This exo-arm is designed to help people who have disabilities in the upper limbs. Possible uses would be to assist with physiotherapy exercices or to provide daily assistence for those with limited strength.

The structure of the arm is based on mechanical parts, respectively an electric actuator, an aluminum support system and a support base for the arm made from PVC pipes of various sizes.

The motor used is a linear actuator, that is an electric motor that under stimulation moves a shaft back and forth in relation to the polarity supplied, the current of use is about 2 Amps at 12 Volts.
The horizontal movement is converted to rotary using the person's elbow as a pivot and a system of belts and pins. There are 3 straps, one that adjusts the PVC tube that adheres to the forearm, one that is at the elbow that keeps the forearm attached to the aluminum plate below and finally a strap that adjusts a PVC tube that adheres to the arm. The PVC tube of the forearm is 120 mm in diameter while that of the arm is larger, the diameter is 140 mm. The middle strap was added after the other two, since we realized that the forearm did not adhere well to the plate, limiting the arch of the arm.

Supplies

-Wooden pieces

-x2 Alluminum bar 29x50x0.5cm

-Plastic tube of 12cm diameter and 10cm long.

-Plastic tube of 14cm diameter and 14cm long.

-Foam

-Velcro

-Linear actuator https://www.amazon.it/Justech-Multifunzione-Solle...

-Electric wires

-Harness for trimmer https://www.amazon.it/Justech-Multifunzione-Solle...

-Hot Glue

-Bolts and screws

-Metal angular brakets

Step 1: Prototype

We made a cardboard prototype.

Step 2: Enclosures for the Arm

Cut a circular section of about 1/5 of it out of the two plastic tubes.

Drill two holes in the middle of the smaller tube at about 5cm away from each other. Do the same to the bigger tube, ut keeping the holes at 10cm away.

Using hot glue, attach foam on the inside of them.

If you'd like to, wrap colored cloth around the outside of the tubes, aways using hot glue.

Cut two wooden rectangles 6x4x0.5cm and cut section of 4x2cm out of them. Drill two holes a couple of cm away from one side of the tubes, at about 5cm of distnce from each other and do the same on two corners of the rectangles. Attach the two rectangles to the tubes using screws and bolts.

Step 3: Metal Bars

Drill two holes a couple of centimeters away from a side of a bar, those two holes should correspond to the holes of the brackets. Attach the brackets as shown in the third picture, using screws and bolts.

On the same bar drill two holes, having them corresponding to the holes in the smaller plastic tube. Attach that metal bar to the plastic tube using screws and bolts.

On the other metal bar drill two holes, having them corresponding to the holes in the bigger plastic tube. Attach that metal bar to the plastic tube using screws and bolts. Leave about 12cm of space on the top of the bar and cut an hole in that part, it's going to be used to attach the arm to the harness. On the other side of the bar attach a little metal support as shown in the firts picture, it's purpose will be to keep the actuator in place.

Step 4: Wooden Parts

Cut four pieces of wood with the shape shown on figure (circle on top and rectangle as a body). Three of them should be 24cm long, the other one just 20cm long.

Attach the wooden pieces with screws and bolts to the sides of the plastic tubes, drilling holes on the bodies of the wooden pieces and on the plastic tubes.

Step 5: Attaching the Linear Actuator

Fix the linear actuator to the bottom side of the bigger aluminum bar, using the electrician's clamps.

Use screws and bolts to connect the moving part of the actuator to the angular brackets on the smaller aluminum bar, as shown in the picture.

Step 6: The Harness

Fix the exo-arm to the harness, using cloth or the material more easy to find and use for you. The final result should be similar to the picture, but it can also deviate a little.

Step 7: Finally, the Electronics

As a power supply for the arm we used a 2200 mA battery from a drone, it is small in size and modest weight but we are still able to get a continuous operation. It powers linear actuator. The direction of the actuator will be obtained from a DPDT (double pow double trow) rocker switch, which is wired to allow reversing the direction of the current. It has 3 modes, a central rest in which the arm stays in place, one by pressing the button upwards which will make the arm go upwards and one downwards which will make the arm go downwards.

Step A: connect the actuator. Use suitable gauge wire in order to carry the modest current required by the actuator and connect the same to the actuator via connector such as mammuth connector, forbox connector, wire nut or wago terminal (We strongly don’t recommend the last two because can sometimes be inadequate to hold big wire or generally will make unreliable connections).

Step B: the switch. Get a DPDT switch with a suitable current rating, at least double the current required by the actuator, and solder the wires to the two sides of a contact of the switch. Using the same wire, connect the other contact by cross wiring the two, effectively connecting from one side to the other the contacts. With the same wire, solder two pieces to the central terminals of the switch (long enough to reach the location of the power source). Fix the wire to the structure with any suitable glue or with zip ties or any other method you like.

Step C: decoration. Get a handful of LEDs of at least 2 different colours. Solder together a strip of LEDs of the same colour, as many as needed to withstand the supply voltage (for ex. using a bright red LED, each LED takes 2.0V so we can use 6 of them in series), paying attention to solder each cathode to an anode. With 2 strings, connect them in antiparallel by the extreme. Solder two smaller section wires to the extreme of the assembly, glue the assembly to a nice location on the project, cut the wire to length in order to reach the switch with a bit of spare and solder the extreme to the wire going to the actuator.

Step 4 test. Connect a source of 12V to the system and by operating the switch the actuator will move and the LED will light up. Note that when you release the switch there will be a brief flash on the LED of the other color, that’s caused by the inductive behavior of the motor that when it’s decelerating, after interrupting the circuit via the switch, it generate an inverse polarity spike of voltage that is visible on the LED.