6 October/November 2024 | Uncrewed Systems Technology Mission-critical info for uncrewed systems professionals Platform one Ultra Motion has developed its first commercial linear servo actuator with a redundant architecture for safety-critical UAV applications, writes Nick Flaherty. The L-Series Linear Servo electromechanical actuators are equipped with integrated, brushless DC control electronics, CAN 2.0B and RS-485 serial communication protocols, and contactless absolute-position feedback. The actuators use a high powerdensity brushless DC (BLDC) motor with slotless windings on the stator to provide improved thermal performance over previous designs. The motor is paired with a planetary gearhead to generate high torque in a compact package. Along with the gearhead assembly, the motor transmits power through a spur geartrain to the precision ground and preloaded ballscrew. A duplexed, angular-contact bearing handles all thrust loads, and it provides zero axial backlash for improved mechanical reliability and control. The L-Series’ design lends itself to dual-redundant implementation with redundant motors, control electronics and a triplicated absolute-position sensor for voting. Users gain complete control over the redundant operation with no arbitration, voting or redundancy decisions being made by the actuator, and there is no internal communication between the redundant controllers or the triplicated sensor, so the actuators are 100% electrically isolated. This allows a host controller to use the redundant actuator in a cold standby design, to take over when required, or a load-sharing arrangement. “In the redundancy architecture there are two BLDC motors with three absolute sensors, all isolated,” said Tom Quartararo, electromechanical engineer at Ultra Motion. “There is no arbitration at the actuator up to the user. There are two optically isolated interfaces that directly control a high-side MOSFET bridge. This bridge allows the bus voltage to the motor, and with that disabled there is no power. This provides a direct hardware switch that provides protection against a CPU error. “This means you can disable the motor in software, but if for some reason there is a problem, you can also disable the servo. The three position sensors feed into the flight computer, so you know which one to trust. That guarantees there is no lock-up nor can one issue create a failure.” The CAN 2.0B and RS-485 serial protocols allow for complete control over the actuator’s position and phase currents, while also providing detailed telemetry regarding the actuator’s health and state. An optically isolated enable input is provided as a hardware disable that removes power from the motor bridge when inactive, bypassing the onboard control electronics completely. The zero axial backlash of the L-series servo design makes it suitable for swashplate and tail-rotor control or fixed-wing control surfaces in UAVs, or rudder control or accessory deployment in uncrewed ships. Control systems Servo actuator’s safety boost The L-series high-reliability servo (Image courtesy of Ultra Motion)
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