What Is a Controller?
A controller is a device used to adjust a machine’s output automatically.
Often sold under indicating regulators or temperature regulators, many devices marketed as controllers can also control parameters other than temperature.
Physical parameters such as temperature, humidity, pressure, and flow rate are input from sensors to indicate a value. Feedback is applied to match this with the target value you set. Most products have a display for monitoring both the current value of the parameter being adjusted and the set value.
If the display uses an analog pointer, it is termed an analog controller. If it features a 7-segment display, it is called a digital controller. Currently, digital controllers are the most common type. In analog controllers, the display may omit the current value.
Uses of Controllers
Controllers are widely used in applications ranging from industrial equipment to household appliances, as exemplified below:
- Temperature control in commercial water tanks and liquid storage tanks
- Temperature regulation in air conditioners
- Humidity management in dehumidifiers and humidifiers
- Flow regulation of carbon dioxide gas in CO2 incubators for cell culture
- Pressure control in local exhaust systems
Controllers are utilized to maintain constant parameters and may be used alongside computers to log monitored data.
Principle of Controllers
A controller consists of a sensor, a processing/monitoring part, and an output section. The sensor inputs physical quantities such as temperature (using a thermometer) or humidity (using a hygrometer), among others.
The processing and monitoring section adjusts the output while monitoring the value. A built-in comparator or similar device compares the measured value to the target value to determine the output, which is then sent to the output section. Various feedback control methods are employed.
1. ON/OFF Operation
Output is activated when the measured value is below the target value and turned off when above. This repeated operation maintains the measured value near the target.
2. Proportional Operation (P Operation)
This operation adjusts the control output proportionally to the deviation from the target value, enabling smoother regulation than ON/OFF operation, though with relatively large fluctuations near the target. Integral operations, etc., adjust the stabilization offset slightly off the target value.
3. Integral Operation (I Operation)
Based on the time integral of the difference between the target and measured values, this operation is often used to eliminate offset appearing in proportional operation, typically in conjunction with P operation.
4. Differential Operation (D Operation)
This operation outputs based on the rate of change (derivative) of the measured value. PID operation, combining P, I, and D operations, integrates the benefits of each method.
How to Select a Controller
Controllers are selected based on mounting size, input specifications, output specifications, communication methods, and other factors.
1. Mounting Size
Consider the panel size where the controller will be mounted. Select a product that fits the available installation space, opting for smaller sizes if mounting multiple controllers in a row.
2. Input Specifications
Choose based on the controlled physical quantity’s input signals and number of input points. For temperature inputs, resistance thermometers or thermocouples (PT100Ω, K, R, B types) are common, with 4-20 mA or 1-5 V inputs for other parameters.
3. Output Specifications
Output specifications match the signal transmitted to the output device. Analog control typically uses 4-20 mA or 1-5 V outputs, while ON-OFF control employs contact or transistor outputs, selected based on the output device’s specifications.
4. Communication Method
For interfacing with computers, communication methods facilitate transmitting measured or output values to a computer or altering target values from a computer. Ethernet communication has become mainstream, succeeding older serial communication standards like RS422 or RS485.