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I am using a RCS-Model-M2313 R/C Servo and I have managed to create a PWM. My problem is that my motor doesn't react as I was expecting. I would like to control the motor angle with the duty cycle but it doesn't work. Indeed, with a 28% duty cycle, a 550 Hz frequency, 1.7V offset, a 3.3V signal from my MCU I can control it to come back to the same position. I have powered the servo with a 3.7 tension. When I change the duty cycle, the motor turn but doesn't stop anymore.

The datasheet doesn't give any information, I really need your advices. Thank you. Regards, Mattew

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  • \$\begingroup\$ Personally, I advise testing with an oscilloscope if you haven't already. That way you can make sure you're getting an output of what you expect. \$\endgroup\$
    – Ci3
    Jun 27 '12 at 8:42
  • \$\begingroup\$ On the oscilloscope, the signal is perfect, I just cannot find the good parameters to control the angle \$\endgroup\$
    – Mattew
    Jun 27 '12 at 8:47
  • \$\begingroup\$ Please provide web references to motor data and anything else related and available. \$\endgroup\$
    – Russell McMahon
    Jun 27 '12 at 8:48
  • \$\begingroup\$ rcecho.com/… \$\endgroup\$
    – Mattew
    Jun 27 '12 at 9:10
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    \$\begingroup\$ @kenny - His "Motor" is actually a radio-control servo, which should have position feedback already built in. \$\endgroup\$ Jun 27 '12 at 11:57
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Your frequency is way too high, or conversely, you are not waiting long enough between pulses. You should be sending a 1 to 2 ms pulse every 20 to 50 ms. Note that this means 50 Hz maximum.

Also, your "1.7 V offset" makes no sense. These pulses should generally be 0 to 5 V digital signals. I followed the link you provided, but that only goes to a product sell page not a datasheet, so we can't tell exactly what voltage levels the digital signal must be. When in doubt with hobby servo units, use 5 V logic.

Try a 1 ms pulse every 20 ms. That should send the arm to one end. Then try 2 ms every 20 ms. That should send the arm to the other end. Try it with the 3.3 V signal straight out of your processor and see if that works. If it doesn't, make it a 0-5 V signal instead. That really should work if everything else is hooked up right. If you don't have a level translator chip, you can rig up something with a NPN transistor and pullup resistor. That will invert, but just invert the processor output to compensate. A HCT (note the T) logic gate powered at 5 V can also be used as a 3.3 V to 5 V converter.

Are you sure the processor power is properly filtered so that the current spikes the motor draws doesn't effect it? A schematic and scope traces with a clear description of what happens in each case would help a lot.

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  • \$\begingroup\$ Thank you very much, I cannot find a real datasheet, that's the main point. I have tried with my signal generator: 20Hz, it doesn't turn on the motor anymore. I cannot go bellow than 20% duty cycle. I am going to try with my MCU tomorrow I will keep you in touch. \$\endgroup\$
    – Mattew
    Jun 27 '12 at 12:39
  • \$\begingroup\$ @Mattew: This is not about duty cycle but about pulse width. The servo mechanism is driven by the duration of the high-going pulses. The length of the low level doesn't matter much as long as the pulses repeat every 20-50 ms or so. \$\endgroup\$ Jun 27 '12 at 12:50
  • \$\begingroup\$ I have bought a new motor, it works very well now, thank you ! \$\endgroup\$
    – Mattew
    Jul 4 '12 at 8:13

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