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Old Jun 11, 2010, 07:58 PM
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Sensored Brushless Motors and ESC's: How do they work?

I'm trying to do a little R&D work for a Motor In Wheel crawler (I know not an aircraft but question applies to brushless in general) i'm going to attempt to build while i'm in a machining class this winter. So theres the some background info.

As of now i'm just trying to get everything worked out, so far i've got a basic idea of everything. Now i'm trying to work out details like the stall in between each wheel which will act like open diffs. Not very good for crawlers.

So after a little research i found that a sensored brushless motors could be a good candidate to eliminate the stall. I've read that the sensors in a sensored brushless motor actually will sensor speed that the output shaft is spinning.

So for you motor gurus could you please explain to me how the esc determines the power going to the motor and if it goes off the speed of the motor or is it another variable that determines. If it is another variable could it be used to eliminate the stall?

Is there a way to directly relate the speed of the motor with the throttle position on my radio: For example; 25% throttle gives me 25% of speed that the motor is capable of even under a torque load
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Old Jun 12, 2010, 06:05 PM
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You have much to learn, Grasshopper.

An esc does not control the amount of power going to the motor, it only controls the speed at which the motor spins. Therefore, it has no need to determine the amount of power going to the motor. The only information it needs from the motor is the relative position of the rotor with respect to the stator so it knows which stator coils to energize at any given time. On a sensored system this feedback is supplied by hall effect sensors the output voltage of which is proportional to the strength of the magnetic field in its vicinity. On a sensorless system the feedback is supplied by a sine wave shaped voltage that is induced into the unenergized winding by the rotating magnetic field of the rotor.

The advantage of the sensorless system is that the motor does not have to be spinning for the esc to know ther rotor position. The relative outputs of the three hall sensors supply that data. Therefore the motor speed can accelerate smoothly from 0 rpm to the rpm dictated by the esc output voltage. The sensorless motor, however, must be spinning at a minimal speed for the magnetic field to induce a voltage into the unenergized winding that is high enough for the esc to sense. The result is that the sensorless motor will stutter and not start rotating smoothly until that minimum speed is met. This is usually at some throttle position a few clicks above minimum and it will vary slightly from motor to motor.

I'm not quite sure, but I assume when you are talking about stall between each wheel that you are using a separate motor to drive each wheel. If such is the case then for the reasons I stated above, when starting from 0 rpm you will not be able to get each wheel to start spinning at exactly the same time with a sensorless system.

As to the motor speed with relation to throttle position. The motor will attempt to spin at the same speed for a given throttle position, but the actual speed will vary with the load on the motor. With a crawler that load is quite variable depending on the terrain and obstacles.

Larry
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Old Jun 12, 2010, 10:49 PM
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Ok, so is there a way to guarantee that the motor will spin at the speed its told to even under a load
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Old Jun 13, 2010, 10:39 AM
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The simple answer is no. A motor will always attempt to spin at an RPM that is equal to the kv of the motor times the voltage appllied to the motor. At the same time, any load on the motor will try to slow it down. As a result the motor will draw more current in an attempt to overcome the the reduced RPM caused by the load. These opposing forces will eventually stabilize with a lower RPM and a higher current draw than the no load condition. The higher the load, the lower the RPM and the higher the current draw.

Larry
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Old Jun 13, 2010, 11:39 AM
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ok, next question do you think there is a way to eliminate or greatly reduce the stall inbetween each motor. Compared to a high $ handwound brushed motor to a sensored brushless what would be the difference inbetween stall due to loads. Which one would have less
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Old Jun 13, 2010, 12:38 PM
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Just a guess, but I would say six of one and half a dozen of the other. The brused motor might have a little bit of an edge.

Just what is your planned set up? Are you using a motor for each wheel or are you using one motor to drive both front wheels and another motor to drive both back wheels? Too me the optimum set up would be a separate motor for each axle using locked diffs. That way you wouldn't have to worry about stall between motors. Each motor would draw the current it needed to drive its respective axle. A small difference in speed wouldn't amount to anything. The best thing about the sensored motors is the smooth start up and operation at low crawling speeds.

By the way, you may be interested to know that I am part owner of Moab 4X4 Outpost. I am familiar with crawling in both full size and model size.

Larry
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Old Jun 13, 2010, 01:50 PM
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No its a motor in wheel so a motor in each wheel. so 4 motors. I have a MOA Motor On Axle and you get stall there but it isn't that big of a deal but i'm sure having stall in each wheel is going to be a problem.

Cool, i live in Grand Junction, CO and you guys just opened up a M4O down herewith Andy i think??? I've heard a something about you guys opening a RC Crawler store, is this true?
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Old Jun 13, 2010, 05:20 PM
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Yes, Andy Muhr runs the Junction store. We've had a few RC crawlers and parts but we haven't restocked and are essentially sold out. The full size vehicles keep us busy enough.

Larry
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Old Jun 13, 2010, 05:28 PM
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I knew it was andy something, my dad has an axle down there getting 4 link mounts setup on it i think. You guys would make some money having like a rc crawler shop and carry the stuff that most hobby shops don't carry like rc4wd stuff for ex.

The whole idea about the MIW setup is to have maximum axle clearance and belly clearance since links would be needed
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Old Jun 14, 2010, 08:56 AM
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Not familiar with MIW. Maybe motor-in-wheel? Anyway, sounds pretty complicated to me. Individual gearing in each wheel plus four esc's if you go with brushless. With a set up like that why even use an axle? Go with independent suspension. Seems like that would give you maximum ground clearance.

Last time I was at the Junction store there was a full size crawler with independent suspension. You might want to check it out. Actually, it's for sale. You might even want to buy it

Larry
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Old Jun 14, 2010, 02:11 PM
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Yea motor in wheel as said in first post, Axle wise i'll have basically a straight rod screwed to the motor mounts going vertically up giving as much axle clearance as needed. Might go the independent suspension but then i'd have to redesign the how everything works. As of now it will be more of a torsion chassis the way i have it set up now. 4 wheel steer, 4 independent driven wheels, Worm gear drive train, I've also been considering some how to do like an axle flip so if i turtle my\self a flip of a switch will pivot the axle assembly upside down. But as of right now none of this will happen unless i figure out my electronics problem, the radio isn't a problem cause i can build a custom one, but i'm not sure motor/esc wise

Well i'd like to buy a full size crawler, but me being 15 there isn't no way i could afford it, I've got a 1990 Toyota right now, here in a year or two I plan on converting it to a straight axle and a few other things to where it can actually crawl a little but thats one i get some $ saved up.
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Old Jun 14, 2010, 06:10 PM
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I know what you're saying. When I was 15 I couldn't even afford night crawlers.

Sounds like an interesting project. Hope you get everything worked out OK.

Larry
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Old Jun 18, 2010, 08:16 AM
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Quote:
Originally Posted by Lnagel View Post
The simple answer is no. A motor will always attempt to spin at an RPM that is equal to the kv of the motor times the voltage appllied to the motor. At the same time, any load on the motor will try to slow it down. As a result the motor will draw more current in an attempt to overcome the the reduced RPM caused by the load. These opposing forces will eventually stabilize with a lower RPM and a higher current draw than the no load condition. The higher the load, the lower the RPM and the higher the current draw.

Larry
Not true at all...a good esc (sensored) with a double loop double PI regulator (speed and current) can keep the speed costant to the reference speed until the power requested (speed*torque) is bigger than the power the motor can supply.
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Old Jun 18, 2010, 11:04 AM
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Cool, so it is possible, what would the chances be that one is ever made, also how hard would it be to build one with general electrical knowledge (don't have to be super small)
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Old Jul 11, 2010, 11:20 PM
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Ok well I've been doing a little research and i haven't actually found a esc that has a double loop pi regulator, but i came across this and i think this is what i need except its software and it gives me some diagrams of hardware. Here's the link http://www.powersimtech.com/manual/T...p%20Design.pdf is this something i should look more into, look at the last page i think this is proving what i want to do
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