I/EC
Special Feature
This basic engineering guide to control purposes, modes and characteristics, and controller choice will show you . . .
How to Select a Process Controller by ROBERT B. NICKERSON, Weston Instruments Div., Daystrom, Inc., Newark, N. J.
MOST
INDUSTRIAL
PROCESSES will
produce a required end product well and efficiently only when certain variables are held within given limits. This regulating function usually can be performed automatically using a controller that will measure the variable and operate a final control element to hold the deviation of that variable within prescribed limits. For some applications, a simple on-off controller is enough; others need a more complex control form. For the engineer contemplating use of a controller, the most important consideration is to view the process as an integrated whole. All functions and operations used in the control of a characteristic or treatment of a material must be considered from an over-all aspect.
The temperature controller performs four basic,' continuous functions: Measurement of variable—hot water temperature. Comparison with desired value— measured temperature compared with desired temperature set point. Determine needed correction. Correction by positioning steam control valve through pneumatic signal. These four functions operate continuously to hold the water temperature within given limits by making corrections such as adjusting position of the steam control valve to the
process in accordance with the signal, hot water temperature, fed into the controller. Such a control system is called the closed loop of control, which is shown with the four functions related to the process (Figure 2). Although this example used a pneumatically operated controller, this closed loop concept can be applied equally well to an electrically operated controller. BASIC PROCESS CHARACTERISTICS
Certain processes have physical characteristics that can delay and retard needed changes. Generally,
TYPICAL PROCESS
A typical process—heat exchange —is shown in Figure 1. The tank with its steam coils and pipes comprises the apparatus within which the heating process takes place. Other components of this control system are : • Controlled variable—temperature of the hot water. • Manipulated variable—flow of steam through the coils. • Controller—pneumatically operated temperature recordercontroller with a filled thermal system consisting of a temperature bulb, capillary, and bourdon spring within the recorder. • Final control clement—airoperated diaphragm control valve. 48 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE 1
Two-Position (On-Off) Control MANIPULATED VARIABLE (STEAM PRESSURE)
COLD WATER INPUT
STEAM INPUT
HOT WATER OUTPUT
CONTROLLED VARIABLE (HOT WATER TEMP.)
ERROR SIGNAL
MECHANICAL MOTION
In two-position control, the final control element is moved from one to two fixed positions to the other. Usually, as in a heating process, the final control element is full on when the variable is below the set point and full off when above it. No intermedite position is provided. Corrective action is either greater or less than exact, causing the con trolled variable to cycle above and below the set point. This control form is discussed here. A plot of temperature and energy input against time for this control
SET POINT
FIGURE 2 these are termed process time lags and may be present as: Capacity lags—lags resulting from energy-storing factors. In the ex ample the walls of the steam coils and the water in the tank can store heat energy. If warmer water is needed, it is provided by increasing the incoming steam flow, but again there is a lag in time before input energy from the steam meets the temperature need. Resistance lags—lags caused by those parts of a process which resist the transfer of materials or energy. An illustration is scale on the inside and rust on the outside of the steam coils of a water heater. The thermal conductivity of the fluids also can be an important consideration. Dead time—time required to carry a change from one point to another in the process, during which no change occurs. Referring again to the illustrated hot water system as an example, a drop in temperature of the incoming water will have no effect until the water travels through the tank and is sensed by the tem perature bulb at the outlet, A. If the bulb were removed to position d, this time would be increased bydistance, divided by the velocity of the hot water output. This type of lag is also called distance-velocity or transportation lag. LOAD CHANGES
Two types of load change may be present in a process: supply-load change, or change in energy input to a process; and demand-load
change, or change in the energy output or production rate of a process. Referring again to the typical process an increase of plant steam pressure is an example of a supply-load change, in this case resulting in more energy input for a given valve opening than before. Changing the flow of hot water required is an example of a demandload change. The magnitude and duration of load changes can affect the ease of control of a process. MODES OF CONTROL
A mode of control is the type of corrective action a control instrument takes when the controlled variable deviates from the set point. This discussion is concerned with the following controller modes:
• Two Position • Three Position • Differential Gap • Single Speed Floating with Adjustable Neutral Zone • Proportional Position with Manual Reset • Proportional Position Plus Automatic Reset • Time Proportional Plus Automatic Reset • Proportional Position Plus Automatic Reset Plus Rate Control
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FIGURE 3 is shown in Figure 3. Two-position control is best suited to high capacity processes with minimum dead time. Three-Position Control With this control mode, the final control element (or elements) as sumes one of three fixed positions or values, each corresponding to a selected zone of the controlled vari able. This zone or range is deter mined by a relation of the high or low contact settings to the set point. Cycling usually is present, as any of the three positions can rarely produce an exact correction for a given load condition. See Figure 4.
HIGH CONTACT SETTING
Each mode of control has its characteristic advantages and limita tions. Usually, the more difficult the control problem, the further down in the list one must go to find the best suited mode of control.
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FIGURE 4 VOL. 52, NO. 12
·
DECEMBER 1960
49 A
Differential Gap Control
Proportional Position Control
This mode of control causes the final control element to be moved from one of two fixed positions or values to the other. It is similar to two position control except for the existence of differential gap which is determined by the relation of the high and low trip points, or contact settings, to the set point.
All of the control forms previously mentioned produce a step type action resulting in cycling of some amount. Many industrial applications, how ever, require a form of control that will maintain the variable at a con stant value. This requires a means of continuous control action which is throttling in its effect. Proportional control gives such an effect by pro ducing a correction which is directly proportional to the departure of the variable from the set point. The greater the departure or deviation, the greater the corrective action. For a given condition, this means the final control element moves the same amount for each unit of deviation. In other words, the final control element assumes a definite position for each value of the controlled variable. The resulting control, when properly adjusted, gives a modulating or throttling effect with out cycling.
•DIFFERENTIAL GAP
0 0 CONTROLLED VARIABLE PERCENT OF FULL SCALE
FIGURE 5 Referring to Figure 5, the final control element moves from its first position to its second, shown by black arrows, when the controlled variable reaches a set value, 60% of scale, from one direction. It can return to its first position, shown by white arrows, only after the variable has passed in the opposite direction through a range of values, 60% down to 40% of scale, to a second value, 40% of scale. This differ ential gap is usually adjustable.
Proportional control adjustment is calculated in per cent proportional band, or in terms of gain. Propor tional band, or throttling range, is defined as the change in value of the controlled variable, as related to full scale of the controller, necessary to cause full travel of the final control element. The proportional band of a particular instrument is usually expressed as a per cent of full scale of the instrument. For example, in Figure 6, if the full scale of the instrument is 100° F.
100
Single Speed Floating Control Single speed floating control with adjustable neutral zone is a form of electric three-position control used with a relatively slow, single-speed, reversible motor. When the con trolled variable is within the adjust able neutral zone, no contact is made and the motor remains motionless. Outside of this neutral zone, the motor operates to correct for the deviation. It is not frequently used, however, because of the difficulty in choosing the optimum rate of movement of the final control ele ment. If moved too fast, the effect is like that of two-position control. If moved too slowly, the controlled variable may drift appreciably from the set point before adequate correc tion is made. 50 A
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FIGURE 7 is 50° F. in 100° F. or 50% P.B. Figure 8 also illustrates the concept of proportional band. Note that proportional bands over 100% can not cause full valve travel even for full scale change in controlled vari able. In combination with auto matic reset, proportional bands ex ceeding 100% are used to control highly stable processes with long lags. Proportional Position Plus Manual Reset Control In the diagrams used to illustrate proportional control (Figures 6-8) the valve has been shown as 50% open when the controlled variable is at the set point. On an actual application, it would be pure co incidence if this 50% valve opening would exactly maintain the variable at the set point. Usually, some other
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FIGURE 6
FIGURE 8
and a 100° F. change in temperature causes full valve travel, the per cent proportional band is 100° F. in 100° F. or 100% P.B. In Figure 7, a 50° F. change in temperature produces full valve travel. In this case, the per cent proportional band
valve opening is required. This will cause the controlled variable to depart from the set point in a direc tion and by some amount until equilibrium is reached. This departure or deviation from the set point is called offset or droop.
INDUSTRIAL AND ENGINEERING CHEMISTRY
S P E C I A L FEATURE The set point and the controlled variable will coincide for one load condition only. For all other loads, there must be some offset. This offset can be eliminated by a reset adjustment which causes the proportional band to be shifted either up or down scale, forcing the controlled variable to coincide with the set point. This is shown in OR I G I H A L P O S I T I P R O P O R T I O N A L BAN! FIGURE 8
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FIGURE 9 Figure 9 where a permanent load change has taken place so that the 50° F. temperature will be main tained by a 2 5 % valve opening, instead of the 50% opening in Figure 8. Note that the propor tional band has been shifted, or reset, to satisfy this new load condi tion. Reset did not change the width of the band which was, and still is, 50° in 100° F., or 50%. A common everyday occurrence illustrates the principle of reset action. Driving a car on a level road at a constant 45 m.p.h. re quires a certain position of the accelerator. When starting to climb a hill, a load change, it is necessary to depress the accelerator further and further, a new valve position, in order to maintain 45 m.p.h. When the hill levels off, car returns to original load, gradually returning the accelerator to its original position will maintain the speed of the car at 45 m.p.h. Purpose of reset is to eliminate offset and to maintain exactly the variable at the desired point by pro viding for proper valve opening or power level at the set point. Proportional controllers with manual reset are satisfactory where
. DEVIATION
load changes, which can change the amount of offset, are small and in frequent and where manual adjust ment to compensate for a load change is practical.
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PROPORTIONAL
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Proportional Position Plus Automatic Reset Control
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This control form produces a correction by combining propor tional control and automatic reset control. Addition of automatic reset does automatically and continuously what manual reset does by hand. It shifts the proportional band forc ing the variable back to the set point after a load change, giving an exact correction and eliminating offset or droop. Two adjustments are provided, proportional band and reset time. Reset time is the time it takes for the reset action to duplicate the proportional action that occurred as a result of a load change or upset. This duplicating effect will continue until the final control element as sumes some position to cause the controlled variable to return to the set point. The best way to explain this type of control is to picture each component separately and observe what each contributes to the resultant valve position Figure 10.
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FIGURE 10
Reset time shown is the time it takes for reset to change the valve position by distance C which is the same "amount of correction made by the proportional action. In this particular illustration, the process is not shown on control. If it were, the valve would continue closing until the temperature returned to the set point. Time Proportional Plus Automatic Reset Control Many industrial applications that require proportional control cannot use proportional position control because they require on-off type final control elements. For these applications, the electric Time Proproportional Control with automatic reset may be used. In this control mode, there is a pulse cycle which is the " o n " time PROPORTIONAL BAND
SET POINT ADJUSTMENT
CONTACTS POWER SUPPLY
1
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FIGURE 11 VOL. 52, NO. 12
·
DECEMBER 1960
51 A
TABLE I. Lags
CAPACITY
Description Exists where there is a considerable storage or inventory of controlled medium
PROCESS LAGS Typical Applications Level control of large storage tanks. Pickling or cleaning baths
RESISTANCE
Exists when opposition to corrective action due to resistive nature of process elements or sensing element is present
DEAD TIME
Exists when the measure Temperature control where ment point is some distance bulb is a considerable dis downstream of final control tance downstream of heat element. Equals distance exchanger divided by stream velocity
plus the "off" time. The percentage of on time to off time is proportional to the position of the controlled variable within the proportional band. Λ simplified mechanical ver sion of this control is shown in Figure 11. The on-off time would vary from continuously on to con tinuously off if the controlled variable should pass completely through the proportional band. The on plus off time or pulse cycle is adjustable. Reset eliminates droop or offset and produces an exact correction after a load change. A form of rate action is available with this control mode. It does not conform exactly to the rate action described under the next mode type.
Temperature control of process elements such as exchangers particularly when using sensing element mounted in thick well
Three adjustments are provided, proportional band, reset time, and rate time. Rate time is expressed in minutes and is the time the rate action speeds up or advances the effect of the proportional action. For this reason, it is sometimes called antici patory control. In effect, it pinches off the proportional control
Controller Mode If not accompanied by other lags, is easiest to control. On-off or proportional con trol with narrow propor tional band is suitable Requires a very wide proportional band with a "slow" automatic reset, Do not add rate action, Try to eliminate dead timee.g., by relocating temperature bulb Requires a wider propor tional band to eliminate cycling. Add manual or automatic reset to eliminate offset. Consider use of rate action if resistance lag is main lag present
may slow down and over-corrects his speed temporarily to gain more momentum. This is rate action. On the way up the hill, depressing the accelerator further and further to hold a constant speed is reset action. All three control components re sponding to an abrupt load change 1
PROPORTIONAL ACTION ONLY
PROPORTIONAL COMPONENT
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RESET COMPONENT
Proportional Position Plus Automatic Reset Plus Rate Control
RATE COMPONENT SET POINT
The use of a proportional plus automatic reset may not be satis factory in a process which has sudden upsets, appreciable transfer lag, or a small to moderate amount of dead time. In such situations, a con troller with proportional plus auto matic reset plus rate action may be required. This type of controller acts in accordance with the sum of these three individual control actions. The rate action makes a correction in valve position or power in propor tion to the speed at which the con trolled variable is changing. 52 A
RESULTANT VALVE POSITION
TIME
FIGURE 12 action temporarily causing it to act more like an on-off controller. Fig ure 12 shows rate advancing the pro portional action by time t. The hill climbing example of reset action also provides an example of rate action. On starting up a very steep hill at a constant 45 m.p.h. the driver anticipates that his car
INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE 13 are shown in Figure 13. The cumu lative effect of proportional, reset, and rate action gives the resultant valve position shown at the bottom of the illustration. To sum up, proportional plus automatic reset plus rate actions function as follows: Proportional action responds in proportion to the deviation.
S P E C I A L FEATURE TABLE II.
CONTROL CHARACTERISTICS
Can Handle Load Change Size
Speed
Direction of departure from set-poinf
Maximum immediate "one step" correction produces cycling
Any
Any
Three Position
Direction and magnitude of departure from setpoinf
Maximum correctiontwo steps—produces cycling
Moderate Slow
Differential Gap
Direction and magnitude of departure from setpoinf
Same as two position Any after passing through the differential gap—produces
Controller Mode Two Position (0n-0ff)
Effect Produced
Responsive To
Any
cycling
Single Speed Floating with Adjustable Neutral Zone
Direction and magnitude of departure from set-
poinf
Constant speed correction outside of dead band. Very difficult to adjust
Any
Slow
Typical
Applications
Large capacity, easily controlled temperature and level applications Batch heating requiring rapid rise with minimum overshoot, as in small electric furnaces Temperature control of oil storage tanks where close control not required. Prevents too rapid cycling and consequent wear of contactors or valve operator Not frequently used, could be applied to process with small dead time requiring a rotary motion type of final element
Proportional Position
Direction and magnitude of departure from setpoint
Proportional Position Plus Automatic Reset
Direction, magnitude, and Same as proportional Large duration of departure except reset will eliminate from set-point offset
Slow to Temperature, flow rate Moderate and many other processes requiring close throttling control where offset is not permissible
Time Proportional Plus Automatic Reset
Direction, magnitude, and duration of departure from set-point
Similar to proportional plus reset above, except used with a two-position final control element
Slow to Temperature control Moderate where offset not permissible and electrical two position final control element is being used
Proportional Position Plus Automatic Reset Plus Rate
Direction, magnitude, duration, and rate of speed of departure from set-point
Same as proportional plus Large reset plus an accelerated response to sudden upsets
Proportional to deviation— Small produces throttling or modulating control. Offset generally present and manually corrected
As applied to "single" capacity processes with a slow reaction rate. As applied to a "single" capacity process having a fast reaction rate and considerable amount of
Reset action eliminates the offset accompanying the proportional action. Rate action is anticipatory in effort and, by advancing the effect of the proportional action, reduces the time away from the set point after a load change or upset.
Moderate
Large
Fast
Easily controlled pressure, temperature, and level applications where close throttling control is required
For difficult control applications with sudden upsets where offset is not permissible and throttling control is required
self-regulation.
SUMMARY
discussion are given in Table II.
A summary of the three types of process lags that may be presentcapacity lag, transfer lag, and dead time—are given in Table I. And a summary of the various controller forms outlined in this
It should be emphasized that these tables are general guides only. They should be used as approximate aids only in selecting the proper control mode for a particular application. VOL. 52, N O . 12
·
DECEMBER 1960
53 A
