Gerald, You're right about me being off on a tangent. That's often the case. In fact, I can often go for days on end going from one interesting problem to another before finally getting back to the original problem.
Now back to the subject at hand . . . As I see it, the goal is to command the machine to start at an exact point on a line segment, to move an exact distance and then to stop exactly at the desired point on that line segment. (I'm using the term 'line segment' on purpose because it's my understanding that even the smallest circles are cut by commanding the machine to move through many straight line segments, with most of the complexity needed to convert a circle or curve to line segments being handled by the tool path program or the native instruction set of the machine.)
Since a stepper motor, like a car, needs to accelerate to get to speed, a programed ramp is normally used. Trying to accerate a car too quickly either stalls the engine, if the engine doesn't have enough power to do what the driver commands, or the tires lose traction and 'burn rubber'. In either case. A stepper that is commanded to move too quickly either completely stalls or loses steps, with neither condition being acceptable. Also, a stepper motor, like a car needs to slow down gradually or a 'skid, i.e. overshoot occurs. Just like you wrote, an experienced driver knows his car and compensates with his skills to make the car do exactly what he desires. That point being conceded, let's try to figure out whether mechanical control, i.e. sensor feedback and automatic compensation, or programming would work best to control acceleration ramping and deceleration ramping on a CNC machine.
Programmed ramping to accelerate a stepper motor, if convervative ramping is used, assures that the motor with neither stall nor lose steps. It also guarantees that the motor will not be running at its peak speed because the ramp will have to be conservative enough to allow for unknown differences in the material or cutter sharpness, etc. On the other hand, a sensor based system, would sense a missed step and then automatically slow the motor down. An intelligent sensor system would compensate for missed steps to assure accuracy. In other words, a sensor based system could be commanded to ramp at an unrealisticly high rate and then compensate for all known and all unknown factors automatically.
Deceleration is another matter entirely. In my experience, if a motor is not ramped down properly, overshoot or skidding occurs. In this case a sensor based system would only know that the motor had gone too far. An intelligent sensor system could reverse the motor until the commanded stop point was reached, but the damage to the material would have already occurred.
So, in my mind, to follow the programmed tool path, sensor based acceleration would be optimum to get the motor to speed in the shortest distance, but programmed deceleration would be needed to prevent overshoot (skidding) at the end of each line segment. Whether programmed deceleration is left to the machine's operator to figure out or whether it is a function of the machine's control software is another matter entirely. I would assume that deceleration ramping should be a function of the machine's control software (which is why I suggested cutting a rectangle with various corner radii, in the post above - that test shows that Shopbot has done a very good job of incorporating ramping into the control software). Optimizing the deceleration ramping to give optimum performance, equal to sensor based acceleration, would be complex and probably require computer hardware far beyond that supplied with any moderately priced CNC controller.
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