So i know that neos have a maximum rpm rating. But do they ever actually reach those rpms? Lets say 4 neos are moving a chassis. And i run the neos at “100%” in code. Will the actual measured rpm of the neos be the maximum rpm they are rated at?
TLDR-No
Rev lists the “Empirical Free Speed: 5676 RPM” . In a frictionless environment with enough time you could get really close to that speed. If you look at the charts on the REV site (below) you can see the power peaks at about half the max RPM. With any game if it is for a drivetrain or flywheel shooter is is also about acceleration. High top speed on a drivetrain will mean lower acceleration. This makes you easier to block. On a flywheel shooter it means more waiting between each ball shot. https://www.revrobotics.com/content/docs/REV-21-1650-DS.pdf
2 Likes
No.
Check out the torque curves. The “free (or maximum) speed” occurs when there is no torque output. Even the slightest load (including friction in the gearbox) will slow that speed down. If you are trying to design a mechanism, a good rule of thumb is to design for 25-50% of the “free speed”, so there’s a good amount of torque headroom leftover for acceleration and errors in your math.
2 Likes
So is the power peak where it usually actually caps out in rpm? Im new to this stuff me and my team never really though of these things much
No, it is typically about 50% of max free speed.
" Torque is a measure of the force that can cause an object to rotate about an axis. Torque (article) | Khan Academy
Power is speed x torque.
For many functions, acceleration can be more important that top speed.
2 Likes
You won’t see the max speed because of losses. The force output of the motor (torque) decreases as it increases in speed. So any little bit of friction (even the bearings themselves) will slow down the motor.
You will get close. How close? Like most things in engineering it depends.
Wouldn’t it be more likely to have the top speed be >4000 RPM given a current limit of 60A? (I think drivetrain is typically current limited from 50A-70A per motor)
The torque seems like it would stay the same until 4000 RPM so it must at least keep accelerating until there, right?
The graph is taken from reca.lc’s motor playground
I’m still learning a lot about motors so I could be completely wrong.
Doesn’t this graph go completely against what rev’s graph says? I thought that the higher the rpm the less torque but here it only seems to decrease after 4k rpm. However, rev’s graph decreases torque as it goes linearly.
This chart agrees with the REV charts that includes current limiting. The Motor Playground chart includes current limiting. Current limiting reduces the torque at low RPM. The stall current is 105A. Multiple stalled motors at stall current could trip the breaker if sustained for too long.
Metaphorically, let’s walk up to the edge of the chasm and take a peek down.
The answer to “how fast will it go” is the result of a differential equation.
Start with the basics:
Sum of Torques = Moment of Inertia * Angular acceleration (newton’s law 2nd for rotating things)
Angular acceleration is the derivative of angular velocity (“RPM”)
In most models of DC motors, current flowing through the coils produces positive torque to accelerate the motor. Torque is proportional to the amount of current.
Additionally, you have to add in whatever load torque is applied to the motor - friction, for example
Current flowing through the motor is proportional to the voltage across the windings (Ohm’s law).
The voltage across the windings is the difference between how much voltage you apply with the motor controller (the “100% in code” thing you mention earlier), and an effect called “Back-EMF”. The fact that coils are spinning in a magnetic field produces a voltage in them (think how generators work). This voltage is called Back-EMF.
Back-EMF is proportional to velocity. Which, (look back up a few paragraphs), creates a “circular dependency” of variables on their derivatives. Hence, the differential equation.
The motor will achieve a “steady state” velocity when the three factors of applied voltage, speed, and load force balance each other out.
Notable things that are further down in this chasm:
- How to construct and solve this differential equation
- Assumptions that are violated when forces, voltage, and currents are changing rapidly
- Details of how current limits get applied for certain motor controllers
- How to apply this knowledge to make your robot software work better.
- What in the heck these “voltage” and “current” things are
- Heat buildup, failure modes
- Details on how to measure each of these quantities
…
Bring a parachute
1 Like