### Author Topic: 2 motors vs 4 motors?  (Read 1103 times)

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#### chanathanlo

• Beginner
• Posts: 3
##### 2 motors vs 4 motors?
« on: October 15, 2013, 06:49:58 PM »
I'm starting to build a robot and I looking into motor speed.  I was wondering how does the a four wheel motor robot compare to a 2 wheel motor robot with caster wheel in terms of speed.  The problem is that is velocity of the wheel going to be the same as the velocity of the robot itself?
I got a motor of 197 rpm
torque to be 1.4 kgf-cm
acceleration to be 1 m/s2
velocity to be 0.667 m/s

Will this be the velocity of my robot with 2 wheels attached to the motor.  What would change if i added another 2 wheel (making it a 4 wheel robot) attached to 2 more motor, what would change in the speed?

#### jwatte

• Supreme Robot
• Posts: 1,345
##### Re: 2 motors vs 4 motors?
« Reply #1 on: October 15, 2013, 10:26:37 PM »
You need the weight of the robot, the radius of the wheel, and the maximum incline for which you want to achieve those values to be able to run the numbers.

Two versus four driving wheels affect the maximum torque that can be applied between the robot and the ground, assuming you have non-slipping wheels.

#### chanathanlo

• Beginner
• Posts: 3
##### Re: 2 motors vs 4 motors?
« Reply #2 on: October 16, 2013, 06:46:23 AM »
Hi jwatte
So would that mean that the max torque on the 4 wheel motor have twice the torque of the 2 wheel motor?
How does the weight of my robot play into the speed?
I know that to torque=forceŚradius of wheel but torque is given by the motor. Does this mean that the 4 wheel motor will be able to achieve its max speed faster?
The robot I'm planning to build is about 5 lb and the wheel diameter is 4 in.  There is no incline.

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: 2 motors vs 4 motors?
« Reply #3 on: October 16, 2013, 09:57:25 AM »
The robot weight plays into the acceleration and the speed on an incline (which is an acceleration against gravity). Try using the motor calculator found by clicking "Robot Tutorials" in the upper right corner of the web page.

#### chanathanlo

• Beginner
• Posts: 3
##### Re: 2 motors vs 4 motors?
« Reply #4 on: October 16, 2013, 11:52:27 AM »
Hi waltr
Are you talking about the rmf calculator or is there an actual motor calculator?
Also with no incline how would the weight play into acceleration other than friction.  so right now I have a motor with 18 in-oz(1.45 kgf-cm) torque motor with 197 rpm and a 4 in diameter wheel.  I found out that the velocity would be about 2 ft/s and the acceleration to be 4 ft/s with no slipping.  does this sound right?  so would this mean that my robot will be traveling 2 ft/s with an acceleration of 4 ft/s?  that seems pretty fast and i know that this is with the motor running at full potential.  The robot should weigh about 5 lb and i don't know how to incorporate that into my calculation? any suggestion?

#### bdeuell

• Robot Overlord
• Posts: 183
##### Re: 2 motors vs 4 motors?
« Reply #5 on: October 16, 2013, 12:17:08 PM »
Also with no incline how would the weight play into acceleration other than friction.  so right now I have a motor with 18 in-oz(1.45 kgf-cm) torque motor with 197 rpm and a 4 in diameter wheel.  I found out that the velocity would be about 2 ft/s and the acceleration to be 4 ft/s with no slipping.  does this sound right?  so would this mean that my robot will be traveling 2 ft/s with an acceleration of 4 ft/s?  that seems pretty fast and i know that this is with the motor running at full potential.  The robot should weigh about 5 lb and i don't know how to incorporate that into my calculation? any suggestion?

F=M*a

F = T/r

thus

a = (T/r)/m

acceleration = (number of motors) * ((torque of motor) / (radius of wheel)) * / (mass of robot)

*assuming level ground, no rolling friction, constant torque, no air resistance, 100% efficiency, and no inertia for rotating parts
« Last Edit: October 16, 2013, 12:21:05 PM by bdeuell »

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: 2 motors vs 4 motors?
« Reply #6 on: October 16, 2013, 01:17:46 PM »
The rmf calculator. By playing with the numbers (trying different values) you can get an idea of the motor performance.
There is also an article explaining to details. Its a good read to understand the concepts.

The 4ft/s accel is only until it reaches the top speed of 2ft/s, then the accel is zero. If these number are correct then it would not take much time to reach full speed. But this allows for software control of the accel and also allows for maintaining speed up an incline.

#### jwatte

• Supreme Robot
• Posts: 1,345
##### Re: 2 motors vs 4 motors?
« Reply #7 on: October 16, 2013, 01:50:37 PM »
Quote
assuming level ground, no rolling friction, constant torque, no air resistance, 100% efficiency, and no inertia for rotating parts

"Consider a spherical cow of uniform density..."