Teach you how to implement PID control in S7-1200

[Introduction] Share how to implement PID control in S7-1200.

Overview

[I] PID control

PID function is used to control the closed-loop process. PID control is applicable to physical quantities such as temperature, pressure and flow. It is the most widely used control method in industrial sites. Its principle is to set a given value for the controlled object and then measure the actual value, compare it with the given value, and send the difference to the PID controller. The PID controller calculates the result according to certain operation rules, which is the output value and sends it to the actuator for adjustment. P, I and D refer to proportion, integration and differentiation, which is a closed-loop control algorithm. Through these parameters, the controlled object can follow the changes of the given value and make the system stable, automatically eliminating the influence of various interferences on the control process.

【I】PID control of S7-1200

The number of PID controller loops provided by the S7-1200 CPU is limited by the CPU's working memory and the number of supported DB blocks. Strictly speaking, there is no specific limit, but in actual application, it is recommended that customers do not exceed 16 PID loops. Loop control can be performed simultaneously. Users can manually debug parameters or use the self-tuning function. Two self-tuning methods are provided for the PID controller to automatically debug parameters. In addition, STEP7 Basic also provides a debugging panel, so that users can intuitively understand the status of the controller and the controlled object. The PID controller function is mainly implemented by three parts: the loop interrupt block, the PID instruction block, and the process object background data block. When calling the PID instruction block, the user needs to define its background data block, and this background data block needs to be added in the process object, which is called the process object background data block. The PID instruction block and its corresponding process object background data block are used in combination to form a complete PID controller. Since Software Portal V13, S7-1200 with firmware 4.0 or higher can use the PID_Compact V2.2 instruction. For S7-1200 V4.0 and above, and TIA Portal V13 SP1 and above, you can use S7-PLCSIM V13 SP1 to simulate PLC programs, but it does not support the simulation of technological functions (high-speed counters, motion control, PID regulation).

(1) The following table shows which CPU each version of PID_Compact can be used for:

(2) Typical CPU processing time of PID_Compact V2.0 technology object (depending on the CPU type):

(3) Memory requirements for the background data block of the PID_Compact V2.0 technology object:

Characteristics of analog closed-loop control system-PID control:

It does not require a mathematical model of the controlled object, has a simple structure, is easy to implement, is easy to use, and has strong flexibility and adaptability.

Tuning PID controller using debug window - Functions of debug window

1. Use the "First Startup Self-Tuning" function to optimize the controller

2. Use the "self-adjustment during operation" function to optimize the controller to achieve the best adjustment

3. Used for trend view to monitor current closed-loop control

4. Test the process by manually setting the controller output value