servo motor or differential drive

[anilg]

Hi,

Friends and myself are planning a robot car, and was wondering what the best method for control should be.

Our robot would end up weighing ~1.5Kg, and would be controlled by a lead acid 12v battery. What would be the recommended method for steering ?

The options I have are a 3 wheel robot (2 rear wheels controlled by a DC motor +  1 servo motor controlled steering) or a 4 wheel robot (the 2 on the left side controlled by one DC motor, the 2 on the right by another. Steering should be done by differential turning)

[SmAsH]

you could make it like most rc cars see pic. but thats hard. i recommend diferential drive for it. its a lot easier to do and do you plan on putting in any sensors? making it autonomous? if so read up on the tutorials. and fell free to email me at [email protected] or pm me if you have further questions.


~smash

[airman00]

differential is the easiest but what I would do is have two modified servos and one wheel on each side of the bot. Then one caster wheel for balance. Meaning, differential but with one wheel on each side

[SmAsH]

lol thats like in one of the tutorials right?


~smash

[anilg]

differential is the easiest but what I would do is have two modified servos and one wheel on each side of the bot. Then one caster wheel for balance. Meaning, differential but with one wheel on each side

Is this design better than a tricycle like design with a motor controlling the backwheels and a servo controlling the caster? How so?

[anilg]

Hmm..

Setting up shouldnt be an issue. I'm using Sun spots (sunspotworld.com) to build this robot.. it will be wirelessly controlled by hand. We might add autonomy later on. We're participating in a race-and-pick-objects-up contest and the idea is to get the best possible steering out of it.

I'm not much worried about the ease as the Spots allow for very easy programming.
More details about the robo:

Size : app 20x20 cm (6x6 in)
it will have a 6" crane mounted that has pulleys to control a rope that will hoist/lower objects.

We thus need a stable car that will turn easily. I'm partial towards using a servo as it will give me better  control over steering but I'd change my mind if someone who has used only differential drive will vouch for the design's stability in moving around.

I can provide any additional details that you'd like.

[airman00]

lol thats like in one of the tutorials right?


~smash

I dunno

but thats what I've always used ( except once , when I was scrapping R/C cars ), ill upload some pics of some of my bots and r/c stuff tomorrow

[airman00]

ill vouch for differential drive


it allows sharp turns , imagine a tank turning   , it can turn on the spot   

just the human operator has to practice with driving it .....

[SmAsH]

yea dif drive rocks for sharp turning but isnt normally as fast as normal drive. but yea turning isquite hard as it turns quickly, and yay i love ot see pics of scrapped r/c's ill post some now

[anilg]

Ok.. so I'll go ahead with the dif drive then.

This probably needs a new post but..

I'll now need 3 motors 2 to drive the wheels, and one to run the pulley to pick up objects. Now Spots have eight output pins. Vcc and Ground will be points on the 12V lead acid battery. What is the minimum number of Hbridges I have to buy to control these 3 motors forward and backward? Any better plan than 3 different h-bridges?

[SmAsH]

YAY for diff drive!!! moving on....
well for the h-bridges 3 how do you plan on picking up the objects? like claw? magnet? suction

[anilg]

YAY for diff drive!!! moving on....
well for the h-bridges ..  how do you plan on picking up the objects? like claw? magnet? suction

hmm.. just a  motor to controll a claw like lever.. this motor should turn both clockwise and anti-clockwise.. I also saw something called the L293 chip that can be used for controlling? How good is that..

[SmAsH]

its a h-bridge so yes it allows it to go both ways...how do you plan on making a claw...theyre extremely hard.. i would recommend suction or magnetics.


~smash

[anilg]

its a h-bridge so yes it allows it to go both ways...how do you plan on making a claw...theyre extremely hard.. i would recommend suction or magnetics.

Mine is a very primitive claw .. just a long stick with a bent edge..this will have to be guided into a ring that is attached over the object to be picked. I can put out a small sketch of the car once we have it ready.

[SmAsH]

oh ok good luck

~smash

[Fredrik Andersson]

YAY for diff drive!!! moving on....
well for the h-bridges ..  how do you plan on picking up the objects? like claw? magnet? suction

hmm.. just a  motor to controll a claw like lever.. this motor should turn both clockwise and anti-clockwise.. I also saw something called the L293 chip that can be used for controlling? How good is that..

Have you looked at servos yet? I think that would be the optimal to go for in this case. An unmodified servo is made for these kind of movements. It will give you a simple interface to control: you simply send the position (perhaps angle would be more accurate) to the servo will align itself to that position.

[anilg]

Have you looked at servos yet? I think that would be the optimal to go for in this case. An unmodified servo is made for these kind of movements. It will give you a simple interface to control: you simply send the position (perhaps angle would be more accurate) to the servo will align itself to that position.

That was the point of this thread.. do you have experience with both.. and in that case what would you recommend? servo or differential drive?

Regards
Anil

[airman00]

That was the point of this thread.. do you have experience with both.. and in that case what would you recommend? servo or differential drive?

You don't understand. You can modify a servo for it to go like a regular motor  , the advantage is the servo already has built in gearing and drive circuitry  http://www.societyofrobots.com/actuators_modifyservo.shtml

So you have two modified servos for the drive wheels and have a servo for the claw.

[anilg]

You don't understand. You can modify a servo for it to go like a regular motor  , the advantage is the servo already has built in gearing and drive circuitry  http://www.societyofrobots.com/actuators_modifyservo.shtml

So you have two modified servos for the drive wheels and have a servo for the claw.

Oh.. I see now. This is cool.. a couple of questions..

*Will I get the same torque (speed of the car) as a DC motor?
*Will I be able to rotate oth ways (Ex 0-180 is anticlockwise and 181-360 is clockwise)

[ed1380]

about servo's being as strong as DC motors. if price stays the same usually not.  you'lll probably need $50 servo's to be able to handle the weight load.
but dc motors need h-bridges. also kinda expensive.


servo's turn either 90*  or 180*
and that 90/180 is full movement. meaning 45* right, 45* left. etc

to use them for driving, you need to modify them for 360* movement