experiment 5_on-off controller

Experiment No 5 Mohit Verma 9001020

1

On – off controller

Aim

To find out the effect of differential gap on the on-off frequency of the on-off controller.

Apparatus

1. A test tank with liquid level sensor and a solenoid valve on the outlet side. 2. A liquid circulating system consisting of a storage tank, constant level over head tank,

collection tank 3. A pump to pump the water

4. Valves at the entry and the exit of the tank to regulate the inflow and outflow rate

5. Stop watch

6. Three probes that basically enable the working of the on-off controller

Procedure

1. Open both the valves, and start the pump to begin the flow of liquid.

2. Place the test tank below the outlet tube of the constant level constant level over head tank

and the collection tank below the outlet of the test tank

3. Now keep the controller top most probe at level 1 (5cm gap) and find the time that is taken

by the liquid to rise to the level of top most probe. This would be time for which the

controller is on state.

4. when the liquid reaches the top most probe, the pump would be off. Now find the time

that would be taken for the liquid level to drop to the bottom most probe. This would be

the time for which the controller is in off state.

5. Now raise the top most probe to level 2 (10 cm gap) and find the on time and the off time

for the controller.

6. Once you have noted down both the timings for this particular flow rate and at both levels,

change the flow rate by adjusting one of the valves and pump speed

7. Record the on time and off time for both the levels.

Theory

On- off controller is the simplest of all the controllers. It can also be considered as a proportional controller with a very high gain. As the name suggests the controller sets the manipulated variable either at full value or at zero value. The difference between upper and lower limit of the controlled variable is known as the differential gap. The controller does not take any action between these two limits. Depending on the settings when the controlled parameter reaches the upper limit the manipulated variable is fully on (if it was off till that time) or it is fully off (if it was on till that time). For the liquid level control system, in the present case, we have made output flow rate from the tank as the manipulated variable. The upper level and the lower level limits can be set


2

by adjusting the electrodes suitably. As long as the level in the tank remains below the upper limit the solenoid valve be opened and liquid level will start falling down. (for this to happen it is obvious that the output flow rate from the tank has to be greater than the input flow rate). As the liquid level falls to the lower limit the solenoid valve will close and no liquid can flow out from the tank. Since there is still the liquid input to the tank liquid level in the tank will start rising and the cycle will repeat.

If the differential gap is kept small then the time required for the liquid level to rise from the lower limit to the upper limit will be small. Similarly for the liquid level to fall from the upper limit to lower limit will also be small and the on-off frequency will increase. The opposite will be the effect for large differential gap. Thus the differential gap will have an effect on the on-off frequency. Larger the differential lower will be the on-off frequency and vice-versa. However this is valid for a given time constant. If time constant of the system is large for the same differential gap on0off frequency will be lower one. Thus the on-off frequency is a function of differential gap as well as the time constant of the process. On-off frequency will decrease with any one or both these parameters while it will increase with decrease in any one or both these parameters.

Observations

Distance = 5 cm ( nominal flow and nominal discharge )

Level (cm) Time (sec) (off-on/on-off) time (sec)

5 0

0 23.89 23.89

5 65.27 41.38

0 89.39 24.12

5 130.83 41.44


3

0 154.96 24.13

5 197.92 42.96

Average time off-on 24.04666667

Average time on-off 41.93

Distance = 10 cm ( nominal flow and nominal discharge )


10 0

0 31.52 31.52

10 81.8 50.28

0 112.99 31.19

10 162.18 49.19

0 193.37 31.19

10 246.77 53.4



Distance = 10 cm ( keeping discharge same, at low flow rate )


10 0

0 30.93 30.93

10 128.02 97.09

0 158.3 30.28

10 245.68 87.38

0 276.18 30.5


0

5

10

0 50 100 150 200 250 300

Leve

l (cm

)

Time (sec)

Level vs Time (fixed flow rate and fixed discharge)


4


Distance = 10 cm ( keeping discharge same, at high flow rate )


10 0

0 27.15 27.15

10 73.81 46.66

0 103.2 29.39

10 160.97 57.77

0 194.09 33.12

10 250.36 56.27



0

10

0 50 100 150 200 250 300

Leve

l (cm

)

Time (sec)

Level vs time (on decreasing flow rate)


5

Distance = 10 cm ( at that high flow rate, making discharge low )


10 0

0 53.14 53.14

10 74.4 21.26

0 128.07 53.67

10 149.67 21.6

0 203.17 53.5

10 224.91 21.74



0

10

0 50 100 150 200 250 300

Leve

l (cm

)

Time (sec)

Level vs Time ( keeping discharge same,

at high flow rate )


6

Distance = 10 cm ( at that high flow rate, making discharge high )


10 0

0 18.33 18.33

10 327.83 309.5

0 344.52 16.69

10 725.65 381.13



0

10

0 50 100 150 200 250 300

Leve

l (cm

)

Time (sec)

Level vs Time (on high flow rate, making discharge low)

0

10

0 100 200 300 400 500 600 700 800

Leve

l (cm

)

Time (sec)

Level vs Time (on high flow rate, making discharge high)


7

Result

We found the on-off frequency of an on-off controller will decrease with increasing differential gap

Differential

gap [cms]

5 (nominal

flow and

nominal

discharge)

10 (nominal

flow and

nominal

discharge)

10 (keeping

discharge

same, at

low flow

rate)

10 (keeping

discharge

same, at

high flow

rate)

10( at high

flow rate,

making

discharge

low )

10 (at high

flow rate,

making

discharge

high)

On-off

frequency

[cycles/min]

0.909 0.729 0.488 0.719 0.8003 0.165

Conclusions

1. It is observed that time taken by the controller to respond changes with change in level, top

flow rate and down flow rate

2. Here we observed that as the level of the third probe is raised the time taken by the

controller to respond increases and hence the frequency of the controller decreases.

Further we observe that an on-off controller is not very accurate to maintain the set point

and can be used in places where very prompt control is not needed. These kinds of

controller can be very widely used in domestic overhead water tanks.

experiment 5_on-off controller

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