Basic Control Loop Components

Learning Outcome

Upon successful completion of this module, the student will be able to describe the basic types and functions of transmitters, recorders, controllers, and control valves.

Enabling Objectives

The student will be able to:

(a) Describe the operation and construction of a pneumatic transmitter.

(b) List controller types and terms, and describe the application of each.

(c) Describe the basic types of process recorders.

(d) Describe the construction of control valves.

INTRODUCTION

In Learning Module 027-11-02-06, Introduction to Process Measurement, some common measurement devices were discussed. This module will present a brief survey of the other major components of control loops, with some typical examples of each. It will deal mainly with pneumatic control loop devices, since these are among the simplest to understand. Although most control loops today are electronic or electronic/digital, their principles are similar to those of the earlier pneumatic devices. Furthermore, many control loops, such as local level controls, still employ pneumatic components.

TRANSMITTERS

Transmitters are used to send information about some process variable to a desired location, such as a central control room, when the sensing elements are located at various distant locations.

Pneumatic transmitters similar to those shown in Fig.1 can be used to transmit signals over long distances. The mechanism basic to all pneumatic transmitters is the flappernozzle-restrictor assembly shown in Fig. 1 (a). Process conditions affect the position on the link, and this motion is converted into a varying pneumatic output.

A regulated air supply equivalent to about 140 kPa (20 psi) is furnished continuously, and allowed to expand through a restriction which has a smaller bore than that of the nozzle. If the flapper is moved far away from the nozzle, the air will bleed out of the nozzle faster than it can pass through the restriction, so the pressure below the capsule will be zero. When the flapper is moved towards the nozzle, the increased nozzle pressure will cause the capsule to move upward slightly to reduce the exhaust opening, and to increase the opening for the air supply to pass to the output and the feedback bellows. An increase in output pressure will cause the feedback bellows to expand slightly to the left to reposition the top end of the flapper. Reverse action takes place if the flapper-nozzle clearance is increased. The output of the transmitter is directly proportional to the flapper-nozzle clearance and nozzle pressure.

The link of this basic transmitter is connected to a primary sensing element, such as a bourdon tube or bellows. As the process variable changes from minimum to maximum desired value, the output of the transmitter will increase in 20 to 100 kPa (3 – 15 psi).

Figure 1: Basic Pneumatic Transmitter

 

Fig. 1(b) illustrates a simplified commercial differential pressure transmitter that can be used for level and flow measurement. For flow measurement, the high-pressure side would be connected to the upstream side of the flow restriction in the line. An increase in flow will lead to an increase in differential pressure. This, in turn, will cause the capsule or diaphragm to move slightly to the left, and there will be slight clockwise rotation of the force bar that acts as a flapper.

This action causes a proportional decrease in the flapper-nozzle clearance, and an increase in the transmitter output line pressure that will be proportional to the increase in differential pressure across an orifice plate. The feedback bellows will expand slightly with an increase in output pressure to increase the flapper-nozzle clearance a small amount to ensure proportional action of the transmitter.

When this transmitter is applied to level measurement, the high-pressure side is usually applied to the tank level, so that an increase in level will cause a proportional increase in transmitter output.

Fig. 2 shows how a differential pressure transmitter looks when installed in a flow measurement system. Note the sensing lines from the high and low-pressure sides of the orifice plate connecting to the transmitter, and the supply and output air lines going to and from the transmitter.

Figure 2: Pneumatic Flow Transmitter

INDICATORS AND RECORDERS

There are many ways of presenting or indicating the values of variables that are measured. At times a simple mechanical indicator such as a Bourdon tube is quite adequate, but at other times it may be desirable to have a permanent record of performance that can be used for future reference.

Recorders

Recorders are used to maintain a permanent record of certain variables for future reference. The two most common types are the circular chart and the strip chart recorders.

1. Circular Chart Recorders

The circular chart recorder is an earlier design than the strip chart type. One hat is designed to measure static pressure is illustrated in Fig. 3.

Figure 3: Circular Chart Recorder

A spiral Bourdon tube is connected to a recording pen through a link-lever mechanism. When the pressure in the sensing element increases, the movement of the pen will be proportional to the change in pressure in the Bourdon tube.

The chart is in the form of a circular paper disc with a hole in the centre which fits over the chart drive motor. It also has concentric circles that form the scale on which the variable is read. The "time arcs" laid out at uniform distances divide the full or concentric circles into appropriate time intervals of the total period. The chart usually makes one revolution every 24 hours, although some charts rotate in up to 7 days. The clock may be powered by manually wound springs, by electricity, or by pressurized air or gas.

2. Strip Chart Recorders

Over the years, very little has been done to reduce the size of circular chart recorders, but in the design of strip chart recorders, similar to the one in Fig. 4, there has been a marked trend to size reduction.

Figure 4: Strip Chart Recorder

The standard strip chart recorder is characterized by the uniform linear motion of a strip of paper in a vertical or horizontal direction, as shown in Fig. 4. The time lines always run perpendicular to the direction of motion of the chart, while the measurement lines can be straight or curved.

Illustrated here is a three pen recorder that can record three different variables at once. Each pen on this type of recorder is generally operated by a different transmitter output. Instead of placing the primary sensing element directly in the recorder, as in the circular chart type, the output of a transmitter, which is proportional to the process variable, is admitted to the recorder. In most pneumatic recorders of this type, a bellows or capsule is used to position the recording pen.

CRT Trend Displays

Computerized control systems can display process conditions on the monitor or screen (often referred to by its technical name, a Cathode Ray Tube or CRT). The display looks similar to a strip chart record; several trends can be placed on the same screen and a printout can be taken if desired.

This system has both advantages and limitations when compared to the dedicated, paper type recorders discussed above. The operator has the flexibility to assign any transmitter output from the process to the trend display - provided, of course, that the transmitter is connected to the computer system. This means the operator can see the trend on almost any process condition; including the trend on things which are not normally recorded, such as valve positions, ambient weather conditions, and machinery speeds. Time periods can be assigned for the graph, stretching or compressing it, to get a closer look at Rapidly changing conditions.

Graph limits can be set higher or lower, so that small changes can be closely watched. The operator can also select the process conditions that appear on the screen, placing one above or below the other. This makes it possible to diagnose operating problems, since, by visual comparison, the first upset condition and then the subsequent effects can be determined.

The major limitation of this system is that the computer can only show one screen at a time. If it is necessary to access other displays, such as control loops, the operator may need to go back and forth between screens; this is a time consuming procedure when plant conditions are upset or changing quickly. For this reason, computer control panels are often supplied with a few dedicated recorders, which monitor critical operating conditions on a continual basis.

CONTROLLERS

As mentioned in Learning Module 027-11-02-04, Introduction to Instrumentation, the controller is a key device in a control loop. It compares the actual process conditions to a desired operating value, called the setpoint. If there is any difference between setpoint and actual conditions, it sends a corrective signal out to a control valve or other similar device.

Controllers can be organized into several categories, depending on how they react to maintain a setpoint. The first category includes on-off, two-position, and multi-position controllers. The second category, sometimes referred to as "modulating", includes proportional, integral, and derivative controllers. A detailed examination of these types is beyond the Fourth Class level; however, brief look at how each of these controllers responds is included here.

On-Off, Two-Position, and Multi-Position Controllers

These controllers compare actual conditions to setpoint values, and respond by either starting or stopping a final control element. Familiar examples are the common household thermostats on furnaces and refrigerators. The furnace remains off until a minimum temperature is reached. When the furnace turns on, fuel is admitted to the main burner at a constant rate. The main fuel valve is wide open during this period. This continues until the temperature reaches a higher cut-out point, when the main fuel valve closes completely.

Two-position and multi-position controllers react the same way, except that their output signals may be high-low, or high-medium-low, rather than on-off.

Proportional Controllers

A proportional controller also compares actual conditions to setpoint; however, unlike the on-off type, it sends a variable signal to the control valve, causing it to be open, closed, or somewhere in between. The control valve position is modulated. The controller output signal will vary depending on how much the actual conditions differ from setpoint. The controller output is thus proportional to the amount of error. The output signal from a pneumatic controller to a control valve will be between 20 - 100 kPa (3 - 15 psi). Both electronic analog and computer type controllers send out 20 -100 mA signals.

Proportional Plus Integral (Reset) Controllers

For reasons beyond the scope of this module, proportional controllers are not always capable of operating the process exactly on setpoint, although they are generally close. To operate exactly on setpoint, a modification called integral, or reset, action is required.

Proportional Plus Integral plus Derivative Controllers

Further modifications can be made to the proportional plus integral controller by adding derivative (also called "rate") action. This enables the controller to respond more quickly, almost like a temporary on-off controller, if the process begins to deviate widely from the setpoint. Derivative action is usually restricted to slow-responding processes such as heating systems.

Additional Controller Terms

The student should also become familiar with the following controller terms

2. Auto/Manual Control

Most (but not all) controllers have the option of selecting either automatic or manual control. Automatic, as discussed above, means the controller responds to differences between setpoint and actual conditions. Manual control means that the operator selects the controller output; the controller does not respond on its own to the process. Manual control is necessary, for example, when the control loop transmitter is being repaired or calibrated.

3. Feedback/ Feedforward

Most control loops respond to a specific process condition after the condition has changed. Since this information is fed back to the controller, it is called a feedback system. In some specialized control systems, the controller is alerted ahead of time that a change in the process is about to occur. The controller responds before the change takes place. This is referred to as feedforward control.

4. Cascade Control

In some control systems the output signal from one controller becomes the setpoint signal for another controller. Fig. 5 shows the relationship between single loop and cascade control.

Figure 5: Cascade Control