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| Learn More | What's In An Actuator |
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| Actuator Types |
| Pneumatic Actuators |
| Hydraulic Actuators |
| Electric Actuators |
| DC Motor Figure |
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| Actuator Types |
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Components inside an actuator vary according to the type of actuator. Usually power for robot motion is provided by electrical, hydraulic, or pneumatic actuators.
Current robotic actuators are refinements of industrial actuators, with the main difference being the requirement of a high power-to-size ratio. Each actuator type has its own advantages and disadvantages.
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| Pneumatic Actuators |
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The most common applications of pneumatic actuators is the opening and closing of grippers. Some of these actuators achieve limited rotary motion by using the linear actuator to turn a shaft with a rack and pinion gearing system, or with a chain.
Compressed air is commonly available in most factory applications. Unfortunately, gas is highly compressible which leads to unwanted compliance in the actuator. This compliance makes stiffness and dynamic speed hard to attain and makes control more difficult.
The difficulties associated with its control mixed with the easy availability of air within a factory floor makes it ideal for end effectors and tools, but a poor choice for high precision robotic actuators.
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| Hydraulic Actuators |
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In comparison to air, fluids are almost incompressible, giving hydraulic actuators different properties from pneumatic actuators. Hydraulic fluid cannot be used for energy storage, so another power source must be used to create pressure in the oil. Also, hydraulic oil cannot be vented into the atmosphere, so a return path must be provided for the used oil.
Rotary hydraulic servo-drives are often used in robotics. Well-designed actuators include oil paths through the actuator shafts, so that oil can flow through the actuators from one link to the next, eliminating external hydraulic cables.
Servo-drives are also controlled by a servo-valve, but the output is rotational, requiring a different drive mechanism from linear actuators and a rotary valve transducer. A typical hydraulic motor uses nine pistons to apply force to a swash plate.
A swash plate is a circular plate, concentric with the shaft, but attached at at angle to it. Oil is released into a piston when the swash plate is closest to the end of the piston. As each piston extends, it applies a force to the plate causing it to turn to reduce the force by moving the swash plate away from the cylinder, allowing the cylinder to extend.
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Flow of oil into the cylinder is controlled by a distributor - a set of tubes which rotate with the shaft. The distributor connects two groups of four sequential pistons to the inlet and outlet servo-valve ports.
As the shaft rotates, pistons move from one control port to the other depending upon whether they are extending or contracting. The ninth piston is always in transition between one control port and the other.
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| Electric Actuators |
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Many robot manufacturers choose to use electric motors. Two types of electric motors are in use: stepper motors and direct current (DC) motors. Stepper motors are driven by a train of electrical pulses.
The stator is wound as two separate coils, which produce magnetic fields offset angularly by half a rotor pole. These coils are pulsed alternately to produce a rotating magnetic field.
The rotor, which is polarized with alternating north and south poles, aligns itself with this field, and rotates with it. Each pulse turns the rotor through a fixed angle (one step) or consequently.
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| DC motors are more common in industrial robots. Many applications use brushless DC motors, which have the same torque-speed characteristics as conventional DC motors with a much smaller size. Their operation, though, is more complex.
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