In the previous part I started designing the fingers in the Autodesk 123D CAD package, and 3d printed one of these fingers out. In this part, I’d like to find a way to electrically control the movement of the finger.

The finger rotates around the knuckle joint – this part of the mechanism is too small to allow a motor to fit into it, so I decided to try another linkage which would allow a linear movement to be translated into rotation.

I thought a good transducer would be a small solenoid – this would allow me to convert electrical energy (from a battery or power supply) into a linear movement. When the current flows through the circuit, the solenoid’s coil wire is magnetised, which then pulls a metal core into the coil of wire. When the current is removed, a spring forces the metal core back into it’s original position. I can attach a link from the finger mechanism to the end of the metal core, so switching on the current will make the finger move.

Three Solenoids

I ordered three types of solenoid – I didn’t really have any idea of the suitability of hardware I was going to receive – each of these purchases was really just a shot in the dark, hoping something would be suitable.

3-12V 0.08-0.35A Push-Pull Type DC Open Frame Linear Solenoid

Purchased from eBay
  • Voltage:3-12v;
  • Current:0.08-0.35A;
  • Dimensions: 11 x 12 x 20.3mm

DC 12V 2.1Kg Force 10mm Push Pull Type Electric Solenoid Electromagnet

Purchased from Amazon


  • Voltage:12v;
  • Dimensions: 30 x 15 x 13mm;
  • Force: 2.1kg (I know force is measured in Newtons, but this is from the spec)

1kg Force 10mm Stroke Push Open Frame Solenoid Electromagnet DC 12V

Purchased from Amazon
  • Voltage:12v;
  • Dimensions: 40 x 29 x 24mm;
  • Force: 1kg (again, I know force is measured in Newtons, but this is from the spec)


I photographed all three of these solenoids side by side, and placed them beside a UK 10p piece to show their relative sizes.


I think this makes the differences pretty clear!

I applied 12v to each of these and tested their strength by pressing against them.

  • The smallest solenoid is incredibly weak, and obviously unsuitable –  it presented almost no resistance. Also, there’s nothing to stop the metal core coming out of the main solenoid body;
  • The middle sized solenoid was quite a lot stronger, but this definitely would not be able to pick up 2.1kg – still pretty weak;
  • The largest solenoid was – obviously enough – the strongest of the three. I thought this would be strong enough to make the mechanism work.

I found this video on YouTube about another solenoid, which is similar to the smallest solenoid.

I designed a jig in Autodesk 123D to test the existing printed finger with this largest solenoid (shown below). The large regtangular pad in the main green part is where the solenoid will sit, and the solenoid plunger will screw into the yellow linkages protruding from the blue part.

finger with base jig (full)

The image below shows the same finger and jig with a few parts hidden so that the internal linkage mechanism is clearer.

finger with base jig

And here’s the complete test apparatus part after printing the green jig and new yellow linkages. I attached the knuckle to the jig by acetone welding the two parts together – I don’t want to do this again, so I’ll redesign the knuckle to allow it to be bolted to any attachments.


Here’s what happens after 12V is applied to the solenoid.


Here’s what happens after the 12V is removed from the solenoid…no change.


Looks like the solenoid spring isn’t strong enough to return the mechanism to it’s original position. Fortunately I had suspected that might happen, so deliberately designed a couple of holes at the back of the proximal phalanges (blue parts in the CAD diagrams above) for a spring.

I added a spring – you can see this at the bottom left of the picture below. This allowed the finger mechanism to return to its original position, although it does make it a bit harder for the solenoid to pull the metal core into the body of the solenoid.


I think this is a good enough proof of concept for version one, so I’ll proceed with a few more of the solenoids used above. Next time, I’ll design more fingers and consider how they knuckles will be positioned on the main part of the hand.