How should the VESC "feel" on a board?

General topics and discussions about the VESC and its development.
hexakopter
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Location: Germany

Re: How should the VESC "feel" on a board?

Postby hexakopter » 10 May 2017, 19:39

benjamin wrote:Also, regarding the 160A measurement limit, it is true that any current above 160A will not be measured by the VESC. If you set the current limit that high you are just lucky if things don't fail, or your throttle does not provide the full range. In the new firmware I have added support for individual current limits for every hardware version, so trying to set them higher in VESC Tool will show a dialog that the current limits have been truncated and the truncated value. Both the VESC4 and VESC6 have this truncation at 120A for the motor and battery current, and 155A for the abs max current. I might increase these limits a bit after more testing, but currently I don't have motors that can take more than 120A without becoming too non-linear. For reference, the 168kv 6374 motors can take 100A quite fine. The 192kv version can probably take a bit more.


Hey Benjamin,

would it be possible that we BETA testers get the sources to change the 120A limit? When I remembered correctly the VESC 6.0 hardware should be able to measure around 300A in theory. I am using a motor with the current specs right now:
Max. Continuous Phase Current: 55A
Overheat in 10 minutes: 85A Phase Current
Overheat in 60 seconds: 242A Phase Current

With the VESC 4.12 the motor stays cool (not even hand warm), while the VESC reaches the temperature limit really quick. I use it with the settings of 130A max motor current and 50A max battery current right now and the 120A barrier would be to little for that motor.
When you don't want to discuss that in public feel free to contact me when you have time. I think you know my e-mail address. :lol:
Thanks a lot.

benjamin
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Re: How should the VESC "feel" on a board?

Postby benjamin » 10 May 2017, 20:26

There is actually a firmware included with limits disabled that is shown when the "show non-default firmwares" box is checked. The measurement limit is still 160A though and the FETs and traces can get problems handling too high currents, so there is a risk of damage when going beyond 120A. Also, the IRF7749 has slightly lower amp rating than the FETs on the VESC4 (140A at 100 degC), so I would not go higher than 140A.

When I have sent the test bench to trampa (it is more or less finished) I will spend my time trying to finish the vesc project website and prepare for the release, so the official release should not be far from now.

devin
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Re: How should the VESC "feel" on a board?

Postby devin » 10 May 2017, 21:48

benjamin wrote:What exactly are you trying to achieve? Was it not acceleration that feels even? I get the feeling that you are completely ignoring what I'm writing...


@Benjamin I learned something new from what you said about the current being recycled through the motor during the duty cycle off time, and also the operating frequency is a lot higher than I expected so thank you for clarifying these points! I learned a great deal.

To answer what am I trying to achieve:

The operating RPM range under it's own electrical power for a BLDC motor is from 0 RPM to no load RPM.

Considering No Load RPM as 100% RPM and 0 RPM as 0% RPM...

There are a range of RPM %'s that 10 battery amps @ 50V DC can be theoretically "fed" into the motor.

Above a certain % RPM, 10 battery amps @ 50V pack voltage is no longer possible because of Back EMF and therefore above a certain % RPM, 500 watts electrical is simply not possible.

Close to 100% RPM (No Load RPM), very close to 0 battery amps is the most battery amps possible to be "fed" into the motor, even full throttle.

From 0% RPM until a below a certain % RPM at which Duty Cycle = 95% @ 500W @ 50V Pack Volts electrical, it is possible to "feed" 10 battery amps or more into the motor, because it is not prohibited by Back EMF from 0rpm until this certain %rpm determined by kv and motor physics.

My goal is this:

During full throttle acceleration, all the way from as close as possible to 0rpm... all the way through the RPM at which Duty Cycle = 95% @ 500w electrical, I want to "feed" precisely 10 battery amps into motor whenever the throttle is fully depressed. I know fully that due to back EMF, above a certain RPM, 10 battery amps is not possible.

There are 2 AMP limits... this means NEITHER LIMIT CAN EVER BE EXCEEDED according to the programming.

If I want 10 battery amps, and 10 battery amps is 500w, then the motor amp limit must also be high enough that it permits at least 500w electrical at all possible rpms, including very close to 0rpm during full throttle acceleration from very close to standstill.

If the motor amp LIMIT when multiplied by the PWM effective motor voltage at 20 RPM is equivalent to ONLY 200w at 20 RPM, then the true battery amp limit at 20 RPM is equivalent to ONLY 200w / 50v pack V = ONLY 4a battery amps... ONLY 200w... not 10A battery amps and not enough for 500W at 20rpm because wattage is limited to 200w by the motor amp limit at 20rpm.

So to recap, my goal is 10 battery amps & 500w full throttle at all physically possible RPMS, but if the motor amp limit = 200w @ 20rpm, then at 20 rpm, the BATTERY LIMIT IS EFFECTIVELY 4A NOT 10A. If the battery limit is 4A, then my goal of 10A battery amps is certainly prevented at 20rpm and below, by a large margin.

Assuming a 0.0415ohm VESC detected motor ( 0.0415ohm x 2 = 0.0830ohm lead to lead ) and 50V Pure DC Supply:

10A Battery Amps = 500W / 50V DC <-battery amps LIMIT must be 10 amps

77.61A Motor Amps x 6.44V Effective PWM Volts = 500W <-motor amps LIMIT must be 77.61A motor amps to allow full 10A battery amps (500w) very close to 0rpm, full throttle

77.61A Motor Amps x 0.0830ohm Winding Resistance = 6.44V Effective PWM Volts

----------------------------------

10 Battery Amps x 50V DC =
500W =
(77.61A Motor Amps x 0.0830ohm Winding Resistance) x 77.61A Motor Amps [I^2R] =
77.61A Motor Amps x 6.44V Effective PWM Volts

--------------------------------
In simple terms it's physically possible to choose "10 battery amps" and "feed" exactly that into the motor at full throttle while accelerating from ~0% rpm until X% rpm @ 95% duty @ 500w... above X% rpm, less than 500w electrical and less than 10 battery amps is the most possible.

Simpler stated my goal is full throttle = 10 battery amps from the lowest possible rpm, through the highest possible rpm allowed by Back EMF, but no more than 10 battery amps at any RPM full throttle.

To do this I think it would be necessary to use the following amp LIMITS:

10A/77.61A/100A battery / motor / absolute amp LIMIT settings

assuming 0.0830ohm winding lead-to lead, 50v dc supply, 10 battery amps and therefore 500w electrical desired at full throttle for all physically possible rpms (0%RPM-X%RPM) not prevented by back emf.

Hummie
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Re: How should the VESC "feel" on a board?

Postby Hummie » 11 May 2017, 00:57

with .13 motor winding resistance and max speed still being likely only 80% of the no-load speed I don't think you'd have a problem getting 10 amps as the back voltage and front only need a 1.5 voltage difference to get it. Maybe it's not as simple as ohms law though.

Sadly the new 6 is then less able to put out low speed torque then it seems. I think hub motors are hugely more effected by this, or at least hub motors w high kv and top speed. Pulley setups have no problem flooring you from standstill but hubs not so much

JTAG
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Re: How should the VESC "feel" on a board?

Postby JTAG » 11 May 2017, 02:35

Both of you are to ignorant for this forum, please consider going back to Jasons **ertion club :geek: . STOP WASTING OUR TIME.

rew
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Re: How should the VESC "feel" on a board?

Postby rew » 11 May 2017, 06:34

devin wrote:
benjamin wrote:If I want 10 battery amps, and 10 battery amps is 500w, then the motor amp limit must also be high enough that it permits at least 500w electrical at all possible rpms, including very close to 0rpm during full throttle acceleration from very close to standstill.
Just set your battery amp limit to 10. Then, in theory the VESC will try to push "infinite AMPS" through the motor at 0RPM. To keep the VESC in one piece you need to set a motor (=VESC) current limit too. (the VESC heats up proportional to the MOTOR current, not the battery current).

So if, in this case you'd set the motor current limit to 50A, then from zero to about 20% RPM (10V effective) the motor will run in a constant torque mode at a constant 50A. From then on it will run in hte constant-power mode you want.

Note that if you weigh 100kg (with vehicle), and you have "average" tires, you'll be able to provide about 1000N of non-vertical force using the wheels. Now at 1m/s, that equates to 1000W. This is linear with the speed. So, below 0.5m/s, 500W would spin the wheels.

(The friction coefficient of 1 is quite normal. racing tires can easily go over 2 or 3. But it does mean that you need 'all-wheel-drive' to achieve the calculated acceleration....)

arvidb
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Re: How should the VESC "feel" on a board?

Postby arvidb » 11 May 2017, 09:19

JTAG wrote:Both of you are to ignorant for this forum, please consider going back to Jasons **ertion club :geek: . STOP WASTING OUR TIME.

There's a very useful ignore function on the forum: click the user name you want to ignore to get to the user's profile, then click "Add foe". Now you no longer have to see that user's posts! I've actually never felt the need to use this before on any forum, but today I added my second "foe" here on this board. Strange that they seem to be aquainted - could it be they are one and the same? :) Whatever, both are now ignored.

devin
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Location: San Francisco, California, US

Re: How should the VESC "feel" on a board?

Postby devin » 11 May 2017, 11:25

rew wrote:Just set your battery amp limit to 10. Then, in theory the VESC will try to push "infinite AMPS" through the motor at 0RPM. To keep the VESC in one piece you need to set a motor (=VESC) current limit too. (the VESC heats up proportional to the MOTOR current, not the battery current).

So if, in this case you'd set the motor current limit to 50A, then from zero to about 20% RPM (10V effective) the motor will run in a constant torque mode at a constant 50A. From then on it will run in hte constant-power mode you want.

Note that if you weigh 100kg (with vehicle), and you have "average" tires, you'll be able to provide about 1000N of non-vertical force using the wheels. Now at 1m/s, that equates to 1000W. This is linear with the speed. So, below 0.5m/s, 500W would spin the wheels.

(The friction coefficient of 1 is quite normal. racing tires can easily go over 2 or 3. But it does mean that you need 'all-wheel-drive' to achieve the calculated acceleration....)


@rew thanks for this thoughtful response!

the reason @hummie and I need very high startup torque as close as possible to standstill is we are designing skateboard hub motors for use on san francisco hills.

any time the rider "starts out" from a standstill pointing up hill, high torque is needed almost from 0 rpm with the board loaded just to get the rider moving.

if the motor amp limit is set too low, and electrical wattage close to 0rpm is too limited, then the motor simply stutters instead of pushing the rider anywhere.

so the key for getting enough startup torque for hub motor skateboards in san francisco is setting the motor amp limit high enough to allow the startup electrical wattage we need to start rolling up the hill we want to climb.

simply high mechanical load close to 0rpm calls for high motor amp limit setting close to 0rpm.

another key feature is using the proper ratio between motor amps and battery amps, the acceleration electrical wattage remains constant across the range of rpms during acceleration, which makes the acceleration "feel even" even though it isn't even due to wind drag, etc.

the interesting part is for acceleration, a person could choose "500w" "cruising mode" or "1000w" "power mode" or "1500w" "insane mode" or whatever wattage they choose, up to the electrical limits of the vesc by choosing the proper batt/motor limits for acceleration.

Image

markorman
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Re: How should the VESC "feel" on a board?

Postby markorman » 11 May 2017, 13:54

devin wrote:the reason @hummie and I need very high startup torque as close as possible to standstill is we are designing skateboard hub motors for use on san francisco hills.

any time the rider "starts out" from a standstill pointing up hill, high torque is needed almost from 0 rpm with the board loaded just to get the rider moving.


Torque is defined by motor current, rpms doesn't matter here.

if the motor amp limit is set too low, and electrical wattage close to 0rpm is too limited, then the motor simply stutters instead of pushing the rider anywhere.


Motor wattage has nothing to do with torque or acceleration produced by motor. If your board doesn't start moving, you need more torque - more motor current.

so the key for getting enough startup torque for hub motor skateboards in san francisco is setting the motor amp limit high enough to allow the startup electrical wattage we need to start rolling up the hill we want to climb.


You almost got it. Yes you need enough motor current to produce required torque. Again power has nothing to with the acceleration, it will only limit the maximum speed.

Maybe you will understand with this:
Power is at what speed you will hit the wall,
Torque is how far you will push the wall.

Hummie
Posts: 110
Joined: 10 May 2016, 04:05

Re: How should the VESC "feel" on a board?

Postby Hummie » 11 May 2017, 14:31

Nope it's me solo. if my nubiiness is too much for you block me but the original poster asked a question which is valuable to me and Devin and anyone who wants low speed torque and it didn't seem fully answered. The startup boost is said to jump the early duty cycle and be too abrupt, far from smooth, and the motor amp limit and what's possible with it seems hugely important. So many people have asked this same question.
Last edited by Hummie on 11 May 2017, 14:55, edited 1 time in total.


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