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Introduction to Positioning Equipment
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This tutorial introduces opto-mechanical component design basics and will familiarize the reader with the issues that need consideration in the selection of the most commonly used tabletop components. It emphasizes practical issues, not mathematical derivations.
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Performance Parameters
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There are many different measures of performance one must consider when choosing a particular positioning component. Understanding the definitions of the various parameters and how they affect performance will simplify the selection process.
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Materials and Stability
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The majority of all opto-mechanical components are made from aluminum, brass, or stainless steel. Therefore, the properties of these materials will be directly compared in the following discussion.
Materials are sometimes selected by emphasizing a single property, such as thermal expansion. This is unwise as it is not likely to result in a good choice for general use. Therefore all relevant material properties should be considered.
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Stiffness
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Stiffness is a measure of the amount of stress (force/area) required to cause a given amount of strain (normalized deformation). Stress and strain are proportional and related by the equation s = Ee, where s and e are stress and strain respectively and E is Young’s Modulus, which is material dependent. A material is stiffer for larger values of E and more compliant for smaller values. For example, stainless steel is approximately three times stiffer than that of aluminum (see table). Aluminum, on the other hand, is 1.3 times more compliant than brass. Specific stiffness (Young’s Modulus divided by the material density) is important when settling time or vibration immunity is an issue. Components with the same shape and specific stiffness will have the same fundamental resonant frequencies. Higher specific stiffness results in higher resonant frequencies, faster settling times, and a reduction in vibration disturbances.
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Thermal Expansion
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Temperature changes cause size and shape changes in a mounting component. The amount of size and shape change is dependent on the size of the component, the amount 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. The thermal expansion of stainless steel is roughly half that of aluminum. This can be important when the mounting component is being used in an application requiring interferometric stability. Note that aluminum is the best choice when temperature change across the component is non-uniform. This situation arises when mounting a power 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 component. The thermal conductivity of aluminum is ten times greater than that of stainless steel so heat can be dissipated more readily, thus reducing the magnitude of the thermal gradients and distortion. The distortion caused by non-uniform temperature changes is proportional to the coefficient of thermal expansion divided by the coefficient of thermal conductivity. Thus, aluminum distorts on the order of three times less than stainless steel in a non-uniform temperature environment. Brass - though nearly as thermally expansive as aluminum, has a coefficient of thermal conduction nearly a factor of two worse and thus is not as good as aluminum in this situation.
If the ambient operating temperature of the component is much different from 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 when the temperature changes and the stage’s bearings may lose preload or the stage may warp due to stresses that build up at the aluminum-steel interface.
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| Material Properties |
Aluminum |
Stainless Steel |
Brass |
| Stiffness, k (MPSI) |
10.5 |
28 |
14 |
| Density, r (lb/in.3) |
0.097 |
0.277 |
0.307 |
| Specific Stiffness, k/r (M in.) |
108 |
101 |
45.6 |
| Coefficient of Thermal Expansion, a (m in./in./°F) |
12.4 |
5.6 |
11.4 |
| Coefficient of Thermal Conduction, c (BTU/hr-ft-°F) |
104 |
15.6 |
67 |
| Relative Thermal Distortion (a/c) |
0.12 |
0.36 |
0.17 |
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Material Instability
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Material instability is the change of physical dimension with time — so 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.
Usually, the mechanical design of the component contributes much more to the instability than does the choice of material. For example, the lubrication on the micrometer’s threads can begin to migrate over time causing a slight shift in the micrometer. Alternately, if a translation stage’s bearings have not been sufficiently hardened, the stage can shift after not moving for extended periods as the bearings deflect locally at their points of contact.
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Summary of Materials Evaluation
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Each of the materials used in positioning components have their own unique set of advantages and disadvantages. Unfortunately, a universal material that meets all requirements does not exist. We summarize here the characteristics of the materials outlined in the chart.
Aluminum: 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 acclimatization to temperature changes is required. Aluminum is fast machining, cost effective, and widely used in component structures. Aluminum is non-rusting and generally corrosion-resistant in a laboratory environment, even when the surface is unprotected. It has an excellent finish when anodized. However, anodized surfaces are highly porous, making them unsuitable for use in high vacuum. Vacuum applications require the use of unfinished aluminum surfaces.
Stainless Steel: 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, making it an excellent choice in typical laboratory environments where there are uniform changes in temperature. Machining of steel is much slower than aluminum, making steel components considerably more expensive. Corrosion of steel is a serious problem. Stainless steel alloys avoid the corrosion problems of other steels. Stainless steel is well suited to high vacuum applications, but the design of the component must also incorporate other factors. (Please the Vacuum Compatability section below)
Brass: Brass is a heavy material, denser than steel, fast machining, but with a less desirable stiffness-to-weight ratio than either aluminum or steel. The thermal expansion of brass is similar to that of aluminum, but its thermal conductivity is nearly a factor of two worse. It is, however, a good wear material. The main use of brass is in wear reduction; it is often used as a dissimilar metal to avoid self-welding effects with steel or stainless steel 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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Exterior Finish
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Anodized aluminum provides excellent corrosion resistance and a good finish. Black is the color most often used on optical mounts. The anodized surface is highly porous. For this reason, only unanodized aluminum is used in high vacuum applications. However, this porosity results in a matte surface that does not specularly reflect light, adding to its value in optical mounts. Anodizing hardens the surface; improving scratch and wear resistance.
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. Painted components should be avoided. Paint will eventually flake off, contaminating the optics or the moving parts of the positioner.
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 that does not specularly reflect.
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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Vacuum Compatibility
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Many products within this catalog can be vacuum prepared. Please look for the “Vacuum Compatible” statement on the specific product page. “Vacuum Compatible” products are prepared for 10-6 Torr. If you require products specially prepared for 10-3 Torr, or greater than 10-6 Torr environments, please contact our technical staff for a quotation.
For those products not designated as vacuum compatible, we may still be able to prepare for vacuum environments. However, these would require special quoting. Please contact our technical staff to discuss your needs.
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Vacuum Preparation
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Preparation for vacuum environments depends on the vacuum you wish to maintain. The word “vacuum” does not adequately specify the conditions for a specific application. Acceptable levels of outgassing, mass loss and volatile condensable materials can vary with the application, pumping capacity, temperature, etc. It is, therefore, essential that the specific requirements be reviewed and understood prior to placing any component in a vacuum environment.
10-3 Torr environments: In general terms, a vacuum of 10-3 Torr requires minimal change to many of our products with the exception of possible lubricants. In this environment, it is not uncommon to use anodized parts and limited use of plastics should not pose any problems.
10-6 Torr environments: Components used in a vacuum of 10-6 Torr are specially prepared for this environment. Many of the materials used in standard components will outgas in a high vacuum, resulting in a “virtual” leak, which limits the ability to maintain a high vacuum. Highly porous anodized aluminum surfaces can trap large amounts of air molecules, resulting in significant outgassing. For those components, within this catalog, specified as “Vacuum Compatible” we perform the following in preparation:
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Products with anodized aluminum are created without anodize. As such, we only use unanodized aluminum, stainless steel or equivalent materials.- Plastic knobs and handles are either removed or replaced (at additional cost) with high vacuum materials such as steel or Delrin. In some cases, you may choose to maintain the plastic knob due to incremental costs associated with producing an alternative design. In spite of plastics permeability it is common to use plastics in vacuum systems because of their insulating properties and price.
- Holes not tapped through are vented; or special vented hardware is used.
- Hardware and lubricants are changed to special vacuum compatible materials.
- Finished units are completely cleaned and sealed in appropriate packaging material.
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Additional vacuum preparation steps, or preparation for vacuums of greater than 10-6, require special handling, including baking the product in a vacuum. If you have requirements at this level, please contact our technical staff to discuss the options available and, if appropriate, pricing.
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Kinematic Mounts
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Any optical mount’s position can be defined uniquely in terms of six independent coordinates - three translations and three rotations, with respect to some arbitrary fixed coordinate system. A mount is said to be kinematic when the number of degrees of freedom (axes of free motion) and the number of physical constraints applied to the mount total six. This is equivalent to saying that any physical constraints applied are independent (non-redundant). A kinematic optical mount, therefore, has six independent constraints.
The most common type of kinematic mount is the cone, groove, and flat mount schematically illustrated in Figure 1. Consider the optic as being attached to the coordinate system of the three spheres in the figure and its corresponding mount having the cone, vee, and flat. If the optic is first seated in the cone, three degrees of freedom (x, y, and z translations) are eliminated without redundancy.
At this point, the optic can still rotate freely about all axes. Next, the second sphere is seated in the groove, which is aligned towards the cone. This constrains or eliminates two more degrees of freedom, pitch and yaw, as shown in the Figure. The alignment of the groove towards the cone is important in order not to over-constrain one or more of the translation degrees of freedom. Finally, there is only one degree of freedom left to constrain, roll about the x-axis. This is accomplished by seating the third sphere on the flat. Six non-redundant constraints make this a kinematic mount.
The advantages of a kinematic mount are: increased stability, distortion free optical mounting and, in the case of a kinematic base, removable and repeatable re-positioning. It is easy to imagine with the mount of Figure 1, that the plate containing the cone, vee, and flat could thermally expand at a different rate than the optic without introducing any distorting load to the optic. The mount would simply expand about the ball and cone. The second and third balls would both slide on the groove and flat, respectively. If, on the other hand, the mount were non-kinematic (for example, if the groove were rotated 90° about z) as the plate expanded or contracted along the x direction, it would tend to warp the optic since the second ball could no longer slide in the groove.
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Figure 1—Kinematic Constraints
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Gimbal Mounts
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It is common to make kinematic mounts adjustable, by attaching a screw drive to the second and third spheres (Figure 1) to provide angular adjustment of the optic with respect to the base. This is the basis of design for Newport’s Kinematic Mirror Mounts. One drawback to this type of mount is the location and orientation of the axes of rotation of the mount. They are usually behind the optic and non-stationary. That is to say that the axes move with every adjustment. This introduces two problems that must be overcome. First, since the axes move, they do not stay orthogonal to the optical axis, so cross-coupled motion occurs during adjustment. Rotation purely in one of the directions orthogonal to the optical axis requires adjusting both axes of the kinematic mount. Secondly, since the axes of rotation are behind the optic when adjustments are made, both rotation and translation of the optic occur.
Use of a gimbal mount eliminates both of these problems. A gimbal mount is defined as a mount whose axes of rotation is orthogonal and fixed in space. Also, when the axes are made to intersect at the center of the front surface of the optic in the mount, this allows for simple non-coupled rotation adjustment of the optic without translation as shown in Figure 2.
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Figure 2—Gimbal Mounting
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Backlash: Non-responsiveness on reversal of input. For example, a simple motorizer with motor-mounted encoder might exhibit several microns of position display change on reversal before its output position actually begins to change. Other terms frequently used to describe this or similar behaviors include: dead zone, stiction, looseness, slop and free play.
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Cross-Coupling: Amount of motion in one axis due to the adjustment of a different axis in multiple axis devices; such as XY stages or kinematic mirror mounts. For example, the amount of X motion when the Y drive is adjusted in an XY stage. Also known as crosstalk.
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Hysteresis: Non-repeatability on reversal of input. For most motion devices, backlash and stiction are the most significant contributors. However, non-recovery of static deflection is possible, with greatest consequence for some submicron applications when inappropriate materials are used in a motion device’s design. In piezo devices, hysteresis is a characteristic property of the material.
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Minimum Incremental Motion: The smallest motion a device is capable of delivering. Not to be confused with resolution claims, which are typically based on the smallest display increment and which can be more than an order of magnitude more impressive than the actual motion a system is capable of producing. This is a key specification but, unfortunately, is rarely disclosed.
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Pitch: Rotation about the transverse, or Y axis. This is also known as elevation, particularly in gimbal-type mounts.
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Play: Uncontrolled movement due to looseness of mechanical parts. Very small in a well built component, it can increase as a component grows older, especially if it is roughly handled or overloaded.
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Precision: Range of deviations in output position that will occur for the same error-free input. Precision is also known as repeatability. Although often confused in common parlance, accuracy and precision are not the same.
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Repeatability: The ability of a motion system to achieve a commanded position over many attempts. Manufacturers often specify unidirectional repeatability, meaning the ability to repeat a motion increment in one direction. This side-steps issues of backlash, hysteresis, etc., and therefore is fundamentally irrelevant. A much more significant specification is bidirectional repeatability. Unfortunately, few manufacturers publicize this much tougher measure of motion performance.
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Resolution, Display: The smallest incremental step that can be displayed or read from an actuator. For most micrometer type actuators, the display resolution is the smallest graduation either standard or Vernier. For example, a standard micrometer such as the SM-25 has standard graduation of 10mm but a Vernier graduation of 1mm so the best display resolution would be 1mm. The sensitivity of most micrometers is typically the same as or better than the display resolution or graduation.
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Roll: Rotation about the longitudinal, or X, axis of travel.
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Sensitivity: With reference to a manually actuated device, the smallest linear or angular movement that can be discriminated. This term is sometimes referred to as resolution and often confused with graduation or display resolution. Since manual dexterity varies from person to person, it is assumed that your fingertips are sensitive enough to be able to make 1° rotations of an adjustment screw and therefore, when you see sensitivity quoted for an adjustment screw, it is the travel associated with a 1° turn of the screw. Sensitivity is limited primarily by stiction.
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Stiction: Occurs because the coefficient of static friction is always greater than the coefficient of moving friction. When a stage is at rest and force is first applied and slowly increased, no motion occurs. At some threshold motion suddenly begins so that the first move of the component will be a jump, giving non-linear and non-repeatable motion. This effect is what limits the smallest incremental movement.
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Yaw: In-plane rotation about the vertical, or Z axis. This is also known as azimuth. This term is also used to refer to the rotation of optics in optic mounts.
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