3D Printed Bionic Claws with MyoWare Muscle Sensors

First off, grab the 3D files (in STL format) from our GitHub repository and either print them at home if you’re lucky enough to have a 3D printer, or have them printed from a 3D print shop.

We use a local print shop, Fineline. To find a machine, makerspace, or service you can use, check out Make:‘s page Where to Get Digital Fabrication Tool Access.

Your printed parts might require some sanding and finishing depending on the quality of the print.

Gather all your materials and tools within reach in a clean work area.

Cut off a 6-pin section from the right-angle headers, and solder these to the 6-hole programming header on the end of the Arduino Pro Mini.

The Y-harness allows you to connect both the Arduino and the servo to the 7.2V battery.

Cut off the female connector of the Y-harness (keep the 2 socket connector ends — you’ll need them to easily connect the servo and battery), then split and strip the wires.

Solder the red wire to the RAW pin on the Arduino, the black/brown wire to the GND pin, and the yellow/orange wire to digital I/O pin 3. Clip off any excess wire.

NOTE: If you find that the Arduino’s LED flickers when the servo extends the claws, you’ll need to add a large capacitor between the RAW and GND pins and/or a diode between the Y-harness positive wire and the RAW pin. The capacitor is needed if the servo is pulling more current than the battery can supply. The diode will keep the servo from creating reverse current.

Before you move on, check to make sure you wired the Y-harness correctly.

Plug the battery into one of the 2 sockets of the Y-harness. The Arduino power light should turn on. If it doesn’t, quickly disconnect the battery and recheck your wiring.

Now unplug the battery, and plug the servo into the other socket.

Grab the SweepMod.ino sketch from our GitHub repository and open it in the Arduino IDE. In the Arduino IDE, click on Tools–>Boards and make sure the board is set to “Arduino Pro or Pro Mini (5V, 16MHz) w/ATmega328.”

Upload the sketch to the Arduino. (If you aren’t familiar with the Arduino Pro Mini, head over to SparkFun and read their Getting Started tutorial.)

Disconnect the Arduino cable and plug the battery back into the remaining socket. The Arduino should turn on and start moving the servo back and forth. If the servo doesn’t move, disconnect the battery and check your wiring.

Now disconnect the battery and servo and you’re ready to move on.

Measure the distance from your wrist to your elbow, cut off an equal length of servo cable, and strip both ends.

Insert the servo cable’s yellow wire into the muscle sensor’s output signal pin (SIG), the red wire into the power (+) pin, and the black wire into the ground (–) pin.
Note that the servo wire color order does not match the sensor’s pin order.

Solder the wires to the muscle sensor board and clip off any excess wire.

You could alternatively use a 3-pin Molex, JST, or other type of connector so the sensor can be easily disconnected from the rest of the setup.

Since the 7.2V output of the battery is too high for the sensor, we’ll use the Arduino’s built-in 5V regulator to power the MyoWare.

Split the servo cable that you wired to the MyoWare and solder the red wire to the Arduino’s Vcc pin, the black wire to the ground pin, and the yellow wire to analog pin A3. Clip off any excess wire.

​You’ll want to test the MyoWare to be sure it’s set up correctly before uploading the new Arduino code.

Snap 3 new electrodes onto the MyoWare sensor.

Peel off the the electrodes’ backing, and stick the MyoWare sensor to your forearm:

• Place one of the electrodes that are connected directly to the MyoWare board in the middle of your forearm muscle with the other electrode placed along the length of the muscle.
• Place the reference electrode nonadjacent to the muscle body that the other electrodes were placed.

Plug in the battery and turn the MyoWare sensor on, if it isn’t already.

• The MyoWare sensor power LED should turn on at this point. If it doesn’t, check your connections and make sure the battery is charged.
• The second LED represents the output signal. If this LED does not turn off when your muscle is relaxed, check your connections and electrode placement.

Next, make sure the MyoWare sensor is set up correctly by flexing your forearm muscle.

• The second LED on the sensor board should light up when you flex your muscle and turn off when you relax. If it doesn’t, check your connections and electrode placement.

Connect the 5V FDTI programming cable to the Arduino and your computer. (Note that this image from Arduino shows ordinary vertical headers, not the right-angle headers we’re using.)

Grab the BionicClaws.ino sketch from our GitHub repository and open it in the Arduino IDE. Again, click on Tools–>Boards and make sure the board is set to “Arduino Pro or Pro Mini (5V, 16MHz) w/ATmega328.”

Upload the sketch to the Arduino.

Disconnect the FDTI cable.

​Before you start final assembly, make sure the electronics are working as expected.

Put on the MyoWare sensor and turn the system on.

Try flexing your musclel. The servo should extend when you do, and retract when you relax.

If the servo doesn’t move, there are a few things to check:

• Check the battery voltage. You don’t want to waste hours trying to troubleshoot only to realize things aren’t working because your battery is dead.
• Make sure the MyoWare LED is turning on when you flex (indicating it is working). If not, make sure its switch is turned on and the power LED is on.
• Make sure the Arduino power LED is on when you connect the battery. If not, then check the wiring again.

Now you can move on to assembling the 3D printed parts. Take the enclosure back piece and insert it into the front piece, lining up the center-punch holes on the tabs of the front piece with the matching through-holes on the back piece.

Drill 4 holes in the tabs, concentric to the 4 through-holes.

Next, use the tap to create the screw threads in the tab holes. Make sure to keep the tap perpendicular to enclosure side or the screw might not fit flush.

Grab the 3/16″ screws and test out the new screw holes by screwing them in. Don’t over-tighten them or you can easily strip the plastic screw threads on the tab.

Remove the screws before proceeding.

The servo will be mounted in the rear portion of the back enclosure piece. There are 2 holes already present but they need to be tapped for the screws.

Carefully use the tap to carve out the screw threads in these holes.

Place the servo brace on top of the holes and screw in the 3/4″ screws to make sure the threads are in working order.

Cut off a couple ½” long strips of the sticky-backed foam and adhere them to the bottom of the back enclosure between these 2 screw holes. This will keep the servo from sliding on the smooth plastic when torque is applied. You can do the same for the servo brace.

You need a way to mount the enclosure to the wrist brace sleeve, but you want to make it temporary so you can remove the enclosure if something breaks or needs tweaked.

Take the wide sticky-backed velcro strips and adhere the hooks side (the scratchy side) to the underside of both the back and front enclosure pieces. You’ll need to trim along the curves to make sure you get the velcro to cover the front piece completely.

The 2.5″ servo horn unfortunately comes with holes too small for the servo linkage but that’s easily fixed. Simply drill out the hole closest to the servo horn tip.

Attach the servo horn to the servo and install the servo linkage to the hole you just drilled.

Actually the claws don’t need much prep. You should try sliding the claws along the tracks of the front enclosure piece to make sure they run smoothly. If not, use some very fine sandpaper and smooth out any parts that catch or bite into the tracks.

Then grab the two 3D-printed tabs and a servo linkage. Hold the servo linkage in the bracket at the back of the claws and insert a tab into each side of the bracket. You might want to dab a drop of super glue on them to keep them from falling out.

Now we’re getting somewhere — time to start putting the pieces together!

Insert the claws into the tracks of the front enclosure piece. Attach the front and back enclosure pieces together and screw them in place.

NOTE: Don’t screw these in all the way or they’ll interfere with the claws running up and down the track. Move the claws until they’re in front of the holes, screw in the screws until you feel them contact the claws, and then back the screws out just enough to give some clearance.

Insert the servo into the servo mount and secure it with the brace and screws. The servo should be positioned with the arm closest to the rear of the enclosure and pretty close to the centerline.

Move the servo arm to the fully extended position: turn on the electronics, flex your muscle to extend the servo, then turn the system off while it’s in this position. Now move the claws so that they’re fully extended as well.

Measure the straight-line distance between the 2 servo linkages, then add a little extra length to allow for threading the rod into the linkages (should be around 4″).

Cut off a piece of threaded rod with this length and carefully screw it into the linkage on the claws.

Detach the linkage from the servo (using the thumb screw) and screw it onto the other end of the threaded rod.

Move the servo arm through extension and retraction a couple times to make sure the claws move appropriately.

Finally, to prepare for inserting the electronics in the next step, detach the linkage from the servo arm and remove the servo from the mount.

Now you need to squeeze all the electronics into the side pocket of the back enclosure piece, opposite the servo mount.

Cut off a reasonable square of sticky-backed velcro and stick it onto the bottom of the battery. Stick the other side in the back corner of the enclosure pocket.

Mount the battery in the back corner.

Slide the Arduino into the pocket just in front of the battery with the MyoWare wire running out the slot at the back end of the enclosure and under the servo arm. The servo wire should be threaded through this slot as well. You could velcro the Arduino board in place but it’s really not necessary — it stays reasonably in place on its own.

Remount the servo and reassemble the servo linkage.

Plug the servo cable into one of the Y-harness sockets. Do not plug the battery in until you’re ready to use the completed system.

TIPS:

• Cut out some of the length of the servo cable and Y-harness to keep things less cluttered.
• Super-glue the 2 Y-harness sockets together to make connecting the battery and servo cable easier once everything is in place.
• Remember the electronics don’t have a switch built in, so whenever you connect the battery, the system will turn on. We’ve added a delay so you can do this without the servo pinching your fingers on accident, but it might be a good idea to add a switch between your battery and the Y-harness.
• If you want to get fancy, you can run the MyoWare wire behind the battery so it doesn’t interact with the servo arm at all.

Last part of the assembly is to attach the enclosure to the wrist sleeve.

Remove the plastic wrist brace from the cloth sleeve.

Place the enclosure on the sleeve. You might have to remove and reattach a few times to get the positioning and alignment just right.

Although not necessary, we added velcro straps to securely fix the enclosure to the arm. Try it without first, and then if you feel the enclosure wobble around or it just doesn’t feel secure, add the straps.

That’s it! Your Bionic Claw system should be fully operational.

• Flex to extend the claws.
• Relax to retract them.
• If you flex for 2 seconds straight, you’ll trigger the lock mechanism and the claws will stay extended when you relax afterwards.
• To take the lock off, simply flex for another 2 seconds and the claws will retract when the unlock is triggered.

Now get started on the other hand!