
Basic Instruments
In this section we will read about

Control System

Control Valve
 I/P Converter (Current to pressure Converter)
 Valve positioner
 Parts of a pneumatic control valve positioner
 Control Valve and its types
 Solenoid Valve
 Pneumatic Cylinder
 Butterfly Valve
 Pneumatic Globe Valve
 Work of different Parts of a Control Valve
 Pneumatic globe valve types
 Characteristics of Control Valve
 Class of a control valve
 Pressure Transmitter
 Working principle of pressure transmitter
 Pin type pressure Transmitter
 Connections of a pressure transmitter
 How signal is checked in a pressure transmitter
PID Theory
In this lesson, we will study the PID Theory.
 Proportional Response
The proportional component depends only on the difference between the setpoint and the process variable.  This difference is referred to as the Error term. The proportional gain (Kp) determines the ratio of output response to the error signal.
 In general, increasing the proportional gain will increase the speed of the control system response.
 However, if the proportional gain is too large, the process variable will begin to oscillate.
 If Kp is increased further, the oscillations will become larger and the system will become unstable and may even oscillate out of control.
Integral Response
 The integral component sums the error term over time. The result is that even a small error term will cause the integral component to increase slowly.
 The integral response will continually increase over time unless the error is zero. so the effect is to drive the SteadyState error to zero. SteadyState error is the final difference between the process variable and setpoint.
With Integral Action, the Controller output is proportional to the amount of time the error is present.
Integral action eliminates offset.
Derivative Response
 The derivative component causes the output to decrease if the process variable is increasing rapidly.
 The derivative response is proportional to the rate of change of the process variable. Increasing the derivative time (Td) parameter will cause the control system to react more strongly to changes in the error term and will increase the speed of the overall control system response.
 Most practical control systems use very small derivative time (Td) because the Derivative Response is highly sensitive to noise in the process variable signal. If the sensor feedback signal is noisy or if the control loop rate is too slow, the derivative response can make the control system unstable.
Controller Output:
Y= E(t)(Kp+1/Ti( ∫ E(t)dt)+ Td( d/dt E(t)/dt)).
It is clear from the above equation that Controller Output increases with Kp and Td and decreases with Ti.
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PID Controller