A jet engine is the very definition of “mission-critical.” A critical failure could cause a serious accident that endangers hundreds of lives. To prevent this from happening, manufacturers perform extensive testing on jet engine components, systems, and manufacturing processes.
Jet engine tests involve subjecting the unit under test to extreme temperatures, jets of water, simulated hail, severe vibration, and other harsh conditions. These tests can be very costly and take years to develop. Engineers must measure many different parameters during these tests, including temperature, flow, pressure, rotation, strain, and vibration.
Selecting the right data acquisition hardware and software is a critical step in ensuring accurate data for jet engine testing. Since jet engine testing has some unique requirements, test engineers must choose data acquisition products that not only make high-quality measurements but offer high reliability as well. To withstand the rigors of jet engine testing within the test cell, you need a ruggedized data acquisition system.
The Traditional Approach
As shown in Figure 1, the traditional way to set up a jet engine test is to wheel it into a test chamber and then install the sensors and cables. The problem with this approach is that there are often hundreds if not thousands of sensors and transducers, and the extreme test conditions mean that the data acquisition instruments cannot be positioned too close to the test cell. This means long cable runs. Each of the hundreds or thousands of cables must make its way from the engine inside the test chamber to the instrumentation, with of course the correct labeling. This process takes many hours to complete and is prone to errors.
In addition, long cable lengths make test signals vulnerable to noise and interference from the high temperatures, shock, vibration, and other components, such as motors and ignitors, while a test is running. One solution is to use high excitation voltages to improve the signal-to-noise ratio. Using higher excitation voltages makes it easier for the instrumentation to distinguish noise from data, but the higher voltages also produce more heat and can shorten equipment life.
Even more concerning is the fact that those thousands of test cables can come loose or break. That means shutting the whole testing process down, finding and repairing broken wires, perhaps re-routing them carefully away from potential danger zones, and then getting everything back up and running. Periodic maintenance, such as checking that cables are still in working order, cable jackets are intact, connections are tight, and that everything is operational, can take many hours.
A More Innovative Test Approach
A more innovative approach to testing in harsh environments is to use ruggedized data acquisition hardware instead of laboratory-grade equipment. Using ruggedized data acquisition equipment enables engineers to place measurement instruments directly in the test chamber, close to the jet engine, and sometimes on the engine itself.
Figure 2 shows an example of some ruggedized test equipment. These data acquisition instruments are able to withstand extreme temperatures from -20°C to 60°C and are designed to withstand high vibration applications. In addition, many instruments are either IP65 or IP66 rated. An IP65 rating means that the instrument is packaged so that dust cannot get in and will withstand low pressure water jets. An IP66 rating means that the instrument's packaging totally blocks dust and powerful water jets.
LXI, IEEE 1588 allow distributed, synchronized measurements
The ruggedized test equipment described in this article supports the LXI industry standard for connecting distributed instrumentation and the IEEE 1588 precision time protocol standard. Both of these standards make it possible to use ruggedized test equipment inside a test chamber.
IEEE 1588-2008, “IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems” specifies a protocol that enables precise synchronization of clocks in measurement and control systems. When used in an LXI system, the protocol specifies how the LXI instruments on a network synchronize over the network. In simplest terms, the process goes something like this:
At power-up, all LXI devices begin intercommunication.
Together, they determine which device has the most accurate and precise clock source.
That device is deemed the master of the system with respect to time.
Periodically, the master imposes its notion of time onto all of its slaves and corrects for errors.
Using IEEE 1588 precision time protocol, distributed LXI instruments on a network can be synchronized as closely as traditional backplane architectures, such as VXI and PXIe. The result is that synchronized instruments can be placed virtually anywhere without the constraints imposed by a physical rigid backplane and still offer superb performance.
Figure 3 shows how to configure a test setup using ruggedized instrumentation. Instead of pushing the jet engine under test into the chamber and then configuring sensors inside, engineers can now install all the sensors, transducers, and data acquisition equipment directly on the engine before pushing it into the test chamber. Rather than having to set up the engine and separately configure the test equipment, engineers can do both steps at once. Because the instrumentation is located close to the transducers and sensors, cable lengths are substantially reduced. This reduces not only setup time, cost, and the potential for human error, but it also ensures higher quality signals.
The shorter cable lengths, and the reduced interference and signal loss that comes with that, helps the engineer use lower excitation voltages and, therefore, extend equipment lifespans. Environmental noise and interference are less of a concern. The data received and used in this innovative testing approach is of a far higher quality and more reliable.
There is also no need to manage thousands of long cables and carefully keep track of them; worrying about whether cables are securely connected or seeing if any one of them has broken now means a quick check rather than tracing thousands of long cables.
Using rugged instrumentation saves time, money, and frustration. Test setup is much simpler, and measurement accuracy is improved because shorter cable lengths mean lower excitation voltages and less interference. Cable maintenance is simpler, too, and this means greater uptime. All in all, a much more effective approach than traditional test approaches.
This article was written by Jon Semancik, Director of Marketing, AMETEK/VTI Instruments (Irvine, CA). For more information, visit here .