DC Electric Motor Sizing. (help needed)

BFassassin

Hey
I could do with getting some help sizing a motor for a project im doing.
Its a lamppost climbing robot which uses two tracks. Currently it weighs 3.5kg but I could potentially reduce that weight a little bit.
I did some calclations a few months ago but they clearly were not correct as I got a motor size of 1W. I therefore used an old motor I had at home thinking it could work. After building the prototype however it can barely turn one track and cant even move the second one. Here is a picture of the prototype just so you can get an idea of what im at. http://boards.ie/vbulletin/attachment.php?attachmentid=112268&stc=1&d=1272457967

Basically I need to recalculate the motor required and I dont have a clue on where to start. I have some gears in place but cant remember off hand what diameter and what teeth they have.

Can someone point me in the right direction?

DublinDilbert

Trial and error will probably be the easiest way to do this, as theres so many variables. Its going to be all about gearing, look at the amount of torque they get out of a 12V electric drill / screw driver.

Do you have to climb at a certain speed? Can you change the gear ratio? cause you can keep increasing the gear ratio till you get enough torque.

You should also monitor the motor current, to ensure your not over loading it.

FoxT

Say you want a 4 kg unit to climb a 6 metre vertical pole in say 10 seconds.

When it is at the top of the pole, its potential energy = mgh = 4x10x6 = 240 Joules.

So, to get to the top of the pole in 10 seconds, you need to apply 24 Joules/sec = 24 Watts.

Your can vary the above obviously depending on weight & speed. Also make an allowance for friction - which could easily consume 20-30% of the motor's power.

BFassassin

Cheers guys. I have to climb a 12m pole in under 10 mins.
That roughly means that my calculations were around right I just never added in the inefficiencies and friction. I used the angular velocity of a single wheel to work it out before.
If all thats right I need to get looking for a motor now.


edit:http://www.argos.ie/webapp/wcs/stores/servlet/Search?storeId=30001&catalogId=1500002201&langId=-1&searchTerms=7112946
This drill could do the job for me. What do you think?

Forgot to mention I am running it off a 7.2V 2100mAh battery.

spideog7

If you expect it to lift 4kgs but it can't even turn the tracks then either your calculations were way off or there's something wrong somewhere in your gearing! Check to make sure everything is running freely and the motor is getting enough voltage. If you're putting a big current load on the battery your voltage could be collapsing entirely. Try running it off a bench supply before using the battery.

BFassassin

Just to complete the picture I have it geared like this in the pictures.

The driven gear is 35mm dia. with 33 teeth and pushed directly on to the motor shaft
The white gear is 50mm dia. with 50 teeth
Then two small 20mm diameter gears driving the wheels each with 21 teeth.

If I use the 7.2v battery on the 12v drill will it work with reduced power or will it not work at all?

Thanks

spideog7

BFassassin wrote: »

If I use the 7.2v battery on the 12v drill will it work with reduced power or will it not work at all?

It may work but poorly or it may not work at all. You can step up the voltage so long as your battery can supply the current. 2100mAh battery can technically run for an hour supplying 2A but that doesn't mean it can supply 2A if you know what I mean. I'm not sure how you can find the maximum current rating for a particular battery it's probably online for each manufacturer or battery type.

Usually it depends on the type of battery, lithium ones have built in circuits to control them or else they'll blow up (I think 7.2v means li-ion cells??) and other varieties have high internal resistance that can be a limitation.

I definitely think using a bench supply is the best approach, if it changes nothing then at least you're narrowing down the possible problems.