AN2577 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'AN2577' PDF : 34 Pages View PDF
System design
AN2577
3.2.2
Original TRR value:
720 RPM * 1/60 * 36 = 432 teeth per second => 2.315 msec/tooth => 23.15 sec/tick
Speed change over the first tooth:
5000 RPM/sec * 0.002315 sec = 11.6 RPM
Second TRR value:
(720 + 11.6) RPM * 1/60 * 36 = 439 teeth per second => 2.278 msec/tooth => 22.78
sec/tick
Second tick count:
2.278 msec/23.15 sec = 98.4 ticks
The edge arrives when the count is 98, therefore count 99 and 100 (tooth count) occur
at 0.08 and 0.16 sec after the tooth arrives. The largest tick error would be tick 98
which would occur about (100 - 98.4) 1.6 ticks (0.16 degrees) late.
Speed change over the second tooth:
5000 RPM/sec * 0.002278 sec = 11.4 RPM
Third TRR value:
(731.6 + 11.4) RPM * 1/60 * 36 = 446 teeth per second => 2.243 msec/tooth => 22.43
sec/tick
Third tick count:
2.243 msec/22.78 sec = 98.5 ticks
...which we must adjust for the high rate counts plus the error in the first tick position:
(0.16 sec + (22.78sec - 22.43sec))/ 22.43 sec/tick = 0.02 ticks
The edge arrives when the count is about 98.5, therefore count 99 and 100 (tooth
count) occur at 0.08 and 0.16 sec after the tooth arrives. The largest tick error would
be tick 98 which would occur about 1.5 tick positions (0.15 degrees) late.
As can be seen from the example, the angle clock can track a typical automotive toothed
wheel quite well under normal operating conditions. Also, the faster the speed, the less the
effect of a given acceleration rate.
Deceleration
When the angle clock completes the tick count and the next tooth does not arrive, it goes
into halt mode. Then when the next tooth does arrive it is counted and the tick count starts
again with the previous low TRR value. When a new TRR value is calculated, the tick rate is
decreased. The angle tracking under deceleration is similar to acceleration tracking.
Example
In the example above, what is the error when the wheel decelerated from 731.6 RPM to
720 RPM in one tooth time?
Second count theoretical minus second count actual:
99 * 23.15 sec - 99 * 22.78 sec = 36.6 sec => 36.6 / 22.78 = 1.6 ticks
The 99th tick arrives 36.6 sec or 1.6 ticks early. This is the largest angle error (0.16
degrees).
See also discussion of excessive acceleration and deceleration in Section 3.4: Anomalies
on page 19.
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