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.

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.

3.  As described, the system is open loop with no mechanism for sensors
to feedback information to the controller.

4.  Limited control bandwidth.  Spacing between control frequencies
limits the rate at which you can change signal amplitude without
spilling over onto adjacent channels.

5.  All channels consume signalling power (and bandwidth) continuously
(whether it's needed or not).

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.

7.  All systems drive through the stops in one direction if transmit
power is lost at the controller.

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.

Best regards,

Dave

Robert B. wrote:

>I recently had an idea (probably ages old) for the transmission of data on a
>continual basis using analog voltages, and would like to get some criticisms
>on it before I get too deep into trying to make it work.  The basic idea is
>to use frequency-dependent filters to determine the amplitude of some analog
>signal at a given frequency.  Perhaps the best way to explain the idea is
>with an example:
>
>Example:  Take a robot with 40 separate actuator "muscles".  For now just
>consider the actuation control and disregard the feedback mechanisms. To
>establish real-time control of 40 actuators on a digital link would not be
>impossible, but would perhaps restrict expandability, etc.  So we assign
>each "muscle" a control frequency based around a signal amplitude.  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.  The actuators are
>tuned to listen on a specific frequency, which is then smoothed to a
>relatively DC voltage.  All 40 actuators are installed as said, at perhaps
>22khz, 24khz, ... and so on (no calcs yet, but assume there is enough
>bandwidth), each with an independent filter to see what component of its
>tuned control frequency is present.  So now to control all 40 actuators at
>once it would (only?) be necessary to sum 40 separate control signals into
>one analog signal, and inject that to the backbone where each actuator will
>single out its respective command.
>
>The apparent advantage of this method (to me) would be the increased amount
>of data transfer on a single wire, with the primary disadvantage being
>somewhat imprecise control all around.
>
>The way I would envision it working is having a powerful processor to
>generate the analog stream, and multiple PICS spread around the network to
>read the filtered frequency and do the actuator control, then perhaps
>eventually "capturing" analog streams to memory for replaying, for example,
>a walking routine for a humanoid robot.
>
>Is there any major problem I'm overlooking in a network like this?  It seems
>pretty sound to me, but I'd really like to get some expert opinions, or
>maybe advice from someone experienced with a similar network.  If it looks
>feasible I very well may be turning it into an "open hardware" project for
>robotic control, but I'd hate to take it that far and have a miserable
>failure!
>
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