Unmanned Systems Technology Dec/Jan 2020 | Phoenix UAS | Sonar focus | Construction insight | InterGeo 2019 | Supacat ATMP | Adelan fuel cell | Oregon tour | DSEI 2019 | Copperstone Helix | Power management focus

90 Focus | Power management Connectivity Once the voltage or pressure measurements have been made, they have to be sent to a local controller. At the moment, that is handled by a galvanically isolated two-wire link. The galvanic isolation (either via a transformer or optical isolation) allows the low-voltage 5 V signal in the link to operate reliably in the 400 or 800 V environment that is typical of EVs. A low data rate, of 1-5 Mbit/s, is also more reliable than a high-speed link. A typical EV will have a control loop measuring the battery every 10-100 ms, looking for changes in the state of charge or state of health of the pack. The controller will combine the voltage, current and temperature into a model, but processing that data takes more than a single cycle, so higher data rates such as 100 Mbit/s are unnecessary. The topology of the link is also important. Previous battery management system (BMS) designs have used a single, unidirectional chain from the sensor in one cell to the next, called a daisy chain. If there was a problem with one sensor or the link, none of the sensor nodes further down the chain could communicate with the controller. More modern systems however now use a two-way daisy chain so that nodes on the other side of a break can communicate with the controller. That allows the system to keep operating more safely than a unidirectional chain. Wireless BMSs The links from all the cells in a large battery pack represent a complex, heavy wiring harness. As the data rate is not high, this could be replaced by a wireless link, potentially saving weight. It would also avoid the need for galvanic isolation. Initial demonstrations of such a wireless BMS over the past two years have used a robust mesh topology in a similar way to the daisy chain, with each wireless sensor node passing on data from its neighbour. Again, if one node fails then the data is routed to another neighbouring node to relay to the controller. However, such a system has to demonstrate reliability in an environment with lots of radio noise, which has been achieved for data rate links of up to 1 Mbit/s. The challenge now is to get to data rates of 5-10 Mbit/s, which requires compression technology in the node, requiring more processing power. The biggest concern with such a wireless BMS system though is security. Once the data is wireless there is a greater risk of hacking and remote access – and an insecure system is not a safe one. That means security, using encryption at the node level, has to be demonstrated. This has required extra processing power and more complex certification to demonstrate the safety. Such systems are expected to be on the market and in use by large vehicle manufacturers in the near future. Aircraft Unmanned aircraft are also now having to address reliability and security in the power management system, mostly as a result of the demands of regulators. If there is a power failure in a UAV December/January 2020 | Unmanned Systems Technology A dual redundant power distribution unit uses interleaved buck-boost converters to ensure power to an avionics system is always available (Courtesy of Millswood Engineering) A dual redundant PDU board includes temperature sensors at both ends and in the middle to identify thermal issues that might compromise performance (Courtesy of Millswood Engineering)