Re: stepper vs servo

<elson@p...> wrote: ........

Hi Jon and the others,

Thanks for the educational, well-informed and spirited explanation.
You have put forth very well your points. Let's see if I have
followed your explanations correctly:

1. Steppers resolution and accuracy are inferior to servos (major
point of argument)
2. servo resolution are as fine as you can make the encoders?
3. stepper paths are like jagged staircase
4. Servos can interpolate between encoder points? <Servos DO, indeed,
cruise smoothly between encoder points>

<First, think of the accuracy of the magnet alignment in a stepper.
The stated accuracy is generally +/- 5%. Therefore, the positioning
of any step is within 5% of a full rotation, or 18 DEGREES! Assuming
one full step = .001" with a 5 TPI leadscrew, then an 18 degree error
equals .010" error.>

THE POSITIONING OF ANY STEP IS WITHIN 5% OF A FULL ROTATION. I do not
understand this part. When the accuracy of a step is 5% does this
mean 5% of a single step or 5% of a full revolution? I am still
learning about steppers and servos, but I find it hard to visualise
how a 1.8 degree stepper in moving 1 step (1.8 degree) can
misposition itself 18 degrees. Again my feeble mind tells me that
inaccuracy in steps over a full revolution are not cummalative
(unlike missed steps). Any inaccuracy of a particular step should
only affect the accuracy of the positioning where that particular
step occured. The sum total of inaccuracy in 200 steps of a full
revolution will zero themself out, meaning if for example step no. 30
is off by +1 degree, then somewhere in the sum total of the other 199
steps it must be off by -1 degree. I stand ready to be corrected.

Anyway, what I was trying to point out was that (I think) a servo is
always correcting itself (a servo does overcorrect itself doen't it)
leading to a wavy path, although the wave may be very microscopic.
The jagged steps of a stepper in reality may be of larger magnitude
than the waviness of a servo, but, theoretically anyway, if the steps
of is sufficient small, the path of a stepper will not exhibit the
waviness of a servo.

Perpectual student