Identify Autotune PID algorithm

[Vindhyachal.takniki]

1. I have checked autotune PID for Delta Temperature PID controller for maintaining temperature of oven.
2. We set autotune temperature to 75C & current oven temperature is 38C. In attached graph, y axis is reading of temperature divided by 10 so 700 represent 70.0C & x axis is equal interval of reading every 500millisecond.

3. We started the autotune algorithm and measure temperature with our temperature sensor placed closely to delta sensor and with CRO measured the output signal which turn on/off the relay.

4. Here it goes:
a) First relay is given 100% duty cycle and fully on, upto 75C & temperature keep on increasing
b) At 75C, relay is off completely. Temperature keeps on rising as heater is already heating, it falls to 75 in second stage as in pic.
c) Now again delta PID turn on the relay 100% duty cycle, temperature goes below a bit, since it was already cooling & then start rising again in third stage.
d) In fourth stage, it completely turn off the relay, temperature goes up a bit & then falls down to 75C, at this point delta PID shows Autotune complete
e) Then it try to maintain the temperature by maintaining duty cycle according to PID values calculated. Time period is 20seconds & duty cycle varies.

5) Do anyone know which algorithm it may be?  Since no sine in output is observed so most of algo for PID tune are rejected. So it may be:
a) Cohen-Coon Method (Open-loop Test)
b) Ziegler-Nichols Method( open -loop test)
c) Or it may be Delta own algo
d) ANy other algo which fits such kind of graph


6. If its Delta Algo then its ok, otherwise anyone know what else algo it may be? Or how to tune PID values from it

[Vindhyachal.takniki]

this seems to me like ZS open loop method: http://techteach.no/publications/articles/zn_open_loop_method/zn_open_loop_method.pdf

Curve in post#1 has two curves as mentioned in pdf here.

1. Now I trying to understand on which of curve this tangent method is applied?

2 Also page 4 mentions the formula for PID values where Kp, Ti & Td are mentioned. Surprisingly delta PID autotune parameters after its completed are: P = 18.6 , I = 251 , D = 62
In Delta Electronics PID you can check calculated PID values.

3. if we look on formula page 4
Ti = 4L
Td = 0.5L


Calculated PID values from Delta PID are always consistent with this formula, we tried very different set point values.
But Delta PID says its PID values while formula in pdf says its KP,Ti,Td values, but there is striking matching between values & their formulas.

[Kleinstein]

For just the ZN open loop test, one would not need the 2nd half of the test. They may still use the simple test, but also do a few more checks in the 2nd half if the simple formula is OK.