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

reading every other sensor to provide an output that is four times faster. For rotor measurements, the array typically measures one line as the rotor is rotating, as it doesn’t make sense to measure the whole 2D array. The field is cylindrical, so there is one line that is key. This gives one measurement line at a particular angle that determines the entire field. Similarly, the number of pixels can be adjusted in each line, for example by using every other pixel for twice the speed. Longer lines can be used to measure a rotor in fewer revolutions. A rotor might be 150 mm long, so to cover the whole surface with the 12.7 mm array would require multiple devices on a PCB to test a rotor in one revolution. Motor makers are now looking to add this type of camera array to production lines for 100% inline quality control of the magnets in the motors. The sensor array can generate an image of the full 2D surface, which can identify issues such as cogging torque in the rotor that cannot be detected in other ways. Cogging, or no-current, torque is that resulting from the interaction between the permanent magnets of the rotor and the stator slots. This is position-dependent, and its periodicity per revolution depends on the number of magnetic poles and the number of teeth on the stator. This can be prominent at lower speeds, producing jerkiness, and using the array can identify the fluctuations in the magnetic field during assembly. Rotary encoders Rotary encoder magnets are a very important class of sensors for automotive applications. The magnets are typically two-pole magnetised and work with a magnetic field sensor. The quality of the magnet plays a crucial role in the accuracy of the finished encoder system, and the performance is typically measured at a few millimetres from the sensor This is used to measure the field around high-performance individual magnets or those assembled in a complete motor. Scanner systems can map a larger area using a robot. This moves the sensor in steps and stitches the data together to satisfy the boundary conditions for larger magnets. The array is built using a standard CMOS silicon process that allows the signal processing electronics to be integrated. The Hall plates that act as pixels are cross-shaped flat resistors of n-type doped silicon with four terminals, and each pixel can be selected individually without cross-talk in a patented innovation. A current is sent in one direction through the cross, and the Hall voltage is read out across the other two terminals, producing an analogue signal that is proportional to the field at that location. Each pixel sensor is surrounded by a number of selection and read-out transistors across four lines per row and four lines per column, and the data is read out by multiplexing the sensors. This produces an output analogue signal that is amplified and digitised. The field that can be measured is typically around 1 Gauss, which is 0.1 mT, and is calibrated in a stable environment using a calibration magnet in a temperature-stabilised chamber. This chamber can also be used by customers for recurring calibration. The fields detected are typically for magnets used in rotary encoders. This type of camera is moving from the research lab to production, which requires a faster output. A number of measurements are averaged to get a better result with less noise, but for a faster measurement the number of sensors can be reduced, for example This magnetic camera uses an array of 16,384 Hall effect sensors (Courtesy of Magcam) Autumn 2022 | E-Mobility Engineering 39 Focus | Hall effect and magnetic sensors