High torque, high speed and high precision motors

Other Hardware Motors, Mechanics, Power and CNC
1

Hello,

I'm not sure this is the right place to ask. It's not even directly related to arduino, altohough I plan to use one. ^^

I'd like to build a robot that is basically a (reversable) 4x4 but that would also be able to erect itself on two wheels in a segway-like fashion. And this, just by providing a high acceleration (no additional arm).

My question is what kind of motors would I need to do this?

I would need high speed motors because in the 4-wheels configuration I want it to be "fast", like 5 or 10 km/h.
I would need high precision motors (a few degrees or less) in order to balance the robot in 2-wheels configuration.
I would need high torque motors (actually torque + acceleration) in order to lift the robot from 4 to 2 wheels configuration.

So, first: brushed, brushless or stepper motors?
And then: how can I compute the torque needed to lift the robot?
I have the feeling that what I actually need to compute is the torque with respect to the speed. So that initial torque can start to lift the robot, and with the speed it aquired while doing this, it must continue to produce some torque in order to continue lifting the robot and so on.

But I'm not sure how to compute it, my physics is a bit rusted. And I'm not sure where I could find a seller that would documentation that precise about their motors.

And of course, I'm just studying this project right now. I don't have any part yet. So if the motors or battery needed is too expensive, I may retract on something simpler. ^^

Thanks in advance for any piece of advice you may provide. :slight_smile:

2

Start by polishing up your physics a bit - some sort of basic diagram of the layout is needed,
post it here for people to comment perhaps, together with approximate weights of things.

Precision for a balancing robot is nothing to do with the motors, it comes from the IMU
unit that is sensing the orientation - couple this to a PID feedback look to control the motor
driver(s) to get closed-loop control.

You will need to avoid backlash in the mechanical linkage if any, slop in the system makes
feedback loop harder to stabilise.

Brushed or not isn't too important I think - get the motor sized right is. The parameters of
DC motors are much simpler than you think, you need enough power, enough torque
and a motor driver that can handle the load.

I reckon your idea of doing a wheelie to get from 4-wheel to 2-wheel drive is going to be the
hard part - both ensuring enough friction to allow it to happen, and switching from wheelie
to balance mode will be tricky and need some experimentation.

I think you'll need to have most of the weight on the balance-wheel axles to allow wheelies to work.

3

MarkT:
Start by polishing up your physics a bit - some sort of basic diagram of the layout is needed,
post it here for people to comment perhaps, together with approximate weights of things.

It is at the end of this message.

MarkT:
Precision for a balancing robot is nothing to do with the motors, it comes from the IMU
unit that is sensing the orientation - couple this to a PID feedback look to control the motor
driver(s) to get closed-loop control.

Really? But if the motors (like brushless motors) only give me a positioning precision of a quarter turn it's going to be hard to keep balance.

And I think a PID control is too simple. :slight_smile: I was more thinking about some proactive non-linear control. I like challenges. :slight_smile:
However, of course, a PID will be a first step.

MarkT:
I think you'll need to have most of the weight on the balance-wheel axles to allow wheelies to work.

Well... I would like it to be fully reversible (able to lift from any of the 4 sides). But again, that may be a solution to make it lighter and cheaper.

And here is the schema of the robot with the forces that will lift it up.

T is the contact point between the ground and the wheel. G is the center of mass. The forces are the red arrow, of course.

I'm not sure about how to estimate weight. It will depend on the weight of the battery and the size of the battery will depend on what the motors need. And the motors needs depends on the weight of the battery. Anyway, I think a total weight of 1 or 2kg may be a reasonable upper limit.

And the physics of the wheelie is that the torque is converted to a traction force by the friction of the wheels on the ground. Against that force, there is the inertia. And the goal is to produce a force strong enough so that the vector gravity+inertia is past the line TG.

I guess we can also model the wheelie as a torque on the robot as a whole. Where the two vectors gravity+inertia and traction+normal_reaction_of_the_ground_on_the_back_wheel fight each other and try to produce torque that would rotate the robot... or not.

But all this is very static. In reality, when the motors will start applying torque on the wheels, the robot will start to move. And the torque provided by the motors may not be constant with speed. And every motor have a max speed. So if gravity+inertia is "just past" TG, it is possible that the motors wouldn't be able to produce torque long enough (as the robot gain speed) to erect it completely. (Not sure if this is clear.)

I can sense an integral coming somewhere. But I don't exactly know how yet.