Mechanics and Construction > Mechanics and Construction

Choosing the right motor

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**BADBOY13**:

Hi,

I'm building a robot that weighs ard 60kg and can climb stairs.

However, i'm having some problems with finding the right motors for it.

I suppose i need to calculate the amount of force needed to move the robot up the stairs.

(Please correct me if i'm wrong)

To make calculations easier, this is wad i did:

Assume the flight of stairs to be a slope of the same gradient(gradient of stairs is taken to be gradient of a line drawn from the edge-to-edge of 2 steps).

Take angel of elevation = 40 degrees

weight of robot = 600N

Then, force needed to move robot up the "slope" (assuming "slope" is smooth):

Sin40 / 600 = 1600N

However, since internal resistance of the the system and resistance due to the "uneven-ness" of the stairs cannot be ignore, thus, i assume the total resistive force to the motion to be 400N.

thus, the total force needed = 2000N

to have the robot moving up the stairs at a speed of 1.2m/s,

power need: 2000 X 1.2 = 2400J/s

~ 3.3 hp

i'm looking for a motor that is 40mm or less in diameter and ard 10 cm in length.

I'll be using 4 motors on the robot, meaning i'll need each motor to have the out put of 3/4 - 1 hp.

Does anyone have anything to recommend?

I've checked out many catalogs, i realized there's 2 different torques given, on is the torque at max. efficiency and the other is the stall torque.

Using the formula, (torque X rpm) / 5252 = horsepower,

i realized that if i used the stall torque in the calculation, many motors can provide the hp i need.

but if i were to use the torque at max. efficiency, no motor in the size i need have the amt of power i need.

Am i supposed to use the stall torque or torque at max. efficiency in my calculation to choose the motor i need?

**Admin**:

Try my .xls calculator on this page:

http://www.societyofrobots.com/mechanics_dynamics.shtml

(it includes the angle calculation for you for the stairs)

--- Quote ---i realized that if i used the stall torque in the calculation, many motors can provide the hp i need.

but if i were to use the torque at max. efficiency, no motor in the size i need have the amt of power i need.

--- End quote ---

Be aware that the max efficiency situation will never occur, and the stall torque situation should never ever occur. You should operate somewhere in between, depending on your requirements.

--- Quote ---Am i supposed to use the stall torque or torque at max. efficiency in my calculation to choose the motor i need?

--- End quote ---

Ideally, you are supposed to look at the motor datasheet. It will, at several voltages, give you an rpm vs torque curve. Possibly even power draw. Then you can decide if the rpm at a specific torque is acceptable to you. But unfortunately the cheaper motors do not come with this data and its all guess work for you . . . So basically, there is no good way to do the calculations unless you have a datasheet.

**Ro-Bot-X**:

I just saw this post, since I was also looking for motors. This is what I have found that might work for you:

http://www.robotshop.ca/home/products/robot-parts/motors/gear-motors/planetary-gearmotors/rbban32-banebots-planetary-gearmotor.html

I think it is too powerfull and too power hungry so I was looking at this one instead:

http://www.robotshop.ca/home/products/robot-parts/motors/gear-motors/gear-motors-with-back-shaft/lynxmotion-ghm-15-gear-head-motor.html

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