FOC poor efficiency at high speed

Discussions regarding the Field Oriented Control (FOC) implementation on the VESC.
protontim
Posts: 7
Joined: 26 Sep 2017, 18:50

FOC poor efficiency at high speed

Postby protontim » 26 Sep 2017, 19:15

I have a Tiger U8 motor 100Kv (21 pole pairs) with a custom 4 bladed propeller that I am driving with a VESC 4.12 at 12S. With a "standard" configuration I could only run the system to about 30% duty cycle (27000 erpm) before the motor would squeal and the current would go way up. In order to get it to run at the higher speeds, I had to increase my observer gain way up from ~120 to 2000, and to make it stable at low speeds I reduced the observer gain to 5% at minimum duty. This seems to work, but when I run to ~60% duty cycle I am drawing 13.7A, but when running in BLDC mode at 60% duty cycle I draw around 9.3A. And as I increase speed from there FOC current goes so high that it goes into current overload (~24A) well before it reaches 95% duty cycle. The BLDC mode goes to 95% duty cycle no problem (BLDC draws about 17.2A at full throttle and spins at 66000 eRPM) . Any ideas what might be going on here and how to fix it?

pf26
Posts: 296
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: FOC poor efficiency at high speed

Postby pf26 » 27 Sep 2017, 09:21

Playing with relatively low currents at high speed is difficult for the VESC. I fear you need to change the shunts to higher values like 5mOhms if you want reliable high speed FOC. If you have a look at the current samples using BLDC tools, you probably will see it isn't very clean. Higher loads would help by increasing currents and making it easier for the algorythm to determine precisely the position at all time.

protontim
Posts: 7
Joined: 26 Sep 2017, 18:50

Re: FOC poor efficiency at high speed

Postby protontim » 27 Sep 2017, 14:03

Below is the current plot. As you can see the system starts to go unstable around 60% throttle. Does this look like higher value shunts would help? If so, what setting do I need to change in the VESC to indicate a different shunt value? Really appreciate the help!
VESC4 FOC Current.JPG
VESC4 FOC Current.JPG (102.53 KiB) Viewed 1603 times

ThierryGTLTS
Posts: 86
Joined: 09 Aug 2017, 11:10

Re: FOC poor efficiency at high speed

Postby ThierryGTLTS » 27 Sep 2017, 15:21

Same motor, same problems but can reach 42000erpm with very light load.

I have received my 3 mohms shunt, and I hope to test them tomorrow.

Stay tuned.

pf26
Posts: 296
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: FOC poor efficiency at high speed

Postby pf26 » 28 Sep 2017, 06:45

@ protontim: I was meaning to look at the "sampled data" VESC tool graphs to acquire at a faster rate, and visualize the current samples "sinewaves". Changing shunt requires more than a different setting, it requires a new firmware.

protontim
Posts: 7
Joined: 26 Sep 2017, 18:50

Re: FOC poor efficiency at high speed

Postby protontim » 28 Sep 2017, 23:01

@pf26 I checked the phase currents and they indeed look very jagged. I am going to order some different value shunts to play with. I will also work on setting up the STMicro tools to compile the complete project. Is it pretty straight forward to change the shunt value in the code? Is that documented somewhere? Thanks for your help!

pf26
Posts: 296
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: FOC poor efficiency at high speed

Postby pf26 » 29 Sep 2017, 10:21

In conf_general.h, we should uncomment and change the line like this :
#define CURRENT_SHUNT_RES 0.003 // 3 mOhms shunt
Or maybe someone can send you an already compiled version like this. (I have many other changes in the code, so I better no send you mine)

protontim
Posts: 7
Joined: 26 Sep 2017, 18:50

Re: FOC poor efficiency at high speed

Postby protontim » 30 Sep 2017, 16:10

@pf26 - I loaded Chibistudio and was able to compile and load the latest code. I have 0.003, 0.004 and 0.005 shunts on order so I should be all set. Thanks for your help.

@ThierryGTLTS - Were you able to test your U8 motor with the higher value shunts? Any luck?

ThierryGTLTS
Posts: 86
Joined: 09 Aug 2017, 11:10

Re: FOC poor efficiency at high speed

Postby ThierryGTLTS » 30 Sep 2017, 16:59

Yes I've soldered the shunts.

But I've problem because the modified firmware for 3 mohms doesn't work.

I have to inquire why :!: :!:

Have a Nice Day.

Thierry

rew
Posts: 940
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: FOC poor efficiency at high speed

Postby rew » 01 Oct 2017, 09:10

yes!


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