By St.J. Dixon-Warren and Rob Williamson, Chipworks, Inc.
With the new generation iPhone 3GS that was just announced, Apple is trying to stay ahead of the pack for smartphones. The new feature that captured our eyes is the electronic compass (also called a magnetometer.) This is another example of a relatively simple technology being elegantly applied with a sophisticated software interface.
One of the leaders in the magnetometer space is MEMSIC, and while they did not win the iPhone socket (won by the AKM AK8973), they are a forerunners to get design wins as more electronics integrate this technology.
The target device discussed here is the Electronic Compass Board (ECB) evaluation module from MEMSIC (figure 1), and it contains both their novel thermal accelerometer and an electronic compass.
Figure 1: MEMSIC eval board
The creation of a MEMS magnet is almost a perfect example of elegant simplicity. Does anyone remember a book called, “101 Things a Boy Can Make” by author Arthur C Horth? Somewhere, in the middle was a project for building an electromagnet using a screwdriver, some wire, and a battery. Many a young engineer did just such a project. Well, MEMSIC certainly didn’t build something quite that simple, but its engineers must have has a touch of nostalgia in taking a simple concept to a whole new level to meet the complex demands of hand-held devices.
One of these is the need to have it detect the earth’s magnetic field, regardless of what direction the device is being carried or used, since most consumers would not tolerate an application that forces them to hold the compass perfectly still and level. To achieve this MEMSIC has used three sensor chips, and simply ‘bent’ the circuit board to achieve the 3rd axis, as seen in figure 2. Despite this, the package is only 1.2mm thick.
Figure 2 – X-Ray of Magnetic Sensor
According to MEMSIC the magnetic sensors are anisotropic magnetoresistive (AMR) sensors. They feature special resistors made of a permalloy thin film, which during manufacture are exposed to a strong magnetic field to orient the magnetic domains uni-directionally, establishing a magnetization vector. An external field such as the earth’s makes the magnetization rotate, and this changes the film’s resistance.
The magnetoresistive sensors are arranged within a Wheatstone bridge circuit, so that the change in resistance is detected as a change in differential voltage, so that the strength of the applied magnetic field can be inferred.
A very strong external magnetic field could upset, or flip the polarity of the film, changing the sensor characteristics. To allow for this a strong restoring magnetic field must be applied. This is enabled on chip with a magnetically coupled strap.
A compass feature combined with inertial sensors promises to improve the dead reckoning capabilities of mobile devices, and reduce the energy drain caused by GPS. It will be very interesting to see what new apps for the iPhone 3GS will appear, now that it will contain a true eCompass.