Navigation:  Equipments (Devices) > PLC200 > Program > Control > PID2 Controlador Proporcional-Integral-Derivativo com Sintonia Automática (Auto Tuning)

Block that performs auto tuning of a discrete PID controller. From the input variables, it calculates the corresponding controller output PID or PI throw relay method. This block also implements the obtained controller or another that the user wishes.

 

 

Ladder Representation

 

 

Block Structure

 

Table1

 

Operation

 

The operation of the PID2 block is divided into two parts: Control and Automatic Tuning.

The Tune variable defines which mode of operation. Whenever Tune = 0, the system operates in control mode, with values of the user defined gains, in the same way as the PID block.

The control part is exactly the same as the PID block, see the PID Help.

 

When Tune receives a value other than 0, the automatic tuning process starts. Some rules need to be obeyed so that tuning can occur. They will be displayed in the sequence.

 

The Auto-Tuning

For the Automatic Tuning, the Relay Method is used, which is based on obtaining the critical gain and critical period , which can be used to obtain PID controller gains through several tuning rules: Ziegler-Nichols, Ciancone-Marlin, Tyreus- Luyben, ITAE, among others.

During the tuning process, the control is switched off and a "relay" determines the output of the controller (Output), as shown below.

 

At start, the relay (Output) goes to the value and after a period the output (Feedback) starts to increase. When the output becomes higher than the value of the setpoint + hysteresis, the relay switches to, and so on, as shown below.

 

Since the value of the manipulated variable (Output) stabilized at the point of operation, we have:

In the case of image above,

From the obtained subtraction curve, the necessary parameters are obtained to obtain the controller.

 

Criteria for tuning to be performed

In order for automatic tuning to be performed properly, two criteria must be obeyed:

1) place the system at the operating point to be calibrated (the difference must be less than 20%);

2) variable , which is the manipulated variable, may not be showing oscillation greater than 20%;

OBS: you can use the Manual mode of the block to reach the operating point without large oscillations (via the variable ).

 

Step by step to perform tuning  (portuguese only)

1) must start with the value zero;

2) Set a value for . An initial value of 10% of the manipulated variable value is recommended () which reached the system setpoint;

Ex: Assuming that to stabilize at the point of operation = 12.3. In this case, use

3) Set an initial value for . This value should be slightly larger than the noise present in the system. An initial value of 2% of the Setpoint value can be used if no noise is known;

Ex: for = 60.0, use

4) Set a value in milliseconds () to . This value depends on the system being applied to the block. Slower systems will require a longer time. It should be sufficient for at least 10 cycles of the relay to be complete, although with 5 cycles the system generally already stabilizes;

5) Place the system in the operating point. If a pre-control has already been done, it can be applied. Another option is to use the manual mode and vary input until the system stabilizes at the point of operation, that is, as close as possible to ;

6) Enable tuning by choosing the type of control (PI or PID and the rule used) through the variable , as shown in Table 2;

7) The output will inform you the status of the automatic tuning process, as shown in Table 9. At the end of the process, the output value will be received if the calibration is performed successfully and the outputs , and will be updated with the values obtained in the tuning process.

Table 2

 

Table 3

The Tyreus-Luyben methods are recommended for systems with the dominant time constant in relation to the transport delay. Ciancone-Marlin methods are recommended for systems with a long transport delay. The minimum ITAE method is for intermediary situations.

It is recommended for initial tuning or for users without some practice with the method of the relays, the use of automatic 1 or 2 for , leaving the system to choose the most appropriate way.

After tuning, the method can be changed causing a new controller to be calculated without re-tuning, since the required data is stored internally. This allows other methods to be easily experienced.

For a new tuning to be made, need to receive the value zero.

 

 

Block Flowchart

N/A

 

 

 

Example in Ladder

 

 

 

 

 

Example in ST

 

 The example below displays the instructions for applying the example above in the ST language.