In this blog post, we will examine how control technology is being applied in industrial settings and daily life to bring about change.
The Importance and Applications of Control Technology
Control technology refers to the technology used to regulate physical quantities such as temperature, pressure, flow rate, and rotational speed so that machines and equipment operate as intended. There are various methods of control technology that adjust the output control quantity to match the measured value of the current physical quantity of the control target with the desired target value. Control technology plays an essential role in various fields of modern industry, and its importance is increasing day by day.
Basic control methods: On/Off switch method
The simplest method is the “on/off switch method,” commonly used in temperature control devices for boilers to regulate water temperature. In this device, when the current temperature is lower than the desired temperature, the switch turns on, supplying power to the heater. When the temperature exceeds the desired value, the switch turns off, cutting off power to the heater.
When the switch is on, the control output is 100%, and when the switch is off, the control output is 0%. When the heater first starts, it maintains the on state to raise the water temperature. However, at some point, an ‘overshoot’ occurs where the water temperature exceeds the set value. To prevent this from causing strain on the system, the switch is repeatedly turned on and off until the current temperature reaches the set value. Water temperature, like pressure or flow rate, has a continuous analog property where changes in physical quantities are not instantaneous. Therefore, even if the switch is turned off when the water temperature rises, it does not immediately decrease. As a result, repeatedly switching the switch on and off causes the water temperature to oscillate above and below the setpoint in a steady manner, known as “hunting.”
Hunting problem and PID control method
The on/off switch method causes overshoot and hunting, making it difficult to precisely control the physical quantity of the control target. To compensate for the shortcomings of the on/off switch method, the “PID control method” is used. The PID control method utilizes P (proportional) control, I (integral) control, and D (derivative) control to precisely control the physical quantity of the control target.
However, depending on the purpose, P control, PI control, and PD control may also be used.
Characteristics of P control
P control sets a constant proportional band above and below the setpoint value. Within this proportional band, the control output is proportional to the deviation between the setpoint value and the measured value. For example, in a boiler temperature control device using P control, if the current temperature is below the lower limit of the proportional band, the control output is 100% until the current temperature reaches the lower limit of the proportional band, keeping the switch in the on state. Once the current temperature exceeds the lower limit of the proportional band, the system enters a proportional cycle, where the switch repeatedly switches between on and off. Specifically, until the current temperature, which has exceeded the lower limit of the proportional band, reaches the setpoint, the on time is longer than the off time, and this cycle repeats periodically. When the current temperature reaches the setpoint, a 50% control output is generated, and the on and off times are equal (1:1), repeating this cycle. If the current temperature rises above the setpoint, the off time is longer than the on time, and this cycle repeats periodically. When the current temperature exceeds the upper limit of the proportional band, the system remains in the off state. By utilizing P control in this way, the measured value can be precisely approached to the setpoint, significantly reducing hunting compared to using only an on/off switch. However, in P control, even when the measured value reaches a stable state where it remains constant, a constant error relative to the setpoint inevitably occurs above or below the setpoint, which is referred to as “residual deviation.” When P control is applied to a boiler temperature control device, setting the proportional band wider lowers the temperature at which the on/off cycling for heating begins, thereby increasing the time it takes for the current temperature to approach the setpoint and enlarging the residual deviation, but hunting occurs rarely. On the other hand, the narrower the proportional band, the shorter the time it takes for the current temperature to approach the setpoint, and the smaller the residual deviation, but hunting is more likely to occur.
Application of PI control
I control can be used in conjunction with P control to eliminate residual deviation, resulting in measured values that are almost identical to the setpoint. The integral action of PI control outputs a control quantity proportional to the integral value of the deviation between the measured value and the setpoint. The integral time, which indicates the strength of the integral action, is used to adjust the intensity of the action. Shortening the integral time strengthens the action that corrects changes in the state of the controlled object, allowing residual deviation to be eliminated in a short time, but this can also cause hunting. Conversely, increasing the integral time weakens the corrective action, preventing hunting but requiring a longer time to eliminate residual deviation.
Completion of the PID control method
However, when using only P control or PI control, external shocks or vibrations can cause the state of the controlled object to change rapidly, resulting in a long time for the measured value to return to the setpoint. In such cases, D control can be used to quickly return the measured value to the setpoint. When external shocks or vibrations occur, the deviation between the measured value and the setpoint increases. The derivative action of PD control or PID control outputs the control signal in proportion to the rate of change in the deviation between the measured value and the setpoint. The strength of the derivative action is adjusted through the derivative time. Shortening the derivative time weakens the corrective action on the controlled object’s state changes, resulting in a longer time for the measured value to reach the setpoint but preventing overshoot. Conversely, lengthening the derivative time strengthens the corrective action, reducing the time for the measured value to reach the setpoint but increasing the likelihood of overshoot.
Applications and Future of Control Technology
Control technology is widely applied in various fields, ranging from simple mechanical devices to complex industrial systems. For example, it is utilized in aircraft autopilot systems, automotive stability control systems, and chemical plant process control systems. In particular, the importance of control technology has become increasingly prominent with the development of industrial automation and smart factories. Additionally, control technology combined with artificial intelligence (AI) is opening new possibilities in areas such as autonomous vehicles, drones, and robots.
The advancement of control technology will not only make our lives more convenient and safer but also significantly enhance industrial efficiency and productivity. In the future, control technology will continue to evolve, driving innovative changes across various fields. Through these changes, we will embrace a more prosperous and advanced future.
