Model SR7020
i
Wall Mount Platform Scale
Operating and Service Manual
Part No.MAN7020I_MICROBAN_200520
Page 11 of 16
S
Instruments, Inc.
,
600 Young Street, Tonawanda, NY 14150
Tel: 716-693-5977 Fax: 716-693-5854 URL:
email:
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 the
micro-controller performs checks for signal stability 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 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.