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Service manual
Brief description of the DMS module
Introduction
The DMS modules are used to determine weights using a load cell with strain gauges. The DMS low
end module also accommodates the display and the operating unit. To allow the modules to be
adapted to different scales, an EEPROM is integrated which is used to configure the module. 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.
Load cell
Weight measurement
The DMS modules use load cells with strain gauges in a double bridge circuit as sensors.
The entire analog part runs on its own supply voltage. This voltage can be switched by the µC to 3V,
5V or 10V. This voltage is also supplied to the load cells. At full load, they deliver a bridge output
voltage of 2mV/V.
This voltage is measured by a dual slope a/d converter. The DMS ext module can evaluate up to four
load cells which are multiplexed accordingly. Only one load cell can be connected to the other types of
DMS modules.
The conversion time is selected so that a new measured value is available every 0.1 s.
The resolution of the a/d converter is 0.05 µV/V. If several load cells are connected, keep in mind that
the resolution relates to the mean output signal of all the load cells connected.
When these measured values are within the expected range, they are converted into a weight value
using a third order polynomial. In order to increase accuracy, eight measured values are used to
calculate the weight to be displayed.
The first value obtained after switch-on is used as the zero point. If the values obtained later on do not
deviate strongly from this value, the zero point is corrected in order to compensate for possible drifts.
(Please also refer to temperature measurement.)
05.08.03 /Jensen / Rei
A/D Converter
Voltage
control
Temperature
measurement
Voltage
measurement
Description of the electronics
Display
I 0 0 0 0
.
µC
Voltage
control
Mains connection
Battery/rechargeable
batteries
1
Operation
EEPROM
Quartz
SeSAM Bus
Power On
Reset
34-34-00-645 b
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