VB Gen 8.1 Avionics

ValBal Generation 8.1 Avionics
Part of the ValBal series
Chief Designer	Aria Tedjarati
Technology Line	Balloons ValBal Architecture
Version	Generation 8.1
Name	ValBaby

The ValBal Gen 8.1 Avionics is the eighth generation and twelfth iteration of of the ValBal core electrical architecture. It is the most up-to-date and highest performing ValBal avionics version as of September 2017, consisting of two 3-inch by 3-inch FR4 PCB's that are stacked on top of one another above 33 AA non-rechargeable lithium primary batteries, in addition to a small breakout PCB that is exposed to the air. The architecture makes significant changes and improvements on the ValBal Generation 8.0 architecture, including changes in physical size, stacking connectors, power regulation and efficiency, cost, wireless communication, and more.

The details of the ValBal generation 8.1 avionics are outlined below in exhaustive detail.

Introduction

ValBal is an actively altitude-controlled, low-cost, high-endurance, autonomous latex high altitude balloon system. It requires a complex set of avionics hardware to enable it to complete its mission. In general, at a bare minimum, any ValBal avionics system must have the following set of features to ensure a successful flight.

• Half-duplex or full-duplex wireless communication
• Dual bidirectional high power motor control
• High altitude GPS location acquisition capability
• Power regulation and subsystem protection
• Temperature regulation
• Barometric pressure & temperature sensing, filtering, and error detection
• Powerful microprocessor with autonomous flight code
• Current & voltage sensing of all system parameters
• Weigh no more than approximately 1 kilogram
• Have a form factor of less than 3 x 3 x 6 inches

The 8.1 generation of ValBal avionics goes above and beyond these minimum requirements. BATTERY stuff.

Mainboard

Power Regulation and Control

ValBal 8.1 Mainboard Power Schematic

After power handoff, there are two parallel power paths in the Radio Board. One is from the output of the diode chip to the 3.3V line, and one from the output of the diode chip to the 5V line. The 3.3V line needs no control and will run automatically. However, because of the bake-in of the Iridium module, and the battery current sensitivity at low temperatures, an actively controlled power system for the 5V line is needed to ensure batteries do not die from over-current. This is because ValBal's high current devices run off of the 5V line (i.e. Iridium, motors), and can easily pull 2A at any given short burst of time, which could destroy the batteries at low temperatures (i.e. degrade their voltage so low as to not recover until they warm up again under no electrical load). Therefore, instead of acquiring high power, short-burst energy from our cells, we will instead store this energy in a super-capacitor and draw the high burst energy from them instead, when we need it. The parallel 5V line works as follows: the output of the diode chip (at 4.5V nominal assuming flight batteries) is connected directly to a super-capacitor charger. This super-capacitor charger determines when, if, and how quickly to charge the super-capacitor (given inputs from the micro-controller). This charger is required because charging the super-capacitor too quickly is just as dangerous as any other high current application, so we must regulate the super-capacitor charge rate and voltage (the voltage is hard set to 5V). From a code perspective, there are two things needed to control this super-capacitor charger. One, there is a super-capacitor charger enable pin, which is active HIGH. The enable pin has a pulldown resistor, so upon startup the super-capacitor charger will remain off. Second, the code must control the current limit of the super-capacitor. This is done in the form of controlling a digital potentiometer that operates over I2C. The mainboard uses the 100kOhm AD5246 digital potentiometer, with 128 different selectable resistances. Each resistance naturally corresponds to a different current limit. The potentiometer is linked to the super-capacitor charging rate by the following equation: Current into super-capacitor = 10,000/Rpotentiometer, where everything is in standard units (i.e. amps, ohms). This means that a charging current of 100mA or 0.1A corresponds to a potentiometer value of 100kOhm, or 100,000 ohms. The maximum current limit for the super-capacitor charger is 1500mA, and the minimum current limit is 100mA. The super-capacitor itself is a 5V rated, 5F capacitor with a total maximum stored energy of 62.5 Joules. It takes time to charge this super-capacitor, and the voltage across the super-capacitor will always be changing given the demands of our system. For example, during charging at initialization, the voltage across the super-capacitor will start at 0V, and will increase to 5V slowly as per the super-capacitor charger current limit. If our initial current limit is 100mA, for example, it will take 125 seconds to charge the super-capacitor to its fully rated 5V. The voltage across the super-capacitor will increase quickly at first, and then roll off slowly to 5V. Now, the since the super-capacitor voltage changes a lot, especially if we drain it with energy required for the Iridium modem, for example, it is not suited to produce a well regulated 5V line. Therefore, this super-capacitor must be connected to a voltage regulator, in the form of a buck-boost switching converter to produce a clean 5V line. This is the last step of the 5V power system. The super-capacitor is connected to a TPS63070 buck-boost switching converter designed to regulate its output voltage to 5V. The 5V line has an enable pin (active HIGH), which controls the TPS63070. This is because it allows us the ability to shut off all our 5V systems if they are drawing too much energy, and also allows us to charge up the super-capacitor fully before turning on the 5V line that would drain it. Temperature directly affects the maximum allowable charging current of the super-capacitor, because this current comes from the batteries. At low temperatures, we cannot charge the super-capacitor as fast as we can charge it at high temperatures. The buck-boost switching regulators have a minimum input voltage of 2V, meaning that the super-capacitor voltage must always be above this value in order for the 5V line to be stable. In addition, the super-capacitor charging current must always be above the 5V line load current (i.e. current drawn from Iridium modem and motors) on average in order to assure that the super-capacitor will not be discharged over time. This means that we cannot constantly charge the super-capacitor at 100mA if our 5V system current is 120mA. While we do have current sensors on all the 5V line devices, there is an easier way to determine this: by measuring super-capacitor voltage. If the voltage drops on average over a relatively long period of time, we know we aren’t charging it at the same rate that it is being discharged at.

Radio Board

The purpose of the ValBal v8.1 Radio Board is to act as an independent payload that interfaces with the ValBal v8.1 Mainboard, and is responsible for all of ValBal's radio communication to and from the ground excluding satellite communication. It is an additive board, meaning that it is not flight critical and is not necessary for a ValBal flight to be successful. However, its inclusion in the ValBal avionics allows for significant advantages over a simple "Mainboard-only" system due to the extremely slow speeds and bandwidth of the Iridium satellite communication system. The ValBal v8.1 Radio Board allows for transmission speeds of over 100,000 times that of the Iridium system, enabling flight controllers on the ground to receive much more data more quickly in the form of flight telemetry, pictures, videos, music, and automated packets, at the expense of power, weight, and cost. The board has two transceivers: one is responsible for transmitting and receiving Ultra-High-Frequency (UHF) digital data modulated with GFSK, FSK, or OOK, and the other is responsible for transmitting and receiving Very-High-Frequency (VHF) analog data modulated with FM.

Microcontroller

The micro-controller onboard the ValBal v8.1 Radio Board is an embedded Teensy 3.2 (identical to the Mainboard except it uses 0603 sized components instead of 0402).

Power Regulation and Control

The power regulation and control of the ValBal v8.1 Radio Board is identical to that of the Mainboard except with a few additions. First, the Radio Board has three super-capacitors in parallel in order to store more energy. The total amount of usable energy in the capacitors is 1/2*(15F)*(5V)^2 - 1/2*(15F)*(2V)^2 = 157.5 Joules. This means that, for example, if we drew a continuous 2.5W of power from the capacitors (assuming no charging), we would have approximately 63 seconds of usable energy time. Second, none of the subsystems on the Radio Board run off of 5V - the closest thing with high current draw is the VHF radio (nicknamed Dorji) that needs 4V. Therefore, after the 5V buck-boost regulator there is a 4V LDO linear regulator that feeds into the Dorji system. Finally, the last difference is that by default the Radio Board charging circuitry charges the super-capacitors as slowly as possible (i.e. minimum current limit is 100mA) to avoid damaging the batteries.

UHF Transceiver and Antenna

Runs on SPI. Takes 3.3V as an input. Can modulate with 4GFSK, 2GFSK, 2FSK, or OOK. -30 dBm to +20 dBm output RF power. We use it at 435 MHz. 30 MHz TCXO. Current draw 20mA recv 90mA tx. Independent of supercaps. Bandwidth programmable. Can choose between Zero-IF, Fixed-IF, and Scaled-IF. Receiver sensitivity changes with a lot of things. BER? Chip has own LO and does mixing itself. Single ended tx output. Matching. Low pass filtering. Choke inductor for power. Receiver is differential --> needs balun/match. Direct tie. Single antenna.

VHF Transceiver and Antenna

Communicates on UART. Takes 4V as an input. DAC input from MCU is mixed with LO in chip