E-Mobility Engineering 015 l EMotive Scarab off-road truck dossier l In Conversation: Giulio Ornella l Hall effect and magnetic sensors focus l Challenge of batteries for heavy-duty EVs l Alpha Motor Corporation digest l Automated charging insight l HVAC systems focus
with a 50 mT field, which is where the sensor algorithm is very practical. This allows a field of typically 200 mT to be measured at a distance of 0.5 mm to provide a noise-free image of the field at 50 mT. This allows the camera to go down to the microTesla range and measure the angles of the field in the rotary encoder. If there is any lack of uniformity in the magnets then there will be ripples in the field at a particular angle, and that can be used to characterise the magnet and predict the angle deviations that will be seen in the encoder systems. This is possible because the array measures the whole surface of the magnet and the strength and orientation of the field at each position. That gives the angle error that the encoder will see, so it can be predicted before the whole system is assembled. This can compensate for inaccuracies in the magnet in the encoder, but there can also be a large variation in the quality of magnets, and with some magnets it may not be possible to apply that compensation. The variations might come from plastic-bonded magnets that have magnetic material mixed in the plastic and then moulded; any non-uniformity in the plastic can create variations in the magnetic field. The array can also detect magnets that are not well-magnetised, for example those that have a double pole. Instead of a North-South pole there is a transition from North-South to North-South. This can happen if the magnetising yoke that creates the magnet is not well-positioned. Quality control of magnets is becoming more important, and the array sensor can be used to sort magnets into different versions, from premium ones for high-quality encoders requiring high levels of stability and accuracy to those suitable only for certain applications with compensation. This also helps to identify poorly performing magnets that cannot be used, and the sorting can be done by the magnet manufacturer or encoder developer. The array sensing is also increasingly important for assessing the field in the axial motors used for in-wheel motors or electric aircraft, where higher magnetic fields are used. With an Focus | Hall effect and magnetic sensors Zero Crossing Latch sensors A Zero Crossing Latch (ZCL) Hall effect sensor uses a latch in the chip to change the output signal at the zero crossing point where the polarity changes. It can be used with brushless DC (BLDC) motors as it is not susceptible to changes in the magnetic flux density and temperature, and so provides stable detection. It also allows much greater flexibility in the positioning of the Hall effect sensor in the motor. A conventional Hall effect IC detects the polarity and intensity of the applied magnetic flux density and switches the output signal. The ZCL Hall effect IC detects when the South pole of the applied magnetic flux density changes to a North pole or vice versa, and switches the output signal. This prevents a reduction in efficiency owing to temperature changes and manufacturing variations, and simplifies the control software required for smooth, high-speed revolution. The ambient temperature of a Hall effect IC used as a position sensor in a BLDC motor changes significantly with the environment and load fluctuations. As the load increases, the coil generates heat that raises the temperature of the magnet, resulting in a change in the magnetic flux density that affects the IC. In a conventional Hall effect sensor, the change in the magnetic flux density caused by the temperature rise in the magnet means the signal can miss the optimal timing for output, leading to a vicious circle of reduced motor efficiency and further rises in temperature. This is where a lot of work is done on the signal processing. A ZCL Hall effect device senses when the polarity changes, and is capable of optimal timing output even if a magnet’s characteristics change as temperature rises. This prevents the successive reduction in efficiency that occurs when a Hall effect IC misses the optimal timing for output. For various reasons, the distance between the rotor of the BLDC motor and a Hall effect IC is often not uniform. The machining accuracy of the rotor, the assembly accuracy of the rotation axis, bearings, rotor and other parts, the accuracy of mounting components on the sensor PCB and other factors during manufacture – in addition to vibrations during motor rotation – can also cause the distance between the magnet and sensor to vary. For these reasons, mass-produced motors that had been expected to provide satisfactory efficiency at the design stage might exhibit individual variations in efficiency, also leading to variations in efficiency during operation. A ZCL Hall effect device outputs a signal at the right time though, regardless of variations in the distance between the magnet and the sensor. Accurate rotational angle sensing is key to ensuring smooth, high-speed rotation in the motor. This requires integrating advance angle and delay angle control into the legacy motor control software, providing a temperature- based timing table, making calibrations to adjust for and absorb manufacturing variations. However, if the factors that change magnetic flux density are used for detecting motor rotation, the software will need to performmore granular control to check a greater number of conditions. Since a ZCL Hall effect device can detect a polarity change regardless of external causes, there is no need to perform calibration to account for changes in magnetic flux density. That means simpler software can be used to provide smooth rotation control without calibration. The ZCL technique provides more accurate Hall effect sensing (Courtesy of ABLIC) 40 Autumn 2022 | E-Mobility Engineering
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