Model: SR630
i
-KG Pediatric Scale
Operating and Service Manual
Part No.: MAN630
i
-KG_200915
Page 14 of 20
S
Instruments, Inc.
,
600 Young Street, Tonawanda, NY 14150
Tel: 716-693-5977 Fax: 716-693-5854 URL:
email:
S
Instruments, Inc.
THEORY OF OPERATION
SR Instruments patient weighing systems are digital scales. Strain-gauge force cells convert the force
of an applied weight into an analog signal. This signal is amplified by an operational amplifier and
converted to a digital signal by an analog to digital converter. The digital signal is transferred to a
micro-controller where it is filtered, converted to appropriate units and displayed on a liquid crystal
display.
Strain-gauge force cells each contain four strain gauges mounted in a full Wheatstone-bridge
configuration. These bridges convert the physical movement of the force cell, due to the applied mass
on the system, into minute changes in electrical resistance. These changes in resistance produce a
voltage difference across the Wheatstone-bridge, which is amplified by the operational amplifier. The
amplifier is configured to current sum the output of each cell.
The output of the operational amplifier is digitized by the analog to digital converter. The sigma-delta
converter sums a rapid sequence of 0’s (0 volts) and 1’s (reference voltage) to achieve balance with the
input from the amplifier.
The micro-controller averages and filters the digital output of the analog to digital converter, subtracts
the value saved during the system zero operation and scales the filtered output, then displays the result
on the liquid crystal display. The micro-controller performs a moving-median filter of data for
continuous weigh and, for AutoHold, the micro-controller performs checks for signal stability before
locking in on the reading. If the data variance is greater than 0.1% in the AutoHold mode, the micro-
controller will reset the filter and start a new filtering period.
The micro-controller can be placed in a calibration mode, where the system can be re-calibrated. In
the calibration mode, the system slope is calculated from two points (zero and full scale) in the 2-point
calibration mode, or the slope and change in slope is calculated from three points (zero, half and full
scale) in the 3-point calibration mode.
