Measurement Technology from Beckhoff Takes the Uncertainty Out of Multi-Component Calibration
The test-and-measurement experts at GTM leveraged PC-based control, EtherCAT and servo tech to design a hexapod testbench for calibrating multidimensional force sensors – reducing measurement uncertainty by a factor of five
Calibrating multidimensional force sensors is one of the greatest challenges for measurement and testing technology. But GTM Testing and Metrology GmbH thrives on tackling these unique applications. And the test-and-measurement experts are taking multi-component calibration to the next level with a new approach – a hexapod testbench that leverages servo technology and high-end measurement tech from Beckhoff.
GTM isn’t just pushing the limits in terms of measuring range, design and size of the measurement equipment, but also the automation technology used. In the past, the company used legacy technologies from a large vendor in test and measurement, or GTM built specialized devices to meet unique application needs.
More recently, the company has harnessed PC-based automation from Beckhoff. GTM has combined the worlds of industrial automation and metrology for incredible results. In the hexapod system, PC-based control, EtherCAT communication and servo drive technology regulate the force application. At the same time, precision measurement I/O terminals from the ELM series record all forces and torques in real time and with maximum accuracy.
GTM is one of the leading suppliers of strain gauge-based precision transducers and measuring equipment, precision electronics and calibration services for forces and torques. Back in 2005, the company became the world’s first DIN EN ISO/IEC 17025-accredited laboratory for multi-component measurement technology. Since then, the company – based in Bickenbach, Germany, near Frankfurt – has been calibrating force and torque transducers as well as multi-component transducers.
There are two possibilities for these tasks. First, using measuring equipment with one motion axis, you could calculate the magnitude of the force very precisely for each individual component. But you would have to merely estimate, with as much precision as possible, the other variables such as force application points, force direction and lever arms.
“Another option is to measure the force vector in its entirety,” says Daniel Schwind, managing director and technical director at GTM, pointing out a second approach much discussed in metrology. The advantage here is that the only source of measurement uncertainty is the measurement setup.
GTM believes complete vector measurement will become the established industry standard in the long term. Consequently, the company invested in a new, fully automatic system to work alongside its manual multi-component reference standard measuring equipment – which has been in operation since 2005 – was a logical development step for GTM.
New approaches to testing offer major advantages
With its high-tech measuring equipment, GTM has blazed a trail into difficult terrain – metrologically speaking, that is. Existing multi-component calibration methods have weak points, such as their achievable measurement uncertainty or the calibration effort. “The multi-component reference standard measuring equipment, as we have implemented it, was terra incognita,” says Schwind.
As a result, basic research was necessary in some areas. GTM completed its work on the comparative measurement and verification in advance of and, to some extent, in parallel with the development of the measuring equipment. “The discussions with the assessor about our procedure and the traceability of our measuring equipment during accreditation were very fruitful,” recalls Torsten Hahn, deputy head of the calibration laboratory.
Deliberations about the measuring equipment began as early as mid-2016. Back then, inquiries about GTM’s for multi-component calibration had increased exponentially. So it was no longer possible to fulfill all orders on the existing measuring equipment and with the existing calibration procedure.
After all, the manual calibration process used to date is very time-consuming: the calibration object must be precisely installed in the measuring equipment and repositioned several times for the measurements. This results in excessive setup time for the entire calibration. “Previously, such a calibration took around two days,” says Marcel Richter, director of product management & marketing at GTM.
On the new measuring equipment, the time requirement drops considerably. The only steps involved are clamping the test specimen, configuring the measuring equipment for the calibration object, and configuring the automatic calibration process in the operating software. This offers major advantages, Hahn explains: “Due to the integration of all components, calibration can be carried out fully automatically overnight.”
Measurement uncertainty improved from 0.5% to 0.1%
The measuring equipment, which was developed completely in-house, dramatically reduces calibration uncertainty. The system enhances calibration by a factor of five compared to the previous accreditation and makes GTM a pioneer in the field of multi-component measurement technology. At the same time, the calibration range has been extended significantly – with realistic and application-specific installation conditions for the calibration object. “To achieve the necessary flexibility, we have designed a very large test chamber of 4.5 cubic meters for the customer’s setup,” says Martin Eller, measurement systems team leader and the responsible design engineer.
“We are at the top of the calibration hierarchy with a proven measurement uncertainty of 0.1%,” says Daniel Schwind, “for forces from 4 to 500 kN axial and 2 to 200 kN transverse.”
The measuring equipment can apply and precisely measure torques between 2 and 50 kNm. “The only other way to achieve these large measuring ranges with such a low measurement uncertainty is with uniaxial measuring equipment,” adds Eller.
GTM invested significant resources and engineering effort into the design right from the start to maximize data acquisition capabilities. The core element is made up of three different basic measuring platforms to ensure maximum flexibility with regard to the size of the calibration objects, equipped with high-precision K series force transducers specially adapted to the new measuring equipment. Furthermore, it quickly became apparent during the design and construction phase that there were no suitable mechanical components – such as a hexapod including control system – on the market.
“We initially planned to purchase key mechanical parts, but in the end, we had to develop them ourselves. With PC-based control and the drive technology from Beckhoff, we found a suitable, high-performance platform that integrates well – including the precision measurement electronics,” emphasizes Martin Urbanski, who is responsible for the software development of the measuring equipment.
The measuring equipment is servo-electrically adapted to the calibration object via three spindles. During the calibration process, six screw drives driven by the AX8000 multi-axis servo system and AM8042 servomotors generate the required forces and torques – with maximum precision. “With a measurement uncertainty of 0.1%, we are playing in a different league and enabling users to take new approaches in order to improve their applications,” says Richter, pointing out the motivation for GTM.
With much more precise calibration, guesses become accurate measurements. This more reliable knowledge provides a foundation for GTM customers to further optimize their processes and manufacturing tolerances. “More precise multi-component transducers enable the aerospace industry to measure more details in their aerodynamic tests in a wind tunnel, for example, and to evaluate the effect of modifications to the aircraft,” says Christian Lindemann, product manager for precision measurement technology at Beckhoff.
Even small improvements to the models can significantly increase the efficiency of an aircraft in subsequent operation. The same applies to rotor blades of wind turbines, ship propulsion systems or rolling resistance measurements on car tires.
Another advantage of the design is that GTM can reproduce gravity very precisely in any direction via the hexapod’s six axes and calibrate the customer-specific application virtually under real-life installation conditions. “This is an innovative and trend-setting solution for customer-specific multi-component measurement technology that is not available anywhere else,” Schwind emphasizes.
Calibration depends on the entire measurement chain
The calibration sequence – loading, unloading, load changes, measuring ranges as well as repetitions – is configured by measurement technicians via GTM’s proprietary WebForceManager operating software. This controls the measuring equipment fully automatically on the basis of the calibration sequence used, which consists of sequential commands. These commands are transferred to TwinCAT automation software and processed further. Furthermore, program sections that had to fulfill a real-time requirement, such as the realization of the hexapod kinematics and the simultaneous acquisition of all measuring signals, are also implemented in TwinCAT.
When it comes to control hardware, GTM relies on embedded PCs and ELM3504 high-end measurement terminals. All measurement terminals are designed with six-wire technology. The system is supplemented by the matching EKM1101 EtherCAT Coupler and the ELM9410 power supply terminals, both infrastructure components that ensure a high-quality supply to the measurement terminals.
“We are pleased that GTM took a very close look at our industrial measurement terminals and selected them for this demanding calibration system,” Lindemann says.
The ELM3504 high-end measurement terminals faced tough competition from established measurement technology suppliers and GTM’s in-house measurement electronics. The ELM terminals had to prove their suitability for the task at hand, including as a measurement chain in combination with the high-precision K series force transducers used.
“We put the Beckhoff Terminals through their paces together with our high-precision force sensors and found that the I/O terminals work much more precisely under our ambient conditions in the calibration laboratory than specified in the data sheet,” says Schwind. GTM had initially considered making its own measurement electronics EtherCAT-capable, he adds: “However, once we had the ELM measurement terminals on the table, we were able to save ourselves the effort.”
“We are always conservative with our specifications and only promise something if our components can still deliver it under more difficult practical conditions,” adds Beckhoff product manager Christian Lindemann.
A total of six ELM high-end measurement terminals with four channels each are used in the measuring equipment for recording the force and torque input via the hexapod and for measuring the calibration object. Further EtherCAT Terminals from the compatible EL series are also used. “Their standard resolution is perfectly sufficient for this,” emphasizes Holger Schneider, who is responsible for the electronic components in the measuring equipment. They are used, among other things, to monitor the actuators for compliance with the maximum forces via strain gauges.
The six hexapod cylinders are driven by servo drives and the AX8000 multi-axis servo system. The power transmission – also an in-house development by GTM – involves a gearbox and a screw drive. As a result, the relatively small servomotors generate high forces and torques – “and with the highest precision,” adds Martin Eller.
The compact multi-axis drive system from Beckhoff in conjunction with PC-based control and simultaneous data transmission of the measured values in real time via EtherCAT helps here. “This has simplified our task of controlling all drives on the basis of the measured values,” Urbanski adds.
The selected control architecture also has positive effects elsewhere – in the control cabinet. Since GTM obtains almost all of the components from Beckhoff, from the measuring amplifiers and other I/O components to the industrial PCs and drive technology, the electronics are much more compact overall. “With our initial plans for a control concept based on separate servo and measurement technology from different suppliers, we would have needed three control cabinets,” Schneider explains. Not to mention the integration and engineering effort required for the different systems.
Interested in optimizing your test and measurement applications with industrial automation solutions? Contact your local Beckhoff sales engineer today.
Azad Jafari is the I/O Product Manager at Beckhoff Automation LLC.