Hi Rally,
A servo is by definition a closed loop system. In the case of Mach3 and Geckodrives the loop is closed at the servodrive (G340 in your case). What that means is that Mach3 doesn't really know where the machine is but relies on the servodrive to make it go there. If it ends up to far off target due to overload or whatever the drive shuts down and (optionaly) lets the computer know that something is wrong.
Now, regarding the lost steps.... It is true that underpowered stepmotor systems can lose steps when driven outside of there specification. But to say that a servo system can't lose steps is not entirely true. If for example there is noice on the encoder signal there can be a problem and if the step/dir signals doesn't meet the specification there is exatly the same risk of losing steps as with a step motor.
There are pros and cons to both types of systems. An overloaded step motor stalls (loses steps) but no harm is done to the motor. An overloaded servo motor may burn up if not protected. A step motor is generally more appropriate for low/medium power CNC due to it's torque/speed curve while a high power servo generally needs some kind of reduction (belt drive, gearbox etc) to make use of all the available power. They (the servo system) can alos be more problematic to tune but it doesn't have to be. The servo system can also run away in case of lost encoder signal or shorted outputstage of the drive.
As to your powersupply, if all motors are the same voltage then a single powersupply will be the best/cheapest option but you can use one for each if you like. If you want absoulte max power from your motors you need to carefully select the voltage of your transformer. In that case you need to concider the max voltage of the motor, max voltage of the drive and the PWM modulation of the drive. Some drives have a max PWM modulation of ~85% which means that if you have 100V powersupply the max voltage the motor will "see" is 85V which would be quite a good fit for a 90V motor. On the other hand IF the drive should fail with a shorted output transistor the motor COULD see the full 100V which MAY be more than the absoulte max on the motor and could make permanent damage to it.
All this needs to be concidered when designing a servosystem.
There is a new device, currently undergoing beta-testing called the Smoothstepper. What I've read is that it should act and look just like two ordinary LPT-ports but it interfaces to Mach3 via USB. I don't know much more than that but if you have a look at the Mach3 forum there should be some info there, just search for Smootstepper. There's also the NCPod but again I'm not really up to speed on the status of it.
Hope it helps!
/Henrik Olsson.  |