Customer Solutions

Automating An Optical Pickup Unit Beam Measurement Test System Using NI IMAQ and LabWindows/CVI

Author(s):

Cheng Woon Fai, Philips Optical Storage Singapore

Industry:

Electronics

Product:

Data Acquisition, LabWindows/CVI, Vision

The Challenge:

Developing a low-cost automated visual inspection system for monitoring the quality of laser beam and correcting alignment of components in the optical pickup unit (OPC) for CD and DVD players.

The Solution:

Using National Instruments IMAQ Vision driver software to perform image processing, as well as data acquisition (DAQ), IMAQ, image acquisition hardware and LabWindows/CVI to acquire signal and image data, and develop a graphical user interface.

Introduction
To ensure good quality of the OPU, the beam coming out of the collimator lens must be of good quality. Therefore, we measure the beam size, also shape, angle, parallelism, and light power distribution.We use these parameters to detect and determine the error caused by optical component or misalignment of these components in the OPU. This equipment is useful for fine tuning the OPU housing design and controlling the quality of OPU during production.

Measurement Specification
Based on product OPU specifications, we established the following criteria for the test system:
• Complete test time less than nine seconds to achieve production output.
• Beam conjugate distance error has a measurement range of ±12 mrad and achieves accuracy of ±0.1 mrad
• Beam coming out parallelism has a measurement range of ±450 µm and achieves accuracy of ±20 µm
• X, Y Pupil distribution (light power intensity) has a measurement range of 100 percent, and achieves accuracy of ±0.5 percent

Imaging Processing Technique
We use imaging processing techniques like spatial filtering, quantitative analysis, and threshold for designing the measurement methodology of the system. We can easily implement these measurement methods by using the IMAQ vision driver.

System Setup
The host computer for the system is a 550MHz Pentium III, running Windows NT, with a total of 128 MB of RAM. We use the AT-MIO-16E analog and digital output for the laser and diodes to control CCD cameras switching control, and laser current measurement. By adding an Optical Sensing Device (OSD) sensor to measure light output intensity, the laser power for both CD and DVD laser diodes in OPU can be easily controlled to certain light power.

The PCI-IMAQ 1407 plug-in image acquisition board acquires the beam images from three monochrome Sony XC-ST30CE 1/3 in. CCD cameras (752 H x 582 V). The acquired image from the first camera measures the beam size, beam shape, and laser power distribution. For each OPU, the beam is a circle with diameter 5.0 mm.

The active area of measurement has a diameter of 5.0 mm. By using thresholds and applying a lookup table to adjust the contrast and brightness of the image, we can determine the size, shape, and position of the beam, as well as the dirt level. We can also calculate the laser power distribution also by measuring the horizontal and vertical pixel values in the beam.

By switching the input from camera 1 to camera 2, the acquired image measures beam parallelism. After applying a threshold and spatial filtering to the image and continuing with partial analysis we use the measure beam height and width to calculate the beam parallelism.

The acquired image from camera 3 measures the beam angle by applying a threshold and partial analysis to the image. The beam angle correlates to the coordinates of the center of gravity of the beam. We use the x, y coordinate or the beam calculates the beam angle. By using this approach, we can precisely measure the beam angle compared to the visual inspection performed by human.

Once the operator scans and loads the OPU, the measurement process will start. All the measurement data is stored in database and linked to the server in the production line for easy process monitoring, tracking, and analysis.

Developing for the Future
The equipment provides improved beam dirt measurement to increase measurement accuracy and capability. IMAQ Vision development modules, the algorithms, like edge detection were used to recognize various types of beam dirt. We also plan to increase the accuracy of the beam angle to 0.03 mrad and conjugate distance error accuracy to 5 µm for product drift monitoring.

Conclusion
System development becomes much easier and faster with the versatility of LabWindows/CVI and IMAQ Vision. IMAQ Vision provides almost all the image processing solution for the imaging processing in this application. The development of LabWindows/CVI and IMAQ-based beam measurement equipment was completed in approximately three months. We achieved an estimated 50 percent reduction in development time and cost by replacing a solution based on C++ and Allegro image acquisition system with the LabWindows/CVI and NI IMAQ vision-based solution. We are currently using the beam measurement equipment in the manufacturing of DVD OPU at Philips Optical Storage in ShangHai OPU-51 production.

For more information, contact
Cheng Woon Fai,
Philips Optical Storage Singapore,
Business Line Audio/Video,
Innovation – Engineering,
Bldg. TP4, 5th Floor,
620A Lorong 1 Toa Payoh,
Singapore, 319762
Tel: 65 - 6882 4260
Fax: 65 - 6882 9839
E-Mail: cheng.woon.fai@philips.com

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