Technology Update
Measuring up to aerospace engineering's needs
 Multilateration is used to measure a BAE Systems Hawk military trainer. Knowledge of precise dimensions can lead to performance improvements. |
Achieving large-scale 3-D measurements to very high levels of accuracy is a necessary part of aerospace manufacture, but it may present major difficulties, not the least of which is the time taken and the subsequent costs involved. But in the UK, the National Physical Laboratory (NPL), BAE Systems, and Leica Geosystems have successfully demonstrated "a major advance in large-scale 3-D measurement."
Aerospace Engineering has previously described the early phase of the collaborative program. Now, at BAE's Advanced Technology Demonstration Center at Warton in northwest England, the three partners have developed what they describe as an innovative way of measuring large objects using four laser trackers. Laser trackers are already used extensively in the aerospace industry for measuring the size and shape of very large structures and components.
According to NPL, the new coordinated measuring system achieves a positional accuracy of one part in a million—10 mm for components of the order of 10 m in size. This accuracy exceeds the performance of conventional instruments measuring large parts.
The system is based on a measurement technique that NPL refers to as "multilateration." A high-accuracy virtual coordinate measuring machine (CMM) can be created using four laser trackers without the need for major structures with long-term dimensional stability. Moreover, the laser trackers can be returned to their normal duties when the measurements are completed. Once the physical arrangement of the laser trackers is decided, the system is calibrated and their precise locations established, so no additional equipment is needed to measure the geometry of the system.
NPL and the two companies say that industries seeking ultra-precise measurements where size and shape are vital to achieve technical performance will welcome this new method. And with more sophisticated arrangements to probe a wider variety of surfaces and components, the technique could eventually replace conventional CMMs altogether, believes the group.
The technique depends on collecting length measurements between a target, which is placed on the object to be measured, and four commercially available laser trackers. The locations of the target and the trackers can be deduced from computations using just the length data. The principal advantage of a laser tracker is that it is portable, enabling it to be brought to the part rather than the part being taken to a conventional large CMM.
However, the uncertainty of measurement from a single instrument can be 10 ppm or more, due to the use of angle measurement and the negative effects that the environment can have on this, says NPL. By combining only the length measurements from four laser trackers to measure a single target, it is possible for the accuracy to be improved to a constant one ppm. In addition, by using four trackers, sufficient measurement data is automatically collected to enable a precise estimate of the uncertainty of a measurement being made.
NPL is the UK's national standards laboratory. It carries out research—primarily on behalf of the British government—to ensure that the UK maintains measurement standards and facilities for physical quantities (mass, length, time, temperature, etc.) and also related work in engineering materials and information technology. Leica Geosystems, based in Switzerland, is a worldwide manufacturer of optical systems and 3-D metrology applied to large industrial objects, while BAE Systems designs and manufactures civil and military aircraft, space systems, radar, avionics, communications, electronics, guided weapons, and other products.
- Stuart Birch
Lord continues with active balancing technology
Initial ground testing of a new active balancing technology developed by Lord Corp. has shown that the system is capable of minimizing vibrations during typical flight cycles. The ground test was conducted in September on the NP2000 Hamilton Sundstrand propeller that is intended to replace the 54H60 series for the E-2, C-2, C-130, and P-3 series aircraft. NAVAIR supported Lord and Hamilton Sundstrand in conducting what was the first field demonstration of the active balancing technology for aircraft propellers. According to Lord, the once-per-revolution vibration levels were reduced by almost an order of magnitude over the full power range from reverse thrust to full takeoff power.
Active balancing, defined as instantaneously correcting an imbalance in rotating equipment while in operation, is faster and more effective than conventional "off-line" balancing techniques. By operating online and automatically, the Lord Active Balancing System provides optimal operation by compensating for variations in balance throughout the entire operating range of rotating equipment.
The system includes a small electronic controller that not only achieves online balancing, but also can serve as a predictive maintenance tool. As the system nears maximum balancing capacity, the controller triggers a call for maintenance before the rotating equipment actually needs to be repaired. Benefits of the system are less fatigue for on-engine and onboard equipment, lower failure rates, and reduced operating costs.
"Propellers are somewhat unique in that different amounts of imbalance result from their variable blade pitch, which can cause changes in both mass imbalance and aerodynamic imbalance," said Jim Pike, Lord Manager of Advanced Engineering. "A variation in the required balance correction amplitude and angular position was indeed observed during the test. The Active Balancer was able to respond to these variations and implement such variation in the balance correction, keeping the propeller vibration at minimal levels."
The traditional method of propeller balancing optimizes balance for one condition, usually for the ground. In contrast, Lord's active propeller balancing system operates online, automatically adjusting to all the variation and conditions of a typical flight cycle. Since it is automatic, the process occurs continuously without human intervention or downtime. Furthermore, the controller module constantly monitors vibration and triggers a balance correction only when the vibration reaches a predetermined level. Any power shutdown or controller shutdown leaves the balance correction at its latest optimal value, thus providing for graceful performance degradation and fail-safe operation.
- Frank Bokulich
