National Instruments NI DIDS-2103 User Manual page 113

CrankCount
During sync, the EPT function provides the latest crank tooth count at the
0. Tooth 0 is determined according to the pattern that is being tracked and is discussed within the Pattern-Specific
Information section below.
were no missing or extra teeth in the pattern. For example, if the pattern type is N-M, NumberOfCrankTeeth = 60 and
Stroke = TRUE, then CrankCount
Crank Position
During sync, the EPT function calculates the latest crankshaft position in terms of absolute Crank Angle Ticks (CAT)
over a complete engine cycle, referenced to position 0. It is internally converted to a crank angle position in terms of
Crank Angle Degrees (CAD), referenced to position 0, so that injection angles may be specified in CAD. Position 0
is determined according to the pattern that is being tracked and is discussed within the Pattern-Specific Information
section below. The CurrentPosition in CAD is calculated according to the following:
CurrentPosition(CAD) = [CurrentPosition(CAT) * Stroke * 360] / MAX_CAT
Where Stroke = 2 if 4-stroke, and Stroke = 1 if 2-stroke.
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
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 4.6.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
calculated MaxCAT value is displayed on the EPT tab.
Pattern Type
EPT ENC 2 (max)
EPT ENC 2 (min)
EPT ENC 4 (max)
EPT ENC 4 (min)
CrankCount is provided in terms of a complete engine cycle and is reported as if there
will range from 0 to 119.
should not exceed a value that causes
NumberOfCrankTeeth
3600
180
720
45
CrankCount
CrankCount is for monitoring purposes only.
EPT_Type drop-down list. Each EPT function type is
MaxCAT to exceed 16-bit representation. The
Stroke Extrapolation
4 2
4 2
4 4
4 4
NI DIDS-2100 Series User Manual | © National Instruments | 113
indicator, referenced to tooth
MaxCAT CAD / CAT
28800 0.025
1440 0.500
23040 0.031
1440 0.500