Page 122 - Instrumentation Reference Book 3E

Force-balance methods 107

I-

Knife edge
/
& Unknown \i b /
mars
\ Unknown 1 force
Known
force
Fl Pointer attached to beam F2 \
to indicate null Position
Figure 7.1 Equal-lever balance.

up to about lQOOkg with high accuracy if
required.

7.3.2 Unequal-lever balance Figure 7.3 Compound-lever balance.
Figure 7.2 shows a typical arrangement of an
unequal-lever balance which can be used for meas- 7.4 Force-balance methods
uring large masses or forces. The balance is
obtained by sliding a known mass along the lever. Figure 7.4 shows an electronic force-balance type
At equilibrium, we have
force-measuring system. The displacement caused
Fla = Ejb by the applied force is sensed by a displacement
F1 z= Fzbla transducer. The displacement transducer output
is fed to a servo amplifier to give an output
... Fi x h current which flows through a restoring coil and
The right-hand side of the beam can therefore be exerts a force on the permanent magnet. This
used as a measure of force. force adjusts itself always io balance the applied
force. In equilibrium, the current flowing through
the force coil is directly proportional to the
applied force. The same current flows through
the resistor R and the voltage drop across this
resistor is a measure of the applied force. Being a
Unknown n feedback system, such a device must be thor-
force
oughly checked for stability.
The force-balance system gives high stability,
high accuracy, and negligible displacement and is
b Calibrated in terms of F.

Applied
force
Pointer attached to beam
'to indicate null position
Figure 7.2 Unequal-lever balance Restoring coil
/
This type of balance is extensively used in
materials-testing machines and weight measure-
ment. The balance is normally bulky and heavy,
but can be made very accurate. R ut voltage a
ed force
Permanent
7.3.3 Compound lever balance magnet I
Figure 7.3 shows a compound lever balance that
is used fo~ measuring very large masses or forces.
Using a number of ratio levers, the applied force
/
is reduced to a level which is just sufficient to Displacement transduce1
actuate a spring within the indicator dial head. (capacitive or L V.D.T.)
The balance is calibrated in units of mass. Figure 7.4 Force-balancesystem