# Society of Robots - Robot Forum

## Electronics => Electronics => Topic started by: Mozes on March 15, 2007, 12:59:34 PM

Title: Choosing a Motor (new here)
Post by: Mozes on March 15, 2007, 12:59:34 PM
hey all,
i'm pretty new in this field of electronics and robotics, but hope to get it quickly.

here are my calculations:
I want a robot with:
velocity: 2 [m/s]
acceleration: 0.8 [m/s]
mass: 20 [kg]

so rps is about 2.2 => RPM=127.
RMF > (20 * 0.8 * 2)/(2*pi)
RMF > 5.09
Torque = RMF/rps = 5.09/2.2 = 2.31.

isn't the torque too low for 20 kg?
and why all RPMs are so high? anyplace I searched motors were like 4000 RPM! and I need only 130..
any ideas?
thanks

Title: Re: Choosing a Motor (new here)
Post by: Somchaya on March 15, 2007, 01:02:42 PM
So I'm not very good with the calculations (I tried it with my robot but I did something wrong), but usually motors have really high RPM without load. Once you put a load on it, the RPM will drop.

Also, the torque on these motors are usually really low, so what you'd want to do is to gear it down so the final RPM is lower, but the torque is higher.

Alternatively, you can use a modified servo (which has a RPM of around 60) and maybe gear it up if you really need to. Servos have a lot more torque than motors for their size I believe, and it's much easier to work with imo.
Title: Re: Choosing a Motor (new here)
Post by: hgordon on March 15, 2007, 01:13:17 PM
http://www.robotmarketplace.com/marketplace_gearmotors.html
Title: Re: Choosing a Motor (new here)
Post by: Mozes on March 15, 2007, 06:11:41 PM
in that site they use (for torque) in/lbs, means inches per pounds, and my result was 2.31, but what units? is it newton metre? N*m ?
anyway I do I convert the units?
Title: Re: Choosing a Motor (new here)
Post by: Jon_Wayne on March 15, 2007, 06:58:43 PM
im a begginer also, but ive looked into this for myself. important factor: how many wheels that you are using are motor driven? i will assume 2. also, im using a spreadsheet i made and with two wheels you need a motor that can put out 43218 g-cm of torque or 600 oz-inches. thats about right. but also the speed of the wheel will be 125 rpm. motor like that will be hard to find. if all you can find is 4000, look for a gearhead motor that is geared 30:1. or make your own gear system, but that i imagine can be pretty hard and wont be as effiecient. if you use 4 motor driven wheels, torque will only have to be 21600 g-cm(300 oz in). maybe use "power wheels" motors? you know the little toy for kids to ride around in.
Title: Re: Choosing a Motor (new here)
Post by: hgordon on March 15, 2007, 09:46:44 PM
use this for conversion -

http://www.onlineconversion.com/torque.htm

by the way, it's not inches / pounds, it's inches * pounds.  the slash on the motor specs is an error
Title: Re: Choosing a Motor (new here)
Post by: Mozes on March 16, 2007, 05:47:25 AM
thanks.

Jon_Wayne, can i have this spreadsheet of your?
86434 [g-cm] = 75 [pound-inch] - that's the sum of all motored-wheels' torque. make sense.
about the RPM, I guess i would decrease the size of my wheels.. no?
Title: Re: Choosing a Motor (new here)
Post by: Tsukubadaisei on March 22, 2007, 06:53:42 AM
My advice is, when you are selecting a motor, search for the torque. Makers usually have pdf files with some usefull data like torque, proper voltage and rpm. They also have this info written on the product packages. So for your case you should buy (by my calculations) a 2.4 N.m motor. But here is the problem: the unity you are using is metric. Just look again in your original post and you will see that you wrote 20kg and 0.15m and so on ( your result gets, then, 2.31N.m) and those units belong to the metric system  Unfortunately in U.S. and Britain they don't use metric system. From your post I could interpret that you misunderstood you result and interpreted it as being 2.31 g.cm, which is not enough. Converting the units 2.4N.m=240000g.cm and a motor with such a power is not affordable at all.  :'(
Since your description is lacking of lot of data I calculated the torque using the simple definition of force and torque Torque = force * radius = mass * accel * radius = 2.4 N.m.
If you want an advice, mine is: reduce the weight of your robot. He is too fat. 20kg is a monster(those are 4 packs of rice)(traumatized foreigner living in Japan who is sick of eating rive for breakfast, lunch and dinner :'( ). I have no idea of the purpose of your robot but you said you are a beginner so I don't think it will be something complex full of heavystuff like batteries and lots of motors and so on. Simple robots can weight about 1kg or even less. Use plastic, correct batteries, an adequate microcontroller and plan well. I am sure you can reduce 80% of this weight. ;)
Use not only 1 but 2 motors(one on each wheel)(in that case I recommend servos or stepper motor that are easier to control) or lots of motors connected to an axis. By doing this you will be able use weaker motors and/or get some extra torque.
Usually cheap DC motors and even some servos don't have enough torque but most of them have high RPM. By using gearboxes you can "convert" the excessive RPM in the necessary torque.
And if you really want to make a big and heavy robots, consider using bigger wheels as well. But be aware: avoid heavy wheels. Not only because of the mass but one should consider the rotational inertia as well, which will drain some extra power.

Anyway, I respect a beginner who recognizes the necessity of planning beforehand and calculating important aspects of a robot such as the torque of the wheel motor.Keep going and you might build something great ;). Unfortunately, the calculations on the tutorials are very simple (no offence ;)). When building a robot there are a lot more variables to be considered like the number and efficiency of motors, friction, variable mass( in case the robot collect stuff), momentum, rotational inertia and momentum and so on. There are cases that one will have to solve diff. equations and/or look for data in tables. Of course, I am actually considering writing myself a more sophisticated, but relatively easy to understand, tutorial with some detailed explanations for the curious ones, both for mechanics and electronics.
Title: Re: Choosing a Motor (new here)
Post by: mhalavo on March 28, 2007, 08:36:01 AM

Here's a good source for some cheaper gear motors:

http://www.trossenrobotics.com/store/c/2882-DC-Motors.aspx