manual pid tuning

8/6/2019 Manual PID Tuning

1/14

1

YOKOGAWA+Y oko ga w a C orp o ra tio n o f A me rica

GUIDE TO MANUALLY TUNEA THREE MODE-(PID) CONTROLLER

TRIAL AND ERROR METHOD

Yokogawa's universal temperature and profile controllers provide an "auto-tuning" function asa standard feature. Auto-tuning is a function whereby the controller automatically measuresthe process characteristics and calculates the best PID parameters for optimum control.Auto-tuning will set optimum PIDs in most applications, however, there are processes wherefast loops (also referred to as step response loops where no dead time is detected) such-as somepressure, flow or processes whose characteristics are not repeatable. Auto-tuning shouldalways be attempted first, ifE200 (auto-tuning error) is displayed or if improvement in PIDsselected by auto-tuning are desired, the controller must be tuned manually.The-following guide is a trial and error method to manually tune a P.I.D. controller on twodistinctly different control loops: fast loops and loops with significant dead time.


2/14

~--

Definitions:

1) Cycle time - The total length of time for the controller to complete -one on/off cycleof the relay or voltage pulse output is the "cycle time n or "duty cycle". The fractionof the cvcle time during which the output is ON is proportional the output display -value. If output display value is 50% and cycle time is set at 20 seconds, outputwould be on for 10 seconds, (50% X 20 sec. = 10), and off for 10 seconds.Reducing the cycle time (ON/OFF period) results in faster cycling and finer control,however it also reduces the life of the relay. For relay output cycle time is usually setat 10-30 seconds.

Comparison of operation for same control output (50%)

Cycle time = 10 sec Cycle time = 20 sec Cycle time = 40 sec

O N I I I IOFF

O N ' _ 'OFF

O N

-2) Proportional band - The range over which the output is adjusted from 0% to 100% iscalled the PB. The PB~s expressed in % of the operating span for full output changeand is centered around the setpoint when MR (Manual Reset) is set at 50%. Ameasuring range of 0 - 1000 with a PBof 5% would equal a PB of.50 (1000 X 5%= 50) degrees wide and extend 25 degrees above and below the setpoint.* Proportional - control mode uses information about the magnitude of the error.

2


3/14

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... ...'. . . . . . . . . . . . . . . . . . . . . . . . . " .... . . ; . . . . . . . . .:.

801

TEMP

601

401

201

375 S.P. 400 ~211-- TEMP~P.B . 50

3) Integral, also known as reset, is added to proportional action to overcome the offsetor error, from the setpoint. It responds to the error signal of the feedback system justas proportional action does, but this time it responds to both the duration and themagnitude of the error. A large change in the load on a system will cause thecontrolled variable to experience a large deviation from the setpoint. If the flow ofmaterial through a heat exchanger is increased, the temperature of the material willdrop before the control system can increase the input of steam to the new load. Asthe change in the temperature of the controlled variable diminishes, the error signalbecomes smaller and the position for the control element gets closer to the pointrequired to maintain a constant value.* Integral - Control mode uses informatiron about the magnitude of eror over a

period of time.

1 ; ; ; ; . ~ : . : . s : , : I PV

PB .~.I.,.L.l.J.~LL,LLLJ.../...J.J.J....UJ_JJ~J.L.J.. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. .

TIMEPROC ES S WI TH T EM PE RAT UR E OFFSETIIII

3


4/14

i S.I Af _ _ _ ,_ _ _ p o I _ " _ 1 e- . . .V . ~ 11m.

4) Derivative action, also known as rate action, responds to rapid changes in the errorsignal. It will anticipate the rise or fall of the ProcessVariable and automatically adjustthe proportional band to minimize overshoot or undershoot.' Derivative action providesa signal proportional to the rate of change of the error signal. When the controlledvariable is steady, the derivative signal is zero. When the value of the controlledvariable is changing rapidly, the derivative signal is large. The derivative signalchanges the output of the controller. In this way a larger control signal is producedwhen there is a rapid change in the controlled variable. In turn, the final controlelement receives a larger input signal. The net result is a faster response to loadchanges to prevent or limit overshoot/undershoot. . -* Derivative - control mode uses information about the rate of error change.

-If D is too large short period oscillationsappear in the Process Veriable

LOOPS WITH SIGNIFICANT DEAD TIMEDefined as a measurable time delay, minutes or hours, before a response in P.V. is observeddue to an output change. Most ovens and furnaces fall into this catagory1) R.O.T. (Rule Of Thumb) parameter settings.

P - Proportional Band ----- 5%I - Integral -------------- 0%D - Derivative ------------- 0%

2) Start the process.3) The process should be run at a setpoint that will allow the P.V. to stabilize with an

output being generated.

4


5/14

,

PROP_ORTIONAL BAND (P)4) With integral and derivative turned off the P.V. will stabilize with a steady state

deviation, or offset from setpoint. If P.V. oscillates, increase the value of P. from 5%to some larger number until P.V. reaches steady state. Note whether or not there areregular cycles at this temperature by observing the measurement on the display. Acycle, or oscillation, may be as long as an hour and will be much easier to detect if arecorder is used to monitor the P.V.

CA..,., . . act ionexample' WIeleP"'Porlonal bindSet point Narrow proporllonal bindSet pointSet poInt~Wlallon

,..,00'

P-ti0'

....0(ON-OFFlDeYlal10nProporUonal ba nd_ola. apa"__

5) If there are no regular cycles in the process, divide the proportional band by two -narrowing the PB leads to tighter control. Allow the process to stabilize and check foroscillations. If oscillations are not detected continue to divide by two until oscillationsare obtained.If oscillations are observed, immediately multiply the PBby two. If oscillations are stillpresent continue to multiply PB by two until oscillations stop.

6) The PB is now near its critical setting. Fine tune - increase or decrease the PB settinguntil process reaches steady state: just prior to. oscillations occurring.

5


6/14

OSCILLATIONS

PRIMARYPRIMARY PRIMARY SET POINTe SET POINT SET POINT !., ;, ;,ii ! ii. . . . Q QE E E{! ~ ~TIME TIME TIME

P.B. too wid. Thl. I. clo.. to P.B. too narrowdlvld. by 2 perfect tuning. Multiply by 2

Temperature Oscillations

INTEGRAL (RESET)

7) Set integral at 5 (five) times the dead/lag time. Dead/lag time is defined as that periodbetween output change (0 -100%) and P.V. change.

Assume a P.V. of 200 degrees, the process is in manual control and stabilized.Increase the output by 50% to drive P.V. to new setpoint of 250 degrees. If after 10(ten) minutes the P.V. begins to climb, we have determined the dead/lag time is 10(ten) minutes.

6


7/14

27 0

260-

25 0

24 0

'> 23 0IL22 0

21 0

20 0

19 0

Change

Integral is set as seconds per repeat, therefore, integral would be set at 3000: 60 sec.X 10 min. = 600 seconds of dead time X 5 = 3000 setting value.

Note: Maximum Integral Settings:UT14/15UT35UT37UP27UP25

36006000600060006000

8) Derivative is usually set at 1/4 of integral, therefore, if integral is set at 3000:derivative would be set at 750 (3000/4) seconds.

DERIVATIVE

FAST LOOPSDefined as having little or no dead/lag time. A small or large change in output triggers animmediate change in P.V. When manually tuning a fast loop it is usually found desirable toconfigure the instrument as a two mode, PI controller. Derivative, an overshoot/undershootsuppression function, by its nature contributes to instability in a fast control loop. Mostpressure and flow loops fall into this catagory ..

7


8/14

8

1) R.O.T. (Rule Of Thumb) parameter settings.P _Proportional Band ----- 100%I - Integral -------- O~D -Derivative - - . : - = - - - - - - - 0%

2) Start the process.3) The process should be run at a setpoint that will allow the P.V. to stabilize with output

being generated.PROPORTIONALBAND (P)

4) With integral and derivative turned off the P.V. will stabilize with a steady statedeviation, or offset from setpoint. If P.V. oscillates, increase the value of (P) from100% to some larger number until P.V. reaches steady state.

INTEGRAL (I)5) Set as seconds per repeat - start with an integral of 20 seconds. If P.V. begins to

oscillate, make (I) weaker by increasing the value of (I) from 20 seconds to some largernumber until P.V. reaches steady state. If the response is sluggish i.e., offsetdiminishing slowly, strengthen the (I ) action by reducing the value of (I) from 20seconds to some smaller number until P.V. reaches steady state.


9/14

9

CONTROL ELEMENTS

Control output's on the Yokogawa UT/UP single loop controllers take several different forms:Time proportional PID relay/voltage pulse, Continuous PID 4 - 20 mA/1 - 5 V DC, Positionproportional PID and Three position relay"output.Time proportional relay/voltage pulse output applies power to the load for a fixed cycle time,e.g., if, when relay output is being used, cycle time is set at 20 seconds and the outputdisplayed is 30% then the N/O relay would be energized (closed) for 6 (six) seconds and de~-energized (open) for 14 (eighteen) seconds.Analog outputs provide a continuous signal to the control element, e.g., if control outputselected is 4 - 20 rnA and the output displayed is 30% then the continuous signal would be30% X (16mA span) + 4 mA offset = 8.8 mA D.C.Position Outputs: Position proportional output is used to open and close amotor driven valve ..Position proportional, also referred to as slide wire feedback, couples a potentiometer to themotor shaft to convert its angle of rotation to a resistance value (position signal). The UT/UPuses this position feedback signal, displayed in % (0 -1 00%), to monitor and adjust the signalto the valve actuator if the actual valve position does not match the desired valve position.Three Position Control has two relay outputs that provide for on/off control only. A deadband set from 1.0% to 10.0% of the output span is centered around the setpoint. When thedeviation is positive and exceeds 1/2 the width of the dead band: output relay #1 is turnedon. When the deviation is negative and exceeds 1/2 the width of the center dead band:output Relay #2 is turned on. When deviation is less than 1/2 the center dead band widthboth outputs are turned off. Three position control is simply on/off control - Proportionalband, Integral and Derivative are not used, mostly used in level applications ..

POWER CONTROLSA device that applies power in response to the controllers output demand is a powercontroller. There are three common power controls; electromechanical relays, solid staterelays (SSR's) and silicon control rectifiers (SCR's). The first uses magnetic devices toactuate power switching while the latter two use solid state electronics to effect theswitching function.Electromechanical Contactor/Mechanical Relay:An electrical and mechanical device with moving parts. When power [s applied to the relaysolenoid a contact closure is created through movement of the relay's common contact.Because of the moving parts it is susceptible to vibration which eventually leads to mechanicalfailure. The closureof the contacts when powered causes burning and pitting - the primaryreason for relay failure. Electromechanical contactors provide a positive circuit break whereassolid state devices almost always have a small amount of leakage current flow.


10/14

If contacts close when the relay is energized they are called "Make" or "Form A" contacts.If contacts open when the relay is energized they are called "Break" or "Form B" contacts.All UT/UP Controllers and UT15L Limit Controller use "Form C" contacts allowing instrumentto be configured for either N.D. or N.C. contact. Unlike the UT/UP controllers, the UT15Luses a latching relay - a relay with contacts that lock in the de-energized position untilmanually reset - a requirement for F.M. (Factory Mutual) approval.

-1. Electromechanical ContactorScrew

contscts~~. . . . .T I ~5jlJNormalOpenConts

- - - . Serer . & ' _. . . . ', Com

NormallyClosedContactIyctw

IField Coli

monContact

UT/UP 15L:Control output relay contact: 250V AC. 3A

120V AC. 5A24V DC. 3A

Alarm output relay contact: -250V AC. 1A120V AC. 2A24V DC. 2A

* * All U"(/UP controllers have Time Proportional PID (relay output) capabilities for use withmechanical relays.Solid State Relay (SSR):Solid state relays are chosen for their long life and reliability. Unlike mechanical relays theycontain no moving parts resulting in higher failure rates. Because SSR's can operate at muchfaster cycle times than electromechanical relays, the should be employed where extremelytight process control (short cycle times) is required.* * All UTIUP controllers have Time Proportional (voltage pulse) output capabilities for usewith SSR's.

10


11/14

3. Solid State Relays

ACOUlpul

D C " " " "

Silicon Controlled Rectifier (SCR):Silicon Controlled Rectifiers (SCR's) provide efficient, reliable and relatively economical meansof electric power control. An SCR can be thought of as a solid state switch for passingcurrent. Turn it on and current flows - turn it off, and it stops. Since this device is solid statenothing physically moves, so nothing wears out and the operation is silent. The result is finercontrol with less maintenance than an electromechanical contactor.A contactor (mechanical relay) may cycle three times a minute - an SCR can cycle 60 timesper second. Unlike a contactor, the SCR controller modulates small increments of power tothe load which helps to eliminate overshoot and undershoot associated with contactor control.Faster cycling time and small modulated increments of power leads to better control of theprocess. Therefore, the advantages of SCR power controls are: their lack of moving parts,long life and improved controllability. SCRs also typically e_xtendthe life of the heatingelement.

seR Power Control

Input:Tlme Proportioning(ONlOFf) n.n. Switched

outputor~20mA f'--

QPAC 3""2 LegPower Control

11


12/1412

There are two ways to turn on (or fire) and SCR;(A) Zero Firing - These should be used where a purely resistive heater load is involved.Defined as a load in which the alternating voltage is in phase with the alternating current.

(8) Phase Angle Firing - These controllers can be used with aOnyoad but must be used forinductive loads _also refereed to as lagging loads. Defined as a load where the alternatingcurrent lags behind the alternating voltage of the load.

_ _ = Phase Difference

* * All UTIUP controllers have Continuous PID (4 - 20 rnA DC) output capabilities for use withSCR's.


13/14

FINAL CONTROL ELEMENTSThe_"final control element" is the last part of the loop and the "muscle" of the process controlsystem. Every control loop of any kind has to manipulate a flow of material or energy.Control valves are the most common type of final control element, however, there are othertypes of devices i.e., mechanisms that regulate (throttle) electric energy such as SCR's(silicon controlled rectifiers), variable speed drives; -proportioning pumps and dampers.

ValvesThe output of the controller sends a corrective electrical signal to the final control elementwhich governs the control of the manipulated variable. In the vast majority of process controlapplications the final control element is a valve: ~ device that adjusts one or more passagesto change the flow rate or path, or both, for one or more fluid streams. Valves are typicallydriven by motors which are commonly called actuators which in turn drives the stem thatpositions the valve plug.Control Valves:On/Off valve: Operates in binary fashion, fully opens or fully closes one or more flow paths.Also referred to as an isolation or shutoff valve.Automated or Non-Modulating Throttle Valve: An on/off valve modified by the addition of anactuator and positioner and is operated by a pneumatic cylinder. The distinction between"automated" and "control" valves is usually considered to be the ability of the latter to"modulate," i.e., to assume an infinite number of "throttling" or travel positions during normalcontrol operation. The non-modulating valve is adjusted by hand or by a binary controller todifferent positions that remain fixed until the next adjustment.

Modulating Throttling Valve: Usually referred to simply as a modulating valve, is operated byan automatic modulating controller that causes the valve to "modulate" or "travel." Like thenon-modulating throttle valve it is used to limit flow by adjusting the opening, "throttling" or"choking" the flow.

Variable-Speed Control:The flow of materials can be modulated by varying the speed of process equipment that usesvariable-speed drives, Le.; conveying solids by means of a variable-speed screw conveyor orconveyor belt: transporting liquids or gases in a pipe or duct by means of a variable-speedpump or compressor. .Power drives offering variable speed for process control include: electric motors, pneumaticmotors, hydraulic motors, mechanical transmissions and steam turbines.Variable-speed drives can be costly to purchase and maintain. The benefit they provide is toreduce power consumption by using no more power than is needed for pumping at any giventime.

13


14/14

14

Proportioning Pump:A proportioning pump, also known as a metering pump, pumps liquids by means of a pistonor plunger inside a cylinder. Every back and forth stroke cycle of the pump discharges a fixedvolume of liquid. The length of stroke of the pump can be adjusted by the use of a pneumaticor other type of actuator. Each adjustment changes the discharge volume per cycle.Proportioning pumps are usually of low capacity and are frequently used to inject smallvolumes of a chemical solution into a relatively large stream.

Dampers:Industrial fans may be supplied with dampers, louvers or adjustable vanes for manipulatingthe gas or air flow through the fan. The dampers are placed on the inlet side of the fan andact as a 'control valve. They can be actuated automatically as part of a control loop ormanually.

manual pid tuning

Documents