Hi,
the typical maximum pulse input rate to a step/direction servo drive is 500kHz and that assumes differential signaling.
If you wish to maximize both resolution and speed you need to signal the servo drive with differential line
drivers. Commonly it is an added complication that is over the top and not required. We shall see once we do the
calculations.
I assume the servos are rated to 3000 rpm? If they are hooked directly to 10mm pitch screws then at 3000 rpm the axis would
be moving at 30 m/min.....quite respectable for a production machine and WAY quick for a hobby machine.
Lets assume that rather than signaling at 500kHz we chose a much easier signaling rate, say 100kHz. This could be done
without line drivers but just simple open collector outputs.
At a step rate of 100kHz that equates to:
pulses per minute = 100,000 X 60
=6,000,000
the pulses per rev =6,000,000 /3,000
=2000 pulse per revolution.
Linear resolution = 10 (mm pitch) / 2000
= 5um (or 0.0002 inch per step)
Thus if you made choices such that the maximium step pulse rate is 100kHz that would still allow you to spin the servo
at its maximum speed with a steps per revolution of 2000 and with your 10mm pitch screw that works out to 5um
resolution....pretty damned good. You could do better if you wish by either decreasing the maximum speed or attempt to
signal the servo drive faster.
If that sounds good to you I suggest you program the drive to accept 2000 input pulses to cause the servo to complete one revolution.
I presume you have set the servo drive up in Position Mode (Step/Direction)? You can use the Low speed input, it will make it that
much easier. That is parameter P1-00 on page 6-3
You need to confirm that parameter P1-46 is set at its default of 2500. That means the encoder is effectively 2500 lines
and because of quadrature encoding that means 10,000 count per rev.
Now we know what electronic gearing we require, the encoder is 10,000 count per rev whereas we want 2000 input pluses
to cause the servo to revolve once. Thus the ratio is 5:1.
So the Numerator N1 parameter P1-44 page 6-8 should be set to 5000 say and therefore the Denominator M parameter
P1-45 on age 6-8 should be set to 1000.
Now you can set Machs motor tuning parameters. We have set the servo drive up to revolve once with 2000 pulses and the
axis will move 10mm . Thus the steps per unit are 2000 /10 =200. I'm assuming you native units are millimeters.
Your max velocity will be 30,000 mm/min and you could probably set those great big kick-arse servos up for an acceleration of
1000 mm/sec/sec to start with. The servos themselves I'm guessing wont be working at all at that piddling rate. I suspect
unless you have a really big or heavy machine you could take the accel up to 10,000 mm/sec/sec. Your machine is going
to Rock-and-Roll!!!!
Craig