One way to measure force is to measure the distortion it
produces. All the methods discussed on the PIC List so far
assume the use of this methodology.

But it isn't the only methodology available.

There is a feedback method in which an opposing force is applied
until the resultant is zero. It is easy to measure a resultant of
zero.

As an illustration of the technique I will discuss a method used
to measure weight:

The moveable iron core of a small vertical solenoid is linked to a thin
wire that connects to the object to be weighed. The top of the moveable
iron core connects to a thin metal vane. The vane is arranged so that
when it moves UP slightly, it breaks a light beam that is aimed at a
light sensor.

A physical stop mechanism is normally included so that the possible
travel of the solenoid core is restriced to a very small distance.

An electronic feedback circuit controls the current feeding the solenoid.
The solenoid position is maintained at the precise position where the
vane just begins to obscure the light beam. (simple negative feedback).

The circuit is calibrated by recording the current required to bring
the circuit to equilibrium for a variety of weights.

When I say we measure the current, we usually don't measure the current
itself, but the average ON versus OFF time of the circuit. This
is proportional to the average current, and easier to measure with
a PIC.

**********

You can dispense with the light sensor method and make an even cheaper
version by using the stops as switch contacts, with the armature of the
resulting SPDT switch being the metal piece that contacts the stops.

You wire the resulting SPDT switch contacts to the R and S inputs
of an RS flip flop. The armature goes to either + or ground, depending
on the kind of RS flip flop you are using. The R and S inputs will
need either pullups or pulldown resistors.

The output of the RS flip flop controls a transistor that provides
current to the solenoid in ON/OFF fashion. As before, we measure the
average ON/OFF ratio, which is proportional to the applied force.

Proper calibration is important.

Fr. Thomas McGahee

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