Piezo motor

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Piezoelectric motors ( piezomotors for short ) are small motors that use the piezoelectric effect to generate movement. Piezomotors can work both linearly and rotationally. In principle, their travel is unlimited for most of the functional principles. In linear motors, due to the design, it is usually a few centimeters and is therefore significantly greater than the travel range of the piezoelectric solid-state actuators used in the motors .

There are a large number of different functional principles of piezomotors. With all principles, the movement is generated by sliding or static friction between a stationary part (stator) and a moving part (rotor). Many, but not all of these principles are based on a frequently resonant oscillation of the stator generated by piezoelectric solid-state actuators. Since the frequency of these vibrations is mostly in the ultrasonic range, many piezomotors are also referred to as ultrasonic motors .

Advantages of piezoelectric motors over electromagnetic motors are a high force per volume, a high holding force when switched off, as well as very good dynamics and miniaturization. In some applications it is a great advantage that they are suitable for vacuum and work without magnetic fields.

Functional principles of piezomotors

Some commonly used principles are:

  • Traveling wave motor
  • Standing wave motor
  • Inertia motor, also known as a stick-slip motor
  • "Inchworm" engine
  • Walking motor

Traveling wave and standing wave motors and related types are also referred to as vibration motors because they are driven by vibrations generated by piezoelectric solid-state actuators. In contrast to this, inertial, “inchworm” and stepping motors are called (piezoelectric) stepper motors because their movement is divided into clearly defined steps. In the case of inertia motors in particular, however, this classification is not always applicable, since there are now also inertia motors whose principle is based on resonant vibrations.

Traveling wave motors

Traveling wave motors are predominantly rotary motors. They consist of a fixed part, the stator , and a moving part, the rotor . There are at least two piezoelectric transducers in the stator that convert the applied alternating voltage into mechanical vibrations. The transducers are excited out of phase , creating a traveling wave on the stator. This sets the latter in motion via the frictional contact between the stator and rotor. In order to achieve high oscillation amplitudes and thus high speeds, the stator is usually operated in resonance at frequencies in the ultrasonic range. A traveling wave in linear traveling wave motors is significantly more complex to generate, which is why linear traveling wave motors have not been commercially available to date. Traveling wave motors have achieved greater popularity primarily through their use in camera lenses. Examples of this can be found in the article " Traveling wave motor ".

Standing wave motors

Functional principle of a piezoelectric standing wave motor

In standing wave motors, a vibration in the form of a standing wave is generated in a stator by piezoelectric solid-state actuators . The resulting, mostly elliptical, movement of one or more contact points drives a rotor. The contact can be temporarily interrupted in the case of large vibration amplitudes, which leads to shocks. Standing wave motors can take many different shapes and produce both rotary and linear motion. The picture on the right shows a rotary standing wave motor driven by four piezo actuators.

Inertia motors

Opened stick-slip piezo motor.
Functional principle of a linear inertia motor with a moving actuator

Inertia motors use the inertia of the moving object to move it via a frictional contact. In classic inertia motors , there is a phase of slow movement in frictional contact with static friction ; in a phase of rapid movement, the inertial forces become so great that the parts slide on each other . This change between static and sliding friction has led to the widespread term "stick-slip motors" (from English "to stick" = to stick and "to slip" = to slide) (see stick-slip effect ). But there are also inertia motors that work without sticking phases. In these motors, the parts also slide on each other in the drive phases.

Piezoelectric inertia motors can be constructed very simply. In the simplest case, they consist of only three components, as in the example opposite. The numerous forms of inertia motors can be differentiated according to whether the solid-state actuator driving the motor is stationary or whether it moves with the motor. Most inertial motors operate at low frequencies down to a few kHz. However, some resonant inertial motors also work in the ultrasonic range. Inertia motors are z. B. used for sample positioning in microscopy and for image stabilization in digital cameras.

"Inchworm" motors

Functional principle of an "inchworm" motor (clamping and sliding) (1 = housing, 2 = feed actuator, 3 = clamping actuator, 4 = rotor)

So-called “inchworm” motors work according to the “clamping and sliding” principle shown on the right. The caterpillar-like principle of locomotion gave the brand name "Inchworm" (English for caterpillar), which today generally describes this type of engine. The motor shown in the adjacent picture consists (above and below) of two clamping actuators and one feed actuator. Because of the clocked operation, “inchworm” motors work at low frequencies in the audible range. They are designed for great forces and precision, less for high speed.

Stepping motors

Functional principle of a piezoelectric stepping motor (1 = housing, 2 = piezo actuator, 3 = intermediate layer, 4 = contact point, 5 = rotor)

In contrast to “inchworm” motors, in so-called stepping motors, the clamping and propulsion are carried out by the same and not by different actuators. In the example shown in the picture opposite, two bending actuators in bimorph design (two actuators plus intermediate layer each) are used. The contact points at their tips would perform an elliptical motion when moving freely. In fact, they press on part of this path against the "rotor", the element to be driven, and push it in the desired direction. Due to the phase-shifted movement of the actuators, at least one of them always clamps the rotor so that it never frees.

Web links

Individual evidence

  1. ↑ a b c d M. Hunstig: Concept, control and properties of fast piezoelectric inertia motors. Writings of the Chair of Mechatronics and Dynamics, Volume 2, Shaker 2014. Zgl. Dissertation, University of Paderborn, 2014
  2. ↑ a b c Tobias Hemsel: Investigation and further development of linear piezoelectric vibration drives . HNI publishing series, Volume 101, 2001. Zgl. Dissertation, University of Paderborn, 2001, ISBN 3-935433-10-7
  3. ↑ SteadyShot INSIDE - Body-integrated Image Stabilization System. Sony website. ( Memento of May 10, 2012 in the Internet Archive ) Retrieved May 10, 2012, archived
  4. ↑ a b J. Twiefel - Experimental and model-based investigation of standing wave drives. Reports from IDS, Volume 05/2010. Zgl. Dissertation, Gottfried Wilhelm Leibniz University Hannover, 2011