Optimal roller distance for dynamometer

[yawmid]

TL;DR Summary

building a dyno, need to determine the best roller distance

I am currently building a dynamometer to measure the power consumption and rpm of a mars prototype. The plan is to place each wheel of the rover onto a pair of rollers but I am currently struggling with determining the optimal roller separation. If they're too far apart, the wheel might not fully rotate the rollers and if they're too close they might roll over the top of the rollers.

 

[berkeman]

Welcome to PF.

What research have you done on this problem? Dynos are commonplace, so you should be able to see how others have solved this problem.

Also, can you show us your Free Body Diagram (FBD) for one of the wheels on the rollers and show all forces and torques involved? What is the range of weights and wheel torques that you have to accommodate?

Finally, you may want to rethink the mechanism that you have drawn. You probably need a bit of space between the wheel and the side guards, and some way to keep the vehicle and wheels tracking straight so they don't try to climb up and off of the dyno.

 

[yawmid]

Thank you for replying,

In terms of research I haven't seen anything that discusses this specifically.

I'm not quite sure how to approach the problem, below is the FBD diagram I have so far. I don't know the torque of the motor being used. The max weight on a roller pair will be around 120N.

You make a good point about the space required for the wheel and I'll be sure fix that. Keeping the vehicle straight is not an issue, I should have described the design in more detail which I will do now.

We are building a test rig for our prototype rover which needs to simulate various terrains and measure the power consumption/rpm of each of the 6 wheels. To do this, we are building essentially building dynos on top of scissor lifts (which does sound absurd without context). The scissor lifts will be able to move up and down to mimic inclines. A more detailed CAD model of the set up is shown below.

 

[berkeman]

Thanks, that helps some. But I'm not sure relying on the separation of the rollers at each wheel will be enough to keep the vehicle from climbing off the setup. I did a Google search on how hold back vehicle on dyno and got lots of helpful hits. Have you considered other hold-back mechanisms like those?

Paging @jrmichler @Ranger Mike @Baluncore

 

[berkeman]

And just to add -- what range of accelerations and slopes are you planning on testing to? How are you going to generate the counter-torques / loads at the rollers to perform that testing?

 

[Baluncore]

You might increase the roller diameter, then place a slack loop of belt over the rollers, that will conform to and support the vehicle wheel.