Model FG1235 Scale Kit
Rev 021210
Figure 3. Analog to digital converter timing diagram (from Maxim MAX132 data sheet).
Figure 2. Force Cell bending with applied weight (displacement exaggerated for clarity)
SECTION I
FG1235 Theory of Operation
1.3
The FG1235 patient weighing system is a digital weigh scale. 4 strain-gauge force cells convert the force of a patient’s weight into an
analog signal. This signal is amplified by a chopper-stabilized operational amplifier and converted to a digital signal by a quad slope 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 dot-
matrix liquid crystal display.
The transducer of the FG1235 patient weighing system contains 4 strain gauge based force
cells (Analog board J1..4). Each cell contains four strain gauges mounted in a full Whetstone-bridge
configuration (Figure 1.) These bridges convert the physical bending of the force cell (Figure 2.), due to
the patient’s 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 chopper-
stabilized operational amplifier (Analog board U1). 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 (Analog board R3) 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 quad slope analog to digital converter (Analog
board U5). The converter integrates the analog signal onto the integrating capacitor (Analog board C9) 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 (Analog board C9) is then multiplied by a factor of eight and again discharged at a rate proportional to the reference voltage (Figure
3.). The residual voltage from this discharge is again multiplied by
eight and again discharged. The time taken to discharge the capacitor
is proportional to the voltage from the operational amplifier, which is
proportional to the weight 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 tare
operation and scales the filtered output to the appropriate units (Kg or Lb.) then displays the result on the dot-matrix liquid crystal display. The
micro-controller performs a rolling average of two
seconds of data for continuous weigh and, for AutoHold,
the micro-controller averages two seconds of data,
minimum, 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 entered by the decimal keypad.
This new calibration factor is then stored in the non-volatile
memory.
Page 2
