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Which one is the best: PID Controller or ON/OFF Controller? 
There are many people have this question, and don't know which one should be choose to use to their systems.
I have made two tests to show what are the difference of them, comparing their accuracy, temperature stability and switching frequency of change.
   (P2: ON/OFF Controller, ITC-2000)
(P1: PID Controller, ITC-100VH)   

Mate​rials for Test:


Materials for test 1:

  • Fuzzy logic enhanced PID controller INKBIRD ITC-100VH;
  • 25A SSR;
  • Waterproof PT100 sensor;
  • Heater (2000W, 220V)
  • INKBIRD temperature and humidity data logger;
  • The bucket and the water

 


Materials for test 2:
  • ON/OFF controller INKBIRD ITC-2000;
  • Heater (2000W, 220V)
  • INKBIRD temperature and humidity data logger;
  • The bucket and the water
Testing records:

Test 1: Fuzzy logic enhanced PID controller INKBIRD ITC-100VH

When I did test 1, the room temperature was 26.5°C and water temperature was 25.9°C.
First, I wired the ITC-100VH and other device as below photo shows:
(P4: ITC-100VH wiring diagram)   
(P5: ITC-100VH wiring)
After wiring and power it on, I set the controller,
THC-4 Parameter setting:
  • SV=40°C,
  • CTRL=3 Self-adjustment (We used self-adjusting function to determine the control parameters (M50, P and T parameter), so it can obtain ideal controlling.),
  • SN=21 (PT100=21),
  • CF=2 (Factory Default),
  • Run=2 (Factory Default).

Remarks: we just use it to heat and don’t connect it to the alarm system.


THC-4 Setting:
  • Temperature Upper: 41°C, Temperature Lower: 39°C;
  • Interval: 1minute
  • Temperature Calibration: -0.5°C (To make sure the temperature is the same as ITC-100VH)

Remarks: All the setting is via PC software.

(P6: THC-4 Setting board)
After Setting, we tied the sensors together to sure that their measuring area to be the same, and then put them in the water.

During the test, we observed the temperature and the frequency of output of ITC-100VH.
(P7: Testing ITC-100VH)
Results test 1:

We upload the data from THC-4 to PC, and here is the result shows:


Temperature Records:
(P8: THC-4 ITC-100VH data)
There were total 63 records in 63 minutes. It took 12 minutes to reach 40°C and become stable.

After that the temperatures were between 39.9 to 40.3°C, the average temperature was 40.08 in these 51 data.
Times

Max Value

Min Value

Average Value

51

40.3°C39.9°C40.08°C
(P9)
The maximum deviation is 40.3°C, only appear once.

Temp.

Statistical Number

39.9°C

2
40.0°C14
40.1°C28
40.2°C
6

40.3°C

1
(P10)
(P11)

The frequency records:
At the beginning, the output from ITC-100VH lasted for 12 minutes.
After the temperature reached about 40°C, the frequency became quick, it changed about every 10seconds to 30seconds, The time of every output lasted between 1S to 5S.
 
Summary:

Contrast

PID Controller

Accuracy (max deviation)

0.3°C
Frequency of ChangingQuickly
Output Lasting Time1-3 s
(P12)
Remarks: The result changes if using different sensor and connecting different heating/cooling device

Test 2: On/Off INKBIRD ITC-2000


The room temperature is 27.2°C and water temperature is 21.5°C.
Wiring the ITC-2000 as below photo shows:
(P13: ITC-2000 Wiring)
ITC-2000 Parameter setting:
SV=40°C,
DS=0.5 (Min),

Remarks: we just use it to heat and don’t connect it to the alarm system.​


THC-4 Setting:
Temperature Upper: 41°C, Temperature Lower: 39°C;
Interval: 1minute

Temperature Calibration: -2°C ( To make sure the temperature is the same as ITC-100VH)


Remarks: All the setting is via PC software.
After Setting, we tied the sensors together to sure that their measuring area to be the same, and then put them in the water.
During the test, we observed the temperature and the frequency of output of ITC-2000.
(P14: Testing 2)
Results test 2:
Here are the temperature results for test 2:
We upload the data from THC-4 to PC, and here is the result shows:
Temperature Records:
(P15: THC-4 ITC-2000 data)
There are total 63 records in 63 minutes. It took 12 minutes to reach 40°C and become stable. After that the temperatures were between 39.6 to 40.7°C, the average temperature was 40.16 in these 51 data.
Times

Max Value

Min Value

Average Value

51

40.7°C39.6°C40.16°C
(P16)
The maximum deviation is 0.7°C, only appear once.

Temp.

Statistical Number

39.6°C

2
39.7°C5
39.8°C3
39.9°C5
40.0°C
1
40.1°C5
40.2°C7
40.3°C9
40.4°C5
40.5°C5
40.6°C2

40.7°C

2
(P18)
(P17)
The frequency records:
At the beginning, the output from ITC-2000 lasted for 10minutes.
After the temperature reached about 40°C,
The relay output regularly fluctuated back and forth between 39.6 ° C -40.9 ° C, the cycle was about 9 minutes; the output lasted about 10-15S.
Summary:

Contrast

PID Controller

Accuracy (max deviation)

0.7°C
Frequency of ChangingSlowly
Output Lasting Time10-15 s
(P19)
Remarks: The result changes if using different sensor and connecting different heating/cooling device
Overall results:
Which one is the best: PID Controller or ON/OFF Controller?
Actually, there is no standard answer.
It depends on where you need to use and what your control target is.
So, before deciding, please ask yourself:
What is your control target?
What system do you use with the controller?
Hope this small test can help you to make the decision.
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