One of the most challenging aspects of the RoboTag project is the detection of other robots. How does a robot know where to aim the tagging device? How does a predator hunt down a prey? How does the prey escape? Obviously there needs to be a method of detecting other robots.

It would be extremely difficult for robots to detect each other without active communication. Sure, a predator could stay still and use a motion detector to 'watch' the surroundings, but who's to say that they prey couldn't also sit still? Other detection methods like vision systems, heat detectors, and etc, would be much too difficult to implement for this course.

So each robot needs to emit a signal, and each robot needs to be able to detect a signal. The design objectives are as follows:

• robots must be able to detect signals from up to 7 other robots
• robots must be able to detect signals within 1/10th of a second
• robots must not detect their own signal
• the signal should only work within line-of-sight
• the signal must be detectable from 3 inches to 12 feet
• the signal must not interfere with any other sensor

Infrared radiation lies between the visible and microwave portions of the electromagnetic spectrum. We'll be using a detector that has peak sensitivity at a wavelength of 940nm, so we'll use IR LEDs that emit at 940nm.

However, there are a number of problems with an infrared signal:

• a number of robots are using infrared navigation sensors
• fluorescent lights, the sun, cameras, etc, emit large amounts of IR
• infrared signals 'bounce' off surfaces
  • (which is how IR navigation sensors work)

It's important to use modulated IR light to avoid interference from other IR sensors and from lights, the sun, etc. For example, if we were using plain infrared light (ie: we emitted light at 940nm), it would be very difficult to determine the difference between another robot or a desk lamp. Modulated IR is commonly used in IR navigation sensors.

To avoid conflicting with IR navigation sensors, we will be modulating our infrared signal at 56.8kHz. Since we're interfacing with digital devices (and we're not doing any mixing, filtering, etc), we'll use a square-wave signal. So our modulated IR signal would look something like this:

There's still the problem of infrared signals bouncing off the arena walls, which is difficult to address. If we use a material that doesn't reflect infrared light then the IR navigation sensors won't work. The solution? There isn't really one. Even slowly moving robots wouldn't see an infrared reflection from another robot for very long, so it won't be too bad.

Robots which do see the reflection of another robot's beacon can obviously use this to their advantage. The scenario above shows the robot on the right seeing the signal from the robot on the left (and vice versa). Remember that both robots won't be able to tell whether or not the other robot is in the original location, or the mirrored location (shown in red).

The technical specification is based around the design objectives of the beacon:

• robots must be able to detect signals from up to 7 other robots
• robots must be able to detect signals within 1/10th of a second
• robots must not detect their own signal
• the signal should only work within line-of-sight
• the signal must be detectable from 3 inches to 12 feet
• the signal must not interfere with any other sensor

When building the specification it was important to consider the number of other robots that must be detected, and the time in which the detection must occur. I used the 0.1 second detection time based on the average reaction time of a human -- if we can react to something within 100msec, then our robots should be able to do the same.

Having each beacon be able to detect 7 other beacons means that we can have eight robots playing at a time! That was the maximum number of 'bots that I could imagine in one 12'x12' arena. "Pandemonium" just isn't accurate enough.

It was also important to ensure that a robot wouldn't be able to detect its own beacon. Because the beacons reflect off the arena walls it is important to turn off the beacon detector when emitting the beacon signal. This works perfectly with the non-multitasking PIC microprocessor which can only perform one task at a time anyway.

In regards to the detection range, I recommend multiple high-output IR LEDs (like the one I recommend later in this document) driven with 50-100mA. While the continuous rating on the LEDs is only 50mA, you could probably drive them at 2A without much problem. Remember our 6.25ms bursts of IR are modulated, so the LED wouldn't be on for very long. In my lab tests, I've detected another beacon at 12'+ with as little as 60mA of LED current.

So the technical specification of the beacon is as follows:

• consecutively, each beacon must:
  • emit 6.25ms of modulated infrared light
    • 940nm light modulated at ~56.8kHz (55.5kHz is okay)
    • emitted 360-degrees around the robot
      (minimum of 50% beam power in any direction)
    • must be detectable at 12+ feet with a standard device (50+mA/LED)
  • looks for infrared light from another robot
    (for N blocks of 6.25ms each, N is assigned by course coordinator)
  • emit 6.25ms of modulated infrared light (as spec'd above)
  • looks for infrared light from another robot
    (for 14-N blocks of 6.25ms each)
  • repeat the process

But this means that when the beacon is transmitting, it won't detect other beacons that are also transmitting. (ie: the beacon can't 'talk' and 'listen' at the same time) One way around this problem would be to synchronize the beacons to start at the same time. This, however, is a real pain in the ass, so we'll use a timing solution instead.

The timing solution (shown below) works if you consider the following worst-case scenario. Assume all eight robots are within line-of-sight of each other. Assume that, by luck, the first 6.25ms pulse of infrared light occurs at the same time. By changing the time between pulses (ie: each robot waits a different amount of time before sending out another pulse), we can ensure that each robot will detect ALL the others, within 100ms.

This can be shown in a chart form as follows:

The above chart shows each robot transmitting from 0 to 6.25ms, then 'looking' for a differing amount of time before transmitting another 6.25ms pulse. This allows us to have eight robots in the same area that will ALWAYS detect each other within 100ms.

In case you're wondering, yes, that is the maximum number of robots that can interact (given the 6.25ms operating time with a 100ms loop time). If we added a ninth robot that transmitted from 0-6.25ms and from 56.25-62.5ms, it could 'collide' with robot #7.

My proof-of-concept solution features two beacons that each have a single detector and single emitter. Each beacon, detailed below, uses a 4MHz PIC to drive a single IR LED and detect a signal from a single LITEON receiver. You will obviously want to extend my work to handle multiple IR LEDs (additional hardware), and multiple receivers (additional hardware and software). You should consider using the 20MHz PIC16F84 to interface with more than one detector.

The schematic, below, shows how the PIC hardware interfaces to the LITEON IR detector module (LTM-97AS-56) via RA0. Note the power supply noise suppression in the RC filter connected to the LITEON detector. The PIC drives a red LED whenever another beacon is detected - this output is connected to RB2.

The PIC also drives the IR LED, connected to RB0, via a simple BJT. I use a 1kohm resistor to limit the current required to drive the transistor, and a 39ohm transistor to limit the current flowing through the IR LED. You will need to develop a more robust biasing network for additional LEDs, of course.

Beacon schematic:

Making the infrared LEDs and the LITEON device work in harmony is difficult. The range requirements are 12+ feet, but you should aim for 5 meters in full sunlight. To help, I've put together the following recommendations.

• Don't drive the LEDs and LITEON from the same power source, the LITEON really doesn't like the power supply fluctuations caused by the high current-drawing LEDs. You can drive the LEDs specified below with 12V and 2A, but it's not required -- a better idea would be to use your motor batteries (whatever the voltage) and aim for 60mA per LED.
  
  
• Shield the LITEON from any overhead fluorescent lights, they'll put out enough IR that some will appear to the LITEON to be modulated at your frequency. Remember to shield all around the LITEON, even behind and to the sides. The metal casing doesn't completely block IR.
  
  
• Don't let your LED shine anywhere near your LITEON. Even though I was transmitting and receiving at different times, the LED would 'flood' the LITEON when transmitting and the LITEON wouldn't recover in time before the LED was transmitting again. Even with 42ms between transmissions, the LITEON wouldn't recover enough to detect a faint signal. If you shield the LEDs from the LITEON, the operating range increases.
  
  
• Of course, drive the LEDs through transistors. Because we're making a 55.5kHz signal (from the PIC) with a total burst time of 6.25ms - you could hit each IR LED with up to 2A of peak forward current! Yes, two amps! If you decide to do this (it's not really needed), you should consider optoisolating your PIC.

Other recommendations and miscellaneous advice:

• use the recommended parts I specify below
• be sure to use the LITEON-recommended power supply noise rejection scheme
  (the 47ohm resistor and the 47uF capacitor - check the datasheet/schematic)
• use a darlington pair buffer (like the ULN-2803A) to drive the LEDs
  • amplify the output from the PIC, then have that current drive nine other darlington pairs (one for each LED)
• make sure your LED power supply can handle the current draw!
  • 9 LEDs at 100mA each is almost 1A! That's a lot of current.

Selecting the best parts for the IR beacon is important. If you're using an IR LED that doesn't transmit at 940nm, for example, the LITEON receiver module won't work as well.

The parts I've used in my lab tests (and which I recommend), are:

• LITEON 56.8kHz Infrared Receiver Module - LTM-97AS-56 (DigiKey: 160-1155-ND)

  The LITEON IR module can detect modulated IR at 8 meters (30-degree cone) or 6 meters in a 45-degree cone. However, it is susceptable to other infrared light, so you should take care to correctly shield your detectors.

• 940nm IR emitting diode - LTE-302-M (DigiKey: 160-1063-ND)

  The emitter I recommend gives 100% intensity at the beam center, 90% intensity at 10-degrees off center, and 50% intensity at 20-degrees off center. So when they advertise a 40-degree beam width, they mean 50% intensity at 20-degrees from beam center. Yes, this does mean you'll need lots of the LEDs. I'd recommend nine. Yes, this does mean you may need to provide more than 1A (peak) to the emitters.