set-point temperature determined by the geometry of the bimetal and switch packaging. Examples include switches found in consumer products. More sophisticated devices of industrial usages may incorporate calibration mechanisms for adjusting temperature sensitivity or switch-response times. These mechanisms typically set the separation between contacts as a means of changing the operating parameters.

(3) Bimetal elements can also be disc shaped as in Fig. 1.2(c). These types often incorporate a dimple as a means of producing a snap action (not appropriately plotted in this figure). Disc configurations tend to handle shock and vibration better than cantilevered bimetallic switches.

1.1.1.3 Application Guide

Bimetallic devices are generally specified for temperatures from − 65 °F to several hundred degrees Fahrenheit. Specialized devices can handle upward of 2000 °F. Set-point tolerance and repeatability is generally on the order of ± 5 °F, and set-point drift is usually negligible.

(1) Choosing the right thermal control. The rate of temperature rise, location of the thermal control, the electrical load, and the mass of the application can each greatly affect cycling (operational) characteristics of a thermal control. Because of these variables, it is strongly recommended that you conduct testing of the switches specifically in your application. Certain aspects should be taken into consideration when applying both creep and snap action devices. Careful attention must be paid to input voltage, load currents, and the characteristics of the load. Final design criteria should be based upon results of the testing of our devices in your application, at your facility.

(2) Choosing the right bimetallic switches. It has been realized that each application for thermal controls is unique in one form or another. Because of this, there is no standard product. A wide range of options is offered, including the calibration temperature range and tolerances. The length of the lead wires and the type of insulation material also require deliberate consideration. You should require samples for your application testing before deciding to use bimetallic switches.

(3) Snap action configurations. Snap action bimetal elements are used in applications where an action is required at a threshold temperature. As such, they are not temperature-measuring devices, but rather temperature-activated devices. The typical temperature change to activate a snap action device is several degrees and is determined by the geometry of the device. When the element activates, a connection is generally made or broken, and a gap between the two contacts exists for a period of time. For a mechanical system, there is no problem; however, for an electrical system, the gap can result in a spark that can lead to premature aging and corrosion of the device. Having the switch activate quickly, hence the use of snap action devices, reduces the amount and duration of spark. Snap action elements also incorporate a certain amount of hysteresis into the system, which is useful in applications that would otherwise result in an oscillation about the set point. It should be noted, however, that special design of creep action bimetals can also lead to different ON/ OFF points, such as in the reverse lap-welded bimetal.

(4) Sensitivity and accuracy. Modern techniques are more useful where sensitivity and accuracy are concerned for making a temperature measurement; however, bimetals find application in industrial temperature control where an action is required without external connections. Evidently, geometry is important for bimetal systems as the sensitivity is determined by the design, and a mechanical advantage can be used to yield a large movement per degree temperature change.

1.1.1.4 Calibration

Temperature range calibration can be conducted with the following two methods:

(1) The ice method. Immerse the temperature probe at least 2 in. into a glass of finely crushed ice. Add cold tap water to remove air pockets. Wait at least 1 min. The gauge should read 32 °F. If it does not, turn the adjustment nut on the back of the reading dial with a pair of pliers until the dial reads 32 °F. Wait at least 1 min to verify correct adjustment.

(2) The boiling method. Submerge the probe into boiling water. Wait until the needle stops moving, then adjust the calibration nut until the dial reads 212 °F. Since the boiling point of water decreases as altitude increases, this method may not be as accurate as the ice method at altitudes above sea level unless the exact boiling point temperature is known.

Calibration is a broad topic and includes the ultimate reference sources, such as the national metrology laboratories, which are the custodians of the International Temperature Scale, and those services that are directly traceable to the national standards. For example, this is the scale that the national labs, or those affiliated to those labs, refer to in the calibration certificates of reference devices that may be used in corporation or university or other measurement laboratories that provide a more local service, such as to working instruments in a process plant or experimental apparatus.

1.1.2 Color Sensors

Color sensors that can operate in real time under various environmental conditions can benefit many applications, including quality control, chemical sensing, food production, medical diagnostics, energy conservation, a...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Preface
• 1: Sensors and Actuators for Industrial Control
• 2: Computer Hardware for Industrial Control
• 3: System Interfaces for Industrial Control
• 4: Digital Controllers for Industrial Control
• 5: Application Software for Industrial Control
• 6: Data Communications in Distributed Control System
• 7: System Routines in Industrial Control
• Index