Using magnetic field sensors to improve the control of electric drives

Magnetic field sensors can enhance the control of electric drives by enabling precise, contactless measurements. Based on the measured magnetic field, it is possible to compute important operating variables, such as the rotor position.

Advances in sensor technology – including graphene-based sensors and emerging quantum sensors – offer significantly higher sensitivity and spatial resolution, enabling finer detection of magnetic field variations. This increased resolution can translate into more accurate torque control or higher efficiency, particularly in applications with strict constraints on available installation space. Within the Cynergy4MIE project ZF investigates possible future uses of magnetic field sensors in electrical drives with permanent magnet synchronous machine (PMSM).

State of the art

A cost-effective, robust, and reliable solution for providing the position and speed feedback required for the control of electric drives has long been established using Hall sensors (Figure 1), a specific type of magnetic field sensor. When the sensor is exposed to a magnetic field, a separation of electrical charges in the hall plates happens. The resulting voltage is amplified and filtered. As output one obtains a voltage signal which is proportional to the magnetic field penetrating the hall plates and which can be acquired by an analog to digital converter.

Figure 1: Illustration of a typical magnetic field sensor that measures the magnetic field by utilizing the Hall effect

Future applications for magnetic field sensors

However, magnetic field sensors could also be used to measure the magnetic field that is relevant for torque generation. Since it is difficult to place a sensor directly in the air gap, another option is the measurement of a flux leakage component that is in anyhow related to the main flux. Figure 2 shows an option for magnetic field sensor placement in an outrunner permanent magnet synchronous motor. In the space over the teeth heads magnetic stray fields exist which are depending on the current operating conditions of the motor.

Figure 2: Images from a finite element analysis tool used to investigate the magnetic fields in a PMSM outrunner motor

The main challenge here is to find suitable locations for sensor positioning so that it is possible to draw a connection between measured magnetic stray fields and the magnetic airgap field which is relevant for torque production.

Improving the control of electrical drives

For ZF, the project objective is to utilize signals from magnetic field sensors to enhance the control of electrical drives with PMSM. This can be achieved in several ways: information derived from the magnetic field sensors may be used to improve functional safety, for example as a fallback for other sensors like current or position sensors. Magnetic field measurements could also contribute to improved control performance, such as increasing the accuracy of the generated torque. Another potential approach is to replace more expensive sensors, thereby reducing costs. To achieve this, the next steps will involve identifying suitable sensor locations and then developing appropriate algorithms that can reliably extract the necessary information from the sensor signals.

Blog signed by: ZF team