[Split Thread] One Motor Drivetrains

I can see it now. 1 motor swerve drive with multiple power take offs.

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See I was thinking monopod…



the one-motor robot is a creation I would find joy and horror in simultaneously.

1 motor robots I can think of steer by tilting only. Like D-O


The IRL toy does not turn well at all. The one with a rc remote actually has tiny clear casters and cheats. The smartphone app enabled one can balance but falls down a lot. On smooth flooring it works better to turn but then it slips and falls. On carpet it stays up but then can’t turn well. The head tilts on a servo and tips the body enough to turn as it drives forward. Obviously no turning in place.

Would not recommend for your FRC chassis unless you are going for a pity award. The thing looks very sad when it falls on its face for the 60th time.


But how do you tilt without a second motor?

Small servo that tilts a mass inside the chassis or in the case of D-O the head above the chassis tilts side to side. A servo isnt a motor in my mind. While servos contain a small motor it is a seperate and distinct component in its own class. You wouldn’t use a servo to drive ever because they aren’t typically continuously rotating or strong enough. So a robot with 1 true motor is possible, just also has other components to aid it, just not more motors. So when they say Only motor you’ll need. I guess that’s still true. Of course Vex and CTRE dont sell servos so there’s that issue now.


Whether you recognize that a servo contains a motor and is a mechanical actuator, It is also a controllable DoF and consumes one of the available GPIO lines from your micro, I guess that’s what I was questioning.

Continuous rotation servos are often used as drive motors on small robots, and large servo motors are frequently employed in industrial robotics - see Boston Dynamics.

Yes but we’re not talking about industrial servos and in those scenarios it’s still not being used as a drive wheel. There are servo drive motors, but not for FRC at all. You could accomplish the same effect with a linear actuator that forces the head to tilt on a cam, but I guess you’d argue that’s still a motor. Okay so pneumatic air bags that fill up on the base to tilt? But the air compressor has a motor. Where do you draw the line on motor versus a component that has a motor in it, but isn’t deemed a motor in the way they meant.

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I’m not trying to get into a pedantic argument with you. I was really just trying to understand how a robot with only one apparent DoF could control both forward/backwards motion and steering. You clarified that there is a second DoF in the form of a servo.

The pneumatic solution uses one motor for the compressor, but it also uses a solenoid to control the valve. The compressor motor can be hand-waved away by just using a pre-pressurized system, but the solenoid requires a control signal from the micro, so again represents a controllable DoF.

Again, I’m not trying to have an argument. I was just trying to understand the control theory you were describing.

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“Meltybrain” robots can control heading & translate in 2 directions using only a single motor & single wheel, rigidly mounted to the chassis. No need for pivots, power takeoffs(, or sometimes even gearboxes)!

Probably less than advisable in the context of FRC…


If you really wanted to do something with a single motor, could you have one drive motor that is connected to a perpendicular spinning disk, so you can use the change in speed to tilt. Controlling would be really hard though.

best. split. thread. ever. Truly peak summer CD.

The nuclear apocalypses has happened. The supply chain is down. You cannot purchase new motors. Your cannot borrow from your friends, lest the zombies get you. But all hope is not lost - You have EXACTLY ONE MOTOR. And, strangely, an unlimited supply of other mechanical and electrical components (thank you disaster-prep-minded team lead who also got a 99% on all high school and college tests).

Somehow, FRC is still happening. And, regardless of the apocalypse situation, Karthik’s wises words continue to ring true through the shambles. Your number-one priority is: DRIVE.

The stage is set. Go wild.

What do you do with that one motor?


Bonus points if said robot design can double as a lawn mower.


You could probably move a robot around by varying the torque a single motor applies as it goes around each turn (phase angle). Something like this shashplate-less helicopter, or a meltybrain combat robot.


If it can drive in one direction forwards, tied to a rope, wrapped around a pole it’ll naturally be drawn inwards each loop.


One fairly simple way of doing this would be to have the motor drive a shaft with two ratches attached to wheels, so when the motor spins in one direction, one wheel moves forward, then when you spin the motor in the opposite direction, the other wheel also spins forward. This does have the downside of not being able to move backward at all, but forward movement would be possible if you oscillate the motor enough, and turning would just be spinning the motor in one direction.


I was waiting for someone to point this out.

There is a fairly common mechanism used in very cheap toy RC cars that drives both wheels forward, but only one wheel in reverse.

That’s probably the most reasonable way to get single motor drive without fundamentally rethinking the drive layout.


Another really-cheap toy method:

Motor firmly attached to a rigid axle in the back. Allow the front axel to have a pivot with two endstops such that when it goes forward, the direction of travel is straight. However, when in reverse, the axle pivots to cause rotational motion too.

Granted, it has zoolander syndrome.


Y’all do know most skid steer bulldozers only have one motor…

You could also use hydrostatic drives if you could explain away the hydraulic fluid issue.

A much more complicated method of a single motor drivetrain could be using a modified Crab drive. By using a similar method as my above post, have one motor direction spin the modules, and have the other direction spin the wheels. Also by having one side of the robot’s modules spinning in reverse, the robot could move both forward, backward, and also turn.

Foward Turning Backward