Page 158 - Concise Encyclopedia of Robotics

Hydraulic Drive
is broken down into several components, or subtasks. Each subtask has
its own subplan. Once the robot has begun executing the job, it carries
out the plan and the subplans subject to modifications that may be
necessary as the work environment changes. These changes are the results
of signals from the sensors.
In a typical robot that uses the hybrid paradigm, deliberations occur
at intervals of several seconds, while reactions take place at a rate of many
times per second. Compare HIERARCHICAL PARADIGM and REACTIVE PARADIGM.
HYDRAULIC DRIVE
A hydraulic drive is a method of providing movement to a robot manip-
ulator. It uses a special hydraulic fluid, usually oil-based, to transfer forces
to various joints, telescoping sections, and end effectors.
The hydraulic drive consists of a power supply, one or more motors, a
set of pistons and valves, and a feedback loop. The valves and pistons
control the movement of the hydraulic fluid. Because the hydraulic fluid
is practically incompressible, it is possible to generate large mechanical
forces over small surface areas, or, conversely, to position large-area pistons
with extreme accuracy. The feedback loop consists of one or more force
sensors that provide error correction and ensure that the manipulator
follows its intended path.
Hydraulically driven manipulators are used when motions must be
rapid, precise, and repeated numerous times. Hydraulic systems are also
noted for the ability to impart considerable force, so they are good for
applications involving heavy lifting or the application of large amounts of
pressure or torque. In addition, hydraulically driven robot manipulators
resist unwanted movement in the presence of external forces. Compare
PNEUMATIC DRIVE.
HYSTERESIS LOOP
A hysteresis loop (the word is pronounced “his-ta-REE-sis”) is a graph
that shows the sluggishness of response in a servomechanism.
The illustration shows a hysteresis loop for a typical thermostat, used
for control of the indoor air temperature in a house. The horizontal scale
shows the room temperature in degrees Celsius (°C). On/off conditions
for heating and cooling are shown on the vertical scales. Notice that there
is a small range of temperatures, from about 18.5°C to 21.5°C, within
which the temperature fluctuates. This prevents the system from rapidly
oscillating back and forth between heating and cooling states, but it is a
narrow enough temperature range so that the people in the room don’t
get too hot or cold.

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