Load Cell Damage - Adam Equipment CBD Series Service Manual

9.0

LOAD CELL DAMAGE

The most common reason a scale fails is that the load cell has been damaged.
The damage can be from 2 primary causes. The first is physical damage due to
an overload or an impact from the side and the second is damage due to the
environment, such as moisture, extreme heat or a cut cable if the cables are
exposed.
PRINCIPLE OF OPERATION
The stain gauge load cell is a method of measuring the amount of stress put
onto a metal structure due to a weight being supported by the structure. The
stress is measured using 4 strain gauge sensors mounted in a particular way on
the metal structure.
These 4 strain gauges are connected in such a way that they form a wheatstone
bridge. See figure below.
resistances of the strain gauges to consider. Normally without a load on the
load cell all the resistances are the same. However when the load cell has a
weight on it, two of the strain gauges will be in compression (A and D) and the
resistance will decrease and two will be in tension (B and C) and their resistance
will increase.
10 VOLT POWER
SUPPLY
+S
This will cause the bridge to become unbalanced. When the bridge has a
voltage across it from +E to -E then the signal output at +S and -S will show a
voltage of zero volts with no load and a small voltage proportional to the load as
the load is increased. Typical load cells show 20-30mv of signal if the excitation
voltage is 10volts and the load cell is fully loaded.
@Adam Equipment Company 2007
In the simplest load cells there are only the 4
+E
A
B
-S
C
D
Vout = O VOLTS,
BETWEEN +S AND -s
-E
BALANCED BRIDGE
10 VOLT POWER
SUPPLY
+S
12
+E
A
B
-S
C
D
Vout = 0.028 VOLTS (28mv),
BETWEEN +S AND -s
-E
UNBALANCED BRIDGE
FULL CAPACITY LOAD
ON THE LOAD CELL