Characteristic Analysis of Switched Hall Effect Sensor
The switch-type Hall effect sensor is a magnetic proximity sensor with flexible application, wide operating voltage range and high sampling frequency. It is a high-reliability, non-contact clean sensor, which has been obtained in position sensing and rotation measurement. widely used. Switch-type Hall-effect sensors are mainly divided into single-pole proximity and bipolar latch types. The basic principles and applications are not explained. The following describes some special applications in position sensing and rotation measurement. 1 bipolar switch type Hall effect sensor The finished Hall sensor has no uniform standard when the chip is packaged, or it is triggered by the S pole of the magnet, or triggered by the N pole. When the magnet is embedded and fixed, the user needs to discriminate the polarity of the magnet during maintenance and replacement, which is inconvenient to apply. . The bipolar switching Hall sensor eliminates the need to discriminate the polarity of the magnet and provides great flexibility and convenience. 1.1 Basic principles One of the sensing faces of the Hall chip A3144 is sensitive to the S pole of the magnet. Assuming that the face is the s-plane, the other face must be N-polar sensitive, assuming that the face is called the N-face. The two switch-type Hall chips A and B are placed close together, the A chip is in front and the s-face is facing outward, the B-chip is behind and the N-face is facing outward, and the open outputs 0a and 0b of the two chips are connected as one output end. The same power source is shared and packaged in a copper bolt device. The active surface of the chip faces the magnetic pole N or S of the magnet. See Figure 1 for the structural principle. 1.2 Implementation method Because the A and B chips are stacked one on top of the other, the magnetic field should pass through the sensitive surface of the A chip to the B chip. Obviously, the sensitivity of the B chip will decrease. Experiments have shown that the trigger distance of the magnet to the bottom B chip is reduced by about 1.5 mm compared with the A chip. To compensate for the decrease in the sensitivity of the B chip, it can be compensated by adding a small ferromagnetic material to the back of the chip. The magnetic field of the rare earth magnet can pass through the A chip without triggering and can act to trigger the B chip. When the S pole is close to the sensor, the A chip outputs, and when the N pole approaches the sensor, the B chip outputs. Thus, regardless of which magnetic pole of the magnet is close to the sensor, there is always a chip output, and there is no need to care about the polarity of the magnet. The advantage of the bipolar switch type Hall effect sensor is that it does not need to care about the polarity of the magnet, the interchangeability of the sensor is strong, and the replacement and maintenance are convenient. The shortcoming is that two chips are used, the cost is slightly increased, and the working distance of the B chip is slightly reduced. 2 Hall effect high speed rotary encoder Accurate measurement of the rotation angle is often achieved by using a photoelectric rotary encoder. However, in the case of harsh environment and low precision requirements, the use of photoelectric rotary encoders will result in excessive system cost. In order to improve reliability, multiple inexpensive and reliable switches are generally used. The Hall effect sensors are mounted at different angular positions and are sampled by rotating magnets across the active surface of the sensor. When there are many sampling points, this method uses more sensors and signal lines, is inconvenient to install, and occupies more control system interface resources. 2.1 The basic principle is the same as the bipolar switch type Hall effect sensor. The difference is that the two chips A and B are output separately. Among them, chip A is used as the initial "zero point" pulse output, and chip B is used as the rotation angle "sequential" pulse output. No matter what the sampling point is, only one rotary code sensor is installed, and a four-core cable line occupies two system interfaces. See Figure 1 for the structural principle. 2.2 Implementation method When installing the magnet, only one of them, such as the s pole, faces the active surface of the sensor, and the other magnets, the N pole, face the active surface. When the magnet rotates, the s pole acts on the switch-type Hall-effect rotary encoder. A zero pulse, and the N-pole rotary encoder is the sequence pulse output, which is judged and measured by the control system, see Figure 1. Due to the large gap between the magnets, the zero pulse does not overlap with the sequence pulse. With the two-wire input, the software processing of the control system is very simple. The advantage of this type of sensor is that it replaces multiple sensors with one sensor. The installation is simple and low cost, and the more sampling points, the more obvious the superiority. The downside is that the pulse width of the two outputs is related to the rotational speed; when there are more sampling points, a larger diameter magnet mounting wheel is required. In the program control system of the printing equipment, the switch type Hall effect rotary encoder is adopted, which simplifies the system design and reduces the number of sensors. 3 magnetic bias Hall effect high speed gear proximity switch Capacitive or inductive proximity switches are difficult to meet high-speed rotation measurements due to their low operating frequency. Gear sensors that can meet high-speed measurements are relatively expensive. An inexpensive high-speed Hall-effect proximity switch can be designed using a switched Hall effect sensor chip, see Figure 2. 3.1 Basic principle The switch-type Hall effect sensor has a critical value for the magnetic induction intensity. When the magnetic induction intensity exceeds the critical value, the Hall chip is triggered to output. The use of paramagnetic materials or increased magnetic flux enhances the magnetic induction, and iron gears can meet this requirement. 3.2 Implementation Method A small high-strength magnet and a Hall chip are packaged in one sensor. The distance between the magnet and the chip is slightly larger than the critical trigger distance. At this time, the chip has no output, which is equivalent to pre-applying a magnetic bias to the chip. When the ferromagnetic material is close to the sensor, the ferromagnetic material generates a strong additional magnetic field, which is the same as the original magnetic induction intensity direction, and strengthens the magnetic induction intensity acting on the Hall chip. When the intensity is greater than the critical triggering intensity, the chip has a close signal output. The advantage of this type of sensor is that the measurement speed is up to 10KHz and the cost is low. In the harsh environment of high temperature, high humidity, and heavy dust falling, the performance is stable and the effect is good. The shortcoming is that this design can only detect ferromagnetic properties, and the application distance is a little closer. It is necessary to use a magnet with long-term stability of magnetic induction. 4 Hall effect reversible metering sensor Photoelectric reversible metering sensors have relatively complicated conversion circuits and mechanical transmission devices, which are relatively expensive, require high installation precision, and have transmission friction, which is not conducive to high-speed continuous operation. Reversible metering can be realized by using a latch-type Hall effect sensor with a simple conversion circuit, which overcomes the shortcomings of the photoelectric reversible metering sensor and has a high performance-price ratio. 4.1 Basic principle The latch type Hall effect sensor A3290 has the characteristics of trigger latch. It needs to use the N and s poles of the magnet to trigger the signal output alternately. The unipolar pole trigger cannot output continuous pulses, so the N and S pole spacing Affects the output pulse width. With this feature, the magnetic pole mounting position is reasonably arranged, and the latch type Hall effect sensor can design a steering determination sensor for reversible metering. Figure 3 Hall effect reversible metering sensor circuit principle 4.2 Implementation The dual magnets are mounted asymmetrically, with one magnet facing outward and the other N facing outward, see Figure 3. It is assumed that the s pole sets the Hall sensor HL and the N pole resets the sensor. Obviously, when the magnetic pole passes the HL clockwise and counterclockwise respectively, the output pulse width is different. The IC in the circuit is a six Schmitt trigger, IC-B and IC-C invert the output signal of HL into two signals, one input to the differential circuit composed of R1, C1 and IC-A, and output measurement pulse. The other input is to an integrating circuit composed of R2, C2, and IC-D. Reasonably select the R2 and C2 parameters so that the integral circuit can integrate the wide pulse to the input threshold voltage of IC-D, but not for the narrow pulse. Obviously, the IC-D outputs a high level when the magnet rotates counterclockwise, and the IC-D outputs a low level when it rotates clockwise. The forward and reverse rotation of the sampling wheel is judged, and the reversible metering control level is output. The advantage of this type of sensor is that the circuit is simple and is particularly suitable for high-speed rotary sampling systems. The disadvantage is that low-speed rotation can affect the judgment. In practical application, the shortcomings of the low speed are corrected. The diameter of the sampling wheel is as large as possible, and the s and N poles should be as close as possible. 5 Conclusion Hall effect sensor technology is mature and widely used, but most of its derivatives have special features and are relatively expensive. Several special-purpose sensors based on low-cost switch-type Hall sensors are designed to be simple and can be packaged with the chip. Designing a suitable sampling wheel can achieve the corresponding functions of the above-mentioned sensors, and has practical significance for simplifying design, improving stability, reliability, and reducing control system cost.
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