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Three common design pitfalls in designing with Hall-effect sensors and their solutions

When designing a circuit, the performance of the circuit may not be exactly as expected. This article will help address three common challenges associated with Hall-effect sensors in industrial and automotive applications: rotary encoding, robust signal transfer, and planar magnetic sensing.

When designing a circuit, the performance of the circuit may not be exactly as expected. This article will help address three common challenges associated with Hall-effect sensors in industrial and automotive applications: rotary encoding, robust signal transfer, and planar magnetic sensing.

Challenge 1C Inability to obtain correct quadrature signatures in rotational encoding applications

In rotary encoding applications, when trying to monitor speed and direction (clockwise or counterclockwise), two Hall-effect latches or dual latches are often used. There are several causes of quadrature signature errors, but one of the most common is improper placement and misalignment between the device and the ring magnetic poles.

When using two Hall-effect latches, the proper two-bit quadrature output can be achieved mechanically by separating the Hall-effect sensor from each pole by half the width plus any integer number of widths. As shown in Figure 1b, the sensor 2 is located at the N pole/S pole interface, and the distance between the sensor 1 and the sensor 2 is the width of one full pole plus the half width of the N pole. For dual Hall-effect latches, one device can be used to separate the two sensors by exactly half the width of the poles. Of course, this is very limiting because the spacing must be matched to the ring poles.

Figure 1a shows potential placement issues when using a dual-sensor solution, while Figures 1b and 1c show how such issues can be addressed using two separate sensors or a single-chip solution, respectively. Hall-effect current sensors, such as the TMAG5110 or TMAG5111, can be used to ensure the correct signature is achieved with a variety of ring magnet sizes and pole counts. Furthermore, their simplicity of implementation eliminates any errors that may be introduced during mechanical placement. This accuracy also provides consistently accurate readings for good quadrature signatures.

Figure 1: Dual sensor rotary encoding: Figure 1a is an incorrect sensor arrangement using two latches; Figure 1b is a correct sensor arrangement using two latches; Figure 1c is a multi-position sensor arrangement using a 2D sensor

Rotary encoding applications are commonly used in many automotive and industrial applications. Here are some examples:

Car C Power windows, sunroofs, lift doors, sliding doors and power seats.

Industrial C Garage Doors and Door Openers, Thermostat Dials, Appliance Knobs, Wheel Rotation Sensing, and Motorized Curtains or Blinds.

Challenge 2 C Off-board sensor communication is not robust enough

If this problem occurs in the circuit design, it is very likely that the voltage output of the sensor used is disturbed by magnetic coupling. Although the traces may be short, if significant amounts of electromagnetic interference (EMI) are not considered, the analog signaling process can couple this interference directly into the measurement process. Establishing a reliable link between the sensor and the microcontroller (MCU) allows the MCU to sense the connected or disconnected state of the sensor. When using voltage output devices, the output may be pulled low or completely disconnected and the MCU will not be able to detect this difference.

EMI is difficult to eliminate, and while shielding, rerouting, and other mitigations add to the cost of the design, the proposed solution should focus on the sensor itself. Two-wire current output devices are inherently less sensitive to electrical noise, making them suitable for remote sensing applications using medium length cables. Despite the voltage loss associated with sending signals over extremely long wires, a two-wire current output sensor is acceptable for most industrial and automotive applications.

Figure 2 shows a Hall-effect switch with a two-wire current output, such as the TMAG5124, which can transmit signals over longer distances using a ground connection. In this example, “two-wire” means that VCC and GND must be connected from the sensor to the MCU’s general purpose input/output. Combining the current output characteristics with higher accuracy (2mT difference between the magnetic field operating point and the release point) enables a reliable design.

Figure 2: Implementation of a two-wire current output sensor

Automotive applications using current output sensors include:

Seat belt buckle.

Seat position/occupancy detection.

door latch.

Parking brake.

Sunroof/trunk closed.

brake pedal.

Challenge 3 C Hall-effect sensors are only sensitive to orthogonal magnetic fields

Today, most single-axis Hall-effect sensors can detect magnetic fields perpendicular to the package surface. If you need a sensor that can monitor the magnetic field parallel to the sides of the package, your options are limited.

Figure 3 illustrates various methods of implementing horizontal magnetic field sensing. Although horizontal magnetic field sensing can be achieved using conventional Hall-effect sensors, there are some significant drawbacks. Mounting a standard 3-pin Small Outline Transistor (SOT-23) package onto another smaller printed circuit board adds cost and complexity to assembly (Figure 3a). The transistor outline (TO-92) package has a different assembly process than standard surface mount packages, but this also increases the overall design cost (Figure 3b).

In a similar situation, a planar Hall-effect switch like the TMAG5123-Q1 can be chosen, which can detect magnetic fields on the sides of a surface mount package. Since it is available in a SOT-23 package, it may occupy a smaller space, thus allowing greater freedom and flexibility in mechanical design (Figure 3c).

Figure 3: Horizontal Magnetic Field Sensing: Figure 3a is a conventional sensor in a SOT-23 package; Figure 3b is a conventional sensor in a TO-92 package; Figure 3c is a planar sensor in a SOT-23 package

Design challenges are unavoidable, but can often be solved with a few methods and devices. Hopefully, the methods presented above will address some of the common application challenges encountered when designing with Hall-effect sensors.