Developing a Monitoring and Control System for Structural Aircraft Tests using LabVIEW and PXI

Author(s):
Luca Cambiaso - SITEM s.r.l.

Industry:
Aerospace/Avionics

Products:
LabVIEW,

The Challenge:
Building an easy-to-use program for a rugged control system that can handle structural and fatigue tests on fuselage or other parts such as the wing or rudder.

The Solution:
Creating a data acquisition and control system based on real-time technology using a PXI and LabVIEW Real-Time platform.

A Multifunction System
We developed an application for Piaggio Aero Industries Spa, a leader in designing and building executive aircrafts, for the purpose of monitoring and testing these types of aircrafts.

We divide the software program into two components. One is on a PXI system interfaced towards instrumentation on field test assembly, and the other component is on a standard PC that acts as a supervision unit.

We used an Ethernet link to connect the PC and PXI.

The program can handle up to:

• 128 analog input channels
• 24 analog output channels
• 112 digital input channels
• 40 digital output channels
• 2,000 GPIB channels from instrumentation

Additionally, the program can work with two different kinds of tests:

• A static test or "manual mode"
• A fatigue test or "automatic mode,"
  that is a dynamic test.

Moreover, we developed a third kind of operating mode. This mode helps the operator check the whole system at start-up or in specific situations.

In fact, using this mode, the user can activate every actuator independently from any logic control loop, verifying test assembly behavior at a specific point and/or verifying the correct functionality for every part of the system, such as pressurizing the cabin of the airplane.

Hardware Architecture
With National Instruments LabVIEW Real-Time and PXI System, we can efficiently develop new test consoles within weeks as opposed to months.

The system is composed of a desktop PC (NT 4.0 Workstation OS) connected via Ethernet to the data acquisition and control system based on a real-time PXI. The PXI then connects to third party data acquisition component via GPIB.

The PXI system is composed of a PXI-1000 chassis with a real-time PXI-8156B controller, two general purpose PXI-6071 DAQ boards, three data generation PXI-6713 boards, and, lastly, two PXI-6508 digital boards. Signal connections among the PXI system, the servo control system (analog CYBER PID units), and oleo-pneumatic actuators are made using DIN rail mounted terminal blocks.

Software Architecture
The software program is composed of two different components. The first is a data acquisition and control component that runs on the PXI control unit processor. This component exchanges data and commands with the second one that is a supervision and user interface component running on the PC.

Improved Data Exchange
To improve data exchange between the control unit and the supervisory unit, we used VI reference technology, developing a dedicated VI library. Moreover, to prevent system and operating damages, we added a watchdog mechanism - alarms sound if any communication breakdown occurs.

From the beginning, using the supervision component we can load or edit an .ini configuration file that describes the test. Ini files can be more than 20,000 rows long and are fundamental to the data acquisition process from boards and from GPIB instrumentation. Moreover, they are fundamental in generating stimulus signals for actuators.

Two other kinds of files define the loads matrix used in dynamic fatigue tests and the correlations matrix. These files describe analog input and output, digital input and output, and channels read from GPIB instrumentation. The loads matrix defines all possible load values.

For each of the 24 analog outputs corresponding to 24 actuators, the correlations matrix describes electric and mechanical relationships between analog output channels, exciting jacks servo controls, and acquisition channels with feedback signals.

After loading the correct configuration file, the operator can launch static tests or fatigue tests or can check the system.

Static Test Management
An operator must work onsite during static test sequences. An operator must first run two important procedures such as applying system calibration and removing zeros before applying loads. These procedures bring the system to an optimal initial situation. Then the operator manually moves the system step by step from a predefined load percentage to another one, applying a straight path.

We can do this both from the program GUI through an ad hoc control slider and using a variable resistance potentiometer acquired with an analog input channel. The potentiometer is a very useful device because the operators can continuously and slowly change the load percentage. We developed special routine to avoid the effects of abrupt movements on the potentiometer.

Load percentage values, read from feedback servo controls signals, are shown on video also with values from analog input GPIB channels. We developed an important tracking routine so that during system loads, parameters change from one value to another and the software recognizes tracking alarms generated from servo controls. These alarms generate when servo controls do not reach the desired load values. So, when a tracking alarm occurs, the software reacts by running the tracking routine that raises or lowers stimulus and frequency generation until the alarms stops.

The data generation function, which can manage up to 24 analog output channels, reads tension values from configuration files, directly writes them on DAC FIFO, and generates them at the demanded frequency. Generation frequency is the same for all DACs and may change if tracking alarms conditions occur or not. Tracking alarms are shown in an appropriate window.

The test automatically stops if a major alarm occurs, called a fault alarm. Both the fault alarm and tracking alarms are digital inputs and defined by software (active/tracking/fault etc.) using .ini files and GUI.

Fatigue Test Management
Fatigue test sequences can run without operator supervision.

After loading the configuration file, applying system calibration, and removing zeros, as in static tests, the system can program DAC converters with an opposite function, in order to obtain the excitation curves that perform flight simulation.

Excitation curves are obtained with sinusoidal interpolation between prefixed points. The program can join together two points with sine wave branch interpolations of 100 points. The shift between one point to the next takes three seconds if no alarms occur. Also, in this kind of test, the follow routine is used to handle tracking alarms from servo controls.

Load parameters are stored on ASCII files describing point after point final load conditions, which are interpolated using 100 points. Using specific commands in the same file, we can perform several operations such as a "pause" in the test, "perform complete data Acq," update the number of "simulated flights or flying hours," open and close the pressurization valve, automatically load another test file, and more.

On video parameters, monitors show test status such as generation frequency, number of simulation flights done, and percentage of test done. The test automatically interrupts if any fault alarms occur. To conclude, the supervision program generates a log file to trace events.

For more information, contact:
SITEM s.r.l.
Via Merano 7/1
16154 Genova
Tel: +39 010 6591757
Fax: +39 010 6593722
E-Mail: sitem@sitemnet.it
Web: www.sitemnet.it

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