ECE5320 Mechatronics Assignment#01: Literature Survey on Sensors and Actuators Topic: Sensor: Flash Ladar

1. ECE5320 MechatronicsAssignment#01: Literature Survey on Sensors and Actuators Topic: Sensor: Flash Ladar Prepared by: Brandon Boldt Dept. of Electrical and Computer Engineering Utah State University (Electrical and Computer Engineering Dept.) 3/9/2007

2. Outline • Reference list • To probe further • Overview • Design Considerations • Applications • Issues • Where to Buy ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

3. Reference list • S. B. Gokturk, H. Yalcin, and C. Bamji, “A time-of-flight depth sensor – system description, issues and solutions,” 2004 Conference on Computer Vision and Pattern Recognition Workshop. • S. B. Gokturk and C. Tomasi, “3d head tracking based on recognition and interpolation using a time-of-flight depth sensor,” 2004 IEEE Computer Society Conference on Computer Vision and Pattern Recognition. • Beeck, Wilinski, Fehn, Kauff, Ijsselsteijn, Pollefeys, Gool, Ofek, and Sexton, “Towards an optimized 3D broadcast chain.” • Anderson, Herman, and Kelly, “Experimental Characterization of Commercial Flash Ladar Devices”, The Robotics Institute ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

4. To explore further (survival pointers of web references etc) • http://en.wikipedia.org/wiki/Ladar A wikipedia entry for Ladar • http://www.canesta.com/html/technical_papers.htm Canesta's repository of technical papers • http://www.csem.ch/fs/imaging.htm CSEM's center for imaging • http://www.3dvsystems.com/technology/tech.html 3DV Systems' repository of technical papers ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

5. Overview • Ladar stands for “Laser Detection and Ranging.” • Ladar is descriptive of all devices which use light waves to determine the distance of objects remotely. • The most common type of light used is laser light. Laser Ladar System Object

6. Laser Ladar System Object Overview • Ladar System emit a laser with some type of wave function. • The laser reflects off of the subject of the system and returns to the ladar system. • The ladar system measures phase offset of the returned ladar which is proportional to the distance the ladar traveled.

7. Overview • Flash Ladar Systems have an array of sensors in order to capture a scene of 3D points simultaneously. Generally, an optical lens is needed to focus the lasers from the object onto the array of sensors. Ladar System Lens Object Sensor Array

8. Design Considerations • Unambiguous Range - Because the ladar system measures a phase offset, the system has some ambiguity as multiples of a phase offset of 2 π give the same distance. The unambiguous measurable distance varies with the frequency used in the ladar system. Ladar System Possible Object Possible Object Possible Object

9. Design Considerations • Accuracy – An important consideration for any ladar system is the accuracy of the ladar system. Various factors affect this important characteristic and will be discussed here. Ladar System Accuracy of Object Location

10. Design Considerations: Accuracy • Modulation Frequency – increasing the modulation frequency improves the accuracy of the system, but decreases the unambiguous range. • Power of the laser – increasing the power of the laser improves the accuracy of the system, but the system will consume more power overall. • Ambient light – the less ambient light that is present in the scene, the better the accuracy of the system, however, this may not be a parameter that can be adjusted

11. Design Considerations: Accuracy • Field of View – Decreasing the field of view improves the accuracy of the system, but some objects may require multiple shots and be stitched together. • Integration Time – Increasing the time the sensor integrates the results will improve the accuracy of the system, but the camera will take more time per shot to capture the data. Also, if the scene is not still, a long integration time can have motion blur.

12. Applications: Head Tracking • For security reasons, it is desirable to recognize a person from a database. Flash ladar has been used to characterize 3d features of heads in order to determine the person being captured. The advantage of ladar over visual camera is the ability to easily separate the head from the background. Laser Ladar System Head Complex Background

13. Applications: Robotic Vision • In many robotic applications, obstical avoidance is a necessary quality. The ability to view the 3D world around the robot is a valuable asset. Flash ladar systems allow robots to capture 3D data quickly and the ability to process the data as a group, rather than individual points. Ladar System Mobile Robot Obstacle

14. Applications: Human-Machine Interaction • The ability to interpret human movements remotely is an area of interest to developers in variety of industrial fields. Its application could be used in computer peripheral devices as well as hazardous industrial environments. Flash ladar allows developers to quickly capture human data as a group rather than individual points giving a richer environment to work with. Ladar System Human

15. Applications: Automatic Target Recognition • In combat, the ability to recognize threats over non-threats is crucial. Flash ladar adds a third dimension to camera based target recognition which gives an added advantage when determining the targets on a field. Ladar System Tank Car Person

16. Issues: Noise • Each pixel of a flash ladar system has a certain amount of noise associated with it. Each pixel value can be modeled as a gaussian variable with a particular mean and a particular standard deviation. To improve the noise of a ladar system, temporal or spatial averaging can be used to decrease the noise assuming the pixels to be averaged together have the same mean value. Measured pixel value over time value mean time

17. Issues: Aliasing • For a particular modulation frequency in a flash ladar system, the system has an unambiguous range. When the scene captured has a dynamic range that is larger than the unambiguous range then aliasing arises as a problem. One method to overcome the unambiguous range is to take two or more measurements with different frequencies. This increases the unambiguous range to the least common multiple of the two previous unambiguous ranges. Higher Freq. Lower Freq. Ladar Sytem Ladar Sytem Object Object

18. Issues: Ambient Light • “Ambient Light is considered as unwanted light that has the same wavelength as the light source of the system.” (Gokturk) Ambient Light has the undesired effect of increasing the noise due to interference. In general this effect produces a high-frequency effect, and can be passed through a low-pass filter if thought to be a problem. Ambient Light Ladar Sytem Object

19. Issues: Saturation versus No Data • In a flash ladar system, a pixel only produces a valid value if enough light lands on the pixel; however, too much light can saturate the pixel values, which also will cause the pixel to produce an incorrect result. This effects the dynamic range of the scene as well as the reflectivity dyanmics in the scene. One method to overcome this issue is to use multiple shots will different exposure levels and then stitch the two shots together.

20. Where To Buy • Canesta, Inc. • http://www.canesta.com • Swiss Center for Electronics and Microtechnology • http://www.csem.ch/ • 3DV Systems • http://www.3dvsystems.com/