INTRODUCTION TO PIEZOELECTRIC MOTORS

The word Piezoelectric roots back to the Greek terminology Piezein, meaning to squeeze or shrink something. The research on piezoelectric materials originated in 1881 when Pierre and Jacques determined that crystals made of quartz produce an electric field when a force or pressure is applied. The inverse methodology is applied in piezoelectric motors where application of an electric stress results in a physical motion either in linear or rotary fashion. So these special kinds of electromechanical systems differ from conventional motors which employ the law or electromagnetic induction to yield motion. In modern designs ultrasonic waves serve as a source to drive piezo motors.



 

What are these motors made of? Modern piezoelectric motors are fabricated using piezoelectric ceramic plates. Based on driving mechanisms these motors are categorized in two major groups. The first kind employs locking and motive groups of crystals and the other type uses piezoelectric elements orthogonally attached to a moving assembly. The base plate is excited by electric field, which can be a high frequency sinusoidal or square wave input. The moving part is connected to the stator, usually in the guided path grooved sideways in the stator, and moves in a linear plane. With each oscillatory cycle of the input, the mechanics executes a step in the range of nanometers. In a broader picture we get a glitch free motion with a virtually unlimited travel range.



How do piezo motors work? As mentioned earlier about the inverse piezoelectric effect is used as the driving mechanism for these motors. The piezoceramics produce movement when excited by external electric field. By using the special excitation sources and the shape characteristics of nanomotion motors, longitudinal extension and transverse bending oscillation mode are excited by approximately the same frequency input and this simultaneous application of the input produces an elliptical shaped trajectory of the ceramic edge. This edge is connected to a moving assembly also called a precision edge which plays the role of an actuator. Due to the excitation voltage, vibrations are produced in the piezoceramic material causing the motion. The periodic driving force at frequencies much higher than the resonance frequency of the moving stage causes smooth motion for unlimited travel with high resolution and position accuracy.

What materials are classified as piezoelectric? There are a number of both natural and synthetic materials which exhibit the reversible piezoelectric effect. Naturally found such kind of elements include Berlinite which structurally resembles to quartz, cane sugar, quartz, Rochelle salt, topaz and tourmaline etc. Human bones when dry, are also an example of a piezoelectric material which primarily acts as a sensor for biological forces inside body. The best example of a laboratory synthesized material is the barium titanate. Lead is also used in synthesis process but due to rising environmental concerns, only lead free materials and devices are allowed to manufacture.

Why piezoelectric motors are a preferable alternative? The typical industrial motors known to us use a laminated winding on its stator which when excited produce a torque. For macro applications these motors are a best option but when it comes to precision and accuracy at a rather smaller scale like managing motion steps in micrometers, conventional motors become hard to manufacture due to complexity of winding design process and unachievable performance parameters. In such a situation piezo motor presents itself as the best alternative. These are simple in design and easy to fabricate and yet another plus is the performance that designers wish for.

Where to apply? These motors, due to precise and micro steps, are used in a huge number of applications like manufacturing process control, camera autofocus system, computer hard disk drives, robotic positioning and medicines handling etc. The high precision gimbal that employs a rotary piezo motor in conjunction with an optical encoder having a micro-step resolution of 0.0001 mrad (milli-radians). Apart from these, piezo motors have brought an ease for developing an equipment used in brain tumor treatment. An MRI robot capable of operation in a highly magnetic environment was not possible previously, the art of controls engineering and piezoelectric positioners from piezo motors have revolutionized the medical field by making treatment of tumors with perfection.

 

Leave a Reply