Dr. Mathias Bach, Head of the Piezo Systems Division at PI (Physik Instrumente)
If a target or an actual dimension between two components inside precision machines changes, for instance in laser processing or precision engineering, readjustment may be necessary. That can be the case for example, when the user's machine is put into operation and initial setting or long-term drifting or changes in tolerance need to be compensated locally after installation. Optical or measuring facilities, astronomical devices, machines for processing wafers, chip holders or positioning systems for heavy precision industry are frequently affected by this. In such cases, piezo-based "shims" are a practical solution for the adjusting processes. Once they have been installed into the machine, the active shims not only make it possible to readjust the gap between two components at any time but also achieve this with nanometer precision.
If precision machine components need to be readjusted, the classical solution is to use shims that are ground exactly to the gaps required. However, the fact that they have to be built in mechanically, and often at hard-to-reach locations, is a decisive disadvantage. Manual alignment with mechanical shims requires a lot of effort and is also time-consuming. Furthermore, this type of adjustment is not infinitely possible and once the dimension has been fixed, it is often very difficult to change it afterwards. The new PIRest (image 1) piezo-based shims now available from PI (Physik Instrumente) are the practical alternative and they simplify and speed up adjustment considerably. Due to the actuator's high resolution of only a few nanometers, not only those applications in classical mechanical engineering are included, but also alignment of optical components in astronomy, or material research in synchrotrons, and in semiconductor manufacturing for example.
Active shims with nanometer resolution and long-term stability: Adjusting with piezo-based "shims" (Image: PI)
Set and forget
The piezo-based shims are built into the machine during its construction; they are available – in exactly the same way as other piezo actuators from the same manufacturer – in virtually any shapes and sizes, e.g., as plates, rings, cylinders or even in application-specific shapes (image 2).
The piezo-based shims are built into the machine during its construction; they are available – in exactly the same way as other piezo actuators from the same manufacturer – in virtually any shapes and sizes (Image: PI)
They are also manufactured in the patented multilayer technology as are the proven and durable PICMA® actuators used in industry (image 3). Here, the actual piezoceramic – a monolithic block, whose active layers are made up of thin ceramic film – are surrounded by an all-ceramic insulating layer. This protects them from humidity and failure caused by an increase in leakage current. The multilayer actuators have proven their quality time and again in industry, life sciences, microscopy, medical technology, and research: No failures have ever been recorded in the field. The monolithic piezoceramic block of a PICMA® actuator is very reliable even under extreme ambient conditions such as those encountered during space travel, and this extends the lifetime considerably.
All-ceramic insulated, PICMA® piezo actuators: Durable even under difficult operating conditions (Image: PI)
However, there is one point where the new PIRest actuators are considerably different from the "normal" piezo actuators that are not suitable for readjusting. In the case of classical piezo actuators, the electrical voltage (offset voltage) at the actuator normally needs to be maintained as long as displacement is required. This has two disadvantages for adjusting: An additional stable power supply is required for the machine's equipment and this would have a negative effect on the lifetime of the actuators. PIRest technology works differently in this case: It is still based on piezo actuators but thanks to special control, it is possible for them to not only maintain stable displacement after adjusting but also achieve nanometer precision without the need for an offset voltage.
Active Shims with Nanometer Resolution and Long-Term Stability
The static gap can be set by briefly applying a supply voltage to the PIRest actuator (image 4 and 5). A voltage connector is provided in the active shim that only needs to be connected to a voltage source for each respective adjusting process. PI provides an easy-to-use power supply. The necessary cables can be considered during design of the machine and are then a permanent part of the system. After adjusting, the desired position remains stable without power and the power supply can then be disconnected. The displacement stability only depends on the change of ambient temperature. Long-term tests in an environment within +/- 1 K temperature change using an actuator with 10 µm nominal adjustment range, indicated a position drift of less than +/- 100 nm, irrespective of the displacement.
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Image 4
Piezo-based, active shims can compensate adjusting errors easily and remotely, without the need for a permanent control voltage (Image: PI)
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Image 5
Set-and-forget: Problems may arise with initial settling processes during installation of machines and this makes it necessary to readjust machine components when they exceed a certain tolerance threshold (Image: PI)
In the case of standard products, the maximum displacement is between 5 and 35 µm depending on the size of the actuator; but skillful combination of the active shims makes it possible to adjust in up to six axes. As an option, the actuators can also be equipped with a temperature sensor.
The ability for the active shims to adjust in inaccessible locations has been simplified considerably, even more so because piezo-based shims are able hold heavy loads of several tons. If required, active shims can also be combined with classical piezo actuators, e.g., for dynamic vibration compensation. Typical applications for these types of hybrid systems include for example, readjusting the focal plane during an optical measuring or scanning process as well as controlling a laser beam in measuring technology or materials processing.