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Message |
    
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 7
Registered: 12-2005
| | Posted on Thursday, June 01, 2006 - 12:10 am: | 
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Mike was commenting on his cut quality and it made me try to think of any causes not yet discussed. Lets look at the USB to Serial converter as a suspect. The communication between the computer and the controller is very critical. A standard serial port is not fast enough for the Alpha Machine. The processor in the Alpha has the ability to provide a 30kz step rate compared to the 6khz of the Prt. In order for the serial to run faster the Port needs to be faster. SBs solution was to use a Usb to serial converter. With the advent of USB few computers have serial ports. These converters are faster than standard serial ports but still limit the processors step rate to 20 to 25k. This alone makes me wonder if these are able to sustain high-speed error free transmission. These adapters are common place for PDA s and older equipment. Some do not even work with the Shopbot and specific brands are used. The Iogear brand is the standard supplied adapter and is fairly inexpensive and will provide a 20kz step rate. The Quatech is a higher quality unit and Ted claims 25k. I say claims because I have not been able to get mine over 17k. This unit must be defective. With a clean install of XP and the latest drivers, I still am at this rate. This is something that is completely invisible with out testing it as it runs. One thing to note is when you hit the top rate provided by the adapter say 21K for the IOgear. The steppers start to loose steps and can even stall. This is true for the Quatech the last 1k same thing happens. So is it possible that when you start cutting angles, circles etc. that the communication between the computer increases to the point where the adapter is near it’s limits and is failing to transmit clean signals? I think that might be a possibility.
I am running test on a high speed pci serial card from Quatech . I have no trouble getting 30K step rates. I had install problems, but the new driver I downloaded took care of that. This card is designed for high speed. Here’s a link. http://www.quatech.com/catalog/rs232_pci.php
What do you think?
Dirk |
Evan Curtis
Registered
Username: Evan
Post Number: 4
Registered: 12-2005
| | Posted on Thursday, June 01, 2006 - 01:49 am: |
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I'm way out of my element here, but I'm interested in getting a smoother cut. Are you suggesting that by installing the pci card in the computer then connecting the Alpha control box to this instead of the usb port the result will be cleaner faster signals and therefor smoother running steppers translating into a smoother cut? (pardon the run on question) And if you have already installed this pci card have you noticed a (qualitative or subjective) difference in the performance of your SB? Also is it a PRT or an Alpha? If you've found a performance increase it seems like a good investment at under $200.
Thanks |
Mike Richards
Registered
Username: Richards
Post Number: 4
Registered: 05-2006
| | Posted on Thursday, June 01, 2006 - 01:55 am: |
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Dirk,
I had no idea that the stepper pulses were being generated on the PC side. Frankly, I assumed that the controller card generated them, much like the Gecko G100/101/102 cards and MachIV. Knowing that the PC is providing stepper pulses will make me rethink my hesitation to change over to Mach software. Granted, the step pulses are also generated at the PC with Mach, but the pulse rate is 45K (on my PC). When the Gecko G100 with MachIV is deemed ready, it will easily max out the Gecko driver modules, which are rated at 200khz.
It would really be nice to have a schematic of this stuff so that expectations can matched up with reality. |
Gerald_D
Registered
Username: Gerald_d
Post Number: 102
Registered: 11-2005
| | Posted on Thursday, June 01, 2006 - 07:31 am: |
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On a side note, the chatters are not worse on curves - they are as bad on straight (inclined) cuts. We cut a 5-pointed star the other day and the only smooth faces were the 2 that happened to be parallel to an axis. Here is a thread to look at the rack&pinion as a possible contributor. |
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 8
Registered: 12-2005
| | Posted on Thursday, June 01, 2006 - 11:54 am: |
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I am only testing the step pulse at the drivers.
I think the steps are generated at controller. Here's the quote from Ted.
We clock: IOgear USB 20K max step rate
Qtech USB 28K max step rate
Qtechc High Speed Serial Card (not USB) 33K max step rate
**Dirk, please keep in mind that in curves/arcs/circles and ramps the demands for passing the speed change info to the Control Box make communications much less efficient so that you will not be able to run files at the full speeds that might be suggested by comm rates.
I don't know exactly what’s going on between the computer and controller. Ted’s comment sounded like more data was passing. Maybe getting Ted to expand on this subject would be helpful. |
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