Weight Measurement; Temperature Measurement; Power Supply - Seca 780 Service Manual

Brief description of the DETF module
Introduction
The DETF module belongs to the family ofseca electronic modules and is used to determine weight
by means of a tuning-fork sensor system. The weight obtained is displayed on a separate display
module. To allow the module to be adapted to different scales, an EEPROM is integrated with which
it can be configured. A connection to the SeSAM bus (Seca's Serial Autoconfiguring Multicontroller
bus) is provided via which the scale can be calibrated and additional modules can be connected.
The following block diagram shows the most important function blocks of the force sensor and the
DETF module:
Weight measurement
A tuning-fork sensor screwed to a bending element is used as the weight sensor. If the scale is
loaded, the bending element exerts a tensile force on the tuning-fork sensor, thus increasing its
resonant frequency. By means of the bandpass filter the sensor is set vibrating at this frequency
which is then measured by the microcontroller.
Using the value obtained and the individual calibration data stored in the EEPROM the controller
calculates the weight.
The first value obtained after switch-on is used as the zeropoint. If the values obtained later on do
not deviate strongly from this value, the zeropoint is corrected in order to compensate for possible
drifts.
When the sensor is connected, care must be taken that the wires are not twisted to prevent crosstalk
from the output to the input.
Temperature measurement
As the value measured by the tuning-fork sensor is affected by temperature, the temperature is
measured regularly and compensated for accordingly.
Power supply
Power supply is provided by four AA size batteries and / or a seca mains transformer / desktop
mains unit. From the power supply the voltage is passed via a reverse voltage protection diode to an
in-phase regulator which provides a stabilized 3V supply voltage.
30-34-00-617 1 06.09.1999
EE/Jensen