Control technology is a technique for manipulating physical quantities such as temperature, pressure, and flow to achieve a target value. There are various methods, ranging from on/off switches to PID control, that enable accurate and stable control. Control technology is essential in many industries and plays an important role in increasing efficiency and safety.

The importance and applications of control technology

Control technology is a technology that regulates physical quantities such as temperature, pressure, flow, rotation speed, etc. so that a machine or facility operates according to its purpose. There are many ways in which control technology regulates the amount of manipulation that is output to match a measurement of the current magnitude of the physical quantity being controlled to a set value, which is the desired goal. Control technology plays an essential role in many areas of modern industry, and its importance is growing every day.

Basic Control Techniques: On/Off Switch Methods

The simplest method is the ‘on/off switch method’, which is commonly utilized in thermostats on boilers that are used to adjust the temperature of water. In this device, when the current temperature is lower than the desired temperature, the switch is turned ON to power the heater, and when the temperature is higher than the desired temperature, the switch is turned OFF to cut off power to the heater. When the switch is ON, 100% of the operating volume is output, and when the switch is OFF, 0% of the operating volume is output. When the heater first starts up, it stays ON to raise the water temperature, but at some point the water temperature will “overshoot” the setpoint. This overshoot can strain the system, so the switch is repeatedly turned ON and OFF to bring the current temperature back to the setpoint. Because water temperature is analog in nature, like pressure or flow, it is a continuous change in physical quantity, so if the water temperature rises and you flip the switch to off, it will not immediately fall back down. Therefore, repeatedly flipping the switch on and off will cause the water temperature to “hunt,” which is a constant rise and fall around the setpoint.

The hunting problem and PID control

The on/off switch method is prone to overshoot and hunting, making it difficult to precisely control the physical quantity to be controlled. To compensate for these shortcomings of the on/off switch method, the PID control method is utilized. The PID control method utilizes P (proportional) control, I (integral) control, and D (differential) control to precisely control the physical quantity of the controlled object. However, depending on the purpose, the P control method, PI control method, and PD control method are sometimes utilized.

Characteristics of P control method

P control sets a constant proportional band above and below the set value, and outputs a manipulation amount proportional to the deviation between the set value and the measured value within the proportional band. For example, if the current temperature is below the lower limit of the proportional band, the thermostat of a boiler utilizing P control outputs 100% of the operating force to keep the switch on until the current temperature reaches the lower limit of the proportional band. Then, when the current temperature is above the lower limit of the proportional ratio, it has a proportional cycle, in which the on and off behavior of the switch is repeated in each cycle, i.e., the on time is longer than the off time until the current temperature above the lower limit of the proportional ratio reaches the setpoint. When the current temperature reaches the set value, 50% of the operation amount is output, and the ON and OFF times are 1:1, and the operation is repeated. If the current temperature rises above the set value, the behavior is repeated periodically with the off time being longer than the on time, and if the current temperature exceeds the upper limit of the proportional ratio, the off state is maintained. In this way, P control allows the measured value to be closely approximated to the setpoint, which greatly reduces hunting compared to on/off switching alone. However, P control inevitably results in some error above or below the setpoint, called “residual deviation,” even when the measured value reaches a steady state where it remains constant. When P control is utilized in a boiler’s thermostat, the wider the proportional band, the lower the temperature at which the on and off cycle for heating begins, so the longer the current temperature is close to the setpoint, the larger the residual deviation, but the less hunting occurs. On the other hand, the narrower the proportional band, the shorter the time the current temperature is close to the setpoint and the smaller the residual deviation, but the more prone to hunting.

Application of the PI control method

When I control is utilized in conjunction with P control, the residual deviation can be eliminated, bringing the measured value closer to the setpoint. The integral action of PI control is to output a manipulation amount proportional to the integral of the deviation between the measured value and the set value, and the intensity of the action is controlled by the integration time, which represents the strength of the integral action. Shortening the integration time results in a stronger action to correct the state change of the controlled object, which can eliminate the residual deviation in a short time, but can cause hunting. Conversely, lengthening the integration time makes the corrective behavior weaker, which prevents hunting, but takes a long time to eliminate the residual deviation.

Completing the PID Control Method

However, if only P control or PI control is utilized, it takes a long time for the measured value to return to the set value when the state of the controlled object changes rapidly due to external shocks or vibrations. In this case, D control can be utilized to quickly return to the set value. When an external shock or vibration occurs, the deviation between the measured value and the set value becomes larger, and the differential action of PD control or PID control outputs a manipulation amount proportional to the speed at which the deviation between the measured value and the set value changes. The strength of the differential behavior is controlled by the differential time: shortening the differential time weakens the behavior of correcting the state change of the controlled object, so that the measured value takes longer to reach the setpoint, but no overshoot occurs. On the other hand, lengthening the derivative time results in a stronger correction behavior, which results in a shorter time for the measured value to reach the setpoint, but is more prone to overshoot.

Applications and the future of control technology

Control technology has a wide range of applications, from simple mechanical devices to complex industrial systems. For example, autopilots in airplanes, stability control systems in cars, process control in chemical plants, and more. Control technology is becoming increasingly important as industrial automation and smart factories evolve. In addition, control technology combined with artificial intelligence (AI) is opening up new possibilities for autonomous vehicles, drones, robots, and more.
Advances in control technology will not only make our lives more convenient and safer, but will also greatly improve the efficiency and productivity of industry. In the future, control technology will continue to evolve and drive revolutionary changes in various fields. These changes will bring us a richer and more developed future.