Motor Comparison for Non-engineers

Thought I would take a shot at answering your questions in a straight forward manner.
Unfortunately that means I have to gloss over quite a bit. For the non-glossed over version, read through all the posts above :smiley:

2 NEOs give you more available power than 2CIMs. It could be described as “close” if you are comparing it to 2 MiniCIMs or something.

You are right, that is excessive. You are able to get more power out of 3 NEOs than 3 CIMs, but it probably isn’t making that big of a difference on the field. IFF you put 16 NEOs on each side of your drivetrain, you would be pulling more current than the FRC battery could provide and you would not be getting any benefit. Your robot might not even move! So there is a balance, but 2 NEOs per side is a pretty darn great amount.

Personally, if I want a high level comparison between motors, I look at their Peak Power rating, AND the power they deliver at 40A (like you posted). Are those two numbers the whole story? Definitely not, motors are dynamic and spin differently based on “where” they are on their operating curve, but it gives you a useful metric. Also keep in mind that while each motor is limited by a 40A (max) fuse, those motors can still pull greater than 40A for short periods. The fuses won’t trip until they are over 40A for a bit (you can look up the exact timings) so even the power at 40A number isn’t perfectly accurate.

Not really, no. You can hit peak power of some motors if the motor will only draw current a bit above the 40A limit (60A is probably fine), but the average team probably wouldn’t notice it. You may be able to see better acceleration, etc.

Nope.

It’s more than enough. For the past couple of years teams have been showing that 2 CIM drivetrains perform well enough. 254 and many other high power teams have been running that. I would guess that they will be switching to brushless motors though. Not so much for the extra power (though that is nice) but for the improved efficiency, lower weight, and improved packaging.

Depends on how big your reduction is. What will really matter is how many pulses you get per revolution of your output shaft. But to your point, putting the encoder on the output shaft is usually better anyway.

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Is 2 NEOs or Falcons enough for a drive train that performs well offensively and defensively.
It’s more than enough. For the past couple of years teams have been showing that 2 CIM drivetrains perform well enough. 254 and many other high power teams have been running that. I would guess that they will be switching to brushless motors though. Not so much for the extra power (though that is nice) but for the improved efficiency, lower weight, and improved packaging.

Are you referring to 2 NEOs/Falcons per side, or 2 total? I believe the OP was asking about 2 total. While 1 per side can certainly work, it’s going to put you at a disadvantage to teams running a 4 CIM drivetrain, and you would need to be careful with your gearing, even with the extra power and efficiency that the brushless motors provide.

I was actually meaning 2 per side. Sorry for the confusion.

Is there a drivetrain calculator with the Falcon data out yet?

Not that I know of. Although, you should be able to replace the data for another motor with the data for the falcon

@ambrose @Carlos1425

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I was referring to 2 per side, I think the OP was as well based on the other stuff in his post.

I’d be more inclined to believe that if it was the same price as a NEO setup OR the speed controllers were replaceable. The high price and integrated controller just come off a little anti-consumer for my tastes.

Don’t get me wrong, they’re definitely the best motors in FRC, but 95% of teams aren’t going to be able to use them effectively. I’d go as far as to say the same about NEOs. I feel sorry for the low-budget teams that will blow significant portions of their budgets on these because of the hype from community members. If you have the budget, go for it. But of you don’t, fear not. You can still have a competitive robot with NEOs or CIMs.

:man_facepalming:

image

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In that case, let’s not forget that the Nidec Dynamo brushless motor is the biggest thing in robotics since wheels.

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wasn’t the title of this thread "for non engineers " :laughing:

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Then, starting with the published parameters for each motor:

NEO Falcon
Stall current 105 257 Amps
Stall Torque 2.6 4.69 Nm
No load speed 5676 6380 rpm
594.39 668.11 rad/sec
No load current 1.8 1.5 Amps

Note that these parameters are defined with 12 Volts applied.

We should be able to reach the following parameters:

NEO Falcon
Km 0.0248 0.0182 Nm/Amp
Ra 0.1143 0.0467 Ohms
Ke 0.0198 0.0179 V-sec/rad
Bm 7.499E-05 4.097E-05 Nm-sec/rad

For the condition you were hypothesizing, 0.4 Nm continuous torque:

NEO Falcon
Holding Torque 0.4 0.4 Nm
Current 16.154 21.919 Amps
Power 29.822 22.433 Watts

Yes, the NEO is expected to drop greater power (~25%); however the NEO is expected to draw less current (~25%).

I trust the tested specs on motors.vex.com more than the REV nominal specs, only because the REV specs may be adjusted for the SPARK MAX and don’t have testing methodology published.

Exactly this. I vaguely remember it being mentioned that the Rev empirical data was with a 100A current limit applied. Using the VEX dyno data for both motors means you’re doing more of an apples-to-apples comparison assuming the testing methodology was the same.

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I trusted both suppliers equally.


Where do you think these differences spring from then? These are fairly dramatic.

I’ll point you to a DC motor model
http://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed&section=SystemModeling

And a graph.

Start with two terms to know: Stall and Free Speed.
*Stall - when the motor is at a stop, the shaft is not spinning. If you were to put the motor in a vise and hold the shaft still, apply power to the motor, the motor is at stall (or stalling). The stall torque is the amount of twisting motion the motor will provide. At stall the torque is at a maximum, and the speed is zero.
*Free speed - if you have nothing on the shaft (no load, nothing to slow its rotation), the free speed is how fast the motor’s shaft will turn. At free speed torque is zero.

The output power of the motor is the green line on the graph. If you multiply the torque of the motor by its shaft speed, you get output power.
The yellow line on the graph is motor torque versus speed. Notice the two intercepts on the graph are stall and free speed.

Notice that output power of the motor is zero at stall and free speed: the motor is doing no work when it isn’t moving, and it is doing no work when it’s not applying a force.

If you dig into the motor models (which I highly suggest you do), you’ll notice the motor has something called a back emf. The faster the motor spins, the higher the back emf, the less ability the motor can apply a torque.
You’ll also notice the motor has a torque constant. The more current you’re able to put into the motor, the more torque it will produce. For DC motors, these two constants are equal (this is cool!).

Back to the graph. The blue line is the current versus speed. Notice how it’s a line, a maximum at stall. If you take the motor constant and multiply it by the blue line, you’ll get the yellow line.

Last line on the graph: the red line. If you take the green line, and divide it by the power going into the motor (the blue line multiplied by 12 volts), the resulting curve is the red line. This is how efficient the motor is, how much output power you’re getting for your input power. The remaining power is converted to heat. This is important!

One more thing to know about the 1st graph. It assumes the motor is getting 12V across the wires going into the motor (this is a bad assumption [exercise for you - explain why]). It also assumes the motor is at ambient temperature (this is also a bad assumption).

Take a look at the “Locked Rotor Stall Test”. The rotor is the part of the motor that rotates. For this test, VEX kept the motor at stall for as long as they could.
See how the stall torque goes down over time? This is because all the energy going into the motor is being converted to heat. As the motor heats up, its internal resistance increases, causing the amount of current at the voltage to decrease. This continues until finally the motor burns itself up.

What are you to do with all this information?
Knowing this stuff allows you to better design your robot’s drive-train. You can start to make (better) assumptions about the conditions the motors will see during in the match and course of competition. In the case of a drive-train, you can try to optimize your design to the task (I need to drive 15 feet and stop as quickly as possible, then rotate, drive another 5 feet and stop as quickly as possible AND/OR I want to make sure the my drive-train is traction limited, that if I put any more power to the wheels they would slip instead of the robot accelerating faster, since anything more is wasted power).
Practically, what does it mean in terms of motor choices? You get more options to optimize from. If you repeat the same optimization exercise over and over for each motor, you can determine what’s best for your application.

TLDR: The peak power numbers you’ve cited is the motor operating at exactly ONE data point that maximizes power output. It doesn’t mean a whole lot for your robot.

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They’re not dramatic? the differences are less than ~12%. I would wager that’s within the sampling error between motors, and the noisy readings of the dyno instruments.

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I think that 166 amp stall is more than 12% off of the 105 amps used in phargo’s calculations.

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:man_shrugging: VEX’s Kt value for the REV NEO is 0.0202409 using a ‘down-up’ method of applying torque and

The “down” (brake applied) side is then averaged with the “up” (brake released) side.

VEX provides no further detail in what the tests entail including what RPM the dyno reduces speed to (or whether each motor is reduced to the same speed, same proportional speed, etc), if there is significant difference between “down” and “up”, etc. They also do not address the non-linearity produced by BLDC control (which is the Falcon’s default), even though their marketing is clearly aimed at teams who think their autonomous routines are held back by the same issues which top-level teams’ autonomous routines are.

Richard Wallace’s Kt value for the REV NEO is 0.0251 Nm per RMS Ampere using a very different methodology described in his post. There is also a further explanation 2 posts down about the non-linearity of BLDC control - definitely worth a read (but you may want to be an engineer :smiley: )

REV’s published data has a Kt value of 0.025 Nm per Amp, which is in line with Richard Wallace’s data and thereby more likely to have come from the same methodology. Here is the best-available description of REV’s testing.

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