AN2577 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'AN2577' PDF : 34 Pages View PDF
System design
AN2577
3.3.3
size can also indicate acceleration or deceleration of the wheel. If the size of tooth period is
approximately twice the previous period, this might indicate a gap, or a severe deceleration
of the wheel.
A common way to ensure that a gap has been detected is to confirm the measurement on
the following tooth edge. Whatever test is used to identify a long period can be applied in
reverse to the following period to test for an ABA pattern of periods. The presumption here is
that the wheel would not make a sudden sharp deceleration, followed one tooth later by a
sudden sharp acceleration. Whether this or any other test is valid for a given system must
be determined by the systems engineer. The ABA test is considered valid for automotive
applications because of the significant inertia of the crankshaft.
EXAMPLE
Four consecutive tooth edge times are found to be:
Capture[1] = 0x1B238A
Capture[2] = 0x1CBA70
Capture[3] = 0x1FAB33
Capture[4] = 0x211145
Subtracting adjacent tooth times yields the following periods:
Period[1] = 0x196E6
Period[2] = 0x2F0C3
Period[3] = 0x16612
We can use a “Gap Ratio” algorithm to test if this represents a gap detection.
Use Gap_Ratio = 0.7
Is_Gap = if ((Period[2] * Gap_Ratio > Period[1]) && (Period[3] < Gap_Ratio * Period[2])
Period[2] * Gap_Ratio = 0x203BB
This is larger than Period[1] and Period[3]...
...therefore the Is_Gap condition is satisfied.
Starting the angle clock
Once the gap has been found and verified, the application knows the position of the wheel at
the time of the last tooth edge. The angle clock can now be initialized with this angle. There
are two considerations for the systems engineer at this point. First, if the application is
synchronizing to a 720 degree cycle, there may or may not be sufficient information to
determine the correct half of the engine cycle. Additional information might be required to
completely synchronize the angle clock to the wheel. Second, there is typically some time
lost between the detection of the last edge and the software conclusion that the wheel angle
is verified. If the angle clock is arbitrarily started at this time, there may be some error in the
indicated angle.
If the angle clock is required to track a 720 degree cycle, then it is possible that the
information on which half of the cycle the gap was detected is not yet available to the
application. While full synchronization may not have been achieved, the application can
declare “Half_Sync.” This means that the crank shaft position is known but the cam shaft
position may subsequently require the shaft angle to be corrected by 360 degrees. In an
automotive application, Half_Sync is sufficient to operate the engine using a wasted spark
and split fueling strategy. If the system is operated in a half sync mode until full sync is
possible, it is important to consider the effect on the system of the angle clock getting a
sudden 360 degree correction if this should be required.
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