Topic 5 Enhanced Regulatory Control Strategies

In The Last Lecture  Cascade Control –What is cascade control –Advantages of cascade control –Testing cascade control loops –Tuning cascade control loops

What We Will Cover Topic 1 Introduction To Process Control Topic 2 Introduction To Process Dynamics Topic 3 Plant Testing And Data Analysis Topic 5 Enhanced Regulatory Control Strategies Topic 7 Process Control Hardware Systems Topic 4 Controller Actions And Tuning Topic 8 Control Valves Topic 9 Process Control Troubleshooting

In This Lecture  Feedforward Control –Measured Vs Unmeasured Loads –Purpose of feedforward control –Feedforward gain –Deadtime compensation –Lead-lag compensation –Testing Feedforward loops

Process Control Feedback Control PID controller Proportional Gain, K p Integral Time, τ I Derivative Time, τ D Feedforward Control SS Gain with lead/lag & deadtime compensation Feedforward Gain, KFF Deadtime Compensation, θ FF Lag time, τ Lag Lead time, τ Lead

Feedback control  PID control is a type of feedback control –To control the CV, we measure the CV –PV is “fed back” to controller for comparison with SP –Controller decides and executes new OP –CV is measured...  No knowledge of process disturbances are required –Loads need not be measured (ULOAD)  Controller takes action after CV is disturbed so perfect control is not possible  Good for short deadtime and lag time processes  Not as good for long deadtime and lag time processes but still ok  If we have knowledge of disturbances, can we do anything to improve control?

Feedback Response Measured Load Temperature with only feedback control Desired temperature response

Feedforward control  Knowledge on how loads affect the CV is required –Loads must be measured (MLOAD)  When a disturbance is detected, the feedforward controller takes synchronized action before the CV is disturbed so potential for perfect control exists  Used to supplement PID controller in a long deadtime, lag time process  Requires tuning for perfect synchronization: –Feedforward gain, K FF –Deadtime compensation, θ FF –Lag time, τ Lag –Lead time, τ Lead

Feedforward control If the flow rate increases, the feedforward controller will increase the flow of FG before the FOT is disturbed. All other unmeasured disturbances are taken care of by the PID controller (TIC).

Process dynamics t t TIC.OP TIC.PV K p, θ p, τ p

Process dynamics t t FI.PV TIC.PV K CV/MLOAD, θ CV/MLOAD, τ CV/MLOAD

Feedforward Gain, K FF  Whenever you see the feed flow rate increase by 1%, you want the FG flow rate to increase by 2%  Whenever you see the feed flow rate decrease by 2%, you want the FG flow rate to decrease by 4% and so on….  This relationship is the feedforward gain

Feedforward Gain, K FF  Assume you have the following information:  FI.Span = 100 – 0 = 100 m 3 /h  TIC.Span = 100 – 0 = 100C  You observe that if FI.PV rises by 1 m 3 /h, TIC.PV decreases by 2C  Also, if TIC.OP increases by 1%, TIC.PV increases by 1C  Therefore, if FI.PV rises by 1 m 3 /h, we can prevent TIC.PV from changing by increasing TIC.OP by 2%  We can instruct the feedforward controller to increase TIC.OP accordingly by setting the correct FF Gain

Feedforward Gain, K FF  FI.Span = 100 – 0 = 100 m 3 /h  TIC.Span = 100 – 0 = 100C  You observe that if FI.PV rises by 1 m 3 /h, TIC.PV decreases by 2C  Also, if TIC.OP increases by 1%, TIC.PV increases by 1C  SS Gain between FI.PV (MLOAD) and TIC.PV (CV) –  SS Gain between TIC.OP (MV) and TIC.PV (CV) –

Feedforward Gain, K FF  SS Gain between FI.PV (MLOAD) and TIC.PV (CV) –  SS Gain between TIC.OP (MV) and TIC.PV (CV) –  Feedforward Gain, K FF –

Deadtime Compensation, θ FF  While K FF deals with the disturbance and process at steady-state, it does not take into account the fact that deadtime may exist –Deadtime between FI.PV (MLOAD) and TIC.PV (CV) –Deadtime between TIC.OP (MV) and TIC.PV (CV)  Suppose θ CV/MLOAD = 10 min and θ p = 3 min  When FI.PV changes, TIC.PV will start to change 10 min later  When TIC.OP is adjusted, TIC.PV will start to change 3 min later  For perfect control, when FI.PV changes, we need to adjust TIC.OP after a fixed time (10 – 3 = 7 min) 

Lag Time, τ LAG  Some lag time will certainly exist between FI.PV and TIC.PV  When FI.PV increases, we do not want to increase TIC.OP by a step change  Instead, we want to synchronize the increase in TIC.OP with the way TIC.PV responds to MLOAD (FI.PV)  To do this, we want slow down the change in TIC.OP. We do this by applying a lag of τ CV/MLOAD to the change in TIC.OP  τ LAG = τ CV/MLOAD

Lead Time, τ LEAD  Some lag time will also exist between TIC.OP and TIC.PV  To compensate for this lag, TIC.OP will need to be speeded up  We do this by applying a lead time of τ p to TIC.OP  τ LEAD = τ p

Summary of FF parameters  Feedforward Gain,  Deadtime compensation,  Lag time,  Lead time,

FF parameters may be implemented in a single block

FF parameters may be implemented in separate blocks

Plant Testing For FF Control  There are two sets of dynamics we need to know in order to configure a feedforward control scheme –Between OP and PV –Between MLOAD and PV  Take a furnace example where we already have a FOT controller and we wish to implement a FF controller to guard against disturbances to the feed flow rate

How do we perform our test?

 To get dynamics between TIC.OP (MV) and TIC.PV (CV), perform step change test as per single feedback loop –Put loop to MAN, make step change to OP and collect data on PV response  To get dynamics between FI.PV (MLOAD) and TIC.PV (CV), –Ensure TIC.MODE is in MAN. We do not want the PID controller to interfere with the response of TIC.PV wrt FI.PV –Make a step change in FI.PV (assuming you can control FI.PV) and collect data on TIC.PV response –If you cannot change FI.PV at all, and you cannot obtain appropriate dynamic data, FF control cannot be implemented

When not to use FF control  When there is no MLOADS (all loads are ULOADS)  When θ CV/MLOAD is less than θ p (i.e. θ FF < 0) –However, you may still be able to approximate the ideal FF controller by setting θ FF = 0 and trying to adjust τ LAG and τ LEAD  When it takes too long to get one value of MLOAD –Some analyzers may take minutes to make a measurement, by which time the CV is already showing signs of disturbance  When dynamic models are poor –Your FF control is only as good as your underlying models –Harder to tune FF controllers by trial and error than PID controllers

In This Lecture…  Feedforward Control –Measured Vs Unmeasured Loads –Purpose of feedforward control –Feedforward gain –Deadtime compensation –Lead-lag compensation –Testing feedforward loops –When feedforward control cannot be used

In The Next Lecture…  Split-range control  Override control  Ratio control