Technical Details
Ndrive HP 10/20/30 User’s Manual
3-8
www.aerotech.com
Table 3-10.
PSO Output Sources
PSO Output Type
Maximum
Frequency
Standard or
Option
Requires User
Isolation
High Speed Opto-Isolator - J301
Table 4-14
-IOPSO option
No
Opto-Isolated Output 11 - J205
1 KHz
Standard No
RS-422 Marker output on
Secondary Encoder - J205
10 MHz
Standard Yes
The pre-scaler in the block diagram is used to normalize the resolution of axes with
different machine step sizes or to scale down (divide) the maximum input frequency to
the PSO tracking hardware. The PSO hardware operates in machine counts, so all axes
must be scaled, or normalized to like units. This allows a trigger to be generated from a
true vectorial position change. Each of the three possible tracking channels has a pre-
scaler, which may be used to divide the number of encoder counts on that channel. Each
pre-scaler defaults to 1 and may divide the input feedback pulses by up to 1,023.
The trigger condition may be a software trigger under program control, various other
hardware triggers, or most typically a user-defined change in vectorial position, on one to
three axes. This vectorial position change is monitored via hardware, allowing an output
pulse to be generated in less than 275 nanoseconds (200 nsec. for single axis) after the
trigger is detected, not including the propagation delay of the output device. This is
accomplished via hardware that monitors the designated axes positions, calculates the
sum of the squares of the axes positions and compares it to the desired (squared) position
trigger value. All of the mathematical squaring of the positions occurs in hardware for
speed. The comparison of the squared command to feedback position occurs at an 8 Mhz.
rate for multi-axis firing and at a 20 MHz. rate for single axis firing. Furthermore, this
vectorial position trigger value is queued via a 255 level queue, in hardware to the
triggering hardware, with each trigger advancing thru the queue to the next trigger value.
This allows the trigger value to be specified as a series of incremental trigger points, if
desired.
The absolute accuracy is excellent, although, point-to-point accuracy may vary, due to
the calculated feedback squared firing distance jumping past the (internally squared) user
programmed firing distance. This small error is accumulated and subtracted from the next
firing distance, maintaining absolute accuracy. For example, if the user programs a
trigger to occur after a vectorial change in position of 5 machine steps. Internally, this is
squared, and the axes positions are internally squared and compared to 25. If they are
equal to or greater than 25, a trigger event occurs. The comparison occurs at an 8 Mhz.
rate (125 nano-seconds), so, should the sum of the square of the axes change in position
be equal to 9 during a sample period, no trigger occurs. However, on the next sample, the
sum of the squares of the axes positions could now be equal to 36. A trigger event would
now occur (36 is greater than 25) and the remainder (11) is then stored to be summed
with the squared feedback positions during the next sample period.
The output pulse is also user programmable. It may be a single, or multiple pulses per
trigger event. It too is generated in hardware and fed from a 255 level queue, allowing
trigger events to advance thru the queue, varying the output pulse per trigger event.
The window modes allow firing to occur, or be enabled, based upon axes being within a
user-defined window. This window may be one or two-dimensional. The +/- and
Enter/Exit Detection block within the block diagram is used to prevent false triggering
due to one bit dither on an axis, etc. Window modes may not be pre-scaled as indicated
in the block diagram.
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