Scale weight craziness!

Hello all!

I have an interesting situation that I have never seen before. I have four load cells on a "table." These are in turn connected to a summons box which relays the readings to a Hardy Instruments 2110 Weight Controller. The weight in turn is displayed onto an Allen Bradley HMI with input controls which an operator can adjust. I have a set of test weights that read spot on the money every time I put them on the table. The Hardy Scale reads them correctly and the correct weight is displayed on the AB PLC. When a container is inserted onto the table, everything reads the correct weights and the correct tare weights are established on the AB PLC. I am telling the PLC to load this container up to 3900 lbs. It says it is loading 3900 lbs. The HMI displays that it achieves target weight. The container is shuttled off and transported to an inspection station which has a scale on it with load cells as well. Test weights confirm that those load cells and the scale they are attached to are good. Container with 3900 lbs only has 2800 lbs in it. I have checked load cells, the summons box, the hardy scale and perused through the RSLogix to no avail. I think the problem may lie in the logix but I am completely stumped. Any ideas?

What is the value of test weights? Details on load cells.

Test weights are certified at 4267 lbs. Hardy Instruments load cells rated at 5k each.

check if calculation is correct especially with unsigned integers etc.

Why not just watch the machine while it fills a box? If it moves to the next station before the Hardy controller says its full, the problem is in the PLC.

You said that everything checks out good with your test weights, But then you mention a Tare. Are you measuring Gross on one scale and Tare on the other?

/agree with jawolthuis -
My first thought is that you are mis-applying the tare value, or not actually applying it to get the true product weight.

Are you taking the tare as soon as the empty container is loaded on the scale and stable? That is the point that your basis weight should be 0. If you can check the scale weight when the container moves off, its product weight should effectively be negative (by the weight of the empty container).

This seams a bit odd. Does your container already have material in it? An 1100 lb tare seams a bit much. What is the actual weight of your container?

Are you using an analogue signal from the indicator to the plc
You may not be seeing the tared value

we had a similar issue last year and this is what i did.

inspect the load cells for correct alignment.
inspect the load cells for inteference
inspect the table for interference.
inspect the test weight lowering mechanism for interference and clearance.

results.

load cell had an external weight applying 100 lbs on it. weld joint broke on a delivery tube.

button load cells were at an angle and causing incorrect weights
weight hopper had shifted causing incorrect load.

test weight support cable had stretched causing test weight to apply load.

we now inspect the systems monthly.

since this is a 4 cell table, place 500 -1000 lbs on each corner. if they read correctly, then your problem may be with the tare weight.

if the plc program has been running for a while, why do you think it is in the plc program?

as previously suggested, tare weight or container not being empty could be part of the problem.

load cells do go bad and will cause this problem as well.

regards,
james

While I don't know anything about what type of load cells you are using. I have had problems in the past with adding weight too slowly. The load cell will compress with the weight but it seems that the change is happening so slowly that it does not register.

What I was seeing was I would apply the force, read the force applied, and when I release would show a negative weight. I learned if I applied the force more quickly the issue went away.

My two cents

frorge said:

While I don't know anything about what type of load cells you are using. I have had problems in the past with adding weight too slowly. The load cell will compress with the weight but it seems that the change is happening so slowly that it does not register.

What I was seeing was I would apply the force, read the force applied, and when I release would show a negative weight. I learned if I applied the force more quickly the issue went away.

My two cents

Click to expand...

There is a setting in some scale heads that "auto zeroes" the scale. This is used sometimes on an outdoor truck scale so that when it rains or snows the scale will automatically make up for it.

That would be something for the OP to check.

I think the problem may lie in the logix but I am completely stumped.

Click to expand...

If the PLC program has operated correctly for a long period, it is probably NOT the PLC logic or PLC program.

The Hardy Scale is doing all the weighing anyway, and the PLC is simply being passed the weights and then moves the container and resets at the right time. The Hardy may be malfunctioning, or has lost its calibration. Someone could have tared a non-empty container with about 1100 lbs in it. That would cause the problem, except you say it is tared for every new container. Most of those scale controllers that I have used have an elaborate calibration procedure with scaling equations that depend on the load cells. Someone could have changed the scale factors. That has to be unlikely but there are tamperers in every plant. Another good possibility is that one load cell is bad or malfunctioning.

Hello all and thanks for the many replies and suggestions. After a few days of scratching my head and the help of a trusted maintenance friend, we have solved this problem. It appears that another maintenance technician had performed a calibration of the scales on the station on the days the problem started. He inadverdently fumbled a step on the hard calibration. Looks like instead of using the SPAN function on the Hardy, he accidentally entered the test weight value into the Linear Correction. So instead of the Linear Correction value being set at 0 where it normally is, it was set at 4267. My trusted friend caught this right away as one of the first things he did was to run the self test on the Hardy itself. I did not do this, and did not realize it had one as I am not a maintenance technician, merely someone who likes to learn and can't sit still. So sorry to waste anyone's time. I learned a lot from all of your suggestions and if nothing else, I got some folks' brains to ticking. Hope I can contribute something constructive one day.

don't be sorry mate - it might not have been so easy to fix.
We are here to help no problems
Thanks for saying thanks