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Stewart/Gough Platform Cable Porn (not the kind on pay-per-view)

Imagine a venn diagram. One circle says “cable management”. One circle says “OCD”. One says “Photography”. At the intersection of these three you’ll find Cable Porn, the art of making cables and wires look extra tidy. I had built a Stewart Platform about a year ago and it sat, sad, neglected, and horribly wired for most of that time. A couple days ago I decided I’d fix it up and take it to the next meeting of the Vancouver Robotics Club. I knew right away I had to get the embarrassment that was my wiring under control. Here are the after pictures.

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Stewart/Gough Platform with Firgelli L12 actuators

I’ve updated yesterday’s wiring because some of the wires didn’t match my year-old code. Here’s the latest. I still think it’s pretty sloppy, but it’s better than the rat’s nest I had in version 1 a year ago.

The board in the bottom left is an Arduino MEGA 2560. The three boards at the top are CANAKIT UK1122 H bridges. Each board has two h bridges. The small breadboard at the bottom right has six 5-pin headers where I attach the Firgelli L12-P linear actuators. The green lines running to the right tell the arduino what the actuator is doing (eg how far out it has moved). The green lines running to the h bridges control the speed of each motor. Speed can’t be a negative value, so the blue lines on either side of the green lines going to the h bridges control the direction. Set both directions at once and you apply the breaks on the motor. So far all the wires I’ve mentioned are running at 5v. The h bridges take the 12v coming in at the very top right of the picture and send it along the very long red and black wires to the correct pins in the bottom right. The remaining tiny wires in the bottom right deal with the sensor on the actuators (positive and negative rails for the potentiometers).

So: the Arduino has a model of the Stewart Platform in it’s memory, including where it is and where it can go. I send a serial message like “G00 Z20 A15”. Arduino knows calculates how long each actuator has to be to move up 20mm and tilt 15 degrees, then sends the correct speed and direction commands to the h bridges, which in turn moves the real platform on my desk. The extra tricky part is that all 6 actuators have to move together at once – if even one tries to go AWOL it could seriously damage the machine. Fortunately I have sensors in the actuators that tell me if the real thing and the model are different. If the difference is too big, emergency stop.

So what’s the point? Stewart platforms are used in microsurgery, flight simulators, and even for docking shuttles to the international space station. They can be used to stabilize platforms on a boat or to control your portal turret. Stack them and you can make an awesome robot elephant trunk. There’s some really interesting math that would keep a high school algebra class awake. Can you think of other uses?

I’d really love to make this a kit in my store but I feel the price for actuators is still too high. With stepper-driven actuators I could lose this wiring and use the RAMPS control board from a 3D printer. Spread the word! I’m searching for a bipolar stepper driven linear actuator with a near limit switch. I’m looking for a hobby quality device up to $30, no more. A universal joint on each end would be extra sweet. Please share this and let me know if you find one.

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Store fixed in time for a great 2013

I think I fixed the shop. It was an obscure setting in a back corner of the sub basement under a box marked “live arachnids”. The box had a hole in it. I’ve been able to create an account, checkout, and add to cart in Chrome, one of the browsers that was giving me trouble. I don’t have IE9 to test it. Please try again and let me know if there are any remaining issues.

For all of you who have been patiently waiting… thank you. I’m always here to respond to your feedback and I’m doing my best to please everyone. If you have suggestions or other feedback, please don’t hesitate to let me know.

If you’re a new reader you might not know that one of my goals for 2013 is to sell 500 Makelangelos. I’m gonna need help to make it happen. Here’s crossing my fingers that I don’t get too many more snags like this.

Edit 2012-01-05 @ 12:55 – Nope! Now it’s broken in other ways. Logins are failing and checkouts still don’t work.
2012-01-05 @ 13:14 – Better?

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What are the best robotics for kids?

I’d say start with LEGO, then LEGO Mindstorms, then VEX contests, then Arduino.
Speaking of which, have you heard about Mindstorms’ EV3? It adds iPhone and Android support out of the box. http://www.theverge.com/2013/1/7/3845222/lego-introduces-android-ios-compatible-ev3-mindstorms-robotics

I wanted to connect my iPhone to my Arduino robots two years ago but Apple wouldn’t give me access.

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Inverted Stewart Platform from three Makelangelos

I’ve been trying the last few days to rebuild my old Stewart/Gough platform. I was stuck with the problem of interfacing parts – attaching two parts that don’t normally want go together. Three days ago I was having a brainstorming session with new VHS member Freddie and he mentioned that they’d been built upside down with cables. It was like lightning striking – I suddenly remembered RoboCrane and I almost screamed “Why didn’t I think of that earlier?!”

The model I’ve come up with is a 3-3 platform. That means the strings at the base meet at three points and the strings at the end effector meet at three points. There are also 3-6 and 6-6 platforms.

To build this I used a few pieces of laser-cut acrylic and three Makelangelo kits.

This is very similar to the method Antonio used to mount his Makelangelo – keep all the motors in one place and run extra long strings.

The rest is software, which I’m writing at this very moment. If I had an extra RAMPS board I could run this machine far more efficiently. As it is I’m maxing out the USB connections on my laptop.

When I release the code the laser cutting templates will come with it so you can make your own. I love that I can add this to my store without having to stock a single extra piece of electronic equipment.

Now let’s see if I can get this thing running by Sunday for the monthly meeting of the Vancouver Robotics Club. Wish me luck!