Iván Torrano - Mondragon Unibertsitatea
Alain Martín - Mondragon Unibertsitatea
Manex Martinez - Mondragon Unibertsitatea
Wind tunnels are the most common experimental technique in the field of aerodynamics and are used to study the behavior of a body immersed in air. There are numerous measurement systems that allow obtaining pressure data, speed and resistance forces exerted by the wind on the object of study. It is very important the proper positioning of both velocity and pressure sensors and the model itself to ensure the repeatability and accuracy of the data. The solution adopted by NI hardware and software provides robustness and reliability in the motion control system. In addition, the ease and speed of set-up data acquisition equipment reduces the makeready time of the experimental tests.
The main objective is to develop a system that meets two conditions:
- Check position sensors and/or model to study different positions and configurations quickly and accurately.
- Monitor and record data from speed, pressure, temperature and force balance sensors.
Finally, the aim is to integrate these two functions into a single environment to facilitate the use of acquisition and control system.
Description of the Acquisition and Control System
Figure 1 provides an overview of the wind tunnel. In the central part the test section is shown where the model is installed. At the top it has a 3 axes positioning system. Each axis is driven by a NI ISM 7400 series Nema 23 stepper motor with driver integrated that allows positioning the speed sensor (Pitot probe or hot wire anemometer) at any point (x, y, z) in the space. At the bottom of the test section a fourth motor drives a rotary table that allows to rotate the model at different angles of incidence. Above the rotary table a force balance measures the resistance forces transmitted through the sting mounted in the model.
Figure 2 shows schematically the control and acquisition system. The embedded NI cRIO-9031 controller together with the 4-axis SISU 1004 interface module is responsible for controlling at real-time the stepper motors. Each motor is powered by a NI PS-12 power supply, and is connected to the terminals by SISU 1004 RJ-45.
Additionally it has a 4 slot NI cDAQ-9174 chassis that connects via USB to the PC. To acquire the signals from the sensors the following modules are used; NI 9215 for Pitot probe, NI 9201 to the force balance and NI 9219 for RTD temperature sensor and anemometer.
The following programs have been used:
LabVIEW: The project structure has allowed the cDAQ and cRIO to communicate with the PC generating a single graphical environment from which the movement is controlled while the results are monitored. Programming code by using a state machine enables synchronization tasks. For example, the project generated in which various states are synchronized is shown in Figure 3. Read the coordinate of a text file, move to that coordinate, data acquisition, etc. In this way, one can automate the process for obtaining 2D maps of velocity contours, boundary layer studies, measurements of turbulent wakes developed by the object of study, or any other scanning in space.
NI LabVIEW Real-Time and LabVIEW FPGA: These platforms are essential to compile the generated paths and perform control and real-time feedback in the motors.
NI SoftMotion: The library available for simplified motion profile development has been very useful for the development of predefined movements. It greatly facilitates the configuration of the axes making the set-up of the system simple. In addition, the option to simulate these paths in virtual axes of SolidWorks verifies beforehand that there are no collisions with the model or with the walls of the wind tunnel (Figure 4).
The highly versatile NI products have been fundamental to suit the needs of the project. The solution provides a system with automated motion control integrated with the monitoring of the acquired data, thus meeting the objectives set at the beginning of the project.
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