Model SR755KG Series Wheelchair Scale
Operating and Service Manual - S/N 1913+
Part No.: MAN755KG_100929
Page 10 of 16
S
Instruments, Inc.
,
600 Young Street, Tonawanda, NY 14150
Tel: 716-693-5977 Fax: 716-693-5854 URL:
www.srscales.com
email:
Copyright 2010
S
Instruments, Inc.
-gauge force cells convert the force
of an a
ig
convert
signal by an analog to digital converter. The digital signal is transferred to a
tr
crystal
ration. These bridges convert the ph
nt of the force cell, due to the applied mass
n the system, into m ute changes in elec
. These changes in resistance produce a
oltage difference acro
tional amplifier. The
mplifier is configured
s serving to adjust the
nsitivity (voltage out per unit of weight applied) of each bridge. The offset potentiometer produces a
tput of the amplifier for an unloaded system.
egrating
discharged at a rate proportional to the reference voltage applied to the converter.
ge on the integrating capacitor is then multiplied by a factor and again discharged at
hich is proportional to the applied load on the force
continuous
eigh and, for AutoHold, the micro-controller averages the data before locking in on the reading. If
e data variance is greater t
ontroller will reset the filter
and start a new averaging pe
-
e the system can be re-calibrated. In
t
calibration factor is then stored in the non-volatile
THEORY OF OPERATION
SR Instruments patient weighing systems are digital scales. Strain
pplied we ht into an analog signal. This signal is amplified by an operational amplifier and
ed to a digital
micro-con oller where it is filtered, converted to appropriate units, and displayed on a liquid
display.
Strain-gauge force cells each contain four strain gauges mounted in a full Wheatstone-bridge
configu
ysical moveme
trical resistance
o
in
v
ss the Wheatstone-bridge, which is amplified by the opera
to current sum the output of each cell, with potentiometer
a
se
small current, which nulls the ou
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 int
capacitor is then
The residual volta
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, w
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, then displays the result
on the liquid crystal display. The micro-controller performs a rolling average of data for
w
th
han 0.1% in the AutoHold mode, the micro-c
riod.
The micro controller can be placed in a calibration mode, wher
the calibra ion mode, the result of the weigh operation is scaled to match the value by adjusting the
“up” and “down” calibration buttons. This new
memory.
