SmallKat: An adorable dynamics oriented robot cat

A dynamics oriented quadruped for research and education.

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SmallKat is a 16 DOF low cost, opensource, 3D printed quadrupedal robot. It was originally designed and made by 2 WPI students (Keion Bisland and Xavier Little) along with framework code prepared by WPI staff Kevin Harrington. The intention of the platform was to make a fully open source dynamics quadruped for the education, research, and hobbyist markets. Using hobby servos and common electronics allows the overall price point to remain very low in comparison to its competitors.

In addition to the hardware kits, a full education kit is provided. SmallKat has lecture slides, tutorials and lab procedures for educators to take out of the box and use to teach robot dynamics. Prior knowledge required is merely basic programming skills in Java or Groovy and understanding of Git. The education kit will onboard you to being able to teach the rest.

In addition to performance and ease of use, adorability is a high priority.

Introducing SmallKat!

The problem we wish to solve is that there is a need for hardware to teach dynamic walking robotics. When we say dynamic, we mean both able to sense perturbations, and has processing power and actuator speed fast enough to compensate.  All existing dynamics-capable walking platforms are too expensive, and all affordable platforms are not capable of full dynamics. If students are going to learn how to make walking robots walk they need to learn using hardware. At the moment only *very* well funded universities and businesses can learn dynamics oriented robot walking. 

We are leveraging 3 degrees in robotics engineering, 10 years of product design and robotics experience, and a brand new robotics IDE to provide the best possible teaching tool at the minimal possible price. We have designed a new, 3d printable cat, about the size of an adolescent kitten. We are also using the brand new BowlerStudio robotics IDE for kinematics and dynamics control. This lets students run their code on the workstation in front of them and see it controlling the SmallKat across the room.  In addition to cutting edge tools, we have a comprehensive teaching package. SmallKat was used to teach robot walking dynamics at Worcester Polytechnic Institute (WPI) and those teaching materials are availible alongside the SmallKat sources. The teaching pack includes tutorials, lab procedures, lecture slides with scripts and teacher onboarding tutorials. No other walking platform has such cutting edge architecture, nor prestigious teaching materials. 

What makes this system so unique is the way students write and test code. BowlerStudio lets the students control SmallKat over the rooms WiFi network. Students code runs as scripts on the PC they are using to develop, and the control commands go over WiFi. This code is quick to change and re-test, making the students experience smooth and delay free. BowlerStudio itself is designed with documentation and tutorials front and center. All documentation is visible from inside BowlerStudio, and the code examples are executable. Executable documentation makes the connection to a description of what something should do, and seeing it happen one click away. Not only is the documentation excellent, it has been teach-tested at WPI. The SmallKat platform was used to teach WPI Robotics Engineering students how to write dynamics controllers. This curriculum and teaching materials are all open-source. WPI has agreed to partner with us to promote SmallKat and help us on the distribution.

The functional structure can be broken down into two major blocks, the high level gait & kinematics computation done in bowler studio and the lower level done by the micro controller on board the robot.

Bowler Studio functional diagram:

MCU Functional Diagram:

Our business model is to sell classroom kits of hardware, and individual fully assembled SmallKats. We have the backing of a top tier Robotics Engineering college and will use that backing to promote this platform to aspiring colleges and advanced high schools. The intent is to provide a lifestyle business for the developers, and to leverage resin casting for bulk orders. All SmallKats can be 3d printed, and for initial fulfilment we will provide printed parts. We will also be refining a resin cast process to streamline production and ensure consistent parts. 

The process to bring SmallKat to market is incremental. In the initial stage we will lean source components and print the plastic to order. As we scale up, we will increase the number of printers dedicated to SmallKat printing and hire a member of the local Makerspace to put together kits. If we outgrow this production throughput, or if we got a large lump sum, we will hire a member at our local makerspace who used to make resin molds for Hasbro. We have already discussed feasibility and would just need to engage him to make the molds. As we spool...

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The final print beds for a complete smallKat Grace

x-zip-compressed - 4.65 MB - 11/07/2019 at 16:13


All files and print beds for SmallKat Luna

x-zip-compressed - 17.08 MB - 10/28/2019 at 16:22


Zip file of latest revision stls and CAD

x-zip-compressed - 13.23 MB - 10/01/2019 at 03:47


Calibration Side 2.step

Calibration Block Step file mirrored version of side 1

step - 97.36 kB - 09/19/2019 at 17:49


Calibration Side 1.step

Calibration Block Step file

step - 96.83 kB - 09/19/2019 at 17:49


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View all 6 components

  • SmallKat will be at Open Sauce!

    Kevin Harrington06/09/2024 at 18:37 0 comments

    Come to Open Sauce in San Fransisco next weekend and meet SmallKat in person! We will have live demos and release of the new model, the Marcos!

  • SmallKat Education package availible (Open Courseware)

    Kevin Harrington02/14/2020 at 19:34 0 comments

    Here is the entire content set of the course materials for teaching SmallKat Dynamic Walking class. For High School teachers, stretch out lab one and 2 for the semester. For College level, teach through the dynamics controller assuming a background in Linear Algebra, single limb kinematics, and basic mobile base kinematics. 

    Lesson Plan:

    Recorded lectures:

    Results from the first run of the class:

    System Documentation:

    Source Code:


  • Howto Balance the SmallKat

    Kevin Harrington12/10/2019 at 16:37 0 comments

    I made a video explaining the math governing the SmallKat's balance control system. 

  • SmallKat Math Video!

    Kevin Harrington11/11/2019 at 18:15 0 comments

    I have posted the video for this weeks lecture on the Math of the SmallKat

  • SmallKat Kinematics

    Kevin Harrington11/10/2019 at 23:24 1 comment

    This weeks videos have to do with the math the controls SmallKat. Tune in if you are a math nerd looking to understand robot kinematics!

  • SmallKat Assembly and Calibration Videos!

    Kevin Harrington10/24/2019 at 15:31 0 comments

    I have filmed a set of videos that take you through assembly and calibration of SmallKat Grace. 

  • Thank you!

    xaveagle10/01/2019 at 04:23 0 comments

    As this competition nears its end, we just wanted to take a quick second to thank everyone who has supported this project. When we first started Smallkat it was really just a challenge to see if it was possible to make a super small and adorable quadruped. We really did not expect it to become as big as it has. Smallkat has been a staple for robotics demonstrations at WPI for over a year now bringing smiles and wonderment to both kids and adults. We have encouraged and inspired students to learn more about quadrupeds and robotics in general and take on challenges they might not have thought possible. Smallkat has been a past time, a senior project and a passion of ours for the last two years and we are excited to share this adorable robot with the world and hopefully inspire more people to build, create and get excited about engineering. Thank you to everyone who has shown interest and support, built their own Smalkat, asked questions and most of all, for letting us share our project with you.

  • SmallKat Curriculum!

    Kevin Harrington09/23/2019 at 19:40 0 comments

    Here is a link to the SmallKat teaching curriculum:

    Here are the lab procedures:

    The lectures are being recorded one by one as the class is taught in B term (Mid October through End of December). They will be recorded and posted on youtube. 

  • Design Revision: Update 2

    xaveagle09/20/2019 at 16:51 0 comments

    Its been some time since the latest revision of the cat but we have been busy. We tested the last version from our previous design update and found it to be a little too big and heavy for what we wanted. For this version we essentially combined the original version and last version together. The features of this version include the following updates:

    • Longer legs and a slightly larger body with same setup as orginal
    • Uses M3 hardware for easier procurement and assembly. M2 nuts constantly stripped out of plastic. 
    • Optional encoders for additional feedback
    • Improved calibration 
    • Better aesthetics 

    This version of the cat has more parts than the original however the assembly process is significantly easier and it is more robust. Additionally replacing servos and other parts takes much less time. 

    The next step will be wiring and running this version through some tests with walking and fixing a few small design errors with tolerances. This version is getting closer to what we hope a final and commercial product would like. Still a lot of work and testing to go before we get to that point. For now enjoy the working prototype featured on our page!

  • A friendly user helped out with a wiring diagram!

    Kevin Harrington08/16/2019 at 20:33 0 comments

    Shout out to @jtechs who made a lovely wiring diagram as he builds the FIRST build of SmallKat made by another person!

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Arnold Muralt wrote 07/16/2022 at 19:59 point

for the grace model, what sort of sensor was used for the foot contact sensors? I don't see anything in the BOM

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Scott Bierly wrote 02/13/2020 at 00:17 point

This is super cool guys, very interested in building one! others, I was shocked at the cost of this build, most people will not drop this much money to do this. Then I noticed that nearly all the cost (if you have a 3D printer) is in those servos. And...why did you specify the rip-off Adafruit as the source? If they build a cool board, great, but they don't build this do they? I'm seeing them at 6 for $37 on, are these the same thing? If so, I'm in!

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Kevin Harrington wrote 02/13/2020 at 02:34 point

Yes those *technically* are the same ones, the difference is that Adafruit does QC. The cheap ones you see on banggood either do not have any QC or are literally all of the rejects. When we buy 16 from adafruit, we get 16 working servos. When we buy from banggood we get a 60% yield. ~15% are DOA, and the rest need re-working by taking apart the gear box and cleaning out the metal shavings from inside. If on the rare occasion we get a bad one from adafruit, a replacement is sent free of charge. Given the yead discrepancy and guarantees, the price difference is not as far off as it might seem. 

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Scott Bierly wrote 02/13/2020 at 03:41 point

Hmmm, it's still hard to see that justifying more than doubling the price. I see your point, though, but...I went back and looked, and product reviews are all good (if real), but more importantly "If one or more of the items you received are damaged, different, or not working" and "we can send you a new item free of charge (we will reimburse you the return shipping cost), or you can choose to receive a full refund". I have no bias for or against Banggood, but this sounds like a reasonable plan to me for anybody on a budget. 

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debil1 wrote 11/09/2019 at 21:30 point


I have some questions:

1. what printing setting do you recommend (infill, supports) ?

2. which red board is used for ESP32 ? I can not find it on "components" list?

Or can we use instead it, bread-board? Can I use: ?

3.. will code work with much cheaper bno055  (  as one from "" is not cheap...?

Thank you.

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Kevin Harrington wrote 02/13/2020 at 02:47 point

1) supports everywhere 40% infill

2) That is a breakout board we manufacture whos source is here:

alternatively you can follow the from-scratch instructions and use a generic protoboard. Its just a simple breakout to servo headers from the IO pins

3) No clue, it looks like it might but i haven't tried it. Ill buy a few to find out. 

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Clément wrote 10/04/2019 at 13:42 point

Hi, I decided it's time to make a SmallKat :)

Is there more details about the motherboard (and related assembly) ?

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Kevin Harrington wrote 11/07/2019 at 16:22 point

We have a build instructions pdf in the instructions section, as well as a set of videos showing the entire process here:

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Suyash wrote 09/12/2019 at 03:59 point

Hey.. Love the idea! I was considering making one of these for myself and need some suggestion. The recommended MG92b Micro Servo is quite pricey here in India. Is there any alternative that I can try? Something like a drop-in replacement but not as amazing as the MG92b. Let me know 😅

  Are you sure? yes | no

Kevin Harrington wrote 09/12/2019 at 10:49 point

SmallKats size and weight are such that any cheaper servo would destroy itself in a few minutes of running.  We push the mg92b to its limits in SmallKat and i have little hope of a cheaper one working at all. 

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xaveagle wrote 09/20/2019 at 16:53 point

As kevin said the Mg92bs are really the recommended servo. You could try the MG90Ds as they will fit the design but they are not as powerful and might not last as long.

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dana2048 wrote 09/12/2019 at 00:33 point

awesome project!

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[this comment has been deleted]

Kevin Harrington wrote 04/23/2019 at 14:14 point

Thanks! I will be in touch as we go into development this summer to build the new stand-alone variant. 

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Ken Yap wrote 04/12/2019 at 02:52 point

Love cats. The "ignore you" part of the coding would be simple. But seriously, love cats. Meow!

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Starhawk wrote 04/12/2019 at 02:11 point


...but seriously, I want one. A pity, because I can guarantee you that I can't afford it :(

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Keion Bisland wrote 04/12/2019 at 02:48 point

you might be surprised. The projects open sourced so you can print everything yourself and all the other components only less than $300

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Starhawk wrote 04/12/2019 at 02:50 point

I have a dead maximum of $200/mo disposable income and no 3d printer.

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alireza safdari wrote 04/11/2019 at 17:37 point

We deserve more information. :)

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Kevin Harrington wrote 04/11/2019 at 18:49 point

geez man im posting as fast as i can...

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alireza safdari wrote 04/11/2019 at 18:51 point

@Kevin Harrington  Sorry, did not want to push you. Take it in a positive way, like there are people out here who are interested to know more. :) Thank you for posting.

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Kevin Harrington wrote 04/11/2019 at 18:53 point

no worries! Im excited to get this public facing after a year of under-the-radar developemnt with Keion and Xavier

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alireza safdari wrote 04/12/2019 at 04:18 point

@Kevin Harrington You guys are on fire :D Thank you for posting more information. :)

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Kevin Harrington wrote 04/11/2019 at 22:40 point

I added a bunch more info :)

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alireza safdari wrote 04/11/2019 at 22:41 point

Going to read them now. Thank you.

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alireza safdari wrote 04/11/2019 at 22:46 point

@Kevin Harrington I really love your solution for programming the cat. Am I outdated or this has never been done before?

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Kevin Harrington wrote 04/12/2019 at 12:00 point

This is rather new way to program and control robots, yes. My contribution to the project is the programming framework, [BowlerStudio]( I wrote the programming environment, the kinematics models and the hot-swapping Git based script architecture. 

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Dan Maloney wrote 04/11/2019 at 16:42 point

If it doesn't spend 90% of the time sleeping, 5% puking, and 5% licking itself, it's not a proper cat. ;-)

But it's still really cool, and I'm looking forward to seeing more. Nice work!

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