过程控制-专业英语-第6章

6

AUTOMATIC CONTROL

OBJECTIVES

When you have completed this chapter you will be able to

·Describe the effect of set-point disturbances to a control loop

·Describe the effect of load disturbances to a control loop

·Describe the desired performance of a control loop

·Describe the effects of loop gain on loop stability

·Describe a control loop with a proportional-only controller

·Describe a control loop with a proportional plus integral controller

·Describe a control loop with a proportional plus derivative controller

·Describe a control loop with a proportional plus integral plus derivative controller

·Perform the ultimate cycle procedure for tuning a control loop

·Calculate the proportional gain to achieve quarter amplitude response

·Calculate the proportional plus integral gains to achieve quarter amplitude response

·Calculate the proportional plus derivative gains to achieve quarter amplitude response

·Calculate the proportional plus integral plus derivative gains to achieve quarter amplitude response

·Describe ratio control

·Describe cascade control

6.1 INTRODUCTION TO THE CONTROLLER OF THE

CONTROL LOOP

A typical continuous as opposed to discrete closed control loop is shown in Figure l.1 on p. 4 and described in sections 1.3 to 1 .7 on pp. 3-7. The objectives of a closed control loop are, first, to provide the process operator with a simple method for changing the measured process variable to a desired value (set point) and, second, to maintain the measured process variable at the desired value, even though the process is disturbed when other process variables are changing.

The most important feature of the control loop controller block is the comparison of the measured process variable to the set point. This is done by subtracting the feedback signal from the set-point signal. Generally, 4-20 ma DC is converted to 1 to 5 V by means of a precise 250 Ωresistor, and the two voltages are placed in series so that their difference is the error signal for the controller. The error signal is used to produce the controller output signal that controls the process adjusting device and, as a consequence, the adjusted process variable. The object is to change the measured process variable so that it becomes closer to the set point value. There are several functions

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of the error signal that may be used in various combinations to generate the controller output signal. The most common function of the error signal is the proportional controller output signal. In this case the output of the controller is proportional to a constant number, called the gain, multiplied by the error signal. All of the standard functions of the error signal include the proportional function, either by itself or with the addition of integral (of error) action or derivative (of error) action or both integral and derivative action. The proportional action alone changes the controller output instantaneously to a new value whenever the error signal changes. The integral and derivative actions are functions of the way the error signal changes with time and they produce added changes to the controller output signal depending on

the effects of time on the error.

Each of the controller functions requires tuning in order to achieve the most desirable response of the loop to a disturbance, such as a change in the set point. Tuning requires the setting of a numerical value for each controller function, rather like setting a dial knob for each function. A variety of undesirable responses occur if any of the functions are poorly tuned. These responses are described in the next section.

6.2 LOOP STABILITY

Disturbances

There are two major types of disturbances that affect a closed control loop. The first is due to a set-point change, the second is due to a load change. It is easy to understand how an operator can change a set point, and for the example of level loop 132 in Figure 6.l, when the set point is increased it becomes necessary for the loop to increase the flow into the tank so that it is temporarily more than the flow out until the level reaches the new value of the set point.

Then the loop will cut the flow back so that it is again equal to the flow out of the tank. For a short time the level is not equal to the set point. It may even overshoot the set point before settling down at or near the set point. When the level settles down again to a steady value, the disturbance is considered over.

A load change disturbance also may occur to level loop l32. The flow out of the tank is

considered the load for this loop. If the loop is running smoothly with the flow in equal to the flow out, and the level is steady at or near to the set point, then the loop is in equilibrium. If the flow out is changed due to an operator changing the outflow valve or due to the process downstream automatically opening or closing another valve in order to obtain more or less flow then the loop will be disturbed. For example, assume the flow downstream is reduced by 25 %, the level in the tank will start to rise, and after a short time the control loop 132 will start to close the inflow valve L V-132. The level will first rise above the set point and then fall back to the set point, perhaps dropping down below the set point, before settling down to a steady value at or near the set point. If the level oscillates about the set point for several oscillations, then the frequency and the dampening of the oscillations will be the same for either a set point or a load disturbance. The frequency and damping characterize the response of the measured variable to any type of disturbance that affects the loop.

The tuning of the controller gains provides a variety of responses to the disturbances, some

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