need help controling dc motors

im having a prob. maybe someone here can help.

im going to be making a robotic arm for my wheelchair to lift and hold my tablet pc. im going to be using 4 mg996r servo motors moded to be geard dc motors. i own a (owi robotic arm edge robot) im hoping to use the pc board out of it to control the 4 motors forward and backwards,but i cant seam to figur out how to hook up a 6v acdc adapter to it to do so. here are pic of the edge robot schematics and a link to the manual (pdf) if anyone can help or know a easer way to control 4 motors both ways and not with arduino as i dont know enuff about them yet.

edge robot manual pdf

Picture of need help controling dc motors
robot arm idea.jpg
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The trouble with your proposal is that a servo modified the way you are thinking won't stay where you put it, you need a drive that WILL stay where you put it. so a servo is just what you want.

I have grave doubts you'll have enough torque to even lift the load you are proposing. These are pretty small motors. It has a 10kgcm torque rating, so at 2cm, it will hold 5kg, at 10, 1kg.

skullkeeper80 (author)  steveastrouk6 years ago
thank you here is a link to the servos i have  i have a minimus avr could i run the servos with it? i want to run them manuly.

its only going to be
sholder-elbo 5-6"
elbo-wrist 5-6"
wrist-table 2-3"

what do you recammend cheap and simple?
I think the minimus will work for you. The Ebay ad says the servos are torquier than the link I saw.

SO worst case shoulder-elbow straight out, 150mm, elbow-wrist straight out, 150mm.

Worst case load on elbow/wrist = 1.1 kg@150mm = the limit.
Worst case on shoulder elbow = load of laptop@300mm+load of elbow joint weight@150mm = too much.

Let me have a further think about this.......

I'm thinking you may be better making the servos into an element of a bigger servo. That is, essentially, convert the servo to "constant rotation", add another pair of gears or toothed belts - these servos are pretty damned quick - they'll spin 360 degrees on 0.8seconds, so losing speed is a good plan, unless you want to throw your tablet PC at people.

NOW take the pot from the original servo, or otherwise, put a pot on the joint of your arm, and wire it back to the servo electronics. If we assume you can get a 4:1 reduction in the gears or belts (easy), you end up with 4 x more torque (win) and 4 x slower drive (win x 2)

Would a picture help ? Who's going to build it with you ?


OK, Here are a couple of things, one is a simple Jpeg for you, the other attachment is a 3D PDF file - open it with Acrobat, click the image, and you can then rotate the 3D model to see what it does.
Arm joint.jpg
You can also use belts and pulleys, instead of gears - which WOULD look a whole lot funkier ;-)

I use these guys

A 1:5 reduction would cost you about 12 Euro, using a 12T and 60T pulley.

skullkeeper80 (author)  steveastrouk6 years ago
do i just need to add the gear on sholder-elbo or elbo to wrist also?

do you have any good links to get gears and rods?

what do you think about this

and what program did you use for 3d i like that was very helpfull

The program I use is called "Alibre Design" In its professional version, its over 1000 Euro, but there is a cut-down version for 100.

skullkeeper80 (author)  steveastrouk6 years ago
cool iv got a real good idea how to build it now ill just have to use trial and error but i think i can make it work

now i just need to figure out how to get the motors going

do you know witch instructable is best for making the servos full rotate theres alot of dif ones

thanks ill check out that program
I don't have a particular recommendation, but don't forget that you need to put the pot on the joint attached to each servo.
skullkeeper80 (author)  steveastrouk6 years ago
ok now im lost again what do you mean by a pot on each joint?

see if im right...remove pot from servo solder 3 wires where pot was put pot on joint solder it to the 3 wires from servo. if so how does it go on joint
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