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What Is Output Bias In Control Valve Positioner?

03/21/2017 7:12 AM

after installing a new control valve positioner (fisher DVC6200) & while making auto travelling

it stopped at stage "adjusting output bias" and didnt continue auto travelling .

-what is meant by "output bias" & why it stopped ??

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#1

Re: What Is Output Bias In Control Valve Positioner ?

03/21/2017 11:25 AM

Somewhere at the facility will be the user manual for the specific piece of equipment. If it cannot be found, the equipment manufacturer's telephone number can be found using a simple internet search. Make the call. Either a replacement manual can be had, or one may speak to someone in a Technical Support function that can answer the question. If one is really bold, both avenues can be explored during one telephone call. And it's far quicker than waiting on the off chance that some reader here might both know the answer and be inclined to respond.

There are more telephones on the planet than at any time in its history. So be bold. The telephone is a brilliant tool for getting to exactly where the information can be found and quickly. Because "time is money" - Anon.

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#7
In reply to #1

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 5:23 AM

Great response. In a couple of sentences, you've just made this website obsolete. Close it down guys, all your answers can be found elsewhere. Happy Days!!!!

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#8
In reply to #7

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 5:31 AM

Very good ,I wanted to say that,if I will proceed to contacting the manufacturer so no need for this forum

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#10
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Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 5:57 AM

I'm just sorry you didn't get the response/reaction you were looking for/expecting.

People are very strange beasts!

All the best Ahmed. I hope somebody can give you the response, you deserve.

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#2

Re: What Is Output Bias In Control Valve Positioner ?

03/21/2017 1:14 PM

Do you have a manual for the positioner? Did you read the troubleshooting section? The glossary of terms?

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#4
In reply to #2

Re: What Is Output Bias In Control Valve Positioner ?

03/21/2017 2:50 PM

Yes and found nothing

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#6
In reply to #4

Re: What Is Output Bias In Control Valve Positioner ?

03/21/2017 4:12 PM

Then please tell the forum how the telephone calls with the equipment manufacturer went.

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#3

Re: What Is Output Bias In Control Valve Positioner ?

03/21/2017 2:18 PM

It hasn't been set properly and needs adjustment for your particular application. Please refer to the manual for setting adjustment details.

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#5

Re: What Is Output Bias In Control Valve Positioner ?

03/21/2017 3:46 PM

What have you done???

I've installed at least a dozen of these the past 5 years and I've never seen that screen readout - ever. Of course I don't usually watch it too closely. We buy ones pre-installed by Emerson on one of their valves, so they probably have gone through that portion of the set-up.

I happen to keep a manual every now and then when a customer is buying 2 or more on their skid, and have scanned through it twice now and can't find that particular combination of terms listed anywhere. Definitely NOT in the index.

They have 2 manuals for this, and I didn't try the "quick start" one.

This is a distributorship based company and they expect you to call one of them for help with these beasties. Especially if the Hart programming is involved, and it is here with this positioner.

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#23
In reply to #5

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 3:38 PM

Was that an OMG!!! What have you done!!!??? Or was that a simple "what progress (or disprogress) have you made by interacting with this valve thus far?

If OP does not understand the manual, and/or cannot index the troubleshooting, or valve set up protocol, then he is truly lost in the field. He needs to bring in an expert - a hot shot in valve theory, control operation, and valve programming.

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#24
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Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 4:23 PM

That was an OMG. I've never had an issue with this particular positioner and I've used it quite a few times. I see 2 of them out on the floor right now for an up coming project. We tend to buy them set-up on a valve by the same company, but have used them on other manufacturer's valves once or twice.

He apparently has gotten into the set up protocol, if you follow along in the You Tube video (three times as good as the manual - wished I knew that was available last custom install) that IQ submitted to him and got hung up by an error mid stream. The manual as I have stated is just for set up, not so good for troubleshooting, as that particular company is part of a group that wants you to call them for assistance. They respond quite rapidly and will even entertain the thought of "no charge" even if it is something that is your fault, if your account is important enough to them. I've been advocating calling them and pitching the manual away for several posts now, as it does not address the OP's concerns. Again - the You Tube video comes closer to the explanation than the manual. Unfortunately, we don't know if the OP is close to a distributor for these, as anyone in the USA is, as the positioner may have been bought off e-bay or something like that. In that case, we have a problem here.

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#9

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 5:48 AM

This is proof that the more technology available, the lazier folks have become. Reading seems to be a chore and the idea researching is obsolete.

This is 'output bias'.

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#11
In reply to #9

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 6:12 AM

"Lazier folks",oh thank u,when you face a technical problem you first check the manual and if not solved you can of course make further " reading" or deeper " research " ,but to save time you share your problem with professional people to benefit from their experience, this is the idea of a forum, if you are one of them please help and if not I advice you with more " reading & research " to be helpful

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#12
In reply to #11

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 6:38 AM

I take it you are the entire team, no peer with knowledge of your 'craft', no one else you could have involved, no distributor close by, no phone to call the supplier to get the info, no internet to discover the anything. Yet, you have internet to contact this site. HMM!

Of course you think you are entitled to an answer to save you time, my Precious.

You took on a job you know nothing about. Dipstick takes on a new meaning. Research the net you love so much. AW.

Educational Services

For information on available courses for the DVC6200 digital valve controller, as well as a variety of other products, contact: Emerson Process Management Educational Services - Registration P.O. Box 190 Marshalltown, IA 50158‐2823 Phone: 800‐338‐8158 or 641‐754‐3771 FAX: 641‐754‐3431 e‐mail: education@emerson.com

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#25
In reply to #11

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 4:30 PM

ROFLMAO - that was a witty reply, sir!

Did you get the video IQ sent you?

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#13
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Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 8:07 AM

The more technology exists, the less likely someone will actually research an idea before hurrying to post it as a suggestion - check out my post #5. That terminology is NOT in the doggone manual. Before I criticize someone else I at least do what I am suggesting to be sure I am not making an a** of myself.

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#14
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Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 8:39 AM

Trust that is not a dig at me, as the FW could have gotten the info on line very easily, including an installation manual and set up manual. (PDF download). It took all of 60 seconds to locate the info he wants. Along with the supplier contact details. How long did it take him to post here? 60 seconds!

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#15
In reply to #14

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 8:45 AM

OMG, I tried Utube, No reading required, amazing and in colour too.

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#16
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Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 8:46 AM

It took all of 60 seconds to locate the info he wants

I would be quite interested in having that section pointed out to me - I went through that manual twice and never found the info he wanted. That terminology is NOT in the index and I couldn't locate it elsewhere.

Supplier contacts for an Emerson product are the local distributor, which probably is on line.

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#17
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Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 9:27 AM

It is in the Configuration set up of the positioner and auto programmes during set up. And appears straight after Seeking Hi & Lo Drive Setup. In auto calibrate settings

https://www.youtube.com/watch?v=zYp0ZtVzayU

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#18
In reply to #17

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 1:03 PM

Thanks for the video, I have already done all these steps till "adjusting output bias", please

Refer to the original thread to know what my question is !!!

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#21
In reply to #18

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 2:30 PM

Reset zero point and span and check the positioner against the template supplied. Then rerun the config.

Try here:

https://www.youtube.com/watch?v=mrx9l4iBcK0

This might help too;

https://www.youtube.com/watch?v=NptMmcZai9g

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#27
In reply to #21

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 4:55 PM

There you go - finally. This is about as good as the OP will get until he calls a Fisher distributor or main office. This is what I expected CR4 to do for him.

We have no idea what the OP's have available to them when they post, and I always take the most pessimistic approach and assume they are cash strapped, bought the device on an e-bay fire sale, been hired as an experienced engineer when they are just 2 months post student engineer, and have a PC still running on DOS, until shown otherwise. Too many times we assume the highest tech set up possible and an OP is just lazy. I try real hard to take the opposite view. I guess i am what you call a sucker.

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#30
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Re: What Is Output Bias In Control Valve Positioner ?

03/23/2017 8:17 AM

So here is one of my concerns with this forum. We never hear back from the OP, unless it is one of our regulars.

So, did we help at all, or did we just frustrate you?

Note that the problem you have seems to be a definite flaw in the DVC6200, as no-one has apparently ever seen what you have experienced, which makes a solution from here nearly impossible. Fisher can help you, but we can't know if you can contact them or not.

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#31
In reply to #30

Re: What Is Output Bias In Control Valve Positioner ?

03/23/2017 8:29 AM

thank you and all members for your concern,I will proceed to contacting Fisher ,this is available ....thanks again

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#32
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Re: What Is Output Bias In Control Valve Positioner ?

03/23/2017 8:31 AM

Thank you - keep me posted if you can. This interests me as I use this controller often.

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#20
In reply to #17

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 1:25 PM

Okay - so I see it in the You Tube video, but still can't find it in either manual (normal or quick start - I downloaded the second one after reading your reply) While certainly a high tech "potential" solution, this is not a solution I am to consider here, as You Tube is NOT to be run in my workplace, for obvious reasons. (music videos or WTF was he thinking type videos are pre-supposed) Don't know about the original OP, but possibly he is banned from it too. Company internet use is monitored in most work places. (I've got a special "pass" for this site, as we've used it for company problems numerous times)

But as the OP just asked you - where is the solution to the sequence hanging up on the Adjusting Output Bias readout? I STILL don't see that in any manual. While a very good place to start, the manual is NOT an answer to every problem, especially with the company who made this controller, as the "mother" company wants you calling them for problems. I've dealt with them for 15 years now.

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#26
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Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 4:36 PM

It even had nice fiddle music!!!

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#19

Re: What Is Output Bias In Control Valve Positioner ?

03/22/2017 1:21 PM

Output Bias could be to apply some extra pressure on the valve trim to make sure it stays closed when the trim is under line pressure on one side. So no movement to see, but you should see actuator pressure increase.

A rising stem valve often has a specification of line pressure that tends to lift the valve trim open, when the positioner is calling for closed. A bias could be the extra pressure to ensure the valve actuator has enough pressure on it to keep the valve closed, beyond the pressure needed to put the trim in the mechanically closed position on the bench.

Similarly, this is also done with pre-load on spring closing valves that have trim that can be cracked open by differential line pressure. The spring to close is loaded an extra amount according to the valve suppliers specifications, for line pressure and valve trim design.

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#22

Re: What Is Output Bias In Control Valve Positioner?

03/22/2017 3:35 PM

Output bias in a valve positioner is two-fold (IMHO):

(1) It may simply refer to an offset to be manually or automatically adjusted, so that the valve is physically closed against process, when the operator is being told to push to a negative position (or one more negative than the default "closed" reading.

(2) Bias may be part of linearizing the valve's output throughout its range, so that 10% actually produces 10% of the intended maximum flow, pressure, mass transport, etc. This is not just a slope calculation, it is for a smart valve controller in conjunction with some instrumentation that tells it what the system response for a given level of action open. This is just my humble opinion of the theory of such a valve.

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#28

Re: What Is Output Bias In Control Valve Positioner?

03/23/2017 12:09 AM

From Pearson, Process Control Instrumentation Technology:

"...2.1 Signal-Level and Bias Changes

One of the most common types of signal conditioning involves adjusting the level (magnitude) and bias (zero value) of some voltage representing a process variable. For example, some sensor output voltage may vary from 0.2 to 0.6 V as a process variable changes over a measurement range. However, equipment to which this sensor output must be connected perhaps requires a voltage that varies from 0 to 5 V for the same variation of the process variable.

We perform the required signal conditioning by first changing the zero to occur when the sensor output is 0.2 V. This can be done by simply subtracting 0.2 from the sensor output, which is called a zero shift, or a bias adjustment..."

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#33
In reply to #28

Re: What Is Output Bias In Control Valve Positioner?

03/23/2017 9:16 AM

We used to call that offset. Why the change in terminology? What the hell is bias gain, then?

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#34
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Re: What Is Output Bias In Control Valve Positioner?

03/23/2017 1:57 PM

It's one of those local usage things, but Pearson is one of "the" go-to books. You might think of offset as a deliberate change to the process variable whereas bias is a compensating change to the calibration. I know it's splitting hairs, but nobody said this stuff is easy.

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#35
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Re: What Is Output Bias In Control Valve Positioner?

03/23/2017 2:03 PM

No, it just chews up my arse every time. At least I can calibrate conductivity, ORP, temperature, and pH (among other chemical instruments).

Tweaking valves is harder. Really. My first lesson was making an ORP control loop on an RO system start behaving better when the pH could swing by as much as three pH units due to stupid acid injection system. We have finally found suitable dispersants that remove totally the acid injection requirement on that one.

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#29

Re: What Is Output Bias In Control Valve Positioner?

03/23/2017 1:18 AM

Output Bias is mentioned in the manual, page 78, but it is not defined.

Mr. Stewart's assessment in post #22 is as good a speculation as any, given the absence of documentation from the vendor. Or it could be a minimum pressure needed to unseat the the plug from a closed position. Who knows?

I'd tend to look at the things related to the task stem travel, linkage, to see what's happening, but it'll probably take a call to Emerson to get some guidance.

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#36

Re: What Is Output Bias In Control Valve Positioner?

05/18/2017 11:14 AM

Interestingly, the manual for the Fisher DVC6000 (an older model of "smart" positioner) gives a more detailed explanation of output bias and its adjustment than the manual for the newer Fisher DVC6200. Here is what the 6200 manual says (on page 38):

During calibration, the instrument seeks the high and low end points and the minor loop feedback (MLFB) and output bias. By searching for the end points, the instrument establishes the limits of physical travel, i.e. the actual travel 0 and 100% positions. This also determines how far the relay beam swings to calibrate the sensitivity of the MLFB sensor.

Here is what the older 6000 manual says (on page 6-3) for comparison:

During calibration, the instrument seeks the high and low end points and the minor loop feedback (MLFB) and output bias. By searching for the end points, the instrument establishes the limits of physical travel, i.e., the actual travel 0 and 100% positions. This also determines how far the relay beam swings to calibrate the sensitivity of the beam position sensor.

Adjusting the minor loop feedback bias is done around mid travel. The valve position is briefly moved back and forth to determine the relay beam position at quiescence. Essentially, it establishes the zero point for the Minor Loop Feedback circuit. The back and forth motion is performed to account for hysteresis.

Adjusting the output bias aligns the travel set point with the actual travel by computing the drive signal required to produce 0% error. This is done while the valve is at 50% travel, making very small adjustments.

So, from this we can tell that the output bias is the amount of drive signal necessary to get the actual stem position equal to the travel setpoint. The "Principle of Operation" section in both manuals shows how this "drive signal" fits into the positioner's design: it is the electronic signal stimulating the I/P converter which then drives the pneumatic relay which in turn sources or vents air to the valve's actuator. The Fisher DVC design incorporates feedback between the "printed wiring board" microcontroller and the pneumatic relay which they refer to as Minor Loop Feedback (MLFB). This makes the microcontroller aware of the pneumatic relay's beam position and from that it is able to approximate the amount of air flowing into or out of the valve actuator. This feedback is essentially a cascade control scheme inside the positioner itself which yields excellent response for a wide range of control valve mechanisms, and makes the DVC positioner very versatile. Sadly, the manual never really explains how Minor Loop Feedback works despite referencing it multiple times.

To your point: why is the positioner failing to determine the output bias? From the explanation we may conclude that any circumstance preventing the valve stem from reaching its target position will foil any attempt to determine the correct bias value. My first guesses would be a stem position feedback problem (i.e. something wrong with the magnetic sensor the DVC6200 uses to sense stem position) and/or a pneumatic problem (e.g. insufficient air pressure) preventing the valve from stroking fully, but of course there are other possibilities. I would recommend first checking any "alerts" or errors shown by the HART communicator, checking them against the table of error messages shown in the manual.

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#37

Re: What Is Output Bias In Control Valve Positioner?

05/19/2017 10:10 AM

Tony Kuphalt's post # 32: "Interestingly, the manual for the Fisher DVC6000 (an older model of "smart" positioner) gives a more detailed explanation of output bias and its adjustment than the manual for the newer Fisher DVC6200."

The inclusion of a definition of a term used in the setup and error codes of a device and then the subsequent removal of that term lends huge credence to Phys's comment in post #20: "while a very good place to start, the manual is NOT an answer to every problem, especially with the company who made this controller, as the "mother" company wants you calling them for problems".

The 'theory of operation' chapter that was at the beginning of almost every manual when I started in instrumentation 40 years ago has largely disappeared making it difficult to understand how to fix it if how it's supposed to work is largely an unknown.

In my opinion, the intent of removing information from documentation is solely to force the end-user into a pay-for-service (pay-to-play) situation because even those in the interim channel (distributors) frequently have no greater access than the end-users do.

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#38
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Re: What Is Output Bias In Control Valve Positioner?

05/19/2017 10:58 AM

I could not agree more with the final paragraph of your statement. While this is supposed to "grease" the wheels of commerce, it results in a lot of wasted man-hours searching for even basic information.

Proprietary is one thing, but it gets freaking ridiculous after a while.

Are we all moving to a day when the only "owners" are the OEM's, and the users are just sheeple from door to door? The company that are the end users should then come up with legal protection in terms of performance: " if OEM valve does not immediately perform when installed by the OEM certified technician, the OEM shall be charged for any production outage, for the price on the market prevailing for the goods or power not produced during forced outage, and the price of wasted man-hours lost in attempting to secure a pass-around to rectify the problem shall also be absorbed by the OEM".

Two can play at the game, and another way to play, is don't let the S.O.B.'s with that valve in the front gate to begin with. Tell them go play in the street during rush hour.

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#39
In reply to #38

Re: What Is Output Bias In Control Valve Positioner?

05/19/2017 2:15 PM

Amen!

It's nice when you do encounter manufacturers who write good technical manuals and take pride in their documentation. Schweitzer Engineering Laboratories does an outstanding job with their documentation, but sadly they don't make valve positioners.

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#40
In reply to #39

Re: What Is Output Bias In Control Valve Positioner?

05/19/2017 2:28 PM

Something tells me you work for SEL?

They do make a good product. GE still documents their gas turbine/generators fairly well.

I think some manufacturers think their product lines have been around so long, that "everyone must surely know how this works by now", or they just don't give a plug nickel worth of care.

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#41
In reply to #40

Re: What Is Output Bias In Control Valve Positioner?

05/19/2017 5:25 PM

No, I don't work for SEL, but I do use their products. I'm an instructor at a technical college, and I like to assign SEL literature as technical reading to my students because their documents are so descriptive.

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