AEM 30-71XX Infinity Stand-Alone Programmable Engine Mangement System Full Manual - Manual (Page 278)

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© 2014 AEM Performance Electronics
Infinity User Manual
there are several considerations to take into account when calculating the desired Noise
Cancellation values.
The shortest duration between edges is the first value to determine by examining the trigger
wheels or signal outputs from the Crank Position Sensor and Cam Position Sensor(s). This
means that either the shortest tooth or the shortest gap between teeth needs to be used in the
calculation (whichever is shorter in duration (degrees). What is important is the minimum time
between edge transitions, either high to low, or low to high within the entire trigger wheel. For a
60-2 even spaced trigger wheel, the minimum tooth duration is 6 degrees (360/60). However the
tooth is actually high for 3 degrees and low for 3 degrees. Therefore the useful value here is 3
degrees. This value will be used in Column D in the table below, which uses a 60-2 trigger wheel
as an example.
The second important variable specific to each engine is the maximum Engine Speed (RPM)
that the engine will spin to. This is because the duration of each degree of trigger wheel (in
microseconds) will be reduced as the engine spins to a higher RPM. The maximum desired
RPM needs to be used as the deciding factor, and can be seen in Cell A12 in the table below.
Use the table below to calculate the desired Noise Cancellation value for the Crank Position
Sensor for a 60-2 trigger wheel. The RPM values in Column A represent maximum desired RPM.
Assume for this example that you want to spin the engine to 9500 RPM. Therefore you will use
Row 12 to calculate the Noise Cancellation value.
From left to right, the RPM is used to calculate degrees per millisecond, the milliseconds per
degree, then microseconds between edge transitions, and finally Noise Cancellation values
based on 3.2us per increment. At 9500 RPM and 3 degrees between edge transitions, each
transition will happen in 52.6 microseconds. Therefore you can have a maximum Noise
Cancellation value of 16 (16 * 3.2us = 51.2us).
Another important consideration when determining Noise Cancellation values is that the values
calculated from the table above above assume a noise-free signal. More specifically, if noise
occurs at some location in between edge transitions, the effective tooth duration is split up by the
noise. If a single noise impulse occurs 1.5 degrees into a 3-degree tooth duration, then you will
effectively have two 1.5-degree teeth widths. Therefore the calculation above is only a guideline
and should not be used at the maximum limit. The actual Noise Cancellation value may have to
be 2-3 times smaller than the maximum allowed value to compensate for random noise injected
into the signal. To understand this idea more completely, refer back to the figures above.