Magnetic encoder structure and working principle of magnetic encoder

The magnetic encoder is a new type of sensor device used to measure the angle, displacement, speed and distance. Today, let's take a look at the structure of the magnetic encoder and the working principle of the magnetic encoder.

Magnetic encoder structure

The main part of the magnetic encoder is composed of a magnetoresistive sensor, a magnetic drum, and a signal processing circuit. The magnetic drum is recorded into small magnetic poles at equal intervals. After the magnetic poles are magnetized, they generate periodic spatial leakage magnetic fields when rotating. The magnetic sensor probe converts the changing magnetic field signal into the change of the resistance value through the magnetoresistance effect. Under the action of the applied electric potential, the changed resistance value is converted into the change of the voltage, which is processed by the subsequent signal processing circuit to simulate the voltage signal It is converted into a digital signal that can be recognized by the computer to realize the encoding function of the magnetic rotary encoder.

The purpose of magnetization of the magnetic drum is to magnetize the small magnetic poles on the magnetic drum, so that when the drum rotates with the motor, the magnetic drum can produce cyclically varying spatial magnetic leakage, which acts on the magnetic resistance to realize the encoding function . The number of drum poles determines the resolution of the encoder. The uniformity of the drum poles and the strength of residual magnetism are important parameters that determine the structure of the encoder and the quality of the output signal.

Magnetoresistive sensors are made of magnetoresistive sensitive elements. Magnetoresistive devices can be divided into semiconductor magnetoresistive devices and ferromagnetic magnetoresistive devices. In order to improve the sensitivity of signal sampling, while taking into account the compensation effect of the differential structure on the temperature characteristics of the sensitive components, generally within the magnetization interval λ, two stripes with a phase difference of /2 are etched to form a half-bridge series network.

At the same time, in order to improve the resolution of the encoder, a plurality of magnetoresistive sensitive elements can be arranged in parallel on the magnetic head. When voltage is applied, the magnetoresistive element outputs a corresponding sine wave through the rotation of the drum. The principle can be simply explained: the magnetic drum generates the NS magnetic field for circular motion, and the sensor made of the magnetoresistive element changes its resistance as the magnetic field changes, and senses two voltage waveforms SinA and SinB. The structure of the magnetoresistive sensor is shown in the figure, which is divided into two groups with a distance of 1/4 NS from 8 magnetoresistors. The Sin voltage waveform can be detected at the contacts of Mr1, Mr2, Mr3, and Mr4. The same principle is that the SinB voltage waveform can be detected at the contacts of Mr1’, Mr2’ and Mr3’, Mr4’.

The two-way waveform signal processing circuit output from the magnetoresistive sensor: SinA, SinB After the signal reaches the signal processing circuit, the waveform needs to be adjusted in order to be within the range of cpu sampling. First, the AB phase signal needs to adjust the DC voltage level first, so that the AB phase signal DC level is at the midpoint of the DSP A/D sampling voltage range, and the amplitude does not exceed the sampling voltage range. The AB phase signal passes through the analog filter and digital Filter, after filtering out high frequency and harmonics, the position and speed will be calculated in real time through the high-speed computing capability of DSP. Another processing method is to directly convert SinA and SinB signals into square waves through the signal processing circuit. Enter DSP. The latter may be easier to process with software.

Working principle of magnetic encoder

Magnetic encoder is a new type of angle or displacement measuring device. Its principle is to use magnetoresistance or components to measure the angle or displacement value of changing magnetic materials. Changes in the angle or displacement of magnetic materials will cause a certain resistance or voltage change. , After processing by the single-chip microcomputer, output pulse signal or analog signal to achieve the purpose of measurement. The concentric code track starts from the center of the code disc of the magnetic encoder, and expands outward to the outside of the code disc. Each layer of code track has twice as many partitions as its inner layer. The first layer has only one light-transmitting sector and one opaque sector, and the second layer in the center has two light-transmitting sectors and two opaque sectors; while the third-layer code channel has a light-transmitting sector There are four zones and four opaque sectors. The built-in sensor head is small in size, suitable for small installation spaces, and is designed with high protection level, which can be used in harsh environments. In industrial applications, it can avoid the sensitivity of general optical environmental pollution and increase the long-term stability of the system.

The magnetic encoder consists of a photoelectric code disc with a shaft in the center, on which there are circular and dark engraved lines, and is read by photoelectric transmitting and receiving devices. Four groups of sine wave signals are obtained and combined into A, B, C, D, each A sine wave has a phase difference of 90 degrees. The C and D signals are reversed and superimposed on the A and B phases to enhance the stable signal; in addition, a Z-phase pulse is output every revolution to represent the zero reference position. Since the phases A and B are different by 90 degrees, the forward and reverse rotation of the magnetic encoder can be judged by comparing the phase A or the B phase. The zero reference position can be obtained through the zero pulse. The material of the magnetic encoder code disc includes glass, metal, plastic. The glass code disc is deposited on the glass with very thin score lines, which has good thermal stability and high precision. The metal code disc is directly engraved with and without pass, but because Metal has a certain thickness, its accuracy is limited, and its thermal stability is an order of magnitude worse than that of glass. Plastic code discs are economical.