Reliable Piezo Motion Devices for Industrial and Space Applications

Piezo ceramics convert electrical energy directly into mechanical motion and vice versa, based on crystalline solid state effects. Piezo actuators are quite unique in combining force, speed and precise motion into a small package. To make them more accessible for OEM designers, manufacturers can package the actuators inside an arrangement of flexures providing precision guidance, amplified motion along with a simple mounting interface. Flexures are usually made of aluminum, steel or titanium. With the absence of friction and wear, they can provide 10’s of to 100’s of billions of cycles of maintenance-free service. Piezo motion devices allow position changes to be made in milliseconds or even microseconds and with precision down to sub-nanometers without difficulty. Piezo actuators (not to be confused with piezo motors) provide motion output proportional to a drive voltage – usually up to one millimeter and high dynamics with frequencies in the hundreds to thousands of Hz. There is no mechanical wear and no frictional parts to limit resolution: perfect conditions for many precision motion and positioning applications in industry, life sciences, microscopy, medical engineering and in space.

100 Billion Cycles – Can an Actuator be Designed to Provide this Kind of Reliability?

How to do it Right

The answer lies in optimized materials and manufacturing processes, appropriate design and, last but not least, the choice of suitable insulation. The PICMA® cofired piezo stacks (Figure 1) supplied by PI Ceramic are a good example. Based on a stack of layers of specialized ceramic, only a few dozen microns thick, interleaved with electrodes and sintered into a solid structure.

During manufacturing, first, the ceramic material, which is based on a special modified PZT (a ferroelectric ceramic), is ground to make a slurry from which thin ceramic foils are cast (Figure 2). This material is useful for motion positioning applications because PZT ceramic exhibits a small but almost linear dimensional change as a drive voltage is applied across the electrodes. Next, electrodes are screen printed and the layers are laminated to stacks. The ceramic is compacted to remove the air trapped between the individual layers and sintered together with the electrodes (cofiring technology) to create a monolithic block. This is protected against humidity and failures by a ceramic insulation layer. The monolithic design also increases the material stiffness leading to a higher mechanical resonant frequency, prerequisites to fast mechanical response to electric input and highly dynamic applications. The ceramic is not flexible, the motion is fully based on solid state effects.

The exclusive use of inorganic materials, for insulation, and the electrical contacts (Figure 3), also facilitates use in in ultra-high vacuum, as there is no outgassing.

Figure 4 shows accelerated lifetime testing results under high humidity conditions comparing piezo ceramic actuators with conventional conformal coatings and ceramic-insulated PICMA® actuators. Resistance to humidity is especially important for positioning applications in the semiconductor industry. High and constant offset-drive voltages are usually employed here, for example, to keep wafers in a stable position during lithography and inspection.

To maximize lifetime and minimize stress in such dynamic applications it is essential to reduce to probability of cracks. This is where the patented slot design of the PICMA® actuators comes into play, as it minimizes the tensile load. The lateral slots prevent an increase in mechanical tensile stress in the stack and thus counteract the formation of uncontrolled cracks (Figure 6).

Another feature to consider is the stress on the electrodes. Repeating the same motion 100 billion times can wear you out. To avoid fractures of the termination electrodes, a patented meander-shaped design (Figure 7) ensures all internal electrodes have a stable electrical contact even at extreme dynamic loads. This is why the PICMA® design will survive where other designs fail. Download paper on PICMA® piezo design and reliability: Reliability & Lifetime of Multilayer Piezo Actuators

Low Operating Voltage

PICMA® actuators are designed to run at minimized operating voltages. In contrast to most conventional actuators they achieve their nominal motion output at operating voltages significantly below 150 V, in the case of bender-type actuators as low as +/-30V. This property is achieved with particularly thin ceramic layers.

The motion output of a piezo stack is limited to about 0.1% of its physical length. When longer motion ranges are required along with ease of installation and integration, a flexure actuator with integrated motion amplifier is the right choice. At the heart of these flexure actuators is a piezo stack (Figure 9).

Depending on the design, the motion range can be amplified up to 20X, while at the same time protection from lateral and tensile forces is provided and precision guidance is added. Increasing the motion range however, reduces the stiffness, and a proper balance between maximum displacement and stiffness / force has to be found for the intended application.

Ease of Control

Today, motion system designers can chose among a wide range of piezo motion controllers, spanning the spectrum of capability and cost.

What’s Next

Piezoelectric devices are increasingly being utilized by product designers and incorporated successfully into a widening range of applications where precision motion control is needed. While piezo stack actuators and the latest generation of flexure actuators deliver a broad product development capability for OEM designers, piezo ceramics provide the basis to many more motion control solutions. Piezo motors, for example, a separate class of piezo motion devices for long travel applications, provide fast and precise motion significantly beyond one millimeter. Piezo motors are divided in to several sub-categories such as resonant motors, inertial motors, inchworm-type motors etc. There are linear and rotary motor configurations each with specific performance characteristics. Stay tuned to learn more or click here for some piezo motor product examples.