• ##### Basic Instruments

In this section we will read about

## Instrumentation

### PID Controller

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In this lesson, we will study the PID Controller.

• A proportional–integral–derivative controller (PID controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems.
• PID is the most commonly used feedback controller.
• A PID controller calculates an “error” value as the difference between a measured process variable and a desired setpoint.
• The controller attempts to minimize the error by adjusting the process control inputs.
• the PID controller calculation (algorithm) involves three separate constant parameters and is accordingly sometimes called three-term control: the proportional, the integral and derivative values, denoted P, I, and D.
• these values can be interpreted in terms of time: P depends on the present error, I on the accumulation of past errors, and D is a prediction of future errors, based on the current rate of change. The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve.
•     Process variable (PV)    A process variable, process value or process parameter is the current status of a process under control. In a typical control system, the process variable is the system parameter that needs to be controlled, such as temperature (ºC), pressure (psi), or flow rate (liters/minute). A sensor is used to measure the process variable and provide feedback to the control system.
• Setpoint (sp)

the desired parameter is known as the set-point. The setpoint is the desired or command value for the process variable.

Error

Error is a signal generated from the difference between sp and PV.

Error is the difference between setpoint and process variable

Error=set point (sp)- process variable (pv)

Control Variable (cv)

The control variable is the parameter used to minimize the error signal generated. This control variable signal is given to the control valve. The control variable is also known as a measured variable(mv).

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