# Turn rate vs wheel's speed

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1. Jul 9, 2016

### abdo799

I am an electrical engineering junior and i am helping some freshmen in a project
They are supposed to build a racing car
They don't want to use servo motors for the turning mechanism. They want to use the same mechanism the Segway uses, turn one wheel faster then the other, so i wanted to know if there is a way to calculate the turning rate vs the wheels speeds

2. Jul 9, 2016

### Staff: Mentor

Definitely -- it is a simple matter of geometry. Start by drawing yourself some diagrams of the wheel layout and turning circle.

3. Jul 9, 2016

### abdo799

I don't know how to draw those things...I tried to imagine it but couldn't come up with how to calculate them

4. Jul 9, 2016

### Staff: Mentor

What do you mean? A turning circle of a car is just a circle. The wheels are two circles of their own, on sticks at a set distance apart. When a car turns, the wheels trace-out two concentric circles on the ground. Surely, you must be able to visualize/draw that?

5. Jul 9, 2016

### abdo799

Oh..OK
I thought it was some sort of mechanical engineering drawing
I did draw some simple drawing but couldn't come up with any calculations

6. Jul 9, 2016

### Staff: Mentor

Can you post a scan/photo of the drawing?

7. Jul 10, 2016

### abdo799

actually i was writing this post while sitting on the beach and the drawing is back home :D
i do have one on my laptop but i used paint so it's not very pretty, will it be ok?

8. Jul 10, 2016

### Staff: Mentor

Yes, anything would be a start.

9. Jul 10, 2016

### abdo799

I couldn't upload it using the mobile app, but my drawing is really simple because of my idea of how it works.
I thought maybe I can apply the concept of the relative motion, if one wheel's speed is 10 rpm and the other is 20 rpm then we can say tht one wheel is 10 rpm and the other is stationary, so the diagram will be just a portion of a circle, the radius will be the distance between the 2 wheels and the length of the arc is easily calculated because I know the radius and spend of the wheel..I can now calculate the angle of the arc and the angular velocity.

10. Jul 10, 2016

### Staff: Mentor

I think you've mostly got it. I would not assume one wheel stationary but instead just calculate how far each wheel travels, which then gives you the length of the arc segments.

11. Jul 10, 2016

### abdo799

Yes..so I should get the difference between the arc lengths and consider the slower wheel the center?

12. Jul 10, 2016

### Baluncore

If you stop thinking of RPM and instead think of angle of wheel rotation then you can consider putting a shaft pulse encoder on each wheel. Now count the number of pulses coming from the individual left and right wheels.
The sum of the two counts gives the distance travelled.
The difference of the two counts gives the direction the vehicle is now facing.
The rate of change of the sum is the velocity.
The rate of change of the difference is the rate of turn.

13. Jul 11, 2016

### Nidum

14. Jul 11, 2016

### CWatters

How many wheels will it have?

If it's a conventional 4 wheel car you would still need to turn/angle two of the wheels. I suppose you could use castors or similar but think about the weight balance or you will get bad under/oversteer.

15. Jul 12, 2016

### abdo799

i just spoke to them about the castors thing, and it wont work, so they decided to make it like the segway

16. Jul 12, 2016

### billy_joule

You may want to push the castor idea again.
A segway is not much good for racing, if you apply to much torque the whole thing tips over so acceleration is limited.

Check out how a roomba works, they have two wheel drive like a segway but also have a small castor wheel so tipping isn't an issue:

Notice how the wheel is coloured, it's so an encoder senses it's rotation- if the drive wheels are rotating but the castor isn't, the roomba knows it is stuck.