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Old Today, 12:57 PM
Awsome 2.5D Pilot.
bigbobed's Avatar
Foxfire Village, NC
Joined Nov 2003
1,487 Posts
Originally Posted by JohnRB View Post
I am not looking to start a debate, only to pass on some measurements I took this morning to make me feel more at ease with the way Open FlighStab generates the servo output pulses. This was intentional in Open FlightStab to remove as many interrupt conflicts as possible.

I never looked at the servo currents in relation to the start of the positive pulse before. I know that using an analog servo the motor current will not start to flow until a negative edge is seen by the servo (either from the internal pulse or the pulse from the receiver). This is how analog servos have worked since the 1960's - I was there.

I setup my Taranis to send switch SC to channel 8. Switch SC was set to 2012us, 1500us and 988us (I have my Taranis setup to display actual pulse widths and not percentages).

I used an X8R, sending channel 8 to the servo under test and also to my scope for sync.

Scope (Tektronix MDO4056-6) was set to trigger if the positive pulse width was ever less than 1.25ms or greater than 1.75ms.

Tektronix TCP202 current probe was used on the ground lead of the servo under test to monitor current. The current being measured was total current of servo logic as well as motor current).

Test starts with SC switch in center position (1500us), scope set to single trigger with 5M of acquisition memory. Switch SC was then moved to 988us to capture the servo current. Switch was then return to center position and servo was allowed to settle. Repeat again with SC moving to 2012us position.

I tested with 2 different Hitec HS-5085MG servos because I had some handy in my office. The servos had no load, just sitting on the bench with only output arms attached.

The Hitec servos center at 1520us so when I measured servo movement there was only 56 degrees of travel going to the 988us limit versus 60 degrees of travel going to the 2012us limit. Servo center of 1500us is about 5.5 degrees off of dead center.

I have attached scope captures so you can see when the current starts and when it stops.

Having all servo outputs close to the same time probably will make dual aileron and/or elevator servos closer in sync but I am not sure I would ever notice it with the flying ability I have (very old eyes and slower reflexes).

No conclusions, just sharing information.


PS. Just located some digital (Power HD DSM44) servos. I captured the currents for one of those. With the digital servos I think the closer output start times might make a noticeable difference.
John, nice work.
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Old Today, 03:29 PM
Always Fly 2 Mistakes High!
RGS2010's Avatar
Houston, Texas
Joined Mar 2010
3,658 Posts
Originally Posted by Rotozuk View Post
We have heard from others with this issue also. The nuts are steel, so not too surprised. I think FrSky is looking into making some alloy nuts... Have not heard an update for a while though.


Thanks Wayne!
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