user_concepts.lyx

CNC 的开放码,EMC2 V2.2.8版

#LyX 1.3 created this file. For more info see http://www.lyx.org/
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\layout Chapter

Important Concepts
\layout Standard

This chapter covers important concepts that should be understood before
 attempting to run a g code file.
\layout Section

Trajectory Control
\layout Subsection

Trajectory Planning
\layout Standard

Trajectory planning, in general, is the means by which EMC follows the path
 specified by your G Code program, while still operating within the limits
 of your machinery.
 
\layout Standard

A G Code program can never be fully obeyed.
 For example imagine you specify as a single-line program the following
 move:
\layout LyX-Code

G1X1F10 (G1 is linear move, X1 is the destination, F10 is the speed)
\layout Standard

In reality, the whole move can't be made at F10, since the machine must
 accelerate from a stop, move toward X=1, and then decelerate to stop again.
 Sometimes part of the move is done at F10, but for many moves, especially
 short ones, the specified feed rate is never reached at all.
\layout Standard

The basic acceleration and deceleration described above is not complex and
 there is no compromise to be made.
 In the INI file the specified machine constraints such as maximum axis
 velocity and axis acceleration must be obeyed by the trajectory planner.
\layout Subsection

Path Following
\layout Standard

A less straightforward problem is that of path following.
 When you program a corner in G Code, the trajectory planner can do several
 things, all of which are right in some cases: it can decelerate to a stop
 exactly at the coordinates of the corner, and then accelerate in the new
 direction.
 It can also do what is called blending, which is to keep the feed rate
 up while going through the corner, making it necessary to round the corner
 off in order to obey machine constraints.
 You can see that there is a tradeoff here: you can slow down to get better
 path following, or keep the speed up and have worse path following.
 Depending on the particular cut, the material, the tooling, etc., the programmer
 may want to compromise differently.
\layout Subsection

Programming the Planner
\layout Standard

The trajectory control commands are as follows:
\layout Description

G61 (exact stop mode) G61 tells the planner to come to an exact stop at
 every segment's end.
 This ensures exact path following but the full stops can be harmful to
 the workpiece or the tooling, depending on the particular cut.
 
\layout Description

G64 (blend without tolerance mode) G64 is just blending and the naive cam
 detector is not enabled.
 G64 and G64P0 tell the planner to sacrifice path following accuracy in
 order to keep the feed rate up.
 This is necessary for some types of material or tooling where exact stops
 are harmful, and can work great as long as the programmer is careful to
 keep in mind that the tool's path will be somewhat more curvy than the
 program specifies.
 
\layout Description

G64\SpecialChar ~
Px.xxx (blend tolerance mode) This enables the "naive cam detector" and
 enables blending with a tolerance.
 If you are in inch mode and program G64 P0.05, you tell the planner that
 you want continuous feed, but at programmed corners you want it to slow
 down enough so that the tool path can stay within 0.05 inches of the programmed
 path.
 The exact amount of slowdown depends on the geometry of the programmed
 corner and the machine constraints, but the only thing the programmer needs
 to worry about is the tolerance.
 This gives the programmer complete control over the path following compromise.
 The blend tolerance can be changed throughout the program as necessary.
 Beware that a specification of G64 P0 has the same effect as G64 alone
 (above), which is necessary for backward compatibility for old G Code programs.
 
\layout Description

Blending\SpecialChar ~
without\SpecialChar ~
tolerance The controlled point will touch each specified
 movement at at least one point.
 The machine will never move at such a speed that it cannot come to an exact
 stop at the end of the current movement (or next movement, if you pause
 when blending has already started).
 The distance from the end point of the move is as large as it needs to
 be to keep up the best contouring feed.
\layout Description

Naive\SpecialChar ~
Cam\SpecialChar ~
Detector Successive G1 moves that involve only the XYZ axes that
 deviate less than P- from a straight line are merged into a single straight
 line.
 This merged movement replaces the individual G1 movements for the purposes
 of blending with tolerance.
 Between successive movements, the controlled point will pass no more than
 P- from the actual endpoints of the movements.
 The controlled point will touch at least one point on each movement.
 The machine will never move at such a speed that it cannot come to an exact
 stop at the end of the current movement (or next movement, if you pause
 when blending has already started) 
\layout Subsection

Planning Moves
\layout Standard

Make sure moves are "long enough" to suit your machine/material.
 Principally because of the rule that "the machine will never move at such
 a speed that it cannot come to a complete stop at the end of the current
 movement", there is a minimum movement length that will allow the machine
 to keep up a requested feed rate with a given acceleration setting.
 
\layout Standard

The acceleration and deceleration phase each use half the inifile MAX_ACCELERATI
ON.
 In a blend that is an exact reversal, this causes the total axis acceleration
 to equal the inifile MAX_ACCELERATION.
 In other cases, the actual machine acceleration is somewhat less than the
 inifile acceleration 
\layout Standard

To keep up feed rate, the move must be longer than the distance it takes
 to accelerate from 0 to the desired feed rate and then stop again.
 Using A as 1/2 the inifile MAX_ACCELERATION and F as the feed rate *in
 units per second*, the acceleration time is ta = F/A and the acceleration
 distance is da = (1/2) * F * ta the deceleration time and distance are
 the same, making the critical distance d = da + dd = 2*da = F^2 / A.
 
\layout Standard

For example, for a feed rate of 1 inch per second and an acceleration of
 10 inch/sec^2, the critical distance is 1^2 / 10 = .1 inch.
 For a feed rate of .5 inch per second, the critical distance is .5^2 / 10
 = .025 inch.
 
\the_end