Repurposing a broken Servo360

Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 21,321
edited November 15 in Robotics Vote Up0Vote Down
In a process that can only be described as abuse, I managed to chip a gear tooth in my new Servo360, rendering it kinda cluncky. Rather than discarding it, I realized that its internal encoder still worked just fine but would work better with a free-running shaft, which meant discarding the gear train. That much was easy to do: just remove the four long screws holding everything together, pop the gears out, and put it back together. The only tricky part was getting the middle gear out, since it's underneath the output shaft gear. But that was remedied by removing the middle gear's shaft and just sliding it out. Here's what was left:

no_gears.jpg

Then I thought it would be nice to eliminate the fourth wire lead, leaving a nice compact three-wire cable for the encoder. That's when things got -- well -- messy. Here's what the wiring looked like before the mods I made:

original_wiring.jpg

It would have been simple to remove the yellow wire and the white wire and to solder the white wire where the yellow one was. And that's what I should've done, but I had more ambitious plans. I thought it would be cool to remove the motor/driver assembly to play with later on and proceeded to remove both sets of red and black wires and to solder the ones from the cable directly onto the encoder board. After doing so, I realized that the two sets of red wires were not connected together. There's a voltage regulator on the motor driver board that provides the encoder's power. No biggie, I thought: I'll just always run the encoder on 5 volts. Then I looked at the datasheet for the Servo360, which specifies that the PWM output is 0 and 3.3V. Nuts!

But maybe the encoder can still run on 5V? I looked up the datasheet for AS5600 Hall-effect chip used in the encoder. Lo and behold! It can run on 5V. But there's a catch: to do so it has to be wired differently, along with an additional filter cap. So I checked for a tell-tale short between the chip's pins 1 and 2 and, indeed, it was there. So no chance to run it on 5V without some major surgery. Nuts, again!

So I undid all the wiring I had so patiently soldered and put everything back the way it was, except for soldering the white wire to the encoder board:

rewired.jpg

Finally, for good measure, I jumpered the original pad for the white wire to ground to keep the motor controller from responding to noise:

jumper.jpg

That's all for the wiring. However, since the yellow wire was completely removed, the remaining three wires were loose in the little rubber strain relief. So to remedy that, I added a dab of hot-melt glue to the inside to keep them from pulling out:

hot-melt.jpg

From there, it was just a matter of reassembling and testing. Amazingly it worked. Now I have a nice absolute encoder with a ball-bearing output shaft that turns freely! And it's still a bargain at the Servo360's retail price!

-Phil
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“Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery

Comments

  • 14 Comments sorted by Date Added Votes
  • Thanks for documenting this Phil!
    Whit+

    "We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths." - Walt Disney
  • That's pretty sweet! Thanks for showing us the ropes on this! Wonder what project you will make with it now? :-D
  • Looks very much like the standard Futaba S-148 style of gear set. You can readily get replacements, and they're cheap, though they'll vary based on gear ratio. Might be an alternative next time should another servo bite the dust.

    I'd be interested in looking at the output shaft side under the encoder board...
  • I'd be interested in looking at the output shaft side under the encoder board...
    There's not much to see. The plastic shaft houses a tiny puck-shaped magnet that floats over the Hall-effect sensor. The magnet would have to be polarized cross-wise.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • It's amazing how the kid inside us all comes out as adults when we start taking things apart and playing with them.
    Thanks for documenting this for us tinkerers that were curious.....
  • Nice job! WRT 3.3V vs 5V, the magic smoke remains safely inside the servo housing.

    For now.

    Biding its time...
    "When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."

    - Pablo Picasso
  • The plastic shaft houses a tiny puck-shaped magnet that floats over the Hall-effect sensor. The magnet would have to be polarized cross-wise.

    It would have to be diametrically (cross-wise) magnetized to work for this function, but I was wondering if it was a disc/cylinder or ring magnet. I was thinking for price it could be a ceramic ring magnet, but those would likely need to be larger to provide a good magnetic field. Can you determine the length of the magnet, and does it have a shiny or matte appearance?

    Maybe the sensor chip has good reliability in low-Gauss fields. You can reprogram the thing, you know! Did they bring out any of the I2C pins to solder pads?

  • The magnet is a shiny disc and, IIRC, less than 10mm in diameter. I have no idea how thick is is, since it's embedded in the plastic shaft. Nor can I tell how far above the sensor chip it floats.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • The encoder board has 5 pads:
    1-Black-Gnd
    2-Yellow-Output
    3-Red-VCC
    4-NC
    5-NC

    Are 4 and 5 I2C pads ?
    Can someone from Parallax confirm ?

    You can reprogram the output pin to be analog, or use I2C to read the position.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 21,321
    edited November 16 Vote Up0Vote Down
    ptiago wrote:
    Are 4 and 5 I2C pads ?
    You can determine this yourself by removing the encoder and following the traces -- if any -- to the AS5600 chip. Refer to the AS5600 datasheet to see if those are the I2C's SCL and SDA pins.

    I put mine back together before the question was asked and don't want to take it apart again. 'Sorry.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • @Phil:
    Thanks for the honest response, i didn't want to cause you any trouble.

    Looking to your picture it seems i need to take the screws to see the other side, i don't feel confident that i can put it back, i don't know if it's necessary a specific alignment.

    I hope someone from parallax could provide more details.

  • ptiago wrote:
    Looking to your picture it seems i need to take the screws to see the other side, i don't feel confident that i can put it back, i don't know if it's necessary a specific alignment.
    Don't worry about the alignment: it goes back together only one way. Make sure you have some needle nose pliers or bent-tip tweezers handy to get the little screws back in and started. It's kinda fidgety and the main reason I didn't want to disassemble it again.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • GordonMcCombGordonMcComb Posts: 3,142
    edited November 16 Vote Up0Vote Down
    Unless the chip comes to the factory pre-programmed, the two extra pads on the encoder board may be for programming prior to assembly. There are a couple of ways to do it, and one is to use the I2C interace.

    The datasheet doesn't suggest a stringent alignment beyond the physical mounting restraints, but I wouldn't want to encourage anyone to take theirs apart unless it's a bad/damaged servo, just in case they use a fancy jig or something!
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 21,321
    edited November 16 Vote Up0Vote Down
    Gordon,

    The screw holes in the PCB just fit the screws: they're not slots. So the alignment has to be built into the board layout. Having to use an alignment jig would only add to the assembly cost.

    Beyond that, you'd be right. Any major non-centricity between the magnet and the chip would result in non-linearities in the output.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
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