National Instruments NI EDIDS-2400 Series, User Manual, Page: 171 / 314

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CurrentPosition is not displayed on the EPT tab because it changes very rapidly.
Position Tracking
Between each crank tooth, CurrentPosition is incremented smoothly at a rate proportional to the previous crank
tooth period. In the presence of crankshaft acceleration or deceleration, sync is still properly maintained. If a new
crank tooth is received before CurrentPosition has incremented to the correct position, then CurrentPosition is
incremented at a faster rate until proper synchronization is achieved. CurrentPosition is then further incremented at
a rate proportional to the previous crank tooth period. If CurrentPosition reaches the correct position expected for the
next tooth before the tooth is received, then CurrentPosition holds its value until the next crank tooth.
EPT Function Selection
There are four EPT function selections available in the EPT_Type drop-down list. Each EPT function type is
optimized for a particular range of number of crank teeth. For example, the DI Driver System provides two encoder
pattern EPT functions, each optimized for different ranges of crank tooth counts. The suggested tooth count ranges
are shown below in Table 4.11.b for each pattern type selection. The EPT function extrapolation value, which
is the single-digit suffix of each EPT_Type selection, will affect the CAD resolution of the internally calculated
CurrentPosition. The worst case resolution is 0.500 CAD for very low encoder pattern tooth counts.
For example, if using a 4-stroke 36-1 pattern, then the suggested EPT function is the “EPT N-M 7”. The “7”
component of the selection refers to the number of bits of binary extrapolation of crank angle between each crank
tooth. In this case, CurrentPosition would increment by 2^7, or 128 CAT, between each crank tooth. The range of
CurrentPosition would be from 0 to MAX_CAT = 2*36*128 = 9216. This is the total number of angular positional
ticks that are tracked by the EPT over a complete 4-stroke engine cycle. Therefore CurrentPosition will track
from 0 to 9216 and roll over to 0 at the occurrence of tooth 0. This provides an angular resolution of 0.078 CAD/
CAT. The calculated CurrentPosition is maintained with a 16-bit integer and cannot exceed 65535. Therefore,
NumberOfCrankTeeth should not exceed a value that causes MaxCAT to exceed 16-bit representation. The
calculated MaxCAT value is displayed on the EPT tab.
Pattern Type NumberOfCrankTeeth Stroke Extrapolation MaxCAT CAD / CAT
EPT ENC 2 (max) 3600 4 2 28800 0.025
EPT ENC 2 (min) 180 4 2 1440 0.500
EPT ENC 4 (max) 720 4 4 23040 0.031
EPT ENC 4 (min) 45 4 4 1440 0.500
EPT N-M 7 (max) 120 4 7 30720 0.023
EPT N-M 7 (min) 12 4 7 3072 0.234
EPT N+1 9 (max) 24 4 9 24576 0.029
EPT N+1 9 (min) 4 4 9 4096 0.176
Table 4.11.b, EPT function Extrapolation Values and Suggestion Tooth Count Ranges
Loss of Sync
The EPT will lose sync under normal conditions if the crankshaft speed falls below the stall speed, as specified by
the StallSpeed parameter of the EPT tab. Keep in mind that the EPT function uses a 24-bit, 40MHz timer to measure
the period between each physical tooth. Therefore, a StallSpeed should be specified such that the tooth period timer