LabVIEW and SCXI Provide a Configurable Measurement System for In-Flight Helicopters

Author(s):
Chris Koehler - G Systems

Industry:
Aerospace/Avionics

Products:
LabVIEW

The Challenge:
Providing a system to acquire data from a variable number of channels and various types of transducers on a helicopter during flight.

The Solution:
Developing a configurable LabVIEW application to acquire conditioned signals from SCXI modules and other DAQ boards.

A Flexible System Allowing Different Input Signals
Our company, Global Helicopter Technology, Inc. (GHTI), in Arlington, Texas, provides a variety of services for commercial helicopters, such as upgrading, certifying, and supplying engine integration kits. We work to characterize an aircraft’s physical behavior while in flight, by measuring quantities such as vibration, mechanical strains, and engine temperature. To improve its existing measurement system, We contracted G Systems to develop a new data acquisition system using National Instruments software and hardware. The Huey T53-703 represents a typical of the type of helicopter that GHTI characterizes using the new data acquisition system.

The system required flexibility for different types of input signals, acquisition rates, and hardware configurations. Normally, we cannot obtain all required measurements in a single test flight, so we must make multiple flights, each with a different measurement emphasis. An initial flight might focus on high-speed measurements of vibration, while the second might monitor temperature, strains, and acceleration. The channel count can range from less than ten to several hundred, and acquisition rates may be as high as 20 KHz. Since helicopter space is at a premium, we had to make the hardware as compact as possible. Also, we the software had to produce binary data files with a means of quickly reviewing data after a flight to verify data integrity.

Tests in the Air Controlled from the Ground
To accommodate high channel count and various signal types, we selected an SCXI system along with several stand-alone DAQ cards. We use a standard PC with a high-end CPU and ample RAM. We chose LabVIEW as the development environment to easily build data acquisition compatibility, file I/O, and analysis functionality into a single application. The application detects the data acquisition cards and SCXI modules installed and updates the display accordingly. We can load saved channel configurations and create new configurations as needed. The program notifies us if a previously saved configuration does not match the current set of hardware. We can enter scaling information, comments, and signal conditioning parameters through this single application.

In order to meet the space requirement, we do not use the monitor and keyboard during the flight test. Instead, we use a small, serial-based remote control unit to perform the basic functions of starting and stopping tests during flight. The unit also provides feedback about the state of the acquisition, including current record number, error messages, and remaining hard disk space. The controller has four lines of 20 characters, 16 programmable function keys, and uses standard RS-232 protocol. We can custom label the function keys and program them to beep or send out a string when pressed. In this case, we designated buttons to start and stop a test and request the remaining disk space.

When we finish setting up a test configuration, the application immediately prepares for data acquisition. The application waits for a mouse click on the GUI or for a button push from the controller to begin acquiring data. If the computer reboots for any reason, it automatically restarts the application, loads the last used configuration, and prepares for acquisition in the same way. The remote controller alerts the user with a beep and a message when the application can begin acquiring data again.

Real-Time Audio Synched with Transducer Data
The application produces binary data files to speed up file I/O and save hard disk space while in flight. On the ground, we view the files on the post analysis screen to verify that we can use the acquired data. Then we import the files directly into a third-party program for further analysis. During flight, the application records data as raw voltage or counts and converts it to engineering units (Hz, degrees F, or other) after the flight. We process data after the flight to allow all for use of all resources to acquire data during the flight. Postponing all unnecessary computations until the test finishes, we can achieve the highest possible acquisition rates.
Normally, we designate one of the analog input channels for audio input. We use this channel to record the pilot’s comments about the maneuvers he performs. Deciphering data during analysis is easier since we can play back the audio in real time with the transducer signals. In the post-analysis screen, a cursor sweeps across the data at the same time the audio plays back. The audio eventually converts to a WAV file so we can play back even without the LabVIEW application. We can record audio at any frequency, but sound quality degrades at frequencies below 8 KHz. The application interpolates the waveform to bring it up to 8 KHz (or higher, if possible) in order to create a standard WAV file.

We developed a highly configurable and easy-to-use data acquisition tool using SCXI and LabVIEW to make measurements during helicopter flight. The system can accommodate any combination of available hardware and is scalable to incorporate new hardware. A single application provides conversion to engineering units, the ability to quickly review data, and audio playback. The methodology employed in this system provides GHTI with better data in less time than previously achieved. The measurements derived from this system provides valuable information for characterizing and improving the company’s existing aircraft.

For more information, contact:

Chris Koehler

G Systems

Tel: 972-516-2278

E-mail: chris_koehler@gsystems.com

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