The Flimsy AcceleroMeter: Principle of Operation
Joachim Köppen DF3GJ Kiel/Strasbourg/Illkirch Winter 2004
The FAM is a simple accelerometer which can be constructed with
easily available material and which should give an indication of weightlessness
rapidly enough and accurately enough to enable simple measurements
in an indoor Drop Tower (drop of 2 m height). It is also capable of giving a rapid
and easily visible optical indication when the weight is reduced below
a certain value.
It is constructed from
The construction technique is soldering, as done in electronics, and
some creative bending of wires...
- a piece of printed circuit board,
- coiled springs,
- pieces of copper wire used for low voltage applications,
- a sewing pin,
- some cardboard,
- a light emitting diode, two 1.5 V batteries, and some more wire
The instrument described here is the 4th of a number of rather different
designs, and has been verified to achieve the design goals.
Why and how it works
The weight (i.e. the gravitational acceleration) of a small mass M on
a long arm L1 is counterbalanced by the elastic restoring force from two
counteracting coiled springs S which acts on the short arm L2. This is
essentially a spring balance, and the pointer P indicates the weight of
- two springs are used so that the balance covers the entire
range from 0 to 1 g with the same sensitivity, and is even
capable to measure negative accelerations, i.e. accelerations
- by using a short arm L2 for the springs to act on, their small
displacement is magnified into a large displacement of the long
- since the arm L2 is shorter than arm L1, a small mass M suffices
to make a displacement of the springs
- by being fixed on the arm L2, the mass M can only move on a circle
of radius L2. It does not respond to accelerations that are parallel
to the axis A of rotation.
- in an earlier design, the mass was fixed to one of the springs.
This leads to a sideways displacement of the springs when
undergoing sideways accelerations. The present design is insensitive
to this effect.
However, some approximations had to be accepted:
- the mass M moves on a circle: Ideally it should be allowed to
move only in vertical direction, if one wants to measure vertical
accelerations only. But as we shall use it only in the Drop Tower,
this is o.k.
- the linkage of the short arm L2 to the spring armature SA between the two
springs is at the end of the arm. Hence it will move on a small
circle about the axis A, which causes the arm to raise SA and so
the inner ends of the springs as the arm moves away from the
vertical. The springs are not only compressed or extended in their
length, but move also sideways. This will cause some additional
friction at the springs' end points, and also a downward force
on the axes and its bearings with more friction.
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