Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 11 Proprioceptive and Environmental Sensing Mechanisms and Devices 269
more robust and easier to incorporate, but it is also much less precise
since the sensing arm is necessarily flexible.
The whisker has the special property of detecting an object from
any direction, making it distinctly different from lever switches. Since
it bends out of the way of the sensed object, neither the object nor the
switch is damaged by impact. This trick can also be done with a
roller-ended lever arm, but more care is needed when using a rigid
arm than with the flexible whisker. Figure 11-3 shows a basic whisker
switch.
The last basic type of limit switch is the slide switch. This switch has
a different internal mechanism than the button switch and its variations,
and is considered less reliable. It is also difficult to implement in a robot
and is rarely seen. Figure 11-4 shows a slide switch.
Magnetic limit switches come in several varieties and have the advantage
of being sealed from contamination by dirt or water. The most common
design has a sensitive magnet attached to a hinged contact so that
when a piece of ferrous metal (iron) is nearby on the correct side of the
switch, the magnet is drawn towards a mating contact, closing the electric
circuit. All of the mechanical limit switches discussed in the following
sections can incorporate a magnetic limit switch with some simple
modification of the layouts. Just be sure that the thing being sensed is
ferrous metal and passes close enough to the switch to trip it. Besides
being environmentally sealed, these switches can also be designed to
have no direct contact, reducing wear.
There are several ways to increase the area that is sensed by a mechanical
limit switch. Figures 11-5 and 11-6 show basic layouts that can be
expanded on to add a large surface that moves, which the switch then
senses. There is also a form of mechanical switch whose area is inherently
large. This type is called a membrane switch. These switches usually
are in the shape of a long rectangle, since the internal components
lend themselves to a strip shape. Membrane switches come with many
different contact surfaces, pressure ratings (how hard the surface has to
be pushed before the switch is tripped), and some are even flexible. For
some situations, they are very effective.
The huge variety of limit switches and the many ways they can be
used to sense different things are shown on the following pages in
Figures 11-5 and 11-6. Hopefully these pictures will spur the imagination
to come up with even more clever ways mechanical limit switches
can be used in mobile robots.