LS SERIES SERVICE MANUAL
ISSUE 4.0 LOAD MEASURING SYSTEM 3
4.4 Loadcell Bridge Supply
See diagram 04/1455 sheet 2 in section 17
The Main PCB produces a stable supply of +4V, which is used to power the loadcell bridge
via pins 9 and 10 of the 15 pin D socket. This supply is produced by IC29 and IC7 using the
+2V5 reference supply from the ADC IC6.
The loadcell excitation voltage is measured by the ADC (IC6), via the amplifier IC5 and the
multiplexer IC3. The loadcell output voltage is compared with the excitation voltage to
automatically compensate for component tolerances. Therefore, the loadcell accuracy is not
affected by power supply fluctuations and the load amplifier circuits contain NO presets.
The only calibration that can be performed is by using the Engineers Diagnostic mode to
program calibration data into the loadcell EEPROM.
4.5 Loadcell Amplifier
See diagram 04/1451 in section 17
The loadcell contains 4 strain gauges wired as a full bridge and is fed with a stable +4 Volts
via pad 4 of the loadcell amplifier PCB and is connected to 0 Volts on pad 6. The 2 outputs
of the bridge are fed to a high impedance low drift DC instrumentation amplifier via PCB pads 1
and 2.
The amplifier, IC1, uses high precision, high stability components and requires no calibration
so no gain or zero potentiometers are fitted. The output of the amplifier, in the range of +10V
to -10V, is fed to the Main PCB via pin 7 of the crosshead ribbon cable. The output signal
can be measured on R3 on the loadcell amplifier PCB.
4.6 Loadcell Value
The loadcell plug contains a link between pins 14 and 15. This link connects a ground signal
to the Main PCB to indicate that a loadcell is connected to the machine. The loadcell value
and calibration information is stored in the EEPROM IC2, inside the loadcell plug. This
EEPROM is programmed with the loadcell value and also contains the loadcell calibration data.
When a loadcell is changed, the loadcell value and calibration factor is automatically read by
IC39 on the Main PCB from the SDA (Serial Data) output of the EEPROM (pin 5), i.e. the
loadcells are self-identifying. When the machine is switched on, the loadcell value will be shown
on the LCD display.
The loadcell value is factory programmed into the EEPROM but may be accidentally erased
by large mains spikes or static etc. If a loadcell is fitted but the display shows "No Loadcell"
when a test is started or a jog key is pressed, this indicates that the EEPROM has been
accidentally erased and MUST be reprogrammed by an engineer using the Engineer's
Diagnostic Mode.
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