The straight story on linear actuators
Linear actuators can be powered by pneumatics, hydraulics, or electric motors. Which is best for your job? Let's find out.
More complex cylinders fitted with electronic sensors generate electrical signals that reflect off magnets during motion — a signal that locates piston position along the cylinder stroke. Then, the electronic signal can be processed to determine piston velocity or acceleration, and a servo directional-control valve can control the cylinder piston speed and stop it at any point in the cylinder's stroke.
What's their strength? Hydraulic systems deliver much tighter control than pneumatic systems and their spongy-acting, compressible air. When available space is a concern, hydraulic systems can deliver significantly higher force density.
When driven by a rotary motor, these linear motion systems employ one of four rotary-to-linear conversion systems: ballscrew, roller screw, Acme (lead) screw, or belt drive. But another electromechanical actuator is the linear motor. Let's review these first.
A linear motor is like a rotary motor that has been unwrapped. The motor coils make up the forcer. Depending on the design, one or two rows of magnets comprise the magnet track. Now, in a rotary motor, the rotor spins while the stator is fixed. But in a linear motor, either the forcer or the magnet track can be the moving component, which is then integrated with an appropriate linear bearing. By sending electrical current to the forcer, the resulting magnetic field interacts with the magnet track and drives the linear motor carriage forward and back.
Linear motors have high dynamic performance, with acceleration of greater than 20 gs at velocities of 10 m/sec and higher. Due to the direct drive nature of linear motors, there are no mechanical components to add backlash, torsional windup, or other positioning errors to the system. Sub-micron resolution and repeatability are commonplace. And, because the motor is directly coupled to the load, there are fewer components to fail, which adds long term value.
Application demands typically dictate what type of linear motor is most suitable. Iron core linear motors use one row of magnets and a forcer with windings that are wrapped around iron poles. The nature of these windings provides a very efficient magnetic path in the motor that produces the highest forces within the linear motor family.
On the other hand, in ironless motors, windings are wrapped flatly and ride in a balanced, U-shaped magnet track. So, ironless motors are ideal where smooth motion and a higher degree of accuracy are required, and for applications involving extremely high accelerations.
Slotless linear motors are a hybrid design between ironless and iron core motors. The design uses only one row of magnets on the track, resulting in a motor with lower attractive forces and less cogging than iron core motors. They are lower in cost per package size than ironless motors.
Piezomotors use crystals to actuate a linear stage. Electrical excitation to the motors causes the crystals to slightly change shape and distort, pushing the stage a tiny distance, usually measured in nanometers. Exciting the crystals at a high frequency generates smooth, precise motion, making piezomotor positioners suitable for applications with very fine positioning requirements — metrology and ultra-fine focusing of optical assemblies, for example.
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