Piezoelectric inertia drive motors utilize the body's inertia to move in small strides via a constant friction contact. In the past, piezoelectric actuators were commonly used due to their high resolution and small response time. However, in positioning applications, the main downside of such solid-state actuators is that they deliver short strokes. This causes a standard 10 mm length piezoelectric actuator to achieve the highest stroke value of just 20 μm.
Bending piezoelectric actuator models and other configurations may raise the stroke at the cost of actuating power and stiffness, but even with these designs, strokes of a few millimeters are difficult to achieve. In piezoelectric motors, such short strokes occur repetitively at frequencies of up to many hundreds of kHz and are combined via various mechanisms to gain a macroscopic step. This motion can be rotary or linear and is largely unrestricted. As a result, spherical or orbital paths can be obtained reasonably quickly. In contrast to standard electromagnetic motors, piezoelectric motors offer magnetic insensitivity, powerless holding force, massive torque or force with small size, and are ideal for manufacturing small devices.
Types of piezoelectric motors
Piezoelectric motors are available in four main configurations: traveling wave, standing wave, walking and inertia motors. The first popular piezoelectric motor variants are traveling and standing wave motors. They are also described as ultrasonic motors or resonant motors, since the stator’s resonant motion at the ultrasonic frequencies is generally required to get the required stator contact point’s vibration amplitude. A rotor then goes through a friction contact due to these contact points. Walking piezoelectric motors use the “clamp and feed” phenomenon and work at frequencies less than their resonance frequency. The rotor is clamped by one piezoelectric actuator while the other actuator drives it.
This article focuses on piezoelectric inertia motors. When comparing it with other types, it does show inferior power, velocity or force characteristics. Therefore, their mechanical designs are basic and are normally operated by a single electric signal per tuning parameter and offer fine positioning capabilities with primarily boundless resolution. Such features bring extra benefits for manufacturing small components and make piezoelectric inertia motors ideal for handheld and smartphone cameras. Most of its many versions use a driven component’s inertia to move it via a continuous friction contact. Furthermore, these motors are often referred to as "stick-slip motors" due to their frequent change from sliding to static friction, which exists in various inertia motors.
Construction of piezoelectric inertia motors
Piezoelectric inertia motors are essentially made up of few components that are simple to produce and are usually driven by one electric signal per tuning parameter. On a side note, in resonant motors, it is viable to sequentially utilize many tuning parameters driven only by a single electrical signal when multiple vibration modes can be triggered by changing the excitation signal. A piezoelectric inertia motor having one tuning parameter usually consists of three fundamental components: a combination of driving rod and solid-state actuator developing the motor stator, and a slider traveling parallel to the rod. Therefore, the key features in the construction of a piezoelectric inertia motor are the solid-state actuator, friction contact present among the slider and rod and the electrical signal excitation.
Applications of piezoelectric inertia motors
The original piezoelectric inertia drives were fabricated for horizontal positioning systems, for example, in microscopy. Later on, researchers started making advanced designs of piezoelectric inertia motors. However, all of these advances are also based on microscopic applications, and several of them are associated with the modern scanning tunnel microscopes and more scan probe microscopes. Most inventions were not planned for mass production but for use in the inventor's lab. The documented motor design changes were primarily related to the maximum slope a motor may manage. Afterward, different processes were also used to develop vertical motion and normal force that is not dependent on gravity. There were also successful attempts to the incorporation of multiple tuning parameters into a single motor.
Today, piezoelectric inertia motors are still primarily used in positioning systems of labs such as cell manipulation, micro handling, microscopy or nano handling. For these kinds of applications, several industries have expertise in the manufacturing of accurate piezoelectric inertia motors. Almost all these motors are not using any force or may be using a little of it for positioning applications. However, it is usually beneficial if a motor can also supply a specific force since this ensures that application systems are resilient against wear and debris. Furthermore, it permits fine-positioning applications and non-horizontal functioning, in which the flexure hinges apply a minor restoring force. Therefore, many piezoelectric inertia motors that can also exert force are also available in the market today.
Conclusion
Piezoelectric inertia motors are engineered to be mechanically basic, are normally driven by only one electric signal per tuning parameter, and naturally, supply a fine positioning ability with largely limitless resolution. Such unique advantages make it usable in the modern applications described above. However, just a few examples are available for the usage of piezoelectric inertia motors commercially and on a broad scale. It is due to restricted comprehension of their working and optimum operation and no availability of detailed architecture rules. Some more research work and the sharing of recent findings can assist in resolving these shortcomings.
