exploring override configurations june 12, 2019 in … · – the unselected controller’s...

EXPLORING OVERRIDE CONFIGURATIONS

IN EXPERIONSamuel Congiundi

June 12, 2019

© 2019 by Honeywell International Inc. All rights reserved.

1

Furnace Process

FC

FEED

FC

FUEL

TC

OUT

TC

STACK<Advanced

Control

Scheme

Hydrocarbon

Feed

Fuel


2

Correct Behavior of an Override Scheme

• Smooth approach of override PV to its SP with minimal overshoot

• Override controller output should follow the process load when unselected

• There is leeway for the selected controller to maneuver without interference from the override controller

– Amount of control leeway is dictated by process gain and dynamics


3

Override Configuration Options

• Configuration parameters are located inside the OVRDSEL block

• Three options

– “Enable Override Option”

▪ Override feedback propagation without offset

▪ Override feedback propagation with offset

– Override option disabled (No override feedback propagation)

• The control response is different with each option


4

Override Feedback Propagation Without Offset

• The unselected controller’s output is initialized to the selected controller’s output

– Thus, the unselected controller’s output hovers just above or below the selected output

• The PID controller with the most restrictive incremental output is selected

<

OPn-1 + ΔOPPID 1 OPn-1 + ΔOPPID 2

OPn


5

Override Feedback Propagation With Offset

• The unselected controller’s output is initialized to the selected controller’s output plus an offset

– Offset is equal to one repeat (controller gain times error)

– Thus, the unselected controller’s output hovers above or below the selected output by ~one repeat

• The PID controller incremental output must overcome the offset before it is selected

<

OPn-1 + {S * Kc * Error} + ΔOPPID 1 OPn-1 + {S * Kc * Error} + ΔOPPID 2

OPn

Select Flag

S = 1 if PID is not selected by override

S = 0 if PID is selected by override


6

No Override Feedback Propagation

• The unselected controller’s output is not initialized

– The unselected controller’s anti-windup bit is turned on

– The controller is free to move, but the integral action is disabled

• The PID controller with the most restrictive output is selected

<

OPPID 1 + ΔOPPID 1 OPPID 2 + ΔOPPID 2

OPn


7

Exploring the Control Responses

• A low-select override scheme was dynamically simulated in both Microsoft Excel and in Experion

• The simulation consisted of a ramped OP routed to OVRDSEL.X[1] and a PID output routed to OVRDSEL.X[2]

• A first order plus deadtime model was configured to provide a simulated process for the PID

– Gain = 1

– Lag = 1 min

– Deadtime = 1 min

PIDRAMP

NUMERIC

OVRDSEL

DEADTIME LAG GAIN

OP 1 OP 2

PV

Selected OP


20

25

30

35

40

45

50

55

60

Override SP

Override PV

Override OP

Primary OP

Selected OP

8

Override Feedback Propagation Without Offset


20

25

30

35

40

45

50

55

60

Override SP

Override PV

Override OP

Primary OP

Selected OP

9

Override Feedback Propagation With Offset


20

25

30

35

40

45

50

55

60

Override SP

Override PV

Override OP

Primary OP

Selected OP

10

No Override Feedback Propagation


11

Pitfalls

• Override feedback without offset option does not leave any control leeway

• Override immediately takes over control

20

25

30

35

40

45

50

55

60

Override SP

Override PV

Override OP

Primary OP

Selected OP


12

Pitfalls

• Override feedback with offset option results in significant overshoot

• As the primary output ramp rate was increased, the overshoot increased

20

25

30

35

40

45

50

55

60

Override SP

Ramp Rate = 0.1

Ramp Rate = 0.2

Ramp Rate = 0.3

Ramp Rate = 0.4

Increasing

Ramp Rate


• No override feedback option does not follow the process load

• Consider these two scenarios:

– The primary controller reduces its output, but a process disturbance keeps the override PV constant

– The override output is changed in MAN then placed back in AUTO

20

25

30

35

40

45

50

55

60

Original PV

Override SP

Override PV

Override OP

Primary OP

Selected OP

13

Pitfalls

Selected OP is

lower by 6%


14

Pitfalls

• Recall the three criteria…

– Smooth approach to override SP with minimal overshoot

– Override controller while unselected should follow the process load

– There is leeway for the selected controller to maneuver without swapping control

• Override feedback without offset – no control leeway

• Override feedback with offset – significant overshoot

• No override feedback – does not follow the process load

• Is there a method that satisfies all three criteria?


15

External Reset Feedback: The Industry Standard1

• Method to prevent controller windup

• Achieved by positive feedback of the process load

• Uses an algorithmic approach vs. a rule-based approach

• Satisfies the three criteria

∆𝐶𝑉𝑛 = ∆𝐶𝑉𝑃𝐼𝐷 +∆𝑡

𝑇1𝑅𝐹𝐵 − 𝐶𝑉𝑛−1

Where:

∆𝐶𝑉𝑛 = Final calculated value

∆𝐶𝑉𝑃𝐼𝐷 = Calculated value from PID algorithm

∆𝑡 = Execution frequency

𝑇1 = Integral time

𝑅𝐹𝐵 = Reset feedback signal

1. Shinskey, F. G. Process Control Systems. McGraw-Hill, 1967.


20

25

30

35

40

45

50

55

60

Override SP

Override PV

Override OP

Primary OP

Selected OP

16

Simulation Results with External Reset Feedback

• Smooth approach to setpoint with minimal overshoot


20

25

30

35

40

45

50

55

60

Original PV

Override SP

Override PV

Override OP

Primary OP

Selected OP

17


• Follows the process load, resulting in the same process response

– At steady state, the override output hovers one repeat (gain x error) away from the primary output

one repeat


20

25

30

35

40

45

50

55

60

Override SP

Override PV

Override OP

Primary OP

Selected OP

18


• Leaves control leeway for selected controller to maneuver

one repeat

one repeat


19

Simulating with External Reset Feedback

• Actually, Experion offers a PID with ERFB!

• The dynamic simulation was re-configured with external reset feedback

PIDERRAMP

NUMERIC

OVRDSEL

DEADTIME LAG GAIN

OP 1

RFB PV

Final OP

Final OP

Selected OP


20


• When simulated in Experion, the control response is different… why?

20

25

30

35

40

45

50

55

60

Override SP

Override PV

Override OP

Primary OP

Selected OP


20

25

30

35

40

45

50

55

60

Override SP

Override PV

Override OP

Primary OP

Selected OP

20

25

30

35

40

45

50

55

60

Override SP

Override PV

Override OP

Primary OP

Selected OP

21


• What accounts for this difference?

Honeywell Simulation with ERFBOriginal Simulation with ERFB


22

External Reset Feedback in Experion (PIDER)

• Honeywell incorrectly multiplies the reset feedback term by the controller gain

– Results in the wrong control response

• Correct implementation of ERFB - At steady state, the override PID output hovers one repeat away (gain x error) from the selected output

• Honeywell implementation of ERFB - At steady state, the override PID output hovers away by the magnitude of the controller error

• At steady state, the override controller should hover away by a distance dependent on the process behavior (i.e. one repeat)

– If the output hovers away only by the magnitude of the error, the control response could be very poor depending on the process behavior.

∆𝐶𝑉𝑛 = ∆𝐶𝑉𝑃𝐼𝐷 + 𝐾𝑐∆𝑡



20

25

30

35

40

45

50

55

60

Primary OP

Kp = 1

Kp = 2

Kp = 3

20

25

30

35

40

45

50

55

60

Override SP

Kp = 1

Kp = 2

Kp = 3

23

Honeywell PIDER With Varying Process Gain

• Simulation was repeated with various process gains

PID PV Responses

PID OP Responses


20

25

30

35

40

45

50

55

60

Primary OP

Kp = 1

Kp = 2

Kp = 3

20

25

30

35

40

45

50

55

60

Primary OP

Kp = 1

Kp = 2

Kp = 3

20

25

30

35

40

45

50

55

60

Override SP

Kp = 1

Kp = 2

Kp = 3

20

25

30

35

40

45

50

55

60

Override SP

Kp = 1

Kp = 2

Kp = 3

24

Honeywell ERFB vs. Correct ERFB

PID PV Responses PID PV Responses

PID OP ResponsesPID OP Responses

Honeywell ERFB Correct ERFB


25

Is there any way to achieve the proper response with Experion?

• The Honeywell ERFB multiplies the reset feedback term by another term called reset feedback gain, KRFB

• KRFB can be set equal to the inverse of the controller gain, Kc

– This cancels out the controller gain in the reset feedback term and allows the reset feedback to function properly

• Unfortunately, Honeywell does not allow KRFB to be greater than 1.0

– So, this method is only possible if the controller gain is greater than 1.0

∆𝐶𝑉𝑛 = ∆𝐶𝑉𝑃𝐼𝐷 + 𝐾𝑅𝐹𝐵𝐾𝑐∆𝑡



26

So, what is the best configuration option?

• There is no perfect configuration

• If controller gain is greater than 1.0, use PIDER with KRFB equal to 1/ Kc

• Otherwise, the process behavior must be evaluated to determine the best option

• Consider working with Honeywell via UIS submittal to correct the PIDER algorithm

exploring override configurations june 12, 2019 in … · – the unselected controller’s...

Documents