Delta Tau GEO BRICK LV User Manual

Hardware Capture with Acc-51 Interpolators 
To utilize the hardware-capture feature on triggered moves using sinusoidal encoder feedback through an 
Acc-51 high-resolution interpolator, several additional firmware features (introduced in firmware revision 
V1.940) must be activated.  Because the capture flags must be of the same Servo IC and channel as the 
position loop feedback, Ixx25 must be set to the address of the channel on the interpolator board.  
However, the interpolator board does not provide amplifier-enable and fault flags, or overtravel limit 
flags, so variables Ixx42 and Ixx43, respectively, are used to specify different addresses for these flags 
(probably on the axis card). 

Next, because the Acc-51 produces 10 bits of fractional count data for each increment of the hardware 
counter in the Servo IC, instead of the usual 5 bits, bit 11 of Ixx24 must be set to 1 for the captured 
hardware count value to be scaled properly when converted to motor position. 

Finally, starting with Revision D of the PMAC2-style “DSPGATE1” Servo IC (introduced early in 2002) 
used on the Acc-51E and Acc-51C interpolator boards, hardware capture with sub-count resolution 
became possible.  Note that this capability is not required for hardware capture using the interpolator 
board, and probably should not be used for homing.  To activate this feature for the channel in the Servo 
IC, variable I7mn9 must be set to 1.  To use this fractional data in a move-until-trigger on Motor xx, bit 
12 of Ixx24 must be set to 1. 

Software Capture 

If use of hardware capture for trigger position is not possible for some reason, software capture is 
possible.  Software capture is specified for Motor xx by setting Ixx97 to 1 (with input trigger) or 3 (error 
trigger).  If software capture has been selected, Turbo PMAC software uses the most recent servo cycle’s 
motor actual position as the trigger position, regardless of the source, when the software notices that the 
trigger has occurred. 

When software capture is used, there is a potential delay between the actual trigger and Turbo PMAC’s 
position capture of one background cycle, which could be several milliseconds.  This delay can lead to 
inaccuracies in the captured position; the speed of the motor at the time of the trigger must be kept low 
enough to achieve an accurate enough capture.  For homing, a two-step procedure can be used: a fast, 
inaccurate capture followed by a slow, accurate capture. 

On detection of the trigger, Turbo PMAC will break into the pre-trigger move trajectory and create a 
smooth blend to the trajectory of the post-trigger move as determined by the present acceleration and 
velocity commands.  The post-trigger move ends at a pre-determined distance from the captured position 
– the method for specifying this distance depends on the type of move (see in each section below).  The 
existing trajectory is extended out for Ixx92 milliseconds (default 10 msec) after detection of the trigger; 
during this time the new trajectory is calculated.  The calculations for the post-trigger move must be able 
to complete within Ixx92 msec. 

The blending from the pre-trigger to the post-trigger move, and from the post-trigger move to a stop, will 
use the acceleration parameters Ixx19, Ixx20, and Ixx21 in force when the trigger is found.  The 
magnitude of the velocity of the post-trigger move will be the same as the pre-trigger move, if the post-
trigger move is long enough to reach that velocity.  Usually, the post-trigger move will involve a reversal 
from the pre-trigger move, but this is not necessarily the case. 

The purpose of a homing search move is to establish an absolute position reference when an incremental 
position feedback sensor is used.  The move until trigger construct is ideal for finding the sensor that 
establishes the home position and automatically returning to this position.