A 3D-Printed 7DOF Robotic Arm!

Skyentific, a YouTube sensation and roboticist, has developed a 7-axis, 7-degrees of freedom robotic arm. Despite the backlash and durability issues, Skentific decided to build the robotic arm with 3D-printed components instead of using cables. The plastic gears tend to wear out quite easily, so he’s not a big fan of them.

With identical measurements from shoulder to elbow, the 7DOF robotic arm is about the same size as a human arm. It also has the same joint-proportion in terms of the distance between each joint. To start things off, he uses two Multi-Star 9235 100KU motors, which produces 4.7 meters of torque. However, there is a drawback when using this motor: it can heat up very quickly without a super-efficient cooling system. It’s capable of rotating 20 to 60 degrees in just 30 seconds with a cooling system.

It has two different types of reduction: a belt and planetary gearbox reduction with a ratio of over eight. The robot arm will be using a Quasi Direct Drive joint, which will be very reliable, drivable, and should be able to sustain some impacts as well. A differential system in the arm will enable the motors to work at the same time. There is one brushless motor on each side of the arm, both of which are connected to the belts. A pulley transmission is being used in order to apply tension to the belt. To make the robot more stable and rigid, all the black parts will be replaced by carbon fiber parts. The planetary gearboxes on both sides with the barrel gears make a differential joint. The output on the top will connect to the wrist of the robot. Power and communication cables go through the channel through the bevel gear to the output shaft.

The electronics for the robot joint is capable of powering robot arms in the future. It includes a large 48V power supply, which is used for the drive and provides enough power for two motors. It also contains a small 5V power supply for the Teensy 3.6 board. There are a lot of serial outputs that allows for the controlling of many drive modules simultaneously. This arm will need control from four drive modules to control seven motors, which ensures the arm has 7 degrees of freedom. The big button is basically an emergency switch that turns the electronics on/off. The PSP joysticks are responsible for moving the arm up/down, left/right and rotating it. When joysticks move up and down, the motors move in the opposite direction, and the axis rotates when the joystick is moved from left to right.

Skyentific used 3D-printed parts to put the axis together and changed the pulley to tension the belt. He also created a cover for the base of the axis and made four iterations of the pool. Afterward, he put the cover on by inserting the embedded nuts into the corresponding holes on the top and sides of the cover. There are no strange sounds coming from the case when the motor is rotated, which means there doesn’t appear to be any friction in the case itself. The axis doesn’t seem to stand up very well on its own because the bolt sticks out from the bottom of the cover’s flat surface. To fix this issue, he replaced the bolt with screws that have a smaller head. He designed the actuator with a hole in the middle so that the cables from the arm can go through it.

The next thing to work on is the encoder, which is to be installed on the magnet. To start this process, he removes the plastic screw, glues the magnet in the middle, and puts the screw back on the motor.

The encoder will need to be fixed onto the magnet plate. Before doing this, the pins and wires will need to be soldered to the encoder. First, he solders a wire that will get 3.3VV from the power supply. Next, he solders the output wires, which go to the wall drive. He even uses an old mouse cable for the encoder. Red cable goes to the 3.3V, the yellow cable goes to the B and orange cable goes to the A.

Next, he installs the encoder onto the encoder holder, which is then fixed in place with tiny M2 screws. When it’s fixed into the holder, the encoder chip should be right in the middle.

Finally, he installs it on axis 1, just on top of the magnet. Once it’s in place, he puts the screws in.

Now that axis 1 is complete, everything will need to be connected to the drive. First, he connects a cable to the motor. Since the drive is fragile, he uses a custom-designed and 3D printed case to help protect it. There is a place inside the case that holds the braking resistor, which is where the entire drive is inserted. All the cables go in through the sides, and the case has access to a USB port. Plastic zip ties can be used to hold the cables in place. Next, he screws the braking resistor and solders it in place inside the case.

Then he attaches an x160 connector on the opposite side of the power cable. He also uses a DMX data cable and a res 232 cable and fixes them in place with plastic zip ties. To finish it off, he solders the motor cables to the drive installs the heatsink and assembles the box.

After connecting the cables and switching the power on, he discovers that it doesn’t do anything because it was initially programmed for two motors. To fix this issue, he went back and disabled the second motor and turned it on again. Now the motor moves in one direction and rotates it by using the joystick.

To finish the entire robot arm, he will need to build axes numbers 2, 3, 6 and 7. Later on, his goal is to create more affordable 7DOF robot arms with AI.

Interested is seeing the whole video series? Here are the first four parts!