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AN INDUSTRIAL INSTRUMENTATION TEACHING AID' COLEMAN 1. MAJOR State University of Iowa, Iowa City, Iowa
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teaching of the fundamentals of industrial instrumentation is greatly facilitated by demonstrations of rontrol instruments in actual operation. Commercial controllers may be used for classroom demonstration purposes but the chief disadvantages of these inst,ruments are that they are costly and that they are somewhat too complicated for beginning students t o underxtand. The teachiug aid developed by the author is relat.ively inexpensive t o construct and offers an excellent means of demonstrating the fundamentals of industrial pneumatic instrumentation. Because all of the working parts are exposed on the panel, the student is able to observe how each part functions throughout the control period. The teaching aid may be used for lecture demonst,rations in the classroom or it may be used by students in the laboratory for carrying out various instrumentation experiments. Apparatus. A photograph of the apparatus is shown in Figure 1. A schematic diagram of the apparatus is Prrsentcd at the Symposium mi Process Instrumentation at the 124th Meeting of t,he American Chemical Society, Chicago,
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Figure I.
1n1trvmentatian Teaching Aid
September, 1953.
MAY. 1954
shown in Figure 2. In this particular apparatus liquid level in chamber H i s the controlled variable. The measuring element consists of a float I whose position is transmitted mecbanically to the pen arm K and the air orifice 0. Water enters chamber H through an airoperated control valve B. Chamber G is connected in series with H whenever the effect of process lag is to be studied.
The position of the float determines the distance of the flapper from the orifice, which in turn determines the air pressure on the diaphragm of the control valve. The proportional band is adjusted by either varying the effective length of lever arm M or by varying the effective length of lever arm N. The length of lever arm M is adjusted by sliding panel Q t o the right or left. The length of lever arm N is adjusted by sliding the bellows P t o the right or left along track R. Water leaves chamber H through manually operated valve X , which controls the load on the system. The load is measured by means of rotameter Y. The air-operated control valve B was constructed from an automobile fuel pump and a l/r-inch screwed brass globe valve. The construction of the control valve is shown in Figure 3. The circular chart J makes one revolution per hour and is rotated counterclockwise by means of a motor taken from a dime-store variety alarm clock. The hub which holds the chart was soldered to the shaft that rotates the minute hand of the clock mechanism. In order to obtain a counterclockwise rotation of the chart, the clock mechanism was mounted with its reverse side facing the control panel. With this apparatus it is possible to obtain simple onoff action, proportional action, and proportional with derivative action. Although it is not employed in this apparatus, automatic reset action may be incorporated
by the addition of a second bellows and resistance combination. TYPICAL EXPERIMENTS
Four typical experiments which may be conducted with the use of this teaching aid are described below. (I) Determination ojProportional Band. The width of the proportional band is defined as the number of chart units over which the recording pen must travel in order to cause the control valve to move from the completely closed to the completely open position. The percentage proportional hand is equal to 100 times the ratio of the width of the proportional band to the range of the instrument. In order to determine the proportional band, the operator moves the pen-arm manually until the output pressure on gage T reads 15 psig. This represents the completely closed position of the control valve. The chart reading is noted. The pen-arm is then moved until t,he output pressure on gage Treads 3 psig, which represents the completely open position of the valve. The chart reading is again noted. Intermediate values of output pressure versus chart reading may also be ohtained if desired. Figure 4 shows the record which was obtained when the pen-arm was moved stepwise to produce two psi increments of pressure change on gage T. The data obtained were as follows: Chart reading +9.6 +6.6 +2.8 -1.2 -5.5 -9.6 -13.7
OuCput pressure, psig 15 (valve fully closed) 13 11 9
7
5
3 (valve fully open)
The width of the proportional band in the above case is equal to 9.6 - (- 13.7) = 23.3 units. The range of the instrument is 42 units. The percentage proportional band for this particular setting is therefore equal to 2 S J / r 2 X 100 = 55.5 per cent. Figure 5 shows a similar run, in which the proportional setting of the instrument was 12 per cent. A plot of the data from Figures 4 and 5 is presented in Figure 6. It will be noted that output pressure is essentially a linear function of the chart reading.
rim.. 4.
liquid Lovd at Diffo..nt output Plessur-. tionsl ~ . ~ d
55.5% R o p e -
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(8) Effect of Process Load on Offset. Offset is defined as the difference between the actual value of the controlled variable and the desired value or set-point. The object of the present experiment is to show that offset is produced by a change in process load and t h a t offset decreases as the proportional band is decreased. Figure 7 shows the results of a run in which the load mas varied for a proportional setting of 30 per cent. The run was made without the use of chamber G, i. e., with essentially no process lag. Referring to Figure 7, the load between points 1 and 2 was 100 cc./min. At point 2 the load was suddenly increased to 200 cc./min. I t will be noted that the chart reading or liquid level in chamber H quickly dropped from a value of 8.0 to 7.8
Fig".e 5.
Liquid bd at D i - n t Output Pr-u.-. tiona1 Band
F i g 7 V w i n g Load. 30% Proportional Bend
Fig-
8. Varying Load,
9% P~oportiondBand
point 5 the load was decreased to the original 100 cc./ min. and the pen quickly returned to its original control point of 8.0 units. Figure 8 represents a run similar to that of Figure 7, except that a proportional band of 5 per cent was used. By comparing Figures 7 and 8 it is seen that a decrease in proportional hand caused a decrease in the amount of offset. (3) Effect of Proportional Band on Cycling. Too narrow a proportional band causes undue cycling for cases
12% Propm-
,
units. At point 3 theload was increased to 400 cc./min. with a corresponding drop in liquid level to 7.4 units. At point 4 the load was increased to 800 cc./min. with a corresponding drop in liquid level to 7.0 units. At
Figure 9.
Pros-
with Lag.
1.9% Proportional Band
where an appreciable process time lag exists. The present experiment shows how cycling may be eliminated by increasing the proportional band. Figure 9 represents a run in which chamber G was introduced into the system to produce a process lag. A narrow proportional band of 1.5 per cent was employed. The chart shows the record of the liquid level in chamber H. At point 1 the water was turned on with a process load of 500 cc./min. The level quickly rose to a maxi-
Chart reading Fig"* 6
rig".
10.
Pro"-
with Lag.
55% P.0~0lti.n.1 Band
MAY. 1954 mum a t point 2 and then dropped t o a steady cycling a t point 3. At point 4 the load was suddenly lowered t o 200 cc./min. The pen-arm continued t o cycle hut a t a reduced amplitude and frequency. Figure 10 represents conditions identical t o those in Figure 9 except that the proportional band was in-
creased to 55 per cent. I t will be noted that the increased proportional band completely eliminated the cycling. I t will also he noted that the larger proportional band caused less overshooting of the control point during the startup but that a greater offset was obtained when the load was chanxed - at point 4. 4. In some processes a wide proportional band is necessary in order t o minimize cycling, but toogreat a length of time is required for the process to settle down after an upset. In these cases a controller with derivative action is desirable. Derivative action in the present apparatus is obtained simply by introducing a derivative resistance consisting of a
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laboratory screw-type pinchclamp shown as item S in Figure 2. The pinchclamp is adjusted manually by trial and error until the proper control is obtained. Figure 11 shows the record of a run in which a proportional band of 60 per cent was used. This run was similar to that of Figure 10 except that a greater load and a greater process lag were employed. At point 1 the run was started with a load of 700 cc./min. The liquid level cycled but the amplitude of the cycle kept diminishing. At point 4 the load was suddenly lowered to 250 cc./min. Figure 12 represmts a r_n identical to that of Fig. 11 except that derivative action was employed. I t is seen that derivative actim very successfully reduced
Y,
Fiwm 12. Proc-
with hs. Duivatin, Action. @% Roportionml
~~~d
the time required for the process to reach equilibrium. I t also very materially reduced the overshooting of the control point during the startup and following the loa3 change.
