PIDE bumpless transfer from Operator manual to Program auto

MATT116 · Jan 12, 2013

RS5000 V20
1756-L71
FTview 6.1 with PIDE face plate

My question, is there is a way to get bumpless transfer from operator manual mode to program auto mode?
Right now if Im in program auto mode and my CV is 50% and then I enter operator manual from the face plate then adjust the CV to 75% then return to program auto mode the CV jumps back to 50%. I know it works going program manual to program auto. Any ideas?

Thanks

jkerekes · Jan 12, 2013

Well, when you go back to Auto mode, the PID is going to control depending on what the SP is set for. To have a bumpless transfer from Manual to Auto, you're going to have to make the SP equal to the PV before switching to Auto.

MATT116 · Jan 12, 2013

Yea thats what I was thinking. I guess I was looking for the operator manual to program auto to function like program manual to program auto in which the PIDE calculates the difference between the CV and the calculated so that the output doesn't jump (bump).

g.mccormick · Jan 12, 2013

When going from open loop to closed loop, copy the open loop command to the closed loop integral.

MATT116 · Jan 12, 2013

I'm not sure what you mean by "open loop command" or what parameter this is in the allen bradley PIDE instruction.

Lancie1 · Jan 12, 2013

Just like JK said, set up a logic rung that looks for a Manual to Auto mode change. Before that change, MOVE the PID Process Variable value to the PID Setpoint (scaled to the proper range). Then the PID in Auto mode will think "wow, I am right where I need to be, so I don't need to change anything". Then after going to PID Auto Mode, the operator has to move the Setpoint back up or down to where he wants to operate in Auto Mode, and the PID starts its normal increase or decrease in the Control Variable - but any sudden bump has been avoided.

Mickey · Jan 12, 2013

jkerekes said:
Well, when you go back to Auto mode, the PID is going to control depending on what the SP is set for. To have a bumpless transfer from Manual to Auto, you're going to have to make the SP equal to the PV before switching to Auto.

The "PIDE" instruction has that feature build-in, see pictures below. "PVTracking"

MATT116 · Jan 12, 2013

Thanks everybody for your reply's. I'll just have to tell the operators that the will have to remember the current setpoint before entering operator manual mode.

Ron Beaufort · Jan 13, 2013

"bumpless" - a few of the pros and cons ...

just a word of advice:

you might want to have a little talk with your operators - and find out WHY they'll be putting this thing into the MANUAL mode in the first place ...

in some cases it's because the operator has suddenly noticed an "OH MY GOSH!" situation – and he knows that if he slaps the "manual" control button, the system SHOULD go directly to his favorite default 45% output setting for MANUAL operation ...

oops! ...

that won't happen if the system has been set up for a bumpless transfer from AUTO into MANUAL ... in that case, the system will go into manual alright – but now the operator has to fiddle around with an additional "output" control (dial, slider, etc.) to get the output to go where he really wants it ...

in some (most?) cases, he's definitely not going to coax the CV into position little-by-little ... instead he's going to BUMP it just as fast as he can - right to his desired output setting ...

so (at least in this situation) how would programming a "bumpless" transfer help us? ...

the reason for bringing this up is an experience from the days when I used to work for a systems integrator ... our company got called in to "fix" a system that had just recently been finished by one of our competitors ... after the other guys had turned the system over to the customer, the operators were going nuts – because every time they put the system into "manual" operation – the output wouldn't react the way they expected it to ... specifically, the system just kept chugging along with the same output as it had while in "automatic" ... that was the programmer's "bumpless" transfer in action ... there was really nothing "wrong" with the system – other than it didn't do what the operators EXPECTED – and not what they WANTED ... we took the bumpless transfer out – something that the original programmer had been unwilling to do ... the operators were happy – and we picked up a new customer ...

so again I say, you might want to talk with your operators ...

I am NOT saying that "bumpless" transfer is a bad thing – and in many cases it's something that you actually do want and need ... but in OTHER cases it's a pain in the operators' necks – and they neither NEED it – nor do they WANT it – and they'll curse your name if you force it down their throats ...

moving right along ...

when it comes to providing bumpless transfer when going from MANUAL to AUTOMATIC mode, you probably don't have to even worry about doing anything to achieve that ... in most cases it's "built in" to the PID/PIDE instruction's operation ...

take a look at the trend below ... this shows a "dummy" PV signal being fed into a PIDE instruction as a lab experiment ...

when the chart starts up, the SP is 40% and we'll keep that same setting all the way through ...

and when the chart starts up, the PV is also 40% (right on target) - but we'll ramp that "dummy" signal up and down as the experiment proceeds ...

and also when the chart starts up, the CV is 10% - and the PIDE will manipulate that "output" in the AUTOMATIC mode ...

so ... the sun is shining – the birds are singing – life is lovely ...

now let's suppose that the load on the system has increased ... (maybe someone slid a new batch of cookies into the oven, etc.) ... the PV starts to ramp down in response to the increased load ...

at point A the PIDE (in AUTOMATIC mode) begins to ramp the CV up in an effort to compensate for the increased load ...

at point B the operator (for one reason or another) has decided to put the system into the MANUAL mode ... and he wants/needs/expects the CV to go directly to his favorite/default value of 45% ...

so – notice that I have NOT included bumpless transfer for this change from AUTOMATIC to MANUAL mode ... if I HAD made the transfer bumpless – what do you suppose the operator would do next? ... smart money says that he'd just spin the output dial directly and immediately right to the 45% value that he wants ... so we're STILL going to get a bump to the CV here – and (rightly or wrongly) the OPERATOR (bless his little heart) is going to provide that bump – regardless of your "bumpless" program ...

so – back to our story ...

the CV nosedives from point B down to 45% at point C ... does that sudden change/bump to the CV "upset" anything? ...

well, in SOME systems it might – but in many (most?) systems it won't ... and in SOME systems we might have much bigger things to worry about than an "upset" ... the big question is "Why did the operator go to the MANUAL mode in the first place?" ... if the system was "getting out of hand" in the AUTOMATIC mode – then maybe we really and truly DO need to let the operator instantly scram the CV directly to the value which he desires ... "bump it, Bubba, bump it" ...

anyway ...

at point D the operator has decided to put the system back into AUTOMATIC mode again ... now one more idea and we're finished ...

notice that at point D there is no "bump" when going from MANUAL to AUTOMATIC mode ... that's just the way the PIDE normally functions ... and I've got a hunch that if you'll try it out, that's really the way you want it ...

because ...

let's just suppose that you HAD changed the SP from 40% (its normal day-to-day setting) to match the PV's current value ... (a common way of preventing a bump when going back into AUTOMATIC mode) ...

well, now ... what do you think the operator (bless his little heart) is going to do with the SP when he finally DOES go back to AUTOMATIC again? ... smart money says that he's going to grab that SP dial and crank it right back to his desired setting of 40% - while mumbling something like: "Why won't the darned thing stay where I put it?" ...

and (here's the good news - at least for most systems) there probably won't be a downside to that sudden cranking ... specifically, based on how you've got the system tuned, the PIDE will probably just pick back up controlling the CV with no bump to worry about ...

a quick analogy:

suppose that you're driving along with your car's cruise control set for 65 mph ... you (the operator) tap the brake pedal - and the gas pedal instantly releases ... the car's speed begins coasting downward ... that sudden release of the gas pedal might indeed be considered a "bump" to the system - but did the bump cause a problem? ... nope – you WANTED and EXPECTED the gas pedal to fully release and let the car gradually slow down ... but suppose that the cruise control had been artfully programmed to provide a "bumpless" transfer when going from AUTOMATIC to MANUAL ... in that case, the car would NOT have slowed down when you just tapped the brake – instead it would have maintained its current gas pedal position – in order to prevent a "bump" ... (see where I'm going with this?) ...

now suppose that you've been MANUALLY controlling the gas pedal for a while – and the car is now going 45 mph ... you push the button to "resume" AUTOMATIC mode – and you WANT and EXPECT the gas pedal to automatically be controlled for your previous cruising speed of 65 mph ... but suppose that the cruise control had been programmed to provide a "bumpless" transfer when going from MANUAL to AUTOMATIC ... in that case, the car wouldn't speed back up – instead it would just maintain its current gas pedal position of 45 mph ... you (the operator) would now need to fiddle around with the gas pedal - and then "reset" the Setpoint - in order to get back up to your desired cruising speed ...

so – do we really and truly NEED or WANT bumpless transfer when going from one mode to the next? ... in some cases it's more of a nuisance than an advantage ...

summing up the main point of all of this: however you're planning to program the system, don't forget to incorporate the operators into your plans ... and regardless of how hard you try, you won't be able to fully program THEM – or the effect that their actions will have on the system's operation ...

DISCLAIMER: most of this is based on how many systems work ... each one is different – and your mileage may vary ... in fact, for your system you might indeed NEED to set up bumpless transfer ... in that case, feel free to ignore this post ...

party on ...

.

Lancie1 · Jan 14, 2013

but suppose that the cruise control had been programmed to provide a "bumpless" transfer when going from MANUAL to AUTOMATIC ... in that case, the car wouldn't speed back up – instead it would just maintain its current gas pedal position of 45 mph ... you (the operator) would now need to fiddle around with the gas pedal - and then "reset" the Setpoint - in order to get back up to your desired cruising speed ...

A tuned PID in Auto mode should be able to go from a 45 to 65 Setpoint without overshoooing. A human in Manual mode may produce a few bumps along the way.

PIDE bumpless transfer from Operator manual to Program auto

MATT116

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