XoomBox

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Monday, 25 April 2011

XoomSpeed datalogger/traction control

Since our first event back in 2006, we always wanted to know more about what the car was doing and why one run was faster than another. We had a look around at what was commercially available but felt everything was either not powerful enough, or too expensive for our budget. Since the team includes a lot of expertise in harder and software development, the obvious solution seemed to be to build our own..

Development of the system started in the second half of 2007. Testing of the datalogging was completed in time for the last event of the season in October.

2008 saw the continuing development of the firmware, and the launch and traction control functions were added and tested at the events through the season.. Writing the documentation also started in August '08, and the latest version of that can be viewed and/or downloaded here.

For the 2009 season, 4 more XoomBoxes were built and supplied to other competitors on the Scottish sprint/hillclimb circuit. GPS logging was added, and the functionality of the analysis software, XoomCentre, evolved to provide power display and analysis tools.

At the end of the 2009 season, a new board was produced, slightly smaller and incorporating a no. of modifications we wanted in the light of experience gained with the original.

Anyway, here's the spec.

Datalogger specification

Analogue inputs - 8 x 10bit inputs range selectable 0..2.5V or 0..10V. 6 of the inputs are available on external connectors, the other 2 are internally connected to +/- 2g xy accelerometers.

Frequency inputs - 6 available. This gives us the ability to record all 4 wheel speeds, the engine speed (via the tacho output from the ecu) and also injector frequency and duration.

GPS recording. The RS232 port enables logging from any GPS receiver with standard NMEA output. GPS time, latitude, longitude, altitude, velocity and heading are decoded and recorded from the NMEA output

Shift light - 7 LED segments lit in sequence as the engine rpm approaches the red line. These LEDs are also used to indicate status such as "launch control armed', memory card invalid or read only and so on.

Rev limiter - programmable soft and hard engine cut.

Data Recording - SDRAM card, 1GByte as standard, FAT16 or FAT32 filing system, so can share data with PC and camera or even mp3 players. Importing recorded data to a PC is as simple as copying a file. Logging rate is fixed at one record every 40ms, ie a record is written to the SDRAM card 25 times/second. Each record contains the current value of all 8 analogue channels and the most recently acquired frequency from each of the 6 frequency channels.

Launch control/traction control/flat shift

Having decided to put that lot into the design, it seemed rather obvious that enough info was available to perform launch control and traction control as well. Accordingly outputs were added to the design to permit individual switching of 4 fuel injectors. The circuitry has been designed to permit individual injector pulses to be suppressed and give fine control of the engine's output.

Launch control has two distinct functions.

When sitting at the start line, regulate the engine revs to a preprogrammed value, allowing the driver simply to push the accelerator to the floor. Monitor the undriven wheel speed, until a set speed is reached.
After the car has reached the transition speed, compare the engine speed with the speed of the undriven wheels during acceleration. Regulate the engine's power output to control the amount of spinning of the driven wheels.
Launch control finishes the engine speed reaches a final target value.

Traction control is easier to describe, but just as complicated to implement.

Monitor the speed of both driven and undriven wheels. If the driven wheels are spinning more than a certain amount faster than the undriven wheels, then reduce the engine's output to limit the wheel spin.
This process is complicated when cornering. It is necessary to monitor the left hand and right hand wheels independently and change the control strategy when cornering.

X10der Modules

For the V2 XoomBox, we've added the ability to fit an extension module inside the case. The design of the X10ders means that they can provide additional data channels, included both in the datalog files, and also visible on the PC's live data display. X10der modules already available include

4 additional analogue inputs
Selected gear display for a sequential gearbox. Also provides 3 free analogue inputs.
Pneumatic control for semi automatic, clutchless gear shifts. Also provides selected gear display.

Data Analysis

Being able to view the collected data in helpful ways is critical to the success of the project. Development of the data analysis package (PC based) is essentially complete. Key features are be

Ability to import logfiles from multiple runs and multiple vehicles. The system is able to maintain configuration information for multiple vehicles. When importing a logfile for analysis, the file is automatically associated with the correct vehicle, so that the correct calibration coefficients are applied for that installation. A single analysis file can contain multiple logfiles recorded throughout the day.
Traces from multiple logfiles can be viewed on a single graph - we can now see exactly how David and Stephen's use of the throttle pedal differs, and what difference it makes to speeds at various points round the track.
Traces can be viewed either against time, or against distance covered since the start.
Traces may be independently scaled, and time shifted relative to one another, so we can see why Stephen's 1st lap on his 1st run was faster or slower than a lap from another driver possibly in a different car.
GPS data used to plot the track the car followed. Replay 2 or more runs on the same diagram, or watch the car(s) moving round the track as the you move the mouse over the graph window.
Export graphs, traces or entire logfiles either as pictures (paste into word processor) or as csv (paste into a spreadsheet for further analysis).
Derived traces. If you've got the wheel speeds, then you can calculate slip on the left hand side of the car, on the right hand side, or between axles. A no. of such calculations are built into the system, but we also provide a programming environment to enable you to perform your own calculations and then include the results in graphs just as if they had been measured directly by the logger.
Each time a logfile is imported, a set of calculations can be run automatically. Our standard setup removes unused traces, deletes data more than 1 second before the start of the run (determined by examining the accelerometer channels) adds a calculated vehicle speed channel (averaged from the wheel sensors) and adds a slip calculation between driven and undriven axles. If you're using GPS logging, then calculation of x,y coords from latitude and longitude together with total distance travelled is also performed automatically. Finally, a standard set of graphs is created from the data.
Additional data. As well as the raw input signals, the datalogger also records other information about what it's doing. An obvious one is the degree of engine throttling being applied during launch/traction control. X10der adapters add their own additional data channels.

It was an ambitious project, but as of June 2008, all of the above functions have been tested and verified. More info and results are available from the links at the side of this page. The download of the XoomCentre software includes real data files collected from our car in competition. The photos below show the first batch of boards for the original 2007/8 prototypes.