Unmanned Systems Technology 010 | nuTonomy driverless taxi | Embedded computing | HFE International marine powertrain | Space vehicles | Performance monitoring | Commercial UAV Show Asia report

68 Focus | Performance monitoring Movement sensing MEMS accelerometers sense movement in several ways. One type of technology senses the micro-movement of very small structures embedded in small integrated circuits. The movement of these small ‘diving boards’ changes the amounts of electrical current moving through the structure, indicating a change of position relative to gravity. Another type is the thermal MEMS accelerometer, which is based on the principle of convection of heated gas molecules within a sealed cavity. It does not have moving parts, but instead senses changes in the movement of the gas molecules passing over a small integrated circuit. It works by using highly accurate temperature sensors to detect the change in temperature profile resulting from very small changes in acceleration (or inclination). Since there are no moving parts it is virtually immune to out- of-band vibration and shock. Current sensors In a battery-powered system, power consumption is a key metric for performance management. A current sensor is a device that detects electric current (ac or dc), and generates a signal proportional to it. Typically it will measure the magnetic field generated by the flow of electrical current. Current sensors can be used to monitor and optimise power drain, safe charging of internal batteries, and detect fault conditions with motors or other areas of the system. They can also be used to trigger electrical isolation to reduce power loss and eliminate the risk of electrical shock or damage to the user or system. Sensors with a fast response time and high accuracy optimise the battery life and the mission time of an autonomous system, whether airborne, ground- or sea-based, or submersible. Magnetic sensors Innovation is traditionally high among sensor manufacturers, both in the use of technologies such as MEMS but also in the sensors themselves. Sensors that monitor properties such as temperature, pressure, strain or flow provide an output signal that is directly related to the desired parameter. Magnetic sensors, on the other hand, differ from most of these as very often they do not directly measure the physical property of interest. They detect changes or disturbances in magnetic fields, changes that are created by objects or events. The magnetic fields may therefore carry information about properties such as direction, presence, rotation, angle or electrical currents that is converted into a voltage by the magnetic sensor. The output signal then goes through some signal processing for translation into the desired parameter. A magnetic field distribution depends on distance and the form of the creating or disturbing object or event. It is therefore always important to consider both sensor and object/event in the application design. Although magnetic sensors are more difficult to use, they do provide accurate and reliable data, without physical contact. A leading technology for magnetic, non-contact measurement of the position of a metal component is the anisotropic magneto-resistive (AMR) permalloy sensor. This uses magneto-resistive elements arranged in a Wheatstone bridge to determine the direction of the magnetic field rather than its strength. As a result, AMR sensors are very tolerant with respect to variations in field strength caused by ageing of the magnet, the temperature sensitivity or mechanical fluctuations, and so have superior accuracy and response time characteristics while consuming far less power than alternative technologies. Turnkey solutions provide UAV manufacturers with quality data sensing in a rugged and compact package. Magneto-resistive technology can be used for measuring moving components more effectively than other rotary sensors. These solid-state sensors can measure through large sections of non-ferrous material – the electronics, for example, can be placed on the outside of a October/November 2016 | Unmanned Systems Technology MEMS sensors can be used in a number of different ways for performance monitoring (Courtesy of Xsens) AMR sensors are very tolerant with respect to variations in field strength caused by ageing of the magnet, so have superior accuracy