hopping humanoids, continued

I was reading through some earlier correspondence in this group in search of encoder links and i stumbled upon a "hopping humanoids"
thread
John Nagle was writing this:

Why would you want a mechanical solution to energy reuse problem ? Why doesnt electronics cut it ? I mean, electric vehicles reuse their braking energy, and some types are also reusing energy coming from shocks, i.e. instead of converting that into heat they are dumping the energy into capacitors and recharge the batteries using that later on. So with robots, why wouldnt you dump the energy into capacitors and reuse it ? With springs, you are converting kinetic energy into potential in spring, and then back to kinetic. With electronic solution you would run your motor ( either linear or rotary, doesnt matter ) as a generator when "braking" and dump the energy into capacitors, supercapacitors, batteries, whatever fits the bill best, and reuse it to move the actuator again. I know that when using DC servos energy reuse its tricky to accomplish, but most industrial bots are using AC synchronous servos which are simpler in construction and thus also cheaper, and energy reuse is relatively easy electronically to accomplish. The only catch is that AC control is more complex algorithmically than DC, but its all implemented in controller chip silicon anyway.
I believe that ASIMO and most of the advanced asian humanoid bots are actually using AC servos, as AC servo has all around better characteristics ( better power per kg, better torque curves, cheaper etc ) so i believe they would go the extra mile and reuse the energy as well. Call it "electronic spring", the principle remains the same as mechanical spring, its just a matter of different energy conversions. And AC motor to capacitor and back should yield over 80% efficiency. Also, electronic springs are obviously more controllable.
further discussion welcome :) -kert
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kert wrote:

Regenerative servomotor controllers do exist, although generally for larger motors. Now that we have ultracapacitors, that could be scaled down.
But that's not the problem. The problem is building a low-weight drive system which can usefully absorb the shock of landing. Air and hydraulic cylinders can do that well, but it's hard to do with electric drive. Linear motors remain too heavy for their power (although Aura, before they went bankrupt, was at 10:1 power/weight ratio). Leadscrew drives usually aren't back-driveable, although maybe with a low-pitch ball screw you could make it work. Schemes with strings and belts are usually troublesome under shock loads. Series elastic actuators, with a stiff spring after the motor, can take shock loads but usually can't transmit them usefully back through the leadscrew. Some biologically oriented "robot muscle" approaches are promising but don't really work yet. Motor-spring-clutch systems tend to be too complex mechanically byt have been tried.

"AC synchronous" and "brushless DC" motors are the same thing, and yes, those are widely used in robotics.
                John Nagle
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John, I'm not sure what you mean by this... With a series elastic actuator, the force sensor gives feedback, and the motor controlling the leadscrew can quickly back off the actuator to react to the load.
I see SEA's as the perfect actuator for biped control. For jumping, I see the robot keeping the legs almost straight, like a person would, with the force applied to the actuators almost zero. The instant the legs start to compress, the force-driven actuators would start to quickly increase the force applied to each actuator, until full compression is reached.
The key is that while the force desired for each actuator is low, the leadscrew would be backing off quickly. As the desired force increases, the leadscrew would slow down, until the force applied is at a maximum, and hopefully the springs are not quite fully compressed.
Later, Jon
-------------------------------------------------------------- Jon Hylands snipped-for-privacy@huv.com http://www.huv.com/jon
Project: Micro Raptor (Small Biped Velociraptor Robot) http://www.huv.com/blog
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Jon Hylands wrote:

The original poster was asking about energy storage. Series elastic actuators don't recover much energy, because the spring travel is small relative to the entire actuator travel.
Humans recover about 70-80% of the energy in running through elastic storage. Cheetahs reach about 90%. Without energy recovery, you need bigger motors and power sources to build a dynamic running machine. That's a design constraint, but you can build a running machine that has no elastic energy recovery. The Honda ASIMO doesn't.
Trooby, the closest thing to what Hylands is talking about, does have elastic energy recovery.
                John Nagle
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Ahh, yes, good point.

Well, I think Asimo (and most of the other balancing bipeds) don't actually run, they shuffle quickly. All of the bipeds that I have seen that run do so attached to a pole that keeps them balancing. But energy recovery is something that is interesting to think about. I was under the impression that humans just didn't apply much energy in the first place to walk or run.

Troody was the inspiration, at least physically, for the robot I want to build. The way it walks, in my mind, was incredibly inspiring.
Later, Jon
-------------------------------------------------------------- Jon Hylands snipped-for-privacy@huv.com http://www.huv.com/jon
Project: Micro Raptor (Small Biped Velociraptor Robot) http://www.huv.com/blog
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No, Asimo really can run, with no external support. Well, OK, it's more of a jog than a run, but it does get both feet off the ground briefly.
It's not the only one, either -- the first free-running biped was the much smaller Qrio, in 2003 IIRC.
Here's a useful story: <http://news.bbc.co.uk/1/hi/technology/4098201.stm
Best, - Joe
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I've seen Qrio run, and I still don't really consider that running, both feet off the ground or not.
Probably I'm just being picky, but with both feet constantly level and moving straight forwards and backwards, it looks more like a shuffle than a run.
Later, Jon
-------------------------------------------------------------- Jon Hylands snipped-for-privacy@huv.com http://www.huv.com/jon
Project: Micro Raptor (Small Biped Velociraptor Robot) http://www.huv.com/blog
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Jon Hylands wrote:

Asimo shuffles because it can't really run. Running requires leaping into the air such that all legs are off the ground, at least part of the time. None of the common japanese bipeds, like Asimo or the Robo-1 type, can do this. They are motor-powered 100% of the time, and the motors used just wouldn't allow the feet to all leave the ground simultaneously.
Animals solved this problem, as John.N noted, by storing and releasing much of the required energy for jumping in the elastic elements of the legs. There is a physiological limit for how fast an animal can go, given a particular gait. To do a fast trot, for instance, the legs have to move very fast, and are limited by how fast the muscles can flex and relax. I have exactly the same problem with my servo-driven hexapod. The servos only slew so fast.
To go faster than the fastest trot, the animal has to change gait to a gallop, for instance, where the animal pushes its body off the ground from both rear legs simultaneously. It gains speed mainly by greatly lengthening its stride, rather than by moving its muscles faster. It also pays a large energy cost for this, because trotting animals can trot for hours, while galloping animals, especially cheetahs, can only keep this up for seconds or a minute or two. See how hard the cheetah breathes after its sprint.
Humans have exactly the same problems. They can walk for hours, but can only run very fast for a few seconds. During a 100-yard dash, the legs are moving as fast as possible and the stride is extremely long, and the muscles run out of energy quickly. During a 10-KM run, the legs move much slower and the stride is also shorter.
Walking humans don't do as much energy store-release because their legs are basically held straight, with little flex, during walking. This gait is described using the "inverted-pendulum" model. During running however, the legs flex heavily as they come down to the ground, and straighten during the power stroke. This gait is described using the "spring-mass" model. A lot of this is described here ...
http://polypedal.berkeley.edu/twiki/bin/view/PolyPEDAL/PolypedalPublications
R.J. Full and C.T. Farley, 2000, "Musculoskeletal Dynamics in Rhythmic Systems: a Comparative Approach to Legged Locomotion", in Biomechanics and Neurocontrol of Posture, Springer.
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Dan,
I'm curious why you say this. Are you implying that a motor-drive servo can't make a robot leave the ground?
I've seen videos of a Robo-One robot that is jumping an inch or so in the air, with both feet.
I plan to experiement with this once I get my Bioloid kit. I plan to machine custom feet for the robot, which will include curved heel and toe sections (with rubber grip) to allow push-off and landing. It will be interesting to see what happens...
Later, Jon
-------------------------------------------------------------- Jon Hylands snipped-for-privacy@huv.com http://www.huv.com/jon
Project: Micro Raptor (Small Biped Velociraptor Robot) http://www.huv.com/blog
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That's two of you guys behind the times now. :) Qrio ran in 2003, and Asimo has been doing so since late 2004. There are probably other running bipeds by now, but it's no longer newsworthy. This is using your definition above (which is the standard one) of both feet being off the ground for some part of the stride.
Though I agree with Jon's point that it doesn't look like a run as we informally use the term -- though to me it's clearly a jog, not a shuffle. (A shuffle to me is a gait where you always have at least one foot on the ground, and in fact have both feet on the ground most of the time.) But technically speaking, these bots can run.

Right. And like the others in this thread, I too am very interested in how robots can apply these energy storage and release principles for more efficient locomotion.
Best, - Joe
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Joe Strout wrote:

In humans, I believe that the distinction between a jog and a run is whether most of the rebound energy (stored on each impact) is used in the takeoff. So by your description, jog is the right word.
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dan michaels wrote:

Asimo actually does have a moment of suspension at the run, but you have to look at a video taken from foot level and step through it frame by frame to see the 1cm of ground clearance.
Some of the little Japanese hobbyist machines in the $1000 category have powerful enough actuators to run, but don't have the balance software to do it. See
http://www.robots-dreams.com/2006/12/greased_lightni.html#more
The off the shelf hardware from Japan has become quite good. As soon as those things get 6DOF inertial sensing, the hardware will be ready.
These robots don't have energy recovery in the legs, but for smaller robots, that's less of an issue. Energy recovery is a cube/square scaling issue. (Mass goes up as the cube of length, but strength goes up as the square. That's why insects have skinny legs and elephants have huge ones.)
                John Nagle
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John Nagle wrote:

To answer John.H and Joe, etc, I've seen the videos of the japanese robo-1 that took a hop., and heard that Asimo can get both feet 1 whole cm off the ground at the same time, but this is light years behind actual running in dogs and humans.
The real difference is, as mentioned in John.N's post here, that real animals use significant amounts of energy storage+release in elastic elements in their legs, and also bending spine movements to some lesser degree. This is what needs be incorporated into bots to get a real run.
The digital servos may be just about powerful enough to do it, I'm not sure, but it's much more efficient to use springs/springiness, which translates into getting your extra needed energy from gravity. As I mentioned last time, look up the spring-mass model.
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Well, yeah. When the robot uprising comes and your biped robot is trying to kill you, the best way to get away from it is to walk away at a leisurely pace. :)
Nobody's arguing that today's robots are anywhere remotely close to the performance of live animals. My point was only that we really should stop saying things like "robots can't run" once it's been demonstrated that they can. But you could continue to say that they can't run fast/well/impressively/whatever.

Right. But, returning to the original poster's question, it sounds like electrical energy recovery and storage is problematic, at least with standard servos. Springs don't work very well either for a robot that you want to also be able to stand still (though I've seen them used well on robots that ONLY bounce/run). It could be that the right actuators, or combination of actuators and passive parts, have not yet been developed.
Best, - Joe
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Joe Strout wrote:

HA! You can't scare me. I read the book about the robot uprising. There are many ways to survive a killer robot. Regards Asimo, all I'll have to do is extend a rope "2" meager cm above the ground, and it'll have to stop and think for about 3 minutes, during which time, I'll make my getaway.

I've clearly made the distinction several times about what it takes to perform real running. Walking fast or trotting aren't the same thing. The biomechanics of the different gaits are very different, and it's wrapped up in the idea of the inverted-pendulum versus spring-mass models, and energy utilization, that I've mentioned several times already.

type, and to get real-running will probably require adding in the elastic part. I'm sure Asimo's designers understand this too.
And once this is done, then the batteries will last much longer. As mentioned, the transition from walk-trot gaits to run-gallop gatits occurs at a specific speed range for animals because the energy requirements to walk-trot any faster are unrealistic. The muscles just can't handle. Likewise there is an upper limit to to how fast humans can move in those fast-walking contests [whatever they're called], where they wiggle their hips like ducks.
You should really go read some of Robert Full's papers that I cited. He's the Elvis of locomotion.

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Agreed. But there is a standard, widely accepted definition of running, and it's simply that both feet leave the ground for some part of the cycle. Using words in standard ways aids communication.

You really should quit assuming that I haven't. :)
Best, - Joe
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Joe Strout wrote:

So hopping is running, then.

You might look up the differences in gait biomechanics, vis-aviz foot action and energy untilization. How does the foot pattern differ between walk/trot and gallop. Why does a transition occur. Why doesn't an animal gallop at slow speed. Why can't an animal trot as fast as it can gallop. Why does the inverted-pendulum apply better in one case, while the spring-mass model is better for the other.
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Joe Strout wrote:

There have been a few attempts, but they're mechanically clunky, like schemes where a string winds around a pulley with a corner cut, so that when the string is fully wound, a bit more motor rotation pushes the string off the pulley and the string releases all at once. Some people at Stanford tried that. Sort of worked, but not too well.
The best known solutions for larger robots are either hydraulic or pneumatic. A muscle can be thought of as a spring with an adjustable spring constant and an adjustable zero point. You can get that effect with a double-ended pneumatic cylinder - high pressure on both sides makes a stiff muscle, low pressure makes a limp one. Unequal pressure changes the zero point. That's been tried, too. Works fine, but you need to carry an air compressor or have external hoses.
The same trick can be played with hydraulic cylinders and accumulators, and that's been used in Boston Robotics' Big Dog, which is pony-sized.
For desktop robots, high end servos, maybe with some stiff springyness in the legs to absorb shock loads, seems to be the way to go for now. Right now, we need better control algorithms, not higher efficiency. Energy recovery will improve battery life, but that's not the current top problem.
                John Nagle                 Animats
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John Nagle wrote:

What about pair of magnetic clutches to disconnect the servo transmission and hook it up as a generator ? Sounds clunky and expensive .. Also, what about reversible tranmission types like planetary or harmonic drive ?
-kert
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BioLoid kit is around $1000, 6DOF IMU costs $300 nowadays, 5DOF $100, and ample portable computing power next to nothing, in form of 32-bit micros. http://www.sparkfun.com/commerce/categories.php?cPath#_85
BTW, why only Japanese/Eastern, whats holding back the US ?
-kert
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