Instrumentation
December 1950
i A new control system is especially suited for batch processes and automatic start -up of continuous processes b@ R d p h H.Munch >
control instruments with many types of response are required t o solve the infinite variety of control problems met in industry. The first control instruments to be developed were the simple onoff type. These were satisfactory when lags and supply side capacities were small, but cycled badly when they became larger. Proportional response controllers were the next development. These gave improved control in many cases, but had the disadvantage that load changes produced a deviation from the control point which was progressively worse as the proportional band was widened t o take care of more difficult control problems. T o correct this situation, proportional response to controllers with automatic reset were introduced. Rate or derivative response was the next development. Controllers with this response have been very successful in certain applications where lags are especially bad.
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125' C., and that the proportional band of the controller is 10' C. The steam valve would then be wide open a t all temperatures below 120', half open a t 125', and shut at 130° C., assuming that it was properly sized. The batch would therefore heat a t the maximum rate until it reached 120' C. Then the controller would begin to close the valve. At 125' C. the valve would be half closed. Because of unfavorable heat capacities and lags, the batch temperature would overshoot the control point, causing the controller to throttle the valve still more. This action would cause the batch temperature to cycle about the control point two or three times before a stable equilibrium was reached. The overshoot could be reduced by widening the throttling range. This would have the disadvantages that it would cause more offset with load changes and that it would cause the batch to approach the control temperature more slowly. Better control could be obtained by using a controller with proportional plus rate response. Let us assume the same proportional band width used in the first example plus rate response. In this case, the
FULOW-UP BELLOWS UNIT
I"
NOZZLE
ADJUSTABLE RESTROTIONS
RELbY
s FOLLOW-UP BELLOWS UPilT
ELEMENT
WNTROL PROCESS
Figure 1. Conventional I'roportional Plus Reset Plus R a t e Controller
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Most control problems can be handled with suitable combinations of the responses mentioned above. However, there are still certain control applications which cannot be adequately handled by a single controller embodying these responses. We shall consider two very similar applications of this type-the control of a batch process where the batch must be brought from room temperature to process temperature i n the shortest possible time without overshooting and starting up a continuous operation without overshooting. I n both cases, we wish t o maintain the control point unaffected by load changes which may occur after the proccss has come to temperature. Let us consider first what happens when a batch of material is brought up to temperature, under control of a simple proportional response controller adjusted to give good control after the operating temperature has been reached. We shall assume that the equipment is steam heated, that the operat!ing temperat,ure is t o be
Figure 2.
New Taylor Triaot Control System
controller would start to close the valve as soon as the temperature came within the proportional band, but it would close the valve much more rapidly than a simple proportional response controller because of the rate action. I n this case the batch temperature might approach the desired value and become stable without cycling. However, the approach would be somewhat less rapid within the proportional band of the controller than it would be without rate response. By increasing the rate action, the temperature could be made to approach the desired value more rapidly, cycling slightly, but not exceeding the control point a t any part of the cycle. Still better action might be obtained during the heating-up part of the cycle by narrowing (Continued on page 70 . A ) the proportional band. This
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Instrumentation could \,e done without making the control less stable a t the control point hecbause of the rate action. An additional advantage would be that the offset due to load changes would be reduced. A proportional plus rate controller would give the best control which could he obtained with a conventional controller in an application of this type. However, it would still be unable to maintain the desired temperature if the process load changed. T o maintain the same control point under changing load conditions requires automatic reset if the controller possesses proportional response. Unfortunately, reset response causes trouble in applications of the type we are considering. If the same process we have been using as an example were to be started up under the control of a proportional plus reset controller, the controller would not start to close the steam valve until the process temperature was reached, resulting in a much worse overshoot than in the cases described above. This would happen even if rate action were added. It is a fundamental property of any controller having reset response in the primary controller. The reason is that if the process starts up below the control point, the reset response shifts the proportional band so that its lower limit corresponds with the control point. Because of the situation described above, it is usual to bring a process to the control point manually if good control under varying load conditions is required and no overshoot can be tolerated. R. E. Claridge of The Taylor Instrument Companies, 9,5 Ames Street, Rochester, N. Y., has devised a method of incorporating reset response into a controller in such a way that its desirable qualities can be obtained without its disadvantages. To understand how he accomplishes this, we must compare the operation of the conventional proportional plus reset plus rate controller shown in Figure 1 with that of the new control system shown in Figure 2 . In Figure 1the baffle moved by the primary element varies the input air pressure to the inverse proportional relay. Output air from the relay is fed to the control valve, the follow-up bellows, and the reset bellows. Needle valves in the lines to the two bellows units permit adjustment of the reset and rate responses; the proportional band width or sensitivity is adjusted mechanically. The unit is therefore a single-stage controller with three responses. I n the controller shown in Figure 2 , the air pressure supplied by the baffle and nozzle is fed t o a follow-up bellows through a needle valve to produce derivative response and to a reset relay which supplies air to the control valve. This system is essentially two controllers in series, the first with proportional and rate response, the second with proportional and reset responses. There are therefore four adjustments-two sensitives, a rate response adjustment, and a reset rate adjustment. As the primary controller has only proportional and rate response, it initiates rapid control action as soon as the controlled variable enters its proportional band. This type controller has been shown to be well adapted t o applications where the process is to be brought to operating conditions as rapidly as possible with no overshoot and kept at the temperature selected in spite of load changes. 70 A
