March 7, 2021

Enabling asynchronous motor commutation with a multi-purpose Hall-effect speed sensor

4 min read
Infineon’s sensor brand and family XENSIVTM provides a wide range of sensors for various industries,...

Infineon’s sensor brand and family XENSIVTM provides a wide range of sensors for various industries, reliably serving automotive and industrial customers, and general consumers. The portfolio ranges from determining magnetic position, current and speed via pressure sensors to sensors for automated driving and smart things.

When it comes to magnetic speed sensors, Infineon’s customers rely on proven Hall and GMR technology with a focus on wheel, transmission, and crankshaft and camshaft sensors. Typical applications for those sensors are ABS and powertrain.

In order to address needs besides typical automotive applications, Infineon has designed the TLE4922, a multi-purpose Hall speed sensor based on an active mono cell. It is perfectly suited to detecting the motion and position of ferromagnetic and permanent magnetic structures. An additional self-calibration module is implemented to achieve optimum accuracy during normal running operation. With its typical frequency range of 8 kHz it is ideally suited to industrial and two-wheeler applications. “Typical” is the right word in this context, because new measurements prove an enlarged frequency range, opening up new potential applications and areas of use.

A new application is the feedback loop in the drive of an asynchronous motor

While the drone market currently goes for the cheap solution – without feedback from the propeller, this will soon be mandatory, as the camera will be replaced by a passenger cabin. Where safety is a priority, feedback from the propeller will tell the system if the multicopter is working properly.

Also important for the asynchronous motor is commuting the drive unit of an electric vehicle. In the 100 kW range, efficiency and torque are critical parameters. Speed sensors are the best devices to enable accurate control of the torque of the motor. In contrast to an angular sensor, which needs a second derivative (from angle to speed to acceleration), a speed sensor needs only one derivative to provide the acceleration information.

Unfortunately, the speed of an asynchronous motor is much higher compared to the usual application such as crankshaft speed or transmission speed. The standard speed goes up to 10 kHz or even 15 kHz in transmission. An asynchronous motor requires minimum 25 kHz and even up to 40 kHz.

Figure 1: The wheel must spin at 9260 rpm to achieve 25 kHz tooth frequency.

From this point of view, the sensor will not work. But there is some buffer in terms of allowed air gap between the sensor and the trigger wheel, or the customer may be able to live with slightly reduced performance in terms of repeatability (jitter performance). Infineon investigated a dedicated setup to evaluate the performance of multipurpose speed sensor TLE4922.

The first step to attain such a high speed in terms of tooth frequency was a specially manufactured wheel with 162 teeth, as shown in Figure 1. The highly accurate jitter test benches are typically limited at a rotational speed of 10,000 rpm, which is already very fast, and reflects the crankshaft speed of a typical sports car.

The planned restrictions are in air-gap performance. Typical automotive air gaps range from 1.0 mm to 3.0 mm and reflect the needs of a combustion engine. However, the asynchronous motor takes up less space; all parts are a little smaller. So the working assumption is to use a maximum air gap of 2.0 mm instead of 3 mm. The modulation of the magnetic field is much higher at 2 mm air gap compared to 3 mm.

Further border conditions on the wheel used are as follows:

  • The toothed wheel has a circumference of 310 mm, and the length of each tooth is 2.5 mm.
  • The NdFe magnet on the back of TLE4922 is 6 mm in diameter and 4 mm in height. The flux density at the sensing element of TLE4922 is roughly 300 mT measured without the presence of a toothed wheel. Please note that TLE4922 is also capable of operating magnets based on Fe or SmCo.
  • Investigated temperatures are -40°C, 25°C, 125°C and 150°C. As the toothed wheel acts like a fan, 150°C cannot be reached at high frequency. For this reason, the following results do not show the maximum temperature at which TLE4922 is capable of operating.

Figure 2: Maximum air-gap performance of TLE4922 over-frequency.

As shown, the maximum air gap of 2.2 mm can be achieved even at maximum frequency. The degradation from 2.5 mm at 10,000 rpm to 25,000 rpm is only 0.3 mm.

For more detail on repeatability, figures 3 to 5 show the jitter performance at different air gaps.

Figure 3: Repeatability at 1 mm air gap over-frequency.

Figure 4: Repeatability at 1.5 mm air gap over-frequency.

Figure 5: Repeatability at 2 mm air gap over-frequency.

The worst case is high temperature and high frequency. The three charts shown give an indication of typical behavior, and should help with deciding maximum frequency and maximum temperature to achieve the necessary jitter performance.

The post Enabling asynchronous motor commutation with a multi-purpose Hall-effect speed sensor appeared first on EETimes.

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