Process Control & Instrumentation

Process Control Instrumentation monitors the state of a process parameter or variable detecting deviations (errors) from the desired state and taking corrective action. Control can be manual or automatic, discrete or analog, and periodic or continuous. The following definitions apply to some terms commonly used in describing control systems.

Variable in the Process

The parameter to be controlled is the process variable.

Temperature measurement in a location, pressure produced by a pumping system, or voltage maintained by a standby generator are all examples of process variables in control systems.

To be controlled, the process variable must be able to be measured and converted into a signal that the controller can act on.

Transducers and sensors

Transducers and sensors are the instruments that measure a process variable.

A watt transducer, for instance, transforms a measurement of the electrical power generated by a generator into a low current signal proportionate to power. Other examples include a pressure switch that closes a series of contacts when air pressure in a supply line drops below a predetermined threshold. The process variable sensor frequently comprises of a separate signal processor called a transmitter and a direct measurement device called an sensible element.

Using a resistive temperature detector, or RTD, as the element and a temperature transmitter, which converts the RTD's fluctuating resistance value into a current or voltage proportionate to temperature, represents one method to measure temperature.

Setpoint

The setpoint, which is often entered into the control system by an operator or obtained as an output of another control calculation, is the desired value of the process variable. The basis for control action is the error signal, which is the difference between the process variable and the setpoint. The controller is the component that analyzes the error signal, chooses the necessary control action, and gives the process a control output.

Proportional, Integral, and Derived (PID) Controller

PID controllers, or proportional plus integral plus derivative, add a component to the control action that is proportionate to the error signal's derivative, or rate of change. This form of control is advantageous for systems that require extremely quick response times or are intrinsically unstable without the controller because it enables the controller to anticipate changes in the process variable by enhancing control action for rapid changes. In a PID controller, the rate is the programmable value of the derivative constant.

Various Controllers

Single-loop controllers are standalone devices that can be used to implement control. Multiple control loops can also be combined into a single bigger controller.

A process variable input signal, a control output signal, setpoint adjustment, fine-tuning of the PID control parameters, and often some sort of display of the value of the process variable and the setpoint are all features of single-loop controllers. They are small, panel-mounted devices that can be employed efficiently in situations where there aren't many control loops.

Programmable logic controllers (PLCs), which are microprocessor-based systems with provisions for multiple inputs and outputs, both discrete and analog control capability, advanced human-machine interfaces (HMIs), and network communications capability, should be the fundamental controller used in control systems.