coordinates.lyx

CNC 的开放码,EMC2 V2.2.8版

\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 5
\layout Quote


\begin_inset LatexCommand \index{G59}

\end_inset 

G59\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 6
\layout Quote

G59.1
\begin_inset LatexCommand \index{G59.1}

\end_inset 

\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 7
\layout Quote

G59.2
\begin_inset LatexCommand \index{G59.2}

\end_inset 

\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 8
\layout Quote

G59.3
\begin_inset LatexCommand \index{G59.3}

\end_inset 

\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 9
\layout Subsection

Default coordinate system
\layout Standard

One other variable in the VAR file becomes important when we think about
 offset systems.
 This variable is named 5220.
 In the default files its value is set to 1.00000.
 This means that when the EMC starts up it should use the first coordinate
 system as its default.
 If you set this to 9.00000 it would use the nineth offset system as its
 default for startup and reset.
 Any value other than an interger (decimal really) between 1 and 9 will
 cause the EMC to fault on startup.
 
\layout Subsection

Setting coordinate system values within G-code.
\layout Standard

In the general programming chapter we listed a G10 command word.
 This command can be used to change the values of the offsets in a coordinate
 system.
 (add here)
\layout Section

G92
\begin_inset LatexCommand \index{G92}

\end_inset 

 Offsets
\begin_inset LatexCommand \label{sec:G92-Offsets}

\end_inset 


\layout Standard

The way that it works has changed just a bit from the early days to the
 current releases.
 It should be thought of as a temporary offset that is applied to all other
 offsets.
 
\layout Subsection

The G92 commands
\layout Standard

This set of commands include;
\layout Description

G92 This command, when used with axis names, sets values to offset variables.
\layout Description

G92.1
\begin_inset LatexCommand \index{G92.1}

\end_inset 

 This command sets zero values to the g92 variables.
\layout Description

G92.2
\begin_inset LatexCommand \index{G92.2}

\end_inset 

 This command suspends but does not zero out the g92 variables.
\layout Description

G92.3
\begin_inset LatexCommand \index{G92.3}

\end_inset 

 This command applies offset values that have been suspended.
\layout Standard

When the commands are used as described above, they will work pretty much
 as you would expect.
\layout Quote

A user must understand the correct ways that the g92 values work.
 They are set based upon the location of each axis when the g92 command
 is invoked.
 The NIST document is clear that, 
\begin_inset Quotes eld
\end_inset 

To make the
\noun on 
 
\noun default 
current point have the coordinates
\begin_inset Quotes erd
\end_inset 

 x0, y0, and z0 you would use g92 x0 y0 z0.
 G92 
\emph on 
does not work from absolute machine coordinates
\emph default 
.
 It works from 
\emph on 
current location
\emph default 
.
 
\layout Quote

G92 also works from current location as modified by any other offsets that
 are in effect when the g92 command is invoked.
 While testing for differences between work offsets and actual offsets it
 was found that a g54 offset could cancel out a g92 and thus give the appearance
 that no offsets were in effect.
 However, the g92 was still in effect for all coordinates and did produce
 expected work offsets for the other coordinate systems.
 
\layout Quote

It is likely that the absence of home switches and proper home procedures
 will result in very large errors in the application of g92 values if they
 exist in the var file.
 Many EMC users do not have home switches in place on their machines.
 For them home should be found by moving each axis to a location and issuing
 the home command.
 When each axis is in a known location, the home command will recalculate
 how the g92 values are applied and will produce consistent results.
 Without a home sequence, the values are applied to the position of the
 machine when the EMC begins to run.
\layout Subsection

Setting G92 values
\layout Standard

There are at least two ways to set G92 values.
\layout Itemize

right mouse click on position displays of tkemc will popup a window into
 which you can type a value.
 
\layout Itemize

the g92 command
\layout Standard

Both of these work from the current location of the axis to which the offset
 is to be applied.
 
\layout Quote

Issuing g92 x y z a b c does in fact set values to the g92 variables such
 that each axis takes on the value associated with its name.
 These values are assigned to the current position of the machine axis.
 These results satisfy paragraphs one and two of the NIST document.
\layout Quote

G92 commands work from current axis location and add and subtract correctly
 to give the current axis position the value assigned by the g92 command.
 The effects work even though previous offsets are in.
\layout Standard

So if the X axis is currently showing 2.0000 as its position a G92 x0 will
 set an offset of -2.0000 so that the current location of X becomes zero.
 A G92 X2 will set an offset of 0.0000 and the displayed position will not
 change.
 A G92 X5.0000 will set an offset of 3.0000 so that the current displayed
 position becomes 5.0000.
 
\layout Subsection

G92 Cautions
\layout Standard

Sometimes the values of a G92 offset will remain in the VAR file.
 This can happen when a file is aborted during processing that has G92 offsets
 in effect.
 When this happens reset or a startup will cause them to become active again.
 The variables are named 
\layout Quote

5211\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5212\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5213\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5214\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5215\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5216\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Standard

where 5211 is the X axis offset and so on.
 If you are seeing unexpected positions as the result of a commanded move,
 or unexpected numbers in the position displays when you start up, issue
 a G92.1 in the MDI widow the problems should go away.
\layout Quote

With these tests we can see that reset returns g92 to the condition that
 it had when the interpreter started up.
 The reader should note that we have established ...
 that no write of these values occurs during a normal run so if no g92 was
 set at the startup, none will be read in during a reset.
\layout Quote

It may be that this is the heart of the problem that some have experienced
 with differences between the old and the new interpreter.
 It may well be, but I leave it to others to test, that the old interpreter
 and task programs immediately wrote values to the var file and then found
 those values during a reset.
 
\layout Standard

On the other hand, if G92 values existed in the VAR file when the EMC started
 up
\layout Quote

...
 starting the EMC with g92 values in the var file is that it will apply
 the values to current location of each axis.
 If this is home position and home position is set as machine zero everything
 will be correct.
 Once home has been established using real machine switches or moving each
 axis to a known home position and issuing an axis home command, g92 commands
 and values work as advertised.
\layout Quote

These tests did not study the effect of re-reading the var file while they
 contain numbers.
 This could cause problems if g92 offsets had been removed with g92.1 but
 the var file still contained the previous numbers.
 
\layout Standard

It is this complexity that causes us to say that G92 values must be treated
 as temporary.
 They should be used to set global short term offsets.
 The G54-59.3 coordinate systems should be used whenever long lasting and
 predictable offsets are needed.
\layout Section

Sample Program Using Offsets
\layout Standard

This sample engraving project mills a set of four .1 radius circles in roughly
 a star shape around a center circle.
 We can setup the individual circle pattern like this.
\layout Quote
\noindent 
G10 L2 P1 x0 y0 z0 (ensure that g54 is set to machine zero) 
\layout Quote
\noindent 
g0 x-.1 y0 z0
\layout Quote
\noindent 
g1 f1 z-.25
\layout Quote
\noindent 
g3 x-.1 y0 i.1 j0
\layout Quote
\noindent 
g0 z0
\layout Quote

m2
\layout Standard

We can issue a set of commands to create offsets for the four other circles
 like this.
\layout Quote

G10 L2 P2 x0.5 (offsets g55 x value by 0.5 inch) 
\layout Quote

G10 L2 P3 x-0.5 (offsets g56 x value by -0.5 inch) 
\layout Quote

G10 L2 P4 y0.5 (offsets g57 y value by 0.5 inch) 
\layout Quote

G10 L2 P5 y-0.5 (offsets g58 y value by -0.5 inch) 
\layout Standard

We put these together in the following program.
\layout Quote

(a program for milling five small circles in a diamond shape)
\layout Quote

G10 L2 P1 x0 y0 z0 (ensure that g54 is machine zero)
\layout Quote

G10 L2 P2 x0.5 (offsets g55 x value by 0.5 inch) 
\layout Quote

G10 L2 P3 x-0.5 (offsets g56 x value by -0.5 inch) 
\layout Quote

G10 L2 P4 y0.5 (offsets g57 y value by 0.5 inch) 
\layout Quote

G10 L2 P5 y-0.5 (offsets g58 y value by -0.5 inch)
\layout Quote

g54 g0 x-.1 y0 z0 (center circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g0 z0
\layout Quote

g55 g0 x-.1 y0 z0 (first offset circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g0 z0
\layout Quote

g56 g0 x-.1 y0 z0 (second offset circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g0 z0
\layout Quote

g57 g0 x-.1 y0 z0 (third offset circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g0 z0
\layout Quote

g58 g0 x-.1 y0 z0 (fourth offset circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g54 g0 x0 y0 z0
\layout Quote

m2
\layout Standard

Now comes the time when we might apply a set of G92 offsets to this program.
 You'll see that it is running in each case at z0.
 If the mill were at the zero position, a g92 z1.0000 issued at the head
 of the program would shift everything down an inch.
 You might also shift the whole pattern around in the XY plane by adding
 some x and y offsets with g92.
 If you do this you should add a G92.1 command just before the m2 that ends
 the program.
 If you do not, other programs that you might run after this one will also
 use that g92 offset.
 Furthermore it would save the g92 values when you shut down the EMC and
 they will be recalled when you start up again.
\the_end