Sunday, August 14, 2011

Dancing Robots vs Robot Dancing

There must be something characteristic about robot motion that it could inspire distinct hip-hop dance moves. 

So what is it?  Some might say it is the fluidity of motion, or lack thereof.  Machine motion often strikes us as jerky, abruptly stopping and starting, while animal motion and in particular human motion appears smoother to us

Is there a technical way to define "fluidity of motion"?  One aspect of fluidity of motion comes from the physics notion of critical dampingThat is, in order to get a mechanical system to execute smooth starts and stops, the accelerations that are applied to the system must be "tuned" to the natural resonance of that system. 

Often machine motion tends to be characterized by a short startup, followed by constant velocity motion, and ending with a short deceleration to a stop; the quick deceleration often causing the system to wobble.  In contrast, the human body has the natural ability to control its motion to accomplish this effect of critical damping. 

Another aspect of fluidity comes about from the fact that robots are often programmed to move from one stationarily stable configuration to another via a path which itself is a continuous sequence of stationarily stable configurations. 

Human movement, in contrast, depends on dynamic stability.  For humans, and animals in general, the positions we go through as we move are not stationarily stable.  That is, if you or I tried to freeze our motion at any one point and hold it, we would fall over; our movement depends on our body’s momentum to carry it through.

One more thing that characterizes machine motion is that part of the machine will stay fixed while other parts move.  But if you watch a field worker, their whole body will be in motion; that is, their locomotion and picking motions are often one continuous body movement.  And this is the point I’m trying to get at in this post; that for an efficient robot harvester/tender, its locomotion will not be separate from its picking motion. 

One of the reasons that field workers can do this work, day in and day out, week in and week out, and as quickly as they do, is the smoothness of their hand and body movements.  They don't waste any motions.  The smoother the motion, the less energy it requires, the faster it can go, and the less wear and tear on the system.  And this observation applies to machines just as much as it applies to humans, too.

I doubt that any robot harvester/tender will be able to go much faster [1] than an experienced picker starting fresh in the morning.  But the advantage that robotics bring to the field is that a robot can sustain that speed all day long without a break.  Speed for a robot harvester/tender isn't the issue.  It's endurance.

The question then is what methods of robotic locomotion lend themselves to the kind of whole body movements that humans do?  Legs offer the ability for the torso to move as part of the picking/hand/eye motion.  I don't see this being a possibility with wheels or tracks integrated with a simple suspension system.  In those cases, you end up with a rigid torso, which essentially disconnects a robot’s locomotion from its arm/hand/vision-system motion.  I imagine that with a more complicated "active" suspension system, wheels or tracks can be made to mimic the whole-body motion of a legged walker.  But in that case, the robot's locomotion system will end up being just as complicated as a straightfoward walker mechanism.

Factory floor-mounted robots do have a fluidity of motion.  But they do repetitive tasks, which gives the programmers the opportunity to "dial in” the motion.  For harvester/tenders, the field's surface is constantly changing and the plants are never the same, so there is no opportunity, ahead of time, for a robot harvester/tender’s motion to be programmed for efficiency.

I found some examples of hexapod-walker motion on the web, but in all cases the motion is very crude.  The only examples I could find, showing a fluidity of motion, were animations such as this of a quad-walker.

Future Post:  This raises the question is animation a good indicator of the kinds of motion that a robot could potentially be capable of?

[1]  Regarding the speed of hand movement for a human picker: having watched pick-and-place machines in action doing PCB assembly, I have no doubt that a robot harvester/tender could have hand movements considerably faster than a human’s. The problem isn’t the human hand speed; it’s the plant’s ability to move in response to that hand movement. The stalk, branches, and leaves of a plant form a passive mechanical system with its own natural response time scales. A system can only respond so quickly to outside forces before something breaks or tears. You can try to go faster than a human picker, but I’m afraid, at that point, you risk damaging the plant itself.

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