Calibrating multidimensional force sensors is considered the supreme discipline of measurement and testing technology. GTM Testing and Metrology in Hesse, Germany is taking multi-component calibration to the next level with a new approach. It is not just the measuring range, design and size of the measuring equipment that are unique, but also the automation concept. PC-based control, EtherCAT, and servo drive technology from Beckhoff control and regulate the force application via a hexapod, while precision measurement terminals from the ELM series record all forces and torques in real time and with maximum accuracy.
When it comes to multi-component calibration, no one can match the metrology specialists at GTM, a leading supplier of strain gauge-based precision transducers and measuring equipment, precision electronics, and calibration services for forces and torques. There are two possibilities. The first is to determine the magnitude of the force very precisely for each individual component; the other variables such as force application points, force direction and lever arms are merely estimated as precisely as possible. These measurements take place on uniaxial measuring equipment. Another option is to measure the force vector in its entirety. The advantage here is that the only source of measurement uncertainty is the measurement setup.
GTM believes that this complete vector measurement will become established in the long term at government institutes such as the Physikalisch-Technische Bundesanstalt (PTB) and as an industry standard. The company therefore invested in a new, fully automatic system to work alongside its manual multi-component reference standard measuring equipment.
Accuracy improved from 0.5 to 0.1%
The measuring equipment, which was developed completely in-house, reduces the measurement uncertainty of the calibration by a factor of five compared to the previous accreditation and makes GTM a pioneer in the field of multi-component measurement technology and calibration. At the same time, the calibration range has been extended significantly. “In order to achieve the necessary flexibility, we designed a very large test chamber of 4,5 m3 for the customer’s setup,” says Martin Eller, measurement systems team leader and the responsible design engineer.
GTM is also breaking new ground in terms of measurement uncertainty. “We are at the top of the calibration hierarchy with a proven measurement uncertainty of 0,1% for forces from 4 to 500 kN axial and 2 to 200 kN transverse,” says Daniel Schwind, managing director and technical director at GTM. The measuring equipment can apply and precisely measure torques between 2 and 50 kNm.
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. It quickly became apparent during the design and construction phase that there were no suitable mechanical components – such as a hexapod with control system – on the market. “This unit was initially planned to use purchased parts, but in the end we had to develop it 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,” explains 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 Urbanski. With more precise calibration, guesses become more accurate measurements, and more reliable knowledge, providing the basis for GTM customers to further optimise their processes and manufacturing tolerances. “More precise multi-component transducers enable the aerospace industry to measure more details in their aerodynamic tests in the 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 tyres. Another advantage of the design is that GTM can reproduce gravity very precisely in any direction via the hexapod’s six axes, and thus 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 emphasises.
Calibration depends on the entire measurement chain
The calibration sequence – loading, unloading, load changes, measuring ranges and 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 and processed further. Furthermore, program sections that had to fulfill a real-time requirement, such as the realisation 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 high-end ELM3504 measurement terminals. All measurement terminals are designed with six-wire technology. The measurement technology is supplemented by the EKM1101 EtherCAT Coupler and the ELM9410 power supply terminals. Both are infrastructure components that ensure a high quality supply to the measurement terminals. The ELM3504 high-end measurement terminals faced tough competition from established measurement technology suppliers and GTM’s own measurement electronics, and 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 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. “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 Lindemann.
A total of six four-channel ELM high-end measurement terminals 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 EL series are also used. “Their standard resolution is perfectly sufficient for this,” emphasises Holger Schneider, who is responsible for the electronic components in the measuring equipment. The terminals 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 with the highest precision. “The compact multi-axis 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,” explains Urbanski.
The selected control architecture also has a positive effect 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 otherwise have needed three control cabinets, not to mention the integration and engineering effort required for the different systems,” concludes Schneider.
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