Boosting Motor Performance with Magnetic Position Sensors

There are many ways to sense motor position today, and optical encoders are favored by motor control system designers because of their height and standardized “ABI” outputs that are easily controlled by a microcontroller.

There are many ways to sense motor position today, and optical encoders are favored by motor control system designers because of their height and standardized “ABI” outputs that are easily controlled by a microcontroller.

But contactless magnetic position sensors are now the better choice for a number of reasons. Due to their smaller size and resistance to contaminants such as dust, grease, and moisture, magnetic position sensors can be used in applications with higher size and/or reliability requirements.

In the past, there has been a trend against magnetic position sensors: new brushless DC (BLDC) motors have overall high efficiency goals to reduce power consumption. At the same time, designers were tasked with adding torque to the new motor, enabling low-speed operation of the motor to support direct-drive systems. Eventually, the transmission will no longer be a necessity, which greatly reduces bill of materials.

To maximize torque and efficiency, a brushless DC motor must have a very high degree of engine rotation angle data at high speeds—which is difficult to obtain with traditional magnetic sensors. Now, a new generation of products has achieved a major breakthrough in sensor design, they can almost completely measure the rotation angle at high speed.

How to implement angle measurement

A brushless DC motor consists of a permanent magnet motor (rotor) and three or more equidistant fixed coils (stator). A magnetic field of any direction and size can be formed by controlling the current in the fixed coil. Moment is the attraction and repulsion between the rotor running on the rotating shaft and the stationary coil.

The torque reaches a value when the stationary coil magnetic field and the rotor magnetic field are perpendicular to each other. The measured rotor angle is fed back to a system that controls current through fixed coils (see Figure 1), creating a vertical magnetic field.

Boosting Motor Performance with Magnetic Position Sensors
Figure 1: A brushless DC motor control system requires either a magnetic position sensor (usually used in the automotive field) or an optical position

In most high-end applications, brushless DC motors are being replaced by permanent magnet synchronous motors (PMSM). Permanent magnet synchronous motors replace the modular commutation scheme affected by torque ripple in brushless DC motors, and can switch freely between coils, reducing vibration and achieving higher efficiency.

Of course, while industrial and automotive electric motor designs must often be optimized for efficiency and reliability, many other electric motors, especially those in consumer products, are cost-conscious. For a simple motor, the Hall switch array provides the proper position measurement and also generates the proper torque for smooth operation.

However, the degree and accuracy of the Hall switch array often cannot meet the torque and utilization requirements of high-performance engines. Conversely, a magnetic encoder (a semiconductor that integrates a Hall sensor into a silicon chip) produces high-resolution, high-resolution position data. It enables measurements on shafts at rest or at low rotational speeds. Unlike optical encoders commonly used in industrial applications, magnetic position sensors are immune to contamination and have a small footprint.

On the other hand, most Hall sensor chips suffer from two major drawbacks: dynamic angle errors at high rotational speeds caused by propagation delays; and shielding measures required in stray magnetic field environments.

These defects increase system cost and impair system performance. Dynamic angle error compensation requires a lot of processing power, and additional protection of the IC from stray magnetic fields also increases the hardware bill of materials.

Causes of Dynamic Angle Errors

The Hall sensor chip continuously samples the magnetic field strength of the magnet on the rotating shaft. The chip is mounted in a fixed position with its surface parallel to the surface of the rotating magnet, typically with a 1 to 2 mm gap between the chip and the magnet.

The chip contains a signal conditioning and processing circuit that converts the measured magnetic field strength to the angular position of the rotor (in degrees). The time required for this transition is the chip’s fixed propagation delay (see Figure 2). The duration of the delay varies from chip to chip, but chip propagation delays on the market today are typically between 10µs and 400µs.

Boosting Motor Performance with Magnetic Position Sensors
Figure 2: Signal processing in magnetic position sensors causes propagation delays

The problem of propagation delay causes dynamic angle errors as the rotor turns. The dynamic angle error increases linearly with speed; the higher the propagation delay and speed, the larger the dynamic angle error. (See Figure 3).

Figure 3 shows the increase in dynamic angle error. Suppose the chip reads the magnetic field strength when the rotor is at the red line position, and the propagation delay of the chip is 100 μs while the rotor is rotating. When the chip converts the magnetic field strength to an angle, the rotor uses 100? s time goes to the blue line – but the chip shows the ECU or MCU that the rotor is still on the red line.

Boosting Motor Performance with Magnetic Position Sensors
Figure 3: Linear relationship between dynamic angle error and rotational speed

In the absence of error compensation, the current in the adjustment scheme will go to the start coil at the red line position instead of the blue position. As a result, the system cannot be torqued, thus wasting energy and reducing system efficiency.

If the propagation delay of the chip is 100 μs and the speed of the engine is 1000 rpm, then the dynamic angle error is 1.2 degrees. If the rotational speed of the rotor is increased to 10,000 revolutions per second, the dynamic angle error increases to 12 degrees.

Boosting Motor Performance with Magnetic Position Sensors
Figure 4: How Propagation Delay Increases Dynamic Angle Error

Propagation delay is characteristic of all magnetic position sensors, so system design engineers try to apply compensation algorithms to reduce dynamic angle errors. Unfortunately, the compensation of several thousand data samples per second places a severe burden on the host ECU, even requiring an additional custom MCU dedicated to error compensation.

The design team didn’t want to inherently increase the bill of materials, nor did they want to spend too much time developing, testing, and revising their compensation algorithms.

New sensor reduces dynamic angle error

As just mentioned, the propagation delay of the magnetic position sensor is fixed, and the value of the dynamic angle error depends on the time and rotation speed of the propagation delay.

Now, Austria Microelectronics has developed a new compensation scheme applied to the magnetic sensor, the scheme is pending. This new internal compensation technique, called DAEC (Dynamic Angle Error Compensation), was first tested on the 47 series of magnetic sensors. DAEC can effectively reduce the propagation delay error of the automotive position sensor AS5147 to only 1.9μs. This means that the dynamic angle error of the AS5147 at 14,500 rpm is only 0.17 degrees, which is almost negligible.

Boosting Motor Performance with Magnetic Position Sensors
Figure 5: Sensor output with integrated compensation scheme (left) and without integrated compensation scheme (right)

Figure 6 shows the difference between the measurement output of the AS5147 (left) and a conventional magnetic position sensor (right), with some optical encoder output as a reference. The graph to the right shows that the sensor output is affected by a 200µs propagation delay, resulting in a dynamic angular error of 18 degrees at 14,500 rpm.

In contrast, the error of the AS5147 is almost negligible, which means that its signal can be used directly to tune the controller without external compensation. In fact, the dynamic angle error produced by internal compensation with DAEC technology may be smaller than external compensation because sampling errors are often present in ECUs and MCUs.

Of course, the internal compensation of the sensor can also reduce the cost of the system, because there is no additional MCU, or because a lower power ECU can be used.

Protection against stray magnetic fields

Another disadvantage of many magnetic sensors is their susceptibility to stray magnetic fields. Magnetic field disturbances other than the rotor magnets can corrupt the chip’s angle measurement at any time, and such random errors cannot be remedied by the host ECU or MCU. Therefore, the user has to take shielding measures for the chip, which increases the cost of materials and assembly; and may also violate the structural design of space-demanding applications.

According to the ISO 26262 automotive functional safety standard, immunity from stray magnetic fields has become a mandatory requirement for engine systems.

“Differential Sensing” technology is used in all Austrian Microelectronics magnetic position sensors, including the 47 series, making the sensor immune to stray magnetic fields up to a value of 25,000A/m. Below this threshold, no shielding is required.

in conclusion

The introduction of DAEC technology from Austria Microelectronics means that manufacturers of brushless DC motors and permanent magnet synchronous motors can use extreme position data to maximize torque in high-speed applications, while reducing motor size and improving reliability through magnetic position sensors .

DAEC technology is now available in AS5147* single-layer wafers) and AS5247 (double-layer redundant wafers) automotive magnetic position sensors (AEC-Q100 Phase 0 automotive applications), supporting brushless DC motors in automotive applications such as electronics Power steering (EPS), transmission (gearbox, actuators), pumps and brakes.

In industrial applications, the AS5047D with DAEC technology is also in use, providing a decimal ABI output, ideal for replacing optical encoders.

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