re:Home / Limit switches

Carlos, list,

Lotta comments already. My $.02

We had a thread re: this subject awhile back. Might be worth a re-
read.

Alan R's use of a single switch with "bumps" is often used. For HARD
limits. It is a good idea to mount the switch in a fixed position,
and have the "bumps" move...

The "classic", most expensive setup has 5 switches. One home, 2 soft
limits, and 2 hard limits.

The soft limits are wired to the control, and are there to recover
from poor programming which tries to exceed the machine travel. The
controller reads these inputs, and decelerates in a recoverable way.

Hard limits work independant of the control system, and "should" be
hard wired into the emergency stop circuit. Their function is to
prevent mechanical damage from an axis runaway. They "should" drop
power, and 'signal' the controller, "houston, we have a problem!".

Neither of these two uses require any REAL repeatability from the
switch (beyond standard microswitch capabilities).

Home switches are DESIGNED to be EXTREMELY repeatable. There is a
HUGE difference in price and performance of a switch DESIGNED
for "homeswitch" use! As Les mentioned, standard microswitches are
often 'combined' with the encoder "index" pulse in servo machines.

In stepper systems, separate home inputs are provided to accomodate
the repeatability needs of the home switch compared to the soft
limits. Often these are called hi limit, lo limit, and home. It would
be equally possible to use the last direction traveled (which is
known by the software) to reduce this to 2 inputs; home and soft
limit. The appropriate message "should" be presented to the operator,
although many just say "soft limit reached" and YOU have to figure
out which one...

IF you use high quality switches (read repeatable), you can use
a "limit loop" and let the control software decipher what happened,
based on what it knows about what it was doing when the input pulse
appeared. Of course, this means the controller software must be
written with this in mind!

Many machine safety standards will require the use of hard limits.

For most of our smaller, less powerful machines, mechanical "stop"
type axis limits are used and the "limit switches" are really 'soft'
limits. Whether or not one of these is pressed into service as a home
switch will depend on software/controller design.

Hope this helps.

Ballendo

P.S. Servo machines often eliminate the soft limit switches,
replacing them with "stored stroke limits". These are control
parameters which relate to how much axis travel is "available". This
technique requires that the machine be homed before use, if the soft
limits are to be useful. Some commercial controls will "alarm" if the
machine is not homed before use.In this way, commercial machines can
save the cost of some switches and their attendant wiring.

>The current discussion on g-code and coordinate systems has me
>wondering about what switches are really needed on the machine
>axes. The typical setup sounds like 2 limit switches and a home
>switch per axis. Why is the home switch necessary? Can't you
>just reference one of the limit switches and add ½ travel to get
>to the center? That shouldn't even be needed if you're going to
>just go and set your part origin somewhere else (so it seems).
>Does anybody out there omit the home switch?