RC Groups

RC Groups
    Multirotor Talk
        Discussion ESC update frequency

#1 milo12 Sep 12, 2011 05:48 PM

ESC update frequency
 
1 Attachment(s)
I have seen it stated many times that 400 Hz is a desirable update frequency and that 200 Hz is borderline unstable.

Can someone point me to testing that can support this? I am skeptical because of the basic physics of accelerating and decelerating the air mass out of the propeller. I doubt you can influence it that quickly, after all a propeller is not a speaker.

For example a 6000 rpm prop is spinning at 100 Hz. With a 400Hz update you are trying to change the rpm every 1/4 revolution. I find it hard to believe that any significant acceleration or deceleration can happen in 1/4 revolution, 2.5 milliseconds.

A typical prop weighs 20 - 25 grams. It is spinning at 6000 rpm therefore it has a certain angular momentum and it seems hard to believe it can slow down or speed up any significant amount in 1/4 turn. After all the prop has to change it's velocity enough to change the amount of air it is pushing down. This means the thrust needs to change. How much? I don't know.

Here is a plot from my eagle tree logger collected at 50 Hz. That is not fast enough to capture 200+ hz activity but it shows very little prop rpm change over this 1/2 second. An interesting bit is the watts shows a lot of activity over the same period. I don't know if that is noise/ ripples in the power or what.

#2 jes1111 Sep 13, 2011 10:40 AM

Not a popular question, then! Amazing how the really important stuff can get marked as "uninteresting", isn't it? ;)

I can't explain the technical fine points of this, but apparently it makes a considerable difference to the responsiveness of a multirotor. Remember that similar improvements can be seen with specialised gyro/servo sets for heli tails. I'd say that, as with any control system, your signalling rate must exceed the response capability of your actuator in order to get the best possible result from that actuator.

However, most current ESCs filter/smooth their input signal, adding a non-configurable ramp/delay in throttle response. That's why OpenPilot is designing their own ESC which will be optimised to the task.

See: http://wiki.openpilot.org/display/Doc/TurboPWM for some (brief) info about OpenPilot's 400Hz PWM capability.

#3 jesolins Sep 13, 2011 11:43 AM

Yes indeed a critical specification for stable multicopter performance. I agree the physics of it is like measuring with a micrometer(gyros and accelerometers) and marking with a grease pencil(FCBs) than cutting with an axe(ESCs/motors/props);) Most agree decent multicopter performance starts at about 200hz ESC update frequency. The known good performing PWM ESCs for multicopters are pretty well known now along with the poor performers. The I2c ESCs have always been the quick answer for faster is better. The MK update is about 500hz and the XBLCs of the famous lab dancing quads are updated at 1000hz. So despite the irony of the physics, faster does seem to perform better on multicopters.

Now if someone could only follow-up on dynamic braking ESCs to make the response even more accurate we would probably be seeing another performance quality jump. Airbotix had started on that path here: http://www.rcgroups.com/forums/showthread.php?t=958957

You might also ask some of the bright Gents like Quax and SimonK and Timecop and others who frequent the ESC conversion thread: http://www.rcgroups.com/forums/showthread.php?t=766589

Kaptienkuk had some good ESC update frequency info here: http://www.rcgroups.com/forums/showt...date+frequency

Cheers,
Jim
Quadrocopter and Tricopter Mega Link Index

#4 Justin Stuart Sep 24, 2011 04:04 PM

Quote:

The I2c ESCs have always been the quick answer for faster is better. The MK update is about 500hz and the XBLCs of the famous lab dancing quads are updated at 1000hz.
Who makes an ESC that refreshes at 1000Hz?

#5 witespy Sep 24, 2011 04:16 PM

http://i1143.photobucket.com/albums/...w/IMAG3645.jpg

too bad they killed the hobby line :(

my first real quad, next to the TT board on the shelf

#6 milo12 Sep 24, 2011 04:56 PM

Is this the same? It looks like it is still for sale.

http://translate.google.com/translat...5%2526&act=url

#7 jesolins Sep 24, 2011 05:30 PM

Hi Justin,
The Ascending Technologies/Astec X-BL/Funpilot/Falcon, etc and X-BLCs have 1000hz update rate. Those are Astec quads you see flying in the laboratory doing amazing control movements. Of course they are controlled in those videos by a separate computer that uses IR sensing and positioning mapping algorithms to the tune of about $500K+.
Cheers,
Jim
Quadrocopter and Tricopter Mega Link Index

Quote:

Originally Posted by Justin Stuart (Post 19410437)
Who makes an ESC that refreshes at 1000Hz?


#8 Justin Stuart Sep 24, 2011 07:09 PM

Does the open pilot (Copter Control) have the ability to make use of a 1000Hz refresh rate on the ESCs?

#9 milo12 Sep 24, 2011 07:53 PM

To add to Justin's question - What controllers can update at 400hz or greater?

#10 timecop Sep 24, 2011 08:59 PM

Nothing public or cheap.

#11 Justin Stuart Sep 24, 2011 09:41 PM

What update frequency do the most common control boards max out at?

#12 SeismicCWave Sep 25, 2011 02:11 AM

Quote:

Originally Posted by Justin Stuart (Post 19411655)
Does the open pilot (Copter Control) have the ability to make use of a 1000Hz refresh rate on the ESCs?

No, as of right now 400 Hz is the max.

#13 XsenseX Sep 25, 2011 02:42 AM

Whats Turnigy plush ESC refresh rate anyways, didn't see any write up on that.

#14 timecop Sep 25, 2011 03:19 AM

Quote:

Originally Posted by XsenseX (Post 19414267)
Whats Turnigy plush ESC refresh rate anyways, didn't see any write up on that.

Doesnt matter; it takes up to 150ms to reach a given throttle setpoint, its artificially ramped/slowed.

#15 Justin Stuart Sep 25, 2011 08:26 AM

I keep seeing ads appearing at the top of the screen for a Maytech 30A ESC with a 400Hz refresh rate for $30. Is this about as good as we're going to get for right now? It seems that what they are using in the UPenn GRASP lab is pretty much out of reach for any reasonable price.

http://montorc.com/MT30ABEC.aspx


All times are GMT -5. The time now is 04:03 PM.