The purpose of an actuator is to convert electrical energy into mechanical energy. This electrical to mechanical conversion is seen in speakers or motors. Piezoelectric actuators are used for their precise control and efficiency in a wide variety of applications, with precision often to a microscopic scale (down to the micrometer). There are multiple types of actuators (explained below) which suit different needs based on their unique properties.
Piezoelectric actuators are very simple in design with little or no maintenance or lubrication required. There are virtually no moving parts and they are quite reliable. For this reason, they are best suited for applications requiring precision movement, for example in motors and a wide growing variety of other practical uses.
A stack actuator is comprised of a stack of multiple slabs of smaller pieces of piezoelectric material. This can be accomplished in two ways: either the components are fired separately and connected later via adhesive (discrete stacks; the components are normally discs), or they are co-fired and bound by subjecting them to intense heat and pressure, or sintering (monolithic stacks).
The underlying principle of stack actuators is the intrinsic quality of constancy. The charge generated is independent of the size of the ceramic a user is working with (this is mathematically modeled by the longitudinal effect). Thus, putting many pieces together creates a multiplicative effect on their transformations’ magnitude for the same voltage, amplifying their power.
Differences between Discrete and Monolithic Stacks
The main difference between these two types of stack actuators is their operational voltage. Discrete stacks are generally operable between approximately 500 and 1000 Volts, whereas monolithic stacks are operable around 150 and 200 Volts. Due to their similar compositions, these two types of stacks have negligible differences.
Benefits and Shortcomings
Stack actuators can cover a wide range of requirements, both in terms of voltage and power. Furthermore, they can deal with many extreme types of environments that conventional motors cannot operate in, such as very low temperatures, vacuums, or powerful magnetic fields. They are extremely efficient relative to the conventional motor and have conveniently simple designs.
One shortcoming of stack actuators is that any stress applied to the actuator must be along the cylindrical axis – any other than compressive force may cause complications.
In a strip actuator, two pieces of piezoelectric material are bonded together electrically and are polarized in opposite directions. When electricity is applied to the actuator it causes one piece to contract and the other to expand, thus creating a bending motion. Through this electrical generated action very large displacements can be achieved.
Strip Actuators are best for situations that require high sensitivity or large forces and displacements. They have been used in accelerometers, small pumps, switches, and many other applications. It is worth noting that, if not coated, strip actuators can be affected by the environment such as dust, thus limiting the lifespan of these components.
There are two types of disk actuators – unimorph and bimorph. Unimorph actuators are comprised of one layer, whereas bimorph actuators are comprised of two distinct layers. In both, a metal plate is used as a base with one or two pieces of piezoelectric material attached, respectively. When current is passed, the piezoelectric material shrinks or contracts, and bends the metal with it. When this current is released, the metal springs back to its original state.
These actuators are normally used for sound production and use AC power instead of DC power. Coupling their displacement’s power with rapid vibration makes for a clear sound with potent sound pressure. This can be observed in products like fire alarms. Other than the production of sound, however, disk actuators cannot compare to the power of the strip actuator, in terms of force.
While piezoelectric components are not as viable in many types of machines (this is because they don’t produce as much force as their counterparts), their volatility and precise control gives them distinct advantages, especially in a growing number of microscopic applications.
Microelectromechanical Systems (MEMS)
Also known as micromachines or micro systems technology, MEMS involves mechanisms operating at the microscopic level, in precision micro-moving parts. Piezo actuators are now being used at these extremely small scales in many common applications where precision and simplicity is in ever greater demand.
One example of such an application is inkjet printers. While consumer printers generally use thermal systems to deposit ink, most industrial printers utilize piezoelectricity to do so. By putting ink in a nozzle and a piezoelectric transducer at the base, when a charge is applied, the piezoelectric component creates a pressure wave through the ink, forcing a few drops out.
MEMS precision has been used in applications including precision focusing in high power microscopes. Due to the extremely small field of view these microscopes provide, users naturally need equally precise control over the focus and movement of the lens. Piezoelectric components are a very viable solution, with precision to the micrometer range.