PID Controller Part-2
In this post, we will discuss the PID Controller more briefly.
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Modes of PID Controller
PID Controller has 2 modes.
In Auto mode, a particular set point is set and according to this set point, PID Controller adjusts its process variable automatically.
In manual mode, no set point is set and we manually open the control valve by adjusting control variable. This mode is selected mostly for checking control valve opening.
PID Control Action
PID Controller has 2 Actions.
1. Direct Action:
A PID is said to be direct action PID if After Set Point (SP) on Increasing PV if CV is Increases. For Direct Action PID Kp is +ive.
A Temperature Reducing Valve. After SP when temperature increases, CV is increased
- Reverse Action:
A PID Is said to be reverse Action PID if, After Set Point (SP) On increasing PV, CV Decreases. For Reverse Action, PID Kp Is negative (–ive.)
A Pressure Reducing Valve. After SP on increasing PV, CV decreases.
PID Controller Control Diagram
As shown in the figure process variable (pv) comes from field side i.e. from any transmitter or sensor. If the signal coming from the transmitter the signal is 4 – 20 m Amp. The controller has an indicator in it also. In this indicator zero and span are set. According to these zero and span, this indicator shows the process variable. Now, this process variable is compared with set point. An error signal is then generated, PID Controller tries to minimize this error. Now by using these P, I and D values PID Controller use this Control Variable (CV) to open the control valve to minimize error or to zero this error.
This 4 – 20 m amp output is used to open the control valve.
m Amp Control valve opening
4 m Amp 0%
8 m Amp 25%
12 m Amp 50%
16 m Amp 100%
Cascaded PID Control
In single-loop control, the controller’s setpoint is set by an operator, and its output drives a final control element. For example, a level controller driving a control valve to keep the level at its set point.
When one process have affected the other process then cascaded PID is used.
In a cascade control arrangement, there are two controllers of which one controller’s output drives the set point of another controller. For example, a level controller driving the set point of a flow controller to keep the level at its set point. The flow controller, in turn, drives a control valve to match the flow with the set point the level controller is requesting.
The controller driving the set point (the level controller in the example above) is called the primary, outer, or master controller. The controller receiving the set point (flow controller in the example) is called the secondary, inner or slave controller.
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