practical flow essentials

Practical Flow Essentials

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Basic Principles of Control Systems

Basic functions & definitions On/Off control Modulation control Principle of closed loop control PID control modes Exercises - #2 & #3

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Definitions PV (Process Variable) - Variable we want to control.

MV (Manipulated Variable) – Variable to manipulate(to control the PV) Simple controllers – OP – controller output signal

SP(Set Point Value) – Target / Ideal Process Variable

Disturbances Other random inputs to the process may be measured and predicted

Hysteresis Output reaction delays to a stimulus response based on direction of approach

Multivariate Systems based on more than one PV may also have more than one

MV (OP)

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On/Off Control Good where fluctuations of PV are acceptable Wear/tear of controlling element is a problem Increase band of PV to reduce switching action Output of controller is a digital signal

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Response of a Two Positional Controller

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Simple on/off Control

Enables a very simple form of temperature control Common form is a domestic gas-fired heater

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Modulating Control

Output of the controller is an analog value Not digital value as in on/off control

Can be used in open or closed loop control Use when more precise control of PV required Less hysteresis (band) of PV variation

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Open Loop Control Control action is not a direct function of PV No self-correction when the PV drifts Allows pre-emptive reaction to disturbance Control is based on measured disturbances

(E.g. Feedforward control)

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Simple Process Block

PV changes are a function of the control action

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Open Loop Control Control Action = Position of Accelerator Control result = Speed of Car

BUT: Speed could vary with changing load

Identify – Disturbances, PV, MV

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Feedforward Control Form of Open loop control Value to be controlled (PV) is not used to calculate the

control action Basic principle is:

Manipulate a variable of the process block so that it compensates for impact of any input disturbances.

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Feedforward Control

Feedforward block diagram

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Feedforward Control

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Feedforward Control

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Feedforward Control

Objective is to keep level constant Manipulated variable = inlet valve position Control strategy: Inlet flow = Outlet flow If Feedforward control is the only control

PV will drift over time

Because of: Natural Evaporation Delay in valve positioning etc.

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Control object -constant outlet

temperature

Feed Heater Example - Analyze Components

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Closed Loop PID Closed loop Control strategy

Measure PV Compare PV with SP (ERR) Calculate OP

Closed Loop PID has 3 Modes: Proportional Control

React to the amount of ERR Integral Control

Eliminate residual ERR Derivative Control

Correct dynamic stability

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Direct or Reverse Controllers

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Manual Feedback Control

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Closed Loop Block Diagram

PV is now part of the control action

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Feedforward and Feedback Combination

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Process Response Curve

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Sample and Hold Algorithms

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Process Response

Response process gain ratio of the change in the process variable to the change in the

manipulated variable.

Response dead time Response process lag

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Process Response

 Example: Process response related to a step change of the input value

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