12-12-2012, 11:43 AM
Hydraulic and Pneumatic Actuators and their Application Areas
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Abstract
Modern robotic systems are difficult. drives are a mechanical part of this systems. Three
types of drives are basically used now: electric, pneumatic and hydraulic. Each type has its
own advantages and disadvantages.
In this paper I’m going to tell you about pneumatic and hydraulic actuators, about their
advantages and disadvantages, about their types and variants of design.
Introduction
Modern robotic systems are difficult. drives are a mechanical part of this systems. Three
types of drives are basically used now: electric, pneumatic and hydraulic. Each type has its
own advantages and disadvantages.
In this paper I’m going to tell you about pneumatic and hydraulic actuators, about their
advantages and disadvantages, about their types and variants of design.
Functional schemes of pneumatic and hydraulic drives are similar. We’ll discuss one of
them.
Different kinds of Pneumatic Actuators
Classification of pneumatic actuators
A set of devices into with one or more pneumoengines, which are determined to start mechanisms
or some other objects by means of pressed working gas is called pneumatic actuator,
or pneumoactuator.
The devices intended for transformation of potential and kinetic energy of the stream of
compressed gas in mechanical energy of the output link that can be, for example, a rod of the
piston, a shaft of the turbine or the case of the jet device is called pneumatic engines of the
automated actuator.
All pneumatic actuators can be subdivided into the following types:
• diaphragm pneumatic actuators;
• pneumatic power cylinders;
• gas-engine pneumatic actuators;
• turbine pneumatic actuators;
• jet-stream pneumatic actuators;
• pneumomuscles;
• combined pneumatic actuators.
Diaphragm pneumatic actuators
Diaphragm pneumatic actuators include membrane and sylphon pneumoactuators. Potential
energy of gas stream, i.e. energy of static pressure is used in diaphragm pneumatic actuators.
The difference of static pressure in receivers of the actuator transformes into the effort on the
output element of the drive - the rod. Pneumatic actuators can be executed both on one-sided
(not reversive), and on double-sided (reversive) scheme.
The basic scheme of the membrane pneumatic actuator is shown in figure 2 A.
Work of the pneumatic actuator consists in moving of a rod under influence of the difference
of pressure p1 − p2 in cavities that is formed due to the difference of gas charges G1 − G2
Pneumatic power cylinders
Air, or pneumatic cylinders are devices that convert power of compressed air into mechanical
energy. This mechanical energy produces linear or rotary motion. In this way, the air cylinder
functions as the actuator in the pneumatic system, so it is also known as a pneumatic linear
actuator.
Devices with forward linear movement are divided into single-acting and double-acting
pneumocylinders, with rod and rodless. Rod pneumocylinders in turn can have a through-pass
or a no-go rod. Devices with rotary (rotational) movement are divided into pneumocylinders
with rotary movement of the output link and rotational pneumocylinders.
Structurally pneumatic power cylinders can be piston, membrane, sylphon and hose.
The air cylinder consists of steel or stainless steel piston, piston rod, cylinder barrel and
end covers.
In piston pneumatic cylinders, as well as in diaphragm, potential energy of the compressed
gas is used, but presence of the piston with mobile condensation allows to reach big movings
of the output link.
As compressed air moves into the cylinder, it pushes the piston along the length of the
cylinder. Compressed air or the spring, located at the rod end of the cylinder, pushes the
piston back.
The combined pneumatic actuators
In particular, electropneumatic actuators considered to be combined drives. In electropneumatic
drives executive engines, electromechanical converters and amplifiers are applied.
For example, we shall consider the circuit of an electropneumatic follower drive, in which hot
gas serves as a work environment (fig. 10.).
The drive consume gas from the source of constant pressure. Basic elements of the drive
are the amplifier 1 of direct current, the electromechanical converter 2 with Ø-shaped stator,
choker 3 that is elastic fixed on stator, nozzles 4, throttles 5, executive pneumocylinders 6
and 7, sensors 8 and 9 of feedback by speed and on position of a conducted link - the shaft
connected to loading. Item loading is given by a spring 10, and inertial loading - a flywheel
11. Besides it is supposed, that the loading from hydraulic friction is also created.
In the given drive, the executive engines are pneumocylinders that are connected directly to
the channels of the pneumoamplifier with managing element of nozzle- choker type, therefore
due to the use of energy produced by gas which is under pressure only one step of amplification
is carried out.
Recuperation of energy in drives
Drives of the robots work basically in transitive cyclic modes dispersal - braking type. Therefore
as one of the most important ways to economise on energy at them is to use the idea of
energy recuperation. It is especially important for mobile robots with independent power supply.
Moreover the speed of operation also raises frequently. Recuperation of energy is based
on its storage during braking and feedback at the subsequent acceleration. There are two
basic ways of such storage of energy: storage of mechanical energy (with the help of flywheels,
springs) and electric (in accumulators, condensers, inductive coils).
A principle of mechanical energy recuperation with the help of springs has got application
in Russian cyclic industrial robots. That is why they have very good power characteristics.
The spring that provides cyclic movement of the manipulator in the mode of resonant not
fading fluctuations with zero speed in extreme points is installed into the cyclic drive. The
drive engine carries out the energy charging in the middle of the way at the maximal speed
of movement, replenishing the energy loss by the drive while performing its work. Thus, in
the final points of movement there is no impact on useless dispersion of the kinetic energy
saved up by a drive. Thus, it is possible to reduce energy consumption by 3-4 times and the
capacity of the engine can be accordingly reduced. The similar effect can be reach in drives
of manipulators’ claw devices.