AN-2610: ADMT4000 Magnetic Reset

Introduction

A magnetic reset of the ADMT4000 giant magneto resistance (GMR) turn count sensor is required whenever the magnetic domain wall pattern is corrupted. This can occur after exposure to magnetic fields exceeding B_MAX during transport or factory assembly. It is mandatory to reset the GMR turn count sensor after the ADMT4000 is assembled into the final system with the application magnet installed. Resetting the device before magnet installation can lead to incorrect domain wall population.

Correct mechanical endstop design is a prerequisite for a successful and repeatable reset procedure and must be implemented before performing any reset operation. Before performing any magnetic reset of the ADMT4000, the following conditions must be satisfied:

• The application/system magnet must be installed.
• Mechanical endstops must be present and verified.
• The actuator must not be located within the turn count wrap or exclusion region.
• The system magnet must be aligned close to the 315° orientation.

Mechanical Endstops

Mechanical endstops must be incorporated in the system design to prevent the ADMT4000 from operating beyond its valid 0 to 46 turn range. If the turn count wraps, the reported turn count becomes incorrect once the sensor returns to the valid range, resulting in permanent loss of absolute position. For example, if the part is rotated to 47 turns, the sensor wraps to 46. When the sensor starts to count down, it has a one-turn offset versus correct system positioning. Turning the sensor beyond the intended range must be avoided in the application to preserve the correct system position information. This can be achieved by the appropriate design of mechanical gearing and mechanical endstops.

⚠ WARNING

If the mechanical endstop is located close to the sensor wrap angle, the ADMT4000 may wrap immediately after a reset or during normal operation, leading to incorrect turn count reporting.

Placement accuracy should be considered when choosing the location of the endstops such that the magnetic field observed by the ADMT4000 never exceeds the range of 10o to 16550o during normal operation, to avoid the risk of data loss due to the wrapping within the sensor. The endstop tolerance further reduces the normal operating range of the part. The endstop accuracy should be such that the part can still achieve 16515o (45 turns plus 315 o) to allow the part to be reset, when resetting at the maximum turn count position.

If the endstop location relative to the motor shaft angle is unknown, the endstop location relative to the wrap point is also unknown. Under worst-case conditions, the endstop is located at the sensor wrap angle. In this condition, the part can reset to just under 46.5 turns (Figure 1). Any slight rotation beyond the reset point wraps the sensor, causing incorrect turn count information.

To avoid this situation, the motor shaft angle at the endstop location must be known. Endstops should be placed such that the system operates within the valid range of the sensor, avoiding wrapping.

After a successful reset, the actuator position can be moved to the endstop, in a direction that increases the ADMT4000 turn count. At the endstop, it is important that sufficient margin is maintained relative to the ADMT4000 wrapping angle. Figure 2 shows the angle range to avoid at the endstop.

If the endstop is located in the excluded angle region, the actuator should be backed away from the endstop to allow the insertion of soft endstops. If this step is not performed, there is a risk of the ADMT4000 wrapping at the endstop during application use. The wrapping event can occur anywhere in this highlighted region, and small variations in orientation relative to the endstop can cause wrapping at a later time.

The soft endstop should be installed such that the angle is outside the shaded region in Figure 2. After the installation of the soft endstop, the motor angle should be brought back below 130o to ensure the final domain wall is not trapped in the last sensor segment. After this procedure is followed, the ADMT4000 reports a predictable turn count and angle at the soft endstop, in the range of 45 turns + 230o to 359o, or 46 turns + up to 170°, based on the soft endstop location.

Magnetic Reset Methods

The are two methods to perform the reset:

• Reset by overturning the system magnet.
• Reset by applying a magnetic field.

Reset by Overturning

The GMR turn count sensor can be reset by overturning by a minimum of 46 turns. The reset requires a continuous 46 full rotations in the clockwise (CW) direction, regardless of the current reported turn count. Rotating beyond the valid range without completing a full 46 turns does not guarantee a reset. This method ensures the GMR spiral is fully populated with magnetic domain walls.

Reset Using a Magnetic Field

The GMR turn count sensor can be reset by applying a magnetic field in the 315° orientation greater than 60 mT. Following the reset, the sensor spiral will be filled with magnetic domain walls resulting in a turn count of 45 plus the angle at which the reset occurred. There are four key methods to apply the reset magnetic field:

• Use an external coil to generate a magnetic field around the ADMT4000.
• Bring an external fixed magnet close to the ADMT4000.
• Move the system magnet^₁ closer to the ADMT4000.
• Use a planar electromagnetic coil embedded in the application PCB.

The system or application magnet should be in place before the reset is performed to ensure that the GMR turn count sensor is not corrupted when the application magnet is installed.

Only the embedded reset coil method is covered in this document.

Planar Embedded Reset Coil Reset Method

 
With the embedded reset coil method makes uses of a planar electromagnetic coil implemented in the application PCB. The resulting reset field (B_RESET) is the vector sum of the magnetic field generated by the embedded coil(B_COIL) and the application magnet (B_APP). This configuration allows the GMR reset procedure to be carried out in the application.

Alignment of B_APP is required in the 315° orientation. The angular range of the system magnet have a range centred around the 315° orientation where a reset may occur. The angular range at which a reset can occur is dependent on:

• The strength of B_COIL.
• The strength of B_APP.
• The temperature of the GMR turn count sensor.

The allowable angular range around the 315° orientation in which a reset can occur is system dependent and varies with temperature. It is therefore mandatory to characterize the final application across the full operating temperature range.

Embedded Reset Coil

• Figure 3 shows the ADMT4000 in the TSSOP package with the coil design shown in ADMT4000_MAGNETIC_RESET_COIL_V2.dxf.
• Figure 4 shows the ADMT4000 in the LFCSP package with the coil design shown in ADMT4000_MAGNETIC_RESET_COIL_V3.dxf.

The coil drawings are available from analog.com in the dxf format (ADMT4000_MAGNETIC_RESET_COIL_V2_1.dxf).

• The coil should be laid out with a 2 oz Cu trace on the PCB layer immediately below the ADMT4000.
• The center of the embedded reset coil should align with the center of GMR turn count sensor in the ADMT4000 package.
• The orientation of the coil should match Figure 3.

 

Embedded Reset Coil Pulse Generator

Figure 5 shows a typical circuit to generate a current pulse to excite the embedded reset coil (L2). The step-up DC-DC converter, VR1, is used to boost the supply voltage VDD (3.3 V) to charge the primary discharge capacitor C5. When C5 is fully charged, it can be discharged through L2 using the MOSFET Q1. To generate the required current pulse with a minimal VOUT voltage, the series resistance of the discharge circuit should be minimized. Care should be taken to ensure the following:

• L2 is constructed, as described in the Embedded Reset Coil section.
• C5 is a low ESR capacitor, in this example, C5 has an ESR of 22 mΩ.
• The MOSFET Q1 has a low R_ON. In this example, U1 is used to:
  • Boost COIL_RS (3.3 V logic) to 5 V.
  • Provide a fast edge to enable a rapid turn on of the MOSFET.

To fully characterize the circuit, the current through L2 can be measured by monitoring the voltage over an in-line resistor using a differential probe (for example, the Tektronik P6247) (Figure 7). The magenta trace in Figure 5 shows the resulting current pulse (229 A peak) from a voltage pulse of 28 V. VOUT can be modified by altering the resistive divider formed by R2 and R3. See the LT3467 data sheet for further information. On a standard four-layer PCB, the transfer function for the magnetic field produced by the coil at the GMR turn count sensor is 0.44 mT/A.

⚠ DESIGN CAUTION

The reset pulse is intended as a single-shot or very low duty-cycle event. Repeated or high-frequency pulsing can result in excessive heating of the PCB coil, MOSFET, or discharge capacitor. Verify the MOSFET safe operating area (SOA) and PCB copper temperature rise for the intended use case.

Table 2. Recommend Parts for the Differential Probe Connection

Reference Designator	Value	Description	Manufacturer	Part Number
C6	0.1µF	Ceramic capacitor, 0.1µF, 50V, 10%, X8R, 0603, AEC-Q200	TDK	CGA3E3X8R1H104K080AB
C7	0.1µF	Ceramic capacitor, 0.1µF, 50V, 10%, X8R, 0603, AEC-Q200	TDK	CGA3E3X8R1H104K080AB
P1	3A per Contact	PCB connector, 3-positions, unshrouded, header, pitch-mating 2.54mm	Amphenol	77311-118-03LF
R6	1MΩ	Resistor, SMD, 1MΩ, 0.1%, 1/16W, 0603	TE Connectivity	CPF0603B1M0E1
R7	1MΩ	Resistor, SMD, 1MΩ, 0.1%, 1/16W, 0603	TE Connectivity	CPF0603B1M0E1
R8	0.005Ω	Metal alloy surface mount fixed resistor, 1206 (3216M), 0.5%, 0.5W	Ohmite	LVK25R005FER