Spine-like Robot Arm With Gripper

I once had the idea of an artificial spine made of 3d printed parts. After a few different prototypes with rubber bands or springs, I realized that it could also be used as a robotic arm or tentacle. (Doctor Octopus sends his regards).

So I made some prototypes moveable in three and four directions.

3D printed parts

3 servos MG90

~2 m. strong fishing line, nylon (0.45mm)

6 small screws

hot glue

ESP32 microcontroller + power supply

optional: red LED

Prototypes that looked good, but with less success.The last prototype could be moved in three directions, but is less stable. That's why the latest actual version can now move in four directions and only needs two servos.

The current version basically consists of two spheres: The negative curve of an upper element slides on the positive curve of the lower element. The whole thing is held together by a spherical clip on a small rod inside the element. The movement of each spinal element is limited to ~15 degrees, but is possible in any direction. This means that just 6 elements result in a 90 degree change of direction.

The parts simply have to be plugged together.

The spine does not withstand tensile forces well, but the controlling nylon threads hold the structure together. However, the spine is quite pressure-resistant and the ball mounts do not lose their hold easily, even when bent sharply.

The whole backbone is also quite light: the entire structure you see here with 9 intermediate elements, a gripper element and the base plates weighs just 127 grams.

The servos with the support for the nylon are attached to the base plate. The plate can also be attached to other superstructures or backpacks, depending on where it is used and the purpose of the movement. I added a socket to look everything nice and compact.

Four nylon strings are pulled through the side flaps of each spine section. These later control the arm movements by the servos.

The gripper is a simple but self-designed model. Three gripper fingers with a gear wheel are opened and closed by a servo-driven gear rack (gear-cylinder). A piece of solid wire (paper clip) acts as a lever to transfer the lift to the gear cylinder. The gripper is attached to a special spine part. This is also where the nylon cords end.

1. Place the cylinder in the centre of the mounting plate.
2. Fit the fingers to the brackets, using small screws.
3. Attach the servo to the underside of the mounting plate with hot glue.
4. Use a piece of strong wire (paper clip) to connect the servo arm to the cylinder.

To make it more interesting, I also added a red LED. It is operated with the current of the servo, a 330 Ohm resistor is pre-wired to the LED.

Theoretically, it is possible to control the servos via a joystick or sensors. However, I opted for control via a previous project, the BLE control device on the wrist (BLE Control From the Wrist).

Two of the XYZ values of the accelerometer an XIAO nRF are used for the arm, another value for the gripper mechanism.

The base ESP32 receives the signals via Bluetooth BLE and converts them to the servos.

For the control unit, see this project: BLE Control From the Wrist.

Seeedstudio.com gave me the opportunity to try out the XIAO nRF sense. It's easy to use and works really well, even with this project. Thank you!

However, you need another sketch that measures and transmits all XY and Z values.

If you prefer to control the arm directly, you can also do this with a joystick on the ESP32.

The servos should not be supplied via the ESP32, but via an external power source. Note the maximum 5 volts for this type of servo. Attention, GND of the ESP32 and GND of the power supply must be connected!

I have opted for a cylindrical Li-Ion battery in a charging unit.

If the robot arm is to be used as a portable unit, I recommend using the power supply from this project (Servo Shoulder Support (Partial Exoskeleton)), steps 3 and 4.