Instrumentation Sensors Book

Ground Wire Res.

Figure 15.14 Effect of resistance and lead capacitance on current signals.

higher than with analog data. Another advantage is that digital transmitters and receivers require much less power than analog transmission devices.

The IEEE defines communication standards for digital transmission between computers and peripheral equipment. The standards are the IEEE-488 or RS-232 [6]. However, several other standards have been developed and are now in use. The IEEE-488 standard specifies that a digital “1” level will be represented by a voltage of 2V or greater, and a digital “0” level will be represented by a voltage of 0.8V or less. The signal format to be used is also specified. The RS-232 standard specifies that a digital “1” level will be represented by a voltage of between +3V and +25V, and a digital “0” level will be represented by a voltage of between −3V and −25V. The signal format to be used is also specified. Fiber optics are now being extensively used to give very high speed transmission over long distances, and are not affected by electromagnetic or RF pickup.

Digital signals can be transmitted without loss of accuracy, and can contain codes for limited automatic error correction or for automatic requests of data retransmission. These networks are known as LANs when used in a limited area, and as WANs when used as a global system. A typical LAN network is shown in Figure 15.15. Engineers, programmers, supervisors, and operators can communicate with the process controllers over the LAN, for functions such as process monitoring, cost information scheduling and entering or modifying programs.

15.7.2Foundation Fieldbus and Profibus

The development of the smart sensor has caused a revolution in the method of communication between the sensing elements, actuators, and controllers. The controller now takes on a supervisory role of the smart sensor. The controller receives data

LAN

Figure 15.15 A LAN network.

from the smart sensor, and updates the smart sensor as required, using a digital serial bus [7].

The Foundation Fieldbus (FF) and Profibus are the two most universal serial data bus formats that have been developed for interfacing between a central processor and smart sensing devices in a process control system. The FF is primarily used in the United States and the Profibus format is primarily used in Europe. Efforts are being made for a universal acceptance of one bus system. Process control equipment is presently being manufactured for either one system or the other, but global acceptance of equipment standards would be preferred. A serial data bus is a single pair of twisted copper wires, which enables communication between a central processing computer and many monitoring points and actuators when smart sensors are used. This is shown in Figure 15.16. Although initially more expensive than direct lead connections, the advantages of the serial bus are minimal bus cost and installation labor. The system replaces the leads to all the monitoring points by one pair of leads. New units can be added to the bus with no extra wiring (i.e., a plug and play feature), giving faster control, and programming is the same for all systems. Accuracies are obtained that are higher than from using analog, and more powerful diagnostics are available. As the cost of integration and development decreases, the bus system with its features will become more cost-effective than the present systems. This was discussion in Section 6.5.1.

The bus system uses time division multiplexing, in which the serial data word from the central processor contains the address of the peripheral unit being addressed in a given time slot, and the data being sent. In the FF, current from a constant current supply is digitally modulated. Information on the FF is given in the ISA 50.02 standards.

One disadvantage of the FF is that a failure of the bus, such as a broken wire, can shut down the entire process, whereas with the direct connection method, only one sensor is disabled. This disadvantage can be overcome by the use of a redundant or a backup bus in parallel to the first bus, so that if one bus malfunctions, then the backup bus can be used [8].

A comparison of the characteristics of the serial data buses is given in Table 15.1. The original Foundation Fieldbus was designated H1. A new generation of the H1 is the HSE, which will use an Ethernet LAN bus to provide operation under the TCP/IP protocol used for the internet. The advantages are increased speed, unlimited addresses, and standardization [9].

Figure 15.16 Foundation Fieldbus cable connection.

15.8.3Width Modulation

Width-coded signals or PWM are blocks of RF energy whose width is proportional to the amplitude of the instrumentation data. Upon reception, the width can be accurately measured and the amplitude of the instrumentation signal reconstituted. Figure 15.17(a) shows the relation between the voltage amplitude of the instrumentation signal and the width of the transmitted pulses, when transmitting a series of 1V signals and a series of voltages through 10V.

For further power saving, PWM can be modified to pulse position modulation (PPM). Figure 15.17(b) shows a typical PWM modulation and the equivalent PPM signal. The PWM signal shows an Off period for synchronization of transmitter and receiver. The receiver then synchronizes on the rising edge of the transmitted zero. The first three pulses of the transmission are calibration pulses, followed by a stream of width-modulated data pulses. In the case of the PPM, narrow synchronization pulses are sent, and then only a pulse corresponding to the lagging edge of the width-modulated data is sent. Once synchronized, the receiver knows the position of the rising edge of the data pulses, so that only the information on the lagging edge is required for the receiver to regenerate the data. This form of transmission has the advantage of greatly reducing power consumption and extending battery life [10].

15.8.4Frequency Modulation

When using frequency modulation, the unmodulated carrier has fixed amplitude and frequency. When modulated, the frequency of the transmitted signal is varied in

1 V

Time

(a)

Figure 15.17 (a) Amplitude modulated waveform, in which the width of the modulations corresponds to voltage levels; and (b) comparison of PWM and PPM waveforms.

C H A P T E R 1 6

Process Control

16.1Introduction

The two basic modes of process control are sequential control, or On/Off action, and continuous control action. This chapter deals with process control, discussing both sequential and continuous control. Sequential control is an event-based process, in which one event follows another until a process sequence is complete. Many process control functions are sequential.

Continuous control requires continuous monitoring of the process variables, so that they can be held at set levels. Process control systems vary extensively in complexity and industrial application. For example, controllers in the petrochemical industry, automotive industry, and soda processing industry have completely different types of control functions. The control loops can be very complex, requiring microprocessor supervision. Some of the functions need to be very tightly controlled, with tight tolerance on the variables and a quick response time, while in other areas, the tolerances and response times are not so critical. These systems are closed loop systems. The output variable level is monitored against a set reference level, and any difference detected between the two is amplified and used to control an input variable that will maintain the output at the set reference level.

16.2Sequential Control

There are many processes in which a variable does not have to be controlled, but a sequence of events has to be controlled. The sequential process, or batch process, can be event-based, time-based, or a combination of both. An event-based system is one in which the occurrence of an event causes an action to take place. For example, a level sensor detects that a container is full, and switches its output from On to Off, which sends a signal to the controller to turn off the filling liquid. A time-based sequence is one in which an event is timed by the controller and is not waiting for an input signal. A batch process can be distinguished from a continuous process as follows:

•All actions can be defined by On/Off states.

•Discrete quantities are handled individually at each step, rather than as a continuous flow.

•Each quantity is kept as a separate unit and can be individually identified.

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•Each unit, unlike in continuous processing, can see different process steps.

•Each unit will not start a new step until the previous step has been completed.

•Batch processes can have process variations for each batch of new material, depending on previous processing.

It should be noted that a continuous process could be part of, or a step in, a discrete process.

Example 16.1

The On/Off controller action has many applications in industry. An example of some of these uses is shown in Figure 16.1. In this case, jars on a conveyor belt are being filled from a silo. When a jar is full, it is sensed by the level sensor, which sends a signal to the controller to turn Off the hot liquid flowing from the silo, and to start the conveyer moving. As the next jar moves into the filling position, it is sensed by the position sensor, which sends a signal to the controller to stop the conveyer belt, and to start filling the jar. Once it is full, the cycle repeats itself [1].

Level sensors in the hopper senses when the hopper is full, and when it is almost empty. When empty, the sensor sends a signal to the controller to turn On the feed valve to the silo; when full, the level is detected and a signal is sent to the controller to turn the feed to the silo Off. When the silo is full, the heater is turned On to bring the liquid up to temperature before the jars can be filled. An On/Off temperature sensor senses the temperature, which informs the controller when to resume filling, and to stop the operation if the temperature in the silo falls below the minimum set temperature.

On/off temperature sensor

Silo fill control

M

Motor control

On/off level sensor

On/off position sensor

Figure 16.1 Example of the use of On/Off controls used for jar filling.

16.3Discontinuous Control

Discontinuous control mode is frequently used in process control, and could be considered as a discrete On/Off mode. In reality, discontinuous control is the basis of continuous control, in its simplest and least expensive form. It is used where its disadvantages can be tolerated. Such systems have high inertia, which prevents the system from continuously cycling [2].

16.3.1Discontinuous On/Off Action

The measured variable is compared to a set reference. When the variable is above the reference, the system is turned On, and when below the reference, the system is turned Off, or vice versa, depending upon the system design. This could make for rapid changes in switching between states. However, some systems, such as heating systems, normally have a great deal of inertia or momentum, which produces overswings and introduces long delays or lag times before the variable again reaches the reference level. Figure 16.2 shows an example of a simple room heating system. The top graph shows the room temperature or measured variable, and the lower graph shows the actuator signal. The room temperature reference is set at 70°F. When the air is being heated, the temperature in the center of the room has already reached above 73°F before the temperature at the sensor reaches the reference temperature of 70°F. Similarly, as the room cools, the temperature in the room will drop to below 67°F before the temperature at the sensor reaches 70°F. The room temperature will go approximately from 66°F to 74°F due to the inertia in the system.

16.3.2Differential Closed Loop Action

Differential or delayed On/Off action is a mode of operation where the simple On/Off action has hysteresis or a dead-band built in. Figure 16.3 shows an example of the fluctuations in a room heating system with delayed action. This is similar to Figure 16.2, except that, instead of the thermostat turning On and Off at the set reference of 70°F, the switching points are delayed by ±3°F. As can be seen in the top

Time

Figure 16.2 Temperature fluctuations using On/Off action in a room heating system.

Figure 16.3 Temperature fluctuations using differential On/Off action in a room heating system.

graph, the room temperature reaches 73°F before the thermostat turns “Off the actuator, and the room temperature falls to 67°F before the actuator is turned On, giving a built-in hysteresis of 6°F. There is, of course, still some inertia. The room temperature will go approximately from 65°F to 75°F.

16.3.3On/Off Action Controller

There are many ways of designing and implementing a two-position controller. An example of a typical bimetallic On/Off switch used in a room temperature controller is shown in Figure 16.4. The bimetallic element operates a mercury switch, as shown in Figure 16.4(a). At low temperatures, the bimetallic element tilts the mercury switch down, causing the mercury to flow to the end of the glass envelope, making an electrical short between the two contacts, which would operate a low voltage relay to turn On the blower motor and the heating element. When the room temperature rises to a predetermined set point, the bimetallic strip tilts the mercury switch back, as shown in Figure 16.4(b), causing the mercury to flow away from the contacts. The low voltage electrical circuit is turned “Off, the relay opens, and the

Figure 16.4 Simple On/Off room heating controller using a bimetallic switch.