Active piezoelectric shims improve machine precision

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MECHANICAL ENGINEERING

By Dr. Mathias Bach and Stefan Vorndran

Active piezoelectric shims improve machine precision

Due to the piezo element’s high resolution down to the nanometer range, active shims cover applications in classical mechanical precision engineering as well as the alignment of optical components in astronomy, semiconductor manufacturing and in materials research employing beamline instrumentation

Figure 1: Aligning parts that have become misaligned due to stress, creep, or settling processes is an application for active shims, when sub-micron or nanometer precision and stability are required. Courtesy: PI P recision machine tools, laser processing equipment, and optical apparatus are often complex assemblies of different component types. The alignment of these components is critical for the overall precision and function of these systems. If a target dimension between two components changes, readjustment may be unavoidable. That can be the case when a machine is put into operation after delivery and tolerances are out of spec due to initial setting processes.

Moreover, long-term creep or temperature changes can have the same effect. Optical measuring facilities, astronomical devices, wafer processing machines, chip holders, or positioning systems for heavy-precision industrial applications are frequently affected by these issues. Differences on the order of a few microns, or less, can exceed limitations.

The classical solution to fix such differences is to use shims ground exactly to the required tolerances. The fact that they must be installed — often in hardto-reach locations — can be a time-consuming and expensive disadvantage. This type of adjustment is not infinitely possible, and once the dimension has been fixed, it can be very difficult to change it afterward.

Engineers may often have wished for the possibility to change the thickness of such shims remotely to bring the system back into perfect alignment. With the advent of a new technology, this is no longer wishful thinking. In such cases, piezo-based, active shims, or spacers, are a practical solution for the adjustment process. Once installed in a machine, active shims can readjust the gap between two components at any time with nanometer precision (see Figure 1).

Active shims such as the PIRest piezo-based shim, from Physik Instrumente

Figure 2: Piezo-based shims. Courtesy: PI

Figure 2a: Built into the machine during its construction, programmable shims can be manufactured in any geometry and size. Courtesy: PI

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Figure 3: The allceramic insulation of PICMA multilayer piezo ceramic elements protects them from environmental influences. Courtesy: PI Figure 3a: A large variety of standard shapes and sizes of PICMA multilayer piezo ceramic elements are available. Courtesy: PI

Figure 4: Conventional piezo actuators: Typical displacement curves (left) of traditional open loop (no position feedback) piezo actuators, and basic design thereof (right). Displacement is roughly proportional to the electric field and when the drive voltage is removed, the displacement will recede to zero once the element is fully discharged. Courtesy: PI

(PI), are a novel alternative, simplifying and speeding up the adjustment process considerably. Due to the piezo element’s high resolution, down to the nanometer range, it covers applications in classical mechanical precision engineering as well as the alignment of optical components in astronomy, semiconductor manufacturing, and in material research such as conducted with beamline instrumentation.

High-load capacity

The piezo-based spacers are installed in the machine during its construction. They are available in virtually any shape and size, such as plates, rings, and cylinders, and can be designed to hold heavy loads of several tons (see Figures 2 and 2a).

The electro-ceramic core of the piezobased shim is manufactured using a patented PICMA multilayer piezo actuator process — proven in many earthly applications, as well as working successfully on the Mars Rover for several years, after passing 100 billion cycles of a NASA test program without failure. The piezoceramic active element — a monolithic block, whose active layers are made up of thin ceramic film — are protected by an all-ceramic insulating layer to keep environmental influences and humidity out. The durability of this multilayer piezo ceramic technology was proven regularly in industry, life sciences, microscopy, medical technology, and research (see Figures 3 and 3a).

The concept of a piezo ceramic actuator is well understood: Displacement depends on electric charge, and by changing the drive voltage, the actuator expands or contracts in real time (see Figure 4). While drawing negligible power in steady-state operation, the actuator will slowly recede to its “zero” position when the power source is removed.

High-resolution, stable active shims

Displacement of PIRest is programmed with a specific control tool, and remains after disconnection from the power source, comparable to a self-locking screw type actuator, but at much higher precision and without the creep.

A voltage connector for programming is provided with the active shim; it only needs to be connected shortly for each respective adjustment. The necessary cables can be considered during machine design and become a permanent part of the system. After adjusting, the desired position remains stable without power and the power supply can 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-micron nominal adjustment range, indicated a position drift of less than ±100 nm, regardless of the displacement. As an option, the active shims also can be equipped with a temperature sensor. Skillful combination of the active shims makes it possible to adjust in up to six degrees of freedom.

If required, active shims also can be combined with classical piezo actuators. Typical applications for these types of hybrid systems include dynamic vibration compensation, readjusting the focal plane during an optical measuring or scanning process, as well as controlling a laser beam in metrology systems or materials processing. PE

Dr. Mathias Bach is Head of Piezo Systems at PI (Physik Instrumente) L.P.

Stefan Vorndran is vice president of marketing at PI (Physik Instrumente) L.P.