PID Tuning in Practice

1. Adjusting Integral Component I.

Matching the value of the integral component I is associated with the use of the term "response time". The above term indicates how long the integral component I will adjust to the proportional component P. In other words, if the output of component P is, for example, 20%, then the "response time" is the period after which the output of the integral component will also reach 20%. It is important to remember that the higher the value of I, the faster the adjustment of component I to component P.

2. Adjusting Integral Component D.

The combination of the P and I terms is usually sufficient for most applications and that is why the combination of the PI terms is the most popular. PI controllers fulfill their task with a relatively simple structure. Adding the D term allows for higher values of P and I while maintaining the stability of the system and increasing the response speed. The time that defines the D term indicates how far the controller should predict changes in the control signal. The problem remains with disturbances, e.g. (caused by the proximity of working induction motors), due to which the algorithm may no longer work so well. Therefore, we recommend not to worry about the D term until the PI controller proves to be really slow.

A lot of work is also saved by the autotuning function available in the Hynux series controllers, see here

3. Simplified method for determining PID settings.

A simple method that sometimes brings satisfactory results. In the first step, turn off the I and D terms and set the P value to maximum. As a result, we will get oscillations X_osc in time t_osc

_{Figure. Chart of the manual PID tuning method}

For example, the set temperature is 170 C and the obtained temperature is between 174 C and 168 C. X_osc for this case will be 6 C. We measured the period of such oscillations with a stopwatch on the phone and it was t_osc = 5 min

We substitute the values into the following formulas and calculate the settings of the elements.

P = 2.0 * X_osc
I = 1.5 * t_osc
D = I / 5

The calculated parameters may sometimes cause too much damping and consequently a slight reduction may be necessary.

4. Examples of settings.

If the effects of auto tuning are not in line with expectations and the regulation is incorrect, you can try the trial and error method and change the PID settings yourself.

Below are some of the most common control problems:

a) Slow temperature jump response - when the regulator takes a long time to reach the set temperature,
-reduce the range of proportionality P,
-reduce the time I and D

b) Oscillations-when the regulator is unable to maintain the set temperature only fluctuations occur,
-increase the proportional range P,
-increase the integration time I,
-decrease the differentiation time D

c) Overshoot-when there are large deviations from the set temperature,
-increase the proportional range P,
-increase the differentiation time D

d) Instability-when the regulator reaches the temperature in a non-linear way i.e. when the temperature increases or decreases abruptly,
-increase the integration time I

5. Example values of PID parameter settings for popular applications:

NOTE !!! The above settings are only indicative and the values may vary depending on the control system.