In trying to keep with the KISS principle, I've decided that the AC servos are much too complicated and fancy for our CNC mill. I picked up these DC servos from the post today. They are 90 VDC servos with 1000-line encoders. I got them from Dan Mauch/Camtronics.
To further keep things simple, the idea is to directly couple the motor to a 2.5mm/rev rise ballscrew (HiWin, from metallstore.de).
This will give plenty of feedrate, the motors are rated at max 3000rpm which corresponds to 7500mm/min. I doubt we will need that much ! The lower end might be problematic, since motion could get jerky when there are not enough encoder counts coming in...
The motors are rated for 1.6 Nm of continuous torque. The maximum push force can be estimated using
where F is the force (N), T torque (Nm), eff efficiency (I've asumed 80%), and L the ballscrew lead. If I plug in these numbers I get a bit over 3000 N which should be enough.
Here is the US Digital 1000-line encoder. When counting each edge, we get 4x1000= 4000 edges per revolution, or 0.000625mm of movement for each count. We're aiming for about 0.01mm accuracy on final parts, which means the motion control needs to keep within a 16 count window (or +/- 8 counts) of the commanded position at all times.
The motors will be driven by PWM Servo Amplifiers from Jon Elson/Pico-systems. The PWM signal for the amps is generated by the M5i20 motion control card from Mesa Electronics. All of this is ofcourse controlled by EMC.
One thought on “DC Servos for CNC mill”
Thinking about maximum achievable acceleration:
The 3000 N of force will be used to move the Table and the spindle assembly. Assuming that they weigh about 50kg, the maximum achievable acceleration should be:
a= F/m = 3000N/50kg = 60 m/s^2 ! wow, that's fast...
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