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  1. #81
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    More Good News,

    the seller of the first Servopack (the one that didn't work) has been extremely nice to deal with, and will compensate me in full. If you read this - thanks!

    Arvid

    Last edited by arvidb; 04-16-2004 at 03:59 AM.


  2. #82
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    arvidb- Welcome! I'm a newbie to this board too. You are going to build a converter for step and direction to ±10vdc? If you can accomplish that, I'll trade you some linear bearings and a ballscrew for your lower axis. When the lower axis rail is fully supported, you are less likely to have vibration vertically. I am retrofitting a bridgeport CNC machine, and the controller is dead, but the servo's and drives are 100%. Plus the machine is only 10 years old. The amplifiers that I have are just like yours. The only low cost controllers out there are designed for stepper (step and direction), not the other industry standard of servo (±10vdc input). I work at a company that tosses out lots of linear ways, ballscrews, even motors and drives. Let me know if you are interested.

    By the way, the machine design looks pretty good. One thing that I would suggest is the way you are mounting the duplex angular contact hearings on the end of the ballscrews. Instead of clamping a thin sheet metal looking piece between the bearings, usually they are put into a metal block with a flat bottomed bore, and clamped that way. Usually the bearings are designed to have the inner races touch when you tighten the nut on the ballscrew. The outer races are then squeezed axialy by a clamp plate with a hole in it. If I can figure out how to attach a picture, I'll show you what I mean. For reference, take a look at our catalog. It has lots of tips about servo vs. stepper, and it might help to take a look at some of our standard products. www.neat.com
    [url=http://www.cnczone.com/gallery/showphoto.php/photo/329][img]

    Take a look if this works!

    Neatman

    NEATman
    Basic research is what I'm doing when I don't know what I'm doing. Wernher von Braun


  3. #83
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    Neatman,

    I'm very interested!

    I have some questions though:

    1) How long are you ready to wait for me to finish this thing? (As you can see from my title I'm a Time Optimist )
    2) What's the resolution of your encoders, and how fast do you need to spin your motors?
    3) Since you work with these things I guess you're aware that you'll not get the same performance and features as with an industrial fully closed loop servo system? (The position loop will only be closed to the "converter" card.) That is, no following error graphs, no graphing utilities to help set PI parameters, no feed-forward etc.

    The graphing utilities could be implemented I guess, but I will try to keep it as simple as possible to start with.

    What I have in mind is an FPGA that counts pulses from the parallell port and encoders and compares them through a PI controller. The result would be fed to DACs for +/-10V output. I would probably also include a binary step multiplier so that higher resolution encoders can be used while still getting ok feed rate.

    What do you (all) think about this? What other features would be required to make it work? For instance, how is homing normally done? It would be neat to just move until a limit switch is reached, and then back off until the encoder index pulse is hit - an easy way to get a repeatable home position. But how would the communication between g-code interpreter and hardware look? Guess I have some reading to do

    Arvid



  4. #84
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    ... and thanks for the tips on how to mount the bearings! Unfortunately I don't have access to either a lathe or a mill, so I have tried to design it so that it can be made with normal hand tools. Do you think it could work as I drew it?

    Arvid



  5. #85
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    Default Controller card

    Hi again,

    I've been doing some work on my controller card. But I wonder a bit, what inputs and outputs are useful?

    The card will be set up and controlled via RS232. It will be run with step/dir signals from the parallell port.

    In normal "run" mode it will listen for step/dir signals and encoder signals and make sure they agree by telling the motors how fast to spin. It will also monitor for positive and negative overtravel and disable the servos in case those signals are hit. It will send out error count over RS232 so that one can see how well the actual position coincides with commanded position, and it will map overtravel and home signals to the parallell port somehow - please help me out here to what's practical, I mean, there's four (five?) inputs on the port and three signals per axis that want out...

    There will also be a "home" mode where one can home and reset the counters for the individual axes via commands over RS232. This mode will use limit switches and the encoder index pulse to find a reliable home for each axis. Will this be compatible with the different controller programs (i.e. is it possible to just zero their counter after an "external" homing has been performed?)

    I will also put in a "set-up" mode that will hopefully help set the parameters for the PID controller - here the motors will be run open loop so that one can watch their step response.

    So all in all, the inputs and outputs to the card will be:
    * A, B, Z encoder inputs for each axis
    * pos, neg overtravel inputs for each axis
    * a home switch input for each axis (if one would like to home with the parallell port controller program without the aid of index pulses - or for Z maybe this could be used for a 0 plate?)
    * +/-10V motor speed outputs for each axis
    * One Servo Enable output that will be active as long as there is no fault condition (will be tripped by encoder errors, excessive position errors, overtravel - anything else? )

    Sorry for another one of my long posts... and please comment! What would you be missing if this were your controller card?

    Arvid



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Arvid's first CNC machine!

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