The Challenge

To build a powerful, modular, flexible user friendly platform by integrating Vision, Motion and Data Acquisition for real time Measurement and Control for Opto-Electronic Interferometric Measurement System for Static and Dynamic Measurements.

The Solution

Development of a Virtual Instrument using LabView Software with IMAQ and DAQ cards for the real time measurement and control for static and dynamic applications in conjunction with the Opto-Electronic interferometric system.

Introduction

It is very important to measure the displacement/slope of mechanical components under static and dynamic environments. The dynamic load case includes the measurement of modal parameters such as natural frequencies and mode shapes. Laser based digital speckle interferometric techniques find many applications in mechanical testing. These techniques, because of their high displacement sensitivity( l /2 = 0.3 mm), whole-field and non-contact nature are found to be very useful in many applications. ISRO Satellite Centre and I I T (M) have jointly carried out studies and developed an “Opto-Electronic apparatus for Static and Dynamic Measurement (OEASDM)”. This apparatus is shown in in Fig.1

Image grabbing card with on-board frame subtraction features is normally used for real time interferometric applications. Generally signal generators and Frequency Response Analyzers are used for excitation and input/response measurements for static and dynamic testing purposes. Now a virtual instrument is developed to cater for these applications using IMAQ and DAQ cards under LabView environment by exploring the power of MMX capabilities.

Description

Opto-Electronic Interferometric system measures the out of plane micro-displacement/ slope and a suitable static or dynamic loading instrument is needed for stressing the component. The Virtual Instrument caters for the Opto-electronic interferometric system as well as the static or dynamic loading system. The loading system could be a Material Tester or an electro-dynamic shaker or a thermal heater or even a vacuum/pressurizing system. The main requirement for OEASDM is that there should not be any relative movement between the interferometric system and the test component or disturbance from the ground. Hence a rigid base plate is used with a pneumatic suspension system.

The schematic of a typical Virtual Instrument along with the Opto-electronic interferometric system for a simple demo/calibration set-up for static and dynamic applications is shown in Fig.1. The same set-up could also be used for mode shape, static stiffness and Non- Destructive Evaluation measurements of different mechanical components using different techniques.

Further, the Virtual Instrument can be very easily re-configured for different applications as Lab View, the visual programming system has flexible Graphical User Interface. For example, in this setup, the Opto-electronic interferometric system itself is being augmented for phase unwrapping measurements with a phase shifting piezo-mirror along with a high voltage DC-DC converter which can be easily controlled by DAC2 as shown in the dotted line in Fig.1.

The set-up (Fig1) consists of a rigid base plate with a pneumatic suspension system on which the Opto-electronic interferometric system and the test component (a mild steel diaphragm of 150 mm of diameter, 1mm thickness) are mounted. A standard PC with LabView software houses the IMAQ and DAQ cards. The IMAQ card is connected to the camera in the interferometric system. The DAQ cards consists of 8 ADCs, 2 DACs and 8 DIO lines and are connected to speaker, stepper motor, input and response measurements selectively as shown in the simple demo/calibration set-up (Fig 1).

LabView Environment

LabVIEW, A G programming language along with IMAQ Vision is used as the software environment for Opto-Electronic Interferometric Measurement System (Fig2). This tool acquires the image of the undisturbed state of the test component (diaphragm) using the IMAQ Card-1407 and NI-IMAQ Driver. This captured image is stored in the image buffer. Static and dynamic tests are then performed on the test component (diaphragm) by giving excitation using Data acquisition cards, which are synchronized with the image acquisition. The image corresponding to the disturbed state of the test component is continuously subtracted from the undisturbed state stored in the image buffer to get the images of different mode shapes or displacement /slope patterns.

IMAQ & PCI-1407- The Image Acquisition & Processing

Though the actual subtraction is done by the software (Lab VIEW +IMAQ Vision) the speed is fairly comparable to an onboard subtraction because of the advanced features of the PCI-1407 and IMAQ Vision. The NI-1407 has on-board decimation and LUT (Look up table) processing and other features such as partial image scanning and programmable gain & offset for optimizing the input signal range. Also an 8- bit Flash A/D converter performs the image digitization and the result is passed to a 256 by 8 RAM LUT. The LUT can implement simple imaging operations such as contrast enhancements, gamma corrections etc.

The image is then transferred to from the on-board FIFO to the PC RAM using the on-board DMA controller. This controller performs scatter-gather DMA, which means the DMA controller, can reconfigure on the fly and perform continuous image transfer to either contiguous or fragmented buffers at up to 132Mbytes/s.

The IMAQ Vision is then used to perform image-processing functions. IMAQ Vision takes advantage of Intel MMX Technology. MMX codes in the Intel processor accelerates integer or fixed point arithmetic functions that process 8-bit images. IMAQ Vision functions can be used to filter, manipulate, smooth and quantify the image. Arithmetic, Logical and Statistical functions can be performed.

In addition IMAQ Vision has the ability to read and write images to PNG, BMP, JPEG and TIFF file formats.

NI-DAQ & PCI-6024E- The Data Acquisition & Control

The PCI-6024E DAQ Card has 8 different analog inputs, 2 analog outputs, 8 digital input/outputs and counter Timers. Though it sits on a separate PCI slot of the PC it can be synchronized with the IMAQ card using the built in RTSI BUS. The data acquisition /control can thus be synchronized to the image acquisition

Generating sinusoidal signals on one analog output channel (DAC1) of the DAQ Card (PCI-6024E) carries out dynamic excitation. Different signal frequencies are generated (using Labview) so as to obtain the natural frequency modes of the test component.

The generated signal is fed to a speaker to excite the test component and the feed back from it is acquired on one of the input channels (ADC2) of DAQ Card through an accelerometer and its signal conditioner.

The generated sinusoidal signal and the feedback signal (vibration response) are plotted on a graph to obtain the Lissajous pattern. By an online frequency sweep a circle on the graph is obtained between the signals corresponding to the input excitation and the response, which marks the natural frequency of the test component. Other required natural frequencies of the test component can be obtained by getting the circle in the Lissajous patterns (Fig3).

In this simple demo/calibration set-up, a stepper motor is rotated by sequentially energizing the controller using 4 DO lines and a micrometer head/load cell connected to the stepper motor deforms the diaphragm. The out of plane deformation and slope measurements for a small diaphragm are given in Fig.4 and Fig.5 respectively.

Summary

We were able to use LabVIEW, IMAQ Vision, DAQ, LabVIEW and the power of the Intel Pentium III PC (with MMX Technology) to simultaneously acquire data and images, necessary for measurement and control of an Opto-Electronic Interferometric System. The data acquisition /control can thus be synchronized to the image acquisition therefore removing the need for expensive on-board Image processing hardware and separate external data acquisition and control hardware. The data acquisition and Control Card can be synchronized with the IMAQ card using the built in RTSI BUS.

Thus a powerful, modular, flexible and user-friendly platform for Test and Measurement applications has been developed using a building block approach.