PILine® linear piezomotors are based on a novel, patented ultrasonic drive developed by PI. They can be as small as 9x6x2 mm (P-652) or integrated in multiaxis translation stages such as the M-686 open frame XY stage. At the heart of the system is a rectangular monolithic piezoceramic plate (the stator), segmented on one side by two electrodes (see Fig. 2). Depending on the desired direction of motion, the left or right electrode of the piezoceramic plate is excited to produce high-frequency eigenmode oscillations of tens to hundreds of kilohertz. An alumina friction tip (pusher) attached to the plate moves along an inclined linear path at the eigenmode frequency (see Fig. 5)
Through its contact with the friction bar, it provides micro-impulses and drives the moving part of the mechanics (slider, turntable, etc.) forward or backwards. With each oscillatory cycle, the mechanics executes a step of a few nanometers; the macroscopic result is smooth motion with a virtually unlimited travel range. The driving force is taken from the energy in the longitudinal oscillation component. The transverse component serves to increase/ decrease the pressure of the friction tip against the friction bar. The transverse oscillation energy determines the maximum frictional force and hence the holding and driving force of the motor.
State-of-the-art ultrasonic motors can produce accelerations to 20 g and velocities to 800 mm/s. Force generation, once a drawback of resonant motors, has also recently made a big step forward. Single-element motors can achieve pushing forces to 50 N (see Fig. 3).
Ultrasonic motors cannot provide the unlimited resolution of linear piezo actuators (see link) and flexure-guided piezo positioning stages (see p. 4-32). It has to be understood that these motors transfer motion through friction, which is why their repeatability is limited to about 50 nm. Much higher resolution and holding forces can be achieved with NEXLINE® piezomotors / actuators, see fig. 10, here.
Features and Advantages of PILine® Ultrasonic Piezomotors
- Compact Size: The direct-drive principle allows the de.sign of ultra-compact translation stages. The M-662, for example, provides 20 mm travel in a 28 x 28 x 8 mm package.
- Low Inertia: High Acceleration, Speed and Resolution: PILine® drives achieve velocities to 800 mm/s and accelerations to 20 g. They are also very stiff, a prerequisite for their fast step-and-settle timeson the order of a few millisecondsand provide resolution to 0.05 µm. The lack of a leadscrew means no lubricant flow and material relaxation to cause submicron creep. There is also no rotational inertia to limit acceleration and deceleration.
- Excellent Power-to-Weight Ratio: PILine® drives are optimized for high performance in a minimum package. No comparable drive can offer the same combination of acceleration, speed and precision.
- Safe: The minimum inertia of the moving platform together with the "slip clutch" effect of the friction drive, provide better protection of precision fixtures / devices than with leadscrew-driven stages. De.spite the high speeds and accelerations, there is a much lower risk of pinching fingers or other injuries than with conventional drives. This means users may not need interlocks, light curtains or other measures to keep them safe.
- AutoLock Feature: PILine® drives create a braking force when not energized without causing the position shift common with conventional mechanical brakes. Other benefits of the AutoLock feature are the elimination of servo dither and steady-state heat dissipation.
- Vacuum Compatibility: Vacuum compatible versions of PILine® drives are available.
- Negligible EMI: PILine® drives do not create magnetic fields nor are they influenced by them, a decisive advantage in many applications.
- Custom Solutions / Flexibility for OEMs: PILine® drives are available in open-loop and closed-loop translation stages and as OEM components. PI develops and manufactures all piezo ceramic components in-house. This gives us the flexibility to provide custom motors (size, force, environmental conditions) for OEM and research applications.
- Quality, Lifetime, Experience: Based on PIs 30+ years of experience with piezo nanopositioning technology, PILine® drives offer exceptional precision and reliability with an MTBF of >20,000 hours. Rotating components such as gears, shafts and moving cables that are prone to failure in conventional motion systems, are not part on the PILine® design.
Fig. 1. Variety of PILine® ultrasonic piezomotors, translation stages and motion controllers.
Fig. 2. State-of-the PILine® ultrasonic motors are based on a simple construction allowing for the design of low-cost drive units and extremely compact, high-speed micro-positioning stages smaller than a matchbox.
Fig. 3. Custom high-force ultrasonic piezo motors can provide forces to 50N.
Fig. 4. Settling behavior of a PILine® M-663 linear stage, 100 µm step. A stable position to within 0.1 µm can be reached in only 10 ms.
Fig. 5. Principle .design of a PILine® ultrasonic piezomotor-driven translation stage.