Calibration Standard of Load Indicating Device

D

dineshquality

#1
Hello Friends !!

I have an automatic machine on which the load is displayed in KN whenever a part is assembled. There is no load sensor / load cell in the machine. My maintenance personnel says it is converting the resistance / current used by the servo drive and displaying it as load.

My question is that is there any international standard available on method of calibration of such load indicators?

I wanted it to be calibrated for the entire range viz. 0- 35 KN (The process requirement is 1KN, 6-8 KN & 35 KN for different operations). But my maintenance personnel is currently calibrating only at 10KN and he says, since it is converting the current to load, the error will be the same whichever be the load range. Hence calibrating at only one load point is sufficient.

This is a recent NC in ISO audit. (as usual:D:D) my ISO auditor is not agreeing to what we currently do !! :(

Can anyone please help..
 
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BradM

Staff member
Admin
#2
Hello there!

Do you have a model number or any information on the load measuring device? The operation method described by your maintenance personnel sounds a bit simplistic. There is some sensor/membrane/mechanism/etc. which senses the weight and translates it to a signal. While the signal to the indicator may indeed be linear and predictable, that doesn't mean the measurement sensor cannot gain error; especially towards it's maximum measurement range.

Unless there are good studies/data to support the notion that the error at, say, 20% of scale will always be the same as the error at 80% of scale, then I would suggest the load indicating system needs to be verified across the range of use.
 

ncwalker

Trusted Information Resource
#3
OK, just putting on my physics hat here ...

The maintenance guys description is that as the "works" of the device have to work harder, the device draws more current. And we can thus use this current as a measure of load.

This is a true statement.

The question is not the validity of making the measurement, but rather the linearity of the calibration. In other words, if you calibrate at 10 kN and get a repeatable system, can you then assume the same "factor" that converts current to kN is indeed the same with 5 kN of load or 50 kN of load.

The physics of the situation is tricky. (Especially because I have no idea of the device). But not only will the load from the "product" increase the load, but EVERY frictional contact involved will get stiffer as well - frictional force is proportional to normal force applied. This relationship is ALSO linear. Also - the frictional effects of the electrons in the wire are different under different loads.

What is unclear is ... is the SUM of all these effects linear. What I'm saying is - there's enough real world noise in the system that I (personally) would be reluctant to say "I expect the system to be linear."

So - you are faced with these choices:
1) Burn a lot of first principles engineering time to theoretically prove the maintenance guys theory.
2) Conduct a relatively simple test and demonstrate that the maintenance guys theory is correct.
3) Do nothing and hope everything is OK.

I know which one I would pick.
 
K

krayworth

#4
If the machine is not equipped with a load cell, it must be equipped with another sensor that can measure loads. If it's a hydraulic frame, the sensor is most often a pressure transducer. Measurement sensors do not have to be calibrated to the full capacity (so a 35kN sensor can be calibrated to 10kN only). However, then the sensor would lose its ability to read loads higher than 10kN. If the tests that you're running are lower capacity tests, you may choose to calibrate the sensor at a lower capacity. Calibration procedures of load sensors are usually performed by ASTM E4 which gives +/- 1% accuracy down to 0.5% of the full capacity, unless otherwise is specified.
 

Marc

Hunkered Down for the Duration
Staff member
Admin
#5
Wikipedia reference-linkPressure_sensors

... I have an automatic machine on which the load is displayed in KN whenever a part is assembled. ...
The real question is - Is the measurement critical to the operation. If it is, it should be on the control plan and calibrated. If it's not, it won't (or shouldn't) be. If it's not critical (and in this case it isn't an "acceptance" measurement) calibration isn't required.
 
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