The Challenge:

Developing an integrated, configurable and reliable test system that can acquire data from various types of sensors like thermocouple, RTD, Pressure, Flow, load cell, speed, vibration and control the loading pattern of the gearbox.

The Solution:

Using the National Instruments PXI, SCXI Platforms, NI DAQmx and LabVIEW Development environment to create a highly flexible, scalable and powerful test system within twelve weeks.

Introduction:

Our client, a leading R&D organization, specializes in developing aircraft engine components wanted to create a test system for the engine gearbox. Gearbox plays an important role in distributing the engine power to the rotors and other power outputs in an aircraft. So before integrating with the actual engine, the gearbox should be subjected to rigorous testing which includes vibration and endurance tests.

The client wanted a test system that would:

• Support signal conditioning for a wide range of sensors like Thermocouple, RTD, pressure, Flow, Load cell, Eddy current speed probes and accelerometers.
• Perform online and post processing on the acquired data.
• Provide facility to monitor and control the tests remotely.
• Provide a software and hardware architecture that supports future expansion.

This meant that we had to use a platform that supported the following features

• Modular architecture that can handle different signal types and will be easy to configure, reconfigure, repair and upgrade.
• Efficient timing and synchronization for better integration between the various measurement modules.
• High processing power and bandwidth to handle the data from around 200 channels.
• Rugged architecture that can withstand the harsh environments in test rigs and maintains signal integrity.
• Support for both Real-time as well as a GUI based operating system

We decided to build the test system based on the open industry standard PXI architecture. PXI provided higher processing power with rugged, modular Eurocard architecture. We used the signal connectivity provided by NI SCXI platform to connect sensors, which required signal conditioning. For certain sensors like RTD, speed, current and digital output PXI based cards were used.

Application:

Endurance testing demonstrates the fatigue life of a gearbox and gives an indication of its reliability using various failure mode tests. The test rig includes a powerful electric motor coupled to a step up gearbox that can produce variable speeds, and which in turn drives the engine gearbox. While some of the power outputs of the gearbox are directly connected to dynamometers some are coupled to them using intermediate gearboxes. Dynamometers are used to vary the load on each power outputs of the gearbox. This is achieved by controlling the flow of water into the dynamometers. The dc-motor was controlled by a PLC based system, which required readiness command, from each of the sub systems to operate. So based on the limits of some analog channels, the tests system should give readiness commands to the PLC.

Accelerometers mounted on the gearbox give an indication of its vibration during the tests. The vibration signals acquired from these sensors were analyzed. Due to the presence of fast rotating mechanical parts and oil mist, our client preferred a system that can be controlled from a station situated around 20 meters away.

System Design:

Taking into account the remote controlling option required for the test setup and the online processing and high data rate requirements for the vibration signals, we divided the test system into two parts. An RT based PXI system that will acquire thermocouple, RTD, pressure, flow and load cell data and a Windows based PXI system which will acquire and process vibration signals from the gearbox. NI’s LabVIEW development environment coupled with NI-DAQmx measurement ready features helped us in reducing the development time significantly

The RT based PXI system was built on a client-server model. The server program runs on a PXI 8186 RT controller while the client program runs on an IBM Laptop. SCXI-1125 isolated input module was used to acquire thermocouple signals while pressure and flow signals were measured using SCXI-1102B. Load cells, which required dc excitation, were conditioned using SCXI-1520, which supported both excitation as well as filtering. Data from all these SCXI modules were acquired using PXI 6052E card. Speed signals from eddy current probes were passed through a Zero Crossing Detector to convert to TTL levels and then measured using PXI 6602 counter card.

The flow of water into the dynamometer was controlled through current controlled valves. These valves were operated using PXI 6704 Analog output card. The operator selects the percentage opening of these valves from the control station using potentiometer knobs, which will generate equivalent voltage signals that were acquired using PXI 6025E card. Based on the voltage value acquired, the server program calculates the amount of current to be generated through PXI 6704. PXI 6527 isolated Digital Output was used to generate readiness commands of each sub system.

The Windows based PXI system, used to acquire and process vibration signals had a test program running on a PXI 8186 Pentium 4M embedded controller and used PXI 4472 DSA card to acquire vibration signals.

The Windows as well as RT systems were interconnected through an Ethernet switch

Software Implementation:

The server program running on the Real-Time controller receives and executes requests from the client program running on the Laptop. The data acquisition and control was performed by the server code based on specific requests from the Client software. The test software included modules for channel configuration, calibration, Engineering Unit configuration, Data transfer from RT to Laptop through FTP, Offline plotting of stored data and Report generation.

The vibration analyzer software running on the Windows based PXI system supported data acquisition from multiple DSA cards. High-speed RTSI lines were user to synchronize data acquisition of multiple cards. Using this software the user was able to do online FFT, RMS and order tracking on the acquired data. In addition to this the user was able to do offline analysis including STFT on the stored data.

The vibration data acquired from this system was transferred to the RT system’s client Laptop through TCP/IP. This helps the user in comparing the effects of vibration during endurance runs.

LabVIEW along with NI DAQmx proved to be a perfect choice for developing the test software. Even though we had to acquire data from a wide range of sensors, the hardware we used along with NI DAQ driver helped us in easily blending all these data together. LabVIEW RT module helped us in making the RT program without getting involved in the system level programming paradigms that are normally involved with such OS. Also toolkits like Sound & Vibration toolkit, Order Analysis Toolkit, Internet toolkit and Report generation toolkit helped us in significantly reduce development time. The great GUI features provided by LabVIEW came handy in creating a user friendly HMI.

Conclusion:

We were able to design and develop a test system that enabled our client to efficiently test his UUT and provided the flexibility to configure it for various kinds of gearboxes. NI LabVIEW development environment and the modularity extended by the PXI-SCXI architecture equipped us in providing a flexible and reliable solution. In addition the analyzed test reports were now ready within minutes of completing a test saving the customer a few days in collating, analyzing and preparing reports.