Piezoelectric Drives - CVI Melles Griot - #1

Text version of the page

Fundamentals of Positioning 8.13 Piezoelectric Drives voltage is applied to such stacks. Note that the increase in length is approximately linear with the applied field and that there is some saturation at higher voltages. Also there is pronounced hysteresis, which is greater with the soft piezoelectric material. Although high voltages are used, power consumption is low, and almost no energy is consumed in maintaining a fixed position with a fixed load. Piezoceramics can respond rapidly to changing input voltages (microsecond time constants), and the positional resolution is limited only by the noise of the power supply. The need for voltages in the 1 to 2 kV range has restricted their utilization because of the cost, electronic noise, reliability, and safety issues involved. Flexure technology provides the capability to design stages and component holders that are completely free from any hysteresis, creep, drift, friction, and stiction. The positioning and alignment capability of a device using well-designed flexures is limited only by the drive mechanism. A highresolution thumbscrew or micrometer will provide precision travel from many millimeters down to about 1 mm. A state-of-the-art differential micrometer will provide a 50-nm resolution over a range of 300 mm, and a modern piezoelectric (PZT) drive will provide movement and resolution from about 200 mm down to the region of 1 nm using position feedback piezoelectric control. PIEZOELECTRIC EFFECT Piezoelectricity, or pressure electricity, a property of some crystalline materials, was discovered by Pierre and Jacques Curie in the 1880s. When these materials are compressed, they produce a voltage proportional to the applied pressure. This effect became known as the piezoelectric effect. Conversely, when an electric field is applied across the material, there is a change of shape. In fact, the change is proportional to the applied electric field, and it is this latter change in shape that is useful when producing the small dimensional changes required for precision positioning. The piezoelectric effect is extremely small in naturally occurring minerals, but present-day materials technology has produced a range of ceramics which can deliver linear extensions of up to 1 percent. Several natural materials exhibit piezoelectric properties, but most devices now use polycrystalline ceramics, such as lead zirconate titanate, and are generally known as PZT ceramics. Typically, linear extensions of up to 200 ìm are obtained when suitable voltages are applied to the appropriate ceramic geometries. Several families of ceramics and types of devices were developed when designers attempted to accentuate the more desirable properties and minimize the less appropriate ones for specific applications. Although similar materials are used, it is proper to refer to the devices that operate in the ferroelectric region below the Curie temperature as piezoelectric and to those that operate in the paraelectric region above the Curie temperature as electrostrictive. Piezoelectric ceramics must be poled for them to exhibit piezoelectric properties. Above the Curie temperature, the electric dipoles are randomly arranged. If a strong electric field is applied when ceramic is cooled below the Curie temperature, the dipoles remain partially aligned and respond collectively to subsequent smaller field changes producing significant dimensional changes. Traditional piezoelectric materials are categorized as soft or hard. Both must be poled. Hard piezoelectrics have Curie temperatures above 300°C with limited dimensional changes; soft piezoelectrics have lower Curie temperatures and greater dimensional changes but depole more easily. However, soft piezoelectrics can be repoled quickly and easily. Traditionally, high voltages (up to 2000 Vdc) have been applied to stacks of thin slices of piezoelectric materials to produce the required extensions. Figure 8.16 illustrates the typical length increase when Piezoelectric drives EXTENSION V soft hard VOLTAGE 0 250 500 750 1000 Figure 8.16 Comparison of soft and hard piezoelectric ceramics The curves show typical hysteresis behavior as the voltage applied to piezoelectric stacks is increased and decreased. The inset diagram shows how the voltage is applied to a stack made from seven slices of piezoelectric ceramic.