All 128 lasers are fired in groups of eight and recharged every 53.3 μs on average. The cycle time for a firing group of eight
lasers is 2.665 μs. In a firing sequence there are 16 firings (16 × 2.665 μs) plus two maintenance periods. After the first
eight firing groups, the first maintenance period occurs, taking 2.665 μs. The second set of eight firing groups then take
place. The firing sequence ends with the second maintenance period taking 7.995 μs. Therefore, the timing cycle to fire
and recharge all 128 lasers is given by (16 × 2.665 μs) + 2.665 μs + 7.995 μs = 53.3 μs. Note that TOH is aligned with the
fourth firing group in a firing sequence. This means that timing offsets for firings before that point will be negative.
Figure 9-5 Firing Sequence Timing
VLS-128 lasers are fired in groups of eight at widely separated angles to minimize IR crosstalk. The sequence of firings is
specified in
Figure 9-5 above
and
Figure 9-8 on page 63
.
To calculate the exact time, in microseconds, of each data point in a packet, first number the points in the firing sequence
from 0 to 127. This becomes the data point index (aka Laser ID) for your calculations. Next, number the firing sequences 0
to 2. This becomes your sequence index.
The timestamp in the packet indicates the time of the 4th firing group in the packet. You’ll need to calculate a time offset for
each data point, add that offset to the timestamp, and then adjust that value by 8.7 µs. See
Figure 9-6 on the next page
for
specific Data Point Timing Offsets (Single Return Mode) and
Figure 9-7 on the next page
for specific Data Point Timing Off-
sets (Dual Return Mode).
The offset equation is given by:
FiringGroupIndex = (DataPointIndex / 8) + (DataPointIndex / 64)
TimeOffset = (53.3 µs * SequenceIndex) + (2.665 µs * FiringGroupIndex)
Note that the division operator above is an integer division, leaving an integer result with no remainder.
To calculate the exact point time, add the TimeOffset to the timestamp.
ExactPointTime = Timestamp + TimeOffset - 8.7
Chapter 9 • Sensor Data 61
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