6 Wire Servo

Could someone explain how a 6 wire servo works? I found one when I opened up an RC toy car from Radioshack.
It has 2 wires that go to the motor and 4 wires that go to some kind of pot I think: on a small board are 2 etched concentric circles which are in contact with a 4-pronged device mated to the spinning gear. Two of the 4 prongs touch the outer circle, the other two the inner circle.
I know how to program 3 wire servos but don't understand the purpose of the extra "pot" of the 6 wire servo.

redbrickhat wrote:
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might it be some sort of quadrature encoder?

fulliautomatix wrote:
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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?

redbrickhat wrote:
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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.

wrote:
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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.

redbrickhat wrote:
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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

redbrickhat wrote:
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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.

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redbrickhat,
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
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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. -- Robert Conquest, "The Dragons of Expectation" --