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Closed Loop Control System

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Proportional

Proportional term considers how far PV is from SP at any instant in time and adds or subtracts accordingly (recall e(t) = SP – PV)
Proportional action is the strongest when the distance from the set point is the greatest

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Intregral

Integral term addresses how long and how far PV has been from SP by continually summing e(t) over time
Integral increases its effort with every moment that passes with the variable
Term I accounts for past values of the error and integrates them over time to produce the I term.

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Derivative

Derivative term considers how fast e(t) is changing at any instant using the rate of change or slope of the error curve
rapidly changing e(t) = large derivative = large impact on CO
Derivative doesn’t consider if e(t) is positive, negative or how much time has passed, just how fast e(t) is changing
Derivative makes no effort when the line is flat, even if it is nowhere close to the set point. D will not move the line itself,
Term D is a best estimate of the future trend of the error, based on its current rate of change

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Types of Controller

P Controller
PI Controller
PD Controller
PID Controller
On Off Controller

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Transfer function for controller

P Controller

PD Controller

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Transfer function for controller

PI Controller

PID Controller

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Effects of increasing a parameter independently
Parameter	Rise time	Overshoot	Settling time	Steady-state error	Stability
K p	Decrease	Increase	Small change	Decrease	Degrade
K i	Decrease	Increase	Increase	Eliminate	Degrade
K d	Minor change	Decrease	Decrease	No effect in theory	Improve if Kd small

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Types of Control Problem

Type	Constant	Variable	Analysis	Over All Transfer Function
Servo	Load Variable	Set Point	Control Objective	C/R
Regulator	Set Point	Load Variable	Control Objective	C/U

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Overall Transfer Function

Negative feed back servo mechanism problem
Positive feed back servo mechanism problem
Negative feed back regulator mechanism problem
Positive feed back regulator mechanism problem

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Overall Transfer function

Negative feed back system servo mechanism

Positive feed back system servo mechanism

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Overall Transfer function

Negative feed back system regulator mechanism

Positive feed back system regular mechanism

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Electronic Control System

ADVANTAGES :

No time lag or transmission delay
Linear and quick response and good accuracy
No entrainment or contamination in control medium
Integrated control system and easier interconnections
Low power requirements
Speed of response is substantially instantaneous

DISADVANTAGES :

Complex networks
Difficult maintenance and test kit requirements
Not possible for flame-proof areas
Skilled maintenance regulations
Expensive layout
Good and safe wiring layout required
Susceptible to line fluctuations of voltage and frequency

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Pneumatic Control System

ADVANTAGES :

Simplicity of the components and no complex structure
Easy maintainability
Useful in flame -proof applications
Low cost of installation
Good reliability and reproducibility
Speed of response is relatively slow but steady
Limited power capacity for large mass transfer

LIMITATIONS:

Great distance lag can be a crucial setback with pneumatic controls.
Slow response
Difficult to operate in sub-normal temperatures
Copper piping is vulnerable to damage
Pipe-couplings can give rise to leaks in certain ambient conditions

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Final Control Element

The most common final control element in the process control industries is the control valve.
The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.

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Most commonly used final control element is pneumatic valve

Air operated valve which control the flow through an orifice by positioning the plug appropriately.
The plug is attached at the end of the stem which is supported on a diaphragm at the other end.
The air pressure operates between 3 to 15 psi
Main Parts of the pneumatic valve

Stem
Diaphragm
Plug
Spring

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Different types of Plug

Linear plug
Quick opening
Square root
Equal percentage
Hyperbolic

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Inherent Control Valve Flow Characteristics

Linear - flow capacity increases linearly with valve travel.
Equal percentage - flow capacity increases exponentially with valve trim travel. Equal increments of valve travel produce equal percentage changes in the existing C_v.
A modified parabolic characteristic is approximately midway between linear and equal-percentage characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.
Quick opening provides large changes in flow for very small changes in lift. It usually has too high a valve gain for use in modulating control. So it is limited to on-off service, such as sequential operation in either batch or semi-continuous processes.

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Air to Close

The air pressure above the diaphragm increases, the stem moves down and consequently the plug restricts the flow through the orifice is known as air to close
If the air supply above the diaphragm is lost, the valve will ‘fail open”

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Air to Open

The air pressure above the diaphragm increases, the stem moves down and consequently the plug allow the flow through the orifice is known as air to open
If the air supply above the diaphrag is lost, the valve will ‘fail Closed”

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Transportation Lag

The transportation lag is the delay between. the time an input signal is applied to a system and the time the system reacts to that input signal. Transportation lags are common in industrial applications. They are often called “dead time”.

To eliminate transportation lag

Placing equipment closer together
Proper plant lay out