Roland Jollivet wrote: > Hi > > I have been wondering the last few days on an 'open' data format for > controlling low cost, user built servo's. > > The scenario of using a 'proper' closed loop servo for a small mill is very > appealing over a stepper. > While parallel ports are almost free and sufficient for steppers, to > implement a servo it looks like you have to spend order of $100, mostly > because of the encoder reading, which is a big jump. > > I was wondering on the viability of a PIC (or other) system driven off the > parallel port. An 18F24J10 per axis(~$2) could handle the PWM and quadrature > in. (At 40MHz, probably two axis) The micro would have to count encoder > pulses, set the PWM out, and write/read it's designated data on the bus at > it's allocated slot. Another micro would handle I/O, directly with a 40 pin > chip, or multiplexed. > > John Kasunich gives an excellent response, but didn't mention connectors. I use the CHEAPEST connectors I can find anywhere (except IDC headers) on my 4-axis servo controller. Do you realize that is over $30 in connectors on that little board? The FPGA does digital input, digital output, 4 axes of encoder counters and 4 axes of PWM, plus interfaces to a spindle speed DAC. The FPGA only costs $12, so it is certainly equivalent to the 3 or 4 PICs you'd use. The PC board costs $15 in modest quantity. The total cost of all parts is $97, not counting assembly. You can see I'm definitely NOT getting rich selling these at $250.
What you describe, generally, is what I sell as the Universal PWM Controller, a slight variant of my Universal Stepper Controller (the difference is one makes constant-frequency pulses of varying width, the other makes varying frequency pulses of set width.) See http://pico-systems.com/univpwm.html for a picture and more description. And, getting good throughput and reliable transfers on the average parallel port of arbitrary computers is NO small task! I've been through HELL and back maintaining this product line what with non-compliant IEEE-1284 implementations of Taiwan motherboards, 5 V vs. 3.3 V parallel ports, IDE vs. PCI par-port chips, and so on and on. Jon ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
