Speed vs Torque, Current Curves

[vidam]

Hi, I am looking to depict the following curve as accurately as possible for different robots http://i262.photobucket.com/albums/ii102/melvida/speedvspwmpayloadvspwm.jpg:



What are the variables, equations that influence this curve so that I can estimate it for a robot with certain specifications. I just want the equations, please?

I would also like to know the equations to find the curve of Speed (rpm) vs torque and Current vs Torque to find the optimal point of operations.

[airman00]

you know that this all varies per motor

[vidam]

you know that this all varies per motor

Let's assume I am using anyone of the motors listed on the battlekits page:
http://www.battlekits.com/robot_motors.htm

[airman00]

 dont see what trouble you are having

just take readings for torque, current,and RPM at different voltages and make equations out of it . Plug the numbers into excel and excel will automatically provide you with the formula.

the graph you posted up shows the relationship between voltage and RPM and between voltage and torque ( i think since it says payload) . So as a general rule of thumb as you increase voltage , everything increases. 

Do you have tools to measure the amount of torque exerted?

you can't provide one graph to work for all motors, each motor requires new readings. Thats whay I meant before in my previous post.

[vidam]

Thanks for answering my question.

But, I'm looking for generalized mathematical relationships that show how these variables relate to one another. And it looks like RMF robot motor factor is what I needed from Admin's tutorial. I don't have motor curves in front of me and I don't have motors to take readings of voltage, current. This is all hypothetical at this time.

[airman00]

oh generalized relationships

I'm not sure of any general ones.... hmmm it would depend on the gearing of the motors as well....

the only one I can think of is  y = K(x)   where y = volts,    k > 1     and  x = torque, speed, or current

also there would be a limit to y before the motor broke


A better idea is to get motor readings or look for similar motors online and get their readings from online. I understand this is all theoretical, but general relationships that apply to all motors should not exist ( each motor has different gearing , different ga. wire windings, diff. # of windings, etc.)

Then again , I would love to be proved wrong , because I would also benefit from  a general equation.   

[Admin]

The problem with that chart is that it doesn't factor in motor dynamics (acceleration is also a force). Its not really worth the effort to put it into an equation, because the equation would be wrong except when the motor isn't rotating.

btw, excel can generate equations for you (create trendline option)

[vidam]

There was a related post  http://www.societyofrobots.com/robotforum/index.php?topic=4324.0 which I had forgotten. Admin please delete this post.

[vidam]

   Here is a simple linear equation that relates Power to torque and rotational speed:


    Power (kW) = ( torque (Nm) times 2pi times rotational speed (rpm) ) / 60000

    Substituing Power = V * I

    You can then solve for I (current) and you get how current varies with respect to torque, voltage, and rotational speed.

     http://en.wikipedia.org/wiki/Torque

[Admin]

not entirely true . . . there is a motor constant missing from that equation. that equation only assumes a motor thats 100% efficient