Hello again,
Once again, it looks as if I should have been more specific.. ;)
I will define some terms I used..
Cycles: A complete cycle of BOTH the A and B signals from the sensor.
Pulses: Maybe not an exact term, but this refers to a cycle from ONE of
the signals either A or B.
Counts: A count is derived from the quadrature input and can be any of 1,
2, 3, or 4 increments per cycle. Yes, there are three counts per cycle
actually in use, this provides an alternate wide and narrow step, when
used in conjunction with custom patterns on the encoder wheel can provide
some very useful information, slot machines are among some things that use
this.
When I was talking about 1024 counts per rev, I was referring to an
encoder wheel with 256 slots, holes, magnetic flux changes, capacitive
strips, reflective spots, or conductive spots, I'm sure I left more than
one off this list. :)
As far as your stepper machine with 256,000 steps per inch, I fail to see
why you would need an encoder that matches that resolution (.000000390625
per step), my electron microscope stage is only 50,000 steps per inch.
I can see why you would need so many steps per inch to get the needed
torque, but that has nothing to do with the the encoder.
Also, I'd like to make it clear that I am not attacking anyone's ideas
here, I really appreciate your input, it will help me make better design
decisions. If there is a real need for very high count rates, I will be
forced to include it in the design, however for jogging speeds with very
high speed motors and very fine encoders, the usual technique is to use
only the index pulse (or a second encoder or track on the same
encoder that has a much lower resolution) above a certain speed. For
example when using a 1024 count per rev encoder, when the pulse frequency
gets over 100,000 counts per second, wait for the next index pulse to come
by, then switch to counting only index pulses and either add or subtract
1024 to the total count for each index. As soon as the speed is back to
something lower than 100,000 counts per second, flip back to counting
actual pulses.
As far as generating steps, I don't see a problem with going into the MHZ
on that side..
-Neil Whelchel-
C-Cubed
760 366-0126
- I don't do Window$, that's what the janitor is for -
Diplomacy is to do and say, the nastiest thing in the nicest way.
-- Balfour
On Mon, 31 Mar 2008, Jon Elson wrote:
> Neil Whelchel wrote:
>> Hello,
>> It is too early to tell what the reasonable maximums are, I will have a
>> better idea when I build a few modules. However, I have plans to count 16
>> bits worth of changes, so reading the register 10 times a second would be
>> able to count just about 65536/2*10=327680 per second. With an encoder
>> with 1024 counts per rev, this works out to be 327680/1024*60=19200 rpm!
>> The microcontroller will be able to handle up to about 4.5 MHZ worth of
>> input pulses. I see the major limiting factor here as the response time
>> from the encoder itself, there are very few encoders around that don't
>> start missing pulses at around 150 khz,
> Well, 150 KHz on the actual A and B encoder signals is 600,000
> quadrature counts/second. At 1000 cycles/rev or 4000 quad.
> counts/rev, that is 150 revs/second or 9000 RPM. Will your
> motors be running that fast?
> ones with less counts per rev can
>> usually exceed this however, but look at the resulting RPM..
>> Also, when very large RPM ranges are needed, a common trick is to use 2
>> encoders, one with a high pulse count for fine positioning, and another
>> with a low pulse count for high speed rough position. When the RPM falls
>> below a certain point, a calculation is made to determine the offset to
>> the high res pulses, and the handoff is made. (This is sometimes done in
>> the same physical encoder using only the index pulse above a certain RPM.)
>> What could you possibly need 100,000 counts per second for?
> Well, with a 1000 cycle/rev encoder, that is 25 revs/sec or 1500
> RPM. That is not very fast for a small servo motor.
> I have 1000 cycle/rev encoders on some of the motors I have used
> on my minimill. It has 4:1 belt reduction and 16 TPI screws.
> So, that is 4000 * 4 * 16 = 256000 counts/linear inch motion.
> At 60 IPM=1 Inch/Sec, that is 256000 quadrature counts/second.
> That is also 3840 RPM at the motor.
>
> Jon
>
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