Originally Posted by
joeavaerage
Hi,
I use Delta 750W B2 series servos and they kick arse....better than any stepper ever made, hands down. You are correct of course, they command a premium and is
that premium justified over the cost of steppers?
The principle advantage of servos over steppers is speed and resolution. For instance my servos are rated to 3000rpm, but in fact I use them up to 5000rpm. Most steppers won't even spin
at 1000rpm without load, let alone with a load.
Imagine if you used servos and that you direct coupled them to the screws then at 3000 rpm the X,Y axes would move at 0.010 X 3000 =30m/min and the Z axis at 0.005 x 3000=15m/min.
That is very very fast for a hobby machine. You are also correct that it may well be that you would have to limit the speed of the servos to prevent the ballscrews from whipping, so some
of that really top speed would be lost.
I use 3205 screws in my new mill, and they, being 32mm in diameter don't whip at any reasonable speed, and so I push them to 5000rpm. The axis movement is 0.005 x 5000=25m/min.
25m/min is scary fast, and I seldom use it, in fact 10m/min to 15m/min is entirely fast enough for me.
Servos have this programmable feature called Electronic Gearing. It a rather more flexible but not dissimilar idea to microstepping in steppers. I have programmed my servos to 5000 steps per
revolution, and being direct coupled to the screws results in a linear resolution of 1um per step. This have proven to be very adequate. At 5000 rpm however the pulse rate to the servo
drive is 5000 x 5000 /60=416kHz. So you can see that having high speeds AND fine resolutions requires a very high pulse rate, much higher than the Masso for instance.
I use Mach4 and an Ethernet SmoothStepper. The SmoothStepper can run at 4Mhz....so can handle 416kHz at a canter. I have a servo driven spindle that when I drive at its rated 35000rpm with
an 8000 count/rev encoder requires an even faster pulse rate of 3500 x 8000/60=466.7kHz. Again the SmoothStepper can do this no trouble.
It is entirely possible for me to use the Electronic Gearing feature to reduce the required pulse rates to 100kHz or less, but only at the expense of resolution, and I am loathe to give
up all that resolution that I've paid so dearly for.
This is the reason that Matec has suggested that the Masso controller may be a little slow....it may limit either your speed OR resolution should you ever use servos.
In a related issue; most stepper drives and servo drives are limited to about 200kHz input with the so called 'single ended' signalling, sometimes also called 'open collector'.
To go beyond that speed requires 'differential signalling', which requires a twisted wire pair for each signal (Step/Dir) to the drive. This adds some complexity to the breakout board,
not insurmountable. I suspect Masso have elected to stick with 'single ended signalling' in order to avoid the extra complication. That compromise would be fine for steppers
of modest resolution and speed but might be found to be limiting for servos being faster and higher resolution.
Craig