Moments of inertia calculations for DMM Tech servo tuning.

I buggered about with the DMM Tech servos on my CNC conversion previously and had a fiddle with the DMM Tech "DMMDRV" tuning software. 

The results were encouraging but without some super loving, self tuning function, there is clearly some work to do before I can claim the parameters are optimised.

DMM Tech published an app note showing how to calculate the MoIs of the system and how this can be translated to starting values for the Kp, Ki and Kd gains.

 Here's my spreadsheet that implements those calcs:

I ignored the trivial MoI of the small toothed pulleys and instead included the relatively large MoI of the 10" diameter handwheels. For a handwheel like this, the MoI is essentially Mr^2, whereas for a solid disk, the MoI is Mr^2/2.

The moving mass for the X axis is the table, vise and work. For the Y axis, it is the table, vise and work - plus the saddle. However, with a ballscrew pitch of 5mm, the effect of even this mass is relatively small in terms of total MoI, particularly with a 2.5:1 reduction.

The final outcome is the "Inertia Ratio" which is the ratio of the MoI of the system to the motor. From that it's possible to read off some starting values for the P, I & D gains, namely:

• Inertia Ratio = ~4.9
• Kp = ~78
• Ki = ~24
• Kd = ~28

Next time - try these out.....