Oscmol wrote: Were can i find your tutorial?
Are you using your system on an mill or some other cnc machine?
Is your system using the distributed clocks so all drives are in sync?
Tutorial:
link
Right now I've finished the setup for drives and CNC; I'm working on a retrofit on a small home-scale milling machine (3 axis); Currently I'm waiting for ball screw spindles to replace the old non ball screw ones.
The Setup with the IndraDrives, LinuxCNC and EtherCAT uses DC-clocks and the application time is synced among the slaves.
Oscmol wrote: Do you have any feeling if the precision on the machine will get any better with this kind of system instead of using a MESA card and pulse trains.
As Andy writes in my opinion there is no difference if you transmit your commanded vlaue for the position via bus or pulse train.
Oscmol wrote: My machine is an Deckle FP4A that i am retro fitting with new control system.
On my machine i have glasscales today, how is the best way to do this conversion?
To take analogue output from MESA card to the drives and put them in velocity mode and then take the feedback from the glasscales to the MESA card?
I have bought 3 Heidenhain conversion boxes for the glasscales so i can get TTL outsignals.
I have no experences with the use of the "MESA card" and LinuxCNC (what does this do?, read encoder, write pulse trains?)
But you have to think about the following (case ball screw spindles):
- Every Servo Drive has in most cases 3 control loops (current,speed,position)
- simple setups for machine tool axis use the position feedback from the rotation motor shaft (means: inaccurancy from errors of spindle, backclash, dynamic compliance)
- more advanced position control use the position feedback from linear measurment devices fixed to the axis (e.g. glassscales) and speed feedback from rotating shafts (classical:tachometer e.g. FP4); In such a setup you have to consider some kind of notch filters for the commanded speed value to get over possible resonant vibrations and so on; This is state of the art in the drives to use speed feedback from rotating shaft and position feedback of linear scales.
- most modern position control use both: position feedback of rotating shaft and linear movement (Siemens, Rexroth, state control instead of PID)
If you thinking about closeing the position control loop 'over' the LinuxCNC/MESA card with linear feedback/pulse train and the speed control loop on the drive as it is this will work.
But my guess is that this is not optimal. Because you have two systems with discrete time signal handling. Reading mesacard counters, calculate new speed command, write to pulse train output on the LinuxCNC side and signal processing on the side of the drive (this will result in dead times with 1 or 2 delays --> limits control bandwith of the position loop, assuming the drive is digital and not analog controled approx. 4-8kHz speed control loop)
I think its better that the drive does all the control loops because a 'shorter' control loop is better for machine tool axis with no dynamic coupling in the mechanics.
Also there could be a problem with the frequency-scaling devices for the glassscale (analog to TTL); If you use scaling 1 this coudl result in low resolution for the axis; If you use scaling/interpolation e.g. 64 the resulting TTL-Freqency at max speed is to high for the counters on the MEAS CARD
To conclude:
My experencies and opinions are based on 'high class' cnc machines and could be completly over engineered for the use in many over cases.
Best regards
Andreas
