In this blog post, we’ll explore how control technology is being applied in industrial settings and daily life to drive change.
The Importance and Applications of Control Technology
Control technology refers to the process of regulating physical quantities such as temperature, pressure, flow rate, and rotational speed to ensure that machines and equipment operate as intended. There are various methods of control technology that adjust the output to match the measured value of the current physical quantity of the controlled object with the desired target value. Control technology plays an essential role in various fields of modern industry, and its importance is growing 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, if the current temperature is lower than the desired temperature, the switch turns on to supply power to the heater; if it is higher than the desired temperature, the switch turns off to cut off power to the heater. When the switch is on, 100% of the control output is applied, and when the switch is off, the control output is 0%. When the heater first starts operating, it remains in the on state to raise the water temperature, but at some point, an “overshoot” occurs where the water temperature exceeds the setpoint. Since overshoot can strain the system, the switch is repeatedly turned on and off to bring the current temperature back to the setpoint. Since water temperature, like pressure or flow rate, is a physical quantity that changes continuously (analog), it does not drop immediately just because the switch was turned off after the temperature rose. Therefore, repeatedly turning the switch on and off causes “hunting,” where the water temperature fluctuates up and down around the setpoint.
The Hunting Problem and PID Control
The on/off switch method causes overshoot and hunting, making it difficult to precisely control the physical quantity of the controlled object. To compensate for these shortcomings of the on/off switch method, “PID control” is utilized. PID control employs P (proportional), I (integral), and D (derivative) control to precisely regulate the physical quantity of the controlled object. However, depending on the objective, P control, PI control, or PD control may also be used.
Characteristics of P Control
P control sets a fixed proportional band above and below the setpoint, and within this band, outputs a control signal proportional to the deviation between the setpoint and the measured value. For example, in a boiler temperature control system using P control, if the current temperature is below the lower limit of the proportional band, a 100% control signal is output until the current temperature reaches the lower limit, keeping the switch in the on state. However, once the current temperature rises above the lower limit of the proportional band, a proportional cycle begins, during which the switch alternates between on and off states. Specifically, until the current temperature—which has exceeded the lower limit of the proportional band—reaches the setpoint, a cycle where the on time is longer than the off time repeats periodically. When the current temperature reaches the setpoint, a 50% control signal is output, and a cycle where the on and off times are equal (1:1) repeats. If the current temperature rises above the setpoint, the operation where the off time is longer than the on time repeats periodically, and if the current temperature exceeds the upper limit of the proportional band, the system remains in the off state. In this way, using P control allows the measured value to be brought very close to the setpoint, significantly reducing hunting compared to using only an on/off switch method.
However, even when the measured value reaches a steady state, a certain error relative to the setpoint inevitably occurs either above or below the setpoint; this is called the “residual error.” When P control is used in a boiler temperature control system, setting the proportional band wider lowers the temperature at which the on-off cycling for heating begins. Consequently, the time required for the current temperature to approach the setpoint increases, and the residual error grows; however, hunting occurs almost never. Conversely, the narrower the proportional band is set, the shorter the time it takes for the current temperature to approach the setpoint, and the smaller the residual deviation becomes; however, hunting is more likely to occur.
Application of PI Control
When I-control is used in conjunction with P-control, residual deviation can be eliminated, allowing the measured value to approach the setpoint very closely. The integral action of PI control outputs a control signal proportional to the integral of the deviation between the measured value and the setpoint; the intensity of this action is adjusted via the integral time, which represents the strength of the integral action. Shortening the integral time strengthens the action that corrects changes in the state of the controlled object, allowing residual deviation to be eliminated quickly, but this can cause hunting. Conversely, increasing the integral time weakens the corrective action, preventing hunting but requiring a long time to eliminate the residual error.
The Completion of the PID Control Method
However, when using only P or PI control, it takes a long time for the measured value to return to the setpoint if external shocks or vibrations cause the state of the controlled object to change rapidly. In such cases, using D control allows the system to return to the setpoint quickly. When external shocks or vibrations occur, the deviation between the measured value and the setpoint increases; the derivative action in PD or PID control outputs a control signal proportional to the rate of change of this deviation. The magnitude of the derivative action is adjusted through the derivative time. If the derivative time is shortened, the corrective action to adjust the state of the controlled object weakens, resulting in a longer time for the measured value to reach the setpoint, but overshoot does not occur. Conversely, if the derivative time is lengthened, the corrective action becomes stronger, shortening the time for the measured value to reach the setpoint, but overshoot is more likely to occur.
Applications and the Future of Control Technology
Control technology is widely applied across a broad spectrum, ranging from simple mechanical devices to complex industrial systems. For example, it is utilized in various fields such as aircraft autopilot systems, automotive stability control systems, and process control in chemical plants. In particular, the importance of control technology is becoming increasingly prominent due to the advancement of industrial automation and smart factories. Furthermore, control technology combined with artificial intelligence (AI) is opening up new possibilities in areas such as autonomous vehicles, drones, and robots.
Advances in control technology will not only make our lives more convenient and safer but also significantly improve industrial efficiency and productivity. Control technology will continue to evolve, driving innovative changes across various fields. Through these changes, we will embrace a more prosperous and advanced future.
