Unmanned Systems Technology 004 | Delair-Tech DT18 | Autopilots | Rotron RT600 | Unmanned surface vehicles | AMRC | Motion control | Batteries

71 Motion control | Focus and low ends of the capability spectrum in terms of performance and cost. FOGs are much more accurate and less affected by drift than MEMS sensors, but are much heavier, bulkier and more expensive. Used in applications including driverless cars, mobile mapping and surveying, high-end reconnaissance aircraft, undersea vehicles and humanoid robots, FOGs operate by measuring the interference patterns produced by counter-propagating laser beams travelling around an optical circuit. This provides a very high bandwidth capability, meaning faster response and more accurate measurement of angles and angle rates. However, they can be biased by strong magnetic fields and require magnetometers to measure the field strength and orientation to allow correction. FOG accuracy depends on the total length of the coil of optical fibre, and recent breakthroughs in the reliable production of very thin fibres have enabled the use of packing far longer coils into the same volume. Made from silicon using microchip manufacturing technology, a typical MEMS gyro features a tiny spring- suspended mass that vibrates at tens of kHz and is connected to a capacitative readout. Rotating the gyro exerts a Coriolis force on the mass that is proportional to the distance between the mass and the centre of rotation. Deformation of the spring system due to g -forces affects accuracy but can be corrected. Recent servo evolution Servomotors haven’t changed much in the 25 years or so since the adoption of rare earth magnet materials, which produced a great increase in flux densities, meaning smaller and more powerful but more expensive motors. However, improvements in modelling software have made them easier to customise. Greater changes have come from advances in feedback devices and servo drives, together making the whole system more accurate and responsive. Digital signal processors in servo drives seamlessly handle multiple tasks including motor commutation, feedback information, power device switching, multiple loop controls, drive system functionality and comms. This is often transparent to the user, as the servo system simply does its job and periodically reports its health. With simplified configuration and diagnostics, vehicle builders need not be servo experts. Overall, motion control strategies vary between centralised or distributed and almost any mix in between. In centralised systems, the master controller knows almost everything about each individual axis, along with input/output, sensing, cameras, safety and other mission-critical parameters. In distributed systems, all responsibilities for motion and trajectory control remain at the drive level, with information only reported as requested by the controller. While electric motors are not considered either analogue or digital, their controlling electronics are, and this can affect systems designers’ choice of centralised or distributed architectures. Generally, analogue servo drives live near the controller and use a dedicated command interface, with the drawback that the feedback signal from the distant motor is more vulnerable to noise. In contrast, digital drives can be mounted either near the controller or the motor. With distributed control, digital feedback and command signalling can minimise interference-prone wiring between the master controller and the drive. Dedicated UAV servos In contrast with basic motor technology, dedicated UAV servos have come a long way since the first ‘serious’ devices evolved. For small and medium UAVs with maximum take-off weights between, say, 50 and 150 kg, an early approach was to improve servos originally designed for model aircraft. Typical hobby servos featured brass-steel or aluminium-steel geartrains with two ball bearings, simple plastic cases, transistor- transistor logic/PWM signalling, analogue printed circuit boards (PCBs), potentiometer angle sensors, cheap brushed motors and cable pigtails exiting from a poorly sealed servo case. The first generation of UAV actuators derived from them appeared in the late 1990s, and had EMI-shielded aluminium cases, steel geartrains, brushless motors, high-quality potentiometers, digital PCBs with multiple signal options, appropriate case sealing and D-9 sub-connectors. Many manufacturers subsequently built servos to this pattern. These days, mature UAV servo technology is characterised by CNC- machined aluminium cases, hardened steel four- and five-stage geartrains Unmanned Systems Technology | Autumn 2015 The PA-R-135-4 is among the smallest ‘serious’ UAV actuators available, at 13.5 mm thick and weighing 65 g (Courtesy of Pegasus Actuators)