<h5>27 February 2001</h5>
<h1>Keeping It Clean</h1>
<h5>by <a href=mailto:kylet@primatics.com>Kyle Tomson</a></h5>
<h3>Positioning stage protection systems offer machine designers solutions for contaminated environments.</h3>
<P>Protecting a positioning stage's internal components can become critical in applications with any kind of contamination and can present substantial challenges for the machine designer. Positioning stages have interior mechanical, optical, and electrical components, none of which operate well or very long in contaminated environments.
<p>Applications such as laser trimming and wafer drilling offer particular challenges because not only can the contamination be very abrasive, but also these types of applications often require a high level of accuracy and repeatability. Protection systems must not only reject contamination particles but also introduce minimal levels of friction because of this force's negative impact on positioning accuracy, repeatability, and life.
<h2>Contamination Categories</h2>
<p>Contamination can be categorized into four broad areas based on particle size, weight, and method of transport. <A HREF=/isaolp/journals/g-html/motion/20010320.html>Figure 1</A> shows the relative sizes, weights, and abrasiveness of nonliquid contamination.
<p>Large particles typically aren't abrasive, but they can be produced in large volumes. These particles are generally larger than 2 millimeters (mm) and have an average weight greater than 10 grams (g). Examples can be found in wood, metal, and plastic machining applications. Large metal particles can have edges sharp enough to cut through many kinds of nonmetallic materials.
<p>Large airborne particles range in size from 25 microns (µ) to 2 mm and weigh between 1 milligram and 10 g. Particles in this category can become airborne for short periods. These particles are commonly produced by laser etching, metal grinding, and metal machining. They have varying levels of abrasiveness and typically leave a thin film over the entire work area.
<p>Small airborne particles are smaller than 25 µ in size and of nearly infinitesimal weight. This type can stay airborne for long periods and is small enough to work itself past barrier seals on encoders, ball screws, and bearing systems. These particles are typically silicate or ceramic in nature and highly abrasive. Laser etching of ceramic and silicon substrates, as well as silicon wafer drilling, can produce particles of this type.
<p>The last category is liquid-based corrosive contamination. This can include acids, water, and coolants. These agents not only are corrosive but also carry abrasive particles in stasis that wreak havoc on positioning stage components. This type of contamination, referred to as liquid-based contamination, is typically present in applications such as machine tools and wet bench wafer processing.
<h2>Protection Schemes</h2>
<p>Three protection schemes are commonly used on positioning stages: collapsible bellows, metal strip seals, and recirculating belts. Each has benefits and drawbacks. <A HREF=/isaolp/journals/g-html/motion/20010321-01.html>Figure 2</A> shows a relative comparison of each system's life, cost, and effectiveness on each particle type.
<h2>Collapsible Bellows</h2>
<p>Bellows are the most common positioning stage protection technique. The bellows are typically made from vinyl or some similar material. They're pleated and designed to expand and collapse, depending on the carriage's position. Separate bellows are used on each side of the positioner carriage. Each is attached to both the carriage and end plate by small screws or Velcro. As the stage carriage moves from one side of travel to the other, the bellows collapse or expand to accommodate its position (<A HREF=/isaolp/journals/g-html/motion/20010321-02.html>Figure 3</A>). Bellows are easily adaptable to virtually any kind of positioning device.
<h2>Metal Strip Seals</h2>
<p>Metal strip seals operate much differently than bellows. The load carriage, resembling an inverted "U," travels along two slots (separated by the top plate) that run the length of the stage. In addition, two thin, narrow pieces of steel are attached to both end plates and pass through the carriage via small slots along its sides.
<p>When these steel strips meet the carriage, they're redirected underneath by a pair of rollers. Another set of rollers on the carriage's opposite side routes the strips for an identical egress (<A HREF=/isaolp/journals/g-html/motion/20010322-01.html>Figure 4</A>). Wipers underneath the carriage at the strips' entry and exit points remove any particles resting atop the strips before they enter the stage's interior. With steel stages, the strips can be slightly magnetized to encourage them to lie properly over the stage's slots.
<h2>Recirculating Belts</h2>
<p>Recirculating belt protection systems are similar to metal strip seals. The carriage has approximately the same "U" shape, and the positioning stage has slots running down its length as well (although the slots are typically much narrower). Instead of being rigidly attached at the end plates, the belts are attached to each side of the carriage. Holes in the end plates allow the belts to recirculate through them and emerge below the carriage, but outside of the interior portion of the stage. Belts from either side of the carriage are attached with a small spring to retain constant tension, as shown in <A HREF=/isaolp/journals/g-html/motion/20010322-02.html>Figure 5</A>.
<p>Material resting on the belts is stripped away by wipers in the end plate that route the particles to a chamber inside the end plate. Seals along the side of the carriage and side plate prevent particles from entering alongside the carriage itself. The belts are typically polymer based and have a polyurethane coating. In addition, the side plates and top plate have containment grooves to guide the belts and create a labyrinth seal structure.
<h2>Advantages and Disadvantages</h2>
<p>Collapsible bellow covers are the simplest to implement and are the most common type of protection found on linear positioning stages. Bellows work well in wet environments because moisture wicks through the bellows in only a few locations (additional seals can be added to prevent wicking through vulnerable areas).
<p>Airborne contaminants offer more challenges for bellows, however. Pressure differentials between each side of the carriage can draw in airborne contaminants, requiring the design engineer to construct a closed-loop system that allows air to move freely from one side of the carriage to the other. When using positive pressure, the design engineer must ensure the bellows aren't deflected away from the stage base, allowing an entry point for contamination. The bellow pleats must periodically be cleaned to prevent large particles from collecting and restricting travel. The design engineer must also allow for the lost travel as the bellows collapse down.
<p>Metal strip seals offer superior protection from particles of all sizes and are very compact and inexpensive. Strip seals, however, offer challenges for the designer of machines requiring high throughput or high precision. The rollers that route the metal strip underneath the carriage exert large forces on the strip seal, which can create high friction forces. Friction has a negative impact on a positioning system's accuracy, repeatability, and resolution.
<p>In addition, strip seals have no way of dynamically tensioning themselves, causing the strip seal to become loose and lose seal integrity. Bends and kinks form in the loose strip seal as it passes through the rollers, causing localized areas of very high friction and further erosion of its particle rejection capabilities. Users must plan on regular maintenance to retension the strip seal or replace it every 10 million inches of travel to ensure proper particle rejection.
<p>Recirculating belts are more pliable than a thin metal strip, and they can be turned around a small roller with minimal friction. In addition, they can be dynamically tensioned with a spring, eliminating the need for retensioning. The reduced friction dramatically extends their life to more than 50 million inches of travel. Their primary disadvantage is the possible migration of particles around their outer edges. Positive pressure through the positioning stage prevents this.
<p>In addition, recirculating belts are unsuitable for liquid contamination because liquids can wick around the belts, even in the presence of positive pressure. Recirculating belts are well suited for applications requiring high accuracy, because of their low friction, and for applications of mid- to high-duty cycle, due to their long life.
<p>Protective bellows are the simplest but most expensive protection system available. They work well in wet environments but are less effective with airborne contamination without further design work. Metal strip seals offer superior protection from particles of any size, as well as liquid-based contamination, but require a high level of maintenance and induce too much friction to be considered in high-accuracy applications. Recirculating belts are unsuitable for wet environments, but they work best in applications requiring high accuracy or high throughput. <B>MC</B>
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<h2>Figures and Graphics</h2>
<UL>
<LI><a href="/isaolp/journals/g-html/motion/20010320.html" target="_new">Figure 1. Relative sizes, weights, and abrasion characteristics of nonliquid contamination.</a>
<LI><a href="/isaolp/journals/g-html/motion/20010321-01.html" target="_new">Figure 2. Relative comparison of life, cost, and effectiveness, by contaminant particle type, of bellows, strip seal, and belt systems.</a>
<LI><a href="/isaolp/journals/g-html/motion/20010321-02.html" target="_new">Figure 3. Pleated bellows attach to the end plates and carriage, expanding and collapsing between the extremes of travel.</a>
<LI><a href="/isaolp/journals/g-html/motion/20010322-01.html" target="_new">Figure 4. Steel strip seals mounted to the end plates are lifted above the bearings with rollers inside a hollow carriage plate.</a>
<LI><a href="/isaolp/journals/g-html/motion/20010322-02.html" target="_new">Figure 5. Spring-tensioned polymer recirculating belts attach to and travel with the carriage plate.</a>
<LI><a href="/isaolp/journals/pdf/motion/20010320.pdf" target="_new">PDF of "Keeping It Clean"</a>
</UL>
<h2>Author Information</h2>
<P><A HREF=mailto:kylet@primatics.com>Kyle Tomson</A> is director of sales and marketing for <A HREF=http://www.primatics.com target=_new>Primatics, Inc.</A> He received his BSEE in 1991 from Northwestern University. Contact him at 33877 Eastgate Circle, Corvallis, OR 97333; tel: (888) 754-3111 or (541) 757-9678; fax: (541) 757-9653. <p>
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