Temperature control PID setting and formula management of twin screw extruder
The twin-screw extrusion granulator mainly melts plastic raw materials through heating. The temperature control of the extruder adopts the dual control of heating and cooling, which usually adopts the discontinuous control mode. The heating controls the operation of the electric heater by controlling the on-off of the solid-state relay. When the temperature is higher than the set value, it realizes cooling through water cooling or air cooling, Therefore, the heating and cooling cycle value t should be set. The heating cycle T is generally set to 3-10 seconds, and the cooling cycle T is generally 1-2 seconds.
PID value: the extruder temperature is controlled by heating and cooling dual PID control, in which the p value has the greatest influence. Therefore, first set the proportional value p, that is, the proportional band of the regulator. The larger the P is, the faster the response is, and the greater the overshoot is, the less sensitive it is. The smaller the p value, the slower the response and the smaller the overshoot, but the larger the steady-state error; Generally, when the temperature is lower than the set temperature and the heating signal is not given in time, the heating p value should be increased. When the temperature is higher than the set value and the cooling signal is not given in time, the cooling p value should be increased;
When the heating or cooling overshoot is too large, the corresponding p value should be reduced; The second is to adjust the I value. That is the integration time to remove the steady-state error reached by the proportional control system. If it is too large, it will delay the time for the system to reach equilibrium; If it is too small, it will fluctuate frequently. Finally, adjust the D value, set differential time, predict the disturbance of the system ahead of time, advance the response action, and improve the stability of the control.
At present, the temperature control mode of extruder includes temperature control instrument control and PLC temperature control. Those controlled by temperature control instrument usually use heating voltage output and cooling relay output instruments, such as OMRON E5EC-QR2ASM-828 series temperature control meter, which has self-tuning function. Select heating and cooling dual PID control mode, and start the self-tuning function after the equipment has startup conditions, Enter the self-tuning state. After the self-tuning is completed, the best PID value will be recorded automatically, so as to achieve high control accuracy.
PLC temperature control collects the temperature value through the temperature sensor module, and PLC CPU performs PID operation to heat or cool the output signal to the output module. It is also one of the temperature control methods widely used in the extruder industry at present; In order to reduce interference and wiring, the temperature sensor module is installed on the fuselage box and communicates with CPU through PROFINET. The CPU performs PID operation according to the set value and actual value, and outputs work instructions to PLC module to realize temperature control.
At present, PLC does not have self-tuning function, and PID setting is closely related to temperature control accuracy. In particular, some extruder production lines will be used for material production with different material characteristics. The same PID parameters cannot match different scenarios. Therefore, we propose PID parameter formula function. When a technician produces a material, after the set PID parameters meet the temperature control accuracy requirements, the temperature control PID value can be stored in the PID formula. The PID formula parameters can be directly called the next time the material is produced.