Piezoelectric Actuators Overview

Background

Piezoelectric actuators can be generally characterized by the piezoelectric coefficients that they exploit (namely d33, d31, and d15). For example:

• a 33-mode piezoelectric actuator produces displacement in the same direction as an electric field applied parallel to the material's polarization direction.
• a 31-mode piezoelectric actuator produces displacement perpendicular to an electric field applied parallel to the material's polarization direction. Approximately twice the strain can be obtained from a 33-mode piezoelectric actuator than a 31-mode piezoelectric actuator for the same applied field; this is due to the relative magnitudes of the piezoelectric coefficients.
• a 15-mode piezoelectric actuator exploits the shear strain produced by an electric field applied perpendicular to the material's polarization direction. Shear strain piezoelectric actuators are not as popular as other piezoelectric actuators since they are difficult to manufacture.

The three simplest piezoelectric actuator designs are the cylindrical actuator, the bimorph (and unimorph) actuator, and the multilayer actuator. Cylindrical actuators operate in 31-mode with the electric field applied across the cylinder wall to obtain an elongation of the length of cylinder. Cylindrical actuators can produce precise displacements. The bimorph is a two layer structure operating in 31-mode to obtain large displacements but low generative force. Multilayer actuators that operate in 33-mode can produce higher strains than cylindrical actuators and generate greater forces than bimorphs. For example, a multilayer actuator with a 1 cm x 1 cm cross section can easily transmit 1000 N of force. TRS specializes in producing multilayer piezoelectric actuators.

In a multilayer piezoelectric actuator, each layer acts as a separate actuator connected electrically in parallel with all of the other layers. The displacements produced by each layer are additive. The advantages of using multilayers over one solid block of piezoelectric material can be thought of in two ways:

• for a given applied voltage, the total strain produced by the multilayer will be N times greater (where N is the number of layers) than the strain produced by one solid block of piezoelectric material having the same overall dimensions
• for a given strain level, the required voltage will be N times lower for the multilayer than for a single block having the same overall dimensions

These advantages are critical for practical applications. In particular, thin layers are desired to keep operating voltages as low as possible.

Piezoelectric Actuators at TRS

TRS produces multilayer actuators made from all of its current materials, including piezoelectric single crystals, electrostrictive ceramics, and fine grain piezoceramics. Two main multilayer configurations are under development:

* High force stacked actuators consist of ceramic or single crystal plates that are bonded together. Fine grain ceramics excel in these devices due to the small layer thicknesses that can be realized and due to the fact that the fine grain ceramics can be safely driven to higher fields than conventional ceramics. TRS produces fine grain actuators with both 0.5 and 0.25 mm layer thicknesses that operate at 1000 V and 500 V, respectively. These actuators yield strain levels > 0.2% with no degradation after cycling at maximum voltage to 10E9 cycles (frequency = 300 Hz, prestress > 10MPa). Single crystal stacked actuators can currently be fabricated with layer thicknesses as low as 0.25 mm to produce strain levels comparable with bulk crystals.

Low profile stacked actuators are also available. These actuators operate in 33-mode but mimic the motion of 31-mode patch type actuators. Higher strains are possible in this way.

* Cofired multilayer actuators are produced using standard cofired multilayer capacitor technology and are, thus, an inexpensive alternative to bonded stacks. Layer thicknesses of 0.05 to 0.1 mm are possible. TRS produces both fine grain and electrostrictive cofired actuators with operating voltages of ~ 100 to 250 V. The electrostrictive actuators exhibit very low hysteresis and extremely accurate displacement reproducibility. Individual cofired multilayer actuators are currently 7 x 7 x 10 mm in size. These actuators can be bonded together to produce larger actuators using a proprietary bonding method. This method creates a stiff bond that does not sacrifice strain.

Cylindrical actuators and other complex shapes have also been produced by TRS. All piezoelectric actuators can be custom-designed for your applications. Please contact us for more details.

The following data sheet is available to download:

Data sheets can be viewed using the free Acrobat® Reader.