Re: [Paparazzi-devel] 8 CH PPM Encoder

[Chris Gough]

Hi Jorge,

The reason for recommending not to power anything but a couple of micro servos directly from the autopilot is to avoid autopilot brownout caused by voltage drop under servo load. It sounds to me like you might be experiencing a similar problem with PPM encoder brownout. 

I determine servo power supply this way.

 1) set voltage on lab power supply to equal fully charged battery (or BEC output, etc)

 2) hook servo to power supply

 3) measure current draw at stall torque (fight servo with pliers, careful not to strip gears though)

 4) multiply by total number of servos in the plane

 5) multiply by a safety factor.

Then I ensure that my servo power supply has capacity for at least twice the estimated  current if all servos were stalling (times safety factor). For small foam planes you would probably be fine with 80% safety factor, because under normal conditions the servos don't work as hard as if they were all stalling. But for a plane with severe crash consequences  (dangerously heavy, overflight risks, valuable equipment, etc) you should use a much higher safety margin such as 200%.

Our ~20KG plane has 5 large servos that each draw 7A at stall. We run them at 6V on a supply capable of >420W. 

Chris Gough

P.S. Increasing voltage won't improve a marginal situation because servos work harder at higher voltage. 

On 12/07/2013, at 4:50 AM, Jorge Pantoja <address@hidden> wrote:

Dear everyone,


I was setting up my airframe and I encountered a problem with the 8CH PPM Encoder that I have (http://ardupilot.com/wp-content/uploads/downloads/2013/04/PPM-Encoder-3DR-Manual-v2.3.16.pdf) when I connected the servos (Futaba S3305 Metal Gear Servos). It seems like the servos are drawing too much current from the batteries because whenever I send commands through the radio the PPM Encoder shuts down (there is a voltage drop that I measured with a multimeter on the encoder supply) and the servos did not respond well to the commands I sent from the Radio. I tried with normal servos (Futaba S3003) and I didn't have this problem.

I tried supplying the PPM Encoder and the receiver separately by removing the Jumper provided in the Encoder board but it didn't solved my problem.


I wonder if I just need to change to a higher voltage battery (?). My S3305 Servos can take up to 6V but I don't know if my YAPA2 can take a higher voltage battery. Also, I am thinking of short circuiting the bridge provided in the YAPA2 to power the servos (http://paparazzi.enac.fr/wiki/File:Yapa_v2-00_pinout.png) using the same battery that I use to power the autopilot but it says it is not recommended, can anyone tell me why? And please, if you can give me other ideas on how to solve this I will appreciate it.

Best Regards,
Jorge

P.s: I am using this type of servos (S3305 Metal Gears) because I have a 17kg Airframe with high loads in the control surfaces.

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