The Air Bearing Advantage Kevin McCarthy, Chief Technology Officer Danaher Motion Wood Dale, IL www.danahermotion.com ContactUs@DanaherMotion.com 866-993-2624 All moving objects possess six degrees of freedom three linear and three rotary. The task of a linear motion guideway is to eliminate, as closely as possible, five of these degrees of freedom, leaving a single rotary or linear axis of motion. Air bearings are the purest and highest performance means of defining linear or rotary motion, and have substantial advantages over conventional mechanical guideways. These advantages become more pronounced as the desired resolution increases, and many aspects of high precision positioning are uniquely enhanced when air bearing guideways are chosen. The advantages that our air bearing, direct drive stages offer can be summarized as follows: Completely non-contact bearing ways, motor, and encoder Friction free Extreme resolution capability Much higher throughput (moves per second) Unlimited service life Extremely straight motion Ultra constant velocity Highly consistent servo tuning, both unit to unit and over time Much better at exploiting feedforward terms Built-in, millinewton level force measurement and generation Extremely low particle counts Supports accurate, sub-micron dither motion
As long as we are listing the advantages, it would only be fair to mention a couple of mild disadvantages: Added infrastructure requirements to provide the air supply Lower stiffness than rolling element steel bearing ways Increased susceptibility to amplifier induced or environmental vibration Perceived higher cost Having annunciated the particular strengths and weaknesses of air bearing, direct drive stages, we ll now provide some detail on each of the above points. Completely non-contact bearing ways, motor, and encoder There is a tremendous advantage in having the three critical components of the stage the guideways, the motor/actuator, and the position feedback all be completely non-contact. All of the subsequent advantages in our list stem from this simple fact. In stark contrast to conventional stages, the air bearing, direct drive stage is a nearly perfect physics package it has a mass, and you pass a current through it. In response, it develops a force, in an extremely linear and predictable manner. This force, and the resulting acceleration, is singly and doubly integrated by the velocity and position loops of the servo control, using position data from an equally noncontact linear encoder. Stated simply, contact is corruption. The presence of numerous and over-constrained contacts in traditional stages (not to mention lubricants, preload variations, leadscrews with torque variations, recirculating ball cogging, retainer creep, etc.) prevents them from coming close to achieving either the static or dynamic performance levels of air bearing, direct drive stages. In a number of our single axis air bearing stages, there are no moving cables, and all connectors are mounted in the stationary base. In multi-axis stacks, there will inevitably be some moving cables, and we take pains to ensure that these are as supple and noninfluencing as possible. What residual cable effects that remain are small forces (not friction), for which the servo integrator develops an equal and opposing force. Friction free Friction is a highly non-linear effect, and degrades the performance of servo control loops, based as they are upon linear system theory. The refreshing absence of friction from air bearing, direct drive stages permits much higher static and dynamic performance to be achieved. The absence of friction is also the key to a number of the other advantages listed below. Extreme resolution capability While there are those firms that try, the practical fact is that leadscrews run out of gas for resolution levels at or below 100 nanometers (0.1 micron). While there are tricks that can be used to make leadscrews cooperate in this regime, they require fairly
high-strung and gimmicky tuning, compromise dynamic performance, and are best avoided. Similarly, while a few mechanical bearing systems can be pushed below 100 nanometers, issues such as bearing friction, preload variations, recirculator cogging, and lubricant issues make this too an uphill battle, with the need to wait while the servo loop integrator term papers over the problems. Air bearing, direct drive stages have no intrinsic resolution limit, and we routinely produce air bearing positioning systems with resolutions of 31 picometers (that s less than the classical Bohr radius of the hydrogen atom). A 100 nanometer step move is shown in Fig. 1 below. Fig. 1. 100 nanometer step move Much higher throughput (moves per second) This is an area where air bearing stages provide higher performance than their traditional mechanical bearing counterparts. In a nutshell, their absence of friction allows a substantially shorter settling time, since there is no need to wait for the fairly slow effect of the servo loop integrator term to overcome friction. In addition, feedforward terms in the servo filter can be applied much more accurately, again because of the absence of friction. Unlimited service life Given the totally non-contact nature of the bearing ways, motor, and encoder, the resulting service life of our air bearing, direct drive stages is essentially unlimited.
The absence of contact means the absence of wear, and the air bearing will operate without change over decades of operation. The cable plant is the one part of the stage that can wear out, although this is true only in multi-axis stages; most of our single axis designs feature a completely stationary set of cables. In multi-axis systems, the effective design of our cable plants, with strategically located connectors, allows easy replacement once the flex service life has been exceeded. Despite the unlimited life of an air bearing stage, there is a class of traumas that can damage the stage. Dropping a vise on the air bearing surfaces, for example, will leave a dent that may prevent motion. Pumping oil instead of air into the compressed air line is another failure mechanism, as is putting twenty amps into a five amp linear motor coil, or a 24 volt supply on a five volt encoder. Fortunately, most of these issues can be fairly easily prevented, using fuses, I 2 T current limiting, coalescing filters, voltage clamps, etc. The encoder read head, due to its active electronics, has a small chance of failure over time; solid state circuitry being highly reliable, the MTBF for the encoder read head is a remarkable 170 years. An added benefit that comes with an unlimited service life is extreme reliability, with the ability to run 24/7 for years without service or downtime. This latter attribute is critical in many advanced inspection and production processes, particularly in remote manufacturing sites. Our AirBeam product series, deployed in the thousands in digital printing systems worldwide over the last decade, has demonstrated the intrinsically high production reliability of our air bearing designs. We are seeing applications in which high precision (the traditional driver for the use of air bearings) is not required, and air bearings are being selected strictly for their unlimited, maintenance-free service life. Extremely straight motion As mentioned at the beginning of this section, the job of a guideway is to eliminate, as best as possible, five of the six degrees of freedom. Air bearing guideways do this extremely well. They tend to integrate minor errors in the surface over which they run, and the resulting errors are very low, and of long period. As a representative stage, our FiberBeam 100 has 100 millimeters of travel; pitch and yaw over this range of travel are held to less than 2 arc-seconds. In addition to angular errors, there are the linear errors of flatness (vertical runout) and straightness (horizontal runout). As described in a separate technical note, angular errors and linear errors are closely related. In the above stage, linear departures from straight-line motion are less than 300 nanometers over the 100 mm. of travel. Ultra constant velocity There are a number of applications that require extremely precise constant velocity motion. Due to the intrinsic purity of air bearing, direct drive technology, stages
based upon this design offer the highest possible performance in constant velocity applications. Residual errors in high-end designs are primarily due Abbe errors resulting from very small angular errors, thermal effects, and environmental vibration. With suitable component selection, tracking errors during motion can be held to levels as low as +/- 2 nanometer at low speeds (Fig. 2). Fig. 2. Position tracking error during constant velocity motion, nanometers / seconds Highly consistent servo tuning, both unit to unit and over time Traditional stages, with their assortment of mechanical bearings, leadscrews, nuts, etc. require careful servo tuning to achieve optimum performance. Attempts to minimize move and settle times often lead to tunings that are marginally stable, and are individually hand-tuned for each axis. The problem is exacerbated as the resolution is driven smaller and smaller. Since tuning is dependent on physical parameters such as leadscrew torque, linear bearing preload, and lubricant properties, and these vary along travel in any given unit, as well as from unit to unit, and over time, the result is often unsatisfactory. The impact to the user is variable performance, as well as occasional stages that turn into oscillators. Hmmm it s buzzing again at that end of travel better lower the gains. This is no way to run an precision positioning system. In stark contrast, air bearing, direct drive stages have no contacting parts to wear, and suffer from none of the corrupting nonlinearities that plague conventional stages. The servo tuning drops directly out of a spreadsheet; for any given stage, the only free variables are the desired servo bandwidth, the payload mass, and any structural
resonances. The resulting tuning is not finicky and borderline; it is robust and perpetual. With no parts to wear, the tuning on day one is the tuning at year ten. For that matter, stages with the same payload mass are identically tuned; there is no need to fine-tune each stage to match any unit to unit variations. The purity and simplicity of the design lead directly to highly consistent servo tuning. Much better at exploiting feedforward terms Simple PID loops exhibit significant position lag during acceleration, and position lead during deceleration. At the moment when the trajectory generator has completed its profile, the payload is well past the desired stopping point, with the terminal following error being easily calculated as 4 * Acceleration / (2*π *f 0 ) 2, where f 0 is the servo bandwidth. At 10 meters per second squared acceleration (~1G) and a servo bandwidth of 50 Hz, the terminal following error is 0.4 millimeters (400 microns). The servo loop must then reverse direction and eliminate this terminal error. It is far better to reduce this error by the use of acceleration feedforward, which adds and subtracts a current command during acceleration to cancel the normal position lag and lead. Friction guideways can benefit from feedforward terms in the servo filter, but the highly variable and imprecise level of friction yields only moderate benefit. Direct driven air bearing stages, which are nearly perfect physical plants out to the first structural resonance, can benefit much more from the use of feedforward terms. Terminal following error in air bearing systems can be cut to a few percent of the nominal value by the judicious use of feedforward terms, which provides direct benefits in settling time, and hence throughput. Built-in, millinewton level force measurement and generation The absence of friction in our air bearing stages, together with the extraordinarily linear conversion of current to force in the direct drive linear motor, provides a very useful and unexpected side benefit. These stages, with no additional changes, also function as both precise force generators and force transducers, at the millinewton level. The measurement and generation of forces is quite useful in a number of applications, and can lead to the addition of load cells to the axis stacks. With our air bearing stages, very sensitive load cells are in essence built-in to each stage by design, and provide this very useful function at no extra charge. In addition, these stages can perform very sensitive touch-point detection at extremely low force levels. The ability to detect part locations at the 100 nanometer level offers new process control variables that would otherwise be poorly controlled. Extremely low particle counts Due to the non-contact nature of the bearing ways, linear motor, and linear encoder, these stages are nearly perfect from the perspective of particulate contamination. As long as the air used to pressurize the bearing is dust free (and very conventional
filters offer 99.7% filtration of all particles above 0.1 microns), the only particle source component is the cable plant. Proper selection of qualified materials in the moving cable set has permitted our stages to be used in ultra-critical applications such as wafer steppers. In some cases, pure dry nitrogen is used to operate the air bearings, producing a nearly undetectable level of particulate generation. Supports accurate, sub-micron dither motion As described in detail in the section on our Gradient Engine technology, our high performance, servo-to-optical-peak-power alignment solution determines the gradient of the optical power curve using small amplitude sine and cosine dither motions in the plane perpendicular to the optical axis. This is a far better method than softwarebased search algorithms, and requires the purity and friction-free attributes of air bearing, direct drive stages to succeed. Friction non-linearities and lubricant effects in conventional stages prevent the production of accurate, sub-micron dither motion, and foreclose this superior means of peak locking. Piezo stages are capable of producing dither motions, but their limited travel is a decided disadvantage, typically requiring an additional set of stages below them to achieve macro travels. Disadvantages Added infrastructure requirements to provide the air supply There is a modest increment in system cost and complexity for air bearings over conventional stages due to the need to provide a supply of clean, dry air. In most facilities, compressed air is generally distributed, and the added cost for spot regulation and filtration are negligible. If house air is not provided, there can be a higher level of burden imposed by the need to add a compressed air station, and if it is adjacent to the stage system, the added noise and vibration can be an issue. Lower stiffness than rolling element steel bearing ways Air bearing guideways are less stiff than the rolling steel bearing guideways used by conventional stages. This could, in principle, lead to a lower first resonance and hence a lower servo bandwidth. As it happens, the numerous mechanical elements present in traditional staging are usually the elements that set a limit to the servo bandwidth, and they are absent in air bearing, direct drive stages. We manufacture a full range of mechanical stages, and find that their achievable servo bandwidths are comparable no more, no less than those of our air bearing stages of comparable size. At an equal servo bandwidth, the numerous other benefits of the air bearing design make the decision an easy one. One additional consequence of the use of air bearing guideways relates to torques due to overhung loads. While the load capacity for centered loads of our air bearing stages are quite high, there is a distinct limit to the allowable magnitude of torques due to cantilevered loads. There are a number of
other reasons (Abbe error, for example) why overhung loads are not a good idea in general, but if they cannot be avoided, conventional bearings may be indicated. Increased susceptibility to amplifier induced or environmental vibration Conventional stages have significant amounts of friction, which is in nearly all cases a distinct disadvantage. One positive aspect to friction, however, is that it provides position stability in the presence of external stimuli, and does so without the intervention of the servo loop. In a frictionless, direct drive stage, the servo loop has the sole responsibility of suppressing axial vibration. As it happens, our techniques of sinusoidal encoder interpolation provide very high resolution at low cost, and this, together with the high quality servo loop that air bearing stages achieve, leads to negligible axial jitter. Vibration sources can be due to either the environmental background, or by the servo amplifier, especially if that is of PWM design. For the highest control of stage position, we employ our NanoDrive series of linear amplifiers; together with 4096X encoder interpolation, vibration is limited in most environments to between 1 and 10 nanometers. Perceived higher cost This one is easy. Air bearings have long been considered stages for the rich and famous. The number of suppliers was very small, and if your accuracy or dynamic performance requirements dictated air bearings, you had to pony up. Pricing at that time was not determined by a bottom-up cost build, but by what the market would bear. We have led the way over the last decade in driving down the cost of air bearing stages (to the chagrin of our competitors), with our AirBeam series of products leading the way. Over the last few years, we have extended this capability with a broad range of miniature air bearing, direct drive stages expressly designed for the photonics assembly and alignment market. Our stages are lower in both cost and complexity than competing conventional stages. Their numerous benefits would easily justify a price premium; when they come, in our case, at a discount, the argument in their behalf is compelling. We are now seeing applications with moderate cost and performance targets switch from mechanical bearings to air bearings, simply for their unlimited service life and high speed capability. This work was done by Kevin McCarthy, Chief Technology Officer, Danaher Motion Mr. McCarthy is based in Salem, NH and can be reached directly at 603-893-0588 or Kevin.McCarthy@danahermotion.com.