Delta Tau GEO BRICK LV User Manual

 

 

Turbo PMAC systems can accept signals from sinusoidal (sine/cosine) encoders and generate position 
data of very high resolution.  This is done through special accessory boards and dedicated firmware 
algorithms.  Presently there are two classes of interpolators: low-resolution ones producing 128 or 256 
states per line, and high-resolution ones producing 4096 states per line. 

Many Turbo PMAC users will utilize sinusoidal encoders for position feedback, 
but with the interpolation function performed in an external interpolator module, 
which generates a high-frequency synthesized digital quadrature signal for the 
controller that is connected to the Turbo PMAC.  In this case, the Turbo PMAC 
treats the feedback just as if a real digital quadrature encoder were used. 

The sine and cosine channels of the encoder are connected either as single-ended or differential inputs 
(differential is strongly recommended) into the interpolator accessory according to the instructions for the 
interpolator.  Optionally, the index channel may be connected as well.  Consult the hardware reference 
manual of the particular accessory for details. 

If connecting a low-resolution interpolator into a Turbo PMAC or accessory with PMAC-style Servo ICs, 
the fractional-count data is brought into the Servo IC at TTL levels on the flag signals for the channel 
numbered one higher than that of the encoder signal itself.  Normally, these signals are 12-24V isolated 
flags to use the interpolator, the isolators must be removed and replaced with conducting shunts. 

Encoder Sampling Clock Frequency: E34 – E38, I7m03, MI903, MI907 

For the low-resolution interpolator, the Turbo PMAC’s SCLK encoder-sampling clock drives the analog 
conversion on the interpolator, as well as the encoder functions in the Servo IC.  This clock must be set to 
a frequency of 2.46 MHz.  If the interpolator is connected to a Turbo PMAC or accessory with PMAC-
style Servo ICs, this frequency is set by jumpers on the PMAC or accessory: of the E-points E34 – E38, 
there should only be a jumper on E36. 

If the low-resolution interpolator is connected to a Turbo PMAC2 or accessory with PMAC2-style Servo 
ICs, Servo IC m’s variable I7m03 sets this frequency.  If it is connected into a MACRO Station, Station 
variable MI903 or MI907 sets this frequency.  These variables also set other clock frequencies, but if the 
other frequencies are left at their default (as they usually are), setting the SCLK frequency to 2.46 MHz 
simply requires changing the variable value from its default of 2258 to 2260. 

For an Acc-51x high-resolution interpolator, the SCLK signal just drives the sampling of the synthesized 
quadrature in the interpolator’s own Servo IC.  The default frequency of 9.83 MHz is virtually always 
fine.  On the Acc-51P, the frequency is set permanently to this value.  On high-resolution interpolators 
with PMAC2-style Servo ICs (e.g. Acc-51E, Acc-51C), IC variable I7m03 sets this frequency if the 
interpolator is connected directly to a Turbo PMAC, or by MACRO Station variable MI903 or MI907 if 
the interpolator is installed in a MACRO Station. 

Encoder Decode Control: I7mn0, MI910 

Both styles of interpolator create a digital quadrature signal from the sine/cosine inputs.  This digital 
signal goes into the encoder decoding and count circuitry to produce the whole-count data.  To match the 
whole-count and fractional count resolution properly, the decode-control variable must be set up to times-
4 quadrature decode.  This variable is I7mn0 for Servo IC m Channel n in a Turbo PMAC system.  On a 
MACRO Station, it is node-specific variable MI910.