Page 67 - Robot Builders Source Book - Gordon McComb

56 Concepts and Layouts

Some words of explanation: what we mean by clarity when it comes to mechani-
cal systems may be demonstrated by the following extreme example. When a mecha-
nism is rotated slowly, almost everyone is able to understand how it works, its logic;
and when broken or out of order it is relatively easy to locate the broken or worn part
simply by looking at it. By contrast, in electronic systems, extensive measurement and
special knowledge are needed to pinpoint defects in, say, a mounted plate. This is why
we generally replace the suspect plate by a new one in electronic systems, instead of
replacing a part or even repairing it as in mechanical systems. A purely mechanical
system is usually driven at a single point, the input, and no additional energy supply
is needed along the kinematic chain; in some cases, however, a multidrive system is
more effective. The layout will then include a multiple of electromotors or hydro-
cylinders. (Pneumatic and hydraulic systems, for instance, require air or liquid supply
to every cylinder or valve along the system.)
We have at our disposal a wide range of known and examined solutions for achiev-
ing various movements. Moreover, when electric, hydraulic, or pneumatic drives are
used to effect complex motions, they are generally combined with mechanical devices.
This is because the latter make it possible to achieve accurate displacement thanks to
the rigidity of mechanical parts.
Thanks to the use of high pressure, the transmission of large forces to considerable
distances in hydraulic systems can be realized in small volumes where a purely mechan-
ical solution would entail the use of massive parts, massive supports, massive joints,
etc. The fact that liquid consumption is easily controlled ensures fine control of the
piston speed, while the nature of liquid flow ensures smoothness of piston displace-
ment. (See Table 2.2.) Thanks to the flexibility of the pipes almost any spatial and remote
location of the cylinders can be arranged with ease using pipes made of flexible mate-
rials, including alteration of the location and turning of the elements when the machine
is in use. On the other hand, once designed, a mechanical system is difficult to modify
(at the least any modification would require special devices). Rigorously coordinated
displacements between remotely located elements are problematic. Spatially oriented
displacements of different elements require specially costly means. Hydraulic systems
can use relatively cheap safety valves to prevent breakdown of elements due to acci-

TABLE 2.2 Hydraulic Systems

Advantages Disadvantages
1. Possibility of generating very large forces. 1. Difficulties resulting from the use of
high pressures.
2. Possibility of carrying out slow, smooth
movements. 2. Mechanical supports or complicated
3. Relative simplicity of spatial location of control layout required for accurate
moving elements. displacements.
4. Possibility of changing velocities of 3. Leakages can influence the pressure
displacements in a smooth manner. inside the system.
4. Variation of the working liquid's
5. The fact that it is not explosive (pressure
drops sharply when fluid leaks out). viscosity due to temperature changes.
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