Appendix A: 360° IR Sensor

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

360° IR Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

Theory of 360° IR Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

Description of Detection Experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

Data and Interpretation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Summary

A 360° IR Detector would provide a cost effective way to implement both object avoidance, detection, and any other function that multiple IR detectors are normally used for. By using a single Sharp IR Detector, some cardboard, a lens, and some reflective material, one IR Detector can effectively replace eight or more detectors, saving over $20 in costs on your robot. The single detector is mounted above a lens, at a distance equal to the focal length of the lens (see Fig. 1). This lens collects and focuses IR light coming from all directions up into the single detector. By using a lens with a smaller focal length, the resulting tower can be made smaller, however, it is at the same time advantageous to use a lens with a wide diameter, in order to collect more light. For this experiment a 1 3/16" diameter with a 2.5" focal length was used successfully.

360° IR Detection

The 360° IR Detector consists of 5 major parts: the Reflective Cone, the Lens, the Reflective Cylinder, the Top of the Tube, and the Sharp IR Detector (see Fig.2). The Reflective Cone is constructed out of notebook paper with aluminum foil glued to it. The foil and paper is then cut into a circle, and then the circle is folded into a cone with approximately a 45° slope. The cone is placed underneath the lens and serves the purpose of reflecting IR light coming in parallel to the lens up into the lens. The lens then takes the reflected light from the cone, and any light that hits the lens directly, and focuses it up, through the cylinder, into the IR detector. The Reflective Cylinder is merely cardboard with aluminum foil glued to it, rolled into a cylinder with the foil on the inside. The inside diameter of the cylinder should be equal to the diameter of the lens being used. The cylinder serves a twofold purpose. Primarily, it provides a support to suspend the detector above the lens at a distance equal to the focal length of the lens. Its secondary purpose is to reflect any infrared light not properly refracted by the lens toward the detector. The top of the tube is a cardboard disc, with a diameter slightly greater than the cylinder, with a square hole cut in the top, slightly smaller than the detector. The top serves only as a surface to mount the detector on. Lastly, the Sharp IR Detector is mounted on the top of the cylinder and supplies readings to the control board of the robot. Figure 3 illustrates what the 360° IR Detector looks like when fully assembled.

NOTE: The sensor must be implemented in the orinetation shown for it to function properly.

Theory of 360° IR Detection

The appeal of 360° IR detection is that you can greatly reduce the cost of IR detection. Two major problems face a designer attempting to implement this device, how to get your detector to see in all directions, and how to determine where the signal came from. The only tested implementation of 360° IR detection so far was to attach an IR detector and a mirror to a rotating motor, and using an encoded disc and a cadmium sulfide to determine position of the incoming signal. I thought that I could drastically improve on this design by eliminating the need for any mechanics whatsoever, and using only one IR detector (no CDS needed), which would no only make the sensor easier to build, but also further reduce the cost. To implement vision in all direction, a reflective cone is placed underneath the detector to reflect incoming light from all directions up towards the detector. To insure a sufficient amount of IR light reached the detector, a lens was placed in between the cone and the detector, with the detector placed a distance above the lens equal to the focal length of the lens. This solves the first of the two problems that I faced. To determine the origin of the signal, a ring of IR emitters should be placed around the body of the robot at the eight cardinal directions, and then pulsed on one at a time, with a pause between each of the lights being turned on. By pulsing your IR in this fashion, you can easily tell where the object that reflected the IR light is located.

Description of Detection Experiments

First, I connected the detector to my control board and took a reading with no IR emitters on to get a base reading, which was 85. I then ran three experiments with my 360° IR Detector. In the first one, I placed the base of the detector directly in line with an IR emitter and took readings at varying distances in order to see if the design was even functional (good old Instant Gratification). Then I placed the IR detector on top of my three control boards (where the bridge would be on a Talrik robot) and took readings at various distances. Finally, I placed the detector on the base of my robot (moved the control boards out of the way, see Fig. 4) and took readings from various distances.

Data and Interpretation

All data was collected by placing an object a measured distance from the detector and taking readings while rotating the sensor. Three readings were taking for each orientation of the sensor, and the sensor was rotated four times at each distance, for a total of 12 readings at each distance. The readings were then averaged to determine the following data points. The reader should note that all readings were within 1 unit of the reported average. The results from the first experiment follow:

These results were very encouraging, as all of the readings were above the base reading of 85. I only took readings of up to 12" away because that was how long the data cable connecting the detector to my control boards were. This meant that the cone was successfully reflecting light into the lens, and the lens was properly focusing the light into the detector. The results of the second experiment, however, were not as encouraging, as all of the readings gathered were equal to the base reading of 85, which meant that the detector was not picking up any objects. This was most likely due to two design errors, one, the incoming IR was being blocked by the control boards, and two, the cone was not reflecting the light properly since it was coming in at to extreme of an angle. This thought prompted the third experiment, where the detector was placed directly on the body so that incoming IR light would be properly reflected by the cone. The results of this experiment are shown on the following page. Note that the distance is from the edge of the robot body (where the emitters are) to the object. Since the body has a 10" diameter, 5" must be added to all distances to determine how far the object is from the detector itself. As you can see, the detector worked rather well, giving a reading of 90 when an object was 6" from the body (11" from the detector), which is allows plenty of time for the robot to avoid the object. Unfortunately, because of the design of my robot, I cannot implement this sensor on my robot (as shown by experiment 2). However, for any robot that has a flat top, such as Talrik Jr., the sensor would be ideal, since the only limiting condition I found was that the base of the sensor must be relatively level with the emitters. I have proven that 360° IR detection can be implemented for under $4 ($3 for the Sharp Cam, and $0.50 for the lens), with no complicated mechanics, and with sufficient vision to detect objects far enough away to allow a proper reaction from the robot. Implementing location of the object is as easy as writing code to pulse your IR LEDs on one at a time and then taking a reading, thus knowing where the reflected light oriented from.