Please recommed actuator type for leg joint

Hi,
Im looking for a rotational actuator for a robot leg (knee) joint that would, when its power is disabled, relax the lower leg into the
vertical downward position (according to gravity) - would the best type of actuator be a stepper motor and controller? - I would rather use a servo for ease of control, but I cant figure out a way of disabling the servo to relax the joint.
What kind of resistance to physical force does a stepper motor give once its power is disabled?
Thanks
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a stepper would be innapropriate for that, they just sit there and resist motion, pretty much at the rated force, listed as holding torque. you could use muscle wire, it's rather slow. if it was a big enough project i'd use a rotating hydraulic cylinder. but, if you have reserve power on board then maybe the stepper would work fine, just like any other motor, with feedback.
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snipped-for-privacy@nickhill.info wrote:

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Regards a stepper motor, not much resistance at all.
Regards an r/c servo, as soon as you stop sending pulses to it, it goes limp, and the leg its driving will typically droop under the force of gravity.
You might also take a look at the "passive" knee on Scout II ... it actually uses a solenoid-activated locking detent mechanism (whatever it's called) ...
http://www.cim.mcgill.ca/~arlweb/scoutii/trot/index.html
You will notice from the movies that the knee never actually gets into the "bent" position as shown in the "Scout II with passive knees as rendered in ProEngineer" diagram. Clearance is never that good. The foot actually just drags along the ground. - dan michaels www.oricomtech.com ======================
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Unfortunately, modern actuator technology will perform badly in respect to your desired goal. Both steppers and servos typically incorporate gear reduction, which tends to make slack motions pretty stiff and can even damage the actuator if too much force if applied (not that gravity would apply). You'd probably have better luck with pneumatics or muscle wire in this regard, but these hold disadvantages all their own that usually make them overall worse candidates than electric motors. One of these days (years...decades) artificial muscle technology will finally take off, but until then, I'll say go with servos. It won't be perfect, but at least they're self-contained and easy to interface.
Chris
snipped-for-privacy@nickhill.info wrote:

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Thanks for all your replys.
I have played with servos, but the resistance to external force is too high (plus I hear that it buggers the servo!)
The other option I have been considering is some kind of electronicly activated clutch mechanism (similarish to the "passive" knee on Scout II ) which would physically disable the servo gear from the lower half of the leg joint, enabling it to swing back to vertical when the clutch is enabled (disabled?) Does anyone know of such a device small enough to attach to a servo gear?
It seems that in trying to model natural leg movements, it would be a requirement to allow the joint to be both passive and active. Anyone seen such a thing on a robot leg?
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NIck Hill wrote:

Sure. You just need to do more reading.
Muscles are best modelled as springs whose zero point and spring constant are adjustable. There's still no really good way to build equivalent hardware, but there are some devices that come close.
All serious leg work has back-driveable acuators with some compliance. Early workers like Raibert used pneumatic cylinders, but usually needed an external air supply. Shadow Robotics has played another type of air actuator. If you have good feedback, robust control algorithms, and keep the valves close to the actuator, precision control of air cylinders is possible.
The combination of a true linear motor and a spring has possibilities, but linear motors are hard to find since Aura tanked. And you don't get much energy storage, so running is inefficient. (In running, about 70% to 80% of the takeoff energy is recovered on landing and stored in the spring capacity of muscles.)
Gil Pratt's people at MIT tried screw jacks in series with springs, which, with aggressive control, can be made to behave like muscles. There's force sensing at the spring. When the spring compresses, the motor frantically cranks the jack to maintain the desired force. These aren't back-driveable (much). They don't store energy well either, so they're inefficient for running. But they're buildable from off the shelf components.
The most common solution today is a servomotor and gear train designed to be back-driveable, plus some elastic component to take shock loads. Look at a RoboSapiens for an example.
                John Nagle
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snipped-for-privacy@nickhill.info writes:

We have done this kind of thing using Air Muscles (surprisingly). Use one muscle as in the human thigh to pull the knee straight. When the muscle is vented, the leg will lower under gravity. If you don't need to hold the leg in an intermediate position, you can do it with one valve.
cheers, Rich.
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Rich.
I have looked into using your air muscles previously - They look like a fantastic product. The use of compressed air was an additional complexity that I didnt really want to have to burden, but it looks like the air muscles do satisfy my requirement. I think I could be converted - Thanks for the info.
Nick
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