Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Rotary Axis Travel Limits?
#1
In the Turbo PMAC for a high resolution rotary position axis operated in continuous motion, there are travel limits when using special lookahead would cause discontinuities in the rotary velocity due to apparent changes in velocity due to loss of LSB resolution in the floating point calculations. This occurred long before the reaching rollover of the binary position register. Does the Power PMAC have any similar issues?
Reply
#2
Floating-point representations, with fixed mantissa width, inherently lose resolution when the magnitude gets larger.

To illustrate with the decimal equivalent (that we call scientific notation):

1.234567 E0 has a resolution of one millionth of a unit, but 1.234567 E4 has a resolution of one hundredth of a unit.

Power PMAC uses 64-bit floating-point representations, with a 53-bit (52 + 1 implied) mantissa. The older Turbo PMAC uses 48-bit representations, with a 36-bit mantissa.

The additional 17 bits of the Power PMAC representation provide resolution 2^17 (131,072) times greater than the Turbo PMAC for any value. So you can go over 100,000 times further in Power PMAC before you reach the same issues. I don't think anyone has come close to noticing any issues like this in Power PMAC.
Reply


Forum Jump:


Users browsing this thread: 1 Guest(s)