Hello,
I have her a Fixhawk, which I want to use with the Arduplane-FW. I have notices that the servos run usually harsh and choppy.
One might think that the servos do not get some intermediate values and jump to the "few" setpoints directly.
To make sure that these problems do not come from the Taranis / L9R, I have directly connected the servos to the receiver. The same servos behave here much better with the same BEC. No stuttering and a normal smooth movement.
The log of RCIn and RCOut in the Fixhawk, looks okay ?! Furthermore, I have to reduced the processor load of the Fixhawk by disabling the complete logging - but unfortunately this makes no change.
To further determine the cause I have a RCOut of the Fixhawks and the same channel of the receiver directly connected to my APM2.5 to measure the PWM with the same time stamp.
Her are the results:
Turquoise = signal from the receiver
Purple = signal from the Fixhawk
Total measurement:
Zoomed in different areas:
Basically, a phase delay of the Fixhawk is recognizable. Much worse, however, I believe that the signal from the Fixhawk is clearly more angular than the actual receiver signal. I think these signals are somewhat filtered in the APM, since the servo moves still more rough than it is shown in the measurement.
What could be the reason? Do you agree?
Replies
I wonder if the problem is that the HS65 servo is a pretty old design? When did it first come out? It's analog?
I have 4 helicopters flying, all using digital servos, and all work with no problem. They all use modern digital servos, not very expensive either. Most are various Hobby King offerings. One is Savox.
The action is not as smooth as when being driven by an Rx directly, but with the Pixhawk it's no worse than it was with APM. In fact, maybe it's better with Pixhawk? It's been a long time since I used an APM so I'm starting to forget.
This is just not a true statement. The servos I have tested and work:
- HobbyKing Magnetic Induction
- HobbyKing Magnetic Induction Brushless
- Savox DS16
- KDS N300
- HobbyKing RotorStar RS-1253MG & 1255TG
In fact, I haven't found any that don't work yet, but I don't use analog servos at all.
What I was trying to say is that none of my servos works as good on the fixhawk as on the receiver.
And if i understand you wright you have the same experience:
This shows that there is a problem.. and coming back again to my "measurements", here we can see that something is wrong with the pass thru of the PWM.
Arducopter has no pass-through, so I can't really comment on that part.
But yes, the servo movement is not as smooth as native to Rx. But it's better than APM was. With Pixhawk, it's smooth, the difference is really just audible. It flies totally fine.
I have tested different servos. Cheap and very expensive servos and yes the cheap are more worse than the expensive servos.But there is no servo which works fine at the Fixhawk.
I have also tested the same servos at my wing. One was controlled by the APM2.5 and the other was connected to the Fixhawk, here it is difficult to see a difference with the eyes, but when I put a finger on the flaps I could feel that the Fixhawk servo runs more rough than the APM Servos?!
The other point that you can see at my measurements that the Fixhawk signal is not very nice..
APM2.5 gives 5v PWM.
Check you servo bus voltage, if higher than 5.0v that will increase problems with sensing 3.3v as logic high.
To increase the voltage, you can use 2 transistors, or a servo buffer, servo extender or whatever the readymade stuff is called.
you can also lower the servo line 5V - reducing it to 4.8V would make 3.3 clearly within the limits of logic high, and that's within servos operating voltage.
most application except for hard 3D flying, won't matter the small sped reduction between 5.0 and 4.8volts.
Hi,
thanks for your help! I have reduced the voltage but cannot see any difference. The cheap servos are still terrible and the expensive servos move still a bit rough.
I am not sure if the PWM Voltage is the root cause. My Frsky Receiver has 3.3V PWM too and directly connected servos move smooth?!
Do you have any experience with Hitec Hs65MG servos. I have intially planned to use these with the fixhawk in the skywalker..
HS65 are a bit more prone to electrical noise when fed with 3v3 PWM. But all HS65 I am using, are used two by two for redundancy, and they were purchased at least 2 years ago, so todays servos may be different.
To really compare servo outputs, voltage and frequency, you need a oscilloscope. - there may still be small differences.
What PWM voltage do those stuttering servos expect?
Get better servos, or amplify the PWM voltage.