Hi:
Just to kick in.. ( Thanks brett fro steering me to this post..)
Estop and limit events shoudl not be prevalent on any system, BUT, if for whatever reason they are..
Estop is just that, an emergnecy, all power shoudl be cut, ( which woudl release brakes on a bug system, thats truly safe..). Mach3 keeps track of all pulses at all times. The DRO's are always accurate as to the pulses put out. Doesnt matter what you do. Postion is lost only when a motor cant keep up. If using servo's, the Rogers baord makes it pretty much infalible. Brian has demonstrated at shows with grabbing the axis and stopping it, thus causing an estop, when the rest is pressed, the DRO's go to the actual motor position, and no homing in necessary. That having been said, Homing shoudl never be risky, it shoudl be accurate. Mine is accurate to .01mm's. If yours isnt, then you need better switches.
Now it IS possible to home to an index using macro's. All it takes is a "move to index " macro. A very slow move that stops when an input sees a pulse woudl do. Slow, but do-able, Ive never bothered as my homing always seems so accurate I woudlnt gain from the complexity. KISS is always a good idea if you can do it IMO.
When hitting stop, many steppers will lose position due to abrupt stopping. Many servo's will not. Alot depends on the system.
>>they know how many pusles move the axis how far. BUT if the computer puts out a pulse which the motor misses - for what ever reason, then the DRO IS NO LONGER ACCURATE. As an alternative - if you press E-Stop - and the computer instantly ceases putting out pulses - yet the momentum of your system carries the motor over a few steps (4.8 to the thousanth of an inch on mine) then the DRO (which registers to 1/10th of a thousanth) can no longer be accurat
Both true statements. BUT , and this is where the theory breaks, it is just as likely for a system to miss a step pulse as an encoder pulse. Both are similar in nature, and both happen. Since nothing, ( truly nothing ) can see the missing encoder pulse, no system, closed or open loop, servo or stepper, is immune to lost pulses. Whiel its true that in true closed loop only 1 weak link exists, any chain is only as strong as its weakest link, so closed loop is questionably better. The true ( and to my mind only ) advantage to a closed loop system with fuill encoder feedback, is that Estops, and sudden stop button presses will not lose position. That having been said, that situation shoudl be a rare one. And homing will cure that in a properly setup system. I may make some changes soon in the stop system, but Estop will always be a problem.. and should be. :-)
Art