Customer Solutions

LabVIEW Streamlines Calibration of Pressure Transducers for In-Flight Measurements

Author(s):

Chris Koehler, G Systems

Industry:

Aerospace/Avionics

Product:

LabVIEW

The Challenge:

Automating the processes of calibrating pressure transducers for flight test, performing results analysis, and adding the results to a database.

The Solution:

Developing LabVIEW applications to perform calibrations and manage resultant data files.

Reliably Storing Specifications and Calibration Results
The flight test instrumentation (FTI) group and calibration laboratory of an aeronautics company certify transducers for use on test aircraft undergoing flight-testing. The laboratory calibrates transducers and verifies that they operate within tolerances specified by the FTI group. The success of in-flight measurements depends highly on the reliability and accuracy of the transducers tested by the calibration laboratory. Initially, our company, G Systems, began the project with a new database developed to manage administrative as well as technical information for the FTI group. The database provided a reliable way to store the manufacturer specifications and actual calibration results for the many types of transducers handled. We first processed raw calibration data files and placed the results in the database. A year later, we automated their new pressure transducer calibration station and generated the same format data files in order to integrate with the existing systems. By keeping the original format of the data file, we could convert to the new system in two stages with no interruption in workflow.

Centralized Data, Access from Anywhere
By implementing a database to hold transducer specifications and results, we hoped to centralize the data storage and provide access to the latest results from anywhere in the company. Along the same line of reasoning, we devoted a single server to processing raw data files and placing the results in the database. When generating calibration data files, we share a specific folder on this server. After generating a data file, we copy it into the shared folder, and the processing application (known as TransCalc) automatically detects and processes it, then inserts the results into the database.

Once the TransCalc application detects a new file, it reads a standard header and determines the type of transducer tested and the format of the data. Every type of transducer produces a different set of data and needs to be processed differently. We used the dynamic loading capability of the VI Server in LabVIEW to accommodate future integration of new transducer types.

We created one data-processing VI per transducer type. The TransCalc application loads the correct VI as specified in the data file header. When we add a new transducer type, we do not need to make changes to the main application. We only need to create a new VI to perform the processing. The application generates a JPEG image as part of the processing, and the image contains a graph of calibration results and embeds directly into a database record for later analysis.

Next in the project, we automated the calibration of pressure transducers. While the TransCalc application developed in the first phase could process many types of transducer data, this new application focuses on only pressure transducers. As time goes on, we may automate test stations for other transducers such as accelerometers and vanes. We send results files from these calibrations to the server when the TransCalc application processes them.

Data Entry Errors Eliminated, Technician’s Work Simplified
The software for the new PTCS had to control the pressure and temperature in an environmental chamber across the full range of operation specified by the manufacturer. At each set point, the system must record the transducers output signal as well as measured values of pressure, temperature, and excitation voltage. (The earlier system did not measure the pressure independently of the set point.) The customer supplied the new pressure controller and environmental chamber and we built the instrumentation rack and associated cabling.
Pressure varies over the acceptable input range of the transducer. It begins at the minimum pressure and moves up to the maximum and back to the minimum again so that we can measure the hysteresis along with non-linearity. After a complete pressure cycle, we set the temperature to a new value. After the temperature stabilizes, the pressure cycle repeats.

We perform the pressure cycle at each temperature to verify that the transducer output remains within tolerance as temperature varies. A transducer passes or fails according to the manufacturer’s specification of deviation from true linear output as a function of input pressure. A typical specification might be < 0.1 percent of the full-scale output over the entire pressure and temperature range. By finding the best-fit straight line and measuring deviation, the application determines pass/fail status. The deviation usually follows a rounded curve because of mechanical characteristics of the transducer diaphragm. The transducer measured in this case failed due to major hysteresis at high temperatures.

As a result of automating the pressure and temperature control using LabVIEW, we reduced the time to calibrate a single transducer at multiple temperatures from 24 hours to 10 hours. The application retrieves testing specifications directly from a database and puts the results back. This simplifies the technician’s work because it minimizes paperwork and eliminates manual dialing of pressures/temperatures. In addition, the system virtually eliminates opportunities for data-entry errors, since the software only allows calibration of transducers already existing in the database and the application automatically retrieves setup information from the database.

These new applications reduce bookkeeping, manual labor, and test time. Using the versatility of LabVIEW and associated add-ons, we simplified a complex system for calibrating transducers, analyzing measurement data, and storing results into a single, unified system. We reduce test time and overhead by more than 50 percent in some cases. The power of LabVIEW makes a complex task easier and leaves a reliable, highly maintainable system for its users.

For more information contact:

Chris Koehler

G Systems

Tel: 972-516-2278

E-mail: chris_koehler@gsystems.com

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