Basic Control Loop
The control loop shown in Figure 1 consists of three main components: a sensor, a controller and a controlled device. These three components or functions interact to control a medium. In the example shown in Figure 1, air temperature is the controlled medium. The sensor measures the data, the controller processes the data and the controlled device causes an action.
The Figure 1 could be an example of a pneumatic or electronic control system, where the controller is a separate and distinct piece of hardware. In a DDC system, the controller “function” takes place in software as shown in Figure 2.
The control loop shown in Figure 1 consists of three main components: a sensor, a controller and a controlled device. These three components or functions interact to control a medium. In the example shown in Figure 1, air temperature is the controlled medium. The sensor measures the data, the controller processes the data and the controlled device causes an action.
The Figure 1 could be an example of a pneumatic or electronic control system, where the controller is a separate and distinct piece of hardware. In a DDC system, the controller “function” takes place in software as shown in Figure 2.
Sensor
The sensor measures the controlled medium or other control input in an accurate and repeatable manner. Common HVAC sensors are used to measure temperature, pressure, relative humidity, airflow stateand carbon dioxide. Other variables may also be measured that impact the controller logic. Examples include other temperatures, time-of-day or the current demand condition. Additional input information (sensed data) that influences the control logic may include the status of other parameters (airflow, water flow, current) or safety (fire, smoke, high/low temperature limit or any number of other physical parameters). Sensors are an extremely important part of the control system and can be the first, as well as a major, weak link in the chain of control.
The sensor measures the controlled medium or other control input in an accurate and repeatable manner. Common HVAC sensors are used to measure temperature, pressure, relative humidity, airflow stateand carbon dioxide. Other variables may also be measured that impact the controller logic. Examples include other temperatures, time-of-day or the current demand condition. Additional input information (sensed data) that influences the control logic may include the status of other parameters (airflow, water flow, current) or safety (fire, smoke, high/low temperature limit or any number of other physical parameters). Sensors are an extremely important part of the control system and can be the first, as well as a major, weak link in the chain of control.
Controller
The controller processes data that is input from the sensor, applies the logic of control and causes an output action to be generated. This signal may be sent directly to the controlled device or to other logical control functions and ultimately to the controlled device. The controller’s function is to compare it’s input (from the sensor) with a set of instructions such as setpoint, throttling range and action, then produce an appropriate output signal. This is the logic of control. It usually consists of a control response along with other logical decisions that are unique to the specific control application. How the controller functions is referred to as the control response. Control responses are typically one the following:
The controller processes data that is input from the sensor, applies the logic of control and causes an output action to be generated. This signal may be sent directly to the controlled device or to other logical control functions and ultimately to the controlled device. The controller’s function is to compare it’s input (from the sensor) with a set of instructions such as setpoint, throttling range and action, then produce an appropriate output signal. This is the logic of control. It usually consists of a control response along with other logical decisions that are unique to the specific control application. How the controller functions is referred to as the control response. Control responses are typically one the following:
Floating
Proportional (P only)
Proportional plus Integral (PI)
Proportional plus Integral plus Derivative (PID)
Controlled Device or Output
A controlled device is a device that responds to the signal from the controller, or the control logic, and changes the condition of the controlled medium or the state of the end device. These devices include valve operators, damper operators, electric relays, fans, pumps, compressors and variable speed drives for fan and pump applications.
A controlled device is a device that responds to the signal from the controller, or the control logic, and changes the condition of the controlled medium or the state of the end device. These devices include valve operators, damper operators, electric relays, fans, pumps, compressors and variable speed drives for fan and pump applications.
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