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Setting the standard

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A Hybrid Hexapod being used for camera image stabilisation (Image: ALIO Industries)

As motion control systems reach ever-higher levels of precision, new standards and methods are required to better define their capabilities to end users, Matthew Dale finds

Precision motion systems are used in numerous photonics applications, such as those using lasers and sensors for metrology and manufacturing. Technology innovation and industry advances are driving a greater need for motion systems offering extreme levels of precision.

However, the terminology used to describe the various levels of precision can be confusing for the end user, according to Bill Hennessey, CEO of Alio Industries, mainly because the standards used to test accuracy and precision within the motion control sector are outdated.

Hennessey said that Alio spends a lot of time having to educate end users on how to navigate around vague terminology used to descibe precision for them to get the right motion control technology for their application.
To overcome these issues, he believes that any claim of precision or accuracy must be first meaningful, and second provable. However, as many motion system manufacturers have developed their own testing definitions and procedures, this leads to confusion in industry, as the test results, which are used for comparison and qualification, are not based on a consistent set of principles.

It’s not that the motion control sector is without standards. The problem, according to Hennessey, is that with the innovation and advancements that have taken place within the sector over the past decade, existing standards and methodologies have become outdated and obsolete, while at the same time, new standards have been slow to evolve.

For example, per the current ASME B5.54-2005 standard, motion system manufacturers specify linear repeatability as a single number representing the variation in linear displacement. The one-dimensional nature of the test means that each test position measured can be conceptually viewed as the intersection of a plane with the axis. The test does not provide any information about the pitch, yaw, or roll of the conceptual plane, or where the actual test point is located horizontally or vertically on the plane. Only one piece of information is known: where the assumed orthogonal plane is along an assumed perfectly straight axis – the linear positioning repeatability.

A 6 DOF positioning system with an inverted Hybrid Hexapod used in the metrology of encoder heads and scales. (Image ALIO Industries)

Historically, this practice was valid, as the repeatability specifications were large enough that other error factors were a small percentage of the total error, and could therefore be neglected. However, motion control systems have now improved, moving from micrometre precision to nanometre precision. This means that these ‘other’ errors that were previously neglected become equal or greater contributors to the repeatability performance when all six degrees (6D) of freedom – X,Y,Z, pitch, yaw, and roll – are taken into account. Each of these error sources will affect the three-dimensional repeatability of a functional point attached to a stage. 

‘Each linear (or angular) direction a stage moves (or rotates) and results in a positional error in that direction,’ Hennessey said. ‘That motion, which must not be neglected when nanometre precision is desired, will have an associated repeatability of that error motion. Each point on a stage mounting surface will move in 3D space as of a result of this error motion in six degrees of freedom.’

This is why, when discussing precision and accuracy for motion control, there are often too many variables that are not fully understood by end-users.

While the current standard does include methods to test the multiple repeatability error sources, there is no clear terminology or requirement for specifying repeatability as a multi-dimensional performance metric that accounts for all error sources.

A laser processing workstation based on a Hybrid Hexapod. (Image: ALIO Industries).

‘The axis described by the current standard should actually be shown as bending and twisting through three-dimensional space, and thus the plane visualisation becomes meaningless as it will tip, tilt, and twist as the stage moves along the axis,’ explained Hennessey. ‘The stage moves in 6D space. Thus neglecting these additional error sources can result in a misrepresentation of actual stage repeatability performance.’

For hexapods, because they have six independently controlled links joined together moving a common platform, the motion error of the platform will be a function of the errors of all links and joints. Hexapods are known to have optimum accuracy and repeatability when performing Z-axis moves, because all links perform the same motion at the same relative link angle. However, when any other X, Y, pitch, yaw or roll motion is commanded, accuracy and geometric path performance of the hexapod degrades substantially because all links are performing different motions.

In order to address this issue, Alio has been working with the US’ National Institute of Standards and Technology (NIST) to develop a set of standards that will measure motion control to a point in 3D space – a methodology Alio has dubbed ‘Point Precision’.

Left: The Point Precision methodology incorporates all sources of error at any desired work location into a meaning three-dimensional value; Right: The six degrees of freedom of a motion axis. (Image: ALIO Industries).

‘Each axis of a motion system has errors in six degrees of freedom, and all error sources are statistically significant when characterising the repeatability of a point in three-dimensional space,’ Hennessey explained. ‘Point Precision characterises all of these repeatability error sources, resulting in a spherical repeatability range.’

As a result, the new standards will enable motion system suppliers to guarantee the precision point of their products in a full work envelope and quote a ‘precision number’ to end-users. Hennessey helped visualise this by explaining that the number would measure to an exact point on a wall as if marked by a laser pointer, compared to today’s standard, which would only measure to a wall as if using a flood light.

The standards, which have been a long time in the making according to Hennessey, are now in their final stages and should be published next year. While not much more can be said about them for now, we do know that the standards will cover all positioning equipment, rather than just the stage and hexapod examples discussed in this article. ‘Point Precision truly is a must for many applications, from laser processing to metrology,’ concluded Hennessey.

New compact rotary actuator points to higher-precision positioning devices onboard satellites

EU researchers have developed a miniaturised rotary actuator for the space sector. Moving rapidly and accurately, it will enable satellites to get the most out of communications systems and solar array panels.
The device was created as part of the Pre2Pos Horizon 2020 project, focused on the industrialisation and commercialisation of rotary actuators, using an innovative motor that takes advantage of the micrometric deformation displacement of piezoelectric stacks to achieve infinite rotary or linear motion.

The project’s goal is to include these rotary actuators in key equipment and mechanisms used in spacecraft, where high precision, low weight, energy efficiency and low manufacturing costs are required. The project team members also anticipate the new technology will also be suitable for other industries and markets, such as metrology and optics.

Increasing pointing accuracy of satellite mechanisms

Every kilogram in satellite payload counts. The same applies to the actuators that ensure fine orientation of satellite mechanisms. Satellites in orbit must be oriented to point their antennas, solar array panels and instruments with high accuracy.

With EU funding of the Pre2Pos project, Italy-based start-up Phi Drive and Spanish-based company Arquimea developed a light yet powerful rotary actuator. ‘Our target was to develop smart mechanisms for space applications that rely on piezoelectricity. Our new actuator featuring a high torque-to-mass ratio could sustain high loads and provide high levels of accuracy in the onboard positioning devices,’ said project coordinator Marco Bacciocchi.

Precise orientation of satellite-operated devices is fundamental to satellite operations. ‘Properly aligned antennas improve bandwidth and allow for better communication amongst satellites or between Earth stations and satellites. What’s more, proper tracking of solar array panels enables satellites to harness more power for their operation,’ Bacciocchi added.

Pre2Pos’ actuator is powered by electricity and is purely mechanical – linear motion in one direction gives rise to rotation. It converts an electrical signal into a precisely controlled physical displacement of piezoelectric stacks to achieve a rotary motion. The selected AG-LT model is a resonant piezoelectric motor that makes use of a hybrid longitudinal-torsional vibration in a fixed stator.

When the stator is pressed against the rotor by its longitudinal vibration mode, there is clockwise rotation due to its torsional vibration. Before the stator reverses its rotation direction, it detaches from the rotor by compression. The rotor keeps on rotating thanks to its inertia, until the cycle is repeated.

Requiring no oils or lubricants to operate, the rotary actuator is in line with the standards for contamination and cleanliness control set by the space industry. Furthermore, it lacks mechanical brakes and gears – eliminating backlash effects which are undesirable.

For now, the researchers are working to further increase the torque of the motor. ‘We think that a great advance would be to include a tool for automatically tuning in real time the resonant frequency of the system to always let the actuator work at its maximum performance in every working condition,’ concluded Bacciocchi.

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