CNC Bridgeport mill for sale



Kind of negative. I meant to say that servos have some inherent advantages over steppers, the biggest of which is the ability to close the control loop in software, using encoder feedback.
This is why the world has moved on, and most everyone is using servo technology right now.

Right. This is a servo drive with a closed loop, and easy computer interface that looks like a stepper, so you cna use Mach3.
But what the computer does not know is what is the current following error, so it cannot make decisions involving following error over several dimensions.

What is this electronic gear, is that for threading?

EMC is pretty fancy also.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 6/10/2012 22:27, Ignoramus15748 wrote:

It is just possible to match the pulse output capability of the controller and the servodrive.
At my spindle, I would need 3000RPM * 2000ppr / 60 min/s = 100kHz pulse rate at step input to get full speed. With 100:1 electronic gear, I get the same speed with 1kHz pulse rate, which is realistic with PC parallel port.
Of course, the spindle angular accuracy is also just 2000/100 pulses per revolution. Works for me as I just need the spindle servo control for running tool changer (which is a EMCO hack where tool changer carousel runs with spindle servo motor through a gear touching spindle gear only in tool change position of Z, outside normal Z range).
This kind of little machine (google image found this link, not mine) http://www.ebay.com/itm/EMCO-CNC-Mill-VMC-100-EmcoTronic-TM-02-/190679550995 I replaced the right electronics unit with a little box of modern electronics, bolted to the side of the main unit.
Kristian Ukkonen.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 10 Jun 2012 15:47:19 -0500, Ignoramus15748

pic is still in the garage. Thought you gave that space back to your boss.
Karl
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yes, I moved it to my warehouse.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

    In my opinion, the drivers which accept step and direction pulses to run a servo motor throw away some of the benefits of servo motors, including smooth motion and the ability to traverse intermediate values without leaving steps in the workpiece. After all -- even though the motor is capable of intermediate positions, the controller is not capable of commanding them to reach those positions if it has to treat them as steppers.
    Enjoy,         DoN.
--
Remove oil spill source from e-mail
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 11 Jun 2012 03:29:58 GMT, the renowned "DoN. Nichols"

What difference does it make if the steps are small enough?
Digital commands will always have some finite resolution, even if it's nanometers.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
snipped-for-privacy@interlog.com Info for manufacturers: http://www.trexon.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

    [ ... ]

    Because the steps may *not* be small enough. This is certainly the case with my Emco-Maier Compact-5/CNC lathe. A little lathe made mostly for teaching CNC machining. The minimum step (from the program point of view) is either 0.01mm or 0.001" depending on the position of a front panel switch. The steps are in metric units, so the inch units wind up with patterns like:
    0.001-0.002- -0.004 0.005- -0.007
And -- to make it worse, the cross-feed steps have to translate into diameter, so a slow taper like a Morse is visibly stepped.
    And if you are going to say -- finer steps and step faster, that machine *can't* step fast enough as it is. As an example, when threading with a fairly coarse thread, you have to slow down to 180 RPM (or sometime slower) on the spindle so the CPU can keep up with stepping the longitudinal feed at the right times. The CPU is an ancient 6502, and has little enough memory so finer steps would reduce the nubmer of program lines possible -- because the input is fixed format.

    Of course. But steppers run fast can introduce resonance in the leadscrews -- and the same applies to servos pretending to be steppers.
    Oh yes -- the fineness of the steps with a servo pretending to be a stepper is limited by the resolution of the encoder on the servo's shaft. Used as a true servo, the computer simply outputs a voltage saying "go so many RPM" and the servo amp takes care of keeping that happening. The computer simply checks every so often to make sure that the speed is right, and if it is a little too fast or slow, it changes the output voltage just enough to fix that speed. And this gives a smooth travel between the two end points, even in the areas where the encoders don't have resolution to tell the difference.
    The only disadvantage to true servo operation that I can see is that if the computer hangs up (say a Windows box which experiences BSOD -- Blue Screen Of Death), each axis will keep going at the last commanded speed -- until it hits a limit which should be wired to shut it down fully. With steppers, a hung computer will stop outputing pulses, and the motor will stop. Likely a spoiled job in either case, but less potentially exciting with the steppers.
    And with servos in home machines, it is likely being controlled by what used to be called EMC, and is now LinuxCNC, which runs in a real time engine under the linux OS' kernel, so a hang is less likely.
    Enjoy,         DoN.
--
Remove oil spill source from e-mail
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 6/12/2012 6:58, DoN. Nichols wrote:

There is a solution for this also, pulse input that requires pulses from the controller all the time, and if pulses stop (controller hangs), it disables drives. My mach3 system is built like that. In Mach3 it is called "change pump" output.
Kristian Ukkonen.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

In other parlances, that's called a "dead man signal". Common on all mission-critical computers.
The name comes from the pedal or lever that railroad engineers had to keep depressed all the time for the locomotive to move. If they fell off the "dead man switch", the engine stopped.
Lloyd
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Lloyd E. Sponenburgh wrote:

Unless the engineer put a big piece of wood on it to hold it down.
My dad was a trainman and I rode the engine a few times as a boy.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 12 Jun 2012 22:48:58 +0300, Kristian Ukkonen

I believe you meant "charge pump", Kristian.
-- Doctors prescribe medicine of which they know little, to cure diseases of which they know less, in human beings of which they know nothing. --Francois-Marie Arouet Voltaire, about 250 years ago
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

    But since Mach3 does not do "true servo" operation -- only servos pretending to be steppers, so if the computer hangs up, Mach3 stops producing pulses anyway.
    The "servo pretending to be a stepper" is accomplished by a counter controlled by the step and direction pulses, and another one by the encoder on the servo motor. The Gecko servo driver is an example of this design.
    Typically, there will be a circuit comparing the count in each of the two counters, and producing a voltage proportional to the difference between the two counts. The step and direction pulses get a burst to move to a higher value, and the voltage difference starts the motor moving (speed dependent on the difference) which starts counting encoder pulses until the two are equal again. This circuitry is totally separate from the computer, so if the computer hangs it stops outputting pulses, so there is no problem. (This unless the computer is asked to be part of the driver too, which I have not yet encountered.
    So the only place where I would expect this problem to show up is when using a servo as a true servo -- controller outputs a desired speed voltage, not pulses, and the servo amp (driver) simply runs the motor at that speed until it is changed.
    As for the hanging problem, computers designed to run unix have in their hardware a "watchdog timer" -- the OS pokes at it once every so many milliseconds, and if it does not get a poke in so-many + 10% or so milliseconds, it generates a pulse which resets the computer, starting a reboot, and hopefully, the card generating the speed command voltages will sense that reset and adjust the output voltages to zero. But in any case, the axes with true servos should have limit switches to shut down the servo amps to prevent crashes. Again -- something which does not depend on the computer.
    Enjoy,         DoN.
--
Remove oil spill source from e-mail
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.