Seven-Axis Robot. One calibration with the Radian Laser Tracker

Laser Based Measurement Technology at Trumpf

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A nearly 400-pound seven axis robotic machine was not too complicated for the Radian. API was able to provide a full calibration of the machine, used to manufacture precisely punched sheet metal. Taking 3,000 points per second, the job was completed in one third less time than the typical calibration performed by the manufacturer TRUMPF, a global technology company.

The punch and laser combination machines from Trumpf always attract a lot of attention when they are demonstrated at trade shows. What is most impressive is their speed: steel plates weighing nearly 900 pounds are moved throughout the apparatus in an apparently seamless manner. Spectators often ask the Trumpf staff how their machinery can possibly find the correct punching position in a fraction of a second without any human intervention and control, and how, after the punching sequence (which can only be seen in slow motion), the machine immediately moves to the next punching position. One incorrect position and the entire plate becomes scrap. Clearly Trumpf is able to ensure the performance of its machines with great precision and certainty. After all, the TruMatic 7000, which is built in Ditzingen, near Stuttgart, Germany, is a high-end production machine. For a machine of this calibre, Trumpf’s machine tool and laser technology specialists use laser-based measurement technology for both its assembly and service.

Measurement: The size alone is impressive

The 165 kg-leveled engraving robot was designed for precision engraving work, making its calibration requirements extremely precise. The team needed to not only calibrate the moving trajectory for all axes and make compensations, but also obtain the coordinate points at the ends of every axis so they could later develop a robot control system using this dataMany of the measurements on the TruMatic and similar machines present a challenge simply due to their size. The crossbeam of the punch-combination machine, for example, which moves steel plates in the x-direction, extends more than 21 feet. The tolerances required in measurements on the crossbeam are less than one-hundredth of a millimetre.

The precision required in manufacturing and installation could only be guaranteed with a combination of different measuring instruments. Trumpf technicians were also faced a difficult task when measuring the straightness and alignment of the crossbeam on the assembly line over a length of 21 feet. These tasks cost Trumpf a lot of time in its assembly process with the tools and equipment they used previously. So, they decided to further expand and explore the use of laser-based measurement technology.

Degrees of freedom: Everything at Once

The robot designed by the research team had a very complicated structure with seven axes. The engineers would need to calibrate each axis movement and ensure they were stable.

Using the API Radian to measure and to calibrate the movement trajectory of each axis, an engineer from API first tracked the movement of the robot, by affixing a target to the robot arm and running it through its motions. The engineer was able to get the coordinate value of the points collected during that dynamic measurement at a rate of 3,000 points per second, rapid enough to easily collect enough sample points through the robot’s range.

Trumpf became aware of the XD-Laser laser interferometer from API while visiting a trade show. Its greatest advantage was that it could register the measurement of three degrees of freedom in only a single measurement process. Trumpf set up a test of API’s equipment for its sales engineers when they came to the company factory from API’s Europe offices in Heidelberg. The test consisted of a straightness and a compensation measurement of the X-axis on a TruMatic 7000. The API equipment had already proven its advantages during the first pre-trials on the test machine at Trumpf in Ditzingen. The system provided all three degrees of freedom in just one measurement. The API laser system was also easier to handle than Trumpf’s other measurement tools and was able to accommodate the reflector in the measurement area thanks to its smaller size.

Further tests revealed an additional important advantage to the system: only two measurements were needed to represent the complete crossbeam instead of the old system’s three. The new system, using an interferometer, could cover the full length of the cross beam mainly because it no longer needed beam splitters and because considerably longer measurement distances could be taken. This not only meant considerably less computational effort, but also a higher accuracy due to the lower number of transfer points. Pentaprism were also used for the right-angle measurements.

The sum of these advantages made the case for API. Trumpf ordered more laser interferometers for its Development, Assembly, Service and Trial departments, as well as for Final Assembly and for the Service/Technical Customer Support departments. The Final Assembly area was convinced that the API system would considerably improve the quality of their machines in mass production. With its portability, the system was also used at the customers’ premises by the Service department.

Use in Assembly and Service

Trumpf engineers in the Development and Trials Department automated the cross beam measuring process by programming an evaluation tool to take two measurements, analyze the results and display them graphically. The user is guided through the entire process step-by-step. This automation will also considerably simplify other operations in the overall process.

“The main reason for our decision to acquire new systems was certainly the capability of the system to measure all three degrees of freedom in a single pass” says Michael Theurer.

“The benefits resulting from this are thereby not limited to the use of the laser interferometer in series production. Even when providing on-site service at the customer, a new measurement process with the help of the laser interferometer provides detailed statements about the structure, the alignment and the straightness in the Y and Z axes after only two hours. We can provide all the data in a detailed report.”

Until Trumpf had acquired the API system, the Service department had only been able to carry out an indirect evaluation using a ruler and with the help of sheet metal machining, which required the evaluation of a steel calibration plate. Now, the compact measurement system accompanies Trumpf service engineers on service jobs around the world.

The actual measurement processes themselves require only a few minutes, depending on whether it is a short or long measurement. The short measurement is limited to the relevant points and is frequently used at the start of construction for a preliminary adjustment. The long measurement provides a complete representation of the crossbeam.

The managers of Trumpf’s Development and Trial Department have since started to see that the advantages the API system offers in their punching machines can also be applied to many of their other areas of production. Their tests showed that they could incorporate the interferometer system into many other machines in the production process. Trumpf has already started measuring its bending machines produced in Austria using an interferometer.

At the Limits of Feasible

In the Assembly department, the demand for ever higher accuracies has resulted in extremely tight adjustment tolerances of just a few hundredths of a millimetre. With a crossbeam length of more than 6 meters (18 feet), those tolerances can easily be broken by a small temperature change. Temperature fluctuations, therefore, must be accurately taken into account. Initially, this means more work for Trumpf. They must wait a period for the laser to fully warm up and careful preparations must always also be taken into account. But this additional assembly time will easily be made up and then gained at other locations in the production process. For example, the assembly times for Trumpf bending machines from the factory in Austria in which the measurement systems have been in use for a long time, have been considerably reduced by the use of the new measurement tools.

Better is the Enemy of Good

There is, however, nothing that cannot be improved upon. Trumpf found that the magnetic bases delivered with their laser measurement systems were not flexible enough in use for the specialists from Ditzingen. So, the Development department designed two different devices of their own for different types of machines. These were additionally fitted with quick closing fixtures. As a result, the laser interferometer could be directly mounted on the crossbeams, which had been fitted with tool clamps. The results with regard to straightness and/or positioning were not only used for the mechanical readjustments, but also delivered the compensation tables that were loaded into the machine controller, and thereby set up the machine optimally.

In summary, the measurements (straightness measurements, in particular) on large machine components, combined with the capability of measuring all the three necessary degrees of freedom within a single clamping operation through the use of laser interferometers from API in the Assembly and Service areas at Trumpf have led to major benefits. They guarantee higher accuracy measurements with a system structure that is still simple, and offers vast time savings.

“It was exactly this point that convinced us” said Michael Theurer. “And the benefits are felt everywhere.”