Model SR775L
Service Manual - S/N 1035+
Part No. MAN775L_100929
Page 11 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.
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. The output of the Wheatstone-bridge is digitized by
a sigma-delta analog to digital converter. The data is transferred to the micro-controller.
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 then micro-controller averages the data before locking in on the reading. The
micro-controller can be placed in a calibration mode, where the system can be re-calibrated. In the
calibration mode, the calibration, local gravitational constant, and duration of “on” time can be
adjusted. The new factors are stored in non-volatile memory.
