Re: [CAD_CAM_EDM_DRO] Encoder index and homing

Hi Wally;

I will comment below, hopefully the material won't get lost in the
clutter.

On Tue, 2005-02-01 at 15:28 +0000, cnc_4_me wrote:
>
> In the last few days there has been discussion about using the index
> pulse on an encoder "anded" with a home switch to get ultra precise
> home repeatability. At first I was very excited about this solution
> to the homing problem and had to take several cold showers. But
> after I warmed up and thought about it for awhile, I came to a
> different conclusion.

Fortunately I'm past taking cold showers. I do believe that your
initial excitement was warranted though.
>
> So, as someone that has never actually run a CNC machine I will make
> the following observations and ask the group to correct me if I am
> wrong…
>
> First, in my mind I see 2 different important positions in a CNC
> machine.
>
> 1) Home - which I define as finding the table and spindle limits
> on power up.

Homing has really nothing at all to do with axis limits even though an
axis is often driven to its limit to home. Homing an axis is an event
that brings the axis to a known position. The axis home switch is
often independent of the axis limit switch.


> 2) Zero – which I define as the zero coordinates on your part or
> fixture.

This will probably generate arguments, but zero is where ever you define
it. I've seen lathes where the zero is defined after home out, near
the end of the axis and never changed afterward. It may very well have
nothing at all to do with you part or fixture. In any event at some
point knowing exactly where your tool is with respect to the coordinate
system is very important.
>
> As I see it, home is a rough position to define table limits and zero
> needs to be very precise and repeatable so that each part is machined
> the same. To further define zero I would say it is a position you
> define after the machine homes. You would then find your zero using
> something like an edge finder. Or in other words zero is a user
> defined location independent of limit switches. And the reason this
> would be true is the zero point will change with each setup or every
> time you moved the vise or stops.

I will have to disagree hear a bit but do realize that practice varies a
bit. Generally you will not be able to establish zero points via a
edge finder to the sort of accuracy you will get with using a properly
set up home pulse. Zeroing on the home pulse should get you a couple
of counts accuracy. Depending on your encoder location in the drive
train and leads this can be extremely accurate out in the mechanical
world.

As to vices or fixturing there are a number of approaches that can be
used. Often it is advisable to simply have the origin of the fixture
as an offset from the zero point. Then after each set up you simply
have to define where the fixture is again with respect to zero.

The other option is to drive your axis over to a reference point and
redefine zero. This is common with some types of CNC work.

There are other times though when that sort of resetting is counter
productive. Take for example a case where you have a lathe running
several different parts all in the same fixture. Here is it is a
simple matter of adjusting a few offsets to obtain new radii and
features. You in effect avoid the need to find the mechanical zero.

>
> Home on the other hand is merely a position to find travel limits and
> possibly a position to move to, to get clearance to change parts or
> tools. Because of this it seems to me that home does not need to be
> very repeatable.

Now I see what your problem is. Do not think of home as finding the
travel limits even though home is often established from an axis limit.
Homing is taking the axis to a know state when the controller is unaware
of its state. Homing is a defining moment in the life of a servo
controller. That is with homing the controller becomes aware of axis
positions.

>
> The only instance I can think of were precise home repeatability
> would be of any concern. Is if you command the machine to home after
> you have set your zero and your CNC control software automatically
> zeros its position. And since I have never used any CNC software I
> do not know if this happens…

Precise homing allows one to reestablish machine position rapidly. It
is extremely important when you have tooling that is moving on an axis.
With out precise homing you end up in the position of having to redefine
the position of all your tooling every time the machine homes out.
How important this is is very dependent on the machines usage.

Beyond that solid homing allows one to restart a program that may have
been interrupted for what ever reason without worries about butchering
something in the process.
>
> As for using the encoder index pulse for zero position, this would
> not be feasible. Let's assume the encoder is mounted on a 5TPI
> ballscrew. That means the index pulse comes around 1 time per rev,
> or 1 time every .200". There is no practical way to sync this
> position to your vice or stops.

You just need to put these pieces together in a different manner.
First the home (index) pulse has nothing to do with the vice or stops,
it is reference point for the CNC controller.

Now look at how a common CNC controller might go about homing off a
index pulse. The controller starts off by moving the first safe axis
towards a homing switch of some sort which may or may not be an axis
limit. It moves the axis slowly until the home switch is made. At
this point the CNC controller has a rough idea of where the axis is.
This position accuracy is defined mostly by a mechanical switch and may
be accurate to within a few thousands depending of course on lead.
This however is not where the machine zeros out, simple a step in the
process.

The zeroing process continues by having the motor reverse direction
where upon the CNC controller starts to look for the index pulse. This
pulse it should see within one revolution of the shaft. Ideally the
controller sees the pulse after one half revolution. When the CNC
controller sees the pulse the lead screw position is established to with
in a fraction of a revolution of the encoder. The accuracy of the
position established is dependent on a number such as lead, CPR, and the
width of the pulse.

Lets say the index pulse represents one out of 4096 and is connected to
a leadscrew. This means you just found the angular position of the
lead 1/4096 of a revolution. Off course how much 1/4096 is in the real
world is dependent on the screw lead and what ever other factors are
involved. The point is one can establish the axis position with a
cheap switch to very high precision.

>
> To sum things up, home is a rough position for table and spindle
> limits found with limit switches. And zero is a precise position
> defined by the machine operator using the keyboard.

Well my take on this is a little different. Homing is a very precise
operation in many servo systems. As to the operator establishing the
zero position precisely I do not believe that is the case at all. If
you want to truely establish a precision reference of a fixture or work
piece you would need to have the machine find that position through
probing. Or you could have the operator establish that position
through a bunch of trial an error cuts.

Either way the big issue is how do you determine position of a fixture.
And how would an operator do that reliably.

Dave


>
> Wally
>
>
>
>
>
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
> Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@..., timg@...
> Moderator: pentam@... indigo_red@... davemucha@... [Moderators]
> URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> bill
> List Mom
> List Owner
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>
>