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Application Development
4.2.1
4.2.2
4.2.3
Fuel and Spark Data Tables
As a means to input data used for fuel and spark values, an Application Map Tool based on a spreadsheet
is provided. This tool provides the essential functionality for translating fuel and spark data tables into
content that can be placed into the example software. Specific engine management data can be placed
into the tool using engineering units. This table is then converted to microcontroller units in a C-source
friendly format. Map table sizes can be adjusted to meet application requirements. The Application Map
tool is identified as “Map Tool.xls”. Additionally, reference for an example map is provided in “Scooter
Map.xls”. This provides an example of a completed map as used in the example application.
Modifying Table Sizes
As a first step, the table size should be customized to accommodate the performance and data
requirements. This is accomplished by adjusting the number of load points and RPM points in the table.
In the empty map provided (Map Tool.xls), this is done by changing number of and content of the load
row (green) and the RPM column (yellow) values. Both the number of load and RPM values directly
impact the size of the table and speed at which the table look up is performed.
While more data points gives you better tuning ability, it will increase the size of the application and
increase the worst case time to perform the table look up. Another factor used for sizing the tables is
available data. If a legacy map is used then the simplest starting point is to directly reuse this map. If a
new map is to be created by empirical data through testing, a smaller map is the best starting point.
Fuel and spark maps are independent of each other and the load and RPM points must be customized
for both sets of data. Using the “Fuel Engineering Units (ms)” and the “Spark Engineering Units (BTDC)”
worksheets, enter the desired number of points and values for each point for the load row and RPM
column. Load is input as a percentage from 0 to 100% in ascending order, left to right. RPM is input from
0 to your max RPM in ascending order, top to bottom.
When determining your max RPM, you should consider the performance of the engine as well as the
resolution of the software. For the example application software, a fundamental timing unit is 1.6 μ s. This
means that the highest resolution between RPM measurements is 1.6us. However, RPM, or engine
speed, is determined from the tooth period measurements on the engine's flywheel. This means is that
while the engine is rotating at a given RPM, the measurement taken is at a fraction of this rate.
For example, at 6000 RPM, an engine completes one rotation every 10 ms. The engine controller
monitors position of the engine through the teeth on the flywheel. Each engine will have a specific number
of teeth. For this example the engine has 12 teeth. The result is that the engine controller will measure
the time between two teeth at 6000 RPM as 833 μ s. Looking at our fundamental timing unit, the software
will provide a measurement of 520 (really 520.8 but quantization results in 520).
At 6000 RPM, there is not much sensitivity due to the 1.6 μ s timing unit as there is a count of 520.
However, as the RPM and number of teeth increases so does the sensitivity. This concept is important to
understand and also is relevant for low RPM conditions as well. At low RPM maximum time that can be
measured is 104.5 ms. For the 12 tooth engine example, this would correspond to 47 RPM.
Configuring Data Translation
Before entering any data, the parameters used to translate engineering units to MCU units must be
properly set. This must be done on two worksheets: “Fuel MCU Units (Tics & Counts)” and “Spark MCU
Units (BTDC & Tics).” At the top of these two worksheets are five parameters that each must customized
to each engine system.
Small Engine Reference Design User Manual , Rev. 3.0
Freescale Semiconductor
19
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