Development of an advanced nano-satellite (VELOX-I) - power quality and monitoring system

Field: Engineering (Physical Sciences)

This report focused on 5 areas in the development of an advanced nano-satellite (VELOX-I) namely cell balancing system, power system redundancy, temperature monitoring system, batteries state-of-charge monitoring system and development of battery management system with enhanced kill switch for pico-satellite. The first part of the report covered the introduction of nano-satellite to understand the systems and functions of nano-satellite (VELOX-I). It was followed by literature review on the 5 areas of development. For cell balancing system, it was designed to balance the battery cell voltage and optimise battery performance. This increases the usable capacity of battery and enables the VELOX-I to carry out longer mission. The method described uses resistor to dissipate the charge and balance the battery cells. The cell balancing circuit was able to balance the two battery cells with 90mV difference in 4h10mins. Power distribution module plays the vital role of supplying stable power to the payloads. It was designed to step down voltage from boost converter from 8.4V to 5V for the payloads. The output voltage should be kept constant despite load changes to ensure functionalities of payloads. The voltage output from the existing power distribution module dropped significantly by 18.61% when drawing 6A load. The final design was able to decrease the voltage drop to 3.17% at maximum load, providing better quality power supply to payloads. The back-up power distribution module was replaced by TPS5450 to achieve power system redundancy and minimise disruption due to similar component failure. Temperature fluctuates rapidly during the orbit with temperature ranges from -20°C to 80°C. It is essential to monitor and maintain the operating temperature to ensure optimal performance of devices. The battery cell’s temperature was measured by AD8497 K-type thermocouple amplifier which provides specific point temperature measurement using thermocouple. It reflects a more accurate temperature reading than LM94022BIMG area temperature sensor, as the thermocouple can be tapped on the battery casing. The experimental results showed that AD8497 K-type thermocouple amplifier was able to sense the temperature to within +/- 3°C within the temperature range of -20°C to 80°C.ii State-of-Charge (SOC) of the battery indicates the amount of charge presented in the battery cell. The SOC of the battery for VELOX-I will be estimated by coulomb counting. It measures the current and integrating it with time to estimate the SOC. Coulomb counting requires regular re-calibration to maintain accuracy. The method described uses AD8210YRZ bi-directional current shunt monitor IC to amplifier the voltage drop across sensing resistor and converted to current value. With accurate offset calibration, the lithium-ion battery underwent 24 cycles of charging & discharging. The results showed that with SOC resetting algorithm, the SOC of the battery can be calibrated close to true capacity of lithium-ion battery. The final phase was to develop the battery management system with enhanced kill switch for pico-Satellite with improved power distribution module, Temperature sensors and SOC resetting algorithm. Sensing and control devices were selected to be compatible with the reduced operating voltage of single 4.2V lithium-ion battery. Current inrush and overcharge protection were also implemented to reduce current surge by 32.5% and prevent damage to the battery and sensors. The kill switch design was modified to prevent circulating current, reducing power losses and improves the capabilities of pico-Satellite. Finally, the battery management module was integrated with the power management module to test its performances.