The LIMIT Command [MLEPC]

The LIMIT command is used: to activate and deactivate travel limit checking; to shift or transform the coordinate data for travel limit purposes; to define a preferred rotary alignment of the machine where feasible; to avoid linear overtravel conditions through the application of polar interpolation; and to automatically orient singular rotary axes to position the workpiece so as to avoid linear overtravel conditions. This command is available for all machine types.

Travel Limit Checking

The following command is used to define, activate and deactivate up to 10 different travel limit checking zones:

\textbf{LIMIT}\;\textbf{/}\icamhsp{0.707mm}\raise0.05ex\hbox{$\begin{bmatrix}\!\begin{array}{l}\textbf{ON}\\\textbf{OFF}\end{array}\!\end{bmatrix}$}\icamhsp{0.707mm}\raise0.05ex\hbox{$\begin{bmatrix},\!\begin{array}{l}\textbf{IN}\\\textbf{OUT}\end{array}\!\end{bmatrix}$}\icamhsp{0.707mm}\bigl[,\mathit{zone}\,\bigr]\icamhsp{0.707mm}\raise0.68ex\hbox{$\icamrange{13.1}{1}\!\begin{pmatrix}\!,\!\begin{pmatrix}\!\begin{array}{l}\textbf{XAXIS}\\\textbf{YAXIS}\\\textbf{ZAXIS}\\\textbf{UAXIS}\\\textbf{VAXIS}\\\textbf{WAXIS}\\\textbf{QUILL}\\\textbf{AAXIS}\\\textbf{BAXIS}\\\textbf{CAXIS}\\\textbf{AAXIS},\textbf{TABLE}\\\textbf{BAXIS},\textbf{TABLE}\\\textbf{CAXIS},\textbf{TABLE}\\\textbf{RAM}\,\bigl[,\mathit{n}\,\bigr]\end{array}\!\end{pmatrix}\!,\!\begin{pmatrix}\!\begin{array}{l}\bigl[\,\textbf{ON},\bigr]\begin{pmatrix}\!\begin{array}{l}\mathit{low},\mathit{high}\\\mathit{total}\end{array}\!\end{pmatrix}\\\;\\\textbf{OFF}\end{array}\!\end{pmatrix}\end{pmatrix}$}

The initial ON or OFF indicates whether the travel limit checking should be enabled or disabled respectively for a zone. If omitted, ON is assumed (i.e., travel limit checking is enabled).

IN and OUT describe two types of range checking zones. IN zones define a specific area where the axes are permitted to travel. The axes must remain inside of the limits specified. OUT zones define a specific area where the tool axes are not permitted to travel. The axes must remain outside of the limits specified. If this parameter is omitted, IN is assumed.

If only one value is given for a particular axis, limit checking for that axis will be based on total travel rather than on a travel range. Total travel limit checking is only valid for IN zones.

The zone parameter is a scalar value identifying the travel limit zone. There are 10 zones available: zone one (1) represents the travel limits predefined by the post-processor creator; zones 2 through 10 can be defined by the NC programmer. If the zone number is not specified, one (1) is assumed (in other words, predefined limits).

See here for a table correlating axes name keywords with machine axes.

The low and high parameters specify the minimum and maximum limits in machine coordinates for the named axis. The total parameter specifies a total travel limit in machine coordinates for the named axis. In either case the ON keyword is optional. The OFF keyword can be specified to turn off travel limit checking for the named axis. For example, the following program segment turns off any travel limit checking predefined by the post-processor creator, moves the machine and then turns the predefined travel limit checking back on.

LIMIT/OFF
GOTO/0,0,0
GOTO/0,0,4
LIMIT/ON

This next example defines two new travel zones that are used depending on the circumstances. Initially zone 1 is enabled and zone 2 is disabled.

LIMIT/ON,IN,1,XAXIS,-10,40,ZAXIS,5,15
LIMIT/OFF,IN,2,XAXIS,-3,47,ZAXIS,2,13

The following commands are needed to turn zone 1 checking off and turn zone 2 checking on:

LIMIT/OFF,1
LIMIT/ON,2

Any or all of the ten available zones may be active at one time. The machine axes will be permitted to travel in the intersection of the active zones.

For total travel limits, axis travel is accumulated starting from the time that the limit is activated. If a total travel limit is turned off and then turned on again later, GENER will stop keeping track of total travel while the limit is inactive and will then resume when the limit is re-activated.

Travel Limit Tool Compensation

The following command offsets the linear axes, for travel limit calculation purposes only, in order to account for controller tool compensation using expected gauge lengths or offsets.

\textbf{LIMIT}\;\textbf{/}\;\textbf{LENGTH},\bigl[\,\mathit{x},\mathit{y},\bigr]\,\mathit{z}

The LIMIT/LENGTH command is similar to the TOOLNO/LENGTH option in terms of where the post-processor calculates the final linear axes positions. With TOOLNO/LENGTH, the post-processor adjusts the axes positions to account for the tool length. With LIMIT/LENGTH, this adjustment is done for travel limit purposes only; the NC code is not affected. Code a length offset of 0 (zero) to cancel this feature.

In the following example, the LIMIT/LENGTH,4 command causes the calculated machine axes positions to be offset, for travel limit checking purposes only, along the tool axis by 4 CL units. This limits the Z axis CL input to the range –4 through 6.

LIMIT/ON,2,IN,ZAXIS,0,10
LIMIT/LENGTH,4
GOTO/0,0,6    $$ ok
GOTO/0,0,-4   $$ ok
GOTO/0,0,-5   $$ exceeds minimum
GOTO/0,0,7    $$ exceeds maximum

\textbf{LIMIT}\;\textbf{/}\;\textbf{SETOOL},\mathit{x},\mathit{y},\mathit{z}

Likewise, the LIMIT/SETOOL command is similar to the TOOLNO/SETOOL option. The xyz adjustment is made relative to the spindle control point and will be affected by the rotation of rotary heads. Code an offset of 0,0,0 (all zeroes) to cancel this feature.

In the following example, a LIMIT/SETOOL,0,0,–4 command adjusts the calculated machine axes positions by the inverse of the specified xyz amounts for travel limit checking purposes only. The inverse is used because the SETOOL parameter specifies the relative distance between spindle control point and tool tip. As with the first example, this limits the Z axis CL input to the range –4 through 6.

LIMIT/ON,2,IN,ZAXIS,0,10
LIMIT/SETOOL,0,0,-4
GOTO/0,0,6    $$ ok
GOTO/0,0,-4   $$ ok
GOTO/0,0,-5   $$ exceeds minimum
GOTO/0,0,7    $$ exceeds maximum

Travel Limit Fixture Compensation

The following command offsets the linear axes, for travel limit calculation purposes only, in order to account for controller fixture (i.e. workpiece) compensation amounts.

\textbf{LIMIT}\;\textbf{/}\;\textbf{ADJUST},\mathit{x},\mathit{y},\mathit{z}

The LIMIT/ADJUST option is similar to the LIMIT/SETOOL command described earlier. It applies xyz fixture compensation offset amounts to the linear axes positions for travel limit calculation purposes; NC code is not affected, with one exception (see below). Code an offset of 0,0,0 (all zeroes) to cancel this feature.

The LIMIT/POLAR command (see “Limit Based Cartesian-Polar Switching”) takes the fixture compensation offset amounts into account when computing Cartesian vs. polar interpolation optimizations, since optimization is highly dependent upon the actual placement of the part on the rotary table.

Travel Limit Frame Transformation

The LIMIT/TRFORM command transforms machine linear axes travel information by the specified matrix before testing for limitations and recording minimum and maximum data. This command can be used to properly manage travel limit processing when outputting machine coordinates that have been transformed in some fashion outside of GENER’s normal control.

Note

The LIMIT/TRFORM command should not be used at the same time as the LCS local coordinate transformation command. LCS already manages travel limit transformations.

The syntax is:

\textbf{LIMIT}\;\textbf{/}\;\textbf{TRFORM}\,\bigl[,\textbf{LAST}\,\bigr]\begin{pmatrix}\!,\!\begin{array}{l}\mathtt{<\!matrix\!>}\\\textbf{ON}\\\textbf{OFF}\end{array}\!\end{pmatrix}

Two separate transformations can be applied and removed individually. LIMIT/TRFORM defines the primary transformation, LIMIT/TRFORM,LAST defines a second transformation.

The <matrix> defines the location and orientation of the resultant after-transformation coordinates on the machine, with respect to the normal coordinates. The matrix can be specified using the standard post-processor transformation syntax (see “The ORIGIN Command [MLEPC]”).

When LIMIT transformation is active, the linear axes column displays and travel summaries show coordinate data in the “as transformed” state.

The ON and OFF keywords enable or disable travel limit transformations. OFF ignores the effect of the matrix. ON re-establishes the previously specified matrix.

Preferred Rotary Solution – Modulo-360 Range

The following command is used to define, for a specific rotary axis, a preferred modulo-360 range of rotary travel (typically within 180 degrees) to be used on 5-axis positioning motions, so as to influence the choice of rotary solution (or pose) from the two possible solutions normally available for any given tool axis vector.

\textbf{LIMIT}\;\textbf{/}\icamhsp{0.707mm}\textbf{ORIENT}\icamhsp{0.707mm}\raise0.05ex\hbox{$\begin{bmatrix},\!\begin{array}{l}\textbf{IN}\\\textbf{OUT}\end{array}\!\end{bmatrix}$}\icamhsp{0.707mm}\raise0.31ex\hbox{$\begin{pmatrix}\!,\!\begin{array}{l}\textbf{AAXIS}\\\textbf{BAXIS}\\\textbf{CAXIS}\\\textbf{AAXIS},\textbf{TABLE}\\\textbf{BAXIS},\textbf{TABLE}\\\textbf{CAXIS},\textbf{TABLE}\\\textbf{RAM}\,\bigl[,\mathit{n}\,\bigr]\end{array}\!\end{pmatrix}$}\icamhsp{0.707mm},\mathit{low}\,\bigl[,\mathit{high}\,\bigr]

\textbf{LIMIT}\;\textbf{/}\;\textbf{ORIENT},\!\begin{pmatrix}\!\begin{array}{l}\textbf{ON}\\\textbf{OFF}\end{array}\!\end{pmatrix}

Rotary solution choices are only available on non-linearized (i.e., LINTOL/OFF) and rapid positioning moves, since once the tool is moving at feed it cannot switch from one solution to another without leaving a dwell mark (caused by disengage, rotate and re-engage motions of the rotary-turn-around feature). When there is a choice of rotary solutions, GENER normally selects the one that results in the least amount of motion, with an emphasis on reducing rotary travel before linear travel. The LIMIT/ORIENT command can be used to change this default behavior, by specifying a preferred range of positioning for a specific rotary axis. See here for a table correlating axes name keywords with machine rotary axes.

The low and high parameters define the lower and upper machine coordinate limits of the preferred positioning range for the named axis. The high parameter defaults to low+180 if omitted. All angles are treated as modulo-360. (Use the physical range command described below to instead define an absolute range of preferred travel.)

GENER will choose the solution that respects the following condition for the named axis, where all angles are modulo-360:

low ≤ angle < high

IN and OUT can be used to define an inclusive or exclusive preferred positioning range. IN is the default and defines an inclusive range as shown above. OUT can be used to define an exclusive range, where the preferred solution avoids the defined range.

Once a preferred rotary solution has been specified, ON and OFF can be coded without an axis value to enable or disable preferred rotary solution positioning.

Preferred Rotary Solution – Physical Range

The following command is used to define, for one or more rotary axis, a preferred physical range of rotary travel to be used on 5-axis positioning motions, so as to influence the positioning of the rotary axes, especially in cases where the rotary range is finite but exceeds 360 degrees.

\textbf{LIMIT}\;\textbf{/}\icamhsp{0.707mm}\raise0.05ex\hbox{$\textbf{RANGE}\begin{bmatrix},\!\begin{array}{l}\textbf{IN}\\\textbf{OUT}\end{array}\!\end{bmatrix}$}\icamhsp{0.707mm}\raise0.31ex\hbox{$\begin{pmatrix}\!,\!\begin{array}{l}\textbf{AAXIS}\\\textbf{BAXIS}\\\textbf{CAXIS}\\\textbf{AAXIS},\textbf{TABLE}\\\textbf{BAXIS},\textbf{TABLE}\\\textbf{CAXIS},\textbf{TABLE}\\\textbf{RAM}\,\bigl[,\mathit{n}\,\bigr]\end{array}\!\end{pmatrix}$}\icamhsp{0.707mm}\raise0.10ex\hbox{$\begin{pmatrix}\!,\!\begin{array}{l}\mathit{low}\,\bigl[,\mathit{high}\,\bigr]\\\noalign{\vskip 0.5ex}\textbf{ON}\\\textbf{OFF}\end{array}\!\end{pmatrix}$}

\textbf{LIMIT}\;\textbf{/}\;\textbf{RANGE},\!\begin{pmatrix}\!\begin{array}{l}\textbf{ON}\\\textbf{OFF}\end{array}\!\end{pmatrix}

Rotary range choices are only available on non-linearized (i.e., LINTOL/OFF) and rapid positioning moves, since once the tool is moving at feed it cannot switch from one solution to another without leaving a dwell mark (caused by disengage, rotate and re-engage motions of the rotary-turn-around feature). When there is a choice of rotary solutions, GENER normally selects the one that results in the least amount of motion, with an emphasis on reducing rotary travel before linear travel. The LIMIT/RANGE command can be used to change this default behavior, by specifying a preferred range of travel for one or more rotary axis. See here for a table correlating axes name keywords with machine rotary axes.

The low and high parameters define the lower and upper machine coordinate limits of the preferred positioning range for the named axis. All angles are treated as absolute angles. (Use the modulo-360 range command described above to instead define a modulo-360 based preferred solution.)

GENER will choose the solution that respects the following condition for the named axis, where all angles are absolute:

low ≤ angle < high

IN and OUT can be used to define an inclusive or exclusive preferred positioning range. IN is the default and defines an inclusive range as shown above. OUT can be used to define an exclusive range, where the preferred solution avoids the defined range.

Preferred positioning ranges can be specified for each rotary axis. Once a preferred range specification has been given for a rotary axis, preferred range selection for that specific axis can subsequently be enabled or disabled by coding ON or OFF in place of the range specification.

Once a preferred range has been specified for any rotary axis, ON and OFF can be coded without an axis name to enable or disable preferred range positioning for all axes.

Limit Based Cartesian-Polar Switching

When the tool axis is parallel to one of the rotary axes, this rotary axis is called “singular”. In the case of a rotary table, it then becomes possible to use this singular rotary axis in place of one or more linear axes when positioning or interpolating from one position to the next in the program. The use of a singular rotary axis in place of a linear axis is called “Polar” interpolation (as distinct from normal Cartesian interpolation), which can be controlled using the MODE/POLAR command (see “Polar Interpolation”).

The basic syntax is:

\textbf{LIMIT}\;/\;\textbf{POLAR}\,\ldots

The LIMIT/POLAR command defines a conditional Polar interpolation, which will only be used to avoid overtravel conditions that would otherwise occur with Cartesian interpolation. For example, if the X axis on a mill-turn lathe is restricted to positive values only, the post-processor can switch to polar interpolation during face milling to avoid moving the X axis below the center line. The LIMIT/ADJUST command (see “Travel Limit Fixture Compensation”) fixture compensation offset amounts are taken into account when computing travel limitations for Cartesian versus polar interpolation purposes.

Polar Interpolation Switching

\textbf{LIMIT}\;\textbf{/}\;\textbf{POLAR},\textbf{CNC}

The CNC keyword specifies that machine polar interpolation should be used if necessary to avoid overtravel conditions. This requires GENER to perform a path-planning look-ahead each time it encounters a RAPID or high feed positioning motion. This look-ahead will analyze all of the cutting motions that immediately follow, to determine if polar interpolation is necessary so as to avoid an overtravel condition.

At the completion of the look-ahead, GENER will do one of the following:

  • Activate machine polar interpolation for the positioning motion(s) and all of the following cutting motions.

  • Deactivate polar interpolation if active and optionally pre-position the singular axis if necessary to avoid overtravel for the positioning motion(s) and all of the following cutting motions.

Polar Emulation Switching

\textbf{LIMIT}\;\textbf{/}\icamhsp{0.707mm}\raise0.16ex\hbox{$\textbf{POLAR}\begin{bmatrix},\!\begin{array}{l}\textbf{AXIS},\textbf{AUTO}\\\textbf{AAXIS}\\\textbf{BAXIS}\\\textbf{CAXIS}\end{array}\!\end{bmatrix}$}\icamhsp{0.707mm}\raise0.21ex\hbox{$\icamrange{4.66}{1}\!\begin{pmatrix}\!,\!\begin{array}{l}\begin{pmatrix}\!\begin{array}{l}\textbf{XAXIS}\\\textbf{YAXIS}\\\textbf{ZAXIS}\end{array}\!\end{pmatrix}\,\bigl[,\mathit{value}\,\bigr]\begin{bmatrix},\!\begin{array}{l}\textbf{LARGE}\\\textbf{SMALL}\end{array}\!\end{bmatrix}\\\;\\\textbf{XYZ},\mathit{x},\mathit{y},\mathit{z},\textbf{NORMAL},\mathit{i},\mathit{j},\mathit{k}\end{array}\!\end{pmatrix}$}\icamhsp{0.707mm}\raise0.05ex\hbox{$\begin{bmatrix},\textbf{SCAN},\!\begin{pmatrix}\!\begin{array}{l}\textbf{ON}\\\textbf{OFF}\end{array}\!\end{pmatrix}\end{bmatrix}$}

The AAXIS, BAXIS and CAXIS keywords restrict the automatic use of polar interpolation to the named rotary table axis only. The AXIS,AUTO couplet specifies that any singular rotary table axis can be a candidate for automatic polar emulation. This is the default if a rotary axis is not specified.

The XAXIS, YAXIS and ZAXIS keyword, the optional value and the LARGE-SMALL couplet, all combine to define a “guard plane” and zone of travel where Cartesian positioning is valid. The post-processor will use polar emulation (i.e., rotate the singular axis) to avoid moving the named axis to the opposite side of the guard plane. The current axis position is used if an axis value is not specified. LARGE is assumed if neither LARGE or SMALL is specified. For example, “XAXIS, –0.5,LARGE” specifies that: the machine X-axis valid travel range is –0.5 and larger; and that the post-processor should use polar emulation if an attempt is made to move the X-axis below –0.5.

The guard plane and zone of travel can also be specified given an (x,y,z) point on the plane and an (i,j,k) vector normal to the surface of the plane. Point and vector data must be specified in part (i.e., CL) coordinates. The range of valid travel is defined by the positive sense of the vector. For example, assuming the part and machine origins coincide, “XYZ,–0.5,0,0,NORMAL,1,0,0” defines the same guard plane and valid range of travel as does “XAXIS,–0.5,LARGE” described above.

A maximum of four guard planes can be defined. These planes do not have to be orthogonal (i.e., are not required to be parallel or perpendicular to each other).

The SCAN,ON couplet activates a proactive form of limit based Cartesian-Polar switching. When active, GENER will perform path planning (i.e., will scan forward in the CL file) when it encounters RAPID or high feed positioning motions in an attempt to determine an optimum orientation of the singular rotary axis, so as to maximize the span of Cartesian motions. SCAN,OFF (the default) performs reactive limit based Cartesian-Polar switching, at the point where an attempt is made to move past the guard plane.

When entire path cannot be optimized in one step, having defined RTA sequence allows the motion to be broken just before the overtravel and to apply additional rotation at the safe RTA sequence. The process will be then automatically repeated as many times as needed.

Starting from V25, the Cartesian-Polar travel optimization can be also applied to the sequence of CYCLE points. Set $PLR230=$FALSE to enable this.

Polar Switching Activation

\textbf{LIMIT}\;\textbf{/}\;\textbf{POLAR},\!\begin{pmatrix}\!\begin{array}{l}\textbf{ON}\\\textbf{OFF}\end{array}\!\end{pmatrix}

OFF disables limit based Cartesian-Polar switching. ON re-enables the last specified Cartesian-Polar switching selection (i.e., interpolation or emulation).

The LIMIT/POLAR and MODE/POLAR commands are mutually exclusive. MODE/POLAR unconditionally selects Polar interpolation for subsequent motions, whereas LIMIT/POLAR conditionally selects Polar interpolation to avoid overtravel conditions during Cartesian interpolation.

Limit Based Singular Positioning

When the tool axis is parallel to one of the rotary axes, this rotary axis is called “singular”. In the case of a rotary table, it then becomes possible to use this singular rotary axis in place of one or more linear axes when positioning from one location to the next in the program. The use of a singular rotary axis to position the workpiece is called “Polar” positioning (as distinct from normal Cartesian positioning).

The LIMIT/ROTREF command defines a series of rotary offsets or positions that the post-processor can use to reposition the workpiece, in an attempt to eliminate a linear axis overtravel. For example, if the X-axis on a mill-turn lathe is restricted to positive values only, the post-processor can rotate the C-axis (i.e., workpiece) so that the end-point of the positioning move is always above the center-line (e.g., when face drilling a bolt hole pattern).

The syntax is as follows:

\textbf{LIMIT}\;\textbf{/}\icamhsp{0.707mm}\textbf{ROTREF}\icamhsp{0.707mm}\raise0.10ex\hbox{$\begin{bmatrix},\!\begin{array}{l}\textbf{INCR},\mathit{delta_{1}}\Bigl[,\mathit{delta_{2}}\bigl[,\ldots\,\bigr]\,\Bigr]\\\noalign{\vskip 1.0ex}\textbf{ABSOL},\mathit{value_{1}}\Bigl[,\mathit{value_{2}}\bigl[,\ldots\,\bigr]\,\Bigr]\end{array}\!\end{bmatrix}$}\icamhsp{0.707mm}\raise0.05ex\hbox{$\begin{bmatrix},\textbf{SCAN},\!\begin{pmatrix}\!\begin{array}{l}\textbf{ON}\\\textbf{OFF}\end{array}\!\end{pmatrix}\end{bmatrix}$}

\textbf{LIMIT}\;\textbf{/}\;\textbf{ROTREF},\!\begin{pmatrix}\!\begin{array}{l}\textbf{ON}\\\textbf{OFF}\end{array}\!\end{pmatrix}

The “INCR,delta1,delta2,…” sequence defines rotary offsets that the post-processor should try in an attempt to eliminate an overtravel on a RAPID or high feed positioning motion. The post-processor will test rotate the rotary table from its current position by multiples of the ±delta1 amount, to a maximum of ±360 degrees, in an attempt to find a position that satisfies travel limits. If unsuccessful, the process is repeated, this time using the delta2 amount and so on, until a valid orientation is found. If an orientation cannot be found that corrects the linear overtravel condition, then the rotary table axis is not rotated and a travel limit diagnostic is output as usual.

The “ABSOL,value1,value2,…” sequence is similar to INCR, except that the trial offset values are measured from the rotary zero position instead of from the current rotary table axis position.

The SCAN,ON couplet tells the post-processor to scan forward in the CL file in an attempt to choose a rotation that satisfies the largest span of upcoming positioning motions. SCAN,OFF (the default) positions the workpiece correct overtravel for the current position only. When using SCAN,ON it is recommended to use a delta2 or value2 that is much smaller than delta1 or value1.