We are trying to use the feedback from a small "S" shaped load cell to measure the force applied by a piston to a sanding belt. The feedback
system works fine (. i.e forced against a static stop) until the rotating sanding belt comes in contact with the sanding block attached to the piston. It will feed back acceptable readings until about 14 lbs. pressure. After that it reads 14 lbs no matter what the actual pressure is.
Any suggestions???
Regards,
Art Bourdeau,
[email protected]
Phone518)765-3667
Fax: (518)765-4033
Mobil: (518)573-4745
Fax518)765-4033

 

I suppose the following might be a dumb question, but what is the load cell rating and what are the signal conditioner and end device scaled to? Also, are the "acceptable readings" actually correct? I would suspect a scaling or calibration problem, but it is difficult to be sure with just the above information.
If you were to take some voltmeter readings directly on the output of the signal conditioner and compare these to your calibration reference then this would yield some useful information.
Also, if your signal conditioner is a straight analogue device it is very easy to crank the zero and span pots so far off that you can't
calibrate it properly unless you centre them and try again from the beginning. Double check any scaling jumpers on the signal conditioner. With
some signal conditioners it is possible to set them wrong, but be able to *almost* compensate for this by the amount of adjustment on the span pot. This situation is actually a lot more confusing when you see it in practice than it sounds.

**********************
Michael Griffin
London, Ont. Canada
[email protected]
**********************

 

>>"It will feed back acceptable readings until about 14 lbs. pressure.
After that it reads 14 lbs no matter what the actual pressure is.

Sand a harder material? <smile>

Mark

 

Look at the signal from the load cell on a scope. It probably has large amounts of noise on it. I had as similar task last year and ended up putting an active low pass filter (c/o freq 1Hz) between the LC and LC amplifier. Also averaged
about 250 sequentual samples taken by the A/D before displaying, taking any action.
neil

 

The load cell works fine when the sanding belt is not running, so, we believe the scaling, range, and output of the cell is pretty good. The
problem only occurs when the piece is placed against a moving sanding belt, and then only when larger forces are applied - still within the limits of the device.
We suspect vibration is causing the problem and are looking for non mechanical work arounds.
Regards,
Art Bourdeau,
[email protected]
Phone518)765-3667
Fax: (518)765-4033
Mobil: (518)573-4745
Fax518)765-4033
web: www.bcsco.com

 

Is it possible that the load cell is placed in a
position where friction when the belt is pressed
against the cylinder is producing enough heat that the load cell is locking up due to thermal conditions?

Davis Gentry
Controls Project Engineer
Research and Development
Carpenter Company

 

Art,

Have you looked at the output of the device on an oscilloscope under both conditions? If so, what did you see?

Tony Smith

Tony G. Smith Phone: (505) 844-8371
Sandia National Laboratories Fax: (505) 844-2925
Control Subsystems Dept 2338 e-mail: [email protected]
P.O. Box 5800
Albuquerque, NM 87185-0503

 

I'm not sure how the set-up is constructed, but as the sanding belt moves past the object is it putting enough side loading on the object so that it can't move up and down in whatever holder you are using? This would make the sanded object
"stuck" so that it couldn't move to change the reading on your instruments.

FWIW,

Ed

Speaking for me, not for Armstrong. . .


>>"It will feed back acceptable readings until about 14 lbs. pressure.
After that it reads 14 lbs no matter what the actual pressure is.

Sand a harder material? <smile>

Mark

 

As someone suggested, noise may be the real problem here and the signal goes completely out of range in the peaks.

The noise is probably mechanically inserted. The solving may need to involve one or more of the following things:

- Mecanical low pass filter, if possible, to stop some of the noice from entering the load cell. Mechanical low pass filtering is probably not possible but think about it anyway

- If the signal from the load cell itself is saturated someplace not too far from the actual maximum that have to be fixed.

- A first step of low pass filtering. (on a voltage signal, not a current signal. If necesary convert it twice). This step is optional and only necesary to ensure the next.

- Make sure the (remaining) peaks in the noice can be handled by the A/D converter

- Increase the sampling rate and insert a median filter.

- Add a low pass filter of high enough order.


All the steps are NOT necesary, pick some. I would start with checking the signal with a oscilloscope to se:
1. The actual noise level
2. If it seems symetrical or if it is truncated at the high peaks

My guess is: The high peaks is truncated making the average never reach above 14lbs and that you actually have enough low pass filtering already.


/Johan Bengtsson

----------------------------------------
P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------

 

That is one we didn't think of. We will check it out!
Regards,
Art Bourdeau,
[email protected]
Phone518)765-3667
Fax: (518)765-4033
Mobil: (518)573-4745
Fax518)765-4033
web: www.bcsco.com

----- Original Message -----
From: Davis Gentry <[email protected]>

> Is it possible that the load cell is placed in a
> position where friction when the belt is pressed
> against the cylinder is producing enough heat that the
> load cell is locking up due to thermal conditions?

 

Thanks Tony.
I am trying to get them to do just that as I am a distance from the devices and can't do it myself.
Regards,
Art Bourdeau,
[email protected]
Phone518)765-3667
Fax: (518)765-4033
Mobil: (518)573-4745
Fax518)765-4033
web: www.bcsco.com

----- Original Message -----
From: Smith, Tony G <[email protected]>

> Art,
>
> Have you looked at the output of the device on an oscilloscope under both conditions? If so, what did you see? <

 

You did a very nice job of summing it up and putting it all together....
Thanks.
Regards,
Art Bourdeau,
[email protected]
Phone518)765-3667
Fax: (518)765-4033
Mobil: (518)573-4745
Fax518)765-4033
web: www.bcsco.com

----- Original Message -----
> From: [email protected]
>
> As someone suggested, noise may be the real problem here and the signal goes completely out of range in the peaks.
>
> The noise is probably mechanically inserted. ...<clip>
.

 

Many S-shaped load cells have a built in mechanical stop to prevent damage...is
there any possibility that the load cell is hitting a mechanical stop with
increased pressure?

Bill Gornto

 

It seems that the system you are using has the measuring range upto 14 lbs. First check specification of the load cell. If the measuring range is more than that check span of the signal
conditioning unit. Also check scaling of the signal to your display unit. If your load cell can
measure above 14 lbs you can do it by adjusting the span and scaling. Hope this will help.

Regards.

Tanweer Ahmed
[email protected]

 

Art,

After following thread for a few days am surprised that no one has suggested that something is in "saturation", i.e. some transducer, or variable will work linearly up to a certain level, then becomes limited to that level no matter how much more the input is increased. Perhaps this is because you indicated that everything works fine before you apply device to the sanding belt. (i.e. you are able to apply a load in excess of "14 lbs pressure" and obtain the expected increase in reading). (You could check this in a test fixture or with sanding belt stopped).

Things to look for when saturation is reached:
Does the load cell have the desired range?,
Does the load cell instrument have the desired range?, Is the load cell excitation voltage correct? Does the applied force have the available range? Is there "Mechanical Saturation", e.g. an end stop on the applying mechanism or cylinder piston bottoming out?
Would check out all these obvious possibilities before looking for anything more esoteric.

Good Luck,

Tony Firth, Electrical Eng.,
Quester Technology Inc.,Fremont,CA

 

Tony,
I can't agree with your diagnosis. In my opinion, the problem is most
likely to be due to vibration causing a very noisy signal which is upsetting the
indicator/digitiser unit. That is assuming that the exitation, calibration etc have
been done correctly.

Neil J.


Tony Firth wrote:

> Art,
>
> After following thread for a few days am surprised that no one has suggested
> that something is in "saturation", i.e. some transducer, or variable will work
> linearly up to a certain level, then becomes limited to that level no matter how
> much more the input is increased. ...<clip>

 

Tony:
Sorry I didn't mention that the readings are fine when the belt is not rotating. The problem only occurs when the belt is rotating, contact is made, and pressure is above 14 #/in2.

--
Building Control Systems
[email protected]
Phone (518)765-3667
Fax (518)765-4033
Mobil (518)573-4745

 

I would like to thank the person from Texas that called me yesterday and shared his grinding and load cell experiences. I am re - iterating our conversation for the benefit of the group. I think you have a very valuable point. That is, if the system is vibrating to the point to where the load cell is unable to transmit a readable signal, the "grind" probably isn't being done very well either.
Other points brought up were:
Angle of the grind.
Use of a shock absorber similar to an automotive one.
I am sorry I didn't get a chance to take down your name. Apparently, you were on a cell phone and we got cut off.
THANKS whoever you are.
Regards,
Art Bourdeau,
[email protected]
Phone518)765-3667
Fax: (518)765-4033
Mobil: (518)573-4745
Fax518)765-4033
web: www.bcsco.com