RR1: Real Robot One - a DIY Desktop Robotic Arm

RR1 is an open-source, 6+1 degrees of freedom desktop robotic arm, mostly 3D-printed with joints powered by custom planetary gearboxes.

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RR1 has 6 (6 joints + 1 for the gripper) degrees of freedom and is powered by stepper motors. It is fully closed-loop, i.e. every joint has its own encoder and at any time we know the current angles of all joints. The important feature that distinguishes RR1 from other similar projects is that each joint has its own custom-built 3D-printed planetary gear reducer. The robot consists of two parts: (i) the RR1 arm itself and (ii) a control computer called Real Box One, or RB1 in short. This allows for having lot of electronics separated from the arm and supports modular design.

The first prototype, called "revision 1" is already running. The second prototype "rev. 2" is on the way (you can try to guess its color). As I gained huge experience during the assembly of "rev. 1", the second prototype "rev. 2" will be much improved in many aspects.

RR1 is a DIY desktop robotic arm, my big project in robotics. The overall design follows the idea of being able to produce more of these robots so that multi-robot coordination on the desktop is possible. I think, having a small scale Industry 4.0 on the table could be great for research and testing.

The robot itself is bigger and more capable than toys but it is not too big like industrial robots and not that expensive so you don't need to have a budget for building a factory if you want 10 of these robot.

Thanks to its small size RR1 is not dangerous like industrial robots that need to be enclosed in an inaccessible area. RR1 can be well used as a collaborative robot.

Unlike other robotic projects of similar size, RR1 does not use any belt for torque transmission. All the torque transmission in RR1 is done via gears. All reducers are custom built based on 3D-printed planetary gearboxes. Each joint has its own planetary gearbox, so altogether there are a total of six similar gearboxes of various sizes across the robot. In addition to this, there are three bevel gearboxes to transmit torque through 90° from the motor shaft, also mostly 3D-printed.

The robot is powered by stepper motors, 4 big (nema 23) and 3 small (nema 17). The lower main joint is powered by the biggest nema 23 stepper motor available (112mm length) generating 3Nm connected to 1:40 reducer which in theory generates torque of 120Nm in the main joint, pretty enough to move the robot.

The reducer for the main lower joint is shown in the following figure. It is a planetary gearbox consisting of 3 planets - herringbone gears - and one middle gear connected to the motor axle. The rotating part that moves the arm is fixed using bearing balls into the groove in the front orange part (balls and bolts are not shown).

The following video shows the assembly of the planetary gearbox from 3D-printed parts.  The assembly process focuses on smooth rotation and backlash elimination at the same time which is a very challenging task.

The robot currently exists as the first prototype called "revision 1" or "rev. 1" in short. There is still some backlash in the joints of "rev. 1", I expect to eliminate it in "rev. 2" (almost zero backlash has been indeed achieved in the above video). The repeatability is still improving.

Tests and findings with the first prototype "rev. 1" are posted below.

The first test, called the "Coca-Cola Test" is focused on the dexterity of the robot where all 6 joints plus the gripper are employed. RR1 is lifting 330ml Coca Cola and simulates pouring. The can is full and it turned to be somewhat slippery for the robot's gripper. This can be mitigated by the blue rubber bands on the gripper which has anti-slip effect. It seems I should add more anti-slip components into the next revision (rev.2) of the gripper so that rubber bands can be eliminated.

Another video showing articulation of the robot follows. This test is also focused on checking if all cables are long enough to allow the arm to twist round.

I also did some preliminary weight-lifting tests in which RR1 lifts a water barbell. I was wondering if the robot can handle the full 1kg barbell - in theory it should. The following videos shows the test.The next video shows motion test with an empty barbell - this is very easy to lift:

And the full 1kg barbell test in the next video (Sonic the Hedgehog has been caught by the robot too). As the video shows, RR1 is capable to lift 1kg in its gripper, that is using its wrist. It seem we can meet the projected 2kg payload. The test also discovered some weakness that we need to fix in rev. 2, the gripper has relatively weak grip and the barbell can slip out. We increased the grip by the blue rubber band.

A shorter video showing lifting 1kg without the "Save Sonic the Hedgehog" scene is here (in case you don't like to see Sonic being hurt by the robot):

I have been already assembling "rev. 2" that will feature many improvements, most significantly improved planetary...

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  • Assembling the Planetary Gear Reducer

    Pavel Surynek08/21/2022 at 12:53 0 comments

    Reducers are a key component of the RR1 robot arm. I use my own custom built 3D-printed planetary gearboxes. The following video shows assembly of the gearbox for the upper joint (elbow join) for the second prototype of RR1 ("rev.2"). There are a total of 6 similar gearboxes of various sizes across the robot.

    As the gearboxes in RR1 use herringbone gears for smoother rotation, the assembly must be carried out exactly as shown in the video. All gears must be meshed together well so that there is not much friction and not much backlash at the same time which is especially difficult to achieve with herringbone gears. The video shows that the assembled gearbox rotates smoothly and has no detectable backlash. Such an achievement requires lot of experience and a very good 3D printer (I use Prusa i3 MK3S+).

    And as you can see "rev. 2" will be orange.

  • Speaking about the RR1 Robot

    Pavel Surynek08/14/2022 at 12:06 0 comments

    This is a talk a had at Prusa Lab in Prague in June 2022. It is in Czech so probably only for the Czech audience but at least it shows me how crazy I am about robotics.

  • Planetary Gearbox Published on Github

    Pavel Surynek08/12/2022 at 12:09 0 comments

    Due to some requests I started to publish the files concerning the mechanic construction of RR1. I started the Github repository of the RR1 project where everything will be collected:

    Currently there are the STL files for the planetary gearbox reducer of the main shoulder joint (shown below) for RR1 rev.2 (the one under construction, not the black one from videos). I recommend to use Prusa i3 MKS+ for 3D printing the parts. In addition to this, 7 ball bearings are necessary (16mm outer diameter, 5 mm inner diameter, 6mm height), 3 long bolts for planets (48mm length, 5mm diameter), 1 short bolt for the sun gear (24mm length, 5mm diameter), and 7 small washers (5mm inner diameter), and some 3mm screws, many 3mm square nuts, and very many 6mm bearing balls between the actuator ring and the outer ring. Please note that bolts, screws and bearings are not shown in the figure.

  • Shifting the Arm Forward

    Pavel Surynek08/06/2022 at 12:10 0 comments

    The rev. 2 will not have the arm mounted in the center of the base. The arm will shifted a bit forward which will make the RR1 arm look more like professional robotic arms. The figure shows the part on which the arm will be mounted, this part sits on the base that changes only little from rev. 1. The reason why the arm will be shifted forward is not only visual but most importantly it improves the reach of the arm and makes the working space more natural.

  • Switched from Completed to Ongoing Project

    Pavel Surynek07/25/2022 at 19:02 0 comments

    I decided to switch the project from the completed one to ongoing. Anyway it seems that is true. The first prototype of the robot, revision 1, or rev. 1, is completed, but the revision 2 is on the way, so it seems it is better to mark the project as ongoing.

    I am printing new parts for RR1 rev. 2, everything is done more precisely to minimize backlash everywhere possible. The picture shows printing of the custom bearing for the base of the robot.

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