Limit switch's - best configuration?

[Sparks]

Hi Andy

Thought you had a modbus board, surely using that you have enough i/o to get any combination you want as far as limits and homes are concerned.

I'm still at beginner stage with mach3 so just put me right if i've got the wrong end of the stick!

Dave

[audioandy]

Hi Dave

I do have a modbus board,but it's still in its box! Not used it yet, I am just about to finish the upgrade to my machine (mechanical build wise) then hopefully build a control box with PC and Modbus PLC all in one, so yes you are right once I get it sorted the I/O should not be a problem, but it will be a steep learning curve!

Andy

[HiltonSteve]

OK, just been reading through the Mach3 online instructions and found this -

"Repeatability is very important for a switch used for the Home function. Each axis could therefore need three switches (i.e. two Limit switches at the two ends of travel and a Home switch). Even a basic mill would require nine parallel port inputs for them. This is not practical, as a parallel port has only five inputs! The problem of insufficient inputs can be solved in three ways:

1. Connect the limit switches to external logic (perhaps in the drive electronics), and use this logic to switch off the drives when a limit is reached rather than interfacing them to Mach3. Separate refer-ence switches would still be connected inputs to Mach3.

2. Use one pin to share all the inputs for an axis, and make Mach3 responsible for controlling both limits and detecting Home. For example, if Mach3 was instructed to "move to Home" on a milling machine, it could move the X axis to the left (table to the right) until a switch was triggered. In context, that would be interpreted as "Home." If that same switch was triggered while machining, however, it would be interpreted as "exceeded Limit."

3. Interface the switches by a keyboard emulator.

The first method is best and mandatory for a very large, expensive, or fast machine where you cannot trust software and its configuration to prevent mechanical damage. Switches connected to the drive electronics can be intelligent and only allow motion away from a switch when the limit is hit. This is safer than disabling the limits so a user can jog the machine off the limits, but it does rely on having a sophisticated drive. The second method is suitable for smaller machines with direct operator control. With this setup, it is possible to use only three inputs to Mach3 for a three-axis mill (four for a gantry type machine - see Slaving). Since you (or the operator) are present and can take appropriate action based on the situation, only two switches are required per axis. Both Limit switches can be tied together, and the Home posi-tion can share functionality with one of the Limit switches. All the two switches need to do is signal Stop! The operator can then figure out what to do for recovery."


I'm going for option 2!

[tribbles]

Option 2 would work if:

1) Your home position is one of your limits

2) You do not do any movements which causes the axis to go over the home position

Otherwise, the switch would be trigged when home was reached (crossed), and that would be interpreted as exeeding a limit...

ETA: Although if it homed, and then moved by a known offset, then you wouldn't have this problem

[HiltonSteve]

Yes home position will be one of my axis travel limits.

All home/limit switch's will be positioned at the end of the safe travel for X,Y and Z.

So, should be ok then. (famous last words... I can see a late night coming on!)