Vacuum means the absence of many things – pressure, gases and particles. For conventional motors, vacuum conditions can be a disqualifying factor. Miniature motors from PiezoMotor of Sweden defy these conditions, however, making them the best choice for particle accelerators and electron microscopes.
If particle accelerators could make friends they would choose piezo motors. Designed for demanding environmental conditions – such as a strong magnetic field in a vacuum – these motors, powered by piezoelectricity, operate trouble-free even under harsh conditions. The vacuum compatibility of piezo motors also makes them ideal for use in production systems for the semiconductor industry, as well as in space. On the basis of new technologies with an operating principle that depends on a vacuum environment, the miniature motors are increasingly gaining in importance: in the optics industry, in the manufacture of microelectronic circuits, in aerospace technology, and even in the automotive industry.
Vacuum refers to a condition of a gas in a volume under pressure that is significantly lower than the normal atmospheric pressure. This condition creates a space that is virtually free of particles. Scientists define vacuum levels according to the quantity of remaining matter and the prevailing gas pressure – from a low vacuum with 300 to 1 mbar to ultra-high vacuum with up to 10-10 mbar, which approximately corresponds to the conditions in near-Earth space and in a particle accelerator.
Vacuum can be a problem for the electric motor
Such conditions are too harsh for conventional electric motors, whose materials would require tremendous modifications for use in a vacuum. Outgassing of the motor components in a vacuum destroys their functionality, and the escaping materials precipitate on the surrounding walls and components. “Many oils and greases that are needed in the ball bearings of an electric motor, for example, vaporize in a vacuum”, explains Pontus Fischer of Suna Precision GmbH. The Hamburg-based company is a spin-off of the German Electron Synchrotron Research Centre (DESY) and specializes in nanopositioning, automation and synchrotron radiation systems.
Besides destroying the motor, outgassing results in contamination of the precision optical components and sensitive materials in the vacuum. “We work with electrons and X-ray light, and air disperses the signal like fog scatters the light of a headlamp. Electrons would collide with air molecules and no signal would arrive to the sensors – electrons would be dispersed in a one-millimetre range.” Air molecules can also escape from the motor mounts, windings and metal surfaces of the motor, if the surfaces have not been specially treated. These potential leaks can result in long evacuation times, as well as insufficient vacuum in the long term. Last but not least, undesired corona effects are the result of current flow between unprotected high-voltage conductors through ionized air.
Operation of a motor in a vacuum therefore presents a special challenge for engineers. In the past, motors and drive mechanisms for moving and positioning specimens, mirrors or sensors were generally installed outside of the vacuum chamber. This eliminates outgassing and contamination caused by unsuitable motors. The problem: There are numerous disadvantages to such an external control solution, such as limitations in the precision, repeatability and resolution of positioning systems. External motors require an axle bushing to drive the mechanical components inside the chamber, which in the case of multi-dimensional motions necessitates the use of deflecting couplings and complicated mechanisms that take up valuable space inside the chamber. The bushing seal itself is a wear part and could easily destroy the vacuum.
This is why vacuum applications are increasingly using special motors that defy the unique ambient conditions of a vacuum and can be installed directly in the chamber. Such in-vacuum technologies are complex and require much higher temperature stability than standard motors, which are typically cooled by convection. Whereas piezo motors are predestined for operation in a vacuum. They require no lubricants, and generate neither friction nor dissipated heat. But what makes these motors so extraordinary? The answer is piezoelectricity: a change in electric polarization due to mechanical stress. To generate a movement, the so-called inverse piezoelectric effect comes about when external voltage is applied to a piezoelectric material. This principle was used by the Swedish company PiezoMotor in the development of Piezo-LEGS: ceramic legs that can both be lengthened and laterally curved. Actuating signals effect a synchronized movement of the legs, which are arranged in pairs. This starts a movement in the submicrometre to nanometre range, which drives the linear and rotating motors.
“We started working with PiezoMotor nine years ago”, says Pontus Fischer. The company Suna Precision, which functions as a system integrator for PiezoMotor, offers implementation of the miniature motors in specific applications, including motion control. “We chose PiezoMotor because the motors meet our standards for precision – and because they are designed for trouble-free operation in a vacuum.” Products manufactured by Suna Precision include sliding and rotary tables for vacuum applications. “Piezo motors in a closed control loop are the ideal control elements for the vacuum levels that we need”, explains Pontus Fischer.
“We build the motors with materials corresponding to the required vacuum, with the cleanest possible design”, confirms Mats Bexell, founder of PiezoMotor. “They are made of stainless steel and ceramic, which are vacuum and space compatible.” All-ceramic isolated actuators prevent outgassing and high bake-out temperatures – ideal prerequisites for use in ultra-high vacuum conditions. Since they have no moving parts such as gears or bearings, they are also non-wearing. Long-term studies confirm their suitability.