The Radiometric Sensor Development and Applications Team of the Sensors and Electron Devices Directorate (SEDD) at the U.S. Army Research Laboratory (ARL) has developed a 2D laser profiling scanner system to study its operational characteristics, performance, and effectiveness in detecting targets in the battlefield and homeland security environments. A profiling scanner provides output images that reveal the size, height, and outline or shape of an object. This information can be useful in a wide range of applications, ranging from simple intrusion detection, to monitoring of parts during a manufacturing process for quality control purposes.
The profiling scanner system uses 16 laser diodes, 16 photo sensors, and a personal computer (PC) controller. The scanner is made up of a 6 × 8' aluminum frame with the laser diodes mounted on one side, and the photo sensors mounted on the other. Each laser diode is optically aligned with a photo sensor to form a working pair. Each pair operates independent of all others.
Mounting fixtures are used on the laser diodes to provide beam alignment to the photo sensor on the other side. Photo sensors are mounted directly onto the frame (Figure 1). A coaxial cable is used to carry each output signal to the PC controller. When power is applied to the laser diodes, each diode generates a beam of focused light, which activates a corresponding photo sensor, resulting in 16 channels of sensing beams going across the two legs of the scanner. During operation, when nothing is in between the two legs, each of the photo sensors outputs a constant and continuous analog voltage. The beam is blocked when an object is placed between the laser diode and photo sensor, resulting in a zero voltage output.
Operation of the system primarily involves the continuous sensing, monitoring, and processing of the activities of each of the 16 beams. A PC controller provides controls for all operations.
The laser diode is the M635-5 from USLasers, and operates at 635 nm with 5 mW of output power. The photo diode is the SM05PD1A from Thorlabs. Each laser diode is powered by a continuous DC nominal voltage of 2.5 V. A total of about 500 mA is required to power all 16 laser diodes. The photo diode requires no power. The PC controller is from National Instruments, consisting mainly of the PXIe-8106 embedded controller in a PXIe-1062Q chassis, along with the PXI-6255 data acquisition module (DAQ). Figure 2 shows the complete controller hardware setup.
The interface between the DAQ and the sensing signals is via coaxial cables connecting from the photo sensors to two interface boxes. To minimize noise, the signals are connected in differential mode, with each of the 16 signal and ground pairs taking up one DAQ channel for a total of 32 DAQ channels. The 16 coaxial cables are divided into two 8-coaxial bundles, each connecting to interface boxes. Outputs of the interface boxes connect to the PXI-6255 DAQ module in the PC controller chassis.
A LabVIEW program provides the controls of all system operations; it includes the interfaces, monitoring, detection, processing, and display of all signals from the photo sensors.
The program runs in a continuous mode. During operation, the 16 analog signals are first digitized according to the sampling rate and operating frequency settings on the DAQ, and the resultant signals are then converted to just 1s and 0s, depending on a set threshold level. A 1 here represents when a beam is broken or when something is detected between the laser diodes and the photo sensors, and a 0 means the beam is not broken or no detection. These signals are then stacked together according to their position on the scanner apparatus to form the 16-channel Yaxis on the output display plot, with the X-axis being the scanning time. With appropriate settings on the scan frequency and number of samples, the output plot shows a real-time moving profile image of objects as they move across the scanner apparatus. The area under each of the 16-channel Y plots is filled with a solid color so that a solid profile image of the object is shown.
Output Profiling Images
The key objective in developing the scanner is to see what kind of output profiling images can be obtained, and to determine if the images have the sufficient details needed for target detection applications. Figure 3 shows an example of an image of a person obtained with the scanner.
The vertical resolution of the images is limited here by the number of sensors (equally spaced apart), which is 16. The x-axis time scale is set to accommodate for the speed of objects moving across the scanner at about 4.8 km/h (3 mph), which is about the average walking pace or speed of a person. The scanning rate is about 500 Hz. As shown, the image portrays enough details about the size and height, in addition to the shape of the person, to allow one to distinguish between a person and a chair. The image obviously does not have the detailed resolution that can be obtained from a camera, but in some applications, these lower-resolution profile images may be adequate.
For example, this type of image can be sufficient to distinguish a person from an animal crossing a border when used in a homeland security application, or as a supplement or aid to other sensors for detection and identification confirmation of enemy troop and vehicle movements when used in a battlefield. In these situations, in comparison to systems using a camera, the profiling scanner offers a lower-cost alternative in terms of design complexity and operating data rate or bandwidth requirement.
The ability to classify a target in a detection and identification system is a useful feature. Profile image data from a scanner can be stored as a signature file that can later be used in a classification system to determine target classification. The ability to count the number of objects crossing the scanner is another useful feature in many detection and identification applications.
The count is implemented in software by counting the number of breaks in the beams. A count is accumulated when transition of any beam-break to nobeam- break occurs.
The 2D laser profiling scanner can provide the following capabilities:
- Real-time moving profiling images showing the outline or shape of an object with enough details to distinguish different objects; for example, between a person and a chair.
- Storing the image data into a file (in Excel format) that can be used later as a signature file in classification systems to determine target classification.
- Ability to count the number of object crossings.
These capabilities can be useful in such applications as detection of an illegal immigrant crossing a border, or as a supplement to other sensors in a battlefield for detection and identification confirmation of enemy troop and vehicle movements. Compared to systems using cameras, this can be a lower-cost alternative in terms of system complexity and operating data rate requirements. With further investigation, performance of the scanner system can be enhanced. Following are some possible areas that may be worth looking into:
- Increasing the number of laser diodes and photo sensors to increase image resolution, and/or determining the optimum number needed to reliably classify different target types.
- Use of non-visible IR to minimize detection by the enemy.
- Use of a collocated single scanning emitter and single detector to reduce the physical size of the scanner apparatus.
This article was written by David Y.T. Chiu and Troy Alexander of the Sensors and Electron Devices Directorate, Army Research Laboratory, Adelphi, MD.
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