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How do I simulate a mobility scooter to go up an incline of 15 degrees at 30 km/h with a 300 lb person to see if it can climb a 15 degree

Question asked by Matthew Chan on Apr 30, 2015
Latest reply on Oct 7, 2015 by Samuel Walsh

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I'm trying to simulate an electric mobility scooter with varying loads i.e. heavy person, light person, etc.. etc... and varying ramps of incline of 5, 10 and 15 degrees. I'm trying to simulate absolute fail conditions (i.e. person of 300 lbs will not be able to climb a 15 degree slope starting at 30 km/h) and work backwards to find an optimal motor/battery strength to be able to push a 300 lb guy up a 15 degree slope, if possible.

I've overwritten the masses to match, inputted common road/rubber friction coefficients, and am using an applied force of 300 lbs (136 kg)(800 or so N of force downward) with an overwritten center of gravity over the rear wheel axle as it should be. Everything is mated carefully, tangential mates in the beginning of the simulation with the wheels to make contact with the road and then after a second or so I flip them off.

Overall, I'm not 100% on all the forces I should be applying, whether initial velocity, torque on rear wheel (from motor), adding a rotary motor, or adding a pushing force equal to 30 km/h. Nevertheless I've tried them all. I've thrown on earth gravity, friction, and 300 lb mass, as well as mobility scooter mass (80kg) and a bit of mass for the wheels themselves.

What would give me the most accurate results for my specific scenario. What forces should be applied and how. Geometry is basic, just an overall representation of a bike body and 2 wheels on axles. Unfortunately I don't have any pics or files on this machine.

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Jared Conway Apr 30, 2015 7:37 PM

I think motion will be overkill for this. I'm trying to understand what you're looking to gain from motion over a hand calc?

I don't think you're going to get much resolution of the wheels and ramp to have a huge impact.

if you're accelerating the system you shouldn't need any force but the human's weight.

your optimization problem is probably torque input vs velocity and playing with torque until you get the velocity that you want.

what i'm not clear about is if you're running into an issue running the simulation or if you're looking for the approach. if you're running into an issue, what is the issue?

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Matthew Chan @ Jared Conway on Apr 30, 2015 10:17 PM

I am running into issues where I would simulate 2 different scenario's in which I should get 2 different results but which I only get the same result.

1 example was I ran a simulation with 2 bikes (2 wheeled) side by side, parallel mated so they won't wobble to the side or crash into each other, both with the same force or torque, but with different masses. I would get the exact same result, both reaching the same distance over the incline at the same speed, and would fall back at the same speed as if the overwritten mass, or just mass overall, doesn't apply. Force applied downward I get crazy results, bike doesn't go anywhere just flips backwards like crazy if the force is too high.

What I'm trying to get is a simulation in which I can enter in different battery power or motor power or whatever calculation is needed and determine if it can climb such an incline. As well as with various weights of people. If simulation fails, we try again up the power, see what is needed to achieve such a climb. Of course these are just computer simulations and actual testing would be required.

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Jared Conway @ Matthew Chan on May 1, 2015 11:40 AM

it sounds like a setup issue

i would recommend you start with something dead simple like a block with wheels on it

if you have trouble, post it up

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Alan Thomason @ Jared Conway on May 1, 2015 4:13 PM

Hi Matthew...

Firstly, I agree with Jared that for this application you would be better off just calculating the problem out on paper or in something simple like Excel. If you are determined to do it in Motion, you might have your reasons. Here is what I would do. First, set up a result to monitor of your vehicle speed. You need to do this to be able to use the rotational speed of the shaft. I have shown a wheel, but ideally you would monitor the speed of the motor shaft so that changes in gear ratio can be accounted for.

Next, applying a torque to the motor shaft using an expression similar to the following:

Here, the first term, the 1000 is the torque in N-mm. The next part in parenthesis creates a relationship in which the torque is maximum at zero shaft speed and drops to zero at some maximum speed. The reason for the *60/360 is to convert from the degrees/sec to revolutions / minute. We divide by 100 to normalize the resultant speed to a maximum speed of a motor of 100rpm (hypothetical). This equation is very simplistic..if you have a better characteristic curve, feel free to use it.

This works, I've used this technique for a more complicated application. This will allow you to analyze different motors and gear ratios, but not batteries. To analyse different batteries you need to know the relationship between the current draw of the motor and the voltage output of the battery. Not a terribly complicated set of equations, but it is very difficult to set up interdependent equations in Motion. I consider the equation capacity of Motion to be a surprising disappointment. Motion achieves so much that is quite difficult, and yet seems pretty incapable of integrating fairly moderate analysis.

Best of luck!

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Samuel Walsh @ Alan Thomason on Oct 7, 2015 5:03 PM

Thankyou for such a detailed answer, is there any way I could get you to share your assembly? as I am doing the exact same thing and not sure where to start. (Robot on an incline plane)

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5 Replies

Jared Conway Apr 30, 2015 7:37 PM
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I think motion will be overkill for this. I'm trying to understand what you're looking to gain from motion over a hand calc?

I don't think you're going to get much resolution of the wheels and ramp to have a huge impact.

if you're accelerating the system you shouldn't need any force but the human's weight.

your optimization problem is probably torque input vs velocity and playing with torque until you get the velocity that you want.

what i'm not clear about is if you're running into an issue running the simulation or if you're looking for the approach. if you're running into an issue, what is the issue?
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- Matthew Chan @ Jared Conway on Apr 30, 2015 10:17 PM
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  I am running into issues where I would simulate 2 different scenario's in which I should get 2 different results but which I only get the same result.
  
  1 example was I ran a simulation with 2 bikes (2 wheeled) side by side, parallel mated so they won't wobble to the side or crash into each other, both with the same force or torque, but with different masses. I would get the exact same result, both reaching the same distance over the incline at the same speed, and would fall back at the same speed as if the overwritten mass, or just mass overall, doesn't apply. Force applied downward I get crazy results, bike doesn't go anywhere just flips backwards like crazy if the force is too high.
  
  What I'm trying to get is a simulation in which I can enter in different battery power or motor power or whatever calculation is needed and determine if it can climb such an incline. As well as with various weights of people. If simulation fails, we try again up the power, see what is needed to achieve such a climb. Of course these are just computer simulations and actual testing would be required.
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  - Jared Conway @ Matthew Chan on May 1, 2015 11:40 AM
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    it sounds like a setup issue
    
    i would recommend you start with something dead simple like a block with wheels on it
    
    if you have trouble, post it up
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    - Alan Thomason @ Jared Conway on May 1, 2015 4:13 PM
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      Hi Matthew...
      
      Firstly, I agree with Jared that for this application you would be better off just calculating the problem out on paper or in something simple like Excel. If you are determined to do it in Motion, you might have your reasons. Here is what I would do. First, set up a result to monitor of your vehicle speed. You need to do this to be able to use the rotational speed of the shaft. I have shown a wheel, but ideally you would monitor the speed of the motor shaft so that changes in gear ratio can be accounted for.
      
      Next, applying a torque to the motor shaft using an expression similar to the following:
      
      Here, the first term, the 1000 is the torque in N-mm. The next part in parenthesis creates a relationship in which the torque is maximum at zero shaft speed and drops to zero at some maximum speed. The reason for the *60/360 is to convert from the degrees/sec to revolutions / minute. We divide by 100 to normalize the resultant speed to a maximum speed of a motor of 100rpm (hypothetical). This equation is very simplistic..if you have a better characteristic curve, feel free to use it.
      
      This works, I've used this technique for a more complicated application. This will allow you to analyze different motors and gear ratios, but not batteries. To analyse different batteries you need to know the relationship between the current draw of the motor and the voltage output of the battery. Not a terribly complicated set of equations, but it is very difficult to set up interdependent equations in Motion. I consider the equation capacity of Motion to be a surprising disappointment. Motion achieves so much that is quite difficult, and yet seems pretty incapable of integrating fairly moderate analysis.
      
      Best of luck!
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      - Samuel Walsh @ Alan Thomason on Oct 7, 2015 5:03 PM
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        Thankyou for such a detailed answer, is there any way I could get you to share your assembly? as I am doing the exact same thing and not sure where to start. (Robot on an incline plane)
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How do I simulate a mobility scooter to go up an incline of 15 degrees at 30 km/h with a 300 lb person to see if it can climb a 15 degree

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