A black and yellow robot dog stands in the middle of the floor, with a GoPro camera mounted on its back. A picture-in-picture view in the bottom left corner shows the view from the camera.

An Improved Robot Dog For Senior Design

May 8, 2026 by Aaron Beckendorf 7 Comments

[Aaed Musa] has been building robot dogs for a long time now, so it was only natural that he would make one for the senior design project of his mechanical engineering degree. Since this meant working with potential customers, the requirements were somewhat more stringent than for previous dogs, but [Aaed] and his team were able to deliver CARA 2.0, their most agile, versatile robot yet.

Based on conversations with potential customers, [Aaed] and his team aimed for a price around $1,000 USD, a weight under 20 pounds, and a durable design. Like the original CARA, this used capstan drives to actuate the joints, which reduced costs. The drives were printed in resin and powered by brushless drone motors. These motors were designed for speed, not torque, so the team had to rewind them with more wire, an ordeal which paid off by roughly tripling the torque. As far as durability, one joint motor was tested by running it continuously back and forth, and it lasted for over 1,000 hours without obvious damage.

Since the joints don’t contain any absolute encoders, each motor has to home on startup by extending to its limit, as detected by a rise in motor current. As a happy side effect, this creates a lifelike stretching motion on startup. Compared to the earlier iteration, CARA 2.0 takes shorter, quicker steps, and thanks to angled step movements can turn much more quickly. In testing, it originally skewed to the left, which turned out to be due to an asymmetric leg design. Once corrected, CARA 2.0 could walk in straight lines, walk sideways, turn in place, crouch, jump, and keep its balance on an inclined surface. It didn’t quite make the price goal, but $1,450 is still cheap for such a capable robot dog, and it reached every other customer requirement. Most importantly, all the team graduated.

For another take on a capstan-powered robot dog, check out Stanley. We’ve also taken a look at TOPS, one of [Aaed]’s earlier designs.

Continue reading “An Improved Robot Dog For Senior Design” →

A round, 3D-printed motor housing is shown, with one flattened side holding a fan mount. A circular plate is mounted above the face of the housing, and a cord runs around it and pulleys on the side of the housing.

Tying Up Loose Ends On A Rope-based Robot Actuator

January 5, 2026 by Aaron Beckendorf 9 Comments

One of the perennial challenges of building robots is minimizing the size and weight of drive systems while preserving power. One established way to do this, at least on robots with joints, is to fit each joint with a quasi-direct-drive motor integrating a brushless motor and gearbox in one device. [The 5439 Workshop] wanted to take this approach with his own robot project, but since commercial drives were beyond his budget, he designed his own powerful, printable actuator.

The motor reducing mechanism was the biggest challenge: most quasi-direct drives use a planetary gearbox, but this would have been difficult to 3D-print without either serious backlash or limited torque. A cycloidal drive was an option, but previous printable cycloidal drives seemed to have low efficiency, and they didn’t want to work with a strain-wave gearing. Instead, he decided to use a rope drive (this seems to be another name for a kind of Capstan drive), which doesn’t require particularly strong materials or high precision. These normally use a rope wound around two side-by-side drums, which are difficult to integrate into a compact actuator, but he solved the issue by putting the drums in-line with the motor, with two pairs of pulleys guiding the rope between them in a “C” shaped path.

The actual motor is a hand-wound stator inside a 3D-printed rotor with magnets epoxied into it. The printed rotor proved problematic when the attraction between the rotor and magnets caused it to flex and scrape against the housing, and it eventually had to be reinforced with some thin metal sheets. After fixing this, it reached five Newton-meters of torque at one amp and nine Newton-meters at five amps. The diminishing returns seem to be because the 3D-printed pulley wheels broke under higher torque, which should be easy to fix in the future.

This looks like a promising design, but if you don’t need the output shaft inline with the motors, it’s probably easier to build a simple Capstan drive, the mathematics of which we’ve covered before. Both makers we’ve previously seen build Capstan drives used them to make robot dogs, which says something for their speed and responsiveness.

Mechanical Logic Gates With Amplification

September 20, 2024 by Bryan Cockfield 5 Comments

One of the hardest things about studying electricity, and by extension electronics, is that you generally can’t touch or see anything directly, and if you can you’re generally having a pretty bad day. For teaching something that’s almost always invisible, educators have come up with a number of analogies for helping students understand the inner workings of this mysterious phenomenon like the water analogy or mechanical analogs to electronic circuits. One of [Thomas]’s problems with most of these devices, though, is that they don’t have any amplification or “fan-out” capability like a real electronic circuit would. He’s solved that with a unique mechanical amplifier.

Digital logic circuits generally have input power and ground connections in addition to their logic connection points, so [Thomas]’s main breakthrough here is that the mechanical equivalent should as well. His uses a motor driving a shaft with a set of pulleys, each of which has a fixed string wrapped around the pulley. That string is attached to a second string which is controlled by an input. When the input is moved the string on the pulley moves as well but the pulley adds a considerable amount of power to to the output which can eventually be used to drive a much larger number of inputs. In electronics, the ability to drive a certain number of inputs from a single output is called “fan-out” and this device has an equivalent fan-out of around 10, meaning each output can drive ten inputs.

[Thomas] calls his invention capstan lever logic, presumably named after a type of winch used on sailing vessels. In this case, the capstan is the driven pulley system. The linked video shows him creating a number of equivalent circuits starting with an inverter and working his way up to a half adder and an RS flip-flop. While the amplifier pulley does take a minute to wrap one’s mind around, it really helps make the equivalent electronic circuit more intuitive. We’ve seen similar builds before as well which use pulleys to demonstrate electronic circuits, but in a slightly different manner than this build does.

Continue reading “Mechanical Logic Gates With Amplification” →

Hackaday Links: August 25, 2024

August 25, 2024 by Dan Maloney 8 Comments

The Sun has been remarkably active lately, so much so that it might have set a new sunspot record. According to the sun watchers at the Space Weather Prediction Center, on August 8, the Solar Dynamics Observatory snapped a picture that was positively bedazzled with sunspots. Counting methods vary, but one count put the sunspot number at a whopping 337 that day. That would be the largest number since 2001, during the peak of Solar Cycle 23. The sunspot number is highly correlated with solar storms and coronal mass ejections; more spots mean more magnetic activity and more chance for something to go very, very wrong. We’ve been pretty lucky so far with Solar Cycle 25; despite being much more active than the relatively lazy Cycle 24 and much stronger than predicted, most of this cycle’s outbursts have been directed away from Earth or only dealt us a glancing blow. Seeing all those spots, though, makes us think it’s only a matter of time before we get hit with something that does more than make pretty lights.

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8-Tracks Are Back? They Are In My House

June 10, 2024 by Kristina Panos 56 Comments

What was the worst thing about the 70s? Some might say the oil crisis, inflation, or even disco. Others might tell you it was 8-track tapes, no matter what was on them. I’ve heard that the side of the road was littered with dead 8-tracks. But for a while, they were the only practical way to have music in the car that didn’t come from the AM/FM radio.

If you know me at all, you know that I can’t live without music. I’m always trying to expand my collection by any means necessary, and that includes any format I can play at home. Until recently, that list included vinyl, cassettes, mini-discs, and CDs. I had an 8-track player about 20 years ago — a portable Toyo that stopped working or something. Since then, I’ve wanted another one so I can collect tapes again. Only this time around, I’m trying to do it right by cleaning and restoring them instead of just shoving them in the player willy-nilly.

Update: I Found a Player

A small 8-track player and equally small speakers, plus a stack of VHS tapes. — I have since cleaned it.

A couple of weeks ago, I was at an estate sale and I found a little stereo component player and speakers. There was no receiver in sight. I tested the player with the speakers and bought them for $15 total because it was 75% off day and they were overpriced originally. While I was still at the sale, I hooked it up to the little speakers and made sure it played and changed programs.

Well, I got it home and it no longer made sound or changed programs. I thought about the play head inside and how dirty it must be, based on the smoker residue on the front plate of the player. Sure enough, I blackened a few Q-tips and it started playing sweet tunes again. This is when I figured out it wouldn’t change programs anymore.

I found I couldn’t get very far into the player, but I was able to squirt some contact cleaner into the program selector switch. After many more desperate button presses, it finally started changing programs again. Hooray!

I feel I got lucky. If you want to read about an 8-track player teardown, check out Jenny List’s awesome article. Continue reading “8-Tracks Are Back? They Are In My House” →

Gears Are Old And Busted, Capstans Are Cool

June 3, 2024 by Elliot Williams 45 Comments

Zero backlash, high “gear” reduction, high torque transparency, silent operation, and low cost. What is this miracle speed reduction technology, you ask? Well, it’s shoelaces and a bunch of 3D printed plastic, at least in [Aaed Musa]’s latest installment in his series on developing his own robot dog.

OK, the shoelaces were only used in the first proof of concept. [Aaed] shortly upgrades to steel cable, and finds out that steel fatigues and snaps after a few hours. He settles on Dyneema DM-20, a flexible yet non-stretching synthetic rope.

Before it’s all over, he got a five-bar linkage plotting with a pencil on the table and a quadriped leg jumping up and down on the table — to failure. All in all, it points to a great future, and we can’t wait to see the dog-bot that’s going to come out of this.

There’s nothing secret about using capstan drives, but we often wonder why we don’t see cable-powered robotics used more in the hacker world. [Aaed] makes the case that it pairs better with 3D printing than gears, where the surface irregularities really bind. If you want to get a jumpstart, the test fixture that he’s using is available on GitHub.

Continue reading “Gears Are Old And Busted, Capstans Are Cool” →

There’s Not A Cassingle Thing Missing From This Cassette Deck Masterclass

October 26, 2021 by Kristina Panos 78 Comments

For [ke4mcl], this whole cassette craze of late is not a new discovery so much as it is a personal nostalgia machine. Since [ke4mcl] sees a lot of basic questions go unanswered, they made an incredible beginner’s guide to all things cassette deck. This concise wealth of information covers everything from terminology to operation, basic maintenance like repairing the belt and lubricating the motor, and appropriate cleaning methods for the various parts. Yep, we’re pretty sure this covers everything but the pencil winding technique, which you probably already knew about.

You don’t need a lot of tools and supplies to maintain a cassette deck or twelve (apparently they’re addictive) — mostly just head cleaning fluid, isopropyl, window cleaner, and a bunch of cotton swabs. And given this guide, you’ll enter the enclosure confidently, armed with knowledge about everything from the belts to the capstan to the head. This is valuable information, the kind of stuff your older brother wouldn’t take the time to explain to you in the 80s. But maybe he didn’t know reverse bias from the holes in the top of the tape.