Cleveland Motion Controls ULTRA ISC CANTILEVER TRANSDUCER CLTSCM REV AA - Manual (Page 35)

MAN-70445-0 R EV AA U LTRA S ERIES ISC C ANTILEVER T RANSDUCER T ECHNICAL M ANUAL
 Passing a cord over a roll on its way to the transducer inevitably causes some amount of friction.
The worst case scenario involves passing the working part of a cord over a roll that doesn’t 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!
 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.
 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.
4.14 F INAL C ALIBRATION
Even though the ISC transducer is factory calibrated, calibration is still usually required to accommodate
application variables such as wrap angle and transducer orientation. The customer then accesses these
software parameters using an HMI, keypad, or other device. The transducer is still calibrated using weights
as is done currently.
Final calibration is always generally required to take in to account particular application circumstances of
roll weight, wrap angle and load cell orientation. The final zero and span calibration is most easily
accomplished by numerical scaling within the digital processor of the tension controller. Because each ISC
equipped tension transducer has been factory adjusted for uniform transfer function, load cell inter-
changeability when calibrated this way is readily promoted.
Field calibration still consists of applying known forces in the web, but instead of adjusting a
potentiometer, the observed load cell offset is determined at the controller and then numerically subtracted
from the reported load cell signal.
In the case of Span calibration, a known web force (equal to a moderate portion of the expected operational
force) is applied using test cords routed through the expected web path past the transducer pair. An
appropriate numerical scaling factor is then determined and then used as a scaling factor within the tension
controller.
Hint
An interesting approach to slightly attenuating the load cell output by purely mechanical means can be used
with those load cell styles which accommodate slight rotation relative to the bisector of the wrap angle (i.e.
Cartridge transducers and Slim Cells).
Consider taking advantage of the Cosine relationship (noting that the first 10 degree mechanical shift will
reduce the output span by 1.5%). The deflection of CMC’s strain gage based loadcell is sufficiently small
that tracking will not usually be adversely affected.
27