Choosing a motor for a vehicle

[chessguy103]

TL;DR Summary

Is it bad to choose a motor with too much torque?

I'm designing a 4-wheeled vehicle that will be driven by 2 servo motors (rear-wheel direct drive). The maximum torque required will be about 20 Nm (including a factor of safety). Since there will be two driving wheels, this amounts to 10 Nm per wheel.

Is it bad to choose 2 motors with 15 Nm rated torque and 30 Nm torque?

It seems like a silly question, but I want to make sure that performance won't be affected and parts won't be worn down. As long as we run it lower than the rated torque it should be good, right?

Thanks

 

[jack action]

Depending on the type of motor, there is usually a sweet spot for best efficiency. If you always used it out of this range, it is not really a smart choice. And yes, some motors might not like it if you are always way out of range.

Torque can always be transformed to any value with the help of gears, pulleys, or any other kind of transmission.

What really matters in motor selection is the power output. Does your motor have the power you need in its rpm operating range? Once you are sure of that, it is only a question of having the proper gear/speed reducer to adapt it to your needs (wheel torque & rpm). No matter what are the wheel or motor torque & rpm, their powers will always be the same.

 

[Lnewqban]

In many cases, max available torque will be needed for situations in which the vehicle is stuck; therefore, you may want to verify that the motors can actualy deliver that torque around zero rpm's conditions.

 

[ardnog]

When you say "rear wheel direct drive" I assume you are not driving each rear wheel individually?

If you are, I recommend not doing that. Tanks used to work like that it caused problems when steering as they couldn't keep the left and right synchronised properly when the thing went over rough ground and one side encountered resistance.

 

[jrmichler]

As long as we run it lower than the rated torque it should be good, right?

Yes. Note that most servomotors have two torque ratings - RMS torque and peak torque. If you stay within those ratings, you are good.

If a servomotor is too much oversized, it will be difficult to control because you are using only a very small portion of the available control signal. The motor is also large, heavy, and needs a more powerful controller.

Independent rear wheel drive will work if your servo controller is good enough and the motors are properly sized. I have no experience with low cost hobby motors and controllers, but the good industrial ones will respond to a disturbance within a millisecond or so. That's plenty good enough for good directional control.

 

[chessguy103]

What really matters in motor selection is the power output. Does your motor have the power you need in its rpm operating range?

You're saying that as long as long as it has the right power and rpm, then torque can be manipulated to be whatever I want, right? Just want to make sure I understood correctly.

However, I'm trying to avoid complexity in the design and am hoping to just stick with direct drive from the motor.

When you say "rear wheel direct drive" I assume you are not driving each rear wheel individually?

If you are, I recommend not doing that. Tanks used to work like that it caused problems when steering as they couldn't keep the left and right synchronised properly when the thing went over rough ground and one side encountered resistance.

That is what I mean. The top speed of the vehicle will only be about 15 mph, so hopefully it won't be a problem. The encoder is a 1024 line Incremental Photoelectric Encoder. I'm honestly not sure what that means. Do you have any insight?

Thanks!

 

[jack action]

You're saying that as long as long as it has the right power and rpm, then torque can be manipulated to be whatever I want, right? Just want to make sure I understood correctly.

Yes.

Let me help you visualize the concept, looking at the following figure for a motor for an electric bicycle:

The orange line is the wheel power, i.e. what you actually need. Usually the sum of the rolling resistance, aerodynamic drag, etc.

The blue line is the actual motor output. Where it meets the orange line is where the acceleration is zero. Thus it represents the maximum possible speed (25 MPH, in this case).

The red line is the actual electric power required by the motor. You can see that at low RPM it is really inefficient to convert its electrical power to mechanical power.

The green line is the motor efficiency, i.e. the blue line divided by the red line. Note how in this case, the peak efficiency is past the maximum speed, i.e. it will never be reached.

Normally, you would want your vehicle to be driven in the region between peak power (for best acceleration) and peak efficiency (for cruising speeds). Imagine this motor used on your vehicle with a top speed of 15 MPH, you would constantly be at very low efficiency (wasted heat). And the fact that in that speed range you have so much more power than you need (compared to the orange line), it might even be less efficient since you will have to constantly reduce the motor power output by a large amount (say, cruising at 10 MPH).

But if you would use the same motor with a gearing factor of 2, the orange line would remain the same, but all other lines would shift to the left. So peak power would be at around 6 MPH and peak efficiency would be at around 14 MPH. The blue line would cross the orange line at around 17 MPH, i.e. the new maximum speed.

Here is another similar graph for bicycle electric motors:

You see that for any company, they have their peak power in a similar RPM range, because it matches the efficient human cadence range (60-110 rpm).

 

[hutchphd]

I'm confused about terminology here.
In my vernacular a "servomotor" is typically a brushed DC motor with an encoder and essentially a PID controller to control the voltage (often using PWM)
This is quite different from more modern technique of generating variable frequency triphase AC to drive a brushless motor synchronously. These "brushless DC motors" are now universally used on electric bikes and trikes and can provide regenerative braking as well.
Please ignore if I am missing the point here, but these are quite distinct systems

 

[jrmichler]

My experience is with industrial servo motors. Those motors are all permanent magnet brushless. Brushed DC motors were obsolete over 20 years ago in industry. Major manufacturers of industrial servo motors are Siemens and Allen-Bradley. Siemens servo systems are popular in Europe, Allen-Bradley in the U.S. A listing of Allen-Bradley servo motors is here: https://www.rockwellautomation.com/en-us/products/hardware/allen-bradley/motion-control/servo-motors/rotary-motors.html. Their HPK series motors are their big ones. Their smallest is the MPL-B1510V. All of them come with internal encoders for precision position feedback.

I do not have experience with little hobby motors, such as those found when searching Arduino servo motor. I suspect that most, if not all, of them are less powerful than the smallest A-B servo motor.

 

[hutchphd]

So the encoder is used to provide finer graduations than the multiphase/multipole arrangement allows? Is it also important to detect and compensate against slippage in practice? Only the slippage would seem to me a concern for powering a car .

 

[Tom.G]

Somehow, a Servo Motor seems overkill for this project; wouldn't simple speed control be sufficient?

Cheers,
Tom