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39 Motor controllers | Focus and easiest to implement, and is particularly useful during early vehicle prototyping, to trial new powertrain concepts quickly before optimising and iterating individual subsystems. However, it is also known to be vulnerable to EMI: signals transmitted by PWM can be weakened or corrupted by interference, especially as modern autonomous vehicles integrate higher densities of sensors and other electronics. The past decade has seen faster and more secure comms protocols emerging as new potential standards. For example, OneShot and MultiShot – which like PWM use analogue signals – can achieve signal widths (the time taken to send one data packet) in the tens of microseconds. PWM’s signal width, by contrast, ranges between 1000 and 2000 µs. The newer analogue formats therefore imply an enormous improvement in response times and controllability between autopilot and motors. Newer still is a digital protocol called DShot, which has variants that range between 13 and 106 µs (up to 600 kbit/s). Its variants are faster than OneShot, and while it cannot achieve the peak speeds of MultiShot, its signals are more secure as it enables ESCs to detect and reject data that has been corrupted by EMI, making it more robust against PWM’s most glaring vulnerability. It also allows a wider array of signal types to be transmitted along its bus, and does not need a lot of calibration to prevent oscillation-related drift. The world by and large is moving towards variants of CAN, however, given its even greater capacity than DShot to transmit many signals at once – and reject corrupted signals – with very high fidelity. It ensures the highest available reliability of comms, and huge reductions in the rates of glitches and drops in signals that traditional interfaces can suffer. It also entails galvanic isolation of the comms bus from the power bus, preventing transient noise during acceleration and deceleration from feeding back through to the autopilot. The wide range of telemetry permitted via CAN means faster and more holistic powertrain optimisation, enabling better judgements on energy efficiency, propeller and motor pairings, and thermal management. It also gives vital health indicators that can prevent disaster. For example, a UAV that has suffered an impact to a propeller or aileron before take-off would probably have unusual spikes in current and temperature at the pertinent motor or ESC. CAN prioritises these and other safety- critical parameters in transmission, so that operators (or AI flight controllers) can quickly recognise them and land the craft. Motor controller manufacturers these days use many of CAN’s variants. These include PiccoloCAN, for its compatibility with the Piccolo autopilot (among others based on Ardupilot), UAV-CAN for its high versatility and compatibility with almost every kind of data, as well as CANopen, ToshibaCAN and many more styled after the name of the company developing them. However, end-users in the unmanned systems sector are seeking increasingly different requirements from their CAN stacks. When combined with the growing variation in CAN protocols, this is making it harder for the industry to agree on a single standard that might enable widespread interoperability of ESCs and flight controllers. As well as choosing the control protocol, the selection of microcontroller also ties in closely with the ESC firmware, as the latter is often designed specifically for a given family of microcontrollers. Support for end-users’ mission-critical parameters such as CAN bus or high- resolution timers should be inherent to the type of microcontroller chosen, which must also be rated to the Unmanned Systems Technology | December/January 2021 As well as positioning ESCs downwind of propellers, using a CAN bus for real-time voltage and temperature data is key to thermal management (Courtesy of maxon motor) Thermally conductive pads and CNC- machined aluminium housings are increasingly being adopted in motor controller architectures for passive thermal management (Courtesy of KDE Direct)