Differential Swerve Drives are on the cutting edge of FRC drivetrains. Designs like 971s have inspired many to think about the potential. If you can handle the increased complexity, having two motors driving each wheel instead of one seems like an obvious improvement right?
Well, I’m not so sure anymore with the release of the Neo and Falcon500 brushless motors. With the incredible power these motors provide, I’m wondering if using only one puts you in a situation where you are traction limited, and therefore, unable to utilize the added power of an extra motor.
Assume a 140lb swerve robot with 4" wheels and a 10:1 reduction.
The force each wheel can produce with a Falcon500 at stall is:
(4.69N*m * 10 / 0.0508m) = 923N
The normal force on each wheel (assuming equal weight distribution) is:
140lb / 4 = 622.75N / 4 = 155.6875N
To avoid slipping, we would need a coefficient of friction of
923 / 155.6875 ~= 6. Based on the thread CoF ranges, no common FRC wheels come close to this.
- Is this the right analysis to do to calculate if the configuration is traction limited?
- If a single motor is traction limited, are there any benefits to doing a Differential Swerve over a traditional one? (I suppose it may allow the use of lighter motors)
Remember that you have an overall current limit of 120A too. If you try to run all 8 at full power you’re going to trip the breaker whether you’re using Falcon 500s or CIMs.
That said, more power is still useful since you can gear for faster speeds and still be accelerating as you get closer to free speed.
Additionally, spreading the load over 8 motors significantly improves your thermals.
If you are looking at how traction limited a drivetrain is in a pushing match (motors at full power but no movement) you are confirming that your force from your wheels from the motors is greater than the CoF times the weight of the robot. In other words, do the motors stall before the wheels slip? When doing these calculations you usually calculate for the drivetrain as a whole rather than each wheel.
Motor stall torque * number of motors * gear ratio / wheel radius = drivetrain pushing power from motors
Robot weight * CoF of wheels drivetrain pushing power from traction
Whichever is lower is your actual pushing power and what limits your drivetrain.
I would recommend using JVNs 1 speed drivetrain calculator to experiment with configurations as its much more efficient (and fun) than hand calculations
Another factor that should be considered when choosing motors and ratios is current draw per motor and per drivetrain. This is important for safe operation of motors and staying under the breaker limit*, among other things.
Again, JVN Calc does a good job of estimating your pushing current, or the maximum current your drivetrain will draw.
*Note you can exceed the main breaker and individual breakers in FRC, and this is very common, especially in drivetrain motors. But as a rough guideline, normal operation (I.e. not pushing matches) should stay under the breaker limit.
Remember that not all your wheels are always touching the ground. With swerve it’s relatively common to get pushed onto two wheels in a pushing match.
With differential swerve, this still gives you 4 drive motors over those two wheels. Meaning that you should match up more evenly against a 4 motor tank drive.
Remember that not all your wheels are always touching the ground. With swerve it’s relatively common to get pushed onto two wheels in a pushing match
This. How you calculate pushing matches depends a lot on drivetrain configs, especially swerves.
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