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need help controling dc motors Answered

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


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.


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 = ~15kg.cm...on 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.


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

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.

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

YES. You're using the original servo gears for your new servo, and cheating the electronics into believing everything is still the same.


sorry was that a yes do that mod with resisters and not put pots on joints just gears


yes do mod no resisters extend pots to joints

cool got it thank you i think iv got all this figured out how it works now i think..lol

now i just gota find the gears been serching all day but dont understand all the size,pitch,gap stuff so im lost on witch gears to get that will work with the servos gear

Getting something on the servo is going to be the trick for sure.


i found these but not sure witch one and what gear to go on joints that will match and be big enuff

what do you think

Those look fantastic - I didn't even know you could GET gears with the appropriate splines !!!

These gears look like they are specified in DP not MOD, so be careful to find matching ones.

Look at these 48P gears


- and go for a 5:1 ratio with your servo gear



glad i could help you as you have helped me so much

what do you mean by a 5:1 ratio?

how many teeth should i go on servo gear less or more and tooth,Diameter
on spur gear?

Small gear on servo, large gear on arm. Look at the gears you can get for the servo, try and find one with close to 5 times more teeth for the arm joints.

If you need anything machining for this project, PM me - I have a very large workshop.


could i get your expert opinion on a few things.

to run the motors iv got 3 options so far

option 1 $30 not cheapest thing but no programing needed easy manual control

option 2 $18 cheap but dont know how to write code or wire any of it up for manual control

option 3 already own but dont know how to write hex code for it or wire it to run the servos

also besides the belt gear got any other good links for the gears i need for the joints? i like the gear option over belt.

cool that helped alot so does it mater what size the green gear is in diameter?

i was thinking i had to add a gear to the servo innards so adding that gear fixed servo would give more torque less speed do i got that right?

In a sense, the bigger the better, you get more reduction - and you have speed to throw away from the original servo. 5:1 is a practical ratio in a single reduction though.

Exactly, the gear fixes like this increases your torque, and reduces your speed.


here is the servo specs

Compliant with most standard receiver connector: Futaba, Hitec, Sanwa, GWS etc...
Great for truck, Boat, Racing Car, Helicopter and Airplane.
Power Supply: Through External Adapter.
Stable and Shock Proof. - Metal Gear - Double Ball Bearing - Connector Wire Length 300mm
Dimension : 40mm x 19mm x 43mm
Weight : 55g
Operating Speed : 0.17sec / 60 degrees (4.8V no load)
Operating Speed : 0.13sec / 60 degrees (6.0V no load)
Stall Torque : 13 kg-cm (180.5 oz-in) at 4.8V
Stall Torque : 15 kg-cm (208.3 oz-in) at 6V
Operation Voltage : 4.8 - 7.2Volts
Gear Type: All Metal Gears
Connector Wire: Heavy Duty, 11.81" (300mm

wow you lost me there.lol

yes pic would be very helpful

what parts would i need and how do i do that?

sounds promising so you think my avr can run them sweet

i will be doing it with a friend that helps me i do what i can then she becomes my hands im the brains lol

thank you so much for helping me get this going im hoping to have it sorted befor i finesh my tabled (converting acer aspire one) to tablet and im getin close

You'll have to do some maths to work out whether you can get away with it. My view is that at least one drive won't be man enough. Work out the torque at each joint, for which you'll need to know the mass of the servo and the arms (well, guess a bit for the arms - aluminium ? )

An AVR offers several channels of native PWM four ? but rolling your own code to get more isn't too hard.

Sounds interesting project - how will you manipulate the arm ?


hello steveastrouk i tryed using a servo calc. i found on a website here but dont understand some of it. sorry im new to servos im an artist and a i guess moder. i take stuff apart and mod/adapt it so i can use it with my disability.

im not sure how to read servos realy eather i just know more tourque more lift.

aluminium close im going to be using purfarated strips.
i dont know a thing about code i bot the avr for doing the ps3 mod and just flased it with the premade hex files.

i want to drive each joint/motor manualy if posable with a ps3 thumb joiystick/pot each stick has 2 pots so 2 motors per stick = less bulk on my wheelchair and i have no arm movement so everthing i do is by mouth

is there a any other way besids arduino? if not wichone and what about code.
im just so lost on the hole motor part .

any recamindations from anyone let me know. (strong enuff servos) (a way to run them) but keep it cheap.

thanks everyone for helping

So you'll manipulate the PS3 stick by mouth then ? I think you'll have to get the stick out of the PS3 to do this - interfacing the PS3 controller directly is definitely a project in itself.


yes manipulate the PS3 stick by mouth i have 4 already that i orderd there cheap plus i had a couple my last project was building an arcade type joystick for the ps3 out of a ps3 controller with button layout that i could use plus added push lock buttons for when i need to push and hold a combo of buttons like for resident evil (hold L1 + X) to shoot i can just click L1 locking button then just hit X untill im done then hit L1 to unlock

its almost done just need to wire it up and connect the 3rd joystick. yes 3rd i added a anolog stick moded for the up,down,left,right as its easer to control with the chin then the normal thumb one or big arcade joystick

So this thing is really to just pitch and tilt the monitor for you to see it more easily whereever ? Does it need to be able to extend and lift do you think ?

I'm currently working in Korea - and I have to go to work in a minute - I'm sure we can get something working with those servos.


it will be on the side of my chair the reson for lift and extend is so i can lower it down to the botom side so it is out of the way of transfur

its hard to tell by my sketch but motor 4 is to rotate the table so it will lay flat aginst the side.

Give me some lengths for the joints and let me think about it with you.


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

thank you

thanks for replying im going to be changing the servos to be geard dc motors like here https://www.instructables.com/id/How-to-modify-a-servo-motor-for-continuous-rotatio/ but i was hoping to use the pc board from the edge robot to controlle them and i would be using a 6v 2a adapter the diagram is from the edge robot that has cheap 3v motors and runs on 4d batterys
im just not sure how to run 4 motors forward and backwards without circuits and stuff

Never really worked with servo motors but I think they have to be controlled with PWM pulse width modulation from a microprocessor to work and not just switched on with full power or they burn out. Maybe look at the other servo/robot instructables to figure it out. Good luck.

and someone will comment on your step-down transformer need for 6 v to 3 v as indicated in the diagram.