In this blog post, we will take a look at how control technology is being applied to industrial sites and everyday life to drive change.
The importance and application of control technology
Control technology is the technology that adjusts physical quantities such as temperature, pressure, flow rate, and rotation speed to make machines and equipment operate according to their intended purpose. There are various types of control technology that adjust the manipulated variable output to match the desired setpoint, which is the measured value of the current physical quantity of the control target. Control technology plays an essential role in various fields of modern industry, and its importance is increasing day by day.
Basic control technology: On/Off switch method
The simplest method is the on/off switch method, which is commonly used in the temperature control device of a boiler used to adjust the temperature of water. In this device, if the current temperature is lower than the desired temperature, the switch is turned on to supply power to the heater, and if the current temperature is higher than the desired temperature, the switch is turned off to cut off the power supplied to the heater. When the switch is on, the amount of operation corresponding to 100% is output, and when the switch is off, the amount of operation becomes 0%. When the heater is first turned on, it is kept on to raise the water temperature, but at some point, an “overshoot” occurs in which the water temperature exceeds the set value. If an overshoot occurs, it can strain the system, so the switch is repeatedly turned on and off to bring the current temperature up to the set value. Since the change in physical quantities such as pressure and flow rate is continuous and analog, the water temperature does not drop immediately after the switch is turned off due to the rise in water temperature. Therefore, repeatedly turning the switch on and off causes “hunting,” in which the water temperature rises and falls up and down based on the set value.
Hunting Problem and PID Control Method
The on/off switch method is difficult to precisely control the physical quantity of the control target due to overshoot and hunting. The PID control method is used to overcome the shortcomings of the on/off switch method. The PID control method uses all three types of control (P, I, and D) to precisely control the physical quantity of the control target. However, depending on the purpose, the P control method, PI control method, and PD control method may be used.
Characteristics of the P control method
P control sets a constant proportional band above and below the setpoint, and outputs a manipulated variable proportional to the deviation between the setpoint and the measured value within the proportional band. 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 manipulated variable of 100% is output until the current temperature reaches the lower limit of the proportional band, and the switch is kept in the on state. Then, when the current temperature exceeds the lower limit of the proportional band, it has a proportional cycle, in which the on and off actions of the switch are repeated in each cycle. In other words, the on time is longer than the off time until the current temperature exceeds the lower limit of the proportional band and reaches the set value, and the operation is repeated periodically. When the current temperature reaches the set value, 50% of the manipulated variable is output, and the on and off times are 1:1. If the current temperature rises above the set value, the off time is longer than the on time, and the operation is repeated periodically. If the current temperature exceeds the upper limit of the proportional band, the off state is maintained. As such, P control can be used to bring the measured value close to the set value with precision, which significantly reduces hunting compared to using only the on/off switch method. However, in P control, even in the steady state where the measured value is constant, a certain error is inevitably generated above or below the set value with respect to the set value, which is called “residual deviation.” When P control is used for the temperature control device of the boiler, the wider the proportional band is set, the lower the temperature at which the repeated on and off actions for heating begin, so the time it takes for the current temperature to approach the set value increases and the residual deviation becomes larger, but hunting rarely occurs. On the other hand, 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, but hunting is more likely to occur.
Application of PI control method
If I control is used in conjunction with P control, the residual deviation can be eliminated, and the measured value approaches the setpoint. The integral action of PI control outputs a manipulated variable proportional to the integral value of the deviation between the measured value and the set value, and the strength of the action is controlled by the integral time, which indicates the strength of the integral action. If the integral time is shortened, the action to correct the state change of the controlled object is strengthened, and the residual deviation can be eliminated in a short time, but it may cause hunting. On the other hand, if the integration time is increased, the corrective action becomes weaker and hunting does not occur, but it takes a long time to eliminate residual deviation.
Completion of the PID control method
However, if only P control or PI control is used, it takes a long time for the measured value to return to the set value when the state of the control target changes rapidly due to external shocks or vibrations. In this case, D control can be used to quickly return to the set value. When an external shock or vibration occurs, the deviation between the measured value and the set value increases. The differential operation of PD control or PID control outputs the manipulated variable in proportion to the rate of change in the deviation between the measured value and the set value. The strength of the derivative action is controlled by the derivative time. If the derivative time is shortened, the action to correct the state change of the control target is weakened, and the time it takes for the measured value to reach the set value is longer, but there is no overshoot. On the other hand, if the derivative time is increased, the corrective action is strengthened, and the time it takes for the measured value to reach the set value is shorter, but overshoot is more likely to occur.
Application and Future of Control Technology
Control technology is widely applied to a wide range of applications, from simple mechanical devices to complex industrial systems. For example, control technology is used in various fields, such as the autopilot of an aircraft, the stability control system of a car, and the process control of a chemical plant. In particular, the importance of control technology is becoming more prominent due to the development of industrial automation and smart factories. In addition, control technology combined with artificial intelligence (AI) is opening up new possibilities in autonomous vehicles, drones, and robots.
The development of control technology will not only make our lives more convenient and safer, but also greatly improve the efficiency and productivity of industries. Control technology will continue to evolve and drive innovative changes in various fields. Through these changes, we will be able to welcome a more prosperous and advanced future.
