IO-LC1 zero drift despite auto-zero

[BoB_S]

Hi All,

The loadcell has been calibrated with 4 points (5, 20,50,100 grams) , zero acquired, calibration saved.

Motion band has been set (LC Motion Band = 2)

The auto-zero-tracking is set to pretty tight definitions: LC time = 1, LC tracking band = 1, tracking range = 1, and activated.

Everything works fine for some time - the loadcell output is pretty steady on zero, some minor fluctuations here and there but it seems that the zero correction fires every now and then to correct.

However, in a matter of hours, zero begins to drift ( up to some 18 grams), which makes the whole application null and void. Same happens in both excitation modes, btw - in DC mode zero drifts up (+), in AC mode zero drifts down (-).

Theres no litter buildup on the platform, no significant temperature fluctuation around the loadcell, not much mechanical movement - the construction is quite solid.

What am I missing here ?

[AlexUT]

Hi,

You have to calibrate 0 as well.

If you continue to have problem - send request to support@unitronics.com

B.R.

[BoB_S]

Hi,

The Help file (under Calibration - Calibrating Points clause) states that zero doesn't have to be calibrated.

Nevertheless, I have tried different calibration settings, one of the points was 0 as well - the result is the same.

Additional suggestions before turning to support ?

- Thanks.

[Walkerok]

I guess my question is on the laod cell hardware. Not knowing what it is. Is there any possibility of drift due to a mechanical reason or an expansion and contraction issue due to materials changing temperature over your time frame?

I use the load cell modules myself on several things and I have never experienced waht you are talking about. except when there was a mechanical reason. In my case I am working with large metal containers and thee is a hysteresis when a container is pressed down on and what level it retunes to. But the effect is completely mechanical in nature and not electronic.

For you to have an electronic drift I would think that is an indication of an moving resistance in the strain gauge curcuit, while possible I suppose I do not have a good enough understanding of how that mechanics would actually work and how to prevent it.

I will say that in my calibrations my first point is always the zero weight position. I do this because the zero position is not the load cell with nothing on it. It is the strain gauge with the measuring bins installed so the zero I want is actually the strain gauge with a few hundred pounds of metal bin weight on it.

Keith

[Gabriel Franco]

I suggest you to use a loadcell simulator to narrow the problem. Perhaps a loadcell drifted with a no-steady behaviour

[BoB_S]

Hi Keith (Walkerok),

The terperature changes around the device are insignificant.

The first point of calibration is zero in my case too (with the platform, of course).

If the nature of this drift is indeed mechanical (which is plausible), doesn't the auto zero track FB supposed to counter exactly that ("hysteresis when a container is pressed down on and what level it retunes to") ?

Anyway, I'll try to strip the system to the bare loadcell and see what I get.

[BoB_S]

Well...

Yesterday I left a bare loadcell, calibrated 4 points (0,6,12,24 grams), saved saved the calibration, set the motion band, turned on the auto-zero track.

As before, the "system" was stable on 0 for a while, today I find it showing a -7 grams offset. Nobody has touched the loadcell, so there's no hysteresis issue here. Temperature around the loadcell was constant, around 22 C, give or take 1 degree.

Maybe the auto-zero doesn't work the way I think it is supposed to ?

This is quite frustrating....

[Joe Tauser]

Are you using AC or DC excitation? Do you have another load cell to try?

You're to the point where you need to look at the actual electrical signals to determine whether the problem is in the loadcell or the LC-1. See if you can get your hands on a 5 1/2 digit meter to read the excitation voltage and the signal voltage at the zero point. You'll need something with at least 0.01 mV resolution. Then read it again after the offset shows up.

Without actual electrical input readings to the LC-1 you're just guessing on this kind of problem.

Joe T.

[Damian]

I wonder if the auto-zero function is actually causing the drift. I would be curious to see you perform the same experiment without it. This would at least rule out your load cell, which is very unlikely to be the cause of the issue anyhow.

I have no idea what the algorithm for the zero-tracking function looks like under the hood. But I would expect that it relies on the peaks and valleys you would get from cyclical readings to determine the hypothetical zero point. So having this feature enabled in a static situation might confuse it. And that might be exacerbated if you have any filtering / rounding activated as well.

One of the biggest challenges load cell amp designers struggle with is temperature stability of the electronics. Many recommend that the load cell amp is powered up and runnning a while to stabilize in temperature prior to taking readings.

I also notice that in the examples, the zero-tracking function is interlocked with the "LC function is progress" bits. Did you do this as well?

Also, are you you executing this function with a one-shot, or do you have it executing every scan? I could see where executing it every scan would also have the effect you are experiencing.

Here is some verbage in the HELP on the zero-tracking.

Auto Zero Tracking

When activated, Auto-Zero Tracking zeros the gross weight according to the conditions you set, enabling the module to automatically compensate for small variations at the zero point, such as those caused by a buildup of litter on the scale platform, or by temperature fluctuations near the scale.

Before Auto-zero Tracking can zero the scale:

The Motion Band FB must be included in the application.

The Motion Band must be active.

The In-Motion bit, Bit 0 of the MI linked to LC Hardware Status Messages in Hardware Configuration, must already have turned OFF, indicating that the scale is steady.

Once these conditions have been met, Auto-zero zeros the gross weight.

Notes ♦

Once Auto-Zero tracking is activated, it stays active until the function is suspended by the application.

♦

Auto zero tracking will not function in uV/V representation mode.

[BoB_S]

Damian, I'm executing the auto-zero with a one-shot, it is interlocked with function is progress bits, not using filtering/rounding.

As for having auto-zero enabled in a static situation - isn't it supposed to work either way ("Once Auto-Zero tracking is activated, it stays active until the function is suspended by the application") ?. Nevertheless, I have recalibrated the system (reminder - currently it is a BARE loadcell), saved calibration, acquired zero, saved calibration again and haven't enabled the auto-zero - shall see in a few hours if we get that dreaded drift again.

Joe, I'm using AC exitation mode. The measured voltages were the same for 12g offset (drift) and fresh calibrated zero : E-toE+ = 4.912Vac, SG-toSG+ = 2.7mVac. As per the loadcell (Zemic L6N class C3) datasheet, the recommended exitation voltage is 5-12V. I haven't checked the wiring (done by another person), but I assume it is correct since the loadcell operates fine except for the drift.

[denogoro]

Hi,
I work program for the mixer of powder adhesives and mortars, capacity is 2 tons, complete mixer is set to weight load cell, the mixer is filled from five silos, scales I set the two points, I set the zero and tare, program works very well .PLC (v570)more I have not installed, all functions of trying on the desk and scales I adjusted in grams to be easier to work with simulations of charging and discharging.
So I came to a problem because workers empty mixer in 25 kg bags.

In the first case: Mixer can not throw out the entire material, remains undetermined amount of material of less than 25 kg.
in the latter case: In a mixer remained quantities of less than 25 kg (23,16,18,5 kg) which is insufficient for filling bags.
I'm interested in, in any way solve this problem, because the residual quantity in the mixer affects the recipe of the product.
How do automatic "tara" after every discharge mixer. I had some ideas that were not correct. The requirement for "start" I chose to value the balance is enough for about zero
I ask for a little help

[HigHTech]

On 2/10/2012 at 2:07 AM, BoB_S said:

Damian, I'm executing the auto-zero with a one-shot, it is interlocked with function is progress bits, not using filtering/rounding.

As for having auto-zero enabled in a static situation - isn't it supposed to work either way ("Once Auto-Zero tracking is activated, it stays active until the function is suspended by the application") ?. Nevertheless, I have recalibrated the system (reminder - currently it is a BARE loadcell), saved calibration, acquired zero, saved calibration again and haven't enabled the auto-zero - shall see in a few hours if we get that dreaded drift again.

Joe, I'm using AC exitation mode. The measured voltages were the same for 12g offset (drift) and fresh calibrated zero : E-toE+ = 4.912Vac, SG-toSG+ = 2.7mVac. As per the loadcell (Zemic L6N class C3) datasheet, the recommended exitation voltage is 5-12V. I haven't checked the wiring (done by another person), but I assume it is correct since the loadcell operates fine except for the drift.

Can you check the temperature of the IO-LC module. I had a similar problem a year ago. The IO-LC module I was using was very sensible to temperature, so after a couple of hours of operation the drift started. I change the module and the new one works ok. So in my case the IO-LC had some problem.