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  #16   Spotlight this post!  
Unread 06-22-2012, 10:58 AM
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Re: Choosing a Battery

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Originally Posted by Ether View Post
but the real question is how hard are you going to be driving the motors.

I called my mentor and he said that the design requirments state that the robot should run for 45min but in all honesty the robot is only going to be used in short burst (~5min) but it should be able to handle this type of running for a long period of time on one battery charge.
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Unread 06-22-2012, 11:06 AM
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Re: Choosing a Battery

Eric,
Please keep in mind that in a two wheel configuration, the motors will need to be capable of accelerating to a higher speed than your target in order to accomplish balance. As I remember the Segway is driven via toothed belts and max speed on the police version is I believe 24 MPH.
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Unread 06-22-2012, 11:45 AM
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Re: Choosing a Battery

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Originally Posted by Al Skierkiewicz View Post
Eric,
Please keep in mind that in a two wheel configuration, the motors will need to be capable of accelerating to a higher speed than your target in order to accomplish balance.
@Eric: In other words, when you're cruising along at your desired 3 m/s, you need some headroom to increase the voltage.

Suppose you leave 2 volts headroom, so you're operating at 10 volts at 3 m/s (I have no idea if 2v headroom is enough or too much, this is just for sake of discussion).

CIM free speed at 10v is 4425 rpm. To go 3 m/s at that motor rpm with 12.5" dia wheels, you'd need a gear ratio of 24.5:1, not 34:1.

Of course, at free speed a motor is producing no torque output. So let's say 90% of 10v free speed, or 3983 rpm CIM speed (again, I do not know if that is sufficient1). Now you need a gear ratio of 22.1:1.


1At 10v and 3983 rpm, each CIM produces about 28.6 ozin torque. Run this through the 22.1:1 geartrain and assume 10% torque loss in the drivetrain and you've got about 3 ft-lb torque at each wheel. For a 12.5: dia wheel this translates to about 5.7 lb motive force per wheel, or 11.4 lb total for both wheels. Is that enough to move your 100 lb robot+payload at 3 m/s up a 5% incline on a gravel surface?


Last edited by Ether : 06-22-2012 at 11:47 AM.
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Unread 06-22-2012, 01:47 PM
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Re: Choosing a Battery

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Originally Posted by Ether View Post
@Eric: In other words, when you're cruising along at your desired 3 m/s, you need some headroom to increase the voltage.

Suppose you leave 2 volts headroom, so you're operating at 10 volts at 3 m/s (I have no idea if 2v headroom is enough or too much, this is just for sake of discussion).

CIM free speed at 10v is 4425 rpm. To go 3 m/s at that motor rpm with 12.5" dia wheels, you'd need a gear ratio of 24.5:1, not 34:1.

Of course, at free speed a motor is producing no torque output. So let's say 90% of 10v free speed, or 3983 rpm CIM speed (again, I do not know if that is sufficient1). Now you need a gear ratio of 22.1:1.


1At 10v and 3983 rpm, each CIM produces about 28.6 ozin torque. Run this through the 22.1:1 geartrain and assume 10% torque loss in the drivetrain and you've got about 3 ft-lb torque at each wheel. For a 12.5: dia wheel this translates to about 5.7 lb motive force per wheel, or 11.4 lb total for both wheels. Is that enough to move your 100 lb robot+payload at 3 m/s up a 5% incline on a gravel surface?

@Ether

Suppose all of these calculations are correct and work for the robot. How do I determine if the battery will last 45min?

PS: I'm an electrical engineer intern so I don't really know a lot about the mechanical side of things so this might be an easy question for you to answer, but when you say 11.4 lb total motive force does that mean that the force would be able to move 11.4 lbs? so therefore not enough to move my 100lb robot+payload?
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Last edited by badtkee : 06-22-2012 at 01:52 PM.
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Unread 06-22-2012, 01:59 PM
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Re: Choosing a Battery

The motive force required to move a 100 lb robot is usually not 100 lbs, it's some percentage of that. But it depends on rolling resistance, how much acceleration you want, whether you're going up hill or down hill, if you're turning, etc. And if it's a balancing robot, you're going to have a balancing load...you will probably be driving the motors in an oscillating manner to keep the robot upright, so you will be using power just to stand still.

This is very hard to figure out with numbers....which is why I suggested you do it empirically. Try it and see what happens.
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Unread 06-22-2012, 02:08 PM
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Re: Choosing a Battery

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Originally Posted by squirrel View Post
it depends on rolling resistance, how much acceleration you want, whether you're going up hill or down hill, if you're turning, etc. And if it's a balancing robot, you're going to have a balancing load...you will probably be driving the motors in an oscillating manner to keep the robot upright, so you will be using power just to stand still
All true.

Quote:
Try it and see what happens.
I don't get the impression he has a working prototype to try it on.

If he did, he would be able to provide a lot more detailed answers to the questions being asked.



Last edited by Ether : 06-22-2012 at 02:11 PM.
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Unread 06-22-2012, 02:11 PM
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Re: Choosing a Battery

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Originally Posted by Ether View Post
I don't get the impression he has a working prototype to try it on.

you're 100% correct. I'm in the design stage. I've just been doing research, research and more research and have just started the calculations and trying to figure out specific things
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Unread 06-22-2012, 02:21 PM
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Re: Choosing a Battery

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you're 100% correct. I'm in the design stage. I've just been doing research, research and more research and have just started the calculations and trying to figure out specific things
Here's a motor calculator I wrote that may help with some of the calculations.


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Unread 06-22-2012, 02:41 PM
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Re: Choosing a Battery

Thanks! I just gave a quick glance at the calculator and I'm sure its going to save me a lot of time.
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Unread 06-22-2012, 03:11 PM
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Re: Choosing a Battery

Leave lots of room for batteries on your prototype
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