Hi,
my math:
32mm ball screw diameter / 1um of CNC machine accuracy = 32,000 ppr,
encoder resolution = 32,000 * 4 = 128,000 cpr (below delta encoder spec)
No, not quite.....the diameter of the screw is 32mm, the
pitch is 5mm per rev. Thus for a resolution of 1um
I require an effective pulse rate of 5000 cpr, the numerator/denominator could be:
160,000=5000 x N/D
so N/D =32
Thus the choice of N and D is very wide, for instance if D=1 then N=32 OR
D=10 then N=320 OR
D=25 then N=800 and so on.
At 3000 rpm (50 rev per sec) the pulse rate is:
pulse rate=50 x 5000
=250kHz.
As you point out the ESS has a max rate of 4Mhz, so at 250kHz its cruising. 250kHz is a little faster than the max recommended
single ended (open collector) signaling rate but very comfortably within the 'low speed' differential signaling spec of the servo
drive of 500kHz.
I have some experience with an Allen Bradley servo which I wanted to signal at 466.66kHz. Its servo drive has a recommended
max differential signal rate of 500kHz and a single ended rate of 200kHz. I can assure you that while 466.66kHz
is possibleits much easier to signal at a lower rate. Thus I reduced the angular resolution to 2000 cpr (from the encoder max of 8000cpr)
and got the signaling rate down to 116.66kHz which can be done comfortably with a single ended transistor.
Note that this servo is used as a direct drive spindle which gives a resolution of 10.8 arc min ( with 2000 cpr) from a max
resolution of 2.7 arc min (with 8000 cpr). I practice even 10.8 arc min is much greater than I require. Reducing the resolution
has made the whole thing easier to do and
still exceed my actual requirements.
My existing mini-mill has a linear resolution of 1um and is very adequate. Thus I am quite happy to aim for 1um resolution
in this new build. I could, given the capacity of the servo encoder, have a resolution of 0.03125um ( 31nm!!!) but at the
expense of complexity of the signaling side of the design. I struggle to even measure 1um......let alone 30nm so why
bother?
The rated speed of the 750W B2 series servos is 3000 rpm. Direct coupled to a 5mm pitch ballscrew results in a max axis
speed of 15m/min which is a marked increase from my existing mill of 1.2m/min. I have experimented with pushing the
max speed parameter out to 5000rpm, the servos max speed (cf rated of 3000rpm) and they still work a treat.
Thus I could have rapids (G0's) of 25m/min in field weakened mode and full rated thrust for machining moves (G1's) of
15m/min while still maintaining 1um resolution and a max signaling rate of 416.66kHz, well within the 'low speed'
differential spec of the drives. As far as I'm concerned that is speed
AND resolution beyond what I ever anticipated
I could achieve with a hobby machine. If I want to use the full acceleration potential of my existing mini-mill (I can
tune it to 1g accel easily enough) I have to tie the machine to the wall or the machine tries to dance all over the floor.
The practical limit for the machine is about 0.05 g without undue machine motion and still provides excellent tool path
following (at modest axis speeds).
These 750W servos offer at least a 50 fold improvement in potential over the existing stepper/reduction drives I currently
use. Thus with the X axis bed/ballscrew/rails/vice/workpiece weighing over 200kg if I tune the axis to 1g acceleration
(of which the servos are more than capable of) then I'm going to have to bolt this machine to a concrete floor just to stop
it lurching around threatening to crush any-one or any-thing in its path!
The speed, torque and resolution of these servos continues to exceed my expectations. 15 m/min G1's sounds fine on paper
but when you see the axis machining at 15m/min you realize just how much power and even how much potential damage
can be done.....its scary!! Fun scary.....but still scary.
Craig