Every year, my daughter and I expand our Halloween display in the front yard. Last year, Snotrag the Ghost made his debut. As an animated display, Snotrag was pretty predictable. In 2007, we decided to try to get him an additional degree of freedom.
I had recently been working with Linux EMC and CNC conversions on small three-axis mills. The conversions have gone well, and the tools are invaluable. While discussing the Snotrag project with one of my coworkers, he suggested that using EMC and a tripod configuration might work. I knew Linux EMC had a kinematic translation layer, and it took a little hunting to figure out which elements I'd need to invent.
The first criteria involved basic stuff - fly over the front yard, stay high enough to avoid the munchkins (and the JDs,) stay low enough to avoid the aerial wires, stay out of the street, etc.
It was desirable to have Snotrag move at a brisk pace, so we settled on 6 feet-per-second as the peak velocity. [Note: this project is being implemented in the US, where materials are rarely available in metric form. Dimensions are presented in feet and inches out of convenience.] That, coupled with a 15-foot mast height provided the basis dimensions for space planning. First up, the isometric plot view [pdf]. Images below show the masts installed in the yard.
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Motors were going to be a problem. I have absolute confidence that standard stepper motor drivers won't function properly over the distances required. New motor drivers were built to handle the task. When the components and PCBs were received and assembled, bench testing began. The big bundle of cable in the following photos is 150-feet of Cat-5E cable. The motors are Lin Engineering L4218L-01 bipolar units driven at 2.0A per winding. The motor drivers were the most challenging piece of this build, and I don't mind saying that they work beautifully.
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During testing, there were a number of problems. Resonances in the mechanical system proved to be the most problematic issue. Several posts in the CNC Zone forums indicated that vibration dampers would help alleviated the problems, so I built "rattler-style" dampers. They solved the resonance issues on the 10-foot garage-scale prototype ... but the resonance problem would rear it's ugly head again later in the project.
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The masts were fabricated from 10-foot sections of 1-inch EMT (aka metal conduit.) Two sections are bolted together such that the upper section pivots at the top of the lower section. This proved to be wonderfully convenient, as the motor driver could be pivoted down to a reasonable height for maintenance. The masts are braced with 1/2" EMT sections. All are secured to the ground with 10-inch steel spikes. A safety line was attached to nearby object to prevent the JDs from tipping the masts into the street.
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I had originally planned to place the Linux EMC computer inside the house, and control the system from outside using a wireless USB joypad. However, it rapidly became apparent that I'd need ready access to both the hardware and software components during the initial turn-up and calibration. Plan B involved placing the computer outdoors in a NEMA enclosure. Easily, the NEMA enclosure was the single most expensive component in the system.
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Calibration of the system is difficult, as it requires disassembly of the "platform" to zero the individual joint wires. At present, limit switches are not installed due to a slight oversight on ordering the parts (someone bought the stock out from under me.) Once the individual joints were homed and reconnected, Linux EMC was placed into World Coordinates mode. The machine was controllable in Cartesian coordinates with EMC performing the kinematic translations. The Logitech wireless USB joypad performed flawlessly at an outdoor range of about 100 feet (which is well beyond it's specs.) Snotrag the Ghost hadn't been resurrected at this point, so test flights were performed with Snotrag's ugly second cousin once-removed - Wrenchy, the 7/16 box-end wrench. A video of Wrenchy being jogged around the yard is posted below in both WMV and Quicktime formats:
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As projects go, this one worked remarkably well. Unfortunately, the large-scale installation had additional resonance issues that didn't exist on the smaller scale model. There is a combination of tensile resonance in the joint lines, and bending resonance in the masts. Wrenchy flew beautifully at chest-height - about 4-feet off the ground. When asked to go higher, he'd run up to about the 10-foot service ceiling, bounce, and plummet to the ground in free-fall. This was a catastrophic failure mode, as Linux EMC is totally unaware that anything bad has happened. There's no feedback on the stepper motors, which are run open-loop. If a stepper drops a step, EMC can't infer that the drop has occured. With the joint lines now in an uncalibrated position, the kinematic translation algorithm operates with hideous error relative to the physical machine motion. Recalibration is required at this point.
With the number of rug rats we expected on Halloween, I abandoned the machine for this year. I had low confidence that it wouldn't fail at an inopportune time, and Mr. Murphy is my constant companion, so I knew better than to take the chance. We hustled to get last year's Snotrag hardware installed on this year's masts, which worked out pretty well. The kids seemed to enjoy the ghost zipping along overhead, though several JDs were observed looking for a stick to knock it down. (Trust me, I know a JD when I see one, because I used to be one many moons ago.)
In spite of missing the Halloween deadline, I'm quite pleased with the overall outcome. The hardware and software worked as well as I could have expected. The mechanical resonance issues didn't show up until the final installation, but I have confidence that I can resolve the issues. I know what they are now, and I have several ideas for eliminating them.
People are twitchy. One of my neighbors, call her Blue Haired Station Wagon Woman, lives down the street, and doesn't like anything out of the ordinary. She stopped to ask me "Is that for the dish?" I didn't understand the question initially, but she was concerned that I was installing a DirecTV dish on a mast near the street. I assured her that this was just a Halloween decoration, and that it'd be down the following morning. BHSWW proceeded to drive by the house at least four times, checking on my progress at half-hour intervals. She'd do a slow drive-by with her mouth hanging open (quite literally.) When she'd inevitably make eye contact with me, she'd stomp the throttle and race down the street. I fully expected her to call the City's Planning and Zoning office.
I'd say about half of my neighbors were genuinely interested in the project. All of them certainly think I'm a nut. But I enjoyed building this project, and I hope to have the bugs worked out by Halloween 2008. Snotrag the Ghost will fly in three-space. Mark my works ...
Finally, I'd really like to thank the Linux EMC developers for helping me get the
USB Joypad and HALUI functions working.
Without their help, I would have been dead in the water.
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