Operation

Basic Knowledge of Magnetic Pattern Recognition Sensors
- Sensor Operation -

Magnetic pattern recognition sensors use a bias-type circuit that fixes elements to a permanent magnet.

Sensor Circuit

Equivalent Circuit
Equivalent Circuit

  1. Based on the characteristics of magnetoresistive elements, the sensor operations are described below. The following is a comparison of the characteristics between the magnetic bias method and the non-bias method.
Operation of Magnetic Bias Method
Operation of Non-bias Method
 Operation of Magnetic Bias Method
Operation of Non-bias Method

  • A bias magnetic flux density of approx. 200mT is applied to the elements as the sensor's operating point.
  • The sensor amplifies magnetic information by using the range where the resistance changes linearly.
  • Magnetic information is converted into an electrical signal without saturation, in the range from a minute magnetic flux density (order) to a high magnetic flux density (T order).
  • The magnetic bias method can detect both ferromagnetic and magnetic materials. However, the non-bias method can only detect magnetic materials.

- Sensor Medium Detection Mode -

Magnetic pattern recognition sensors are configured with a combination of a permanent magnet and elements. While a magnetic medium is passing over the sensor's detecting surface, the elements detect a change in the magnetic flux density, which is then converted into an electrical signal. The elements detect the magnetic flux perpendicular to the element surface.

1. The following model shows detection of a change in magnetic flux density (from RMR1 to RMR2).

following model

  1. (A) indicates that the magnetic flux density on the MR1 element is larger than that on the MR2 element. Accordingly, resistance RMR1 is larger than RMR2. (B) indicates that RMR1 is equal to RMR2, and (C) indicates that RMR2 is larger than RMR1.

2. Output voltage (Vout) is expressed by the following formula:

Vout = RMR2 / (RMR1 + RMR2) x Vin

  • Vin is the power supply voltage (fixed voltage). (V)
  • RMR1 and RMR2 are resistances of the elements. (ohm)
  1. The output voltage is proportional to the difference between RMR1 and RMR2 and the power supply voltage.
  2. When RMR1 = RMR2, Vout = Vin/2.

3. While a magnetic medium is passing over the detecting surface, an output voltage is generated according to the detected pattern.

Passage of Magnetic Medium/Output Voltage Model
Passage of Magnetic Medium/Output Voltage Model

  1. A voltage corresponding to the density and shape of the detected pattern is generated. The output voltage is proportional to the magnetic field intensity (ink density) of the target medium.
  2. From a static state to high speeds, the output voltage is constant regardless of the frequency.