Model SR725
i
Wheelchair Scale
Operating and Service Manual
Part No. MAN725
i
_MICROBAN_200603
Page 17 of 24
S
Instruments, Inc.
,
600 Young Street, Tonawanda, NY 14150
Tel: 716-693-5977 Fax: 716-693-5854 URL:
email:
Copyright 2020
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 on-chip analog to digital converter. The digital
signal is filtered, converted to appropriate units, and displayed on the 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, with potentiometers
serving to normalize the sensitivity (voltage out per unit of weight applied) of each bridge. The
offset potentiometer produces a small current, which nulls the output of the amplifier for an
unloaded system.
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 signal from the amplifier.
The micro-controller filters the digital output of the analog to digital converter, subtracts the value
saved during the system zero operation and scales the filtered output, and 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.