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Translation Stage Design
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Each material used for mechanical components in the design of a translation stage has its own unique set of advantages and disadvantages. The following is a summary of the properties for the most common materials used in motion mechanics.
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Stiffness
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Stiffness is a measure of the amount of force required to cause a given amount of deflection. Force and deflection are proportional and related by the equation:
Dx = (1/E) * xF/A
where F and Dx are force and deflection, respectively, x is the nominal length, and A is the surface area perpendicular to the force. E is a material-dependent constant called the modulus of elasticity, Young's modulus, or simply the stiffness of a material. Larger values of E mean greater material stiffness.
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Thermal Expansion
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Thermal expansion is the change in size or shape of an object, such as a stage, due to a change (increase or decrease) in temperature. The amount of change is dependent on the size of the component, the degree of temperature change, and the material used. The equation relating dimensional change to temperature change is:
DL = aLDT
where a is the material-dependent coefficient of thermal expansion.
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Thermal Conductivity
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Some materials, such as aluminum, are good choices when temperature change across the component is non-uniform. This occurs when mounting a heat source, such as a laser diode. Because the diode is hotter than the surrounding environment, it dissipates heat through the mount, setting up a temperature gradient along the stage. If the material does not readily dissipate the heat, then distortions caused by thermal gradients can become significant.
The distortion caused by non-uniform temperature changes is proportional to the coefficient of thermal expansion, a, divided by the coefficient of thermal conductivity, c.
Relative thermal distortion = a/c
If the ambient operating temperature of the component is much different from the room temperature, then close attention should be paid to components made with more than one material. In a linear stage, for example, if the stage is aluminum while the bearings are stainless steel, the aluminum and steel will expand at different rates if the temperature changes, and the stage’s bearings may lose preload or the stage may warp due to stresses built up at the aluminum–steel interface.
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Material Instability
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Material Instability is the change of physical dimension with time (also called cold flow or creep). For aluminum, brass and stainless steel, the period of time required to see this creep may be on the order of months or years.
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Aluminum
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Features
Aluminum is a lightweight material, resistant to cold flow or creep, with good stiffness-to-weight ratio. It has a relatively high coefficient of thermal expansion, but it also has a high thermal conductivity, making it a good choice in applications where there will be thermal gradients or where rapid adjustment to temperature changes is required. Aluminum is fast-machining, cost-effective, and widely used in stage structures. Aluminum does not rust, and corrosion is generally not a problem in a typical user’s environment, even when the surface is unprotected. It has an excellent finish when anodized.
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Limitations
Anodized surfaces are highly porous, making them unsuitable for use in high vacuum.
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Coatings
Anodized aluminum provides excellent corrosion resistance and a good finish. Black is the color most often used. Anodizing hardens the surface, improving scratch and wear resistance. Aluminum may also be painted, with excellent results.
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Steel
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Features
Steel has a high modulus of elasticity, giving it very good stiffness (nearly three times that of aluminum) and good material stability. It also has about half the thermal expansion of aluminum (Figure 8), making it an excellent choice for stability in typical user environments where there are uniform changes in temperature. Stainless steel is well suited to high vacuum applications.
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| Parameter |
Steel |
Aluminum |
Brass |
Granite |
| Young's modulus (stiffness), E (Mpsi) |
28 |
10.5 |
14 |
7 |
| Density, r(lb/in3) |
0.277 |
0.097 |
0.307 |
0.1 |
| Specific Stiffness, E/r |
101 |
108 |
45.6 |
70 |
| Thermal Expansion, a(min/in/°F) |
5.6 |
12.4 |
11.4 |
4 |
| Thermal Conduction, c (BTU/hr-ft-°F) |
15.6 |
104 |
67 |
2 |
| Relative Thermal Distortion, a/c |
0.36 |
0.12 |
0.17 |
2 |
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Limitations
Machining of steel is much slower than aluminum, making steel components considerably more expensive. Corrosion of steel is a serious problem, but stainless steel alloys minimize the corrosion problems of other steels.
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Coatings
Steel parts are generally plated or painted. Platings are often chrome, nickel, rhodium, or cadmium. A black oxide finish is often used on screws and mounting hardware to prevent rust. Stainless steel alloys avoid the rust problems of other steels. They are very clean materials that do not require special surface protection. A glass-bead blasted surface will have a dull finish so that it does not spectacularly reflect.
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Brass
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Features
Brass is a heavy material, denser than steel, and fast machining. The main use of brass is for wear reduction; it is often used as a dissimilar metal to avoid self-welding effects with steel or stainless steel lead-screws or shafts. Brass is used in some high precision applications requiring extremely high resistance to creep and can be diamond turned for extremely smooth surfaces.
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Limitations
Compared to aluminum and steel, brass has a less desirable stiffness-to-weight ratio. Moreover, although the thermal expansion of brass is similar to that of aluminum, its thermal conductivity is nearly a factor of two worse.
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Coatings
For optical use, brass is usually dyed black. In other cases, it may be plated with chrome or nickel for surface durability.
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Bearing and Flexure Mechanisms
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The load and trajectory performance of a translation or rotation stage is primarily determined by the type of bearing or flexure used. Bearings permit smooth low-friction rotary or linear movement between two surfaces. Bearings employ either a sliding (dovetail) or rolling action (ball or crossed-roller). In both cases, the bearing surfaces must be separated by a film of oil or other lubricant for proper performance. Flexures, however, provide a means of translation that requires no lubrication and is virtually free of the stiction normally associated with bearings. This type of mechanism, when used in a translation stage, limits linear travel range to just a few millimeters.
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Dovetail Slides
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Dovetail slides are the simplest types of linear stages and are primarily used for manual positioning. They consist of two flat surfaces sliding against each other with the geometry shown in Figure 9. Dovetail slides can provide long travel, and have relatively high stiffness and load capacity. They are more resistant to shock than other types of bearings and are fairly immune to contamination. However, they do have relatively high stiction, and their friction varies with translation speed, which makes precise control difficult and limits sensitivity.
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Figure 9—Dovetail slide
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Goniometers, like the GON40 and GON65, typically use dovetail bearings.
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Ball Bearings
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Ball bearing slides reduce friction by replacing sliding motion with rolling motion. Balls are captured in guideways by means of vee-ways or hardened steel rods (as shown in Figure 10). The guide ways are externally loaded against the balls to eliminate unwanted runout in the bearings. Even with this preload, the friction is very low resulting in extremely smooth travel. Ball bearing slides are relatively insensitive to contamination because each ball contacts the guideways at only a single point, allowing debris to be pushed out of the way.
With a vee-groove bearing way, ball bearings have a lower load capacity than crossed-roller bearings, since the contact area available to transmit loads is smaller - so in order to carry the same size load, the balls would need to have a larger diameter or be greater in quantity.
If the mating ways are ground with either an arch or circular groove (see Figure 11), the closer conformity to the balls’ radii allows the use of smaller balls than with flat ways. The arch approximates a vee-shaped way with the load effectively split at angles of about 45 degrees with the vertical into two loads on the way.
A circular shaped way has a higher load capacity, but the balls bear the load on the bottom of the groove, which can result in side play for loads that are perpendicular to the direction of motion.
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Figure 10—Ball bearing slides have extremely low friction with moderate load capacity.
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Figure 11—The type of bearing way, ball diameter, and number of balls affect the load capacity of a stage.
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Ball bearings are typically used in general-purpose aluminum translation stages like the 423 and 433.
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Crossed-Roller Bearings
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Crossed-roller bearings (see Figure 12) offer all of the advantages of ball bearings with higher load capacity and higher stiffness. This is a consequence of replacing the point contact of a ball with the line contact of a roller. Due to the averaging characteristics of line contacts, angular and linear deviations are generally below those found in ball bearings. However, crossed-roller bearings require more care during manufacture and assembly, resulting in higher costs. Crossed-roller bearings are also more sensitive to contamination because they are less effective at pushing foreign particles away from contact with the guideways.
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Figure 12—Crossed-roller bearings have all the advantages of ball bearings with greater stiffness and load capacity.
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Flexure Mechanisms
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A flexure is stictionless positioning mechanism which uses the elastic deformation of a material to provide translation. A flexure may be made from various materials but generally is made from high-yield-strength spring steel. A flexure must be designed such that the maximum stress experienced by the material does not exceed its yield strength. If this is not done, the flexure will have permanent deformation and will no longer function normally. Flexures can be used in either single or multi-axis translation stages and, due to their size, can provide several axes of motion in a very compact package. In use, flexure-enabled translation normally approximates a straight-line with a slight circular path, so there is a second-order cross coupling between axes. The stage platform moves vertically as it is displaced longitudinally. This is called arcuate motion and is largest at the limits of the stage’s travel range.
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Flexure mechanisms are used to provide multi-axis translation in stages such as this 466A.
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Drive Options
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Manual actuators are the simplest and lowest cost options for positioning. A manual actuator can be described as a high sensitivity lead screw with a knurled knob. On many manual linear stages, like our 460 or 423 series, the nut of the screw is fixed to the stage body, and the screw itself moves back and forth (in contrast to lead screw driven systems, where the nut moves back and forth). Springs press the carriage against the screw tip to make good contact and to preload the screw to eliminate backlash.
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Fine Adjustment Screws
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Fine adjustment screws, such as the AJS Series, provide high-sensitivity manual actuation for translation stages.
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High resolution, ultra-fine adjustment screws, found in Newport’s AJS Series, use rolled threads for smooth actuation and have a ball tip to reduce wear and minimize undesirable lateral motion. Featuring a pitch between 80 and 127 threads per inch (TPI), these screws permit sub-micron adjustments. When no position readout is required, fine adjustment screws are not only a lower cost option, but they also offer superior sensitivity compared to metric micrometer screws, which have 50 threads per 25 mm. Examples where position readout is unnecessary can be found when position may be monitored from the orientation or position of a laser beam or from the amount of optical power coupled through a system.
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Micrometer Screws
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Micrometers, such as the SM Series, have a scale to provide position information.
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Micrometer screws are used if accurate position read-out, like repeatable positioning, is needed. Standard metric micrometer screws feature 50 threads per 25 mm and have a scale in units of 10 mm. An additional vernier - available on some versions - allows position readings with a resolution of 1 mm.
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Differential Micrometer Screws
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Differential Micrometers, such as this DM-13, provide both coarse and fine adjustment in one actuator.
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When resolution of much less than one micron is needed, a differential screw is recommended. These devices use the difference between two screws of nearly the same pitch to produce very fine motion. The motion achieved equals the difference between the two screw pitches. This is illustrated in Figure 13. The effective pitch is given by
1/Peff = 1/P1 – 1/P2
For example, in the patented DM-13 differential micrometer screw offered by Newport, the two screw pitches used have 20 threads per cm for the standard coarse adjustment micrometer barrel and 21.05 threads per cm (200 threads per 9.5 cm) for the differential barrel. The resultant effective pitch is equivalent to 400 threads per cm. This micrometer has a sensitivity of 0.07 mm.
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Figure 13—Construction of a differential micrometer. Two threads of nearly the same pitch (P1, P2) are used to yield very fine motion.
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Vacuum Compatibility
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Stages used in a vacuum of 10-6 hPa or better must be specially prepared for that environment. Many of the materials used in standard linear stages will outgas in high vacuum, resulting in a “virtual” leak, which limits the ability to maintain adequate vacuum.
Procedures used at Newport to specially prepare products for use in vacuum environments ensure that our products will function as designed at pressure levels down to 10-6 torr and at the same time not release unacceptable quantities of contaminants into the vacuum environment. For proper preparation, more information in addition to operating pressure is needed. Acceptable levels of outgassing, mass loss, and volatile condensable materials can vary with the application, pumping capacity, temperature, etc.
Material issues that must be addressed include the selection of acceptable metals, ceramics, coatings, lubricants, adhesives, rubbers, plastics, and electrical components, etc. For example, highly porous anodized aluminum surfaces trap large amounts of air molecules, resulting in significant outgassing. For this reason, aluminum used in high-vacuum applications is unanodized. Motors must also be specially prepared for vacuum operation.
Machining practices must avoid creating surfaces conducive to trapping gases and other foreign materials that could be released in vacuum conditions. Care must also be taken to ensure that gasses are not trapped in assembly cavities.
In addition to material selection and manufacturing practices, special cleaning, handling, assembly and packaging practices must be followed. These functions are carried out in a clean environment to minimize the possibility of airborne contaminants becoming attached to the components. Newport does not perform bakeout at an elevated temperature.
Performance specifications for products used in a vacuum environment may vary from non-vacuum use. For example, because heat cannot be as readily dissipated, motor duty cycle must be reduced, which in turn may limit the maximum achievable speed. If your application requires vacuum preparation, please contact our Applications Engineering Department to discuss your specific application needs.
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Cleanroom Compatibility
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Newport has facilities to properly prepare products for cleanroom applications. While many of the techniques, practices, procedures and material requirements for cleanroom applications are similar to those for vacuum preparation, each application has its own unique requirements. Please contact our Applications Engineering Department to discuss your specific application needs.
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