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I took apart an RC car recently and found 2 sets of motors. The rear wheels (left) seem to run off an electric DC motor which is understandable.

The front wheels (right) I expected to be controlled by a servo motor because it can only make the front wheels face 3 directions (front, steer left, steer right) and no angles in between.

However, looking at the insides I can see that there are only 2 wires instead of 3. And the servo's that I've worked with before needed 3 wires for Vcc, ground and a control wire.

Could somebody confirm whether or not this is a servo? If not, what's the next best type of motor it could be? And how would I control it (PWM?, etc)

enter image description here

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It's an open loop actuator based either on a motor that stalls at a limit or a solenoid. Controlling it proportionally will probably not work, as it will be too dependent on mechanical load variation. PWM hobby servos are dirt cheap now, well under $10, just get one and rig it in there. Get a spare too for when the gears in the first die. –  Chris Stratton Feb 19 '11 at 15:57

2 Answers 2

It is a DC motor in there, A very simple gear system moves the wheels left and right and a spring centers everything. there is no fine control.

enter image description here

to control it you could use PWM and an H-bridge.

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That is exactly what the RC car we tore down as part of our engineering project last year looked like. Definitely just a motor, gears, and spring. –  pfyon Feb 18 '11 at 15:37
    
Thank you jsolarski, for the picture. I had to fix one of our RC cars here at Radio Shack and couldn't picture how the spring lined up. The peg had broken off inside, so I had to drill a hole in it, pin it, and super glue it back into place. Works like a charm now. –  user34070 Dec 13 '13 at 0:09

Since it can't do angles in between, I would not expect the 3rd control wire. Perhaps it uses a simple geared brushed DC motor? It could drive in one direction until it hits a limit switch. If the user wants to turn in the opposite direction, it just needs to drive current in the opposite direction, so it would only need two wires.

You could also put an oscilloscope on the wires and see what's going on.

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How would it reach middle position? By only a spring? –  AndreKR Feb 18 '11 at 8:54
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@AndreKR & Dave - Correct, but it not a brushed motor. It is basically an solenoid with a magnetic plunger, and spring centering. You drive current through it in one direction, and it repels the plunger, the other and it attracts it, turning toe other direction. When no power is applied, it is centered by the spring. Technically, it's really not a servo, per-se. –  Connor Wolf Feb 18 '11 at 9:02
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@Fake Out of curiosity, how can you tell that it's a solenoid? I haven't seen any models use that, but I also haven't gotten one in a while. –  W5VO Feb 18 '11 at 13:52
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@Fake oh duh, I totally forgot about the cars I've seen in the past. It's really obvious when you see the wheels flick back and forth. Definitely a solenoid. –  Dave Feb 18 '11 at 13:58
    
...and I just threw that little car away about a month ago. Rats! I could have pulled it apart. –  Dave Feb 18 '11 at 13:58

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