On 10/26/2015 11:00 AM, John Kasunich wrote: > > On Mon, Oct 26, 2015, at 11:43 AM, Sebastian Kuzminsky wrote: >> On 10/26/15 8:50 AM, andy pugh wrote: >>> Some systems have many indexes and a way to distinguish between them. >>> Sometimes the indices are different distances apart (some linear scales) or >>> there may be a secodary feedback system (robots with encoder + index and a >>> potentiometer). >>> >>> I don't think that LinuxCNC can work with these systems at the moment as >>> the home_offset is a static value read from the INI. >>> >>> I quick inspection seems to suggest that it would not be a huge change to >>> make an extra pin. >>> it requires an extra entry in hal_joint_t and extra code in control.c to >>> read the value (and in inihome.cc to set the new pin too) >> >> On my machine, each joint has a rotary encoder with one index per >> revolution, and no way to distinguish between them. My homing >> configuration uses the joint's home switch to distinguish between the >> index pulses. >> >> I don't understand what problem you're solving, or what your solution >> entails. Can you give more details? >> > Imagine a lathe Z axis. The leadscrew has an encoder with an index pulse. > > It can be inconvenient to home to a fixed location. If you put the home > switch at the headstock end you might run into the chuck if a larger-than- > normal chuck is mounted. (Or the workpiece, if one is loaded). If you put > the switch at the other end you might run into the tailstock depending on > where it is located. > > Now imagine that you have some kind of coarse absolute position > measurement. Perhaps linear potentiometer connected to the carriage. > It can't measure position to 0.001", but it can do +/- 0.050". The screw > pitch is 0.200". So the pot can tell you where you are to within one turn > of the screw. > > In theory, it should be possible to home by moving the carriage approximately > one turn of the screw in either direction, to find an index pulse. The pot > tells > you what turn you are on, the index tells you exactly where you are in that > turn. > > But there needs to be a mechanism for LinuxCNC to use the info from the pot. > > A related topic is "how do you home when you have absolute encoders?". > > Some absolute encoders literally produce a wide parallel word that tells you > where you are at all times. But others are basically incremental, with the > ability to squirt out an absolute position on request (usually a serial > interface). > As with the pot example, there needs to be a way to get that absolute position > into LinuxCNC. >
I think that Andy and Peter have worked on an SSI or BISS solution for absolute position encoders before. I was going to set one up but the project never happened. Dave --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus ------------------------------------------------------------------------------ _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
