6 Wire Servo

might it be some sort of quadrature encoder?

in a rat shack product?  not bloody likely.  probably just some strange
kind of pot.  the point here is, though, that you're not describing any
electronics, which would be required for servo operation.  it's not all
that hard to build your own but it's probably cheaper and easier to just
rip out the whole mechanism and put a proper rc servo in.

chris

If it's not a "real" servo, do you think the rc car is just reading a
simple 4-bit binary number from the 4 leads rather than calculating
some kind of variable resistance that a true servo would?

The etched circle has a "weird" pattern (not symmetric); could it be a
2-bit gray encoding not a quadrature?

     Then you've got some kind of encoder there.

     Can you post a picture someplace?

     And, actually, a 2-bit Gray encoding IS a quadrature.

                John Nagle

I thought the difference was that in gray encoding the binary output
changes one bit at a time, while in quadrature different phase shifts
are measured.

   With four wires I would guess this is a three-bit encoder (one of
the four wires is a common), encoding eight (2 to the 3rd power)
positions.

It's got 4 wires but there are only 2 concentric circles not 3 like in
a normal 3-bit encoder.
The circles are fragmented and interconnected so I think some kind of
complex logic function is being implemented.

    Oh, that's cute.  It sounds like a simple analog encoder.  Do the
circles have significant resistance?

    I suspect that the idea is that you apply a voltage to one
of the circles, and run the output from the other into an
A to D that can handle from -Vcc to +Vcc. If this is what
I think it is, you'll see a sine wave coming out as the
motor turns.

    This is a good way to cheaply get position from a continuously
rotating shaft.

                John Nagle

  It isn't a Servo as such - The disc just provides basic middle, left &
right positions.
When you move the transmitter stick the receiver puts the steering into
the appropriate position. This arrangement is normally referred to as
'Bang - Bang' steering, although in really cheap cars the motor just
drives against a mechanical end stop and stalls there while turning and
a spring centres the wheels when no steering input is present. These
type usually have an adjusting screw to set the spring to give an
acceptable straight ahead alignment.
I've seen the type of set-up you have on a Radio shack four wheel drive
RC car as they need to exert a little more authority over the steering
when negotiating uneven surfaces. There is usually a sprung coupling
between the motor/gearbox output and the steering rack to absorb heavy
shocks. On the unit I looked at this was like two discs with lugs on
with a loose plastic ring in between them. There was a slot in ring
where the two lugs engaged.
I once started to map out the continuity between the wires for different
wheel positions with a view to writing a PIC program to act as a
proportional steering interface - In the end I just adapted a standard
RC servo to fit in its place, but even then the amount of play in the
steering made it difficult to maintain a reliable straight ahead position.

Hope this helps.
    Jon.

I don't know if what you're describing is what my brother and I ran into
on a RadioShack "Razer", but I've appended a copy of my original post with
our test results on the off chance it may help...

Frank McKenney


Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
--
    Knowledge does not necessarily imply judgment.  All truly
    critical, as against technical, argument is either intuitive or
    hypothetical or partial.  This cannot be compensated for by a
    study of the raw material, however exhaustive.
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