Achieving high precision in linear stages without costly air bearings

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Alexander BrommeManaging Director, Steinmeyer FMD

George JaffeExecutive VP, Steinmeyer Inc.

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© 2012 Steinmeyer GmbH & Co. KG

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Measurement applications in biomedical, semiconductor, and optical industries demand highly accurate linear stages. Typical specifications in applications such as surface topology machines, for example, include a maximum deviation of 1 micrometer in position and straightness/flatness over a travel range of 100 millimeters or more.

Such high-precision requirements are usually satisfied by using air-bearing stages with linear motors — a high-tech but very expensive solution. Besides the basic high cost and larger size of such stages, preventing damage in an emergency requires additional systems to monitor air pressure and motor power, which further increases cost and technical complexity.

By combining many years of experience in manufacturing high-precision stages with the use of special components, Steinmeyer’s FMD division achieves remarkable precision with classical cross-roller bearings and high-quality Steinmeyer ball screws. These solutions are especially cost-effective, smaller in size, and easier to use compared with air-bearing systems.

The key special component is a decoupling system that isolates any transverse move-ment of the ball screw from the linear bearing system and saddle of the stage.

Even though the linear bearings in Steinmeyer’s precision stages are dramatically oversized, their stiffness is not infinite. Disturbance forces can elastically deform the base plate, bearing, and saddle, interfering with the straightness and flatness of the entire stage system.

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External influences, such as the application itself, can introduce such forces, of course, but these are either minimal or nonexistent in most measurement operations. More significant are internal disturbance forces caused by the eccentricity of the ball screw nut while the screw rotates. Since the stage saddle normally connects directly to the nut, transverse forces from the nut affect the stage system adversely by contributing to runout. By applying flexure designs well known in piezo-micropositioning stages, Steinmeyer FMD has developed a solution that virtually eliminates these effects.

Flexures have no play or surface friction, so wear, stick-slip, and lubrication are definitely of no concern. Flexures can, however, overload easily, so precision operation requires careful engineering. The simplest form of a flexure is a single flat spring that is fixed at one side and acted on by a force at the opposite side. This type has low stiffness in the bending direction but very high stiffness in the shear direction, which is the same direction as the ball screw drive.

This transverse force deforms the flat spring, which not only creates lateral movement but produces bending, as evident in Figure 1. When this happens, the upper and lower surfaces of the spring are no longer parallel. While the flexure compensates the trans-verse force, the result is additional and undesirable torque.

This disadvantage can be minimized by combining two of these flat springs to form a parallelogram assembly. This enables lateral compensation without detrimental torque.

Figure 1: simple flat spring — 3D, at rest, under transverse load

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While the upper and lower surfaces of the assembly remain perfectly parallel, there is also a visible reduction of overall height as a side effect of the lateral displacement. In reality, this effect is mostly negligible — in our example, typically a contraction in the range of 10 nanometers.

The exact direction of the transverse force coming from the ball screw, however, can vary. It is always orthogonal to the driving direction, but can be anywhere between horizontal and vertical with respect to the base plate of the stage.

Hence, it is necessary to use two independent, orthogonally arranged flat spring paral-lelogram units as a decoupling arrangement (see Figure 3). Whatever the direction of transverse forces, this arrangement only transmits actuating forces of the ball screw (high shear stiffness) and compensates any disturbance forces (low bending stiffness).

Figure 2: parallelogram spring assembly — 3D, at rest, under transverse load

Figure 3: double parallelogram assembly — 3D, at rest, under load

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Figure 4 shows an example of a linear stage using a decoupling arrangement. The precision linear bearing between the saddle and the base is the main component determining straight-line motion. The side-mounted ball screw drive assembly includes thrust bearing, rear support bearing, coupling, and DC motor. The ball screw nut connects to the saddle by the decoupling device (shown in blue).

Steinmeyer ball screws usually have a transverse nut movement of less than 2 microm-eters. Considering the number of parts and their intricate geometry, this is an amazingly small value! Nevertheless, this 2-micrometer peak-to-peak disturbance causes a systematic periodic waviness with an amplitude of 500 nanometers peak-to-peak or greater. For many high-precision measurement applications, 500 nanometers of waviness is unacceptable.

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Figure 4: ball screw driven linear stage with decoupling mechanism

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Figure 5 shows flatness runout for a travel range of 50 millimeters. Clearly there is a sinusoidal waviness with a wavelength of 2 millimeters that directly matches the ball screw pitch. The measured runout is 500 nanometers peak-to-peak.

Figure 6 shows the same measurement after installation of the decoupling assembly.The previous significant waviness is almost completely gone. The remaining magnitude is less than 50 nanometers, which is 10 times lower than before. The dominant error is now a long-period deviation over full travel, which is negligible in most applications or which can easily be compensated (the two graphs show the same data with and without compensation).

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Figure 5: stage flatness without decoupling unit

Figure 6: stage flatness with decoupling unit

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The decoupling unit (see Figure 7) is assembled from parts made from spring steel, tool steel, and brass. Tests with monolithic designs, where spring elements are cut by wire EDM from a solid steel block, did not yield significant performance or price advantages.

The technology of decoupling devices is standard in the PMT160 stage series from Steinmeyer. Production-proven in more than 1,000 units, this method brings obvious benefit to our customers’ precision applications. With typical deviations in straightness and flatness of 50 nanometers over 50 millimeters travel, these stages represent a cost-effective alternative to air-bearing systems.

A typical application for stages with decoupling devices is high-precision instruments that measure roughness or surface topology. Adding such a stage to a standard two-dimensional profilometric roughness measuring instrument would enhance a 3D-topography system. Instead of a single profile, a number of parallel profiles would be available, which could then be combined in software to yield a three-dimensional image of the object surface.

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Figure 7: actual decoupling assembly from PMT160 stage series

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Steinmeyer, Inc., is the North American subsidiary of the German-based August Stein-meyer group. It offers the most precise industrial positioning systems available today, from Steinmeyer Feinmess Dresden (FMD), a preferred supplier of mechatronic systems worldwide. It also supplies and supports the full line of world-class, high-precision ball screws manufactured by Steinmeyer GmbH & Co., KG. Besides an extensive range of standard offerings, the company specializes in products customized to meet very specific application requirements. The Steinmeyer group has produced engineering innovations of the utmost precision and quality for more than 130 years. To learn more about the company and its products, please visit Steinmeyer.com or contact Steinmeyer, Inc., at 781-273-6220.

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