6605
« on: September 22, 2017, 02:59:24 AM »
Hi,
what you want is a closed loop control....Mach3 is not suitable.
Closed loop control occurs when an encoder/index pulse /tacho measues the current speed of a motor, some electronic device or computer compares the current
speed with the programmed speed and increases or decreases the voltage/current/frequency to the motor to get it closer. While Mach3 can read an encoder
it can only stop and do the comparison and consequent correction about ten times a second, the macro pump rate. The Nyquist sampling theorem tells us that the
best control bandwidth is half that ie 5Hz. A control engineer would say that you'll get no/poor 'discrimination' at 5Hz, in fact it will still be pretty ragged at 1Hz.
Your control bandwidth is 1Hz at best, any variation in speed of your spindle due to load or whatever that occurs faster than once per second cannot be corrected
by Mach3, its just too damn slow.
Mach3 is primarily a Gcode interpretor and trajectory planner and does pretty well, as a closed loop motion control device it sucks.
Mach4 is much faster with the PLC script running every 10ms easily and faster if you wish to program it so. It can achieve a Nyquist rate of 50Hz and pretty good
discrimination at 5Hz. Useful but hardly great, for a spindle I'd like about 5-10 times better and for an axis servo 100-500 times better. Yes axis servo bandwidths
are at least hundreds of Hz, 500Hz is a common number but 1kHz and more is not unheard of even if expensive.
There are a few Mach ready motion control devices that can close a control loop nicely, Galill, HICON, KFLOP and maybe others. All are reasonably pricey. In truth
most are best considered motion control devices that can optionally close a loop but is not their primary claim to fame.
The best device to close a servo loop is a matching servo drive. If you choose a servo and drive from the same manufacturer you can be pretty sure that 1) it
will work and 2) achieve best or near best performance capable of that servo.
With a suitable drive Mach can feed it with an analogue voltage say and the drive/servo loop will maintain its speed to within a small fraction of 1% depending
on the drive/servo pretty much irrespective of the load AND you don't have to do any fancy programming with Mach. Theres a reason control engineers are weird
antisocial chain smoking alcaholics....it comes from trying to program PID control loops on poor gear for an uncaring public..%$#)(*&&&!!! If you value your
health don't be too inclined to program feedback loops.
Craig