Measurement applications in bio-medical, semiconductor and optical industries demand highly accurate linear stages.
Typical specifications in applications such as surface topology
machines include a maximum deviation of 1 micrometer in position and
straightness/flatness over a travel range of 100 mm or more.
Usually such high precision requirements are satisfied by using
airbearing stages with linear motors - a high-tech but very expensive
solution. Besides the basic high cost (and larger size) of such a
stage, additional systems are needed to monitor air pressure and motor
power to prevent damage in an emergency - thus further increasing 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. The 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 movement of the ball screw from the linear bearing system
and saddle of stage.
Even though the linear bearings in Steinmeyer's precision stages are
dramatically oversized, their stiffness is not infinite. Disturbance
forces can elastically deform base plate, bearing and saddle and
interfere in the straightness and flatness of the whole stage system.
Such forces can, of course, be introduced by external influences,
such as the application itself. However, in most measurement
operations, external forces are either minimal or non-existent.
One of the most important internal disturbance forces comes from
eccentricity of the ball screw nut while the screw rotates. Since the
stage saddle is normally directly connected to the nut, any transverse
forces from the nut adversely affect the stage system by contributing
to runout. By applying flexure designs well known in piezo
micropositioning stages, Steinmeyer FMD developed a solution that
virtually eliminates these effects.
Flexures do not
have any play or surface friction, so wear, stick-slip and lubrication
are definitely not of concern. However, flexures can easily be
overloaded, so careful engineering is required.
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 bending direction, but very high stiffness in shear
direction (which is the same direction as the ball screw drive).
As a result, the flat spring deforms from the transverse force.
Unfortunately, there is not only lateral movement but also bending,
which is evident in Figure 1. The upper and lower surfaces of the
spring are not parallel anymore. The flexure compensated the transverse
force but the result was additional and undesirable torque.
This disadvantage can be minimized by combining two of these flat
springs to form a parallelogram assembly. Now lateral compensation is
possible without any detrimental torque.
Obviously, the upper and lower surfaces of the assembly remain
perfectly parallel. But there is also a visible reduction of overall
height as a side effect of the lateral displacement. In reality, this
effect is mostly negligible - typically in our example a contraction in
the range of 10 nm.
But the exact direction of the transverse force coming from the ball
screw can vary. It is always orthogonal to 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 parallelogram units as a decoupling arrangement. 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).
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 is connected to the saddle by
the decoupling device..
Steinmeyer ball screws usually have a transverse nut movement of
less than 2 micrometers. This is an amazingly small value considering
the number of parts and their intricate geometry. Nevertheless, this 2
micrometer peak-to-peak disturbance causes a systematic periodic
waviness with amplitude of 500 nm peak-to-peak or greater. For many
high-precision measurement applications 500 nm waviness is unacceptable.
There is flatness runout for a travel range of 50 mm. Clearly, there
is a sinusoidal waviness with a wavelength of 2 mm that directly
matches the ball screw pitch. The measured runout is 500 nm
peak-to-peak.
The previous significant waviness is almost completely gone. The
remaining magnitude is less than 50 nm, 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 decoupling unit 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 nm over 50 mm travel, these stages represent a cost-effective
alternative to air bearing systems.
A typical application area for stages with decoupling devices is
high-precision measuring instruments for roughness or surface topology.
By adding such a stage to a standard two-dimensional profilometric
roughness measuring instrument, an enhanced 3-D topography system can
be realized. Instead of a single profile, a number of parallel profiles
are available which can be combined in software to now yield a 3-D
image of the object surface.
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