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SiP styles for magnetic position sensors in automotive apps

Posted: 10 May 2016 ?? ?Print Version ?Bookmark and Share

Keywords:magnetic position sensor? MPOS? automotive rotary position sensor? PCB? SiP?

In automotive applications that call for very accurate and precise measurement of angular (rotational) or linear motion, the magnetic position sensor (MPOS) has emerged as the preferred choice of component type. The MPOS helps automotive suppliers to meet car manufacturers' demands for safety and efficiency, enabling OEMs to conform to the specifications of industry standards such as ISO 26262 (for functional safety), as well as government-mandated fuel efficiency standards.

At the same time, the MPOS offers a very high degree of electrical, mechanical and environmental robustness, superior to that of other component types such as resolvers and potentiometers. By using an MPOS, the automotive supplier can develop products that are smaller and more cost-effective and that measure rotation with greater accuracy.

Conventionally, the complete measurement solution consists of an MPOS IC and a small number of external components mounted on a PCB. The PCB is normally fixed perpendicularly to a two-pole magnet mounted on the end of the shaft or rotor which needs to be measured.

Although the total number of components mounted on the PCB might be small, this is still a more complex assembly for the automotive supplier to produce than the equivalent assembly made with a potentiometer.

This article examines the case for using a complete MPOS system produced as a System-in-Package (SiP), thus relieving the automotive supplier of the need to produce a PCB assembly when implementing magnetic position sensing.

More robust package is easier to mount
An MPOS requires a small number of external components in order to function safely and reliably in an electronic circuit. A typical example of an application circuit C this for the AS5162 automotive rotary position sensor from ams C is shown in figure 1.

Figure 1: Typical application circuit board for the AS5162 rotary MPOS from ams.

In order for the automotive supplier to implement a product, such as a pedal position-sensing module, it must design, lay out and assemble the board including the position sensor, resistors, capacitors and any required protection devices, and connect them to the rest of the system.

But if the MPOS were supplied as a SiP, this requirement would be eliminated: the entire application circuit shown in figure 1 would be combined into a single packaged unit.

This has the benefit both of reducing the footprint occupied by the sensor and its associated components, and freeing the manufacturer of the need to develop and assemble a PCB.

The SiP package technology also has an important advantage in terms of the robustness and performance of the application, because of the way that an MPOS solution is mounted within the end product. A position-sensing circuit built with a PCB cannot be completely airtight. This is because the PCB and the housing are separate components of the application, and in general the housing is moulded, and the PCB is connected to the housing's cover, a separate unit (figure 2). So on the production line, two components, the cover and the housing, have to be attached to each other.

But in rotary MPOS applications, the sensor IC must be aligned precisely with the rotating magnet on the shaft or rotor end if it is to make accurate angle measurements. And if the PCB is attached to the cover, any variation in the position of the cover relative to the housing could lead to variations (that is inaccuracy) in the output from the sensor IC.

Figure 2: Typical implementation of a magnetic position sensor in an application measuring rotary position.

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