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


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)


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


Response of a Two Positional Controller


Simple on/off Control

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


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


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)


Simple Process Block

PV changes are a function of the control action


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

BUT: Speed could vary with changing load

Identify – Disturbances, PV, MV


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.


Feedforward Control

Feedforward block diagram


Feedforward Control


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.


Control object -constant outlet

temperature

Feed Heater Example - Analyze Components


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


Direct or Reverse Controllers


Manual Feedback Control


Closed Loop Block Diagram

PV is now part of the control action


Feedforward and Feedback Combination


Process Response Curve


Sample and Hold Algorithms


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


Process Response

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


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Practical Flow Essentials

Engineering

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