Need help adding an adjustable offset to a 0-10V DC signal

Discussion in 'The Projects Forum' started by ChrisAttebery, Jul 3, 2014.

  1. ChrisAttebery

    Thread Starter New Member

    Jul 3, 2014
    8
    0
    Hello,

    I'm hoping that someone here can help me out with a small circuit design issue. I'm an IC layout designer by trade and it's been 20 years since I attended tech school so I can't even claim to be rusty. ;)

    I'm working on controlling the spindle speed on my CNC mill. I have a speed control card that outputs a signal from 0-10VDC to tell the spindle motor controller how fast to turn the motor. The problem is that the spindle motor controller needs some voltage just to get the spindle turning, around .7 volts. So I really need 0-5V output to be .7-5.7V. The speed control card has a top end voltage adjustment, but there isn't any way to adjust the low end output. The speed control card uses a 12VDC single sided supply.

    I thought that a non inverting summing op amp would do the trick. I built a circuit using the LM741 op amp but I've since realized that the LM741 was a bad choice. It wasn't designed to swing rail to rail so the lowest voltage I can get out of it is about 2V. I've attached a drawing of the schematic.

    I either need to find another op amp that can swing 0-10v off of a 12v supply or come up with another solution. Can one of you help me out?

    Thanks,


    Chris
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,059
    Maybe you could add a diode to lower the "ground" of the spindle controller to 0.7V below the "ground" of the control card. So 0V at the card is 0.7V at the spindle controller.
     
  3. eetech00

    Active Member

    Jun 8, 2013
    650
    112
    hi

    I wasn't sure what you needed. You mentioned input 0-10, then mentioned 0-5v :confused: . But anyway, how about this. You could change R2 and R4 to pots to adjust gain and output level.

    eT :)
     
    Last edited: Jul 4, 2014
  4. ChrisAttebery

    Thread Starter New Member

    Jul 3, 2014
    8
    0
    The problem is that the op amp I selected cannot produce a rail to rail output. I got a suggestion on another forum to try a LM324. I'll pick one up this week and try it out.

    Thanks,


    Chris
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    I don't get it.. Its really working as it should.. You DON'T want the spindle to be turning at all when the speed control card is turned (assuming its a knob or something on it) as low as it should go (aka 0V out to spindle)

    If its not a knob on this speed control card what is the control to it to tell it how much voltage to send to the motor controller?
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    The LM324 is a single supply op amp (input and output go to the negative rail) but the inputs and outputs don't work to the positive rail. If you want that you need a full rail-rail op amp such as an LT1637. The are many other rail-rail types available such as here.
     
  7. ChrisAttebery

    Thread Starter New Member

    Jul 3, 2014
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    0
    The motor controller needs a little voltage just to get the spindle turning, about 0.7V. If I was adjusting the speed manually it wouldn't be a problem, but the CNC software isn't smart enough to have an adjustment for the the low end voltage. The speed controller card also has a relay to disable and enable the motor controller, so when the speed is set to zero the spindle is disabled. There isn't any feedback to the CNC software, so it doesn't realize that the rpm at the spindle doesn't match what it has requested. The software assumes that the motor controller is a VFD that can handle all of these trim issues, but I'm using a "dumb" DC motor controller (KBMM)

    Voltage CNC RPM Motor RPM
    0 0 0
    1.0 1000 400
    2.0 2000 1400
    3.0 3000 2600
    4.0 4000 3800
    5.0 5000 5000

    In order to correct this I wanted to use the adder to offset the output of the card by .5-.7V. The adder works, but it can't swing rail to rail, so the output starts at 2V.

    Anyway, I think I have a solution now. I'll buy the LM324 and try it out this week if I have a chance.


    Chris
     
  8. ChrisAttebery

    Thread Starter New Member

    Jul 3, 2014
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    0
    crutschow,

    I have a 0-12v supply available. I only need about 5V max at the output. I'll keep that in mind though.



    Chris
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    or just put some black tape over the RPM display on the motor controller and problem solved ;)

    doesn't the RPM display have an adjustment (trimpot or something).. I'd think so..
     
  10. ChrisAttebery

    Thread Starter New Member

    Jul 3, 2014
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    0
    The problem is that the RPM has to match or the chip load per tooth is off. With as big of a mismatch as exists right now the machine would break tools left and right.
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    simply adjust your calcs for the proper chip load. :cool:
    you know the RPM correction factor..

    There is more to breaking tools than just chipload too.
    Just get to know your specific machines capabilities and you won't break tools.
     
  12. ChrisAttebery

    Thread Starter New Member

    Jul 3, 2014
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    How about we just get the machine working like it's supposed to? I've been running this machine for 18 months with manual spindle control. The whole point of this exercise is to get full control of the spindle via CNC. It would be less PITA to just keep it manual than to start having to convert all of my speeds. :rolleyes:


     
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The 324 will fix you up.
     
  14. ChrisAttebery

    Thread Starter New Member

    Jul 3, 2014
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    The 324 worked like a charm. I was able to get the spindle calibrated to within 50rpm over the entire 500-4500rpm range. :D
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,565
    2,379
    The KBMM and most of the KB drives have a MIN and MAX pot setting?
    IOW, once you enable it, the drive would run at whatever the MIN setting was, regardless of pot or input at zero.
    Are you using I1 & I2 to enable the drive?
    Max.
     
  16. ChrisAttebery

    Thread Starter New Member

    Jul 3, 2014
    8
    0
    The CNC4PC C6 card that I am using creates a 0-10V signal to feed the P2 pin on the KMBB. The pots are not used in this set up.
     
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