A process variable (PV) is any measurable physical condition or property within an industrial or manufacturing operation. These variables represent the current state of a system and must be monitored or regulated to ensure product quality, efficiency, and safety. Common examples include the temperature inside a chemical reactor, the pressure within a boiler, the flow rate of a liquid through a pipe, or the level of fluid in a storage tank. Tracking these conditions allows industrial control systems to determine if the process is operating according to its established guidelines.
Defining the Key Types of Process Variables
Within a complex industrial setting, process variables are categorized based on their role in the control strategy, leading to four primary types. The Controlled Variable (CV) is the specific property the system is designed to maintain at a desired level, known as the setpoint. For instance, if the goal is to keep water at 85 degrees Celsius in a holding tank, the water’s actual temperature is the Controlled Variable.
To influence the Controlled Variable, the system adjusts the Manipulated Variable (MV), which is the input the control mechanism can actively change. In the water heating example, the Manipulated Variable might be the position of a control valve regulating the flow of steam into a heat exchanger. The control system opens this valve wider to raise the temperature or closes it to let the temperature fall, directly affecting the CV.
The Measured Variable is the variable that a sensor is physically reading and transmitting to the controller. In most control loops, the Measured Variable is the same as the Controlled Variable, such as the actual temperature of the water. Sometimes, however, a related property is measured instead, such as measuring the pressure inside a vessel to infer the liquid level within it.
Finally, a Disturbance Variable (DV) is any factor that enters the process unintentionally and causes the Controlled Variable to deviate from its setpoint. These are uncontrolled inputs that the system must compensate for to maintain stability. If a sudden influx of cold feed water enters the holding tank, this change acts as a Disturbance Variable, forcing the control system to adjust the steam valve (MV) to keep the water temperature (CV) steady.
How Process Variables Are Detected and Measured
The Sensor
The ability to control a process relies on the successful conversion of a physical condition into a usable electronic signal. This conversion involves three primary components: the sensor, the transducer, and the transmitter. The sensor is the initial element that contacts the process fluid and detects the physical change. A common example is a thermocouple, which generates a small voltage in response to a change in temperature.
The Transducer
The signal from the sensor is often weak, requiring the use of a transducer, which converts the measured physical quantity into a proportional electrical signal. For example, a pressure transducer uses a diaphragm to convert force per unit area into a corresponding change in electrical capacitance or resistance. This raw electrical signal is then passed to the transmitter.
The Transmitter
The transmitter conditions and amplifies this signal into a standardized format that can be sent reliably over long distances to a central control room. The most common standard for this analog signal is a 4 to 20 milliampere (mA) current loop. In this scaling, the lowest possible value of the process variable (e.g., 0 psi) corresponds to 4 mA, and the highest possible value (e.g., 100 psi) corresponds to 20 mA. This standardization ensures the control system accurately interprets the physical measurement.
The Role of Variables in Automated Control Systems
Process variables gain their purpose within the closed-loop architecture of an automated control system, which works to maintain the process at its desired state. This structure, known as a feedback loop, continuously cycles through measurement, comparison, and adjustment. The process begins when the Measured Variable is sent to the controller, which compares it against the established Setpoint.
The difference between the Measured Variable and the Setpoint generates an error signal, which dictates the necessary corrective action. The controller uses this error to calculate a new output signal, which is sent to a final control element, such as a motor or a control valve. This final control element physically adjusts the Manipulated Variable to drive the Controlled Variable back toward the Setpoint.
Consider a system designed to maintain the pressure in a steam boiler at a constant level. The boiler pressure is the Controlled Variable, and the flow of fuel to the burner is the Manipulated Variable. If a disturbance causes the pressure to drop, the sensor detects this change and the controller registers a negative error. The controller then sends a signal to open the fuel valve (MV) further, increasing the heat and bringing the pressure (CV) back to the Setpoint. This continuous cycle of sensing and correcting ensures the process maintains equilibrium, compensating for disturbances.