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

Focus | Motion control 72 with integrated overload slip clutches, full roller bearing support and oil bath lubrication. They also have controller- managed integrated PCBs, motor and PCB temperature control and feedback, position feedback, brushless dc rare earth magnet motors, contactless angle sensors, multiple digital signal versions and military-spec connectors. A decade ago, using brushless motors was risky because some of the high-power field effect transistors used for commutation were easily damaged, but now they are very tough and reliable. Most UAV servos now comply with MIL-STD- 810F environmental, MIL-STD-461E electromagnetic standards and DO-160F standards for avionics. Redundancy However, first-generation servos lack redundancy, so a single component failure can result in the loss of the UAV, particularly in say a helicopter application where there is little scope to add extra actuators to the main rotor swash plate or tail rotor controls. To overcome this, leading suppliers have begun to offer second-generation UAV servos with high levels of redundancy. Typical features include a pair of motors, a triple-redundant contactless magnetic deflection angle sensor, multiple redundant digital servo controller boards and master controller boards plus a redundant servo signal and power supply. The goal is a system architecture that is not vulnerable to single-point failure. Redundant UAV servo technology is increasingly regarded as a game- changer – a must for system reliability and any serious attempt to certify a vehicle for use in civilian airspace. Another line of development involves adding intelligence to servos so that they can keep a UAV flying safely for some time without input from the autopilot. The autopilot is typically the most advanced component in the vehicle in terms of embedded software, and the more lines of code it runs then the more likely there will be bugs. Embedded software in the servos, a separate comms channel for them and a redundant set of sensors look set to provide an extra layer of safety and reliability, while a radio link to the separate comms channel can allow a remote operator to take control of the lower-level functions in case the higher-level command systems fail. Flutter control There are still some things that electric servomotors struggle with though, one of them being very rapid responses combined with large loads and angular displacements needed to damp out aerodynamic flutter. Important factors here include bandwidth, precise position sensing and geartrain backlash. In this instance, bandwidth is a measure of how fast the servo can react or the frequency at which it can follow set points. For example, if the servo is programmed to move through ± 5°, the set point is a sinusoidal motion profile, and if you increase frequency then there comes a point at which the servo can no longer follow the set point. That frequency defines the servo’s maximum bandwidth. Most tactical UAVs have fairly modest precision requirements of the feedback in their flight controls: representative numbers might be 14-bit resolution, about 0.3-0.5° of output shaft backlash, repeatable positioning of ± 2° at a rotational speed of 150°/s at the rated torque, plus a signal bandwidth of about 6 Hz. However, requirements are emerging for 10 Hz bandwidth, 400° rotational speed at the rated torque and positioning precision better than ± 0.1° including geartrain backlash, plus flutter damping capabilities that need a second control loop and higher resolution. The normal control loop that runs between the controller PCB and the output shaft’s angle deflection sensor is not responsive or accurate enough, in part because of the backlash in the geartrain between the motor and the sensor. Part of the answer is to add a second loop between the controller and the motor axis, along with 16-bit resolution. Non-linear loadings One of the toughest aspects of such high speeds with significant angular deflections is the increase in loading on the system. Actuators capable of active flutter control would be highly stressed: for example the step from 8-10 Hz, depending on the deflection and the load, would represent about 20 times the effort – a highly non-linear jump. In this area, high-pressure hydraulics are superior because the low-mass servo valves can move very fast, with bandwidths of 15-20 Hz. Magnetic field strength in an electromechanical actuator is the equivalent of pressure in a hydraulic system. A hypothetical motor with a high field strength and low rotational inertia in its armature would be able to produce a very high frequency response, and would have cigar-like proportions –long and thin. Also, such a device would probably use a ball screw instead of a geartrain because of a very low backlash of around 0.1 mm. As actuators become more capable, they also become more complex. First- generation devices might have eight axes/degrees of freedom, but second- generation redundant units with features Autumn 2015 | Unmanned Systems Technology The DA 26-D represents a new generation of redundant servo actuators for demanding UAV applications (Courtesy of Volz Servos)