Wednesday, November 23, 2011

Electromagnetic Actuators? Why Not?

Regardless of the novelty of a design concept, the laws of physics will always put limits on what any actuator can do. That good-old F=ma equation tells you that in order to move an actuator mechanism quickly, large forces are required to first accelerate it up to speed and then to de-accelerate it back to a stop.

If electro-magnetic forces are going to be the driver of an actuator's motion, then the equation F=iLBsin(θ) tells you how much wire(L), current(i) and static B-field you need to generate a given force level.

Given a possible 2000-8000 gauss range for permanent magnets, the currents necessary to generate any kind of usable force levels (with speed equivalent to human hand and arm motions) will be on the order of 10's of amps.  Then, with these current levels, the resultant resistive losses from the actuator's "windings" will generate copious amounts of heat that will have to be "dumped" somehow.

I worked on this concept a couple of years ago and for the above reasons, came to the conclusion that it will be impossible, using standard-technology electro-magnetic actuators, to recreate a robotic hand/arm system that mimics the action of a muscle fiber/tendon bundle.

Muscle fiber can only pull, not push. So to accomplish the finger, hand and wrist joint movements that we can, several muscle groups have to act simultaneously, pulling either together or in opposition to each other. Because of this fact, we have control over not just the position, force, and velocity of our hand movements, but also the overall mechanical stiffness of our hand/arm system as well.

Another thing to consider is the force level our muscles are capable of. If you look at our body's muscle-bone attachments and their attendant mechanical disadvantage in terms of forces and lever-arms, you'll see that our muscles don't just generate ounces of force, but 10's and 100's of pounds of force. And in the case of exceptional individuals, like an Olympic power-lifter for example, 1000's of pounds of force! I believe, the only thing in the same category, in terms of force per unit volume, is hydraulics.

The skeletal-muscular system that Mother Nature has evolved is truly extraordinary.

Now back to the topic of this post, if the goal is to create a robotic hand/arm system and do it using actuators that mimic the action of a muscle fiber/tendon bundle, then the following observations should be noted.

First, since a muscle fiber is a pull-only actuator, several different muscles, some in opposition to each other, have to fire smoothly, simultaneously and continuously together to reproduce the range of joint movements we have.

Second, the accuracy to which we can control our finger movements is tied to the rigidity that we can hold our hand/arm system to. There is no "brake" in the human hand/arm that allows it to be locked into a fixed position. The way we do this is to fire opposing muscle groups, in a full-on mode, and thereby force our hand/arm into a state of ridge tension.

These two observations together imply that for a robotic hand/arm system that mimics the action of a muscle fiber/tendon bundle, most, if not all, of the actuators will be in some state of "on"-ness all of the time. If one assumes voice-coil type actuators, operating in the 10's to 100's of watts power range, then multiples that power load by a factor of 10 to 30 (the approximate number of muscles in a human hand and forearm that might be engaged at any one moment), one finds that the total power load skyrockets. And of course along with the accompanying waste-heat cooling problem.

And this is just one hand/arm we're talking about.  Now multiply this by two hand/arms, then add in four to six legs for locomotion, then add in the power to run the harvester/tender's computer systems, and the power requirements for our robot become greater than any reasonably sized "portable" power source will be capable of delivering. 

Or to say it a different way; the reason factory floor mounted robots run just fine using electro-magnetic actuators is that they don't have to carry their power source with them.  Actuator power consumption is not a design constraint when your robot can plug into the power grid.  But once a robot is required to be self contained power-wise, then any excessively large current requirement will become a design show-stopper.  
Which is why, IMHO, that if a robotic hand/arm system is going to be made using actuators that mimic the action of a muscle fiber/tendon bundle, then it won't be done with electro-magnetic based devices.

Topic for a future post  The technological step I‘m looking for in hydraulics is to have a small fast-acting piezo-electrical driven pump that would mount directly onto the cylinder it is driving; thus forming a self-contained unit, a kind of "point-of-load" hydraulic supply.  This would get rid of the need for an external engine-driven hydraulic pump with its accumulator and rats nest of hoses that is typical of current hydraulic systems.

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