Model SR555 Stand-On Scale
Operating and Service Manual - S/N 1800+
Part No. MAN555_060829
Page 12 of 18
S
Instruments, Inc.
,
600 Young Street, Tonawanda, NY 14150
Tel: 716-693-5977
Fax: 716-693-5854
URL:
www.srscales.com
email:
Copyright 2006
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, with potentiometers serving to adjust 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 converter
integrates the analog signal onto the integrating capacitor over a short interval.
The integrating
capacitor is then discharged at a rate proportional to the reference voltage applied to the converter.
The residual voltage on the integrating capacitor is then multiplied by a factor and again discharged at
a rate proportional to the reference voltage.
The residual voltage from this discharge is again
multiplied by a factor and again discharged. The time taken to discharge the capacitor is proportional
to the voltage from the operational amplifier, which is proportional to the applied load on the force
cells. The time is stored as a binary number in the analog to digital converter and is transferred to the
micro-controller when the conversion is complete.
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, and then displays the
result on the liquid crystal display.
The micro-controller performs a rolling average of data for
continuous weigh and, for AutoHold, the micro-controller averages the data 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 averaging period.
The micro-controller can be placed in a calibration mode, where the system can be re-calibrated. In
the calibration mode, the result of the weigh operation is scaled to match the value by adjusting the
“up” and “down” calibration buttons. This new calibration factor is then stored in the non-volatile
memory.
