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

68 Focus | Motion control others use non-contact methods, with the latter now dominating applications that require greater reliability and long service lives. Hall effect sensors Among the simplest digital feedback devices are Hall effect sensors, inexpensive solid-state contactless devices that react to magnetic fields. A passing magnet causes the sensor’s output to change from on to off (‘high’ or ‘low’) in a square-wave signal. In some motors the magnets are on their own wheel, in others the sensors are set up to detect the motor magnets. The sensors are individually identifiable and in known positions, so the rotor position is known, while the time between the pulses provides information for speed control. Encoders For tighter control of speed, particularly at low rpm, optical encoders offer high pulse counts that translate into higher resolution of speed and position. The anatomy of a basic optical encoder includes a light source, a mask to narrow the beam, a disc coded with opaque lines and a set of photo-sensors and associated electronics. As the disc rotates, the light is alternately allowed through and blocked, creating a pattern of light and dark that a sensor detects and electronics turn into a square-wave ‘high/low’ pulse pattern. The resolution available depends on the number of lines etched onto the disc – the more the better. By counting the pulses, the electronics can also determine the shaft’s position relative to where it started. An extra pair of lines 90° apart form two channels that enable the control system to determine the direction of rotation according to which of them reads ‘high’ first. As well as relative encoders as described above, there are also absolute encoders that provide a specific address for every shaft position through 360°, which the electronics derive from multiple independent tracks, each with its own dark/light pattern that then forms a complete binary coded signal when read by a bank of sensors, one per track. Resolvers Another important type of rotary position and speed sensor is the resolver, an electromagnetic inductive device akin to a transformer, which consists of a rotor and a stator with input and output windings. An ac reference signal applied to the rotor changes in direct proportion to the angle through which the rotor moves. The reference signal is applied to the single input winding that induces it into the rotor that, in turn, induces current in the two output windings, which are 90° apart. Their output voltage is proportional to the angle the rotor has moved through, and the order in which they occur Autumn 2015 | Unmanned Systems Technology Flight controls actuators for tactical UAVs, such as this AAI Shadow 152A, have modest precision requirements (Courtesy of the US DoD) Modern servomotors with embedded control feature digital signal processors to handle functions such as commutation, feedback, power device switching and comms seamlessly (Courtesy of Advanced Motion Controls)