The language used by suppliers of technology solutions aimed at precision engineering applications is vague and, in some instances, confusing. The use of traditional language and words such as precision and resolution without any degree of qualification are just meaningless. This article analyzes the concept of precision. It explores new language for defining submicron- and nanometer-level accuracy for motion control and details why new standards are required to indicate the real levels of precision that different motion control solutions can achieve.

A 6D ALIO Hybrid Hexapod.

Technological advancements often necessitate a root and branch change in the language and nature of discussion associated with them. For example when electricity became available in every home, it would have been somewhat perverse if we had all continued to talk in terms of our preferred candles rather than the relative luminescence and longevity of various forms of light bulb. Likewise, when the car began to emerge, and eventually replaced the horse and cart as the most efficient form of transportation, the discussion logically switched from the best hay to feed the horse to the merits of different forms of internal combustion.

A similar shift is happening in the area of motion control. A new technology developed by ALIO Industries has pushed the boundaries of what is deemed possible in terms of precision. The nature of the conversation needs to shift in order to differentiate these advances. Interest in hexapods has increased to address demand for micron- and submicron-level precision in multi-axis motion applications. The new technology, Hybrid Hexapod®, offers motion control technology exists at the ultra-precision end of motion control.

The burgeoning area of more precise motion control is driven by industry demand for technologies that will improve production processes. Industry has called for smarter, smaller, and faster precision motion control and positioning equipment, and demand is especially high in areas such as laser micro machining, micro assembly automation, and optical inspection.

The Hybrid Hexapod represents a quantum step forward in motion control, and for the first time provides the ability to achieve repeatable nano-level accuracy, stimulating innovation and promoting manufacturing efficiency previously considered impossible. It is redefining the area of precision motion control, and the rule book is being rewritten to accommodate it and to position it correctly against industry alternatives.

Fig. 1 - Single point precision using Point Precision® metrology.

One key area for focus is how motion control process suppliers describe the level of precision achievable. Standard industry vernacular talks in terms of micron and submicron precision, but ALIO is now working with NIST to move to a new and more effective methodology of measuring and quantifying motion systems by introducing the concept of Point Precision® (see Figure 1).

Defining Precision

The very nature of the word precision is vague. Engineers are used to hearing descriptions using phrases such as precise and ultra-precise. Also, many suppliers have made claims of “achievable resolution.” But what does resolution really mean and what does it tell you?

In the area of motion control, the focus should and must be on much more exacting criteria: repeatability and accuracy. Precision is actually synonymous with repeatability and accuracy, but too often suppliers hide deficiencies in these areas behind meaningless phrases such as precision, high accuracy, high precision, or ultra-precision. In the case of the Hybrid Hexapod, precision means 10 nm or less — repeatably.

For standard hexapods, claims of precision are best condition, unidirectional one-axis numbers, which don't factor all six-axis error quotients or the backlash, which is the total error of all motion in a hexapod due to the compression and tension of each leg for every move. Unless the word precision is accompanied by such quantifiable and definite statements in terms of what is achievable, it is truly meaningless. While still vague, micron, submicron, and nano precision are better. These terms at least give an illusion to the level of precision that is being claimed.

Point Precision — The New Gold Standard

Point Precision has been introduced by ALIO and has been adopted by NIST as the future standard methodology of measuring and quantifying motion systems.

Point Precision includes all six degrees of freedom of errors of each axis in motion, guaranteeing the precision point in the full work envelop. As an example, the Hybrid Hexapod has a 3D point precision of less than 100 nm repeatability anywhere in its full work zone. With that information, a customer with, for example, a demanding metrology application can be extremely confident in the uncertainty measurement error quotient.

Point Precision allows for a precision number to be noted based on an exact point on the wall (as if a laser pointer was used) whereas the current standard only gives the measurement to the wall as if using a floodlight. The key is not just an ability to be nano precise, but to be nano precise repeatably. For some suppliers, this is impossible, and so they use either false claims or illusory claims often hidden behind phrases such as ultra-precise and high resolution.

Some even go as far as to publish “typical specifications” and “guaranteed specifications.” Typical specifications show what could be possible in a motion control solution and allude to much greater precision than the guaranteed specifications, which is what the supplier will guarantee. In other words, they show what they would like to be able to do repeatably, and then show that what they can actually do. With NIST's adoption of Point Precision for measuring motion systems, referencing performance specifications to a point in space, not the planar methodology current standards use, the language has changed.

Fig. 2 - 6D Point Precision® incorporates all sources of error at any desired work location into a meaningful three-dimensional value.

While there are compensation methods to reduce error sources in conventional six-link hexapods, they do not improve performance at the single-digit micron- or nanometer-level. Motion systems’ straightness and repeatability performance must be analyzed and specified using a point precision methodology that accounts for all 6D spatial errors in order to provide a true representation of nanometer precision (see Figure 2). Point Precision is the gold standard to assess the real precision of alternative motion control solutions.

Summary

There are numerous companies working in the area of micro- and nano-manufacturing that exist because of a passion to lead and to provide industry with solutions that stimulate innovation and advance the chances of achieving success in ever more exacting precision engineering applications. By their very nature, they push the boundaries and strive to provide technology solutions that facilitate greater and greater precision, which is consistently demanded across the medical technology industry.

However, at the same time, some companies attempt to compete with suboptimal solutions, either making false claims of competence or using language that is vague and does not provide meaningful information on the levels of precision that can be achieved.

This new approach addresses the lack of clarity that exists specifically in the area of motion control solutions. The creation with NIST of the Point Precision as the future standard methodology of measuring and quantifying motion systems exemplifies the uniqueness of motion control solutions in the area of nanometer precision.

This article was written by Bill Hennessey, CEO/Founder, ALIO Industries, Arvada, CO. For more information, visit here.