Piezo technology
Piezo history and principle
Short history
The piezoelectric effect (from the Greek"Πιεζώ“ (Piezo) = Pressing) was discovered in 1880 by the brothers Jacques and Pierre Curie, the latter being the husband of Marie Curie. They found that, when subjected to amechanical load, certain non-conductive materials produce electrical charges on their surface, which has been made conductive.
What is a piezo element?
Piezo elements are electromechanical transducers. With the so-called direct piezoelectric effect, a piezo element converts mechanical forces (pressure, tensilestress or acceleration) into a measurable voltage. The inverse piezoelectric effect works in precisely the opposite way: a piezo element is deformed when avoltage is applied to it, thus generating mechanical motion or oscillations.
The principle
Piezoelectric materials, usually special ceramic objects with surfaces which have been rendered conductive, convert electrical energy into mechanical energy and vice versa.The lattice structure of the molecules in piezo ceramics is asymmetrical below the Curie temperature (TC), and is thus a dipole. Using the strong electric fields, it is possible to permanently polarise piezo ceramics, or in other words give them a preferred direction. The ceramic material now has piezoelectric properties and changes shape when a voltage is applied. The physical deformation takes place along field lines. Since the ceramic materials have a constant volume, the materials shrink at right angles to the field lines.
The advantage of piezo-basedd rives lies in the fact that they hardly need any power to be energised. In electrical terms, a piezo element is a capacitor consisting of two electrically conductive plates and the ceramic piezo material which functions as a dielectric. Current only flows while the capacitor is charging, and the flow drops to zero when charging is complete. Since electrical power is calculated as voltage x current, the power will be zero if no more current flows. In applications that need to be extremely energy-efficient, it is even possible to recover the charging energy when the drive is reset. This can then be used again for the next charging operation.
Piezo bender
Types and versions of piezo transducers and their applications
Depending on the application, the effect described earlier can be used in different types of transducers. Bender actuators, disc transducers and piezo stacks are the basic forms on which piezo elements with more or less complex designs can be based. The bender actuator has a rectangular shape. Its main element is a piezo ceramic material with a conductive surface on both sides. This ceramic material is entirely bonded on one side to asubstrate which is also conductive. The conductive surfaces of the ceramic layer and the substrate function as electrodes. If avoltage is now applied to these electrodes, the ceramic material expands in the direction of the electric field. Since bender actuators are attached at the front end in most applications, this results in a bending motion at the end that is not attached. There are many different versions of bender actuators with a variety of forces and actuator motion that are highly suitable for use in pneumatic valves. Typical characteristic data include deflection of several tenths of a millimetre and forces of up to1N. One special variant that is often used is the so-called trimorph, which has a second ceramic layer applied to the back of the substrate. This increases the performance of the transducer and allows it to be used in a wider temperature range thanks to its symmetry. Applications for bender actuators can be found in circular knitting machines, Braille modules and pneumatic valves – in the latter case particularly in proportional valves for regulating pressure and flow rates.
Benefits of piezo valves
Operating piezo valves
The preferred piezo elements used in pneumatic valves are bender actuators. The performance of piezo valves depends on the strength of the electrical field: the greater the field strength, the better the performance of the actuator and the valve. Compared to solenoid valves, piezo valves don't require a holding current to maintain a switching state. The higher supply voltage required by piezo valves in comparison with solenoid valves is only important during the switch-on phase. Even then, the energy consumed during switch-on is much less than the energy generally needed to actuate pneumatics. This switch-on energy "E" can be approximately calculated using the formula E = CU²/2, where C = the capacitance of the transducer and U = the control voltage. Values usually lie between 0.5 and 5 mWs, because the capacitance of the transducer is generally around 30 nF, while the control voltage can reach 300 V DC.
Benefits of piezo technology
- Low energy consumption
- No heat generation
- Proportionality
- Long service life
- Non-magnetic
- Lightweight
- Short response times
- Silent
- Small dimensions
- Savings potential
- 3/3-way function in one valve
Applied in semiconductor gas control
Gas control in the chamber of semiconductor equipment is actually a basic and important subject. The control and power of various gases are crucial to the performance of wafer yield. However, in valve control within the equipment, in addition to numerical performance such as flow rate and pressure, the amount of suspended particles will also have a significant impact on wafer yield. Therefore, how to control the valve quickly and accurately becomes a very important key.
Chris Lu, Head of Technology Management of Festo Taiwan, gave a 20-minute quick explanation of the "piezo technology" used by semiconductor companies in Taiwan, the United States, and South Korea, which can quickly and accurately adjust valves.
Application examples of piezo valves
Careful handling of fragile workpieces
A unique solution for handling sensitive work pieces is the so-called “speed controller” for double-acting pneumatic cylinders. This enables the speed of pneumatic cylinders to be controlled and ensures movements are jerk-free. To achieve this, four piezo valves are combined with a controller unit to form a system that can be connected to a double-acting cylinder. The speed is controlled by maintaining a constant cylinder exhaust flow rate. This low-cost system works without an expensive displacement encoder. What's more, it enables a smooth start and gentle braking.
Accurate and fast dispensing in industrial and laboratory applications
In industrial adhesive applications, a negative pressure must be generated to prevent the adhesive from accidentally dripping after a specified amount has been applied. It is extremely important that liquids are accurately dosed and do not drip, especially in laboratory applications. Conventional applications normally operate with two valves, one for the vacuum and the second one for positive pressure. Piezo valves are much more accurate and faster for these types of applications. Pressure and vacuum are controlled with the same valve, so there is no need for a second valve.
Production of semiconductors: polishing wafers
Piezo valves are extremely accurate and can reach preset set point values quickly, making them ideal for applications in semiconductor production. They need to dose extremely small volumes of air accurately to meet the demanding requirements for precision machining of the workpieces. When polishing wafers, for example, the challenge is to press the wafers on a rotating polishing table with a very accurately controlled pressure. This is the only way to get a perfectly flat surface. In order to achieve a perfect result, several diaphragm rings are pressed onto the wafers with different pressures. The process must be controlled extremely precisely using both vacuum and pressure. Piezo valves combine both these functions in one compact device.