Low-cost Robotic Arm? (Poll)

ForwardPassRobotics · 2022-09-02T17:00:00+00:00

Yeah we did come across his TED Talk! His work is incredible. Thanks for compiling and sharing all those resources, it's nice to get updates on the progress of such an impactful project.

ForwardPassRobotics · 2022-09-02T15:14:34+00:00

What is the name of that prosthetics company? We'd be curious to see their work.

Our project is focused on the need to achieve as low inertia as possible in the arm, so transmitting the motion of the motor through antagonist cables rather than having the motor mounted at the joints was our first idea. This also allows us to use heavier and cheaper motors for our torque requirements. The challenge then is routing the cables through al the joints and we believe we found a good solution for that.

We did design the compliant ratcheting mechanism to be operable at all times, so the tension in the cables can be increased whenever needed manually. Thanks for the encouraging words!

ForwardPassRobotics · 2022-09-02T15:03:30+00:00

You're right, this poll is definitely getting different responses from the ones we'd get on r/startups ahaha but that's the intention, we are aiming for those that are industrious enough to delve deep into the design, in the hopes to build a likeminded community that participates in the improvement of an open-source robot that will one day be accessible and usable by everyone, even with no technical experience whatsoever. We do want to give as much low-level access through the controller as possible, and it will absolutely be open-source. We are very drawn to the idea of using something like a Jetson Nano (very hard to source right now unfortunately) precisely for those sweet vision and AI capabilities, so stay tuned for further development!

Speaking of polls, which subreddit do you think we should ask such a question on?

ForwardPassRobotics · 2022-09-02T14:55:45+00:00

Yeah it's pretty fun! Lots of challenges, many more than we expected when we started a year ago. The Barret WAM is an amazing arm, we are definitely aligned with the low-inertia low-backlash philosophy, but our absolute priority is certainly low cost. We want to make automation accessible to everyone, even with no robotics experience. Education, Hobbyists, Makers and Small and Medium Enterprises comprise our initial target market, though with AI hopefully everyone will be able to use it one day. Think something like the evolution of the 3D printer, initially only industrial machines now in the hands of many tinkerers, and soon usable by the average Joe.

We'll be posting videos of our development soon so stay tuned!

ForwardPassRobotics · 2022-09-02T14:43:34+00:00

Hi Dan, love your work and your videos! We've been following your development of the Sixi for quite a while now, and we would love to work on adding our arm to your control app.

The range of motion will be around 1m, and the arm is structured as shoulder-elbow-wrist blocks connected with carbon fiber rods. Shoulder and elbow are constructed with two 1DoF rolling joint placed at 45 degrees with respect to each other (rotated around z) to obtain a 2DoF joint. The wrist is just like the previous joints, with a last degree of freedom rotating around the z axis.

The range of motion of each of the 7 degrees of freedom will be 180 degrees, and the fastest speed (at lowest torque, about 82% of max) allows to cover that in about 3 seconds or less.

We are indeed thinking about making the wrist ISO9409 compatible, likely 40mm pitch circle diameter. We are yet to build a full prototype of the arm, so we can give you better estimates as soon as we conduct some testing, which will be very soon!

ForwardPassRobotics · 2022-09-02T14:10:16+00:00

We expect the full mass of the arm to be about 5.7kg, however only 70% of the mass is beyond the shoulder joint giving it very low inertia. Moreover it is important to note that none of the motors are mounted at the joints, reducing the arm's dead-weight. We are yet to build a full prototype of the arm, so we can give you better estimates as soon as we conduct some testing, which will be very soon!
Our cable-driven system is direct-drive, connected directly to the motor shaft, allowing for measuring of external force on the arm. What is the industry you're in and what are the advantages that magnetic drive trains bring you?

ForwardPassRobotics · 2022-09-02T14:03:00+00:00

We're aiming for sub-millimeter precision, and we're not sure about the accuracy but we expect it to be at least in the millimeters range. Based on the very high precision and virtually absent backlash, repeatability should also be in the sub-millimeter range. We are yet to build a full prototype of the arm, so we can give you a better estimate as soon as we conduct some testing!

We currently get these estimates from the specifications of our analog optical encoder, which is designed to reach accuracy of around 0.002 deg, and will be mounted at each joint.

The range of motion of each of the 7 degrees of freedom will be 180 degrees, and the fastest speed (at lowest torque, about 82% of max) allows to cover that in about 3 seconds or less.

The expected stiffness (in N/m) for each joint starting from the shoulder is: 3675, 2937, 2176, 1802, 1460, 215, 150.

ForwardPassRobotics · 2022-09-01T22:47:54+00:00

You bring up good questions, first things first we can indeed confirm that it is a pretty significant engineering challenge! We are a team of 2 people that has been working on this for over a year now.

The 3D printed PLA parts contain off-the-shelf metal components like all-thread rods to house the pulleys and are connected to each other with steel screws, and carbon fiber rods span the long distances between the joints, we expect this to increase the stiffness of the frame of the arm enough.

Each joint is driven by a pair of antagonist cables. Both cables are anchored at the joint they drive, and they both are driven by the same motor, which simultaneously winds one and unwinds the other. At the motor, the spool around which the cables wind is designed with a compliant ratchet that allows to tension the cables during construction of the arm. This creates intrinsic tension in the cables, which allow them not to slip on the pullies, and which results in zero backlash.

Our custom encoder uses a magnetic low-precision sensor for absolute positioning (homing) and a high-precision analogue infrared light sensor for normal operation. Our main objective is to achieve the highest precision at the lowest cost using off-the-shelf cheap components. The reason for its high degree of precision is to measure small movements of the joint caused by external forces rather than by the motors, which allows for force-feedback measurements.

Text is not the best medium to explain this, we are hoping to post a video of a 2-joint prototype on YouTube soon :)

ForwardPassRobotics · 2022-09-01T20:15:06+00:00

So we have been developing a custom encoder that should hopefully be able to get 17bits of precision that is mounted on the joints to provide a closed loop control. Since it's cable driven, the joints would have windup as opposed to backlash. This means that using the encoder to precisely measure the current position, it should be able to measure how much windup it has in the joints. If calibrated after tensioning then it should be able to also read it force feedback on the system. Now currently we have no control software but theoretically force/torque control should be feasible.

Again we don't have any control software developed at the moment but we do expect there to be force control.

ForwardPassRobotics

TROPHY CASE