AC servo conversion on CNC Patriot VFD machine - Page 2

Page 2 of 3 FirstFirst 123 LastLast
Results 13 to 24 of 33

Thread: AC servo conversion on CNC Patriot VFD machine

  1. #13
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    It's been awhile, but i'm been working on the conversion here and there. I made a mounting plate from 3/8" thick mild steel plate for the x-axis biggest motor (nema42 frame) using the shopmaster. The others are nema34 and are a direct swap. This was my first milling experience and first on the shopmaster. The biggest hurdle I had was quill vibration (solved by partial tightening the quill stop). Also the spindle speed index marker magnet came off, so I took the one from the lathe and put in on the mill. I ordered a new magnet and put it on for the lathe.

    I had great success with getting accurate encoder readings in mach3 from the glass scale encoders by shielding the tapped wires and grounding each shield to the corresponding axis shield and then all three axes together and then grounding the shielding on the computer end well to the computer ground.

    I believed these drives had the capability of the sought after dual loop (position and velocity feedbacks from resolver and glass scale on shopmaster). I had a breakthrough and was able to get the dual loop to work with the drive. I'm able to make minimum steps of 0.005mm or steps of any factor of 0.005mm because of the glass scale resolution and servo drive capability. Because of the mm glass scale I need to use mm in mach3, otherwise there's a small error that accumulates. The max step input frequency for the pacific scientific drives I have is 500Hz, so with a 25um step size the max speed should be 30in/min....I was able to get close to 20in/min...I'm not sure why it isn't faster. These tests were done with the drive only and a parallel port on/off program for the step/dir signals. For working with mach3 the pulse width needs to be increased from 15us (mach3 max) because of a low pass filter of 20us time constant on the drive input. I'm thinking an arduino board to do the conversion to longer pulse width to see if this will get the drive to read the pulses.

    To improve the safety, I want the emergency stop button to disable the drives as well as cut the main power instead of just cutting the main power. I also want a separate switch for enabling/disabling the drives.

    I'm very happy with the progress and thanks for reading.

    Attached Thumbnails Attached Thumbnails AC servo conversion on CNC Patriot VFD machine-big_motor-001-jpg   AC servo conversion on CNC Patriot VFD machine-big_motor-002-jpg   AC servo conversion on CNC Patriot VFD machine-big_motor-003-jpg  


  2. #14
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    I was able to get 30 in/min speed by changing the servo software program to have an even step size since fraction step size doesn't seem to affect distance. I ordered an arduino board and relays for improving the safety shutdown. I was able to get a response in mach3 using active low; however the response was dependent on pulse width and 15us still isn't long enough for accurate functioning....but it does respond to mach3 input, which is a step forward.



  3. #15
    Registered
    Join Date
    Jan 2008
    Location
    usa
    Posts
    458
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    I've been following your post, but the electronic talk is way beyond my understanding. However, my old Patriot machine could get 40 IPM with the original stepper and the 2-1 drive ratio. Switching to a 1-1 would get you 60-80. It seems that with that monster servo, you should be flying to the point of being scary.



  4. #16
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    Thanks for following! The reason is because of dual feedback loops. Using a single regular velocity feedback loop, very fast speeds are possible. I, however, am attempting to improve positioning accuracy using the patriot glass scales as feedback, which should in theory remove nearly all errors from the ball screw and belt drives.



  5. #17
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    The max. input frequency (steps/sec) is slower for the dual loop setup (i want to set each step to 25um or ~0.001"). With 500 steps/sec, that gives 30"/min. With the single loop velocity feedback setup, the max. input frequency (steps/sec) is much much faster around 1,000,000 steps/sec. Yes, so with this motor very fast acceleration and speeds are possible.



  6. #18
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    I added relays for disabling the drives when the emergency stop button is pressed. I also added a switch so I can easily disable the drives when needed. Still waiting on the arduino board.



  7. #19
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    I got the arduino board for ~$20 (Duemilanove 2009 AVR ATmega328 p-20PU USB version) a couple days ago. It works to take the mach3 pulses and extend the width from 15us to 1500-2000us. When using this between the computer and SC954 drive, I am able to get a huge improvement in reading response. I'm using 20um for a single movement step, and with this ~20in/min is the max. speed. I was trying out the accuracy by jogging at different speeds and just random movements, and it's quite good; it says within about 0.003" (80um). Under certain manual pulsing conditions the error accumulates to 0.004-0.008", which is higher than I want with this setup. I'm thinking it's either out of sync pulse to what the drive will read or something to do with the servo tuning and how it responds under certain conditions or it could be noise in the arduino board. I'm going to try and change the control scheme of the drive to absolute instead of incremental to see effects the performance under mach3 pulse control. What i'm wanting to get is something that is accurate to about 20um or 0.001" along the whole axis travel and all speeds from 0 to 20"/min. This has turned out to be very challenging, but very rewarding. Thanks for reading.



  8. #20
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    I tried the absolute positioning scheme of the drive (SC954 Pacific Scientific) and the same errors accumulated under certain pulsing conditions.



  9. #21
    Member
    Join Date
    Sep 2013
    Posts
    288
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    This is a very elaborate and technical endeavor! very neat stuff here. I hope it works out and will try to stay tuned on this one! Using the output of the glass scales for your positioning was a really neat way to go. On another site, there are a few guys who went with the Magnetic tape and readers by Renishaw for their positioning and it worked out well.
    Keep us posted!
    CG



  10. #22
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    After a spending my time on other projects, I am working on finishing the conversion. I finished the x-axis conversion listed above. I tuned the motor and it responded pretty much on the mark within +-20um. However, when I send the motor random and more erratic jogging motion some position error accumulates, and so far I have not been able to identify the source. I measured the mach3 output and the pulse widening (via arduino MCU) and both are consistent with Mach3 readout. Maybe it's a synchronization error between arduino output (mach3 pulses) and drive pins reading (which is 500Hz). I did order a replacement servostar drive from ebay as an alternative. But first I may try to find for sure if it's a synchronization error and if so then synchronization the two.



  11. #23
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    I wasn't able to get the dual loop position feedback working on the SC954 so I bought two Servostar CD 2 series drives to try. Those drives turned out to be not usable for such input positioning speeds (even though the position loop update rate was 2kHz). I'm think that these two types of drives just aren't meant for that type of input speed under the dual loop configuration. A newer drive may work (kollmorgen recommended a newer drive). For now I got the single loop motor feedback working well in no time with the servostar cd on the x-axis. I'm going to hookup the other two motors next. I'm happy with the speed and resolution of the x-axis. So I have 2 extra drives...and i'm going to use one to replace the lathe motor (as the low speed torque is not the best). Going to sell the other one on ebay or here if anyone is interested SC933 for $200. Just a preview; I was thinking of going with one of those water cooled chinese spindles (3kW) as a replacement for the mill spindle (0-24krpm). That's all for now.



  12. #24
    Registered
    Join Date
    Sep 2006
    Location
    USA
    Posts
    177
    Downloads
    0
    Uploads
    0

    Default Re: AC servo conversion on CNC Patriot VFD machine

    I installed the y-axis motor and it is working well with single loop resolver feedback. One motor to go.

    Attached Thumbnails Attached Thumbnails AC servo conversion on CNC Patriot VFD machine-servo-002-jpg   AC servo conversion on CNC Patriot VFD machine-servo-003-jpg  


Page 2 of 3 FirstFirst 123 LastLast

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

AC servo conversion on CNC Patriot VFD machine

AC servo conversion on CNC Patriot VFD machine

AC servo conversion on CNC Patriot VFD machine