Unmanned Systems Technology 033 l SubSeaSail Gen6 USSV l Servo actuators focus l UAVs insight l Farnborough 2020 update l Transforma XDBOT l Strange Development REVolution l Radio telemetry focus

40 CAN enables multiple data inputs to be carried to multiple end-points on a single bus, with all the required arbitration performed by a CAN chip. That makes sure signals are transmitted correctly and consistently in their order of priority, as was needed by the automotive industry when it first produced CAN in the late 20th century. For example, aileron servos might be programmed with the highest priority to ensure that attitude control is activated persistently and accurately, whereas a payload servo is less safety-critical and could therefore be given a lower priority. CAN also enables the integration of multiple sensors per servo, on position feedback, temperature, voltage, power consumption and other factors that can contribute to accurate diagnostics for the motor, processors and other internals. For servos being designed to work for months at a time (such as – again – on HALE pseudo-satellites), self-testing diagnostic features can be added. These allow the onboard computer to perform, for example, tests of every motor winding, stepping it through different positions to check that the motor’s sensors and windings are all working correctly while still months away from the vehicle’s next ground maintenance check. Naturally, such functionality could also automate and speed up maintenance for UAVs that fly on a daily basis, to ascertain in better time that the servos’ motors, memory, comms, power supply and so on are working. Redundant systems In addition to equipping CAN and other software functions with useful redundancy features, designing actuators with built-in hardware redundancies is also key for improving their inherent safety and reliability. This is likely to become a prerequisite for delivery UAVs and urban air transports operating over cities and industrial assets. It is also important for experimental or highly dynamic vehicles. For example, supersonic target UAVs used by military forces could cause severe collateral damage if a servo seizes up and they lose a degree of freedom. To enable dual redundancy, many servo manufacturers have therefore developed versions of their existing actuators that incorporate a second channel of their internal components. Having a second motor, gearbox, driver chip, microcontroller and so on can considerably increase a servo’s MTBF (as projected by tests performed according to standards such as Mil- Std 217), as the probability of a single component failure causing a total breakdown of the actuator will have greatly decreased. OPV actuators As more OEMs develop urban air taxis (or refit light aircraft for autonomous flight), servos for optionally-piloted vehicles (OPVs) are growing in demand. These are actuated by the autopilot when engaged; when disengaged, their link to the autopilot is cut and the pilot’s yoke becomes the sole source of control. Such servos are also useful for testing new autopilot hardware and software, for example when using manned aircraft or ground testing stations to trial autonomy before flight tests. An established approach is to incorporate a toothless, electromagnetic clutch. These typically mount a disc to the output shaft, with a spring-loaded magnet under the disc. When electrical power is applied, the clutch closes, and when electricity is no longer supplied, the magnet and disc spring apart. However, the position of the output shaft cannot be sensed by the servo’s processors when the clutch is disengaged. That means an external position sensor is needed at the disc end of the shaft, along with some method of feeding the signal to the servo’s PCB. Adding external position sensors will mean enhanced safety, but it will also mean more wiring harnesses and potential points of failure in the actuator. That will make alternative interconnection technologies highly desirable in this area (although which approaches the industry will adopt remains to be seen). August/September 2020 | Unmanned Systems Technology Larger servos for urban air taxis and heavy-lift UAVs are being designed for higher continuous and peak torque (Courtesy of MKS Servos) Adoption of CAN bus (and variants such as UAVCAN and CANopen) in actuator designs is widespread, thanks to the greater security and breadth of useful information the protocol enables (Courtesy of Hitec)