QMT Features: January 2010
iGPS metrology for robots
RWTH WZL, a German manufacturing research centre, investigates how manufacturing quality can be maximized using iGPS enabled industrial robots

In search of versatile production system concepts for German manufacturing industries, RWTH Aachen has invested in Nikon Metrology iGPS metrology. The renowned institution’s “Werkzeugmaschinenlabor WZL”, uses a cell populated with two industrial robots to study how manufacturing quality can be maximized using iGPS.

WZL, RWTH’s Laboratory for Machine Tools and Production Engineering, develops new and optimizes existing solutions through  research and development  with industry clients, helping manufacturers to improve their competitiveness.
 Robots are flexible production entities for handling, mounting, welding or other production processes that can be deployed in production systems that manufacture small series  and can easily switch from one product type to another.

However, their potential is left largely unexploited because too much time is needed to set up robot processes, program robots movement, rig them or align process fixtures. Scientists at WZL confirm that a combination of off-the-shelf robot and metrology technologies yield better economics than expensive bespoke equipment or employing a manual approach. Optical metrology, in combination with industrial robots, is seen as a key enabler to achieve this flexibility at an economic cost.
“The  combination of industrial robot workhorses with innovative large-scale metrology offers distinct advantages compared to equipment specifically designed for a particular manufacturing task,” says J. Jans, Nikon Metrology Executive V.P. Marketing. “Today too many manufacturing tasks are performed by hand, or when automated the task is performed by expensive dedicated designed precision equipment.”

Nikon Metrology has an impressive track record in metrology-assisted production. In a project aimed at stretching the accuracy limits of industrial robots, Airbus applied Nikon Metrology metrology to improve the precision of robotic drilling and rivetting at CAD specified wing locations. As a result, the aerospace giant’s wing drilling and rivetting cell ha become capable of reaching an accuracy level that is 10 times better than previously.

Nikon Metrology iGPS systems can turn an entire manufacturing cell into a single metrology enabled environment. iGPS robot sensors capture their individual elevation and azimuth angles with respect to multiple iGPS transmitters that are within line of sight, based on the timing of the arriving invisible coded laser pulses. This is iGPS’ unique approach: to continuously track robot tool centre point positions, and, possibly, also the part that is clamped by the robot(s). By returning the acquired metrology data to the robots using a closed feedback loop, robot positioning improves drastically. iGPS plays an important role in this innovative metrology-adjusted process, which nearly eliminates the influence of robot warm-up, drift and backlash.

WZL has mastered this concept to improve the manufacturing quality of numerous dynamic industrial applications, including handling, assembly, burnishing, deburring, welding and pleating. Björn Damm, team leader of the imaging processes and coordinate metrology groups,explains that: “Optical metrology technologies will be the eyes and ears of versatile and self-optimizing production systems of the future!”,

Dynamic manufacturing
“Dynamic applications are much more challenging because robots need continuous adjustment when manufacturing tasks are actually taking place,” says Alexander Schönberg, research assistant for the robot cell and member of the imaging processes group. When two robot-clamped parts are being welded, for example, the positioning of the parts ideally requires real-time adjustment, not just at the start and end point of a seam weld. WZL is heavily involved in optimizing this critical real-time adjustment process that determines the precision of dynamic manufacturing procedures. An approach being investigated employs Kalman filtering. 

Originally used to allow the spaceship of the Apollo program to land on the moon, Kalman filtering will guide dynamic robot movement with higher precision. From a mathematical point of view, Kalman is the optimum filter type for vector-based Gauss-Markov-Processes, as no other linear method exists that reaches smaller errors between the actual and predicted state of a system.

Robert Schmitt, Professor of the Chair of Production Metrology and Quality Management, concludes: “Top-notch iGPS metrology, combined with WZL specialist know-how, will lead to flexible and affordable robot technology that may fit numerous manufacturing applications. This very interesting and promising topic will be targeted on a highly scientific level in an upcoming EU-project.”

Candidate partners are welcome to contact WZL,  email: R.Schmitt@wzl.rwth-aachen.de

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