Originally Posted by
joeavaerage
As far as various fault signalling schemes Delta drives are very flexible.
Lets start with limit signals. With most Mach systems, and I imagine similar to UCCNC, limit switches are connected directly to the controller. A limit event will cause the machine to stop.
Delta drives (and incidentally most other modern AC drives) offer another alternative. You can hook a ++limit switch and a --limit switch direct to two inputs on the drive. Should a limit
event occur the servo will stop, either a crash stop or controlled deceleration at your program choice, and also propagate a LimitEvent signal on one of the drive digital outputs
to signal to the controller that a limit event has occurred. If you are required to jog the servo back within limits the drive will know not to allow you to jog++ when the axis is already
exceeded the ++limit, but will allow you to jog--.
You might ask 'why would Delta bother to add complexity to their drive in the form of limit signals and logic when my controller can do it?'
The main reason is that Delta drives are capable of being used in a distributed motion control solution. With such a system you do not have a motion controller at all, your PC is the trajectory planner
and via EtherCat or CanOpen that data is signalled to each drive, and each drive is responsible for controlling its own motion. Thus each drive must be able to handle and control its own
limit and home events. EtherCat is a recent feature addition in Mach4 but is not available with UCCNC.
Technically we do not need the limit handling built in to the drive but I point it out that you may see some of the power and flexibility of the drive.
If you had limited input pins on your controller then having drive handle limits might be an advantage. Instead of two inputs (++ and --)being required
for each axis, you would require now only one, namely the LimitEvent signal. I know that most people combine ++ and -- limits to just one input but then you lose
the capability of determining which way to jog the axis. As the servos are so fast and powerful having a mechanism that absolutely prevents you from jogging in the
wrong direction seems worthwhile to me. I have not decided which method (drive handles limits and jogs verses the controller handling the limits and use a software interlock to prevent
erroneous jogs) to employ on my machine. Given that the drive handled signals are hardwired in the drive they would be less subject to programming error.