cbe495 lecture ii advanced control strategies€¦ · cbe495 lecture ii advanced control ... •...

Korea University 2-1CBE495 Process Control Application

CBE495 LECTURE IIADVANCED CONTROL

STRATEGIES

Professor Dae Ryook Yang

Fall 2013Dept. of Chemical and Biological Engineering

Korea University


Advanced Control Strategies

• High-level application of PID controller– Cascade control– Time-delay compensation– Selective control– Override control– Valve position control– Variable structure control– Split range control– Etc.

• Not anymore advanced• Contemporary advance control strategies

– Model predictive control (MPC)– Nonlinear control techniques and so on


Cascade Control

• Motivations– The FB controller can respond only after some changes appear

in measurement of CV.– The FF controller can act faster, but requires accurate model.– If a secondary measurement can recognize the disturbance,

then the disturbance can be handled more efficiently.

The jacket temp. can be controlled by adjusting cooling water flow rate.

If the CW inlet temp changes, the jacket temp. will change with same flow rate.

The changes in CW temp. in the jacket can be a indication of the disturbance.


• Example: Furnace temperature control

– In conventional FB control, the hot oil temperature is controlled by the fuel gas flow rate.

– The valve characteristics is not linear and results the detrimental effects on performance of PID controller

– If the pressure of fuel gas (disturbance) changes, the flow rate of fuel gas becomes different even at the same valve opening.

– Cascade control• Primary (master) loop: furnace outlet temp. control

– Adjust the fuel pressure set point (directly related to fuel flow)• Secondary (slave) loop: fuel pressure control

– Maintain fuel pressure regardless of fuel feed pressure and valve characteristics


• Advantages of cascade control– Reject the disturbance in the slave loop before it affects the main

process variables.– Linearize the slave process– Improve the dynamics of the slave process

• Implementation of the cascade control– The slave process is at least 3 times as faster as mater loop in terms of

response time.– The slave loop may not need to be controlled exactly at set point. (I-

mode is not necessary in many cases.) It needs to be controlled to provide fast action on disturbance and set point change. (P-mode may be suffice in most cases.)

– Due to some reasons, the slave may not be able to follow the set point for considerable period. Reset feedback is required for output tracking as well as anti-reset windup.

– The slave loop should be tuned first while the master in manual. The tune the master while the slave is in automatic mode.

– If the slave is retuned for some reasons, the master should be retuned.– The master can be transferred to ‘auto’ after the slave becomes ‘auto’.


KcE(s)

1P(s)

++

External Feedback signal,Actual P(s)

Reset Feedback (Review)

• Compensate the integration when the calculated controller output and actual controller output are different due to the output range


• Block diagram of cascade control

Equivalent

By choosing suitable GC2, the disturbance effect on C2 can be minimized.


• Overall transfer function for cascade control

– Characteristic equation

• The stability is affected by both GC1 and GC2.• The slave controller GC2 usually enhance the stability

characteristics of the whole system and thus larger value of master loop gain can be applied.

• Cascade control also makes the closed-loop process less sensitive to model errors.


– Comparisons of the performance

Change in L1

Change in L2


• Flow control with valve positioner– Due to the valve sticking and hysteresis, the opening of the

control valve may not be same even for the same command signal.

– The positioner action needs not to be too fast, but it should not be slower than flow controller.

– Usually P control is used.– It is a cascade control.


Time Delay Compensation

• Delay causes more phase lag– Shift the critical frequency lower.– The longer delay is, the lower critical

frequency is.– Makes less stable.

• Delay causes difficulties in feedback control– Limit the performance of a conventional FB control system.– Application of tuning rules

will be limited if time delayis too big.

– Delay approximation such asPade approximation will notbe valid and cause inaccuratedecisions.


Smith Predictor

• If the process output before time delay (ahead of time) is known, the control performance can be greatly improved.

• Since the process cannot be known exactly, use model.• The mismatch between process ( ) and model ( )

will be fedback to compensate the mismatch.• Smith predictor can be converted

to conventional FB control scheme.


• Closed-loop transfer function– Smith predictor

– For (no modeling error),

• No time delay in the denominator• In the response, the time delay still exist in numerator. After the

time delay, the response will be same as the control performance of the process without time delay.

• Not easy to implement for analog control, while easy for digital control. (Digital version: Generalized analytical predictor)

• If there is modeling error, the time delay will not be vanished in the denominator.

• Tune the Gc with existing tuning methods such as IMC and fine tune for the modeling error.


• Examples

Smith predictor

Comparison for Load Change

Comparison between conventional PID for process without time delay and Smith predictor for process with time delay (No modeling error) with same tuning parameters


Inferential Control

• In some control problems, the process variable to be controlled (CV) can not be measured on-line conveniently or infrequently. (e.g., product composition)

• If other measurements that can be measured rapidly have close relation with the targeted CV, the targeted CV can be inferred from these measurements.– For binary distillation column, there is a unique relation between

composition and tray temperature via Gibbs phase rule.– Density ↔ composition of binary mixture– Turbidity ↔ particle concentration, biomass– A combination of several measurement can be used.– The model is required. Sometimes, it is called “soft sensor”.– The parameters in the correlation may be updated, if necessary, as

actual composition measurement become available.


Selective Control

• Utilize the best suitable measurement among many measurements.

• High Selector (HS), low selector (LS), or median selector (MS) can be used.

• Examples– Hot spot temperature control

• The location of hot spot travels.• Use many distributed sensors.• Or, interpolate to find hot spot.• Auctioneering control

– Enhancing sensor reliability• Exclude faulty measurements.• Use 3 or more same sensors.


Override Control

• Normal control loop will be overridden by another loop if contingency situation occurs.

• Example 1– Objectives:

• regulate level and exit flow rate in a pumping system for a sand-water slurry

• Slurry velocity in the exit line must be kept above the lower limit.( if slurry velocity is too slow, it causes sand sediment.)

Normal: Level controller is working.

Flow is too low: FC adjusts the pump to increase slurry flow rate

Level controller: slow P control with normal set point (tight level control is not required)

Flow controller: fast PI control with high set point and reset feedback


• Example 2: Surge tank flow/level control– Objectives:

• Maintain flow at set point• L > Lmin (too low NPSH causes cavitation)

– Normal: flow controller is working– If L is low, level controller takes over and reduce outlet flow.– Flow controller: PI controller with reset feedback– Level controller: tightly tuned P control with lower limit of

level as set point.– By the reset feedback, the

integration stops while the levelcontroller takes over.

– By adjusting the gain of LC,the switching can be adjusted.


• Example 3: Compressor flow control– Objectives:

• Maintain the flow at set point• P < Pmax (preventing overload)

– Normal: flow controller is working– If P is too high, the pressure controller takes over and reduce

motor speed set point.– Flow controller: PI controller

with reset feedback– Pressure controller: tightly tuned

P control with high limit of press.as set point.


• Example 4: Distillation composition control– Objectives:

• Maintain the temperature at set point (composition control)• (preventing flooding)

– Normal: temperature controller is working– If is too high, the differential pressure (DP) controller

takes over and reduce the steam flow set point.– Temp. controller: PI controller

with reset feedback– DP controller: tightly tuned

P control with flooding limit of DPas set point.

When PI controller is used, if a disturbance comes in, the level of the controller output may be significantly different. In this case, CV should be used instead of controller output.


Valve Position Control• Example 1: Utility compressor control for air

supply– Objectives:

• Supply air to downstream processes as they require• Minimize the energy consumption of compressor

– Cascaded header pressure controller: maintain the pressure of the header as it is directed for enough supply of air

– Valve position controller:• Adjust the header P set

point so that the valveopening of at least one ofthe valves is near “fullyopen”. (slow control)

• The SP should be around90-95% (fully open).


• Example 2: Common heating utility control– Objectives:

• Supply energy to downstream processes as they require• Minimize the energy consumption of heater

– Heater outlet controller: maintain the temp of the heater as it directed for enough supply of energy to down stream processes

– Valve position controller:• Adjust the TC set point

so that the valve opening of at least one of the valves is near “fully open”. (slow control)

• I-control can be used.(slow but no offset)

• The SP should be around90-95% (fully open).


• Example 3: Mixing tank cooling control – Objectives:

• Remove the heat of mixing and maintain mixture temperature• Maximize the production of mixed feed

– Cascaded jacketed tank temp controller: maintain the temp of the mixture by removal of heat of mixing using cooling water

– Valve position controller:• Increase the feed flow of A while cooling capacity allows. (slow

control)• Mix ratio is maintained by

ratio control scheme• The SP should be around

90-95% (fully open).


Variable Structure Control

• Example: Floating pressure of distillation column– Objectives:

• Maintain the column pressure by adjusting cooling area through reflux flow rate control

• If the level is too low (below the heat transfer area), the vent must be open to relieve the column pressure.

– Pressure Controller: control the P byadjusting reflux flow rate.

– Level controller: takes over if level istoo low. (L<Lmin)

– If LC takes over, there is a discrepancy,and it opens the vent valve.

– The K in summing station decides thevent speed. The two cases should besimilar in response time.


Split Range Control

• Depending on the process condition, different MV should be used.

– Use one controller– Controller output will be

split by range splitter.

• Examples– Pressurize or

depressurize

– Heating or cooling

– Acid feed or base feed in neutralization


Other Complex Control Schemes

• Blending process control

– Objectives:• Maintain product composition• Maximize production• The additive flow rates are

limited. (constrained)

– Control scheme:• Override control for max.

production• Ratio control for composition• Valve position control for

additive flow constraint


• Boiler Control

– Objectives:• Maintain drum level• Maintain furnace pressure• Maintain steam flow rate• Maintain the air-fuel ratio• Maintain the excess oxygen

– Control scheme:• FF/FB control for level control• FB control for furnace pressure• FB control for fuel flow rate• Ratio control for air-fuel ratio• Cascade control for excess

oxygen control


• Common header control

– Objectives:• Maintain common header P• Adjust loading ratio separately• Give different loading max.• Allow individual shutdown and

manual operation• Similar dynamics for eachsituation to P controller

– Control scheme:• FB control for P control• HC for separate loading ratio• LS for separate loading max.• Averaging station for separateshutdown/manual operation

• MOC for same dynamics

MOC: Multiple output controller

HC: Hand/Automatic station

(output=K*input+bias)

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