Cleveland Motion Controls Classic Series DIN Rail Amplifier with Isolated Output MWI-13467 REV CA - Manual (Page 23)

MAN-13467 R EV CA C LASSIC S ERIES A MPLIFIER W / I SOLATED O UTPUTS
Page 23 of 30
3.6 A CCURACY C ONSIDERATIONS
The application of an accurate calibration force requires careful attention to minimizing the non-ideal affects of the
real world. Keep the following points in mind:
•
Allow the transducer and amplifier to reach thermal equilibrium before conducting calibration.
Ideally, the temperature should reflect the expected operating conditions.
•
The test force should be a moderate percentage of the intended working force of the transducer,
and never over 100% of the transducer’s Maximum Working Force (MWF) or, you risk
calibrating with an overloaded (“clipped”) transducer signal.
•
Cycle the load on the transducer a number of times with the test force to pre-condition or “set” the
transducer prior to calibration. Repeat this procedure again before calibrating if the transducer has
been disturbed (i.e. bolts re-torqued).
•
With very low force transducers consider that connecting a test mass involves some finite cord
mass.
•
When all else is done correctly, the largest remaining contributor to error is friction. If friction
cannot be reduced, consider determining the magnitude of the friction through measurement and
then making first order corrections numerically.
•
Passing a cord over a roll on its way to the transducer will cause some amount of friction. The
worst case scenario is in passing the working part of a cord over a roll that does not readily
freewheel. A test was conducted to determine the loss on a stationary 4” diameter anodized roll
with a 90 degree wrap angle. It exhibited a 25 to 30% loss in force due to friction!
•
Always apply and remove the test load in a continuously increasing or decreasing manner, so that
the force changes are monotonic. This helps to avoid disturbing any hysteresis component of the
transducers force signal.
•
When calibrating for a particularly wide roll that will always have a narrower product tracking to
one side, consider applying the calibration force at the roll position that represents the center of the
product. This will automatically cancel some of the affects of transducer gain imbalance without
the need to actually re-balance the transducers gains within the amplifier.
•
It is a good practice to verify linear operation of the transducer and amplifier by applying a final
test force that falls somewhere between the zero and full-scale endpoints. The intent is not to
conduct calibration, but to confirm the hardware’s ability to accurately report a measured force.
When conducting a calibration that involves a large mass, it is often practical to use a series of smaller masses
added in succession. Consider performing an initial Zero and Gain adjustment when the first 20% of the
weights have been applied. By performing the calibration using this method, the Zero and Gain adjustments
can be made approximately correct earlier in the calibration effort (before many weights have been handled).
When the full calibration load is applied, there is a better chance that only minor adjustments will be needed.