Drives and Mechanisms Used in Robots
Three main types of actuators are currently used in robots: pneumatic, hydraulic and electric, as well as different combinations of these three. Every robot is fitted with a system of actuators on the arms or in the joints, constituting a drive system.
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b)
Pn¢umalic 1.4,9%
Fig. 9 .I. Percentage shares of different types of actuators used in robots according to [9. 9J and [9.12]: a) in 1977, b) in 1990
9.1.1. Pneumatic drives Pneumatic drives feature the use of compressible fluids, usually compressed air. The advantages of these drives are the an abundantly available and easily accessed power medium (air) that can be released when the task is finished. Their pressme, compared with hydraulic drives, also makes these systems ve1y safe to operate. In addition, unlike all liquids, air has good dynamic propetties, no viscosity and low A. Morecki et al. (eds.), Basics of Robotics © Springer-Verlag Wien 1999
9. Drives and mechanisms used in robots
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stiffness (high compliance). Pneumatic drives are used mainly in small, simple robots performing "pick-and place" tasks. The system shown in Figure 9.2 generally operates with compressed air at a pressure of 0.4+0.8 MPa. A linear (or sometimes rotary) actuator (I) is supplied with air through a two-position, four-way valve (2) controlled by an electric or pneumatic logic system (5). Limit switches (3) transmit a logic signal that arrests the piston at end stops (4). The system usually has only two positions (3). If a pneumatic manipulator with several two-position joints is equipped with these drives, the duration of the cycle can be programmed, but the range of displacement must be changed manually. The system described above is relatively inexpensive, but also much less flexible than a proportional one [9.2]. A simple proportional system is shown in Figure 9.3 . The heart of this system is a two-stage amplifier. The first part is an impact piston amplifier. The second is an air transmitter that provide a large volume flow. The error signal e is used to provide feedback. The balance between the pistons' positions and the feedback is ensured by two springs (of kz and k1 stiffness coefficients) and the suitable ratio of the areas A 1 andA 2 . Another example of a pneumatic conh·ol system is the conb·oller shown in Figure 9.4. It is essentially an enhancement of the system shown in Figure 9.3 . A differentiating, or integrating option can be obtained by modifying the bellows arrangement. Such modifications can lead to a pneumatic PID controller. Each controller can be used to actuate a prismatic joint of a robot. A four-way valve and a cylinder with a piston act as a hydraulic system. It is also possible to actuate a rotational joint by means of a turbine motor. The motor generates a torque propm1ional to the output pressure Po of the conh·ollers described above and independent of the velocity of the lever. High pressure compressed air ensures quick and accurate movements with the use of mechanical end stops at individual joints. When handling large loads that require high lifting capacities and maintaining the desired trajectory, hydraulic actuators must be used. 1 - cylinder
2 - four-way valve 3 - limit switch 2·
3
4 - end stops
5 - logic controller
Fig. 9.2. U-1 pneumatic drive system- principle of operation [ac
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