What is automatic control?
According to the Japanese Industrial Standards (JIS), control is defined as "the application of a required operation to an object in order to make it conform to a certain purpose.
For example, consider a case in which you want to maintain a constant temperature in an aquarium for breeding tropical fish. The water in the aquarium is heated by heaters, but the temperature is affected by outside temperatures and other factors, so the output of the heaters must be constantly adjusted to achieve the proper temperature.
In such cases, keeping the temperature of the aquarium constant is the "purpose", and adjusting the heater output is the "required operation". And the entire system of detecting water temperature and outside temperature and adjusting heater output is called "control".
Control is broadly divided into manual control and automatic control, with automatic control taking the form of feedback control, feedforward control, and sequence control.
Typical methods of control include
This control is performed by simply turning the switch on and off. A typical example is a thermostat.
For example, a heater with a thermostat is placed in an aquarium filled with water. Since there are only ON and OFF operations, the output value moves up and down in the vicinity of the set value. When the power is turned on, the heater stays on until the water temperature reaches a pre-determined level. If the water temperature exceeds the specified temperature, the thermostat will turn off the switch, and if it falls below the temperature, the thermostat will turn the switch on again.
This phenomenon of going above or below the "setpoint" is called overshooting or undershooting. Because it is a very simple mechanism with only two operating volume options (0% and 100%), it is utilized in a variety of devices and locations, such as the heating mechanism of humidifiers.
Unlike ON/OFF control, this control continuously changes the operating volume in response to changes in the control volume. A few percent is set as the proportional band centering on the target set value, and in the proportional band, the operation amount is proportional to the deviation between the current value and the set value.
Taking the water in the aquarium and the heater as an example, the heater heats at 100% output until the water temperature reaches the proportional band, and the proportional control is performed from the point when the water temperature reaches the proportional band. During proportional control, the output is 100% at the lower threshold of the proportional band, 50% at the target value, and 0% at the upper threshold.
Compared to ON/OFF control, there is less overshoot and undershoot, and equilibrium is achieved somewhere in the proportional band. However, it is also characterized by the fact that the stability of control results varies greatly depending on the setting of the proportional band. The guideline for the proportional band is about 2 to 10%, but be careful because it depends on the device.
PID control is one of the feedback controls and is a very common control that is currently used in so many places. P stands for Proportional, I for Integral, and D for Derivative. PID control is the addition of differential and integral control to proportional control.
P operation (proportional control) combined with I and D operation. I-operation and D-operation do not operate independently. The I-operation is responsible for reducing the offset generated during proportional control, while the D-operation is responsible for responding to disturbances.
Three operations of PID control
We explained that PID control is based on a combination of P, I, and D operations. In this section, we will explain specifically how each of these operations works.
- • P-operation
In PID control, P operation (proportional control) is the basic operation. As described in the previous section, proportional control is control in which the deviation between the set value and the current value is proportional to the operating volume within a proportional band.
- • I-operation
I operation is controlled by time integration. In P operation, the set value and the current value do not match due to load fluctuations or inherent characteristics of the equipment, and a permanent discrepancy (deviation) may occur. This is called offset (residual deviation).
Therefore, PI operation, which combines P operation with I operation, can be used to automatically eliminate the offset that has occurred. Using water in an aquarium as an example, PI operation is the fine-tuning of the water to achieve equilibrium at a temperature as close to the set value as possible.
The I operation adjusts the operation amount of the P operation by proportionally accumulating the offset time and the operation amount. If the integration time is short, strong integration will be applied, and the deviation can be corrected in a short time, but hunting in which the current value fluctuates with the set value in between is likely to occur.
Therefore, the point of control is to set the integration time appropriately.
- • D-operation
D-operation is a control to respond to sudden changes due to disturbances, etc., and to correct the output. Based on the example of an aquarium, even if the water temperature is stable once, it will change due to external factors such as a sudden rise or fall in the outside air temperature.
In such a case, the P operation adjusts the output value in proportion to the deviation between the current value and the target value, whereas the PD operation plus the D operation adjusts the output value in proportion to the speed of temperature change. As soon as a trend in temperature change is detected, the corresponding control is performed, shortening the time required for stabilization at the original value.
Example of PID control
PID control is used in a great many fields, including production, such as temperature control, motor control, and robot control. In addition, the equipment, where PID control is used, is mainly equipment with many disturbance factors or equipment that frequently changes target values. For example, there will be many examples of use in electric furnaces. PID control units are sold for integration into equipment, and these may be used to build production facilities and other equipment.
In recent years, the thermal cycler used for Polymerase Chain Reaction (PCR) has been attracting attention as a device that uses PID control. In PCR, the temperature of the reaction solution must be manipulated up and down at a specified temperature in order to replicate the DNA fragment. At this time, PID control is used to adjust the temperature of the PCR solution.