11CONT-0080

Economic Solution for Data Acquisition in a Formula SAE

Race car

Kumar Saurav, Hemen Gogri, Arati Phadke

K.J.S.C.E

Copyright © 2011 SAE International

ABSTRACT

Formula SAE is a student design competition in which a prototype race car is to be evaluated for its potential as a production item. This paper covers the design and development of a cost-effective DAQ (Data Acquisition) system for a Formula SAE race car. The system is capable of logging data from a maximum of 33 sensors and a logging rate of 36Hz has been recorded for 18sensors. The system controls radiator fan and coolant pump based on the sensor readings. The paper further discusses the basic sensor requirements and filter designs for obtaining consistent data. A comparison is drawn between two DAQ systems developed during the same year, to highlight the difference in accuracy and logging rate due to variation in circuit design.

INTRODUCTION

Data acquisition is the process of collecting real time data from a wide variety of sensors and converting it to values which can be interpreted or manipulated by analysis software. When it comes to fine tuning a race car for gaining maximum performance in terms of acceleration, braking, handling and fuel economy, DAQ systems play an imperative role. Without DAQ systems, analyzing and verifying vehicle design would have been impossible.

The DAQ systems currently being used by a majority of Formula student teams are either expensive (DL2 Data Logger priced around 2,085$, used by Dartmouth Formula Racing [2]) or heavy (NI CompactRIO weighing minimum 1.58kg, used by Formula Manipal [3]). This paper covers the development of a DAQ system to aid teams with limited budget in satisfying their data acquisition requirements.

The primary design goal of the 2010 ORI (Orion Racing India) electronics team was to make a robust and reliable DAQ system which aids vehicle analysis and quick

troubleshooting. At the same time the system had to be light-weight, flexible and cost-effective to ensure maximum points at the Formula Student competition. The overall development time of this system was drastically reduced due to use of various simulation software and computer aided design programs.

DESIGN PROCEDURE

To reduce overall development time while maintaining reliability of the final output, a well-defined design procedure was adopted. The circuit was initially modeled and its behavior simulated on a simulation software namely Proteus ISIS (Figure 1). Smaller and much simpler circuits which had to be incorporated into future PCB designs were tested on breadboards. The first versions of PCBs usually require debugging and testing, hence low cost prototype boards were fabricated to ensure proper operation (Figure 2). After few revisions, the final PCBs were fabricated which would be used on the car. This procedure further assisted in identification of many redundant circuits.

Figure 1 - Frequency to voltage convertor simulation in Proteus ISIS

Figure 2 - Prototype PIC development board

Based on team requirements and feedback received from senior members, the electronics system for the 2010 car was

developed into two distinct yet interconnected modules,

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namely DAQ Unit and Dash-Board Unit (Figure 3). CAN (controller area network) bus was used for communication

between the two units.

Figure 3 - Electronics 2010 block diagram

SENSOR REQUIREMENTS, CALIBRATION

Sensors are responsible for conversion of various mechanical parameters into measurable electrical quantities. The sensor selection for every team changes with its design goals. Typical sensors implemented by most teams include three axis accelerometers and sensors to detect wheel speed, shock travel, steering angle, manifold air pressure, coolant temperature, throttle position and gear position.

ACCELEROMETER

When a race car moves with a high velocity on a race track, it experiences g-forces in lateral and longitudinal directions. These forces have a particular threshold value under a given set of conditions, beyond which if the car is accelerated, the forces may increase so much so as to cause loss of traction. To prevent this, the g-forces need to be monitored and the threshold levels identified.

The MMA7260Q, low cost three axis capacitive micro machined accelerometer was selected to monitor the vehicle accelerations. It features signal conditioning, a single-pole low pass filter, temperature compensation and selectable sensitivity (1.5g/2g/4g/6g). Its capacitive approach offers several benefits when compared to the piezoresistive sensors used in many other accelerometers, notably a wider temperature range and response to DC as well as dynamic vibrations. The sensor outputs voltages proportional to the experienced g-force.

WHEEL SPEED SENSOR

All major dynamic parameters in a race car are related to the speed of the vehicle. The theoretical and practical values of parameters such as longitudinal and lateral acceleration, slip angles at different speeds and various steering angles are all

validated using wheel speed sensor readings as an essential input.

Wheel speed is sensed using an NPN type inductive proximity sensor (Figure 4). As a tooth on the wheel passes by the sensor, a pulse is generated in a recurring manner. The pulse train will increase in frequency as the vehicle speeds up and decrease as the vehicle slows down. Therefore, a frequency to voltage converter, LM2907 was used to convert the generated frequency into a corresponding output voltage.

Figure 4 - Wheel speed sensor mounted on car

Low cost inductive proximity sensors have a maximum switching frequency of approximately 600Hz. Hence, assuming maximum vehicle speed to be 120km/hr, the number of teeth in the toothed wheel was selected to be 30. Figure 5 depicts the response of the circuit developed for conversion of wheel speed to voltage.

Figure 5 - Wheel speed sensor response

SHOCK TRAVEL SENSOR

Suspensions play a key role in the vehicle dynamics of a race car. Knowledge of the linear displacement of the shocks for a particular damping setting is critical in evaluating the damping performance in various loading scenarios.

Shock travel sensors used to measure these linear displacements are available in two types: wire wound and

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conductive plastic. Due to un-availability of conductive plastic sensors in the local market, wire wound linear displacement potentiometers were used. The sensors are mounted parallel to the shock absorbers of the race car. The calibration graph of the shock travel sensors is linear.

STEERING ANGLE SENSOR

While maneuvering a race car through chicanes and hairpin bends, it is a critical requirement for the driver that the power from the steering wheel of the race car is efficiently transmitted to the front wheels. At times the car under steers or over steers, which may gradually lead to change in vehicle dynamics, and even mitigate the performance of the vehicle.

A steering wheel angle sensor is used to accurately measure the angle through which the steering wheel is rotated by the driver. A rotary potentiometer was housed inside the steering rack housing and attached to the steering column to measure the steering angle. Figure 6 shows the calibration chart of the steering angle sensor.

Figure 6 - Steering angle sensor response

GEAR POSITION SENSOR

It is a basic requirement in any car, that the gear it is currently in be indicated. For the purpose of gear position indication, a small gear is meshed with the gear drum in the engine. A high quality potentiometer shaft is rotated accordingly by the gear drum using this arrangement. Distinct voltage levels can be obtained for each position, which are then fed to the ADC (Analog to Digital Convertor) channel.

FUEL LEVEL SENSOR

Knowing the amount of fuel in the tank during testing and competition will not only provide information about the efficiency of the engine but will also help determine if any leaks exist in the fuel system and will enable the driver to decide whether or not the race car has enough fuel to finish a race.

A typical fuel gauge comprising of a float that is mounted on the end of a pivot arm was used as a fuel level sensor. The resistance of the fuel gauge changes as the float moves up and down with the fuel level. Errors introduced in the readings due to fuel slosh were reduced by implementing a moving average filter with a window size of 15.

COOLANT TEMPERATURE SENSOR

The engine coolant temperature, which is a direct indicator of the engine temperature, needs to be monitored and accordingly the coolant pump and radiator fan have to be controlled to get optimum speed/torque performance from the engine.

Two negative temperature co-efficient thermistors with brass housing and a temperature range of -40o C to 135o C (Figure 7) were used to measure the coolant temperature. A potential divider network was implemented to get the required output voltage swing. A second order logarithmic curve was fitted to the response of this sensor.

Figure 7 - Coolant temperature sensor

SUPPORTING HARDWARE

COOLING CONTROL BOARD

Every engine is designed to work at maximum efficiency in a certain temperature range. The devices which can be controlled to maintain the engine in the specified temperature range are the coolant pump and the radiator fan. The cooling team provided us with three temperature set-points: 75o, 80 o and 85 o C, which can be selected by the driver.

The hardware used to regulate the engine temperature comprises of a Goodsky GU-SH-112DM relay, used to control the radiator fan and an IRFZ48 power MOSFET, used to control the coolant pump (Figure 8). The control signals generated based on the set point are as shown in Figure 9.

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Figure 8 - Cooling control board

Figure 9 - Cooling control board behavior

POWER CARD

A noise free, light weight power supply is an essential requirement of a portable DAQ System. A Power Card based on low cost linear voltage regulators and equipped with features like surge voltage protection (using Metal Oxide Varistors), over current protection (using Polymeric Positive Temperature Coefficient fuses), reverse voltage and over voltage protection (using silicon avalanche diodes) was implemented to satisfy this requirement(Figure 10).

Figure 10 - Power card

TELEMETRY

Key vehicle parameters, if communicated via wireless transmission to the pit crew may enable quick error detection and troubleshooting. To provide a wireless link between the DAQ Unit and the pits, XBee-Pro RF Modules were selected. These modules communicate with the DAQ units via RS-232. The outdoor range provided by the modules was about 1.5km in line of sight.

DATA ACQUISITION UNIT V1

The DAQ v1 was built around a PIC18F4680 microcontroller (Figure 11). It was designed as a stand-alone system, so as to be exempted from the cost report at the competition. The PIC18F4680 runs in HSPLL (High Speed Phase Locked Loop) mode under which a PLL multiplies the oscillator output frequency of 10MHz by 4 to get maximum possible operating frequency of 40MHz. The10bit ADC channels on PIC were expanded using analog multiplexers (CD4052). A temperature sensor in the form of LM-35 was included to ascertain that the operating temperature was within the hardware limits. CAN interface was provided using MCP2551 CAN transceiver. The firmware for DAQ v1 was developed using embedded C.

Figure 11 - DAQ v1

The CP3838 GPS (Global Positioning System) module was selected to satisfy the vehicle tracking requirements. Data recorded from the GPS receiver played a significant role in validating sensor data. The track shown (Figure 12) was plotted using co-ordinates recorded from the GPS receiver. Interface to an MMC/SD card for logging sensor and GPS data was provided via SPI (Serial Peripheral Interface). The data is stored in as comma separated value file format which can be easily imported into MATLAB for analysis. A logging rate of 33Hz was recorded for 14 sensors interfaced to the module.

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Figure 12 - GPS track plotted using logged data

DATA ACQUISITION UNIT V2

The DAQ v1, although a fully functional stand alone system, was reliant on GPS for generating time-stamps. The 10bit ADC of PIC18F4680 could not provide the degree of precision required by the team. In addition, the analog multiplexer introduced a series resistance of 270ohm and a delay varying from 500-1000ns (nanosecond) on the signal acquisition line. Hence, a real time clock (DS1307) was introduced on-board to get time-stamps at higher frequency and three 12-bit ADCs (TLV2553) were interfaced via SPI to get a higher resolution of the sensor data. The DAQ v2 (Figure 13) also features fourth order Sallen-key based low pass filters and transient voltage suppressors for noise suppression and anti-aliasing. A logging rate of 36Hz was recorded for 18 sensors.

Figure 13 - DAQ v2

Analog filtering was used to remove extraneous noise from the signal lines. In the absence of an analog filter, signals outside half of the sampling bandwidth of the ADC are aliased back into the signal path. Once a signal is aliased during the digitalization process, it is impossible to differentiate between noise with frequencies in band and out of band [8]. The signal-

to-noise ratio of the 12-bit ADC is 74dB and the bandwidth of interest for the analog signal is DC to 1 kHz. Hence, a 4-pole low pass filter was designed to provide an attenuation of 80dB/decade with a cut-off frequency of 1 kHz (Figure 14).

Figure 14 - 4th order low pass Butterworth filter design

CONCLUSION

This Data Acquisition System will be used for data collection by the future FSAE teams. It will also provide a platform for development of faster, complex, in-house designed data acquisition units. The module is extremely robust and reliable which has been concluded by rigorous testing. The logged data can be easily imported into tools like MATLAB for analysis. The system boasts of user-friendliness, flexibility in terms of interfacing multiple sensors and is cost-effective when compared to off the shelf products. The cost of the DAQ unit and sensors combined is approximately 250$.

A possible improvement is to implement the circuitry using SMT (Surface-Mount Technology) and integrate the power card on the same circuit board to reduce the overall system size. The X-bee pro can be substituted with alternate transceivers like Maxstream 9XTend or Microhard n920 for an extended range.

REFERENCES

1. SAE International, “Rules & Important documents”, http://students.sae.org/competitions/formulaseries/rules/, Aug. 2010.

2. Christopher Kane, “Testing and Tuning a Formula SAE Racecar”, Dartmouth Formula Racing, 2006.

3. Khan, S.; Sonti, S.; , "Data acquisition system for a 600cc formula SAE race car," Vehicular Electronics and Safety (ICVES), 2009 IEEE International Conference on , vol., no., pp.46-49, 11-12 Nov. 2009doi: 10.1109/ICVES.2009.5400316

4. Dogan Ibrahim, “Advanced PIC microcontroller projects in C”, Newnes, 2008.

5. Gregory T. French, “Understanding the GPS”, Georesearch, 1996.

6. André Lehmann; Andreas Grebner; Christian Pfeiffer, “The Electronic System of the FP208”, Special issues ATZ-MTZ, 2008.

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7. Microchip, “Compiled Tips ‘N Tricks Guide”, http://ww1.microchip.com/downloads/en/DeviceDoc/01146B.pdf, 2009.

8. Mya Thandar Kyu, Zaw Min Aung, Zaw Min Naing, "Design and Implementation of Active Filter for Data Acquisition System," icime, pp.406-410, 2009 International Conference on Information Management and Engineering, 2009

9. mikroElektronika, “mikroC user’s manual”, http://www.mikroe.com/pdf/mikroc/mikroc_manual.pdf.

10. Rouelle, C., OptimumG, personal communication, Mar. 2010.

11. NMEA reference manual, SiRF Technology, Inc.

CONTACT INFORMATION

Kumar Saurav

kumarsaurav9@yahoo.com

E-7/803, Runwal Estate, Behind R-mall, Manpada, Thane (w)-400607, India.

+971 55 175 2422

ACKNOWLEDGEMENTS

The authors would like to thank the 2010 Orion Racing India team and Orion Racing India alumni for their considerable support throughout the project. We express our gratitude towards MathWorks India, Silicon components and Pankaj potentiometers for sponsoring this initiative.

DEFINITIONS/ABBREVIATIONS

Formula SAE Student design competition organized by the Society of Automotive Engineers

DAQ Data AcquisitionPCB Printed Circuit BoardCAN Controller area networkg-force Acceleration of a body

relative to free fallUnder steer and Over steer

Terms used to describe the sensitivity of a vehicle to steering

ADC Analog to Digital ConvertorRS-232 Recommended standard-

232, an asynchronous serial communication method

MOSFET Metal-Oxide-Semiconductor Field-Effect-Transistor

PLL Phase Locked LoopGPS Global Positioning System

MMC Multimedia CardSD Secure Digital

SPI Serial Peripheral Interfacens NanosecondExtraneous noise Noise in the out of band

frequenciesksps Kilo samples per secondSMT Surface-Mount Technology

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