arod

I am entering a robotics competition where there are two beacons placed in two corners of a square playing field (shown as the "O" below in my ascii art lol).

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The left beacon modulates its IR light at 2.5kHz and the right beacon modulates IR light at 5.0 kHz. I realize it might be possible to get positional information from these two beacons. If you are able to find the angle to each beacon relative to your robot, you could do some trig and get your position. My first question is how would one detect this modulated IR light (50% duty cycle). Would it be as simple as designing a circuit with a phototransistor/photodiode, a comparator for a threshold value, and connecting the output to a microcontroller? Are there readily available modules that will detect and be able to distinguish between the two signals of IR light at these frequencies? Every commercial module that I have found is specifically for a carrier frequency of 38-40kHz. This module could then be mounted to a servo of some type to get the angular position of the light. Is this even what the competition designers had in mind when they set up two separate beacons. Any advice or suggestions are welcome. I am pretty new to robotics, but I am fairly well versed in electronics.

dknguyen

I've never worked with IR, but I think if you just add a filter (I mean frequency-filter, but a wavelength filter might also be helpful) onto the circuit that you said it would work. I also think you would need a circular array and a way to pick out the brightest one- otherwise you would need to increase mechanical complexity by adding a pan unit which would greatly slow down response time. You might need a lenses (IR sensors with beuilt in focused lenses) for good directionality.

I also suppose you could directly measure the frequency of the square waves using an input compare module on a uC, though I am not sure how it would react to two square wave inputs going in at the same time and if you could separate them out. If you had lense I guess you could always assume the particular photo detector was only looking at one beacon and "round" the measured frequency to the closer frequency between the two beacons and assume it is that.

It would have been much preferred if the designers added a third beacons and the beacons had IR+ultrasonic pulse output

jpanhalt

Are the beacons parallel, as shown, or intersecting?

John

dknguyen

I assumed they were omnidirectional.

jpanhalt

They were not drawn that way, so I didn't assume it. My guess is that they are not omnidirectional, but rather directional with a fairly broad dispersion angle of say, 15 to 25 degrees, and intersecting.

Several months ago I suggested in response to one of the line follower-type questions that someone could try a little different approach modeled on the old beacon system for aircraft navigation (sometimes known as the the A&N airway system as A = dit, da and N = da, dit).

See these links:
http://www.navfltsm.addr.com/ndb-nav-history.htm
http://www.electro-tech-online.com/r...-detector.html

If the beacons intersect and have different modulations, it would seem like a good opportunity to try that approach. John

dknguyen

WHat is telling you atht they are directional? I don't see it. Just looks like two mysterious round thing sitting at the corner of a square area.

Nigel Goodwin

A quick google found this:

http://roboflag.carleton.ca/resource...r_beacon.shtml

It's not really what you're after, but I thought it was quite interesting.

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jpanhalt

What was telling you they were omnidirectional apropos your earlier assumtion? So, I asked a simple question of the OP and have yet to receive the answer. In the absence of that informattion, I was simply making a suggestion. John

dknguyen

I was just curious as to the way you were reading the picture. I just figured that uncovered flashing photo emitters were cheaper and simpler than directional ones.

jpanhalt

It was the heavy, dark vertical lines that made me think directional. Moreover, single IRED have broad, but not omnidirectional , emission. John

arod

They are directional IR transmitters. Each beacon has 6 IR LED's that are wired in series (shown below). Sorry for not clarifying that in my initial post. The outer edge of my ascii picture just indicate the physical boundaries of the playing field (4 walls). Thanks for all the input/suggestions.

bobledoux

A micro controller could count the period between light pulses and determine which beacon was being read.

Mount an IR receiver on a small stepper motor. Have it sweep a full circle. Establish a step on its sweep that is "straight ahead," from the robot perspective. Then rotate until the first beacon is read at its strongest strength level. Record that step number. Continue rotating until the second beacon is likewise seen. Record the step number. The difference in step numbers times the motor step angle is the angle between the beacons.

Here is an article with some pertinent math:

http://www.seattlerobotics.org/encod...9/lasernav.htm

jpanhalt

If there were just one emitter on each side, it would seem that intensity measurements (as suggested in the A/N airway analogy) could be relatively easily calibrated. With multiple emitters on each side, I suspect the calibration would be considerably more complex. The beam divergance and intensity for each emitter is probably slightly different, and there has been no mention of how or whether the emitters are aimed.

Thus, I agree that in this scenario, a triangulation method as shown in bobledoux's link is a better approach. John

Pommie

Reading the link posted, it would appear that it is impossible to work out where you are with only 2 beacons.

Arod, do you have a link to the competition?

Mike.

arod

Competition Rules

You should be able to work out position with a little trig since the playing field dimensions are known. This is of course assuming that you can accurately detect the angular position of the two beacons separately. I really like the idea of using a stepper motor or even a couple servos to rotate a small phototransistor circuit. As of right now, we have a big arm on the robot so doing a full 360 degrees of rotation would not be feasible with a stepper motor.

I am still left wondering how to differentiate the two signals from each other. Is it as simple as having a phototransistor circuit with a comparator that goes high at a certain threshold. Then I could feed the output of the comparator to an I/O line a microcontroller and check the time that it is high? It just seems a bit too simple to actually work practically.