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

38 Focus | Servo actuators quantities of sensors and processors dependent on secure connections. Switching to a brushless DC (BLDC) motor not only removes a lot of friction and EMI from the servo’s normal operation, it also greatly reduces the damaging effects of shock, vibration and ingress by environmental contaminants. With less physical contact between parts, such problematic factors are transmitted far less throughout a servo’s internals. It should also be noted that servos must be able to measure the position of their motors, to determine the next correct turning angle or linear movement (and therefore input power) or confirm that the actuation has been performed as required. While the use of BLDC motors does a lot to improve servo lifespans, combining them with a potentiometer for position sensing still leaves the problem of persistent contact being needed for consistent actuation. Potentiometers incorporate resistors with sliding or rotating contacts that will degrade over time, far quicker than contactless sensors. The service life of potentiometers in UAV actuators is often limited to around 100-300 hours, given how shock and vibration can damage such parts. For a UAV flying two to four missions per year, for example, that might be acceptable, but for MALE-class UAVs flying 8-hour missions multiple times per week – let alone HALE-class UAVs flying 365 days a year – having to replace servos so often is unacceptable. Contactless position sensors give far more accurate feedback than potentiometers, through methods such as the Hall effect sensing approach. Switching to contactless sensors is also useful for manufacturing. With no direct coupling between the sensor and the output shaft, there is no risk of misalignment, as there is when trying to mate a potentiometer with the shaft. Lastly, contactless sensors increasingly come with built-in diagnostics, further reducing the risk of failure modes compared with potentiometers. By switching to BLDC motors and contactless position sensors, the geartrain becomes the primary bottleneck in the lifespan of the actuator. If steel gears are used it takes the mean time between failures (MTBF) from about 100 hours to upwards of 2000 hours. It should also be noted that, as well as the first control loop for servo position, BLDC motors can incorporate a second loop for controlling temperature, to enable them to heat themselves when the temperature sensor detects that the actuator is nearing a configurable negative temperature threshold (such as -40 C). This is a common problem with smaller servos, which lack significant air gaps for innate insulation and start to suffer slower dynamic responses below that kind of temperature. Programming considerations Increasing sophistication in actuator firmware is critical for more rugged applications. For example, servos used to actuate engine throttle valves must have sub-degree accuracy to rotate to the correct angle for every performance level, from wide-open throttle to idling. Once the angles and air quantities have been mapped, a UAV manufacturer or operator can change their throttle bodies in the field, upload the maps and find that their new throttle body behaves precisely as the previous one did in normal operations. It has been traditional to set commands and protocols for actuators (in engines and on wings, fins and tails alike) using PWM (pulse width modulation), but the vulnerability of this form of interface is well-established. Between a servo and an autopilot, PWM signals can be exposed to high levels of EMI (or magnetic interference), to which they are highly vulnerable, compared with other available signal standards. CAN, for example, typically incorporates a 15-bit cyclic redundancy check (CRC) that detects accidental changes to the raw data before performing corrections, to prevent corruption. With the CRC providing a safety layer around CAN-transmitted information, servos can far more easily ignore inaccurate comms. As such, more and more UAV servos are adopting CAN bus as their interface standard for electronic speed controllers and data comms. CAN microcontrollers, drivers and other components from the automotive world have served as a broad but vital design basis, and some UAV servo manufacturers have adopted specialised versions such as UAVCAN, CANopen or custom protocols on top of CAN. Different standards and formats of CAN require different degrees of work on integrating and implementing them in associated components. Some servo companies now work directly with autopilot and engine manufacturers for example to ensure common interface standards. August/September 2020 | Unmanned Systems Technology New kinds of contactless position sensors are reducing servo weight and complexity while greatly enhancing their accuracy and reliability (Courtesy of Ultra Motion)