Pick-Up; Check Of The Phase Diff Erence - Bosch WBE 4120 Instructions De Réparation

en 62 | WBE 4120 / WBE 4120 D / WBE 4120 DT | Diagnostics
10.2

Pick-up

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Balancing system
The apparatus used to measure the unbalance of
the wheels has a mechanical system based on a
1° degree lever. The piezo-electric sensors, positi-
oned in the lower part of the shaft at the two ends,
detect the vibrations converting the force in electric
voltage.
Abb. 11: Position of the piezo-electric sensors
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Voltages
The piezo-electric sensors, depending on the po-
sition they are placed, receive a different stress
and charge. The one inside the shaft on the left is
excited with a lower amount compared to the exter-
nal one. It can therefore be affirmed that the ratio
between the two is of 2; or that the voltage measure
by the external pick-up is twice greater than that of
the internal pick-up.
Abb. 12: Stresses distributed on the shaft
10.2.1 Check of the phase difference
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The term phase indicates a particular moment
during the performing of a periodic phenomenon (in
this case, the electric signal caused by the pick-ups
during the wheel cycle). When two signals are con-
sidered (one caused by the internal pick-up and one
by the external) it is the phase difference between
them, meaning the difference of the read voltages.
For example, with reference to fig. 10, if the black
signal represents the voltage of the external pick-up
and the red signal the voltage of the internal pick-up,
it can be affirmed that the voltages are in antiphase,
meaning that the offset is equal to 180° (the peaks
of a signal coincide with the grooves of the other
and vice-versa).
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Abb. 13: Ratio between the voltages measured by the pick-ups
4. Access the self-diagnosis menu (Menu 1).
"
The displays show the values read by the pick-up in
sequence during the last cycle and the phase diffe-
rence in degrees.
5. Fix the wheel of a motor vehicle of average size and
in excellent conditions (e.g. 5.5" wide, 14" diameter)
on to the flange and carry out a perfect balancing.
6. Apply a sample weight (e.g. 100 g) on the external
side of the wheel.
7. Press < I > or close the wheel protection cap.
« Measurement is started.
"
At the end, checking the measured values, the voltage
value of the internal pick-up must always be small com-
pared to the voltage value of the external pick-up and
the ration between the external and internal pick-ups
must be between 1.9 and 2.1.
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To calculation the voltage difference, divide the va-
lue of the external pick-up with that of the internal,
in this way obtaining the ratio between the two.
!
If, when checking the ratio between the pick-up and
the phase difference, out of tolerance values are
found, replace the board (chap. 4.3) or replace the
pick-up sensors (chap. 4.1.4). If after replacement
of both the problem persists, it may be caused by a
too high disturbance of the bearings of the measure-
ment bench, therefore replace everything.
Bosch Automotive Service Solutions GmbH