OpenScale drift

Sensors Force & Weight
1

I’m using OpenScale to read a large 6’ x 4’ commercial 5k-lb scale platform that uses 4 load-sensors(bridged), and generally it has worked well. During Pi-controller software-dev, I use a bathroom scale. On the bathroom scale the readings are rock-solid. After TARE, 0 is always quite close to 0. However, when I move to the commercial scale, after TARE, the 0-pt will slowly drift, and after several hours may be 2-3 lbs off 0. After a day or two, as much as 4-5 lbs.

NOTES:

On the bathroom scale, the sense-wires are only about 4-5 inches, whereas the commercial scale includes a high quality double-braided-shield 15ft cable, where I’ve cut the DIN-connector off and wired straight to OpenScale screw-terminals. The scale makes available its own controller/LCD-display (where the DIN normally connects), but I’m not using it since I want the Pi to drive a large LCD-display monitor.

When the 0-pt drifts, the measured weight drifts the exact same amount - if 0 is now reading -4, a 100lb-wt now reads 96lb.

THINGS I’VE TRIED:

I have the cable’s shield wire connected to OpenScale’s shield terminal, and have tried with and without the shield connected to OpenScale (no diff).

I’ve earth-grounded the big scale platform (no diff).

To experiment with short wires, I stuffed the OpenScale inside the scale platform, and used 4-5" sense-wires, and ran a 15’ USB cable to the Pi-controller (no diff).

Appreciate any thoughts!!!

-mark

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2

Hi Mark.

Is the temperature always the same when you’re taking measurements? A change in temperature could cause the zero point to move. You could write code that deals with change in temperature, but it might be easier to just tare the scale from time to time to reset the zero point if it happens to have moved.

3

It’s normal for a large pallet scale to drift off of zero a few pounds. Most scales and balances have a function that is called different things among various manufacturers but it’s usually some variation on ‘automatic zero tracking.’ When enabled, it keeps a zero reading when the weight changes a small amount over time. Of course, the assumption is that you’re not weighing a one pound object on the 5000 lb platform. The classic scenario is the forklift operator (or whatever) that has to leave the truck to zero/tare 2 pounds every morning.

It’s better described here:

http://www.iscscale.com/blog/weighing-s … gy-part-5/