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Is this spec of motor good enough to run a small robot with wheels of diameter 8cm at a speed of roughly 4m\s

Posted on 2004-11-01
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Is this spec of motor good enough to run a small robot with wheels of diameter 8cm at a speed of roughly 4m\s

MOTOR, STEPPER 18 DEGREE; Angle, indexing: 18°; Centres, fixing: 25mm; Diameter, external: 20mm; Length / Height, external: 17.2mm; Length, shaft: 6.2mm; Phases, number of: 4; Power consumption: 0.74W; Resistance, phase:

If not could someone please give me the correct spec. it's pretty urgent so I would really appreciate it. thanks!
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Question by:AnthonyFee
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Physicistm earned 600 total points
ID: 12463196
Off all the data , the one that worries me the most is the Power consumption: 0.74W, well how fast it will go depends on the load you put on it .. the mass of the robot ... but judging by what I see ...

Not quite sure that such tiny motor could reach that much speed ... nevertheless,
by my calculations you need about 16 turns per second on the wheels.. well including a transmission from the motor to the wheel which should be at least say 3:1 means you need at least 48 turns per second , which means about 2880 turns per minute, which is quite high. If you're using transmission 1:1 then 960 turns per minuta should be enough.

The steping of 18 degree can assure you 20 steps per revolution...which means that this is quite a quick step motor...
Well I think this is possible. But like said depends on the mass of the whole thing ... should probabyl use some reductor
and offcourse you have to watch out for the pulse frequency that you give that motor...
by my calculation you need like at least 20 kHz source - pulse frequency to drive this thing.
I think this is achievable. This should be working, but if it's to heavy you'll need a stronger motor, or use a reductor at higher source frequency (multiplied by the transmission coefficient)

I hope my thinking can help you out here.
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by:AnthonyFee
ID: 12471589
Just if you have a second could you go to this site and just quickly have a look and see which would be most suited keeping in mind i'm on a pretty tight budget. thanks for the help!

http://ie.farnell.com/jsp/endecaSearch/searchPage2.jsp?Ntt=stepper+motor&Nty=1&N=401&Ntk=gensearch
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by:Physicistm
ID: 12471630
Well, it's kind of a hard to predict what you actually need as I don't know what size your robot will be and weather the main load (mass of the robot)on the robot is actually the motor and the batteries itself.
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by:d-glitch
ID: 12504301
You can't tell without some calculations, but they need not be too difficult.

The biggest weight is apt to be the batteries, so you also have to decide what you want for range and operating time

To give yourself some margin, require that your robot be able to climb some small incline angle ( A of 10 deg for example) at some velocity (v) .

Now you can calculate the climbing power the motor must deliver:      

        P  = sin(A)* M* g* v        with     P in  Watts     M in kg     v in m/s     g=9.8 m/s^2


You have to go the motor data sheets to find Torque and angular velocity:

The formula shaft power is

       P  = omega * T                  with    T in N*m       omega in radians/s  ==>  omega = 120 * Pi * (v in RPM)

       

http://www.318ti.org/notebook/torque/
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by:d-glitch
ID: 12507111
Here is a sample calculation for a small robot:

Let the total mass of the robot be 2 kg including 0.3 kg for a 6 volt, 1.3 AH lead acid battery which lists for $8.00

                             http://rocky.digikey.com/WebLib/Panasonic/Web%20data/LC-R061R3P.pdf


We require that the robot climb a 10 deg incline at 1 m/s.  We can find the climbing power:

                            P  = sin(10)* 2 kg*( 9.8 m/s^2) * ( 1 m/s)

                                = (0.173) * 2 * 9.8 * 1

                                = 3.4 watts       ==> So the 0.74 W motor you mentioned in the question could not drive this robot.


The current drain from the battery at 3.4 watts is

                            I  =  P/V  =  3.4/6  =  560 mA


From the battery data sheet (Duration of Discharge vs Discharge Currrent) you can see that the battery would be dead in 1 minute.

On the flat, it might last five times longer.  You might decide on a bigger battery.  Maybe you can cut down on the mass.  Maybe 1 minute is enough.

You may have to go through several rounds of calculations like this to optimize robot performance for your specific requirements.



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by:d-glitch
ID: 12508570
Horrible job reading the data sheet:

The battery will last for  1 HOUR  not 1 minute.

The Y163 motor at 50 Euros would probably work.  A slightly smaller (0.5A rather than 1.0A) would be fine too.  But I didn't see one on that list.
 
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