Possible error in logic of LinuxCNC homing with leadscrew compensation

( DRO sampling of axis realized response to motion commands			)
( Creates or appends to file _.log in the configuration directory		)
( RJK 02/2019									)
(										)
( This program requires the LinuxCNC HAL-reading feature, which exposes		)
( realtime HAL values to G-code programs as G-code parameters.  To enable that	)
( feature, before launching LinuxCNC, edit the FEATURES mask in the .ini	)
( configuration file to assert the value 8, or to add 8 to an existing value,	)
( thus:										)
(										)
(   [RS274NGC]									)
(   FEATURES = 8								)

( ----------------------------------------------------------------------------- )
( Manually configure the constants in this section for the desired sampling run	)
( Observe the consistency directed in the comments; this program does not check	)
( the values for consistency.							)
( ----------------------------------------------------------------------------- )
#<_gmin> = -3.875	( Minimum G53 coordinate to be sampled, < _gmax		)
#<_gmax> = 1.375	( Maximum  "       "     "  "     "   , > _gmin		)
#<_samples> = 256	( Number of samples to be taken across the range, >= 2	)
#<_home> = 0.0		( G53 axis home coordinate, _gmin <= _home <= _gmax	)
#<_feedrate> = 6	( Feedrate most practiced in precise stopping, > 0.0	)
#<_dwell> = 0.5		( Seconds of dwell time to settle mechanically, >= 0.0	)
#<_backlash> = 0.050	( Worst-case estimate of backlash magnitude, >= 0.0	)

			( Additional configuration required:			)

			( Change the configuration-dependent "_hal" names below	)
			( to reflect the appropriate HAL pins for the HAL .ini	)
			( configuration of the machine under test for the	)
			( sampled axis.						)

			( To select which axis is sampled instead of X, edit	)
			( each appearance of capital "X" below to the target	)
			( axis.							)

			( To change the output log file name, edit "_.log"	)
			( below.						)

( ----------------------------------------------------------------------------- )
( Subroutine: log the current realtime sampling state and DRO measurement	)
( Format of log file lines emitted:						)
(     <commanded> <actuated> <dro> <diff>					)
( Where:									)
(   <commanded> = the G53 coordinate						)
(   <actuated> = the coordinate actuated by the realtime motion controller	)
(   <dro> = the DRO reading							)
(   <diff> = <actuated>-<dro>, which is the error at the <commanded> location	)
(	This diff value is logged to aid human inspection of raw results and to	)
(	make plotting a log file such as in gnuplot simpler.			)
(										)
( Note: each sample's commanded and actuated values should be identical,	)
(   *unless* a prior backlash value or compensation table is configured into	)
(   HAL, in which case the values will differ by the position-dependent		)
(   interpolated compensation as applied by the realtime motion controller.	)
(   To temporarily turn off such compensation, comment out the BACKLASH or	)
(   COMP_FILE settings in the .ini file configuration, and restart LinuxCNC.	)
(   LinuxCNC may involve other offsetting factors in the actuated motor		)
(   position for a given commanded position, so the actuated value is not	)
(   necessarily significant other than to confirm whether "something", perhaps	)
(   unexpecting, is going on in the motion control.				)
(										)
( All prior compensation must be turned off to read the basic or uncompensated	)
( error characteristic of the mechanism.  Conversely, a compensation table	)
( processed from a reading of the basic error characteristic must be turned	)
( on, and the now-compensated error characteristic read again, to evaluate the	)
( effectiveness of the compensation.						)
(										)
( This program uses G53 moves, which is the machine coordinate system in which	)
( any compensation table is referenced. In short, this works as close to the	)
( "bare metal" as a G-code program can, which is critical to getting valid	)
( and repeatable characterization of the mechanical response and error.		)
( ----------------------------------------------------------------------------- )

o100 sub
g4 p#<_dwell> ( Dwell )
m66 p0 L0 ( Queue-buster with no other effect )
#<commanded> = #1
#<actuated> = #<_hal[xpos-fb]>
#<dro> = #<_hal[sum2.0.out]>
#<diff> = [ #<commanded> - #<dro> ]
(LOG,#<commanded> #<actuated> #<dro> #<diff>)
o100 endsub

( ----------------------------------------------------------------------------- )
( Subroutine: Step the range #1 .. #2 of axis position, logging the experiment	)
( ----------------------------------------------------------------------------- )

o101 sub
#<start> = [ #1 ]
#<end>   = [ #2 ]
o103 if [ #<start> gt #<end> ]
    (LOG,NEGATIVE-GOING)
    #<min> = [ #<end> ]
    #<max> = [ #<start> ]
    #<incr> = [ - #<_step> ]
  o103 elseif [ #<start> lt #<end> ]
    (LOG,POSITIVE-GOING)
    #<min> = [ #<start> ]
    #<max> = [ #<end> ]
    #<incr> = [ + #<_step> ]
  o103 else
    (LOG,NON-GOING)
  o103 endif
#<loc> = [ #<start> ]
o102 while [ 1 ]
  o104 if [ #<loc> gt #<max> ]
    o101 return
    o104 endif
  o105 if [ #<loc> lt #<min> ]
    o101 return
    o105 endif
  g53 g1 f#<_feedrate> X#<loc>
  o100 call [ #<loc> ]
  #<loc> = [ #<loc> + #<incr> ]
  o102 endwhile
o101 endsub

( ----------------------------------------------------------------------------- )
( Main program									)
( ----------------------------------------------------------------------------- )

(LOGAPPEND,_.log)
(LOG,# Axis error sampling)

( Emit all configuration parameters and conditions to the log output, for use	)
( by post-processing programs							)
(LOG,GMIN #<_gmin>)
(LOG,GMAX #<_gmax>)
(LOG,SAMPLES #<_samples>)
(LOG,HOME #<_home>)
(LOG,FEEDRATE #<_feedrate>)
(LOG,DWELL #<_dwell>)
(LOG,BACKLASH_WORST_CASE_ESTIMATE #<_backlash>)

( Provisional step = the step length that integrally divides full range )
#<_step> = [ [ #<_gmax> - #<_gmin> ] / [ #<_samples> - 1] ]
( Adjust step such that one sample will be taken at G53 home, which will maintain )
( phase with any axis compensation table deduced from one-time error readings )
( taken when calibrated to the automatic homing location. )
( Scaffold: we don't handle the case where _home is within a provisional _step )
( away from _gmax )
#<_step> = [ #<_gmax>/FIX[ [ #<_gmax>-#<_home> ]/#<_step> ] ]
( Adjust backlash up to the next integer multiple of the step )
( This keeps the sample locations aligned in both directions )
#<_backlash> = [ FUP [ #<_backlash> / #<_step> ] * #<_step> ]
(LOG,STEP #<_step>)
(LOG,BACKLASH_ADJUSTED #<_backlash>)
#<_logitem1> = [#<_gmax>/#<_step>]
#<_logitem2> = [[#<_gmax>-#<_gmin>]/#<_step>]
(LOG,POSITIVE_INTERVALS #<_logitem1>)
(LOG,OVERALL_INTERVALS #<_logitem2>)

( Starting move to most-positive end )
g53 g0 X#<_gmax>
( Sample the full range in the negative-going direction )
o101 call [ #<_gmax> - #<_backlash> ] [ #<_gmin> ]
( Sample the full range in the positive-going direction )
g53 g1 f#<_feedrate> X[#<_gmin>+#<_backlash>]
o101 call [ #<_gmin> + #<_backlash> ] [ #<_gmax> ]

( Return to G53 home on the sampled axis )
g53 g0 X[#<_home>+#<_backlash>]
g53 g1 f#<_feedrate> X#<_home>

(LOG,# FINISHED)
(LOGCLOSE)
(MSG, DONE)
m30