Process Control Methods

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Open-Loop Control

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• Process control operations are performed automatically byeither open-loop or closed-loop systems

• Processes controlled only by set-point commands without feedback are open-loop

• Open-loop systems are used in applications where simple processes are performed

• Open-loop systems are relatively inexpensive

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Closed-Loop Control• Closed-loop control systems

are more effective than open- loop systems

• With the addition of a feedback loop they become self-regulating

• Components of a closed-loop system include:– The primary element -

sensor– The controlled

variable– The measured

variable– The control signal– The final correcting

element

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Process Behavior• The objective of process control is to cause a

controlled variable to remain at a constant value at or near some desired set-point

• The controlled variable changes because of:– A disturbance appears– Load demands vary– Set points are adjusted

Process Behavior Example

• Flow through the pipe is theprocess

• Fluid flow rate is the controlled variable

• Valve position is the set point• Demand for the fluid

downstream is the load

• Variance in upstream pressureis the disturbance 5

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Single-Variable Control Loop• Several process variables are controlled at once in a typical

production machine• Usually, only one individual feedback loop is required to control

each variable• Single-variable control loops consist of the following elements:

– Measuring device– Transducer/transmitter– Controller– Final Control Element

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Response Time of the Instrument

• All instruments have a time lag - the time from when a variable is measured till the corrective action is taken

• Factors affecting lag:– Response time of sensor– Time lag of transducer– Distance the signal must

travel– Time required for thecontroller to process– Distance control signal must

travel– Time lag of the final

correcting element

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Time Lag• The controlled variable itself may contribute to

lagproblems because of inertia in the variable

• Lag due to inertia of the variable is referred to aspure lag

• Another factor that affects time lag is dead time - this is the elapsed time from when the deviation occurs and corrective action takes place

Illustration of Pure Lag

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Selecting a Controller• The controller mode selection is based upon the

requirements of the process and one of the following control modes will be used:– On/Off– Proportional– Integral– Derivative

On-Off Control

• Used for slow acting operations where lag is unavoidable

• Final correcting element is either fully-on or fully-off

• The primary drawback of on-off control is the rapid switching of the final control element

• On-off differential or hysteresis isprogrammed into the controller

• Deadband refers to the differing levels at which a controller switches on and off

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Continuous Control• On/Off control is acceptable for process where the variable

isset between two limits

• For processes where the variable needs to be kept at particular setpoint level, proportional control is used

• Proportional action can be accomplished in two ways:– Time Proportioning Method– Amplitude Proportional Method

Time Proportioning• Is a method whereby the

output of the controller is continually switched on and off

• On versus off times are varied dependent upon process requirements

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Amplitude Proportional• Most common technique to produce a

proportional signal• The control signal is proportional in amplitude to

the error signal• The signal may be amplified and the

amplification may be referred to as proportional gain and proportional band

Proportional Gain Comparison

Level control at a gain of 1

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Proportional Gain Comparison

(Gain set at 2)

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Proportional Band

• Proportional band is defined as the percentage change in the controlled variable that causes the final correcting element to go through 100 percent of its range

PB =Controlled Variable % ChangeFinal Correcting Element %

Change

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Integral Control

• Because of the introduction of offset in a control process, proportional control alone is often used in conjunction with Integral control

• Offset is the difference between set point and the measured value after corrective action has taken place

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Reset Action• Integral control is also

referred to as reset control as the set point is continuously reset as long as an error is present

• Integral adjustments that affect the output are labeled 3 ways:– Gain - expressed as a whole

number– Reset - Expressed in

repeats per minute– Integral Time - Expressed in

minutes per reset

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Derivative Mode

• For rapid load changes, the derivative mode is typically used to prevent oscillation in a process system

• The derivative mode responds to the rate of change of the error signal rather than its amplitude

• Derivative mode is never used by itself, but in combination with other modes

• Derivative action cannot remove offset

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Control Mode Summary

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Tuning the Controller• Fine-tuning is the process to optimize the controller

operationby adjusting the following settings:– Gain setting (proportional mode)– Reset rate (integral mode)– Rate (derivative mode)

• Three steps are taken when tuning a systems– Study the control loop– Obtain clearance for tuning procedures– Confirm the correction operation of the system components

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Trial-and-Error Tuning• Does not use mathematical methods, instead a chart

recorder is used and several bump tests are made in the proportional and integral modes

• Trial-and-error tuning is very time consuming and requires considerable experience on the part of the technician or operator

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Ziegler-Nichols Tuning Methods

• Two formal procedures for tuningcontrol loops:–Continuous cycling method–Reaction curve method

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Continuous Cycling Method• The continuous cycling method analyzes the process by

forcing the controlled variable to oscillate in even, continuous cycles

• The time duration of one cycle is called an ultimate period. The proportional setting that causes the cycling is called the ultimate proportional value

• These two values are then used in mathematical formulas tocalculate the controller settings

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Continuous Cycle Calculations• Proportional only controller• Proportional Gain

– Kc = Gu x 0.5• KC = proportional gain,• Gu= ultimate gain

• Proportional Band– PB = Pbu x 2

• PB = proportional band• Pbu = ultimate proportional band

Continuous Cycling Formulas

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Ziegler-Nichols Reaction Curve Tuning Method• This method avoids the forced oscillations that are found in

the continuous cycle tuning method• Cycling should be avoided if the process is hazardous

or critical• This method uses step changes and the rate at which

theprocess reacts is recorded

• The graph produces three different values used in mathematical calculations to determine the proper controller settings

Reaction Curve Tuning Formulas

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Direct Synthesis Method• The direct synthesis method uses the

same procedures to analyze the process times as the Ziegler-Nichols reaction curve method

• One advantage of this method is that it directly matches (synthesizes) synthesizes the process

• The first step in this method is perform a bump test and recording the dynamic settling time

• Depending upon the complexity of the process, one of several dynamic response signals may develop as shown on the right

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Direct Synthesis Tuning Calculations

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Advanced Control Techniques• Some complex manufacturing operations require

more precise control than available with PID controllers

• Four Techniques frequently used are:– Cascade control– Feed forward control– Ratio control– Adaptive control

Cascade Control• Cascade control

systems use a second feedback loop with a separate sensor and controller

• Cascade control is effective in overcoming lag in some systems

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Feedforward Control• Feedforward control

measures a variable that enters a process and takes corrective action if it is affected by a disturbance, reducing or eliminating deviation from the set point

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Ratio Control• Used in mixing systems

where an uncontrolled flow of material (wild flow) is monitored and used to control the second material which is controlled according to the desired ratio between the two components

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Adaptive Control

• To accommodate a nonlinear process, a microelectronic controller uses software that has adaptive control capabilities to compensate for nonlinear transducers and sensors

END

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