stuff going on

[gfish]

I finally got a wire-feed (flux/MIG) welder last week, and a sharp looking (if kind of cheap) rolling cart to keep it all on. It's... just amazing. Normally when you buy a new tool, it gives you a new capability, and that's cool. But sometimes you get the magical experience of a new tool changing how you do everything. This is one of those times. Being able to weld cheaper/faster/better means I'm already turning to welding as a solution for all kinds of things I never would have bothered with when using my old oxy/accetylene set.

Other than cleaning up the workroom and shop last night, my focus continues to be on incrementally working towards the grand hexapod project. I have a working hydraulic testbed now, with a tiny little electric pump driving a small cylinder through a real directional control valve. I just finished mounting the cylinder to a pivoting arm, to give me a complete mockup of a single (1 of 18!) hexapod joint. I have the electronics working to control the valve from an Arduino, a frustrating but very educational process. Next: attaching a position encoder on the arm and implementing precision control. I might make a series of videos showing the relative performance of hysterisis, proportional, derivative and integral control loops, that would be neat. Assuming I can get that working, it's time to start working on a full-size leg. Which means a larger electric pump, amongst other things. That will be the final sanity check before getting absolutely stupid about this project. The point of no return. Whee!

Comments

[nojay.livejournal.com]

Sadly what you're describing isn't as easy as it looks. I was involved in research to do this sort of thing about 25 years ago and the results weren't pretty. We were using pneumatic actuators and valves coupled with microprocessor controllers but the problems with hydraulics are the same, pretty much.

Positioning using a hydraulic cylinder and a proportional valve gets messy when you consider inertia, mass flow through the valve and into the cylinder, the differential characteristics of the two cylinder piston faces (the rod face has less active area than the non-rod face hence the same pressure on both sides of the piston causes differential force and, absent a braking system will cause creep when the system is supposedly static) and a whole load of other bugger factors.

We got it to work, sort of, eventually developing a cheap industrial robotic arm but precision was not its strong point and hunting and overshoot were a fact of life. It was not a commercial success.

If you are seeking rotary motion at the joints rather than linear extension/retraction you might want to consider using vane-type rotary actuators rather than cylinders. They do have the advantage that the vanes have the same pressure/force on each side as the rotating shaft comes out the side of the vane chamber, not through one end of a cylinder. They also simplify the actuation at the joint. Rotary encoders are also cheaper and more robust than linear ones; we got decent results from using wirewound potentiometers to measure angular movement, building a resistance/angle lookup table in the MPU controller's memory for each joint but we ended up with 8-bit Gray encoders in the production version for engineering reasons.

[randomdreams.livejournal.com]

I felt much the same way when I got my welder. Suddenly you just want to stick *everything* together. I used to spend a lot of time casting and fabricating. Now I weld everything. It's glorious.

[happyinmotion.livejournal.com]

I feel the same way about laser cutting now that Ponoko exists. I look at everything and think "how can I make that out of flat bits?"

[gfish.livejournal.com]

Yeah, I'm not expecting it to be easy. I won't be needing quite the precision of an industrial arm, thankfully. It would be nice to have, of course, that would allow for more interesting gaits and behaviors in the final hexapod.

Why were you trying to hold a cylinder static under pressure? (Iirc, there are some clever plumbing arrangements which do allow that, though they reduce the total force the cylinder can exert.) I plan on using center-closed or tandem valves, so the cylinders will be locked even when the power is off. I guess that might not work as well with squishy pneumatics?

Rotary is what I'm going to care about in the end. Existing hexapod code is all written for RC servos, so it's setting the angle of the joint directly. As you say, easier to read that anyway. I have an as5040 (http://www.austriamicrosystems.com/eng/Products/Magnetic-Encoders/Rotary-Encoders/AS5040) that I'm testing for the encoder. Cheap, fast, accurate and dust proof -- very important for the target environment. Other than having to solder up way more SMD chips than I would normally chose to deal with (that is, zero) they should work quite well once I get the mounting worked out.

[gfish.livejournal.com]

I need to get back to playing with that. I was using the local Maker shop's laser cutter fairly regularly last year, and then fell out of the habit. It's a very exciting technology, and I love the look of the final product.

[nojay.livejournal.com]

Pneumatic devices are a lot cheaper than hydraulic ones and we were trying to get hydraulic/electric positioning performance at a lower cost -- what we called a "working man's robot". The compressibility of the working fluid was a major handicap in terms of accurate positioning and dynamic path control though.

There were all sorts of tradeoffs; most factory floors have 7 bar compressed air supplies which meant no dedicated compressors were needed to drive the robot unlike hydraulic devices which would usually require a power pack. Robots in food processing facilities can't use regular hydraulic fluids for fear of contamination and this was one of the target markets for our design; all it needed to drive it was a sterile filtered air source. Not requiring recirculation of working fluid was a big win in terms of plumbing, hoses, leak prevention etc. We were looking at the idea of using pneumatic actuators with a suitable low-pressure hydraulic fluid (7-10 bar) to compensate for the compressibility problem when I left the project.

If you do go to rotary actuators you can buy them off-the-shelf with angular position sensors already fitted and qualified saving you a lot of integration hassle. They can also be sealed better than cylinder actuators since there is only a rotating shaft coming out of the actuator housing rather than a sliding piston rod.

[keystricken.livejournal.com]

I've already signed up for the laser cutting SBU at TechShop next week, so I'm looking forward to solving all my problems with lasers.