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Old Sep 17, 2011, 01:41 AM
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How do servos work?

Hi,

I want to use an RC servo (like the one on my airplanes) to control a pneumatic valve. Its a valve I designed that needs to be rotated 60 degrees one way to open one flow, and -60 degrees (opposite way) to open the second flow.

I have microprocessors which I can program to generate any digital or analog signal, any voltage +/-10V. Problem is, I have no idea how to control these servos, I don't know how they work. There are 3 wires, and I'll guess that one is ground, one is power, and the other one is for a signal.

But I heard different ways how servos are controlled. One way I heard some work is by giving a digital signal 0-255 which corresponds to -90 to +90 or something like that.
Someone else told me that some servos take a digital pulse between 1ms and 2ms in duration. 1ms corresponds to the servo all the way clockwise, while 2ms corresponds to servo all the way counterclockwise, and so on.
I'm guessing there could be analog servos as well.

So how are the RC aircraft servos controlled?
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Old Sep 17, 2011, 02:51 AM
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Staffs, UK
Joined Nov 2003
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Quote:
Originally Posted by motorator View Post
Someone else told me that some servos take a digital pulse between 1ms and 2ms in duration. 1ms corresponds to the servo all the way clockwise, while 2ms corresponds to servo all the way counterclockwise, and so on.
I'm guessing there could be analog servos as well.

So how are the RC aircraft servos controlled?
That's it. Pulse 1-2ms repeated approx every 20ms. You'll have to check the specific servo you intend to use for which pulse width means CW and which CCW because there's no standardisation.

There's no difference between how analog and digital servos are driven, the difference is just in the internal working of the servo.

Steve
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Old Sep 17, 2011, 05:29 AM
A man with too many toys
United States
Joined Feb 2001
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Servos use Pulse Width Modulation (PWM) for control. Servos are designed for 5v or 6v +/-10 volts will instantly burn them out. Look up PWM servo control on google for more information.


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Old Sep 17, 2011, 08:18 AM
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flypaper 2's Avatar
Kingston, Canada
Joined Jun 2004
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A servo tester would also work for what you want. The one I have is a box with a joystick and an ampmeter on it that goes between a batt and the servo.

Gord.
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Old Sep 17, 2011, 08:44 AM
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Moab, Utah, USA
Joined Apr 2003
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As others have said, all servos use the same drive signal whether or not they are analog or digital servos. You are correct in that of the three wires one is ground, usually either black or brown in color, one wire is +Vcc, always the middle one and usually colored red, and the other wire is signal, usually some light color like white or yellow.

The control signal is a square wave that varies from 1 to 2 msec in length. 1 msec will give full deflection in one direction and 2 msec will give full deflection in the other direction. Between the maximum and minimum pulse widths the servo position is proportional to the pulse width. With an input pulse width of 1.5 msec the servo will be centered. The control pulse frequency is normally 50 ppm.

With the standard 1 to 2 msec signal most servos rotate only a maximum of 40 degrees in each direction. Computer transmitters get around that by increasing the maximum pulse length and decreasing the minimum pulse width. So you should be able to modify the pulse width to get the amount of servo travel you require.

Larry
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Old Sep 17, 2011, 09:15 AM
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Florida
Joined Aug 2004
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I believe all the discussion above refers to the typical RC servo. What you may need is something quite different depending on the power you need to rotate your valve. Most RC servos are quite weak relatively speaking. You might want to check into one of the Robotic forums where they often need servos with far more muscle than a typical RC servo can provide. You will find servos that can and do use digital inputs rather than pulse width as RC servos do as well as those using stepper motors which can be made quite powerful and very precise.
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Old Sep 17, 2011, 05:55 PM
Will fly for food
Maryland
Joined Sep 2004
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http://www.servocity.com/html/hs-7980th_servo.html

500 ounce inches at 4.8 volts, 611 at 6 volts.

That is pretty hefty torgue.
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Old Sep 17, 2011, 07:49 PM
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Joined Apr 2007
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Okay thanks a ton guys.

Its a small 1/2" diameter valve that shouldn't require too much torque to twist (I haven't machined it yet but I'll lubricate it well). All I need is +/- 60 degrees, I don't need any precision and I don't need any angle in between. It will be an on/off valve.

Also it needs to be quick, so I want to get from 0 to 60 deg or from 0 to -60 deg as fast as possible. I was looking at something like this:

http://www.hobbyking.com/hobbyking/s...30sec_79g.html

Or if anyone else has other suggestions, I'd highly appreciate that too. Thanks again.
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Old Sep 18, 2011, 04:16 PM
Will fly for food
Maryland
Joined Sep 2004
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Hmm, have to try some of those on a heli at 2S lipo voltage.
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