Steve Blackmore wrote:

>Hi Guys, what's a realistic number of PPR for a lathe spindle encoder
>via a parallel port in EMC. Spindle is capable of 3500 rpm. 
>  
>
It's dependent on your BASE_PERIOD.  You can theoretically count one 
quadrature phase per period, but it's better to have two periods per 
count - gives better noise margin (noise in this case being encoder 
phase issues and latency variations).

>My home made encoder had 90, but I've altered the drive system and
>fitted a 1024 PPR encoder with index. Output is via differential line
>drivers. 
>
>I've changed the scale factor but it starts loosing pulses at 600 rpm or
>so?? 
>  
>
Hmmm - you must have a 256 cycle encoder, which gives you 1024 counts 
per rev.  At 10 RPS, that's 10240 counts per second, which is a 
reasonable number for a 16 kHz base thread.  At 3500 RPM you need close 
to 60000 counts per second.

I'd guess that your PCs latency is 17k or so (calculated as 
(1/10240-62500ns)/2).  You should check the latency, and see how fast 
you can run the base thread.  You can only count reliably if the counts 
come in slower than BASE_PERIOD+2*latency.  If the latency is actually 
17k, you probably can't run the base period faster than 35000-40000 ns, 
which will only increase the top speed to about 1000 RPM, give or take.  
I don't know of any computer that can do 60k counts/sec reliably on the 
parport.

The best thing to do is probably to get a Mesa card, either PCI or 
parallel port connected.  You'll get smoother step generation too.

>Hal file reads
>
>setp encoder.0.position-scale 360.000000
>  
>
This should now be 4096, unless you have a 256 cycle encoder which is 
1024 phases per rev (in which case it should be 1024).

- Steve


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