GX-A/GF-A Maintenance manual
Page 4
2. Principles of operation
The GX-A/GF-A balances work on the principle of "Force Restoration". Any change in the load
on the weighing pan causes a Position Beam Lever to pivot on two Fulcrum Flexures (refer to
diagram below). Attached to this beam is a bobbin (wound with fine wire), called the "Force Coil",
which floats in a permanent magnet, called the "Force Motor". At the end of the Position Beam
Lever there is a small hole which allows light from a Light-Emitting Diode (LED) to pass through
to two Photodiodes (light measuring diodes) as it moves up or down. At zero weight, the light
detected by the upper Photodiode is equal to that detected by the lower Photodiode. These
three diodes make up the Position Detector.
When the Force Coil is pulled up by the leverage exerted from a mass on the weighing pan, the
Position Detector detects a change in the position of the Force Coil as the light reaching the
upper Photodiode will be greater than that reaching the lower one. The balance then feeds the
force coil with more voltage to pull it back until the light measured by the two Photodiodes is
equal again. This is accomplished by the Analog section receiving photocurrent from the
Photodiodes, converting it to voltage, and boosting it back to the Force Coil. As the voltage
increases, so does the magnetic power, pulling the Force Coil back until the Position Detector
reads equilibrium.
The current flowing through the Force Coil generates a voltage proportional to the load weight
on the pan. This is read back through the Analog section, first being filtered - then the
Analog-Digital (A/D) Converter digitalizes this measuring voltage; the resulting value is counted
and then fed to the microprocessor (CPU).
Temperature affects the magnet and weight data. So temperature coefficient for weight is
measured and saved beforehand. The balance eliminates the temperature effect by using the
present temperature measured by the temperature sensor and the coefficient. The output from
the temperature sensor is converted digitally and sent to the CPU by the analog module.
The CPU performs mathematical operations in connection with each parameter, such as
temperature, linearity coefficient, and calibration data. Also, the user can specify how the
calculated information should be displayed by using the keyboard. For example: s/he can have
the CPU perform special functions such as conversion into other measuring units, or counting of
small parts. Finally, the results are displayed on the Fluorescent Display, or sent through the
RS-232C interface.
