EPF 3106 Food Process Engineering

Laboratory III

TITLE : PROCESS CONTROL SIMULATION STATION

EXPERIMENT 7A : OPEN LOOP TEST

Experiment 7A :Open Loop Test

Objective

To perform open loop test at the Simulated liquid Flow Process

Introduction:

Controller tuning relations have been developed that optimize the closed-loop

response for a simple process model and a specified disturbance or set-point change. Tuning

a control loop is the adjustment of its control parameters (gain/proportional band, integral

gain/reset, derivative gain/rate) to the optimum values for the desired control response. PID

tuning is a difficult problem, even though there are only three parameters and in principle is

simple to describe, because it must satisfy complex criteria within the limitations of PID

control. There are accordingly various methods for loop tuning, and more sophisticated

techniques are the subject of patents; this section describes some traditional manual methods

for loop tuning. Process control systems can be categorized as either open loop systems or

closed loop. All elements in a control system have gain. Gain is the ratio of the size of the

output response from a component to the size of the input stimulus and sometimes is known

as sensitivity. The overall purpose of a control system is to minimize the difference of the

controlled process variable from a desired value. The selection on control strategies,

instrumentation and tuning of the control loop are all important elements to attain the desired

end which is in reducing the process variability. Most important thing in reducing the process

variability is placed on the selection of the control algorithm. While the control algorithm

obviously plays an important role in reducing the process variability, it is also important to

pay careful attention to the dynamic performance of the control instrumentation. It is often

incorrectly assumed that the speed of response of flow meters is unimportant in flow control

applications, especially when digital control systems with relatively slow sample rates are

used. Experimental and theoretical analysis have shown that the use of faster responding flow

meters always results in reduced process variability, even when using slow sample rates with

digital controllers.

Materials and Equipment

Liquid Flow Process System : FE21/FT21-FIC21PID-FCY21/PP/FCV21

Procedures

1. The dynamic graphic of the Simulated Liquid Flow Process at the operator workstation

was viewed. The pump 12 was not started to pump. It should be red in colour.

2. The PID controller scheme was selected. FIC21PID was then called up. The controller was

switched to Manual(MAN) mode. The following Gain, Reset and Rate(PID) values were set

from the detail faceplate of FIC21PID.

FIC21PID: Gain=0.4 Reset=3 sec Rate=0 sec

3. A set point of 1.5m3/hr was used for the flow controller FIC21PID.

4. Pump P21 was started. It should be green in colour. Controller FIC21PID was switched to

Auto(AUTO) mode.

5. The Process History View was called up from FIC21PID to view the flow response(red

trend).

6. When the response was fairly steady, FIC21PID was switched to Manual(MAN) mode and

step increase its MV(OUT) by about 5%. The flow response will rise and then gradually

flatten out exponentially. The response was wait until the response was fairly steady at the

new steady state flow rate. The observed flow response was printed to a enclosed together

with the results sheet.

7. The obtained response curve was used to determine the process parameters like Steady

State Gain(Kp), Dead Time(DT) and Response Rate(RR) and Time Constant(p).

8. The controller was switched to Auto(AUTO) mode, with a set point of 3m3/hr and 4.5m

3/hr.

The flow response was viewed at the Process History View. Step 6 and 7 were repeated.

9. For the different operating set points of the liquid flow, the process parameters were

checked whether they were remain constant or vary. The process variability was commented

if the process parameters vary for different operating liquid flow set points.

Results

Table 1 : Process parameter obtained for the Simulated Liquid Flow Process at different

operating setpoint.

Process Parameter Setpoint (SP)

1.5 m3/hr 3.0 m

3/hr 4.5 m

3/hr

Kp 7.55 20.00 9.00

DT(sec) 3.18 2.20 2.70

RR(sec) 0.50 4.55 0.65

15.12 1.11 19.44

Calculations are done on attachments.

Discussion

A controller can be used to control any process which has a measurable output (PV), a

known ideal value for that output (SP) and an input to the process (MV) that will affect the

PV. The output is directly proportional to input thus G1=Kp. The Kp is the gain and this

controls the basic response speed of the process. Dead time (DT) is the delay from when a

controller output signal is issued until when the measured process variable started to respond.

In this experiment, open loop test was performed for the simulated liquid flow process

using three different operating set points of the liquid flow, which are 1.5 m3/hr, 3.0 m

3/hr

and 4.5 m3/hr. The open loop test was tested for set point of 1.5 m

3/hr. The step increased its

manipulated variable MV (OUT) by 5% after the response were fairly steady causing the

flow response to rise and gradually flatten out exponentially. From the flow response we

obtained, the calculated value of steady state gain (Kp), dead time (DT), respond rate (RR)

and time constant ( p) we acquired are 7.55, 3.18 sec, 0.50 sec-1

and 15.12 sec respectively.

After that, the experiment was repeated by using a set point of 3.0 m3/hr and the same

procedure applied where the step increased the MV (OUT) by 5%. From the flow response,

we calculated and obtained the steady state gain (Kp) value as 20 m3/hr, dead time (DT) as

2.20 sec, respond rate (RR) as 4.55 sec-1

and time constant ( p) as 1.11 sec respectively.

For the set point of 4.5 m3/hr, we obtained the steady state gain (Kp) value as 9 m

3/hr,

dead time (DT) as 2.70 sec, respond rate (RR) as 0.65 sec-1

and time constant ( p) as 19.44

sec respectively. The results of all three set points are different and this shows that the

process parameters changes with a change in set point.

From the results, the steady state gain (Kp) decreases from 20 to 9 when the set point

was changed from 3.0 m/ hr to 4.5 m/ hr. This could be due to the valve opening. Ste steady

state process gain decreases as the valve opening increases. The process gain is the sensitivity

of the process output to change the process input. In this experiment MV is the input of the

process whereas PV is the output of the process. Since all the calculated values of steady

state gain is positive it shows positive gain where the increase in process input results in

increasing process output.

Dead time is the amount of time it takes for the process variable to start changing after

changing output as a control valve, variable frequency drive etc. In this experiment, the dead

time (DT) is the time from when the manipulated variable step is made until the time when

the PV starts to show clear response (stage which the responds starts to meet steady state).

The time constant ( p) describe how fast the controlled variable, PV moved in respond to the

change in the manipulated variable. From the results, we can see that the time constant ( p)

for set point of 3.0 m3/hr is 1.11 sec which is the lowest among the three set points, showing

that the response for PV was faster than that of set point of 1.5 m/ hr and 4.5 m/ hr.

Conclusion

Based on the results it clearly shows that the process parameter changes with set point

changes.

Reference

1. http://en.wikipedia.org/wiki/Time_constant

2. http://www.engineeringtoolbox.com/process-control-terms-d_666.html