MTS SWIFT 20 Ultra Sensor - Manual (Page 29)

Calibration
SWIFT 20 Sensors Hardware Overview 29
Calibration
Important The following sections include information related to the Low-
Profile Transducer Interface (TI). For SWIFT transducers designed
to operate with the newer Mini TI, there is a separate manual that
documents the Mini TI software utilities (MTS part number
100214316).
Each transducer is calibrated by MTS before shipment. The transducer and Low-
Profile TI may be returned to MTS for repair and recalibration as required.
Calibration is performed at MTS on a special fixture that is capable of applying
multiple loads to the transducer. During calibration, raw signals are measured.
The calibration gains and cross talk compensation values are computed from this
raw data. These gains are recorded in a calibration file.
A unique calibration file is supplied for each transducer. The serial number of the
Low-Profile TI associated with the transducer is listed at the top of the
calibration file. A label with the serial number of the Low-Profile TI box (and the
SWIFT sensor with which it was originally calibrated) is located at the back of
each Low-Profile TI box.
The calibration file is loaded into the Low-Profile TI non-volatile RAM by MTS
before the transducer is shipped. A copy of the file is also provided on a diskette.
MTS verifies the calibration by applying loads to the transducer, measuring the
main outputs and checking for accuracy. Final calibration reports are provided
with each transducer.
Shunt calibration
At the end of the calibration process, a shunt calibration is performed. During a
shunt calibration, a resistance is introduced into the bridge circuit. The difference
between the shunted and unshunted voltage is the delta shunt reference value for
each bridge. That value is saved in the calibration file, which is downloaded from
a PC or laptop computer and stored in non-volatile memory in the Low-Profile
TI.
At any time afterward, pressing the Shunt button on the front of the Low-Profile
TI causes each of the strain gage bridges to be shunted in sequence, and the
measured shunt voltage (delta shunt measured value) is compared to the
reference value.
An acceptable tolerance range is also loaded into the Low-Profile TI memory
during system calibration. One tolerance value is used for all bridges. This value
is loaded as a percentage of allowable deviation from the delta shunt values. For
example, if the FX1 bridge has a shunt delta reference value of –3.93, and the
tolerance is set at 2 (percent), the acceptable range for the measured value would
be –3.85 to –4.01.