jsegMarch 13, 2017 at 14:23 #12638
Back in August of 2013 one of your designs really helped me out. My institution was working on a museum exhibit about ice age paleontology in Indiana. I was tasked with building a display that highlights the differences in chewing motions between mastodons and mammoths. The project specification was to animate the chewing motions of the two animals using full size casts–ideally such that the teeth would mesh. Initially, we contracted the mechanism out to a local engineering firm, and I worked on making and painting the casts from real specimens in our collection. About six weeks before opening day the engineering firm pulled out of the project. It was too late to design and build a special purpose linkage, so I started looking for any solution that might work. I ended up building the first version of your Stewart Platform as a proof of concept. It looked good, so we scaled it up. The final version was about 2.5 times larger and used Torxis 1600 in*lb servos. With a giant leg up from you we went from nothing to two robots and life size animations of extinct creatures in five weeks. Thank you.
Here are some pictures: http://imgur.com/gallery/16T9v. Sorry about the quality, and the lack of a picture of a robot by itself.
Our visitors loved it. The platforms ran (and the animals chewed) more than 30,000 times during the exhibit’s 8 month run. Now we are working on a renovation of our permanent ice age exhibit gallery and are planning to include an updated version of the “Masticator”. I’m working on the updated design now–mainly to improve reliability and motion smoothness. I’m thinking of using these servos, and your V2 firmware. Would you be interested in chatting with me about this project? I have some questions about the firmware and its performance. In any case, I would like to include attribution on the exhibit label. Could you let me know how you would like your name to appear?
JohnjsegMay 1, 2017 at 13:30 #12977
One of the difficulties for us was that it was very difficult to know what the desired movement should be before the project was started. That’s what made a Stewart Platform a desirable choice–it was much easier to specify a range of motion than a specific motion profile. In the first version, we used a joystick to teach the SP how to move. The joystick consisted of a smaller SP with analog feedback servos with drive gears removed. I basically recorded and filtered the analog values and then played them back on the bigger units. The movement was not ideal. But the bigger problem was the servos I used became unstable after a few months of up-time. They would jitter back and forth and were damaging the casts. It turns out that the feedback potentiometers were wearing around the machine’s home position and producing feedback noise larger than the PID algorithm’s deadband. One solution was to increase the deadband but that decreased resolution an unacceptable amount. In the end we decided to upgrade the servos to Clearpath units. I think that they have a good mix of power density, resolution, and cost. The ones I selected can resolve 6400 steps/rev. I’m hoping that’s enough for smooth motion because so far I have not been able to find reasonably priced gear boxes. I saw your post on r/robotics about your museum bound Stewart Platform and it looks like you are getting an insane 51200 step/rev. It looks really smooth. I’d be curious to know how it does at lower micro-step values. Also, I really like your linkages. Are the biceps custom? (As a museum interactive nerd, I’d love to know more about your project–if you can talk about it. Have you seen the earthquake simulator at the California Academy of Science? It is really great. Built by the Oregon Museum of Science and Industry, I think.)
One major improvement from last time is that now I have a pretty decent CAD model and a simulation to generate the GCODE. Here’s a GIF: That simulation is built in Rhino and controlled with Grasshopper. The curve in the construction plane is a graph of the rotation (x) and twist (y) of the end effector about the red x with the line through it. That curve is controlling the simulation. The resulting motion is 5 axis, so even a hacked together CAM is a major improvement. One thing I’m not clear on with the firmware is how the feedrate works. Is the feedrate a limit of the steps/sec per axis or is it a composite limit the to the end effector velocity? I’ve read the code but I haven’t studied in detail how the motion profiling works yet.
Thank you for all your efforts, and especially for your open source contributions. So you know, I’m planning to make a donation to help support the Stewart Platform firmware. I’d like to get my institution to do it, but if not, I’ll make it personally.
Indiana State Museum and Historic SitesDanMay 1, 2017 at 14:54 #12979
Cool! Path generation for SPs is still a big challenge. I like your joystick solution, similar to
The new SP I made is good,
but it’s weaker than we’d planned. I need a version with 10:1 gearboxes so I can move 500N, which will counter both gravity and the human sitting on top.jsegMay 4, 2017 at 13:10 #12992
In my experience building museum exhibits, it is always better to dramatically over build any machine. If it were me, I’d go for the lowest gearing that the other constraints allow. At my museum I basically assume that 25,000 ornery teenagers/year are going to actively try to destroy whatever I put out on the floor–and stuff still breaks from time to time. Right now I have an interactive that invites guests to “split wood” with a rubber mallet. After installation, I was blown away when both mallets were totally destroyed in the first 3 days. I’ve made some improvements and am up to one set of mallets/7000 visitors. I’m hoping to double that, but never in my life did I expect to spend time thinking about how to make hammers last longer. I realize this is unsolicited advise…so, you know, grain of salt.DanMay 6, 2017 at 09:49 #13027
I appreciate it. Thanks!
I suspect the first point of failure will be the home limit switches.
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