Thanks for the thought-provoking response, but I see that perhaps I didn't explain my intentions as well as I thought I did. (inlined below) ----- Original Message ----- From: "David Minkler" > Robert, > > Some of the obvious issues that I see are: > > 1. Intermodulation causing false signals. Channel 1 and channel 6 mix > to create a false signal on channel 31. Touch tone phone frequencies > were VERY carefully selected to avoid this problem. This is a very valid concern, but it could probably be worked around with some effort. But hey, they cram 40 channels into a standard CB radio (AM modulated, right?), so I'd like to think some of these problems have already been solved in the RF fields, and could be worked around. > 2. No real immunity to network (call it resistive but, it doesn't have > to be) loading. As nodes are added to the network, the amplitude of all > of the signals will probably drop a little causing all actuators to skew > in one direction. Difficult to sense this loading at the controller or > to compensate for its effect at various points along the network. Also easily overcome with a higher-current supply, like an op-amp follower or something to keep it stiff. Certainly not ideal for power conservation though... > 3. As described, the system is open loop with no mechanism for sensors > to feedback information to the controller. I didn't describe any feedback system yet (see note in original post) > 4. Limited control bandwidth. Spacing between control frequencies > limits the rate at which you can change signal amplitude without > spilling over onto adjacent channels. The way I understand this,the signal rate of change would generate sidebands at f1+f2 and f1-f2, so to avoid an unwanted component the rate of change for signals must be significantly less than the control frequency. This wouldn't really be a problem for control of a physical system like I described, perhaps changing the signal at 50Hz with a control frequency in the kHz. > 5. All channels consume signalling power (and bandwidth) continuously > (whether it's needed or not). Yeah thats a biggie, especially for a mobile system. > 6. Difficult (not impossible) to manufacture a system with say, 40 > independent, nearly brickwall bandpass filters at very specific > frequencies. You'll probably start using digital filtering techniques > long before you get to 40 channels. Well lets presume we space the channels wider then, and use a rougher filter. The idea isn't to have maximum (or even reasonable) accuracy in signal interpretation as in digital communications, but rather I would imagine it being more qualitatively interpreted at the uC controlling the actuators. Don't think industrial precision robotics, think cheap imprecise entertainment robotics. > 7. All systems drive through the stops in one direction if transmit > power is lost at the controller. Since the controllers would respond to the RMS signal, 0v would just return everything to a neutral state. In the event of controller failure it would probably be catastrophic regardless without some sort of backup signal generator. Essentially the same thing that would happen if your brain suddenly quit sending signals. :-) > There are appropriate places for such a signalling scheme. One that I > know of uses two different audio tones, transmitted on the same carrier > frequency, by two different high gain antennas pointed in slightly > different directions. A device which wants to follow a path between the > two transmission lobes just servos to maintain the same amplitude > between the two audio tones. The pilots on the list will know what I'm > talking about. > > Before I started looking at actually building a signalling system like > you've described, I'd take a good hard look at CAN bus or, for one way > communication, look at some of the PCM systems that are used in model > aircraft control. Thanks for the advice, I'll look into them. Its purely a hobby type idea at the moment, and I doubt it would ever be marketable in the real world. Again thanks for your post, and interpret all comments in the best of light (lots of smiley faces). -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu