Robert B. wrote: >>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. > > Sort of. While the CB radio uses amplitude modulation, it also uses AGC which would effectively destroy the type of control that you were trying to achieve. >>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... > I don't know what you have in mind for transmission medium (coax, twisted pair?). You can certainly maintain a stiff source. I don't know how precise your control would need to be. Presumably it would vary with actuator. >>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) > Agreed. I was just mentioning it for completeness (and to add a little weight to my later plug for a CAN bus solution (should we maybe change the topic tag to PIC?). >>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. > True. >>5. All channels consume signalling power (and bandwidth) continuously >>(whether it's needed or not). >> >> >Yeah thats a biggie, especially for a mobile system. > >>. 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. > Actually, it was cost that I was trying to address here. >>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. :-) > My brain stops sending signals all the time, just ask my wife. :-) From your initial description: "Muscle 1 gets 20khz, 10vpp centered on 0v. To move it from neutral, the amplitude of the 20khz signal drops to say 5vpp or rises to 15vpp." it sounds like the actuator is centered or neutral at an input signal amplitude of 10Vp-p (for example), extends or servos to a new position (again, for example) at an input signal amplitude of 15Vp-p and contracts with a 5Vp-p signal. If the input signal at the actuator drops to 0Vp-p, as would occur if the controller stops sending then the system servos to a super contracted state (on all channels!). Most of the digital signalling systems that I'm aware of either hold the last commanded position or go into some kind of pre-programmed sequence. >>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. >> I'll plug again now for a CAN bus solution. It's relatively (compared to a 40 channel analog solution) inexpensive, fairly fault tolerant, allows two way communication, allows lots of channels blah blah blah. Did I mention that it's a simple effective alternative to analog signalling systems? :-) Best regards, Dave -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu