Mach EMC Maximum Encoder Count Closed Loop Servo EMC/MACH3

[bad bearings]

I have a shop-made 5 axis CNC based on some surplus stages that are very accurate and have high count linear encoders--roughly 250,000 counts per inch and 20,000 counts per degree--rotary. Is is possible to use Mach3 or EMC to control these stages in closed loop mode and still take advantage of their high accuracy? With their old controller the stages can be moved at 100 IPM. It seems as though EMC and Mach will only allow much lower feedrates. Any advice would be appreciated.

[samco]

Look at mesa or pico hardware.

http://wiki.linuxcnc.org/cgi-bin/emc...orted_Hardware

You will be able to count into the mhz range and close the loop within emc.

What are you going to use for servo drives? what do they take for input? +/-10v? or pwm? or

sam

[bad bearings]

I did some research into the controller, breakout board and amplifier cards. The whole set up seems expensive considering the value of the little CNC: <$2500. I think I'll make do with what I have for the time being. Perhaps writing some more code for the existing controller so that it can accept at least, line, arc and maybe nurbs as G-Code. Thanks for the advice. This way maybe I can find some freeware that will generate the G-Code from dxf or stl or something. Thanks for the info.

[skullworks]

I would think that you could re-use almost all the existing control.

A mesa 7i43, an opto card and maybe the digital to +/- 10v card is about all you should need. ( You may need more I/O for a tool changer.)

Total cost should be under $350 and could well be under $200.

[bad bearings]

Thanks for the info. I took a look at the mesa thing. The old controller has a power supply and eight amplifiers as well as eight LM628 based controller cards. If I understand you right I would bypass the LM628 cards and the existing optically isolated bus and connect through the breakout board and this USB mesa board to the amplifier cards. Is that right? I guess I could still use the power supply. This would be a way to use EMC to control the machine?

[samco]

Sounds right - looks like the lm628 has a DAC. The servo amps must then be analog input. Analog what would be the question (+/-10v or what)

Sounds like a fun project. (so for the mesa you would also need the digital to +/- 10v card as skullworks mentioned) well - actually 2 as each one only has 4.

[skullworks]

Basically you got the idea - Except NO USB - you will either use a 5i20 PCI card or the 7i43 card via a EPP Parallel port. The required number of I/O lines will decide. 7i33 will give 4 channels of digital to +/-10v and to be safe misc I/O would be routed thru a 7i37 for isolation as needed.

[bad bearings]

Thanks "No USB" that's too bad. I thought USB was the connector of the future.

I took a look at the EMC Integrater's guide and they mention that it is optimistic to assume 50Khz quadrature encoder signal processing--Page 87 http://www.linuxcnc.org/docs/EMC2_Integrator_Manual.pdf -- I don't think the old copley amps in the controller can accept "electronic gearing" 50KHz implies a maximum feedrate of 2.5 degrees per second on the rotary table and .2 IPS on the linear stages.

You mentioned being able to control the stages to the "MHz range" -- can you give any suggestions for compatible hardware ie. CPU speeds and interface cards? My guess is that I would need about 250KHz per each of the five axes to get equivalent performances out of the stages as can be achieved with the old controller.

Thanks

[samco]

That is if you are using the software hal encoder module. (encoder hooked directly to the printer port.) The mesa hardware (like the 7I43(parallel) or 5i20 (pci)) counts the encoder within the card itself and can count into the mhz range. The card keeps track of the count and sends the info to emc every ms or so.

USB doesn't work well with emc. Emc wants to be in control. USB isn't realtime enough to send out and read back info consistantly enough. Realtime ethernet will probably be first before any usb device. (for it to work decent you would really have to off-load motion onto the usb device. - not a good match for emc)

sam

[samco]

What copleys are they? Do they take the industry standard +/-10? if so - you should be golden. Emc - mesa hardware (pico also might be an option) would get you a nice closed loop servo machine.

Off the top of my head - the 5i20 + 2 7I33 (analog servo interface for +/-10v) and 7I37 isolate i/o card would give you 8 axis servo + 8 output 16 input. (plus you can use the unused servo i/o)

Do your research

sam

[skullworks]

Sam's right, and also don't get caught up in "step rates" as your amps are velocity based using the +/-10V control signal. The Mesa hardware can sample encoder counts up to 1MHz and (IIRC) there are some new improvements in tracking velocity by encoder counts to make up for not having actual tach voltage feedback (which your existing setup may have).

[bad bearings]

Thanks for all the information.

I have been looking at the UBUNTU live EMC2 disk. It would definitely be an improvement over my interface--especially if someone adds crash detection.

Does the mesa 5i20 card have built in contouring commands? I have only read about trapezoidal profiles in their literature. Does EMC2 produce contours--splines etc...--by periodically updating the trajectories on the mesa board? Will this allow for the continuous motion of the stages while executing spline or circular contours?