BISS-C

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07 Dec 2018 21:14 #122019 by monit111
BISS-C was created by monit111
Hi

was wondering if 7I76E will take an absoulute encoder that is BISS-C?
i have a servo with incremental encoder (10,000 pulses) want to use 5mm ballscrew and add an absolute encoder that ahould give me 3 microns accuracy over 1 metre.

thank you

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07 Dec 2018 21:48 #122024 by PCW
Replied by PCW on topic BISS-C
If its only a single channel and you are not using the 7I76Es serial expansion port, you should be able to use the sserial expansion port to interface to 1 BISS-C device. This would need firmware that has a BISS interface

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15 Dec 2018 11:38 #122438 by monit111
Replied by monit111 on topic BISS-C
Hi
thank you for getting back to me
I have a 7I74 (RS-422) interface would this work?
I need 4 BISS-C devices and possibly even 5

also do you know what expansion board I can use to get an extra 2-3 axis as well as the max 5 axis on the 7i76e?

thank you

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15 Dec 2018 12:09 #122439 by andypugh
Replied by andypugh on topic BISS-C
You can connect another 7i76 (not-E) or 7i78 (cheaper) to the one remaining 25 pin expansion port on the 7i76E, and it should be possible to configure the 7i74 port for mixed BISS and smart-serial. (though with one sserial each on the 7i76E and 7i85/7i76 you might not need to mix them.

BiSS is supported by the LinuxCNC Mesa driver: linuxcnc.org/docs/2.7/html/man/man9/hostmot2.9.html#BiSS
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15 Dec 2018 14:10 #122442 by PCW
Replied by PCW on topic BISS-C
Other options for adding more step/dir outputs are a 7I85S (adds 4 differential step/dir outputs and 5 RS-422 ports (for sserial, BISS/SSI etc)

One of the inexpensive Ebay 5 Axis BOBs will work also if you just need 3-5 more buffered step/dir outputs
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15 Dec 2018 20:12 #122453 by monit111
Replied by monit111 on topic BISS-C
will most likely get the 7I78 since its in stock and all i need are the extra axis

by any chance would you guys know if i have a 10,000 pulse rotary encoder
on a 5mm ball screw do i have enough accuracy to accurately make the required movements the linear encoder is asking for?
encoder has a Resolutions to 1 nm at 100 m/s.

just came to mind now that my servos might not be good enough to correct position?
i need it to remain within 3 microns over a metre. im worried that if the servos cant get to the location then
1 its wasted money on renishaw linear encoders (that i already got)
2 it all might behave very badly with extreme vibrations hunting for position?

thank you

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16 Dec 2018 02:18 #122465 by andypugh
Replied by andypugh on topic BISS-C
10,000 pulses in 5mm is 0.5µm resolution. You can normally count on 4 counts for control, so the motor encoders are good for 2µm on position.
But, it is possible that the secondary feedback can get things closer. You _might_ need to work in torque mode in that the drives won't see enough velocity resolution.

I wonder if an independent Piezo element in the thrust bearing would be useful ?

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16 Dec 2018 19:14 #122489 by monit111
Replied by monit111 on topic BISS-C
so if im understanding correctly

I can only move in 2µm increments?

if so what would happen if the Renishaw encoder wants to move 1µm?


the bellow pasted right up was from guy before me he claims we don't even have the full 10,000 increments but 8,000

im a little confused about the result of this setup and the bellow info if its correct.

Step and Direction
The adtech servo drives only support maximum of 500kHz pulse frequency. LinuxCNC needs about 25% extra frequency headroom to make sure the pulse generation is stable. This limitation made it impossible to achieve the max RPM. To solve the max velocity limit problem I experienced, I had to reduce the number of steps per rotation by reducing the micro-stepping multiplier from 10 to 8 and then setting the electronic gear ratio on the servo drive to 5:4 (parameter 12 set 5 and parameter 13 set to 4). This solution reduced the range of encoder impulses from 10000 to 8000 and reduced the frequency needed to achieve 3000RPM by 25% providing the extra 25% pulse frequency headroom. The servo drive operates at maximum 3000RPM which equals 50RPS. The machine configuration .INI file has MAX_LINEAR_VELOCITY variable under the [DISPLAY] section. This value should equal to 50 when using the 3000RPM servo motors (linuxCNC uses rotations per second instead of the rotations per minute).

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16 Dec 2018 20:18 #122490 by andypugh
Replied by andypugh on topic BISS-C
If the servo drives only support step-dir then you can not move less than one step.
But I am very confused about what the "micro-stepping multiplier" is on a servo drive. You can't micro-step a servo.
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16 Dec 2018 22:36 #122491 by monit111
Replied by monit111 on topic BISS-C
could this have something to do with the ratio?

this driver has a ratio function so maybe he was confused by that
i never understood how 10,000 pulses ever changed.

it is step and direction
it does have 10,000 increments so i think he might have changed the ratio but could not possibly affect the accuracy of the encoder?

so based on your first comment i can move half micron min?

do you recommend i setup 1 axis to test? im really worried about servo hunting due to linear encoder higher accuracy.

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