Let's start with the brutally obvious:
1. robots are not alive.
2. robots do not have a brain.
3. robots do not move.
4. robots are built instead of evolved.
5. robots are all metal parts and some grease.
If someone makes a robot sort of look like a human, that's cute, but functionally unnecessary. What matters functionally is how the parts are constructed and put together.
For a robot made to look like a golfer putting, the usual design has only two parts that matter -- the neck and the shoulder-arms assembly. The shoulder-arms assembly has a hole in the center that fits onto the "pipe" of the neck, so the shoulder-arms assembly can teeter-totter or see-saw about this neck pivot. Other than this functional aspect, the ONLY thing that matters for the sort of stroke a robot makes is the angle the neck pipe makes with the floor. If the pipe parallels the floor, the shoulder-arms assembly makes a straight-back and straight-thru stroke. If the pipe is angled / tilted up a bit, the shoulder-arms assembly makes a stroke that "gates" or "arcs"
3. square-at-only-one-precise-moment-which-needs-to-be-at-impact, then
(A "frowny face" as seen by the golfer.)
If the pipe tilts down off level with the floor, the robot stroke goes:
3. square-at-only-one-precise-moment-which-needs-to-be-at-impact, then
(A "smiley face" as seen by the golfer.) Whatever the orientation of the "pipe" in space, the shoulder-arms assembly MUST and CAN ONLY move in a plane that is perpendicular to the axis of the pipe. Tilting the pipe tilts the plane of motion of the shoulder-arms assembly, and this tilts the stroke of the putter head.
Examining this a bit, we see that the shoulder-arms assembly moves ONLY AS ALLOWED BY THE PIPE. That's because the shoulder-arms assembly is all metal in one unchangeable shape, as is the neck pipe, and because the "fitting" of the shoulder-arms assembly hole onto the pipe can only be done in one way, so the relationship between the neck and the shoulder-arms assembly is fixed and unchangeable. And once the pipe angle is screwed down tight, that is also fixed and unchageable.
Dave Pelz's "Perfy", the perfect putting robot who will teach you how to "putt like the pros" or near enough, wearing a golf cap (scratch golfer!, too!).
Note especially that Perfy does NOT have his hands directly beneath his shoulders, as Pelz claims is required (more on this below). Pelz appears oblivious to this rather obvious conflict with his teaching, and unknown to Pelz, Perfy actually "proves" that Pelz's claim about hand position determining stroke path is false by making straight-back, straight-thru strokes with the hands outside / beyond the line beneath the shoulder sockets.
Harold Swash's "Rail" -- no pretense that it's human, but the "robotic ideal" is still inspired by flawed notions of robotic vs. human biomechanics (see the level setting of the pipe extending out from the vertical base):
The Putting Arc's "Iron Archie" -- named after "Iron Byron" to personalize the robot, but it's the same deal: set the pipe angle to change the shape of the stroke, in this case, 12 degrees off vertical in order to match the shape of the Putting Arc on the ground -- any slight misadjustment of the pipe and the robot cannot trace the shape of the Putting Arc.
Kramski putting robot, another engineer makes a golfer out of metal and bolts, but I don't think Kramksi uses this robot for teaching and instead restricts the use of the robot to testing variables by controlling and eliminating other variables except the one under study (e.g., putter face loft, MOI, etc., uninfluenced by stroke path irregularities):
A human is "slightly" different. The main thing is that there is NOT REALLY a fixed action between the neck angle and the way the shoulders can move. The shoulder frame and indeed the whole upper torso can move with a degree of freedom in a human that has nothing whatsoever to do with the angle the neck makes with the level surface. The acid test for all doubters is that anyone on earth can make a straight-back and straight-thru stroke along the baseboard of a wall WITH ANY NECK ANGLE DESIRED. (If you haven't personally tried this, then don't argue about whether the point is valid or not -- you're not allowed a voice in the debate.) What happens is that the golfer simply moves the shoulder frame in a vertical plane of motion regardless of the angle of the neck. Do that (and only that -- no independent use of arms or hands) and the putter head runs straight back and straight thru; do anything else and it won't.
So teaching a "rule" that the "cervical spine" (neck) needs to parallel the surface is a flawed "robotics" misconceptualiztion of human biomechanics.
The robot shoulder-arms assembly COULD have degrees of freedom introduced at each "joint" if the robot is designed to "look like a human". But functionally the robot doesn't need any "joint" other than the pipe-and-hole-socket where the shoulder-arms assembly fits onto the pipe of the neck. But a human has joints at the shoulders, at the elbows, at the wrists, and at the fingers (not to mention at the hips, knees, ankles and toes). In a well-trained "shoulder stroke" the human does not change any of these joints during the stroke. In an armsy stroke, the human changes the shoulder joint and armpit, as the upper arms change relationship with the shoulder frame and chest during the stroke.
In general, a body part "moves" by changing the joint that is higher up or closer in to the center of the body (more "proximal") than the ("distal") moving body part. The wrist joint moves the hand. The elbow joint moves the forearm and the wrists and the hand. The shoulder joint moves the upper arm and elbow and forearm and wrist and hand. So what moves the shoulder frame (the body part, not the joint)? Answer: the gut muscles, where the upper skeletal frame is "jointed" to the lower skeletal frame by the spine sticking into the pelvis and sheets of muscle and tendon wrap from the rib cage down onto the pelvis in order to make the spine at the "joint" move around in certain ways. This "joint" moves the shoulder frame and the whole upper torso in relation to the lower body. The head and neck can be isolated and withheld from this motion due to the inner ear's connection to the brain, so you can move the shoulder frame without necessarily moving the head and neck too. The spine is willowy and actually bends sideways left-right or forwards-backwards like a young planted tree. That's a funny sort of joint compared to the hingeing and swiveling action of other joints, but it's what happens in the human body when you move the shoulder frame as a whole using the gut muscles.
IF a golfer makes a true "shoulder stroke", he moves the gut joint and this moves the shoulder frame and this moves everything downstream from the shoulders, all as a single unit without independent action at the other joints. The robot does the same because the shoulder-arms assembly CANNOT have independent joint changes other than the see-sawing at the neck.
It's possible to build a robot with all the degrees of freedom, just like a human (see MIT). But in golf no one tries this because they can't use it to show things and making it move is too complicated a job of coordinating vectors. A "putting robot" is moved simply by tipping the shoulder frame down and letting go. The shoulder frame then "rocks" like the pendulum on a grandfather clock and continues for a while as its motion slowly dies out.
So when people use a robot to "illustrate" how a human makes a stroke, they are kidding you and themselves. What they are showing is how the robot makes a stroke, which is defined solely by the fixed character of the parts and the neck angle being the only determining factor.
One of the dumber "rules" supposedly proved by a robot (Perfy, given a name to emphasize how "like" a human he is supposed to be in design and functional properties, which he ain't) is this:
"Unless the hands hang directly beneath the shoulder sockets, the stroke cannot run on a straight-back, straight-thru path."
Two follow-on "rules" are:
"If the hands hang outside the line directly beneath the shoulder sockets, the stroke will necessarily make a "frowny-face shaped" arc inside-square-inside" and
"If the hands hang inside the line directly beneath the shoulder sockets, the stroke will necessarily make a "smiley-face shaped" arc outside-square-outside."
Not one of these three so-called "rules" is true or applies to either robots or humans, which is why they are dumb. Using these rules to teach golfers how to putt is worse than telling a pole vaulting champion he can't clear the bar unless he's chewing Juicy Fruit gum at the time, because it affirmatively messes golfers up trying to comply with a nonsense rule at the expense of learning what really matters for movement and how to do it simply.
Only the pipe angle matters on the robot, not the position of the hands in relation to the shoulder sockets. That is because the shoulder-arms assembly is fixed in shape, and this means that WHATEVER PLANE OF MOTION results from the pipe angle, the shoulder-arms fxied-shape assmebly will transmit the same plane of motion all the way thru to the putter head, regardless of the relationship of the hands and shoulder sockets. So long as the relationship between the hands and shoulder sockets doesn't alter during the stroke, regardless of the relationship's exact form, the putter head will move in a plane that parallels the shoulder-arms assembly. If the pipe is level with the floor, the shoulder-arems assembly can move ONLY in a vertical plane perpendicular to the axis of the pipe, and so does the putterhead, no matter whether the hands hang beneath the shoulder sockets, inside the line, or outside the line. It doesn't matter one tiny bit.
For the human, first of all, the hands DON'T NATURALLY HANG DIRECTLY BENEATH THE SHOULDER SOCKETS. Just look at ANY HUMAN standing still with decent posture -- the hands always are at the end of forearms that angle out of the line directly down from the shoulder sockets, so the hands actually hang directly above the toes. This is due to the fact that muscle development in normal growing up and living creates a steady tension on either side of the elbows that slants the forearm out of this vertical line forward a bit -- usually about 10-20 degrees out of vertical. So getting the hands directly beneath the shoulder sockets at address in putting requires "putting and keeping" them there with tension in the upper arms and pecs to make a certain angle of the upper arm with the shoulder socket OTHER THAN the natural, tension free hanging of the upper arm in the same vertical line beneath the shoulder sockets. The ELBOWS naturally hang directly beneath the shoulder sockets, not the hands. Just look at any relaxed, standing adult human!
Second, the human can move the shoulder frame in a vertical plane regardless of where the hands are positioned in relation to the shoulder sockets. There is no bone connecting the shoulder bones to the spine. There is no bone connecting the shoulder bones to the neck. Check any anatomy book. The shoulder sockets themselves (as opposed to the shoulder frame as a unit) can usually be "hunched" up about 5 inches without bothering the neck at all. And the shoulder frame as a whole can be rocked sideways up and down without moving the head any at all or very, very little. (Incidentally, a really BIG putting stroke only requires one shoulder socket to move down from its starting position and then up above its starting position about 3-4 inches, as the radial arm of the limbs and the putter shaft translate and amplify this small central motion into a big sweep of the putter head back and thru, so you don't really need a lot of independent range of motion of the shoulder frame to putt over a very wide range of distances without involving other body parts to an undesireable degree.)
So these "Hand position determines stroke path" rules are especially dumb, since they don't apply to robots OR humans, and they don't correspond to humans at all anyway.
Robots only illustrate the relationship between the pipe pivot and the shoulder-arms fixed-shape assembly, and even this is not really understood or taught by the people using the robots to teach with. Instead, these good folks confuse matters by suggesting falsely that if you could make yourself into a robot's shape and then mimic the motion the robot makes (and repeat this "perfect practice" 20,000 times), all your problems will be cured. Too bad these folks aren't very clear thinkers or teachers!
There are other differences, such as the "torque" and "coupling" forces in the way the robot "grips" the putter handle and reacts to impact, but these are secondary to the ignorant illusion that the robot illustrates what the human should do. In fact, teachers using these robots never seem to make the point about moving the shoulders in certain planes of motion in order to make related shapes in ther stroke path of the putter head, so apparently they don't know about this. But it's just fifth-grade geometry.
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