Brushed PWM rate and hidden thrust


#41

The frames I use are around 3g each and available at the link below. Scroll to the bottom and go to page 3 and 4. The best ones now are the MFC793, MFC893, and MFC8100 because they are designed to fit all the popular FCs currently in the market including the Aliens.
http://www.armattanproductions.com/pages/shop_product_grid/2223


#42

Nice! Great info!


#43

I think this video is relevant to our PWM rate discussion so I’ll post it up here for reference. Window 1 in this video was a different setup for my own comparison - but windows 2,3,&4 are identical configurations a three different pwm rates. We know it makes more static thrust - but 100% throttle in flight beyond the “static” moment of generating initial lift seems to be unaffected by pwm rate. That said, I do like the way lower rates recover from flips better even though they sound awful! Lol


#44

100% throttle shouldn’t have any PWM going on in the first place really, should just be solid ON to the motors.
I’m not sure how to test partial throttle, maybe mapping throttle to a switch or setting the range on the throttle stick in the TX?


#45

Agreed, I find it interesting that on the bench tests it produces more static thrust though. Must be something with moments before it gets to 100% helping it ramp up. You can feel it for sure at part throttle but in practice it does not affect a full throttle vertical climb.


#46

That makes sense, that transition time is always longer than I expect.
If you don’t get full 100% throttle that’d do it too.
Actually now that I think about it now I doubt level mode gives you full bore no-pwm-happening, the FC wouldn’t have any control.
That’s what I get for replying right after waking up.
Acro mode might get true full throttle though. Sounds like it’s time for me to make a little tiny PWM rate/% logger and send it up.

Or more likely it’s time for me to go to work.


#47

That’s a really interesting thought. My vertical drag races were in angle mode though and it didn’t seem to make a difference on that. I think lower pwm rates get you to full throttle quicker like when recovering from a flip and may provide a little extra thrust for a split second on a climb - but over longer distances like up to the top of a light post - it’s really insignificant. For indoor acro with a confined space - it really feels like it helps. I’m gonna start climbing my pwm up from 400 and see where the horrible noise starts to go away. While this seems to make make a big feel difference in an underpowered whoop - I still have not tried with an 8.5mm craft yet.


#48

Hey guys,
not to revive a dead topic but I had some more thoughts on the PWM on brushless motors; and if you see my other post this is some of what I would like to start researching and trying to optimize in the whoops design.

This is a good start to the issue, the fets do have an on and off time and depending on the drivers we can optimize this for faster speeds. ie. applying a larger voltage to the gate will cause it to change states faster, usually the datasheet will give a range of switching times based on gate source voltage.

the other part of the equation that I think is really the culprit is that an motor is an inductive load, therefore it resists changes in current. my guess is that whats actually happening is the PWM is switching so fast that the voltage no longer has a chance to fully ramp up. I think what I’d like to do is measure the inductance and resistance in different “Kv” motors and find the highest PWM that a certain motor could take and still get to the maximum voltage.


#49

So, I’ve been working on building a simulation model to do some PWM analysis. I’m not sure that I fully trust what I have so I thought I would post what I’ve been working on and where I want to go next.

I found these links on creating a simulation model in SPICE.
http://support.ctc-control.com/customer/elearning/younkin/motorParameters.pdf

http://www.ecircuitcenter.com/Circuits/dc_motor_model/DCmotor_model.htm

Right now when I’ve been trying to watch the velocity output at different PWM values at about 50% throttle (its easy to do a 50% duty cycle). I notice that they all ramp up with about the same slope and level off at about the same value. So in an ideal world maybe there is no difference.

My next step is going to be finishing up the mechanical portion of the model, I need to get an actual value for motor inertia, so I’m going to try and think about building some kind of test rig to measure acceleration and torque from the motor.

Once I finish the motor model I want to start using real world mosfets to deliver power to the motor and see how the change in series resistance and non ideal risetimes effects the motor output potential at different PWM frequencies.

I’ve attached the LTSpice model I’m currently working with if anyone would like to try and play with it.
LTSpiceMotor.zip (676 Bytes)


#50

I anticipate what you’ll find is that higher pwm rates attenuate partial throttle values somewhat. Intuitively 100% duty cycle is the same regardless of pwm rate, as is 0%. So the difference can only be in between. It will be interesting if we can attribute the bulk of this mid throttle attenuation of power delivery to an efficiency loss from switching in the mosfet, or a physical response of the motor. In the end, it’s kind of like a throttle expo… so it comes down to a “feel” that you like.


#51

That’s something that I wasn’t sure about, I couldn’t remember if you could ever get to 100% uptime in the the PWM. Even when I was looking at chips to simulate the PWM you can get them that can go from 0 - 100% and ones that only go from 5 - 95%.

it’s not only going to be the feel, I’m interested to see about the ability for the current to ramp up when the switch time is realistic and also the RDS on resistance. I looked really quick at the mosfets one board was using (maybe the inductrix) and the on resistance was really high.

another thought, more to your point, if we can change the “feel” via the PWM rate, can we change the number of prop blades and get different thrust values and being able to normalize the feel with the PWM rates.