How to hack a servo for continuous rotation - TowerPro MG995

Why would you want or need to modify your perfectly good servo?  It turns out that servo motors are great little gear motors that can connect directly to a microcontroller without the need for a motor driver circuit.  The problem is that servo motors are made to turn between 90 and 180 degrees, for turning the front wheels of a remote control car, turning a rudder, controlling ailerons, etc.

But we want to use these cheap high-torque motors to turn the wheels on our robot.  This instructable will show you how to make your servo spin 360 degrees.

Why the TowerPro MG995?  It's cheap and powerful.  At 4.8 volts, the MG995 produces 138.9 oz/in of torque and spins 360 degrees in 1.2 seconds.  The MG995's are all over eBay for just under ten dollars apiece.  I might not use these to steer a $1,000 RC plane, but they are terrific for powering the drive wheels on your robot.

I highly recommend watching the YouTube video first so you have an overall view of what you will be doing in this project.

Step 1: Parts and tools

Parts and tools

MG 995 Servo
Two 2.2k ohm resistors

Soldering iron
solder
sponge
Needle-nose and diagonal pliers
Flat and phillips head screwdrivers
Helping hands
Electrical tape

**Optional - for testing purposes
Arduino or microcontroller of your choice
3 jumper wires

Comments

[skootles]

Note for others: One problem I had came during the re-assembly. The right-most gear in this picture has a pin on top and for some reason mine came loose when putting it all back together, causing the gear to become dislodged and the motor to not work correctly. I had to open it back up and push down on that pin to make sure it was secure.

Other than that, everything went perfect. Great guide!

[cobourgdave]

Just saw this in April 2013, and I liked your approach. Nice tutorial, clear and concise. Would suggest that a video camera on a tripod would have made the images a little more stable, but what the heck, it WAS viewable. Nice job, hope to see more

[radiogareth]

Nice glossy hack, well done :-)
I found there was space inside to just remove the potentiometer from its drive shaft, leave it in the center of its travel, tape it up along with the metal stop pin and fit it all back inside the servo in case I needed to de-hack it later. Only problem is each servo may have slightly different 'middles' (dead zone), but you could re-assemble the gear train, send it middle pulses and adjust the pot until it stops. HTH :-)
Gareth

[Robin Capehood]

Din't work. Its still doing pulse rotation.

[jduffy54]

I would just like to comment that you can have more than one line per step. Mine are a little long, but most of these steps are one sentence. My internet is slow, so this took like 20 minutes to see the whole instructable.

[meanpc]

Dial-up?

[jduffy54]

No, regular internet. It's not peticularly fast, but its not bad. Main problem was the gazillion steps. Would have helped it there was more than a line of text per step

[meanpc]

Yeh, I can see how that would be annoying. I'll keep that in mind next time.

[Pandalo]

What if i'm not using an arduino? I'm new to servomotors, and i'm about to hack 4 of these pieces. Whats the pulse modulation for these babies when hacked?

[meanpc]

I don't know, but I googled it for you and this was the best I could find:

http://www.societyofrobots.com/robotforum/index.php?topic=5062.5;wap2

[rc jedi]

the drive on the 995's are very robust, but the driver setup buzzes, so they are a great choice for your project. the 996 have better driver circuit, i have used both for some time. servos are so cool. great pictures, thanks for showing it.

[meanpc]

The motor seems to be strong and the gears are metal, so I think it should be pretty robust. Some of the negatives I have read on the net about the MG995 were more on the build quality - especially the quality of the connections of the wiring.

I think they will make good drive motors for a robot though.

Thanks for taking the time to comment!

[jwolski1]

Great job! I just did this with a futaba s3003 controlled by my arduino uno. I didn't want to do the soldering, so I instead just set the pot near the middle. I can still use servo.write(180) and servo.write(0) to go in either direction. To stop, I just do a server.detach().

Thanks for the idea!

[Sandisk1duo]

Is there a way to hack the servo in a way that allows you to read the value of the potentiometer?

Thanks!

[meanpc]

I don't see why not. Couldn't you just hack the potentiometer so that it can rotate 360 degrees, send 5v in one lead, then send the other lead to an analog input on an Arduino or whatever microcontroller you are using? You can test the potentiometer before hand and can tell exactly what position the potentiometer, and thus the connected motor are in. Would be relatively simple I think.

The only tripping point might be that the wipers in the potentiometer would get worn out really quickly, depending on the duty cycle of the motor. Remember the contacts on slot cars back in the day? I imagine the wipers would wear out in similar fashion.

A good idea though, you should try it.

[amandaghassaei]

cool, thanks for posting, I'll have to try this sometime.

[meanpc]

Thanks for looking and commenting. Just checked out your instructables - WOW.

[PeckLauros]

Good tutorial and nice pictures.

[meanpc]

Thanks! I have 4 of these servos, and hacked the first one the other day. I decided to make an instructable with the second. The pictures are still captures of the video my daughter took .