AMR Sensors (Magnetic Sensors)FAQ

Detection methods can be roughly divided into the following.

• Open/close and position detection: Distance between the magnet and AMR sensor changes (Strength of magnetic force lines)
• Rotation detection: Direction of magnet changes (Direction of magnetic force lines)

For suitable AMR sensor candidates based on the positional relationship with the AMR sensor and movement of the magnet, see the Selection Guide. From these candidates, select the supply voltage, response frequency (high frequency type for magnets with quick movements), and current consumption (low current consumption type for battery driving).

(2) Use the Simulation Tool to determine whether the sensor turns on and off based on the positional relationship with the AMR sensor and magnet.

Although the Simulation Tool cannot cover all operation patterns, it can provide a simplified estimate of the ON/OFF level based on the positional relationship between the AMR sensor and any rectangular magnet.

(In the Simulation Tool, the position is an estimate only. Be sure to always check in the actual device.)

The detection directions of magnetic fields are different.

In a Hall effect sensor, when the magnet is directly above the sensor, the axis between the north and south poles of the magnet is perpendicular to the sensor. In an AMR, it is parallel to the sensor.

Because the magnetic force is strong near the north and south poles of the magnet, when credit card or other magnetic data is near, it could affect the card data. And so, AMR sensors are advantageous when the magnet will not be placed perpendicularly in electronic devices such as smartphones and notebook PCs.

(This does not mean that magnetic data is not lost when a horizontal placement is used.)

When the magnet position cannot be changed significantly, the AMR sensor can be turned off by the direction of the magnetic force lines. This can be used for applications in switches with extremely short strokes and in rotation detection.

What are the key points in distinguishing AMR sensors from reed switches?

Reed switches have mechanical contacts, and so the contacts have a limited lifespan, and they are sensitive to impact and vibrations.

(Have you ever had reed switches that failed to work after product delivery? They may have been affected by a strong impact during transportation or when mounting.)

AMR sensors have a higher impact durability than reed switches, have a smaller size, and when the magnet position cannot be changed significantly, the AMR sensor can be turned off by the direction of the magnetic force lines.

This can be used for applications in switches with extremely short strokes and in rotation detection. Also, because the magnetic field detection direction is the same as reed switches, magnets that were used in reed switches may be able to be used without modification in AMR sensors in some cases.

In contrast to reed switches, AMR sensors include built-in switching circuits, and a power supply is required to operate the sensors. Be aware that AMR sensors may have different numbers of wires and current consumptions.

What points should be focused on in the surrounding environment?

(1) When examining placement and conducting design, be aware of the direction and size of the magnetic force lines emanating from the magnet in a 3D space.

Even when a magnet is close, the sensor may be unable to turn on unless the direction of the magnetic force lines matches the sensor detection direction, and the sensitivity may appear to drop if the magnetic force lines reach the sensor at a slant. If the preliminary study is inadequate, unexpected operation could occur in the prototype or mass-production processes.

(2) The strength of the magnetic force lines changes based on the temperature.

(3) Determine the placement so that the magnetic force is sufficiently larger than the sensor ON sensitivity at the ON position and is sufficiently smaller than the OFF sensitivity at the OFF position.

(4) If there is another magnet or magnetic body (such as a metal plate reacting to the magnet), it could affect the sensor and result in an unexpected operation. Because it is difficult to provide guidelines for the affected range, try to make a design that provides as much separation as possible.

Is an actual device assessment required?

〇 It is required.

Be sure to always conduct operation verification of the AMR sensor in an actual device. For both the magnet and AMR sensor, the magnet strength and AMR sensor sensitivity vary based on the environment temperature. Higher temperatures result in shorter detection distances (more difficult to turn on), and lower temperatures result in longer detection distances (more difficult to turn off), and so be sure to also conduct assessment in actual usage environments.

(1) In cases where the output is expected to change when another magnet comes near during usage, check that operation still returns after the nearby magnet moves away.

(2) If there are screws or other magnetic objects in the area around the AMR sensor, the AMR sensor may be affected by the magnetic object.

(3) If products are manufactured without fully conducting operation verification, operation defects could occur in the market.

(4) Be aware that, even if your company requests a parts study from Murata, a "Pass" result may be obtained when operation of the product itself shows no problems, and identifying the cause of operation defects may be difficult in some cases.