Synthetic Sensing: Proximity / Distance Sensors

Transcription

1 Synthetic Sensing: Proximity / Distance Sensors MediaRobotics Lab, February 2010 Proximity detection is dependent on the object of interest. One size does not fit all For non-contact distance measurement, an active sensor which transmits a pilot signal and receives a reflected signal is commonly used. The transmitted energy can be any form of radiation (electromagnetic, acoustic, microwave, optical). References: Fraden: Handbook of Modern Sensors Feynman: Lectures on Physics

2 Proximity detection with polarizing filters, for example, has a different effect on specular reflecting objects (metals) than on non-metallic reflectors (people).

3 Fiberoptic sensors Fiberoptic sensors can be used to detect the level of a liquid. In one sensor design, two fibers and a prism allow light to 'bend around' (total internal reflection) when the sensor is in air, but be diverted into the liquid if the prism is in contact with the liquid. This is due to the the refractive index of a liquid is higher than that of air. The result is a weaker light signal at the other end of the fiber when the prism touches the liquid...

4 Fabry-Perot Sensors Fabry-Perot Sensors are capable of measuring very small distances. A Fabry-Perot sensor consists of a cavity contained by two semi-reflective mirrors facing each other and separated by a distance L. The cavity is injected with a defined light source (such as a laser) and the light bounces around in the cavity. At some frequencies, light passes out through the semi-reflective mirror. The frequency of this light is related to the length of the cavity: dv= c 2 L

5 Linear Optical Senors (PSD) PSDs operate in the near infrared. This type of sensor is often used for auto-focusing in video cameras. The sensor has an active component (LED) and a photo-detective linear sensor. The position of an object is determined by triangulation. The beam of the LED is pulsed at a particular frequency and collimated by a lens to produce a narrow beam.. On striking the object, the beam is reflected back to the detector that generates an output signal proportional to the distance x of the light spot on the surface of the detector (distance from the central position).

6 Ultrasonic sensors Transmission and reception of the ultrasonic energy is used in popular range finders in robotics. Ultrasonic waves are mechanical waves above the range of human sensitivity, but easily 'heard' by cats and dogs. Bats and dolphins also use ultrasonic emission and detection for navigation (biological ranging). When waves fall on an object, part of the waves' energy is reflected, often diffusely (uniformly). The distance L to the object can be calculated through the speed v of the ultrasonic waves in air and the incident angle: L= v t cos theta 2 t: time of flight

7 In ultrasonic sensors, the waves are generated with the compression and expansion of a medium. Commonly, the excitation occurs with a piezoelectric transducer in motor mode, converting electric energy into mechanical energy and acoustic waves.

8 Radar sensors The micro-power impulse radar (mir) developed at Lawrence Livermore Labs has a white noise generator coupled with a pulse generator and a high power radio transmitter. The noise generator triggers a pulse generator (2Mhz+-20%) that creates random bursts in the high energy radio transmitter as pulse frequency modulation (carrier frequency 6.5 GHz). The reflected pulses are demodulated (square wave form is restored) before the time of flight and thus the distance is measured. The spatial distribution of the radar is determined by the type (shape) of the antenna. It can be in the form of a horn, reflector or lens. Dipole antennas create almost 360 deg coverage. Because of the unpredictable modulation pattern (white noise) and the low spectral density, the mir is immune to detection. Also, the mir are cheap and consume little power. Two alkaline batteries can power it continuously for several years (50 μwatt).

9 Range Finders A range finder is a device that measures distance from the observer to a target. Some devices use active methods to measure (such as sonar, laser, or radar). There are other devices that measure distance using trigonometry to determine distance. An IR range finder uses a beam in the low-energy infrared to triangulate the distance to an object. A pulse of light is emitted and then reflected back (or not reflected at all). When the light returns it comes back at an angle that is dependent on the distance of the reflecting object. Triangulation works by detecting this reflected beam angle - by knowing the angle, distance can then be determined. IR finders operate at in limited distance ranges.

10 Laser range scanners Laser range scanners use a laser beam in order to determine the distance to an opaque object. This works by sending a laser pulse in a narrow beam towards the object and measuring how long it takes for the pulse to bounce off the target and return to the sender. A rotating mirror deflects the pulsed light beam to many points in a semi-circle. The precise direction is given by an angular sensor on the mirror. A large number of coordinates measured in this way are put together to form a model of the surrounding area's contours. The accuracy of the instrument is determined by the brevity of the laser pulse and the speed of the receiver. A sensor that uses very short (sharp) laser pulses and has a very fast detector can range on object to within centimeters or less. # Range max.150 m # Range (without supplementary reflectors) up to 30 m # Range with minimum reflectivity 1.8% 4 m # Resolution 10 mm # Statistical error * +/- 15mm range 1 to 8 m * (mm-resolution) reflectivity: 10%-10,000% * +/- 4 cm range 8 to 20 m * (mm-resolution) reflectivity: 30%-10,000% # Angular resolution 100 Scan:0.25 /0.5 /1, 180 Scan:0.5 /1 # Response times 52/26/13 ms depending on angular resolution

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12 A webcam based laser range finder A laser-beam is projected onto an object in the field of view of a camera. This laser beam is ideally parallel to the optical axis of the camera. The dot from the laser is captured along with the rest of the scene by the camera. A simple algorithm is run over the image looking for the brightest pixels. Assuming that the laser is the brightest area of the scene, the dots position in the image frame is known. Then we need to calculate the range to the object based on where along the y axis of the image this laser dot falls. The closer to the center of the image, the farther away the object is. D= h [ tan theta ] theta = pfc rps ro pfc: # of pixels from center of focal plane rpc: radians per pixel pitch ro: offset

13 These parameters need to be derived from calibration data (known distance to target and known # of pixels from center. From these the actual angle can be calculated and with this the offset (r0) and the gain (rpc) by fitting the data to its physical representation (linear relationship assumed) The website give above reports the following data: pixels from center calc D (cm) actual D (cm) % error