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Baja Telemetry System Electrical Engineering, in collaboration with Mechanical Engineering, University of Arkansas Participants in the Mini Baja SAE competition design, build, and race a vehicle for harsh, off-road conditions. Many of the design goals for this year were to make the vehicle lighter and stronger. Real-world data is required to make informed, elegant design. This real-world data is precisely what the Baja team lacked. Arduino based telemetry system. Credit Jason Bailey and Toren Caldwell A University of Arkansas Baja Racer racing during last year’s competition in Oregon. Credit: University of Arkansas Mini Baja SAE Baja Telemetry Schematic Need and Background Approach and Benefits Realization and Results • High Signal Integrity: Schmitt triggers provide clear HIGH and LOW signals. High impedance, buffered inputs provide accurate voltage readings. • Logic Level Shifting: 1.8V or 3.3V logic interfacing with 5V sensors and USB devices. • Massive I/O: 65 possible GPIO, most multiplexed between 7 modes of operation. • Circuit Protection: clamping diodes protect the sensitive ADCs and various GPIO • Battery Power: variable battery voltages are converted to a constant 5V • Swift Software Development: Python controlled GPIO, ADC sampling, Ethernet communication, interrupts, and OS level commands. • Fast Processing Speeds: the AM3358 ARM Cortex-A8 provides 1GHz processing Given = − 2 The experimental RC constant of the low pass filter can be found from the frequency and phase shift angle. Solving for RC: = −2 = −2 = −2 = −2 = 45.36° −2(150) = 5.426 × 10 −6 The current circuit lacks the 1kΩ RTD used during data acquisition. The -3dB frequency must be derived from the experimental values. . = 5.426 × 10 −6 655Ω = 8.285 And equivalent resistance, which includes the RTD, is calculated . = 1 1 655Ω + 1 1000Ω = 395.8Ω Finally, the expected -3dB frequency is calculated for real-world operation: = 1 2 = 1 2 × 395.8Ω × 8.285 = 48.5 Resistance Temperature Detectors (RTD) are used to sense temperatures on the Baja Racer. These sensors are located at various extremities of the vehicle, and subject to noise. This noise will cause spurious readings by the ADC. Thus, a low pass filter was designed to filter this noise. Waveform of input sinusoid (blue) and output sinusoid (yellow) of Low Pass Filter used to find phase shift angle (). Clamping diode in operation. The Analog to Digital Converter of the BeagleBone Black has 12 bit resolution and 5μs sampling time (200KHz sampling rate).[1] It has an maximum 2.0V and minimum -0.5V rating. The figure shows a clamping diode holding voltages between -0.56V and 1.76V. A bias voltage is required to being the minimum clamping voltage up to -0.5V. References Joe Moquin, Electrical Engineering Longer pulse with larger ac component still providing sharp output pulse. Long pulse with small ac component providing sharp output pulse. Schmitt triggers are used to provide clear HIGH or LOW signals to interrupts on the BeagleBone Black. The hysteresis of the Schmitt triggers assist in preventing false LOW signals from high frequency ac signals. These HIGH and LOW signals are provided by Hall-effect sensors on the Baja racer. The signals are used to calculate rotations per minute of wheels and axels. CAD drawing of enclosure. Credit: Asa Thacker Vibration, mud, dust, water, rocks, tree limbs, and a host of unexpected hazards threaten the operation and reliability of the Baja Telemetry system. A weatherproof, robust, and easily mounted enclosure was designed to protect it. Wires and cabling extends through the three cable glands to an external quick- connect for ease of use. All photographs and images from the University of Arkansas, including the University of Arkansas Mini Baja SAE team. [1] Thanks to Jason Bailey, Toren Caldwell, and Asa Thacker for their contributions. Texas Instruments, “AM335x Sitara™ processors (Rev. J) Sitara AM335x ARM Cortex-A8 Microprocessors Silicon Errata (Revs 2.1, 2.0, 1.0) (Rev. G)” AM3359 datasheet, October 2011 [Revised January 2016].
