I'm working on a 6-legged radio-controlled hydraulic walker and I wanted to see if there were folks who had some experience controlling hydraulic spools with servos. I chose hydraulics for the potential for integrating other systems, the look, the engine sound, general vibe. Pics of the project thus far here:
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Here's what I want to do. Forward and backward are easy with the spool valve I have, but I'd like to skid steer the monster right & left which requires a second spool & pump. No problem thus far, one servo per spool vavle, for control over right & left sides.
But the way hexapods walk is by "alternating tripods"; there are always three feet on the ground (one on one side, two on the other, then they alternate). The legs on one side are synched via chain (see pic), but maintaining synch side to side would be important.
Now I could just sort of "nudge" the machine back into sync from the R/C controller manually once I'm done turning, but I was wondering if I could control the servos to "resynch" the sides, perhaps using position sensors on one leg per side...
One low-tech approach that I've seen (not on hydraulic or pneumatic, but on electric motors) is to use a clutch that links the two sides. In your case, your clutch would have a cam face machined into it so that it will "click" into place in only one position of the 360 circle. That position would be the exact tripod gait for the two sides in synchronism. Given the size and weight of your robot, maybe a magnetic clutch would work best. On the motors I've seen it was just a spring loaded mechanism, where the clutch would allow slip if there was enough speed difference between the two sides.
I once worked up a chain-driven hexapod using an electric motor and a large aluminum frame (about 18" by 24"), and the walking efficiency is not great. Something with 2DOF independent control of each legs works better. But linked gait walking does function, and if the machine is large and noisy, it looks/sounds impressive.
It would be great if this could be done with steam!
Thanks for your response. This sounds very much like what I'm looking for, as I'm not necessarily after a high tech solution. I'll have to dig up some info on a suitable clutch.
Do you have any pics of your hexapod? Mine has about a 6 inch "step" (actually a cycloid motion).
One version of such a clutch was used in the Milton Bradley Big Trak. The motors were once a common find on the surplus market but they've long since sold out. Basically the clutch was composed of a spring and a N/S magnet on the inside of the motor shaft (the wheel was mounted on the other side of the motor shaft). The inside shafts faced one another.
If the motors are going the same direction, the two magnets will re-align and tend to "catch" one another. If one motor goes in reverse, or substantially different speed, the magnetic pull is broken, and the sides are no longer in sync. I recall the magnets used in the Big Trak had north and south on the same face, which is unusual. However, you can make the equivalent out of two magnets for each side. Flip one magnet so its N is facing out; flip the other so its S is facing out.
I doubt on a big machine that permanent magnets would work, but maybe larger electromagnets? As you're working with hydraulics maybe the motor running the pump could also run a small DC generator for the electromagnets.
The other approach would be a completely mechanical version using a clutch with a shape machined into each face so that it will align only one way (kind of a cam). You'd still probably want a magnetic clutch to release the clutch (rather than the tension of a spring) or else I think eventually you've have a lot of wear or breakage on the cam.
It's in my book Robot Builder's Bonanza:
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Amazon lets you read though (some) pages. Starts on page 335 (chapter
22). You can also search the book using the phrase 'Walkerbot.' The book is also available at most libraries. Walkerbot is not a great example of a hexapod, and it's made for people with only access to a hardware store, but it might give you some ideas. Again, to really do a hexapod right, you want 2DOF on each leg, which means 12 cylinders and 12 valves. Yikes!
That's really clever. It lets you make a machine that actually goes in a straight line, despite the unavoidable slight differences in motor speed, without having to have encoders and a complex control scheme.
I'm partial to hexapod walkers with 3 DOF *total*. One moves the front and back left leg forward and back; another does the same on the right; and a third tilts the center legs up and down (so that one is always up while the other is down). With that, you can walk forwards, backwards, and turn either direction. It's not graceful, but it's pretty effective.
OK, that's got me thinking about a clutch (almost like a sprag) with a ramp on one face, maybe a spring loaded pin in the other. In one direction it will freewheel & slip, the other way the pin will catch, and off we go...maybe a modified motorcycle clutch. Plenty of spring pressure there.
It is clever, though not the only way of doing it.
I've also seen on some old mechanical toys that use two spring-loaded crown gears facing each other. The WAO robots from OWI work this way (or did). The gears would be cheaper, a little easier to engineer (no funky selection of magnets so get just the right pull/push), but probably not as sturdy over the long run. The teeth of the gear can strip out over time.
The same idea might work with a dimpled clutch plate and one or more spring-loaded ball plunger. Wouldn't suffer the wear problem so much. You'd have to select the ball plungers with just the right springiness, but fortunately you can get them pretty cheap -- under a buck each for the small ones with a plastic press-fit housing.
OK, that's got me thinking about a clutch (almost like a sprag) with a ramp on one face, maybe a spring loaded pin in the other. In one direction it will freewheel & slip, the other way the pin will catch, and off we go...maybe a modified motorcycle clutch. Plenty of spring pressure there.
This thing will have a 6 HP gasoline motor and likely two variable displacment pumps (as transmissions), and two hydraulic motors. Final drive will be very slow, 40-50 RPM at the legs. I'd imagine torque is going to be immense, so anything I make or hack together is going to have to be pretty tough.
Maybe manual skid steer isn't such a bad idea...
I do like the idea of some RPM sync so it WILL go in a straight line; the servos getting some feedback as to what the other side is doing. The Futaba servos do have a port on them for some pulse input...
I see the problem. What you really need to control is the range of motion on each side, not the speed. You always want to keep both tripods in phase, but for turning, you need to shorten the stride on one side. The problem is that one motor controls each tripod, not each side. Right?
So it really only needs one motor. But it needs some way to tweak the motion of at least two legs.
Suppose you spring-loaded the middle leg on each side in the fore-aft direction, so it can be stopped without damaging the mechanism, then provided it with a hydraulic cylinder which keeps it from moving. When you want to turn, you close the block valves for the inside cylinder, which locks the fore-aft movement of the middle leg. The machine then pivots around that leg for half a cycle. No synchronization is really necessary, though it would probably work better if you locked the leg at the midpoint of its travel.
This won't let you turn in place; the best you can do is a tight circle. (Sherman tanks had the same problem; they couldn't run one track forward and one backwards, since they just had clutches.)
While you're adding springs, it would help if the rear legs were given some flexibility in the transverse direction. That keeps them from being overstressed in turns. The "skid steer" thing is tough on pointy legs.
Personally, I'd do the whole thing with hydraulic cylinders, lose the motors and chains, and synch everything in software, but then I'm a controls guy.
That's a good summation. I don't care abuot zero-turn or anything like that but I would like the ability to turn and keep everything in phase.
But no, I foresaw this as each side controls each side, not each tripod. One pump & motor on a side, the legs synced via the gears & chain (see URL of first post). I could arrange it in "per tripod" fashion easily enough with a jackshafts, and sync wouldn't be a problem, but turning would still require some special arrangement.
Can you elucidate a bit on what you mean by "spring loaded"?
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