Delta Tau GEO BRICK LV User Manual

 

 

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If the programmed path itself introduces path error, such as the chordal error of linear interpolation, this 
must be added to the error budget as well.  In addition, if the servo-loop execution adds servo errors, these 
must also be included. 
Calculation Implications 
While smaller Isx13 segmentation times permit higher real maximum block rates and permit more 
accurate interpolation, they increase the Turbo PMAC computational requirements, particularly when 
lookahead is active.  The following table shows the result of benchmarking tests on the Turbo PMAC that 
shows the minimum segmentation times that can be used for a given number of axes executing lookahead 
calculations. 

2 

2000 

1 @ 200% 

3 1000 

1 

4 500 

2 

5 500 

2 

6 500 

2 

8 333 

3 

12 250 

4 

16 200 

5 

Notes: 
1. Tests performed on 80 MHz Turbo PMAC 
2. Tests performed at default 2.25 kHz servo update rate 
3. Tests performed with no PMAC motor commutation or current-loop closure 
4. Higher block rates can be done, but segmentation will smooth out features 

Note that subject to these constraints, the length of the lookahead is subject only to memory limitations in 
the Turbo PMAC. 
In general, the Isx13 segmentation time is set to the largest value that meets user requirements in each of 
the above three concerns.  However, it is seldom set larger than 10 msec. 

Calculating the Required Lookahead Length 

In order for the coordinate system to reach maximum performance, it must be looking ahead for the time 
and distance required for each motor to come to a full stop from maximum speed.  Because the lookahead 
buffer stores motion segments, this lookahead length must be expressed in segments. 
To calculate this value, first compute the worst-case time required to stop for each motor in the coordinate 
system.  This value can be obtained by dividing the maximum motor velocity by the maximum motor 
acceleration.  In terms of Turbo PMAC parameters: 

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Now take the motor with the longest stop time, and divide this time by 2 (because the segments will come 
in at maximum speed, which takes half the time of ramping down to zero speed).  Next, convert this value 
to a number of segments by dividing by the coordinate system segmentation time: 

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This is the number of segments in the lookahead buffer that always must be computed properly ahead of 
time.  Because the Turbo PMAC does not fully recalculate the lookahead buffer every segment, it must 
actually look further ahead than this number of required segments