Determine real values for Step Time, Step Space, Direction Hold and Direction

Dear all,

as described here wiki.linuxcnc.org/cgi-bin/wiki.pl?Tweaki...ftwareStepGeneration, it is important to know the driver specific values for Step Time, Step Space, Direction Hold and Direction setup.

Is there a way in EMC to measure the combined value stepper drive + BOB. Lets say the stepper needs a step Space of 5050 ns this value might be decreased in the software because the BOB adds additional lag time which occurs while propagating through the BOB, e.g. slow optocouplers as in the chinese db25-1205 bobs

Ben

No, matching the drives numbers is usually a bad idea (as these are normally minimums).
The steplength and stepspace numbers should be set as large as possible until they
interfere with the maximum required step rate. 5000 for each is a good starting place.

dirsetup and dirhold can both be quite long without having any noticeable effect
on performance so 20000 is a good universal value for these.

thank you pcw

What is minimum value I can set for Steplen and Stepspace? What will happen if I set them to 100 ns instead of 5000 ns? Will my computer hang?

Suppose I want 0.1 micro-meter resolution, I set 40000 PPR in drive, using 4 mm ball screw, Want Maximum Velocity is 20 mm/sec.

According to formula of

Max Velocity = 1/((steplen+stepspace)*step-scale)

20 = 1/(2*steplen*10000)

steplen = 10^9/400000 = 2500 ns.

Should I use steplen=stepspace=2500 ns?

For 20 mm/sec speed, what value should I set for Maximum Acceleration?

You can set the times of a Mesa Ethernet card down to 10 ns,
and it will work fine, but your drive will not see step pulses that short.

As far as step time and step space go they also need the ~25% headroom
That is, you need to do the timing calculation on the stepgen maxvel number
not the machines maxvel number. That means the steplen/stepspace
would be 2000 ns

Maximum acceleration depends on your drives torque capabilities and the inertia
(both rotary and linear) of your mechanics, but a common starting guess is 10X velocity (so full speed is reached in 100 ms)