Olympic 50 - Theory of Operation; General Overview

Theory of Operation
General Overview
Fig. 4-12 is a simplified block diagram tht illustrates the
essential components of an electronic scale. A weight
sensing device translates a force into an electrical signal.
This analog signal is amplified and translated into a digital
signal for a microcomputer (jC). The uC takes this raw
weight data (usually binary) and performs manipulations
to present it in readable form on a display.
Load cells are used as the weight sensing devices in the
Olympic Pediatric Scale. A load cell contains four strain
gauges connected to form a Wheatstone Bridge (Fig. 4-13).
If an excitation voltage is applied in two opposite corners
of the bridge, a small differential signal occurs at the other
two corners. This signal will be proportional to the applied
weight. The signal is also proportional to the excitation
voltage.
The Pediatric Scale employs “ratiometric” operation. Ifa
small change in excitation voltage occurs causing a signal
change, the gain and zero circuits change in the opposite
direction to counteract the original change.
Pediatric Scale Overview
Refer to the block diagram in Fig. 4-14. The Scale has two
printed circuit boards: the Main Board and the Display
Card. Components are mounted on the chassis. The front
panel has five membrane switches.
Power Supply
Line power is connected through a fuse and RFI filter to the
primary of the power transformer. The secondary AC is
rectified and filtered on the Main Board. The DC is
regulated by a +12 V regulator which provides all current
to the Scale.
The +12 V from the regulator supplies the charging circuit
for the nickel-cadmium batteries. It also provides a signal
indicating to the computer that alternating current (AC) is
present. The +12 Volts are also applied to the on/off latch
circuit. The On/Off membrane switch on the front panel
controls the on/off latching circuit.
Each time the On/Off switch is activated, this circuit
alternately turns a reed relay on and off which controls DC
power to the Scale. The Recall switch can also turn the
Scale on but it cannot turn the latch off.
The power supply has a +5 V regulator for digital power
and another +5 V regulator for analog power. A switching
regulator provides negative voltage to an op-amp which
provides -5 V analog voltage which tracks the +5 V analog
voltage. Separate digital and analog grounds are used.
Signal Flow
Refer to the Block Diagram, Fig. 4-14. The weight on the
platform is sensed by the load cells. The signal from the
load cells is amplified by a chopper stabilized instrumen-
tation amplifier.
The zero control provides ratiometric zero offset to cancel
the tray weight and load cell offset, and provide DC offset
for the A/D converter. Either of two filters can be selected
by the pc.
Firsta light filter is used because it allows rapid acquisition
of the weight data. The nC switches to the heavier filter
when in the Averaging mode. (The pC also performs
software averaging to get the best possible approximation
of the weight.)
The span (gain) control is also ratiometric and controls the
gain of the A/D.
The A/D provides binary data to the pC. An external
EPROM is used to store the software. An I/O expander
receives inputs from the Zero, Lb/Kg and Lock/Unlock
switches on the front panel. Since both the On/Off switch
and the Recall switch tum on the Scale in hardware, the
uC must determine which switch turned it on. Therefore,
these switches are also indirectly tied to the pC. There are
two serial data outputs to display drivers on the Display
Card.
CO pis ==
+ SIGNAL
- SIGNAL
Fig. 4-12 Essential Scale Components
Page 4-7
Fig. 4-13 Wheatstone Bridge
SS50-N1-220V(B09)