104 Focus | Servos with the motor. Different types of contactless sensor are available, with one design using two single-turn magnetic angle sensors coupled through a geartrain, enabling the encoding of revolutions though an interference pattern that is proportional to the gear timing. This approach therefore provides position readings accurate to 1-2 microns and works immediately on actuator start-up. Protocols As mentioned, CAN bus is prevalent across UAV servos because of its diagnostic feedback features and signal integrity, although a few varieties of it are now in use in uncrewed vehicles. These generally fall under the umbrella of the CAN 2.0b standard, which was developed and published by Bosch (the original developer of CAN). It is based on the use of 29-bit identifiers, and is now used in a number of commercial UAV autopilot systems. However, as far as specifications beyond the base CAN go, DroneCAN has become one of the most widely adopted protocols in the UAS industry. DroneCAN originated as an offshoot of UAVCAN, and has become the primary CAN protocol used in Ardupilot and PX4-based flight controllers for comms with servos and other CAN-based subsystems. It comes with advantages such as detailed protocol specifications, dynamic node allocations for assigning CAN node IDs, and a feature-rich GUI for diagnostics and device configurations. The choice between DroneCAN and other standards sometimes comes down simply to the end-user’s choice of autopilot and their engineers’ personal preference. In addition, a growing number of ESCs, GNSS receivers and other components are being designed to communicate using DroneCAN, enabling its wider use in servos through compatibility across onboard component ecosystems. A broader benefit of DroneCAN however is the way it has been consolidated down to a core number of critical features. This is in contrast to CAN-FD, say, which is widely used in cars but is far more complex and multi-faceted than is necessary for uncrewed aircraft, as it has to cover ancillary driver and passenger-related functions such as seatbelt sensors, check engine lights and airbags. On top of that, the growing popularity of DroneCAN is somewhat self-perpetuating, as UAV OEMs know they can swap from one supplier’s servos to another if they both use it. Both sets of actuators will use the same code structure, minimising the integration work for their technicians. Delving into the structure might still be needed to determine and reconfigure node allocations and IDs, but unlike CANFD, say, it would not require exhaustive removal of nodes and IDs that go beyond platform requirements. That adds another layer of consideration regarding the level of customisability or preprogramming that can come with servos. It might be very useful for UAV manufacturers to receive actuators in which the nodes and IDs are initially undesignated, so that systems integrators can custom-define the architecture themselves. However, some actuator suppliers opt to predefine the architecture, as they have found that allowing customers plenty of configuration options too often leads to them introducing errors into the servos’ programming. In the meantime, other protocols are gathering followings. For example, some servo models are now being designed to operate using CANaerospace, a protocol derived from the aviation-specific ARINC 825 standard’s guidelines but in a more streamlined format and now being adopted across some aviation engines for UAVs. That adoption has driven the use of CANaerospace in servos for flight control, to enable all the servos in a given UAV to run on a common protocol. Beyond these three, there is a growing rate of requests for customised CANbased protocols. They deviate from the standards and guidelines to suit individual UAV manufacturers and their unique implementations, and some of them are expected to feed into future variations of CAN that are further tailored to suit the evolving needs of uncrewed commercial and military fleet operators. Future prospects While the hardware and mechanics of servo actuators have no doubt undergone considerable innovation over the past few years, with reconfigurations and redesigns using established August/September 2023 | Uncrewed Systems Technology Programming servos for different versions of CAN bus is key to meeting integrators’ different needs in diagnostics, maintenance and control dynamics (Courtesy of Acutronic)
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