MagnTek· New product | MT6701 Differential Hall magnetic coding chip for DC brushless steering gear and advanced press knobs

MT6701 is the latest three-axis (rotation+pressing) induction magnetic angle encoding chip launched by MagnTek Suitable for unmanned aerial vehicles, gimbals, remote control models, robot joints, etc. related to DC brushless servos, as well as advanced electronic knob scenarios related to smart home appliances.

In the nearly 20-year development process of magnetic angle encoder chips, they have gone through the evolution of various magnetic induction principles, including early horizontal Hall induction, later vertical Hall induction, and the latest high-performance magnetoresistance induction (AMR anisotropic magnetoresistance, GMR giant magnetoresistance, and TMR tunnel magnetoresistance). At present, the application of high-performance magnetic angle encoders is dominated by various magnetic resistance induction technologies. At the same time, ASICs that integrate various complex signal processing and calibration algorithms have become an indispensable core part for improving the overall performance of magnetic encoder chips。

However, MagnTek has unearthed new treasures from early magnetic induction technology. Today, we will introduce a relatively ancient induction method - differential horizontal Hall induction (Hall devices are located in the horizontal plane of the chip surface).

Although in high-performance closed-loop stepper and servo motor applications, this induction method suffers from poor signal-to-noise ratio due to its naturally low sensitivity (at least 10 times lower than magnetic reluctance), resulting in noise and dynamic performance that cannot be compared to angle encoder chips based on magnetic reluctance. However, its natural differential induction method is favored for its excellent resistance to external magnetic field interference and electromagnetic radiation in certain applications; At the same time, this method can not only sense the plane angle of the chip surface, but also sense the magnetic field changes in the vertical direction of the chip, thus achieving a non-contact "knob+button" function.

MagnTek's latest MT6701 magnetic angle encoder chip is based on this principle.

Firstly, let's understand the principle of differential horizontal Hall sensing as shown in the following figure。

Figure 1 Differential Horizontal Hall Induction Principle

01 | Angle detection within the XY plane

Four horizontal Hall disks are arranged at a 90 degree angle to each other. The horizontal Hall disks only sense the magnetic field component Bz perpendicular to the chip surface. Two Hall disks on the diagonal form a differential pair, and the induced magnetic field strength is Bz - (- Bz)=2Bz. This forms two pairs of differential Hall bridges that are 90 degrees apart from each other. As a pair of radially magnetized magnets above the chip rotate, two pairs of differential Hall bridges placed at 90 ° to each other generate voltage signals of Cos θ and Sin θ as shown in the following figure. Due to the low sensitivity of the Hall disk, the voltage signal here is generally only a few mV. Through subsequent amplification, calibration, and signal processing circuit operations, the angle θ can be solved. The angle information θ of MT6701 is output to the user in the form of ABZ, PWM, analog linear voltage, and 14 bit binary digital absolute value.

Figure 2 Differential horizontal Hall angle sensing principle

02 |Differential Hall Disk Resists Magnetic Field Interference in All Directions

Due to the Hall array on the chip, it only senses magnetic fields perpendicular to the surface of the chip. As shown in Figure 1, when a pair of radially magnetized magnets are installed directly above the chip, the magnetic field lines from the north pole to the south pole of the magnet precisely cause a pair of Hall disks placed 180 ° on the chip, with one sensing a magnetic field of+BZ and the other sensing a magnetic field of - BZ. After differential operation, the total induced magnetic field is obtained as 2BZ; If there is interference magnetic field BEX in the X direction or interference magnetic field BEY in the Y direction at this time, the Hall disk will not produce any induction on it; If there is an interference magnetic field BEZ in the Z direction, one Hall disk senses the magnetic field of BEZ+BZ, and the other Hall disk senses the magnetic field of BEZ-BZ. After differential operation, the total induced magnetic field is still 2BZ. Therefore, the differential Hall disk design can perfectly eliminate the interference of magnetic fields in any direction from the outside. This has natural advantages in many applications.

03 |Vertical direction (Z direction) press detection function

In addition to angle rotation detection in the XY plane, MT6701 also integrates vertical (Z direction) pressing detection function.

This function utilizes the voltage signals Cos θ and Sin θ generated by two pairs of 90 ° electric bridges, whose sum of squares A represents the absolute strength of the magnetic field induced by the Hall disk:

A=ASin2θ+ ACos2θ

In the case where the distance between the chip and the magnet is fixed, regardless of the θ angle, the calculated value of A is fixed. However, when the direct distance between the magnet and the chip changes, the magnitude of the magnetic field induced by the Hall disk changes accordingly, and A also changes. It is precisely by utilizing this mathematical relationship that MT6701 has implemented the Z-axis pressing function, and the pressing threshold can be programmed through the chip's built-in EEPROM to facilitate various application designs.

| Convenient and anti-interference design - SSI interface

MT6701 provides a convenient and anti-interference SSI digital transmission interface.

Figure 3 SSI communication interface of MT6701

MT6701 provides a convenient SSI interface, which does not require communication commands compared to I ² C or SPI interfaces. The upper computer only needs to provide chip selection signals and clocks to read data from the chip. MT6701 provides fixed format data of 24 bits per frame on the SSI interface, including 14 bits of absolute angle data, 4 bits of magnetic field status information bits (including weak magnetic, strong magnetic, pressing, speed saturation), and up to 6 bits of CRC data check bits.

The 6-bit CRC data checksum greatly improves the anti-interference ability of data transmission. In many applications, encoders are located near the motor body or power line, and the electromagnetic environment is harsh, making signal transmission highly susceptible to interference. The upper computer can greatly reduce the probability of receiving erroneous data by using a 6-bit checksum for judgment.

01 | DC brushless servo motor

DC brushless servo is a high-end series of servo motors widely used in unmanned aerial vehicles, gimbal, robot joint control and other scenarios.

DC brushless servos are generally in the form of hollow cup motors. Due to space limitations, encoders are usually placed on the side of the motor, and general magnetic encoders are affected by the motor's magnetic field. Only differential Hall type magnetic encoders can easily and perfectly eliminate this interference.

Figure 4 Magnetic field in servo motor

02 | Application of "knob+button" based on magnetic coding

At present, most knobs are implemented using traditional potentiometers, which are inexpensive and have mature applications. However, with people's pursuit of more beautiful, convenient, and reliable smart home appliances, there is a demand to replace traditional potentiometers with magnetic angle encoder chips as knobs.

Figure 5 Traditional potentiometer knob magnetic angle encoder switch

Using magnetic Angle encoders to chip knobs has two advantages

1. Non contact operation, with a much longer service life than potentiometers. For devices that are running all the time, the limited service life of a potentiometer, which is usually tens of thousands of times, may seem inadequate, while the service life of a magnetic angle encoder scheme is independent of the number of times, with a general service life of over 10 years.

Also, due to non-contact operation, the magnet and chip can be separated, making it easy to achieve waterproof design. For example, the PCB board and knob (containing magnets) can be completely isolated with materials such as glass, which can be waterproof and does not affect use at all.

Figure 6 MT6701 implements the "knob+press" function

Click on MT6701 in the article to view the details page. For quick service, please contact our company directly at: info@magntek.com.cn ； Contact number: 021-20965129. Thank you very much for your attention to the above content. We look forward to working with you soon.