Combining 2D and 3D Sensing in a Single Package

In sawmills, optimization is an in process procedure which maximizes output of highest value board size and quality from the limited and environmentally valuable input of random shape logs. In the optimization process, 3D profiles of each raw board are analyzed to position saws to optimally edge the individual boards to width and trim them to length, removing defective areas and providing the optimized quality and productivity at the end of the line. Properly implemented optimization can improve yields by an impressive 15% or more. In a larger sawmill cutting an average of 80,000 board feet per shift, two shifts, results in 40 MMBF (million board feet) per year. An additional 15% results in 6 MMBF per year at $225 per MBF equals a return of $1,350,000 per year.

In board production, traditionally optimization involves mounting banks of 3D laser sensors above and below board conveyor lines to provide high density 3D geometric profiles of each rough board as it passes the inspection station (Figure 1). The differential configuration, with sensors above and below the conveyor line, provides true board profiles even if boards are not sitting flat on the conveyor. Typical inspection rates are up to 120 boards per minute with some systems running as high as 200 boards per minute. Depending on the length of the boards, 20 or more sensors may be implemented for full surface coverage.

Over the past 3 decades, 3D sensing has evolved significantly, with profile resolution improving from 3 inch (76 mm) to 0.3 inch (7.6 mm) spacing, and frame rates have increased to 2 kHz. These improvements provide much higher density data input to the optimization process, detecting smaller defects and making much better decisions on cutting paths.

The next step to improve yield even further is implementing automated detection of surface defects such as knots, splits and stains on the boards, with 2D full color surface inspection. Implemented early in the process, 2D information is analyzed to automatically cut out surface defect areas from the boards, eliminating the cost and time required to process defective board sections, only to discard them at the sorting station at the end of the process stream.

Adding a separate 2D surface color inspection station to the line would be a challenge. The integration task would be significant, involving mounting separate 2D and 3D sensors, cabling all components, transporting and tracking work pieces through the two inspection areas, and synchronizing data from all sensors to match the 2D and 3D data to the same zone on each board to insure proper decision making on each piece. The resulting system would be complex, consisting of many separate elements, with the end user faced with high system acquisition cost, increased floor space requirements, and the need to support and maintain all of the components and their interconnections.

To dramatically simplify the integrators challenge in these types of applications, LMI Technologies has introduced a family of sensors which combine 3D high density profile scanning with true color 2D imaging for automated visual inspection for surface defects into the same sensor package. Built into each sensor are laser projectors for 3D profiling as well as high power LED’s and imagers for surface imaging. With a frame rate of 2 kHz or higher, these sensors provide very high speed inspection capability. One such sensor, an LMI DynaVision® chroma+scan 3300 3D profile and color sensor is shown in Figure 2.

Integrating 2D and 3D inspection capability in a single sensor package is only the first step in simplifying the integrators tasks. Implementing “Smart Sensor” technology further reduces the integrators work. Sensors are provided to the integrator with internal processing which apply the optical calculation equations and factory developed linearization factors, providing automatic gain control to ensure accurate readings independent of the object surface texture and colour, and converting measured values to engineering units.

In many sawmill and other applications, obtaining high resolution requires the implementation of multiple sensors, each inspecting a zone of the work piece. In a system built from discrete and separate sensors, this creates challenges of synchronizing data and stitching multi-sensor data streams into a single data file. To simplify the tasks of synchronization and stitching for the system integrator, LMI has developed a synchronization platform known as FireSync™. This platform is designed to accept and integrate data from multiple vision sensors, as well as other local inputs such as encoders and photocells monitoring the part conveyor.

Combining 3D measurement and 2D color data in the platform also allows for internal parallax correction of the 2D map that can be caused by the work pieces variation in thickness, or, movement towards the sensors.  This ensures that the 3D profile and the 2D color map exactly match, no matter how the work piece is positioned. Figure 3 shows the 2D color and 3D profile data outputs from the sensor. 

Connecting high speed sensors to a host system processor still can present some challenges. Even with smart sensors which internally reduce optical information to engineering measurements, it is common today for sensors to output hundreds or thousands frames per second, with each frame containing hundreds to thousands of data points. Adding complexity is the frequent need to locate the sensors remotely from the system processor, requiring ability to reliably transmit masses of data over long cable lengths. Use of an industrial standard communication protocol simplifies the integrators job and reduces cost and risk of implementation. While a number of standards are available, such as USB 2.0, FireWire, and Camera Link (developed specifically for industrial and scientific imaging) each have useful features, Gigabit Ethernet (GigE) has proven to be broadly accepted and easy to apply, with inexpensive cabling running up to 100 meters without repeaters. Data is sent from the platform over a single GigE output cable to the host computer, reducing wiring costs and improving reliability.

A typical example of chroma+scan 3300 sensors carrying out combined 2D and 3D inspection of boards is shown in Figure 4. In this case ten sensors are used, 5 above and 5 below the conveyor for full surface inspection. Photo courtesy of Comact Equipment Inc, (Saint-Georges, Quebec, Canada).

Conclusion
Sensors with combined 2D and 3D sensing simplify the task of integrating complex multi-function inspection systems. With internal data processing in the sensor, multiple sensor synchronization and data stitching provided by the FireSync platform, and standard GigE communications, risk and development costs for the integrator are minimized. Combined function sensors minimize complexity, mounting and cabling tasks, and reduce system cost. When the system is installed, the end user benefits from system simplicity, with reduced maintenance costs and ease of operation.

Author
Dr. Walter Pastorius is Technical Marketing Adviser for LMI Technologies Inc. He received his doctorate in Mechanical Engineering from the University of Windsor. He has more than 30 years of experience in both marketing and research and development in the machine vision industry. Pastorius has authored more than 100 articles and technical presentations in the field of new applications for vision sensors in manufacturing in automotive, aerospace, lumber, road inspection, foundry and other industries, and is co-holder of several patents.