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

67 Movement and feedback A reasonable working definition of a servo is a device that generates motion in response to a command, then regulates outputs, particularly speed and direction, in response to feedback signals. While there are hydraulic and pneumatic servo systems, the electric servomotor is now centre stage. A generic servomotor system includes a controller (typically an embedded computer), a servo amplifier and the motor itself with an integral feedback device, such as an optical encoder. The controller sends commands to the amplifier, which in turn supplies the motor with the current it needs, commonly using a pulse width modulation (PWM) technique that involves rapidly switching the current on and off while varying the duty cycle to control the speed: the longer the power is on, the faster the motor runs. The motor’s output shaft is connected to an effector such as a propeller or operating arm, either directly or via a transmission mechanism such as a geartrain or a ball screw to convert rotary motion into linear movement. There are also rotary actuators whose output shafts move through a restricted angle rather than spinning freely, and there are direct linear actuators as well, but the principle of control via closed-loop feedback still applies. Waking the beast Unlike simpler electrical machines, servo- based systems need more than just the push of an ‘On’ button before they will do useful work. The wake-up procedure for any servo has three stages. Turning the amplifier on brings control power online. Next comes the main power that the amplifier will control to drive the motor, followed by the final ‘servo enable’ stage, which involves the controller sending a signal to the servomotor that makes it ready to move when commanded. If the vehicle needs one or more of its servomotors to move in forward and reverse, those directions have to be defined in the controller’s programming and carefully coordinated for all axes. Unless the application allows the motor to spin freely, limits to angular and/or linear motion must also be programmed in to prevent hard contact with physical stops or other parts that might suffer damage. Controllers can be programmed to command relative as well as absolute moves. A command for a relative move would tell the servomotor to travel a set distance in a given direction from its current position, while to command an absolute move it would tell it to go to a particular position defined in a look-up table in computer memory. The relative command uses the current position as its zero point for the next move, while the absolute command requires a pre- established reference or zero point. Vehicles subject to aerodynamic forces that threaten their stability need more sophisticated control schemes that calculate outputs in response to inputs from multiple sensors, in addition to following pre-planned routes or responding to external manoeuvre commands, but in all cases servo systems carry them out. For all control, feedback is crucial, and there are many kinds of sensors that can provide it. Some have moving parts, others are solid-state. Some, such as potentiometers, rely on mechanical contact for their measurements, while Motion control | Focus Unmanned Systems Technology | Autumn 2015 Able to fly at over 70,000 ft for weeks at a time, the Zephyr solar-electric UAV has grease-lubricated servo-actuators (Courtesy of Airbus)