The Challenge:

Implementing a PC-based Data Acquisition system to measure pressure and force in a Tri-sonic Wind Tunnel. The acquisition consists of creating calibration data for the pressure sensors both single and multiple (Electronic Pressure Scanners) types. The force measurement is to be calculated using a Calibration matrix for the same.

The Solution:

Developing a reliable and highly accurate Data acquisition system using SCXI- Signal conditioning System (SCXI-1000 CHASSIS, SCXI-1520 MODULE, SCXI-1180 FEED THROUGH PANEL), high speed features of Data Acquisition Card (PCI-6036E), Digital I/O Card PCI-6601 & software using Lab VIEW 7 Express.

Introduction:

The Data acquisition system is designed to perform reliable and highly accurate pressure as well as force measurements. The system first calibrates the Electronic Pressure Scanner and the calibration constants are recorded. The acquisition for the Wind Tunnel System Starts with a Wind-off Phase, for obtaining the offset values in the Tunnel. During the actual run (Wind-on Phase), the tunnel is run for a duration of 20-30 seconds and corresponding Pressure values obtained by means of calibration constants recorded and are displayed. The Electronic Pressure Scanners (Multi port pressure sensors) are continuously switched using DIO lines of the data Acquisition card and the multiplexed analog output from the sensor is acquired using analog input channels of the ADC. The user may perform the wind-on acquisition for n number of times. The pressure values obtained are logged in a Processed data file, where user can relate it with a configuration file, which contains the Transducer details. Force data is also calculated based on the calibration matrix.

System Configuration:

The Data acquisition system uses two SCXI module (SCXI-1520), which are interfaced to the DAQ card (PCI-6036E) in the System. For Calibrating the ESP, the address lines of the Scanner are interfaced to first four lines of DIO Card for addressing. The System uses four channels for Pressure Sensitivities obtained from the pressure Transducers, One Channel for Pressure Scanner and two channels for Force Measurements.

System Implementation:

The software of the system is basically divided into 3 Modules which are Calibration, Configuration and Acquisition Modules.

Calibration Module

This module is used for calibrating the sensors. Depending the Transducer type, the ESP or Pressure Transducer is selected and calibrated. The GUI Screen used when creating new calibration is presented below

The transducer details are stored in the table shown above. The parameters of the calibration settings can be edited when the user presses the settings button. The calibration constants obtained can be used for a pressure check; a utility was created to check for the pressure values obtained before actual run. Once the pressure check is completed the user can visualize the display to decide whether it has passed or Failed based on deviation from actual reading. The Pressure check resultant screen is shown below.

Here the user can enter a sample pressure and define his mode of units as relative or absolute and after he has set the sample pressure and continues, the measured pressure as well as the corresponding deviation is displayed, the user can perform this test n number of times, to check its reliability.

Configuration Module

The configuration module consists of storing the input parameters information in the Fixed Data File (FDT). The user creates a FDT file enters the parameters necessary for measurement and correspondingly selects Calibration Data File (CDT) for specified parameter and assigns specific channels. The corresponding CDT file selected by the user is used for acquisition in obtaining the pressure values. In the input parameter settings the user is provided with the option of selecting the DAQ Device and DIO Device (if Multiple devices are Present), the user is also required to select a FDT file for run. Once the required parameters are set then the user proceeds with the acquisition.

Acquisition Module

Once the user enters this module, the user can start with the acquisition, when the Wind off button is pressed offset values present in the tunnel are obtained. After this Pre-tare condition, the user is prompted to press the Wind-On button for the actual run which normally occurs for 20-30 seconds, the user may perform wind-on operation a number of times. Since the Electronic Pressure Scanner used may have different ports (16 or 32), based on the channels selected for run, ESP will contain corresponding number of channels. The user can also monitor the values of each port in the graphical display provided.

Some additional utility features are provided such as Channel Test utility, in case certain channels are not responding properly. The user clicks the View Parameters option to individually monitor the respective Channel and when X/C plot option is selected. Each port of all ESP are kept in a particular positions over the model have a X/C value (0 to 1) each port has one more parameter called Top, Bottom, Side1, Side2 depending on their positions on the model. For each parameter an XY plot is made.

Conclusion:

The PC-based Data acquisition system is capable of acquiring the Pressure as well as force from the corresponding type of Transducers connected. The report is also generated for either the recent run or also for previous runs. The system is also extremely flexible that if a new module is being added, it automatically detects in the program & performs acquisition.