Share the magnetic circuit design and circuit design of the zero-power magnetic sensor.
The zero-power magnetic sensor is a magnetic sensor developed by using the Wiegand effect. It consists of two parts: 1. a sensitive alloy wire; 2. a detection coil wound around the wire. When the relative movement between the measured magnet and the sensor occurs, an appropriate external magnetic field acts on the alloy wire, and the inner core is magnetized to jump, and the detection coil outputs an induced voltage pulse. Therefore, the voltage pulse can be used to detect the movement of the magnet, and is applied to instruments that indirectly measure various physical quantities, such as a flow meter, a three-meter (water meter, a gas meter, a heat meter), a tachometer, a corner detector, and a passive pulse generator. , no contact limit switch, etc.
Design of zero-power magnetic sensor The zero-power magnetic sensor (Wiegand sensor) can be connected to the signal processing circuit of the latter stage without any positive or negative.
1. When the sensor is working, do not use an external power supply, and completely obtain energy from the measured object (on the order of 0.1μW);
2. The output signal belongs to the intrinsically safe type;
3. The trigger mode is simple;
4, working temperature zone: -50 ° C ~ +180 ° C or even wider;
5, the output signal has obvious personality, not afraid of external interference;
6. The sensor has an extremely wide operating frequency, and the signal characteristics are independent of the moving speed of the object (magnet) to be tested;
7. The characteristics of the sensor signal can show the direction of movement of the object to be tested (magnet);
8, the original signal transmission capability is strong, only 2% attenuation per kilometer;
9, the structure is simple, the reliability is extremely high;
10. The service life is not less than 2 billion times;
11, no mechanical components, no contact, anti-shock;
12, easy to detect, through two output leads for sensor detection;
13, there are certain requirements for the strength of the magnetic field, too high or too low magnetic field will affect its work;
14, signal processing is simple, can be directly connected to the triode, absolute value amplifier, analog to digital conversion chip (ADC).
The key to the zero-power magnetic sensor (Wiegand sensor) and Wiegand module is the magnetic circuit design:
1. The N and S double magnetic poles should be alternately triggered. The distance between the two magnetic poles should be close to or greater than the sensor length. For example, a φ9.5 mm ferrite magnetic ring can be used in a water meter. A pair of magnetic steels can also be used, and the distance between the two magnetic poles is generally not less than 9 mm.
2. The magnet should be installed in such a way that the direction of the magnetic field lines is parallel to the sensor sensitive wire (see triggering method and installation method).
3. The triggering magnetic induction intensity of the position where the sensor sensitive wire is located should be ensured not to be less than the minimum value and not to be greater than the maximum value. It should be noted that in some applications (such as heat meters), the magnetic field strength of the magnet will decrease with the increase of the ambient temperature. The rotating parts may cause the distance between the magnet and the sensor to become far away due to mechanical wear, so the initial design should be The trigger magnetic induction is set to be slightly larger than the value.
4. Pay attention to magnetic shielding to prevent interference from surrounding stray magnetic fields.
Example: In the application of electronic water meter and electronic heat meter, most manufacturers currently use φ9.5mm matching ferrite magnetic ring (magnetic ring is a pair of N, S magnetic poles, the surface maximum magnetic induction intensity is above 90 mT). In order to reduce the influence of mechanical wear and temperature on the magnetic field and ensure long-term reliable operation of the instrument, it is recommended to set the initial working point of the triggering magnetic field to about 8 to 9 mT at the time of design. According to the measurement, the optimum distance from the surface of the magnetic ring to the bottom surface of the sensor (module) is 2.0 mm, and the maximum distance is not more than 2.5 mm.
When using the counting sensor as a flow meter, in order to prevent various possible interferences and ensure accurate counting, the following measures are recommended:
1. The zero-power magnetic sensor (Wiegand sensor) signal is shaped by the shaping circuit and then connected to the microcontroller. The shaping circuit can effectively eliminate signal clutter and make the sensor work more reliably. After shaping, the two output terminals of the circuit sequentially output a pair of high and low level standard square waves. The square wave amplitude is the circuit power supply voltage, the width is about 30 μS, and the square wave frequency is the same as the alternating frequency of the trigger magnetic field. The WG504 Wiegand module developed by the company integrates a zero-power magnetic sensor (Wiegand sensor) with a shaping circuit, which is more convenient to use.
2. The two input terminals of the single-chip microcomputer should separately collect the signals of the two output terminals of the shaping circuit (ie, the two output ends of the Wiegand module), and the rear-stage counter adds “1” whenever a pair of high and low levels are collected.
Note that if only the zero-wave signal of the zero-power magnetic sensor (Wiegand sensor) or an output signal of the shaping circuit is collected, it may cause miscounting.
3. Within 5mS after collecting a square wave signal, the MCU should not immediately collect the signal, and the software arranges the MCU to do other work.