Designing and Implementing a Chassis Control Module for a Formula Type Vehicle Using myRIO

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"The versatility and compatibility of the myRIO unit helped us integrate it alongside the ECU using CAN. This opened up the possibility of interfacing with engine parameters in a much more coherent and efficient way. "

- Dennis Honkanen, Lund Formula Student Engineering

The Challenge:
Designing and implementing a chassis control module for a formula student vehicle that offers easy driver interaction, shift and clutch control, and high-performance data logging.

The Solution:
Using the myRIO device as the main unit in a rugged and high-performing system that continuously logs data from 27 sensors and performs real-time analysis and control to simplify driver interaction and ultimately improve lap times at competitions.

Author(s):
Dennis Honkanen - Lund Formula Student Engineering

Figure 1. Lund University’'s formula student car performing the skidpad event at Hockenheim (Pictures, Courtesy of Formula Student Germany)

Students at Lund University were building their team’s sixth formula student vehicle to compete at Silverstone and Hockenheim in the summer of 2014. They needed a chassis control module (CCM) that could complement the engine control unit (ECU) and offer extensive data logging and rugged control capabilities.

We followed three design criteria for the CCM: extensive data logging, simplified driver interaction, and extreme reliability. Due to the amount of vehicle testing planned, data logging and data management were important to simplifying and optimizing test procedures. By easing and minimizing driver interaction with the vehicle, the driver can focus on the track and improve lap times. For reliability, driver input and control must work flawlessly during operation. Shift and clutch control are essential for vehicle operation. If shifting or clutch actuation fails, the vehicle might have to forfeit the race.

Figure 2. The steering wheel includes shift paddles, clutch actuation lever (not visible), various multipurpose buttons, and a driver feedback display.

System Description

The system consists of two parts with different requirements. The real-time control part, responsible for clutch control, gear engagement with spark cut, brake light triggering, and driver interfacing, focuses on response time and stability. Alongside that part is a high-rate data acquisition system that logs and, to some extent, processes data from 27 sensors for wheel speed, damper movement, accelerometers, and driver behavior, among others. The main focus is to handle all incoming data in an effective way.

Figure 3. A simple schematic of the system shows some of the inputs and outputs.

Hardware Considerations

Embedded systems with different requirements are often separated in hardware to make sure that the timing requirements for the real-time control are met regardless of variations in data rate from the sensors. We considered other solutions, including a simpler ~20 MHz microprocessor running the real-time control due to its deterministic properties and running the data acquisition on a CompactRIO controller. Because of the real-time properties, we realized we could use the LabVIEW reconfigurable I/O (RIO) architecture to run both subsystems with no timing problems or deadlocks. Since weight is a major concern in formula student cars, we decided not to use CompactRIO and instead  chose the myRIO embedded device and LabVIEW software for our final deployment since the myRIO is lighter.

Our systems use 33 of the ports on the myRIO, excluding power and ground connections, which makes the hardware connector types important. They must be reliable and easy to connect. The standard connectors on the myRIO meant that we could connect the PCB containing all sensor prescaling and filtering using a flat cable connector. This simplified installation, alterations, and reparations significantly.

An automotive system must withstand quite an amount of physical abuse, including vibrations, impacts, and a wide temperature range. This creates other hardware concerns. We were most concerned about the vibration part of the system since the 450 cc one-cylinder engine produced substantial vibrations in the chassis, and every component on the control system must withstand this. Several hours of tests showed that the myRIO fulfilled these criteria.

Versatility, Compatibility, and Fast Software Iterations

The versatility and compatibility of the myRIO unit helped us integrate it alongside the ECU using CAN. This opened up the possibility of interfacing with engine parameters in a much more coherent and efficient way. We took advantage of the LabVIEW environment for fast software development and easy reconfiguration between software iterations. This was crucial during testing when we needed to make small software changes in between testing runs. This is partly due to the workability in the LabVIEW environment, but also due to the WiFi capability of the myRIO. We did not have to open up enclosures and physically connect the myRIO with the development laptop, which decreased the time between code iterations out in the field. The myRIO boot duration was about 10–15 seconds due to the combined FPGA and processor startup. This was not a problem for us, but we considered it a minor inconvenience.

Conclusion

The hardware properties of physical robustness, computational power, and embedded functions (A/D converters, selectable pull-up resistors, and more) for the myRIO definitely meets the requirements for our system. We only experienced one minor hardware issue with our implementation of myRIO. The relatively long startup time for the system meant that we needed to implement power backup systems to handle eventual shorter voltage drops, as occur if the engine misfires during startup. From the software side, LabVIEW offered a fast development process with easy debugging and many built-in real-time programming considerations that probably lowered the bug count substantially.

Overall, the NI platform met our expectations. We successfully designed and implemented a CCM for steering wheel-mounted driver inputs that reduce driver fatigue, and rugged and reliable shift and clutch control. Extensive testing (more than 500 km) showed that the myRIO could withstand the harsh automotive racing environment. Our success would not have been possible without the support of the LabVIEW educational community.

Figure 4. Lund University’s formula student car competes in Hockenheim (Picture Courtesy of Formula Student Germany)

Author Information:
Dennis Honkanen
Lund Formula Student Engineering
Sweden
Tel: +46 708 89 55 80
dennishonkanen@gmail.com

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