I built a model of the leg cart/leg test stand in our simulation environment. Leg dimensions, masses, ranges of motion and actuator force limits are all in the model.
On the meatspace implementation the inputs to the system will be valve commands, which roughly correlate to flow rates. In an attempt to model this, we present actuator linear rates as the inputs to the model.
This week we assigned the controls team the task of controlling the cart and making it row itself along in a straight, controlled line. We’ve got groups working on joint level control, forward kinematics, inverse kinematics, trajectory generation and the nebulous “high level”. As a starting point I put together a little demo with the leg cart rowing itself along very poorly:
For reference, the cart weighs about 600 pounds and the pins weigh about 200 pounds and are 6 feet tall. When this thing exists in real life, we will have to be very careful… More videos next week when the student solution starts coming together!
Well, we just wrapped up our second class session. We kept lecture time pretty short – we only did a brief introductory presentation on steel weldments (which you can check out here) and reviewed a couple of homework assignments from the first class – and then spent most of the class on (a) prepping the controls team to work with the upcoming Leg Cart simulation and (b) coordinating the mechanical team’s build progress. James or Dan will post some simulation videos soon, but in the meantime I thought I’d show off some of the mechanical work.
To get started, let’s review the hardware we bought before class, starting with…
One of the sets of decisions we made as instructors early on was deciding on the powerplant. We needed something that provided sufficient power to provide hydraulic fluid for a 1-2 ton robot, that was (a) robust, (b) relatively affordable, and (c) as off-the-shelf as possible. We decided to rip the hydraulic powerplant out of a forklift (specifically, a Toyota FGC-45 from 1991) and repurpose it. The net result is that we have a 135 horsepower, propane-powered powerplant that produces upwards of 35 gallons per minute of hydraulic fluid at 2,500 psi. The fact that it’s propane fired means the robot can operate for brief periods indoors, and also means that we can swap industrial-grade fuel tanks good for 6 hours of runtime without having to custom build or fill any of the fuel system components.
So that’s what we have – what we’re working on now is making the Leg Cart, our prototype hydraulics platform. The idea here is to make a platform on “rails” (i.e., only able to move in one dimension) that has a single leg and a hydraulic powerplant on board (we’re borrowing the pump from our full system and running it off of a giant electric motor). This will be the students’ introduction to hydraulic systems, so we’re building something small (relative to the final system – the cart will still be 600 pounds…) at first so we can screw up and figure out what we’re doing. Without further ado, here’s the progress we made on the cart chassis in the past week:
Spark and Mac have been working hard at it for the past week or two – good work guys! Next up, we have the base weldment for the arm pivot:
Adam and Andy teamed up to work on this over the past week. Unfortunately, some material was backordered, but we’ll be resolving that shortly.
Joel, Joe, Mike and Jona are all working on parts that are heavy on machining and low on welding, so we’ll see updates from them a bit later on in the process.
That’s it for now. Keep an eye out for an update on simulations!
OK! The blog is up, we just had our second class session, metal’s being cut, and simulations are being written. We’re off to a hell of a start, and now it’s time to tell the world.
We are Project Hexapod. We’re a team of 19 people who are trying to pull off the highly improbable; building a giant (1-2 ton, 15 foot diameter), rideable 6-legged hydraulic robot. We’re not some robotics company, and we’re not working off of some big grant or another. We’re tackling this problem in a whole new way; we’re a class based out of Artisan’s Asylum, a community workshop with all the fabrication facilities we could possibly need, where 15 students are learning how to build robots from 3 industry roboticists from Barrett Technologies and other local robotics companies, and 1 national champion Battlebots builder as a teaching assistant.
As I mentioned, we just had our second class session. The class officially kicked off on April 17th, though me, James and Dan have been working on the system-level design for the past 6 months to make sure the whole thing was possible. We’re going to push to make this project as open-source, publicly engaging and widely available as possible, so be sure to check this blog for presentations, source code, design documentation, the works.
And thus, without further ado, let’s talk classes!
Class 1: What Did We Get Ourselves Into?
Our first class was all about shock, awe, and expectation management. We introduced ourselves, the scope of the project, the exercises and design steps we would be taking to achieve our goals, and then did a quick introduction to hydraulic systems to get everyone on the same page. Once all of the lecturing was done, we broke up into a controls team and a mechanical team; the controls team busied themselves in setting up a simulation environment that James and Dan developed for the class, and the mechanical team got launched on making and welding parts together for the Leg Cart (more on that in a bit). You can check out our introductory presentation here: Rideable Hexapod Intro 2012-04-17
After the introductory presentation, I presented an introduction to hydraulic systems. If you’ve never used hydraulics before, I suggest checking this out – they’re a fascinating power transmission technology widely used in farming equipment, earthmoving equipment and high end robots like BigDog, but are almost nonexistent in the hobby/amateur robotics world. There’s certainly a steep learning curve to hydraulics, but hydraulic systems are awesome and you should learn how to use them. Learn more about them with us here: Intro to Hydraulics 2012-04-17 (Warning! There is one gory image of a hand after it’s been hit by a hydraulic leak, which may be NSFW)
Once all the presentations were over, it was time to start work on the Leg Cart! Also known as the Land Barge, Land Gondola, single leg prototype, that-which-cannot-be-stored-easily, and generally-terrible-idea. The mechanical team got a giant cut list to work on for the next two weeks, and the controls team got introductory exercises in our custom simulation environment.
The cart is a giant, 600 pound monstrosity with a half-scale hydraulic leg sticking out the side, a hydraulic power system (using a 100 pound field-wound electric motor attached to our hydraulic pump), all on four fixed casters (yeah… you heard us… we’re going full stupid here). The idea here is to have the students get familiar with working and welding steel, assemble a functional hydraulics system for the first time, wire real hardware with feedback sensors and a computational platform, wrap feedback loops around sensors and the hodge-podge hydraulics system we’ve assembled, and then control custom hydraulics hardware in closed-loop forward and inverse kinematics. If everything goes according to plan, the cart will be able to paddle its way up and down a section of the shop aisle.
OK. That’s the update for class 1. Stay tuned for the next class update, where we post some pictures about our progress on the Leg Cart and maybe a simulation video or two!
Hello world! The first class is nearly upon us. On Tuesday, April 17 2012 we will be having our first full class meeting. The instructors have been working overtime for the past few months roughing out system level design. We are putting it all together in to a presentation, I will bloggify the summary once we are done with the presentation. Oh boy!