Small, Simple, Self-Centering Servo Circuit

Thread Starter

dafremen

Joined Aug 18, 2011
14
Kingspark,

That is precisely what I'm looking for. I never expected to have more than full forward, hover or full reverse..which is what the heli had to begin with, but with much quicker response times.

Can't wait to see what you have in mind. :D

daf
 

SgtWookie

Joined Jul 17, 2007
22,221
I posted a schematic for the tailmotor circuit off of a SYMA S107..similar helicopter (same voltage, smaller current requirement.)

It's here:

Just FYI - I simulated the above circuit the other day, and had very poor results from it. There is a good deal of power wasted in the PNP transistors due to the poor drive scheme.

I came up with a much improved design, but you're probably not interested as it requires more parts.
 

John P

Joined Oct 14, 2008
1,897
I don't see how this circuit will ever "center" the output drive when there's no input. It seems as if it'll go one way or the other, but if you take away the input drive it'll just stop where it is. Or is the output from a pot sent to a microcontroller and that decides whether to drive the motor, until the pot shows a central position?

That phrase "when there is output voltage" is pretty vague. Voltage from where?

How much current does the motor draw? Obviously the design of driver components has to be based on what's needed. Very small motors can run directly on the outputs of a PIC processor (up to 20mA each) but that motor looks bigger.

Edited to say maybe you don't really need a "servo". If the only position you want to drive to is the central one, then you could have a wiper traveling over a circuit board, where it would have two regions of "high" and "low" with a gap between them which would be "center". So you'd only need to sense which region you were in to know which way to drive the motor, or whether to stop it. Would that be enough to do the job?
 
Last edited:

SgtWookie

Joined Jul 17, 2007
22,221
Hiya John_P,
I'm thinking that our OP needs a continuously variable response from the control input, and have the rotor control be some proportion of the tail rotor control.

The motor our OP needs to control was originally supposed to be attached to a control board that had pot tracks on the surface; the position of the motor could then be detected by the controller, and driven at an appropriate rate of speed to where it needed to be rather precisely.

I suggested such a control mechanism using a sliding pot for detecting the center position, and a couple of power opamps to control the motor position based on input from the tail controls and the current motor position, but apparently the OP thought it was too complex.

I personally can't think of a smaller or more simple solution than that - although our OP later revealed that they were dealing with a very low-voltage battery. The low voltage makes it difficult to select a suitable power opamp, as they are generally not rail-to-rail for outputs.
 

Thread Starter

dafremen

Joined Aug 18, 2011
14
I have no idea of the motor ratings. The battery is a 450maH lipo at 3.7v driving 3 motors and a circuit board.

The idea is to take a power signal which normally drives a horizontally mounted tail fan, and use that same signal to drive a motor which will move a linkage up and down using a worm gear.

The worm gear I'm going to use has a brush built in for position sensing, but since ther eis no circuit board with wipers, i want to create a SIMPLE wiper solution.

There is no need for me to detect exact position, since this helicopter's controls only provides for full forward, full backward or hover.

So when i push full forward, i want the tailmotor signal to drive the servo motor up. When I let up on the forward stick, I want the servo motor to re-center itself.

Any new commands (ie. tailmotor current) should get top priority over recentering the servo. To repeat: it ALWAYS Follows the tailmotor signal..no matter what. The only time it tries to recenter itself is when there is NO TAILMOTOR signal present.

The scheme for sensing whether the servo is above or below center is just one idea. If you have another one..I'm all ears/eyes.

Once the servo is centered and there is no tailmotor signal present, the circuit should stop driving the servo motor.

SelfCEnteringCircuitTable.jpg

Thanks again for taking the time to help.

daf
 
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