Achieving right first-time manufacturing requires regular calibration and verification of production machinery. The increasing use of on-machine measurement makes this even more important. Rapid, automated and fully traceable calibration of machine tools is, therefore, an urgent priority for high-value manufacturing. However, the current methods fall some way short of these requirements.
The Etalon Multiline system is set to change all this. It uses frequency scanning interferometry (FSI), a state-of-the-art laser measurement technique, which can be fiber channeled to multiple points within a machine-tool or coordinate measurement machine. To carry out a calibration, the machine must only move its spindle or probe through a preprogrammed path while the lasers make measurements, documenting any deviations from this path. This information is fitted to a kinematic error model of the machine. All of the error parameters are then updated on the machine controller automatically. A complete calibration takes minutes instead of hours and requires no human intervention.
Simultaneous Error Parameter Estimation
Traditional methods of calibrating machine tools involve the careful alignment of granite straight edges, squares and gauge blocks. Highly skilled machine-tool calibrators measure these references with a dial gauge mounted in the machine-tool spindle. This process takes days to complete.
Although these methods have been largely superseded by the use of laser interferometers for machine-tool calibration, the underlying process remains remarkably similar. Current methods involve isolating each degree of freedom for each axis by setting up a carefully aligned test. Even when using a laser interferometer, a machine calibration is likely to take several hours and will usually not provide a complete calibration for every error parameter in the machine’s model.
Etalon takes a completely different approach to machine calibration. It does not attempt to isolate and measure each kinematic error independently. Instead, the 3D coordinates of the machine’s spindle or probe are measured in many positions. The deviation, or difference between the commanded position and measured position, is recorded at each location.
The kinematic model, or equations describing the machine, give these deviations in terms of the error parameters for the machine. The error parameters are the unknown values required to compensate the machine during calibration. When more measured deviations are available than there are unknown error parameters, it is possible to solve a set of simultaneous equations to obtain the error parameters. Typically, there will be considerably more measurements than error parameters. Least-squares minimization will be used to obtain an optimal estimate for the error parameters. Such an approach may be referred to as simultaneous error parameter estimation.
The method the Etalon system uses to measure the 3D coordinates of positions on the machine’s path is known as multilateration. This involves measuring the distance to the position from three or more fixed locations. These multiple length measurements can be combined to give a 3D position, similar to the way three angles may triangulate a position. Normally, four lengths are used for a more robust measurement.
Frequency Scanning Interferometry
To obtain distance measurements Etalon uses FSI, which was first developed for the Large Hadron Collider at CERN. None of the existing measurement technologies were able to measure the very small movements of the particle detectors, which were required in this experiment. The detectors were spread over a volume tens of meters in each dimension. Movements at the micro meter scale had to be recorded.
Researchers at the University of Oxford, led by Dr. Armin Reichold, created FSI to solve this problem. The technique uses a tunable laser that scans across a range of frequencies to identify an absolute distance between reflectors, indicated by interference fringes. FSI is at the cutting-edge of length measurement and is showing up in a number of new instruments, such as the system from NPL that I wrote about recently.
FSI provides direct traceability to the fundamental definition of the meter by referencing the molecular absorption spectrum of a gas cell. Every measurement includes the scanning of this gas cell to re-reference the laser frequency. Independent validation at the U.K.’s National Physical Laboratory (NPL) confirms that the system has an uncertainty of 0.5 ppm for distances ranging from 20mm to 30m. This level of accuracy is similar to the width of a human hair over a kilometer.
Another big advantage of FSI is that a single laser can be split and channeled along many different fiber-optic cables so that many measurements can be carried out in different locations. Powerful signal processing electronics are also required, but they may also be shared with the single laser.
The central FSI system may provide highly accurate and traceable metrology to an entire factory, considerably reducing the cost of deploying this system. Each individual length measurement may be up to 20m with a measurement uncertainty of 0.5µm per meter. The central FSI system is able to provide these measurements at up to 124 individual locations. Typically, an individual machine would require just four of these. Since fiber-optic transmission causes very little power loss or noise, the individual measurements can be located kilometers from the central FSI system.
The Etalon Multiline FSI system is an exciting development for precision engineering. Future factories will be able to host their own fully traceable metrology center, which derives length measurements directly from physical constants. This is something that has, until recently, been the domain of national measurement institutes such as NIST, NPL and PTP. Individual machines, such as milling machines, lathes, robots and coordinate measurement machines will then directly access this metrology center over fiber-optic connections. In a matter of minutes, they will be able to fully calibrate themselves, with no human input. This will greatly improve the accuracy of production operations while also increasing confidence in on-machine measurement.