Finger Printed at last

I was away for a while, and of course when I travel, I don't cary my 3D printer with me. Plus, being away in the desert makes it difficult to comuunicate with internet.
Since I released the finger prosthetic for Bionico, I was impatient to print it. 
Fortunatly Morris, on the other side of the Atlantic had his printer ready for testing. He was the first one to print the finger and got me some feedback. 
You can see his blog here
So I was designing on my computer in the desert and he was printing the parts along, and everytime some thing was not correct in my design, he would notify me and I could check the pictures posted on his blog to modify the design in consequences. This took us a lot of time after all. But we got to move on and thanks a lot to him. The three bellow pictures are from his blog:

Being back from the desert, I was then able to print the finger. There was still things that were not correct. I also ordered another type of motor which is a 6V planetary gear motor with a ration of 154:1 from Pololu.
So the casing had to be redesigned again. There is a very small worm gear that actuate the finger and I printed it in ABS but it is definitly too fragile, so it either needs to be modified or printed in stainless steel. You can get it by Shapeways here.
I left both versions on Thingiverse because it can be handy for other projects.
With the servo Corona DS238HV use these files:

With the planetary gear motor use these files:

The planetary motor:
The dual driver DRV8833

The Arduino card:

Here is a video of the finger in action. The speed isn't really great, but it is a choice of planetary ratio I made for to get higher torque.

It was pretty hard to make that video, because the small wormgear breaks when the finger reach the end of it's run. Unfortunatly I haven't yet figured out how to set a time delay on my script and a reverse of polarity. Using DC motors and servos is not the same when it comes to create a script in the Arduino.

The big news:

InMoov is selected and invited for the European Makerfaire Edition in Roma. So I will be going to Italy and have the pleasure to meet all the italian and european geeks that attend this event. I hope to get some time to make the robot usable, it's going to be something to put the full InMoov in a suitcase.
Myrobotlab is also selected to attend the Makerfaire, Alessandro will be the Ambassador of MRL. Leonardo who has created a superheroe InMoov will attend the faire as well, hopefully his suitcase is big enough for to bring his robot. Leonardo his also working to make us a real website.

And Bionico, of course, is also selected for the Makerfaire, we are currently getting a hand ready for Nicolas. Hugues is hard at work on all the stage levels.
Here is a french article about us in LE MONDE newspaper.


  1. Hi Gael
    If you put a small potentiometer in the finger base to measure the angle of the finger with a PID loop that will give better control and stop breaking the worm I'm trying it now with a old servo potentiometer

  2. Hi Peter, you are right. That was the idea I wanted to do at first with the servo motor. I had planned to extract the pot and use it as part of the shaft.
    Let me know if you experience does good and how you controlled the motor.

  3. Did you try sealing the ABS worm gear with acetone before using it? Did it shear along the print plain?

  4. Hi Gael! 05-06 October I'll come to Rome.
    Thanks for all and W INMOOV ;)

  5. Hi Gael,
    Glad to see that the prothetic work is progressing. I have been researching and experimenting with things involved with the finger. I have found a small magnetic 'slipper clutch' that fits to a small servo that starts slipping when the servo reaches the end of its travel. This should stop the servo from breaking the drive mechanism at either end of its travel.

    I have been thinking a lot about how limitations of travel are done in InMoov and I came to the conclusion that using only some sort of feedback to the controlling computer may not be the optimal method, hence the interest in clutch mechanisms that would physically stop the motor driving at the end of travel. I had several instances of a programming error that caused the system to ignore input from a feedback source, in this case a limit switch, and the result was breakage because the servo kept driving.

    I think I may try to incorporate a clutch with a motor driven finger as you have built just to see what happens.


    1. Hi Morris, I have been looking for a simple clutch method to print but didn't come up with a solution. I had no idea you could buy such small clutch. One thing, I can't figure is how to determine the difference of pressure between the worm reaching the end of it's travel and the finger grabbing a heavy load. I have chosen a motor with a torque of 8.5kg cm so that the finger can grab heavy things (if plastic resist..), but if the clutch let go for a few grams to prevent breaking the worm, it isn't okay.
      Also one problem I have is the length of the motor added to a printed clutch, it's too big, and I can't add the two motors needed for the thumb DOFs.
      I will follow your progress on that field.

  6. Hi Gael.
    FYI here is the info from the order for clutches.

    3 x DAGU - Magnetic Servo Clutches (for Miniature Servos) (DAGU-RS022) = £12.75
    Sub-Total: £12.75
    Airmail (Uninsured) (4-6 business days for Europe (may take longer for other destinations.)NO tracking number.): £13.58
    Total: £26.33

    In thinking about the need to protect the most fragile parts of the finger mechanism (or any mechanism) I considered a stall condition at an end of travel or a stall condition in the middle of travel to be essentially the same thing in terms of using a clutch to disengage the torque. The difference between the two stall conditions would be in how the finger detects the position. I have been thinking in terms of limit switches for the end of travel and pressure sensors for the grasping.

    In addition to this area I have been thinking and researching a bit in the area of driving multiple actions from one motor. In this regard I have been trying to find some sort of small through-shaft electro-magnetic clutch but so far unsuccessful.


  7. Hi Gael. I have one question for you. I'm searching in your blog and I doesnt found the information for theservos. In the head, there are: 3 servos DS929HV for the eyes, in the jaw a HK15298. To move the head right/left and up/down, you use 2 HS-805BB, one in the chest and another in the head?? There are some servo that I left?

    Thanks for your time.

  8. Hello.

    Regarding breaking the gear, you can control how much force the motor can produce by limiting the amount of current supplied to the motor. The feature is called current limiting. Your motor controller board has this feature, but it's disabled by default.

    To enable it, you have to cut a trace on the back of the controller board and solder on small surface mount resistor. The formula is
    I (current) = 200mV / R (Value of resistor.)

    A .4 ohm (R400) would limit at 500mA.
    A .2 ohm (R200) would limit at 1 Amp.

    At 6 volts your motor's No-load current is 250mA, and the maximum is 3.3A. If you limit it at 500mA, there is a pretty good chance it will work without self destructing. Later on when you get metal parts or a limit switch working you can replace the resistor with a jumper to remove the current limit.

    Good luck!

    1. Very interesting,
      I had seen on the driver board pdf some kind of extra feature but had no idea what it was. But it's good because this can be adapted to whatever material is used for to print the finger.
      Since this post, I designed a space for to have a potentiometer as a limit switch. I'm facing a problem on how to connect it with the driver board and the Arduino. I'm no programmer and no electronician, and this is taking too much time in research.
      I will write you an email, if you don't mind.

  9. You speak of printing stainless steel for greater strength, have you considered brass? Easy to machine, fast cooling allows mate to plastic without creating void, and its pretty!

    1. But can we 3D print brass? Machining brass is a lot of work compared to printing, and the parts I have created are hardly machinable I think. Thanks for the suggestion!