of Microprocessors

OrHER INPUT METHODS 323
Related direct input devices are rotary and linear slide potentiometers.
The slide pot, which is a fairly recent development, is of particular
interest, since it is actually a miniature ribbon controller with the ribbon in
constant contact with the track and a handle to simplify the sliding. Their
most common application is in fancy consumer audio equipment and sound
studio control boards but long travel units (3 1/2 inches) also make good
direct input devices. Their low cost (as little as 50 cents each) means that
quite a number can be provided and used simultaneously. Rotary pots,
especially if fitted with a large knob and calibrated scale, have also been
shown to be effective direct input devices.
Joysticks
One limitation of ribbon controllers and various types of potentiometers
is that essentially only one degree of movement freedom is available,
which in turn implies that only one output from the device is present. The
human hand, on the other hand, is capable ofseveral degrees offreedom in its
movement. Devices for direct hand manipulation are called joysticks, a name
derived from a similar, but much larger, device used as an aircraft control.
Joysticks may have only one or several degrees of freedom but the term
usually applies to a two-axis (two degrees of freedom) device.
A joystick usually takes the form of a handle poking through a hole in
the top of the joystick cover. The handle may be 2 to 5 inches long and can be
moved forward and back or sideways or any combination of these motions
always pivoting about the hole in the cover. The joystick produces two
outputs, one proportional to the X component (sideways) of the handle
position and the other proportional to Y (forward to back). The cheap
"joysticks" often seen on electronic games merely close switch contacts when
the handle is moved. Ideally, the unit should offer the same resistance to
motion in any direction but some inexpensive ones may favor motion along
the principal axes. For some applications it is desirable for the handle to
remain in its last position when released, while for others a spring return to
center (X = 0, Y = 0) is appropriate.
A variety of mechanical arrangements is used to separate lever movement
into X and Y components, but the most ingenious is shown in Fig.
10-2. The arrangement of brackets does the separation, while two standard
rotary pots are used to convert the motion into output voltages. Note that
only a fraction of the pots' rotary range is utilized so some postprocessing of
the output voltage will probably be necessary.
Joysticks of reasonable quality are commonly available for about $5.00.
Although these may not be quite as smooth acting as one constructed
according to the figure, they are small and convenient. One nice feature is
that each axis often consists of two pots ganged together. Connecting the two
pots differentially can provide a bipolar output voltage that may not require
additional processing to utilize.