### Author Topic: Calculate velocity of movement  (Read 4328 times)

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#### kennykck

• Jr. Member
• Posts: 43
##### Calculate velocity of movement
« on: December 09, 2007, 09:28:37 AM »
Hi, i have a question. How do i calculate the velocity of the weighted object which is attached to the conveyor belt? Given that i have a Torque versus Speed graph for the motor. Thanks in advance.

#### paulstreats

• Supreme Robot
• Posts: 1,381
##### Re: Calculate velocity of movement
« Reply #1 on: December 09, 2007, 09:39:59 AM »
a quick way is to find the rpm of the motor, then multiply that by the circumference of the drive wheel, it should tell you how fast the belt is moving and therefore the object on it.

so if the motor spins at 40rpm and the drive wheel has a circumference of 12cm, then the velocity is 480cm per minute or 8cm per second.

An accurate way of finding how fast the motor is turning is to use an encoder wheel on it

#### kennykck

• Jr. Member
• Posts: 43
##### Re: Calculate velocity of movement
« Reply #2 on: December 09, 2007, 10:00:49 AM »
So, the weighted object does not influence the velocity of the belt? OK then, what if i wanna know how long it takes to reach the top speed?

#### paulstreats

• Supreme Robot
• Posts: 1,381
##### Re: Calculate velocity of movement
« Reply #3 on: December 09, 2007, 10:13:55 AM »
The only time the weighted object should alter the speed of the motors is when the object is too heavy for the motor rating meaning that the motors havent been chosen properly.Calculate the torque you need to pull the object before you buy the motors.

The best way to find how long it takes for the motors to accelerate to top speed is to also use encoders in the wheels. If you pre-calibrate a top speed, just then set a counter when the motor starts and wait for the encoder feedback to become the same as the pre-calibrated speed

#### kennykck

• Jr. Member
• Posts: 43
##### Re: Calculate velocity of movement
« Reply #4 on: December 09, 2007, 10:17:31 AM »
Err... i dont have any encoder.
Is it possible to calculate the time anyway?

#### ed1380

• Supreme Robot
• Posts: 1,478
##### Re: Calculate velocity of movement
« Reply #5 on: December 09, 2007, 10:40:40 AM »
you can get an encoder from the scroll wheel of a mouse, or there are 2 in the ball type mice
Problems making the \$50 robot circuit board?

#### paulstreats

• Supreme Robot
• Posts: 1,381
##### Re: Calculate velocity of movement
« Reply #6 on: December 09, 2007, 10:45:32 AM »
I dont think that you could effectively calculate it to be accurate. Saying that, most electric motors have very fast acceleration to a point where they are almost full off to full on instantly provided that the load they are carrying is within their torque ratio, this is why when using motors in robots, you have to manually control the acceleration to get smooth starting and stopping.

• Supreme Robot
• Posts: 11,659
##### Re: Calculate velocity of movement
« Reply #7 on: December 09, 2007, 09:29:25 PM »
The weight will affect speed during the initial start up condition until the mass accelerates to your desired maximum velocity.

I derived this equation for you to calculate whatever you want for this situation:

motor_torque * pulley_radius = object_mass * max_velocity / time_to_accelerate

where motor_torque matches max_velocity on your motor datasheet graph

#### kennykck

• Jr. Member
• Posts: 43
##### Re: Calculate velocity of movement
« Reply #8 on: December 10, 2007, 02:19:29 AM »
I think you mean "motor_torque / pulley_radius" for the formula you given is it?

• Supreme Robot
• Posts: 11,659
##### Re: Calculate velocity of movement
« Reply #9 on: December 10, 2007, 05:59:46 AM »
Quote
I think you mean "motor_torque / pulley_radius" for the formula you given is it?
? uhhhh no . . .

this is how I derived it:
force = mass * acceleration
torque = force * distance