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Original question:

[FAQ] TMAG5170: How are 3D Hall-effect Sensors used in Robotics?

TMAG5170: Off-axis magnetic angle sensing - how to maintain angle accuracy despite mechanical tolerances

Lucas Muller
Prodigy 20 points
Part Number: TMAG5170

We are developing a device that uses a TMR sensor that is positioned in-plane (off-axis) with respect to a permanent magnet that is rotating about its center. We use a calibration protocol to map the orientation of the magnet to corresponding output on the sensor.

However, with this sensing configuration we have found that the orientation of the field lines across the sensor are extremely sensitive to mechanical tolerances between the sensor and magnet. In our application, the sensor and magnet are located on different subassemblies which are attached/detachment more than once. This attachment interface between them is expected to result in slight variation in position/alignment of the magnet and sensor, and calibration cannot be performed after each attachment/detachment to account for this potential misalignment. Our requirement is to detect the absolute orientation of the magnet relative to the sensor within +/-3 deg.

We stumbled across TI resource, "Achieving Highest System Angle Sensing Accuracy", which mentions the use of 3-axis hall effect sensors like the TMAG5170 to reduce the impact of off-axis tolerances on angle error (Section 4. Mechanical Error Tolerances). We'd like to ask about the following:

  1. are there ways to use these sensors to actively detect misalignments/tolerances between the magnet's geometric center and the sensor that may occur randomly during use, and
  2. are there ways to use these sensors to ignore the or compensate for the effects of misalignments/tolerances between the magnet and sensor, such that the absolute orientation of the magnet relative to the sensor can be determined accurately despite these misalignments?

Lucas

  • 0
    TI__Genius 15230 points

    Lucas,

    Thank you for reaching out on E2E.  Misalignments can be challenging, especially in the "In-Plane" alignment.  A significant part of this error will result from amplitude mismatch between each axis.  However, certain mechanical errors such as run-out or wobble can create a case where the total vector magnitude is also changing. This will create a complex error profile, which it sounds like you are familiar with. 

    In these cases, a 3D Hall-effect sensor can be used to capture the changing magnitude or to detect when component amplitudes deviate from normal.  For instance, with the sensor "In-Plane" to a rotating diametric magnet, the ideal alignment will have no z-component.  Devices like TMAG5170 or TMAG5173 can be setup with a threshold set on the z-axis output to alert the microcontroller when the field exceeds a particular threshold.  This can be done to actively predict maintenance needs or detect deviation from acceptable alignment.

    The second part of your question is a very complex challenge to address.  Typically in most cases this kind of misalignment error is calibrated out at the end of line, but I believe you are asking if there is a device function to actively detect changes in the field profile and to adjust the device response in such a way that the output is able to continuously track mechanical angle linearly.  Unfortunately, this would require a very complicated algorithm and processing capability.  This is not something that is currently supported by any of our devices.  In the event that the magnet rotation is changed from the original calibrated alignment, it would be necessary to re-calibrate position for optimal linearity.

    Thanks,

    Scott

  • 0 in reply to Scott Bryson
    Prodigy 20 points

    Thanks for the reply Scott. This all makes sense. We were hoping to either find a different magnet/sensor configuration that would be more resistant to tolerances and migration during use within a 45 degree range (all we need for our application), or to find a way to use additional sensors and magnets to detect the extent of magnet or sensor movement that occurs so that we could dynamically compensate for the expected changes to field angle at the sensor. However, as you mentioned, it seems that small offsets in x/y/z or any combination of them can influence the profile of the field angle at the sensor in a way that is not easily predictable (e.g. such as an offset or stretching of the response curve). In our application, while we can calibrate for potential misalignments of the sensor on the PCB and the component that encapsulates it, we cannot perform additional calibration after it has been attached to the magnet-holding component, which will have its own tolerances.

    I'll follow up if we have further questions about the TMAG5170/3. Thanks.