User's Manual

PMAC User Manual
70 Setting Up a Motor
get stability using just the linear scale for both position and velocity loop because there is no direct
information about what the motor is doing. The tachometer can be connected to an A/D converter on an
Acc-28 (e.g. ADC1).
Then this conversion table entry can integrate the A/D value into what is effectively position information
for the servo loop to use. The source and process word would be $50C006; the bias term would be set
empirically to hold the integrated value constant when the motor is still. Ix04 for the motor would point
to the second line of the entry to use the integrated value.
Second, this makes it possible to do cascaded loops inside PMAC. The outer loop, which could be a
force or tension loop acting around a normal position loop, would put out a command value that is a
velocity correction to the inner position loop. The position loop can take a correction as position
information from its master position register. This conversion table entry makes it possible to convert the
velocity output of the outer loop to the position input of the inner loop. The outer loop would direct its
command output to an unused internal memory register (e.g. Y:$07F0) by setting Ix02 to that register
(Ix02=$07F0). The integrated analog entry in the conversion table uses this register as its source
($5007F0). Probably no bias is needed. Ix05 for the inner loop points to the second line of the entry,
where the integrated value is, giving the position loop its correction.
Unsigned Analog
If bit 19 of the analog conversion setup word is set to 1 ($18xxxx for normal analog, $58xxxx for
integrated analog), then PMAC treats the A/D number in the high 16 bits of the source word as an
unsigned number with a range of 0 to 65,535 instead of a signed number with a range of -32,768 to
32,767. The unsigned conversion is required for use of the newer Acc-28B. The usual signed conversion
(bit 19 = 0) is required for use of the older Acc-28 and Acc-28A.
Parallel Position Feedback Conversion
If providing position information to PMAC as a parallel data word — as from an absolute encoder or
processed from a laser interferometer — use one of the conversion formats $2x, $3x, $6x, or $7x.
Formats $2x and $3x get data from the specified source in Y-memory space; $6x and $7x get it from
X-memory space. Usually this data is brought in on an Accessory-14 board, which is in the Y-memory
space, so the $6x and $7x formats are rarely used.
If the x value in the conversion format is 0 or 1 (format word bit 19 = 0), the resulting data is shifted left
five bits so that the least significant bit of the source data appears as one count to the servo algorithms,
which expect five bits of fraction. If the x value is 8 or 9 (format word bit 19 = 1), the resulting data is
not shifted and the LSB appears as 1/32 count to the servo algorithms (see the Unshifted Conversion
section in this manual). If x is 1 or 9 (format word bit 16 = 1), the resulting data is summed with the
result of the previous table entry. If x is 0 or 8 (format word bit 16 = 0), no summing is performed.
Acc-14 Source Registers
When using Acc-14 to bring in the data, the following source addresses would be used:
First Acc-14 Port A (J7): $FFD0
First Acc-14 Port B (J15): $FFD1
Second Acc-14 Port A (J7): $FFD8
Second Acc-14 Port B (J15): $FFD9
Third Acc-14 Port A (J7): $FFE0
Third Acc-14 Port B (J15): $FFE1
Fourth Acc-14 Port A (J7): $FFE8
Fourth Acc-14 Port B (J15): $FFE9
Fifth Acc-14 Port A (J7): $FFF0
Fifth Acc-14 Port B (J15): $FFF1
Sixth Acc-14 Port A (J7): $FFF8