Torque Calculation/Determination-Urgent

[BBray_T1296]

A "torque-ier" drivetrain will have greater acceleration. Your robot will reach its maximum speed faster with more torque. however, as Torque and RPM are inversely related, the more torque you have, generally the slower your top speed. That is, unless you add more motors, increasing the resource pool of your drivetrain in a different way.

Think of it like a car transmission.
You wouldn't want to accelerate out of a stoplight while in 4th gear, because 4th gear is designed to give you a high speed, at the expense of low torque. It would take all day to get up to 100MPH starting in 4th.
1st gear has a large ratio, supplying the torque you need to break static friction and get the car rolling from a standstill. However, due to the ratio, 1st gear has a very low top speed and could only carry your vehicle to maybe 15MPH before reaching dangerous engine RPMs. In 1st gear you would never reach 100MPH within the limits of the system. You shift from 1st into 2nd, then 3rd, then finally 4th, to reach your top speed in efficient time*. A Continuously Variable Transmission (CVT) is most ideal, because you can have an infinite number of ratios to maximize efficiency*.

Torque is also why a semi truck is powered by a 12 cylinder engine instead of a 4 cylinder. While both operate at similar RPMs, a more powerful motor will output more torque. Torque allows the massive, heavy truck to haul a loaded trailer at the same speed as a Prius. Force=mass*acceleration. A truck uses its force (torque=force at along a lever) to accelerate a great mass.
Alternatively, a Veyron uses the torque from its 16 cylinders to accelerate the relatively light vehicle quickly and achieve blisteringly high speeds.

Basically, more torque=more acceleration. Too much output torque, you compromise top speed. Too little output torque, you compromise acceleration. You could direct drive your robot with CIMs, but it would take you months to reach 5000 RPM (if you could move ever at all). You could gear your robot 5000:1, and you would have an ungodly amount of torque, but you could only rotate your wheel once every minute! Of course these are both exaggerations, but there is a careful balance between getting enough top speed, and being able to practically reach that top speed on a 54' field

*When FRC teams use shifting transmissions, they are not using them to improve acceleration, but to allow a selection of high torque or high speed, to deal with match conditions. Depending on the gearbox, acceleration to top speed is only improved by a small fraction of a second when shifting like a car. Their high speed gear may not have the torque to push another robot, so they shift low to assist with dealing with opponents.

[gpetilli]

I am bumping this thread since much of the motor basics are covered well and we hope to extend the basic concepts. Please excuse the thread hijacking.

For our off season project this year, we are considering experimenting with a poor man's Continuously Variable Transmission (CVT) gearbox. The basic concept is to use two motors but to intentionally mismatch free speed by about 2x. The 1st motor would be geared for high torque and would reach its free speed when the wheel was at 1/2 its top speed (lets call this the shift RPM). The 2nd motor would be geared for top speed. The PWM commands to the ESC for the 1st motor would be twice the value of the 2nd motor up to the shift RPM. The 2nd motor will continue increasing PWM values up to full voltage at full speed past the shift RPM. We will have one of these gearboxes on each of four wheels.

We understand that past the shift RPM the 1st motor will act as a generator and load the 2nd motor. The question is how severe is this load? What if we set the ESC to coast and signal the PWM to zero above the shift RPM? Will the flyback diodes in the 1st ESC effectively still load the 2nd motor? Will this damage the 1st ESC (we have blown HBridge with flywheel shooters)? Is it better to continue to drive the 1st ESC to max voltage?

More specifically, what we are considering is:

1st motor: RS775 12:60 (32dp) 14:72 (24dp) direct drive 6" wheel ESC=coast
2nd motor: MiniCIM 12:72 (24dp) same 72T gear ESC=brake

So the combined available stall torque (before gearing) is about the same as a full size CIM. During a pushing match we would drive all motors at full voltage.

We have looked at JVNs motor combiner xls - it predicts favorable results, but does not solve this exact configuration. It appears to assume matched free speed and therefor does not appear to comprehend the 1st motor loading the 2nd motor.

[Monochron]

These guys pretty much have everything covered. I'll just drop this handy reference sheet here which has a couple of equations for calculating maximum torque and other specs:
Motor reference sheet

Maybe you team can get some use out of it.

Edit: (Also, didn't realize this post was 5 months old . . .)

[gpetilli]

Originally Posted by Monochron

These guys pretty much have everything covered. I'll just drop this handy reference sheet here which has a couple of equations for calculating maximum torque and other specs:
Motor reference sheet

Maybe you team can get some use out of it.

Edit: (Also, didn't realize this post was 5 months old . . .)

Thanks for the quick response; it is important to understand the raw motor performance in designing/choosing a gearbox. We have been using a similar table published by JVN.

The task at hand is to combine two dissimilar motors from the table to create a new, wider range super motor.

[Ether]

Originally Posted by gpetilli

During a pushing match we would drive all motors at full voltage.

Has anyone ever run a test:

How long can you stall a 775 at full voltage before the factory-installed smoke escapes?

[gpetilli]

Originally Posted by Ether

Has anyone ever run a test:

How long can you stall a 775 at full voltage before the factory-installed smoke escapes?

I cant find it now, but I believe I saw 30Amps for over a minute. Keep in mind that the RS775 stall current is rated at 18V as 130A. If we design for enough torque to slip wheels when pushing against a wall, the spreadsheet predicts 45A and moderate motor RPM (internal fan running). I dont think we will violate any non-smoking laws, but thats why we are trying this off season.

Any thoughts on back driving the "generator" if the ESC is in coast mode?

[Ether]

Originally Posted by gpetilli

I cant find it now, but I believe I saw 30Amps for over a minute.

Are you implying you're planning on using a 775-12 for this application???

The 775-12 draws 30 amps stalled at 12 volts.

The 775-18 draws 87 amps stalled at 12 volts.

[gpetilli]

I should have specified. We are planning RS775-18 with a 20:1 ratio (plus the added torque from the 6:1 MiniCIM).

Since there is enough torque to overcome the CoF (1.2) of our wheels, the motors never stall and the current per motor is 45A when pushing a wall (with the wheels slipping on the carpet). It is hard to find dynamic CoF for FRC wheels. The only one I did find was 85% of the static CoF so that percentage is what I have been using as an estimate.

BTW: you have a similar issue with full size CIMs - if you ever let them stall you draw too much current and pop breakers. You simply have to design to slip the wheels.

[Ether]

Originally Posted by gpetilli

We are planning RS775-18 with a 20:1 ratio (plus the added torque from the 6:1 MiniCIM).

Since there is enough torque to overcome the CoF (1.2) of our wheels, the motors never stall and the current per motor is 45A when pushing a wall (with the wheels slipping on the carpet).

At 12V, a 775-18 motor draws 45 amps at 6385 RPM

At 12V, a MiniCIM motor draws 45 amps at 3008 RPM.

But if you have them geared 20:1 and 6:1, respectively, they can't simultaneously be at those speeds.

6385/20 = 319 RPM
3008/6 = 150 RPM

[gpetilli]

Originally Posted by Ether

At 12V, a 775-18 motor draws 45 amps at 6385 RPM

At 12V, a MiniCIM motor draws 45 amps at 3008 RPM.

But if you have them geared 20:1 and 6:1, respectively, they can't simultaneously be at those speeds.

6385/20 = 319 RPM
3008/6 = 150 RPM

That's an interesting way to look at it. Since I don't yet have a spreadsheet to dynamically calculate operating points, I used JVN table - which does include voltage droop (which looked like about 7.5V).

We did intend for the RS775 to reach max RPM when the MiniCIM was at 1/2 max RPM, so your estimates make sense. Clearly the wheel RPM will be the same for both and currents will split accordingly. The biggest concern I have is what happens after the RS775 exceeds it free RPM. I know it will behave like a generator and load the MiniCIM.

I do have some serious unanswered questions with this configuration. If we put the ESC in coast does that sufficiently break the generator current path such that it does not significantly load the driving motor? I don't think anyone has tried coasting the motor after it exceeds free RPM but instead continue to drive at full voltage. If we back-drive the RS775 to twice it's free RPM (coast or driven) will the generator blow the ESC?

[Ether]

Originally Posted by gpetilli

We did intend for the RS775 to reach max RPM when the MiniCIM was at 1/2 max RPM...

775-18 free RPM is 13000. With 20:1 reduction that's 650 RPM.

MiniCIM free RPM is 6200. Half of that is 3100 RPM. With 6:1 reduction that's 517 RPM. So you're a bit off with your gearing.

That's an interesting way to look at it. Since I don't yet have a spreadsheet to dynamically calculate operating points, I used JVN table - which does include voltage droop (which looked like about 7.5V).

I was just taking the numbers you posted to see where they would lead. The motor calculator I posted here makes this easy to do.

Here are the results using 7.5V instead of 12V:

At 7.5V, a 775-18 motor draws 45 amps at 1407 RPM

At 1407/20*6 = 422 RPM, a MiniCIM draws 48 amps at 7.5V

See example calculation attached.

I do have some serious unanswered questions with this configuration. If we put the ESC in coast does that sufficiently break the generator current path such that it does not significantly load the driving motor? I don't think anyone has tried coasting the motor after it exceeds free RPM but instead continue to drive at full voltage. If we back-drive the RS775 to twice it's free RPM (coast or driven) will the generator blow the ESC?

Good questions. Maybe the engineers from VAX and CTRE can answer for their respective motor controllers.

There's also the question of the 775-18 spinning at 13000*2 = 26000 RPM. Can its rotor, fan, and the bearings handle this? And even if the the motor leads are open-circuit (zero generated current), how much load does the fan put on the MiniCIM at that speed, through a 20:1 gearbox?

[gpetilli]

Good questions. Maybe the engineers from VAX and CTRE can answer for their respective motor controllers.

There's also the question of the 775-18 spinning at 13000*2 = 26000 RPM. Can its rotor, fan, and the bearings handle this? And even if the the motor leads are open-circuit (zero generated current), how much load does the fan put on the MiniCIM at that speed, through a 20:1 gearbox?



[/quote]

Good point on the fan - I had not thought of that. The 26000 RPM is only 33% higher than the 19,500 free RPM at the rated 18V. The robot will not be at this velocity very often or for long so I think there is sufficient margin.

I assume that the ESC have flyback diodes, so it will never truly be open-circuit. I am not sure what load that will impart on the MiniCIM. If the ESC blows, it expect it will be the flyback diodes that smoke.

[Oblarg]

Originally Posted by Ether

Has anyone ever run a test:

How long can you stall a 775 at full voltage before the factory-installed smoke escapes?

A bit off topic, but while I'm not sure about full voltage, I found this year that they're actually quite tolerant of stalls at partial voltage; they can go a surprisingly long time at 50% - I believe we went the better part of a match with one stalled in such a manner when we had a limit switch fail during the DC regional, with no obvious damage to the motor.