Hardware Reference
In-Depth Information
The Concept
he type of patterns a harmonograph produces are called
Lissajous igures,
which are much
beloved of electronic engineers. In fact the irst lab I did as an undergraduate student was to
apply two signals from separate signal generators to the X and Y delection circuits of a cath-
ode ray oscilloscope to obtain them. I sneakily got a third signal generator and applied it to
the intensity of the beam, a Z axis modulation, and thoroughly confused the supervising
lecturer. However, Lissajous igures are only one simple example of the sort of pattern you
can get from a harmonograph. In a harmonograph you can have multiple signals deining
each axis, and the slow decay of the amplitude of the swing adds greatly to the complexity of
the pattern. So how are you going to computerise this mechanical device? Well the secret lies
in being able to measure rapidly the angle of a pendulum. In the last chapter, “Facebook-
Enabled Roto-Sketch”, you saw how the rotary shaft encoder could be used to measure the
position of a shaft, but the type used there has detents, and requires a relatively large amount
of energy to turn. You can get optical shaft encoders with very little friction, but for any great
resolution these are horrendously expensive. To the rescue comes a new chip which ofers
the possibility of a totally friction-free method of measurement - the Hall efect absolute
rotary shaft encoder.
he idea is to use four pendulums to create your drawing, and a shaft encoder on each will
measure the angle of each pendulum's swing at any instant of time. hen the Raspberry Pi
will plot the information in a window as it comes in, and you will see the picture being plot-
ted in real time. he size of the swing, along with the period of and phase of each pendulum,
will alter the picture you produce. You will be able to set the initial swing conditions and alter
the length of the pendulums to produce an almost ininite variety of pictures.
The Hall Effect
A
Hall efect device
is one that uses the inluence of a magnetic ield on a low of electrons. It
was discovered by
Edwin Hall
in 1879 but has been widely exploited only in the last 30 years
or so with the advent of semiconductors. Basically if current is lowing along a conductor,
there will be no potential diference on either side of that conductor. However, if a magnetic
ield is applied upward through the sample perpendicular to the current low, then the elec-
trons will initially be delected by the ield toward the side of the conductor. hey will build
up there, with a corresponding number of holes (positive charge) on the other side. his will
continue until they build up an electric ield that completely balances the force of the mag-
netic ield. hen the electrons can continue traveling in a straight line through the conduc-
tor. his is shown in Figure 16-2. So by measuring this voltage across the conductor you can
measure the strength of the magnetic ield up through the conductor. his is used in all sorts
of devices such as contactless switches, contactless current measurement, the electronic
compass and angle measurement.
