To keep Blimpduino as cheap and simple as possible, it navigates by looking for signals from a ground-based IR beacons in any one of four directions. There are four IR detectors (N,S,E,W) on the blimp and the ground-based beacons are nothing more than an IR LED transmitting random 1s and 0s at 56KHZ. This being IR, they bounce all over the room and there is loads of IR noise from other sources, but the IR receiver that records the highest number of 1s (highest signal-to-noise ratio) is considered the direction that the beacon is transmitting directly from and we steer accordingly. (This is also the way the Pololu IR beacon/transceiver pairs work)
That's easy for one beacon. But when we want to introduce multiple beacons, each with a unique ID, it gets more complicated. We can't transmit at different light frequencies, because we'd need to add matching IR receivers on the blimp for each beacon we added. We can't use TV remote control codes, because then we can't tell where they're coming from (it's the ratio of signal to noise that tells us direction, but the codes are all signal and work as well if they're bouncing off a wall as when they're aimed directly).
Our instinct is to have a central beacon controller (another Arduino--see diagram above) and sequence them so that you'd be able to tell which beacon is transmitting by when in the beacon sequence you got the signal. But that requires us to synchronize the blimp and the beacons to a common clock, and we're debating how to do that.
My proposal is to do the following, 10 times a second:
- For the first 1-50ms in each cycle: all beacons go on for 30ms, then all off for 20ms ("clock sync pulse").
- 50-60ms: Beacon 1 on
- 60-70ms: Beacon 2 on
- 70-80ms: Beacon 3 on
- 80-90ms: Beacon 4 on
- 90-100ms: Beacon 5 on
- Repeat...
The beacon hub controller would just schedule that sequence. The blimp, meanwhile, would have to detect both the direction of signals and how long they're on. If they're on for 30ms and then off for 20ms, that's the start of a cycle. Then depending on when in the cycle it detects the next signals, it knows which beacon that is.
Jordi's not convinced this will work, and thinks we'll need an RF link to communicate between blimp and beacons, which strikes me as expensive, complicated and unnecessary. What do you guys think?
Is there a better way to have a blimp distinguish between different IR beacons?
Comments
I was able to setup a universal remote to transmit on the RCA protocol. Using the beacon and the remote at 90 degree angles to one another at distances of a couple of meters from each other I was able to distinguish between the two. The IR code will need to do more then just count pulses from each each sensor. Right now the problem I am having is that the two receivers back to back are both picking up the signal. I had some success shrouding them with electrical tape.
I will bring another universal remote in and see if I can consistently distinguish between one of them transmitting a #1 on the remotee and the other a #2 while the remotes are at right angles to each other using the RCA protocol. The RCA protocol is nice because it transmits the complement of each signal after each transmission so a bit of error correction is built in.
Scope pictures and more at:
http://mtsacflight.blogspot.com/2009/01/ir-beacon.html
Thanks!
It also occurs to me that if we used 38Khz IR modules, we could use a standard TV remote as a remote control for the blimp. This would save the cost of the RF gear. I converted a standard RC car to be controlled using an IR remote as described briefly here. http://profmason.com/?p=392 (Near the bottom)
If you choose to add an IR transmitter to the board or 4, which I recommend,
You should be able to multiplex them with for example the motor drivers.
I'm fairly certain the motor driver board includes an enable line.
You will probably need a hi-side driver (a transister) as an enabler for the IR beacons.
To drive the motor, set the enable, then set the drive pins.
To drive the IR, reverse the enable and pulse the beacons.
is the idea fixed to use 4 ground beacons??
or do you just need some procedure for navigation???
Each beacon transmits their id several times with a delay based on their ID. Sometimes they collide, but the delay separates them.
You might try to:
Send a brief ID on frequency, followed by a series of pulses increasingly off-frequency. The cut-off filter in each IR receiver will cause the system to lose sensitivity as the function of frequency excursion. The number of pings from each detector can be used to vector the signal.
This permits very rapid signal acquisition, and so permits sparse signalling.
You might also consider diffusing the detectors so there is some overlap in the field of view. That overlap will allow for rational vectors, in which the signal strength of adjacent detectors can be used to determine the vector with a higher degree of specificity than just the four corners.
Good luck
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