tweaking_steppers.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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Stepper Tuning
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Getting the most out of Software Stepping
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Generating step pulses in software has one very big advantage - it's free.
 Just about every PC has a parallel port that is capable of outputting step
 pulses that are generated by the software.
 However, software step pulses also have some disadvantages:
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limited maximum step rate
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jitter in the generated pulses
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loads the CPU 
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This chapter has some steps that can help you get the best results from
 software generated steps.
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Run a Latency Test
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The new easy way to do a latency test is described in the Getting Started
 Guide.
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Latency is how long it takes the PC to stop what it is doing and respond
 to an external request.
 In our case, the request is the periodic "heartbeat" that serves as a timing
 reference for the step pulses.
 The lower the latency, the faster you can run the heartbeat, and the faster
 and smoother the step pulses will be.
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Latency is far more important than CPU speed.
 A lowly Pentium II that responds to interrupts within 10 microseconds each
 and every time can give better results than the latest and fastest P4 Hyperthre
ading beast.
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The CPU isn't the only factor in determining latency.
 Motherboards, video cards, USB ports, and a number of other things can
 hurt the latency.
 The best way to find out what you are dealing with is to run the RTAI latency
 test.
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DO NOT TRY TO RUN EMC2 WHILE THE TEST IS RUNNING
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On Ubuntu Dapper, you can run the test by opening a shell and doing:
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sudo mkdir /dev/rtf; 
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sudo mknod /dev/rtf/3 c 150 3; 
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sudo mknod /dev/rtf3 c 150 3; 
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cd /usr/realtime*/testsuite/kern/latency; ./run
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and then you should see something like this:
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ubuntu:/usr/realtime-2.6.12-magma/testsuite/kern/latency$ ./run 
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* 
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* 
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* Type ^C to stop this application.
 
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* 
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*
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## RTAI latency calibration tool ## 
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# period = 100000 (ns) 
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# avrgtime = 1 (s) 
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# do not use the FPU 
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# start the timer 
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# timer_mode is oneshot
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RTAI Testsuite - KERNEL latency (all data in nanoseconds) 
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RTH| lat min| ovl min| lat avg| lat max| ovl max| overruns 
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RTD|   -1571|   -1571|    1622|    8446|    8446|      0 
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RTD|   -1558|   -1571|    1607|    7704|    8446|      0 
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RTD|   -1568|   -1571|    1640|    7359|    8446|      0 
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RTD|   -1568|   -1571|    1653|    7594|    8446|      0 
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RTD|   -1568|   -1571|    1640|   10636|   10636|      0 
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RTD|   -1568|   -1571|    1640|   10636|   10636|      0
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While the test is running, you should "abuse" the computer.
 Move windows around on the screen.
 Surf the web.
 Copy some large files around on the disk.
 Play some music.
 Run an OpenGL program such as glxgears.
 The idea is to put the PC through its paces while the latency test checks
 to see what the worst case numbers are.
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The last number in the column labeled "ovl max" is the most important.
 Write it down - you will need it later.
 It contains the worst latency measurement during the entire run of the
 test.
 In the example above, that is 10636 nano-seconds, or 10.6 micro-seconds,
 which is excellent.
 However the example only ran for a few seconds (it prints one line every
 second).
 You should run the test for at least several minutes; sometimes the worst
 case latency doesn't happen very often, or only happens when you do some
 particular action.
 I had one Intel motherboard that worked pretty well most of the time, but
 every 64 seconds it had a very bad 300uS latency.
 Fortunately that is fixable, see FixingDapperSMIIssues in the wiki found
 at wiki.linuxcnc.org.
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So, what do the results mean? If your "ovl max" number is less than about
 15-20 microseconds (15000-20000 nanoseconds), the computer should give
 very nice results with software stepping.
 If the max latency is more like 30-50 microseconds, you can still get good
 results, but your maximum step rate might be a little dissapointing, especially
 if you use microstepping or have very fine pitch leadscrews.
 If the numbers are 100uS or more (100,000 nanoseconds), then the PC is
 not a good candidate for software stepping.
 Numbers over 1 millisecond (1,000,000 nanoseconds) mean the PC is not a
 good candidate for EMC, regardless of whether you use software stepping
 or not.
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Note that if you get high numbers, there may be ways to improve them.
 For example, one PC had very bad latency (several milliseconds) when using
 the onboard video.
 But a $5 used Matrox video card solved the problem - EMC does not require
 bleeding edge hardware.
 
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Figure out what your drives expect
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Different brands of stepper drives have different timing requirements on
 their step and direction inputs.
 So you need to dig out (or Google for) the data sheet that has your drive's
 specs.
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For example, the Gecko G202 manual says this:
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Step Frequency: 0 to 200 kHz 
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Step Pulse 
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0
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 Time: 0.5 uS min (Step on falling edge) 
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Step Pulse 
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1
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 Time: 4.5 uS min 
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