Derek Schmidlkofer PH 464 Project: IR Sensors
Robots need devices that allow them to interpret and interact with the world around them. Infrared sensors are an effective method of accomplishing this task. Infrared sensors can be used in such applications as detecting reflectivity of a surface or distance/range measuring. Infrared light can be used for a variety of applications. A great example would be the use of infrared (IR) light in sensors. These sensors can be quite useful because the IR light spectrum lies outside the human visible spectrum, 750nm to 1,000,000nm, rendering it invisible. IR sensors can be found all around us from security/motion detectors to TV remote controls. I have chosen to perform this research on IR sensors because my primary interest lies in the field of robotics. IR sensors are known to be a cheap and effective means for a robot to interact with the world around it. A robot that I recently constructed (Figure 1) contains three QRB1134 reflective sensors (Figure 2), mounted under the front side of the robot, and two GP2D12 IR range sensors (Figure 3), mounted on the front of the robot. Let us look at the properties and implementation of these IR sensors. Figure 1: SumoBot Figure 2: QRB1134 Figure 3: GP2D12
There are two types of IR sensors, transmissive and reflective. Transmissive IR sensors (Figure 4) consist of an emitter and detector pointed at one another. These sensors can be used to transmit data, like a TV remote control, or take on a passive role where the sensor is only activated when an object interrupts the beam path between the emitter and detector, like a safety detector in an automatic door. Reflective IR sensors (Figure 4) work by having the emitter focus a beam of IR light at an object, the light is reflected off of the object and back to the detector. These types of sensors are used for measuring the reflectivity of an object and for measuring the range/distance to an object. Figure 4: IR sensor types The QRB1134 is an example of a reflectivity measuring sensor. A diode emits a beam of IR light which is reflected off of a surface to a phototransistor where the amount of IR light received by the phototransistor can be directly transferred to a voltage that the robot can interpret (Figure 5). For this application, on the robot, the sensor is used to detect the difference between black and white surfaces. White surfaces reflect more IR light than black surfaces do, higher reflected IR light equals lower voltage on the output of the sensor. The robot can now be programmed to follow lines and/or stay within some type of boundary designated by a change in the color of the surface it is driving over.
Figure 5: QRB1134 layout The GP2D12 is a distance/range measuring IR sensor. There are three ways of measuring range: time of flight (TOF), return signal strength, and triangulation. Time of flight measures the time that it takes for the emitted IR light to return to the detector. These sensors are very accurate, but very expensive. They are expensive because the speed of light makes it difficult to precisely measure the TOF for short distances. The return signal strength method measures the decrease in amplitude of the return signal. This reduction in amplitude correlates directly to the distance the sensor is from the object. The return signal strength method suffers from the predicament that not all surfaces have the same reflectivity. If two objects are placed at the same distance from the sensor, the object with the lower reflectivity may appear closer. The triangulation method is the method used by the GP2D12. It requires that the detector be able to sense multiple beam paths depending on the range of the object. A diode emits IR light which is then focused, using a lens, into a beam (Figure 6). The beam of IR light reflects off of an object and enters the detector portion of the sensor. The lens at the detector end of the sensor focus the beam onto an array of photodiodes, called a Position Sensitive Detector (PSD) (Figure 7). The position at which the beam hits the PSD is translated into a voltage that the robot can interpret. In Figure 8, it can be seen that object A is closer and therefore the reflected light form it enters the detector s
lens at a greater angle than does the light from object B. Upon closer examination of Figure 8, it is also notice that if object A were any closer to the sensor then the IR beam will not land on the PSD. The limited size of the PSD confines the possible range values that this IR sensor is capable of detecting. Figure 9 show that the range of the GP2D12 sensor is limited to 10cm-80cm. If the object is any closer than 10cm, the output registers a false reading. Figure 6: GP2D12 emitter Figure 7: GP2D12 Detector
Figure 8: GP2D12 Figure 9: GP2D12 voltage vs. distance There is IR light/radiation all around us, what s to say that the detector end of an IR sensor is distinguishing the light produced by the emitter from background IR radiation? The solution is simple. In order to differentiate the light emitted by the sensor from the other ambient sources, we modulate the light transmitted from the emitter at a
known frequency, usually 40 khz. We build a filter at the detector end of the sensor that removes all frequencies except for the one that was transmitted (Figure 8). Figure 8: Unmodulated vs. modulated This project has given me a greater understanding of the strengths and weaknesses that come with the use of IR sensors. IR sensors to have limited capabilities when compared to say a sonar transducer or laser range finder. But as long as the application that they are needed for fits within their capabilities they are a cheap, effective means of information gathering for robots.
References: Title: IR Sensor Info Page By: Ben Wirz Web: http://www.wirz.com/info/ir.html Date accessed: 2/11/2003 Title: Sharp GP2D12 IR Range Sensor Web: http://abrobotics.tripod.com/snuffy/gp2d12.htm Date accessed: 2/12/2003 Title: Sharp GP2D12 Web: http://dmtwww.eplf.ch/~jzuffere/sharpgp2d12_e.html Date accessed: 2/12/2003 Title: Data Sheets Web: http://www.junun.org/markiii/datasheets.jsp Date accessed: 1/10/2003