Model SRV712W Marine Scale System
Service Manual
Part No. MAN712W_170816
Page 10 of 14
S
Instruments, Inc.
,
600 Young Street, Tonawanda, NY 14150
Tel: 716-693-5977 Fax: 716-693-5854 URL:
email:
Copyright 2017
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 Whetstone-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 output of the operational amplifier is digitized by the analog to digital converter.
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 continuous
weigh.
