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Original question:

DRV5055: DRV5055 only reports two unique voltages ...no voltage change between the poles.

DRV5055: absolute position using two sensors and a diametric ring

AL
Prodigy 170 points
Part Number: DRV5055

I thought my problems were solved...but I was wrong.

After thinking my original magnet was improperly magnetized...and scrambling to find a new diametric ring magnet...I am disappointed to learn that I'm getting the same performance as before.

I've tested my board with the magnet provided in the DRV5055-ANGLE-EVM kit...it produces a nice sin wave on each DRV5055.

When I do the same test with my custom ring magnet I get a strange valley on the peaks.  

The question is....is this somehow related to the fact that I'm trying to use a ring and not a disk?  I'm a bit naive to what's happening with the flux on these rings and I want to confirm that I'm not trying to solve a problem that can't be solved.

I'm using the least sensitive version of the sensor at this time.

See output from a single DRV5055...this is the reading from my 10-bit ADC input while spinning by hand very slow.  The valley in the peak seems to correlate with the mid point of either the north or south pole...I determined this my using another diametric magnet to locate when it switched from attraction to repulsion and then marking the middle.

Here is the graph from the magnet provided with the EVM kit.

  • 0
    TI__Genius 17990 points

    Hello Anthony,

    Either a ring magnet or a disc magnet should work, as long as it is diametrically magnetized. Can you send me the details of your new magnet? Ideally a product webpage with dimensions, magnetization, etc. 

  • 0 in reply to Mitch M
    Prodigy 170 points

    I'm slowly getting to the bottom of this.

    I'm using a custom designed ring.  

    Here is the simulation output...

    I have the sensor placed directly under the outside edge.

    My current speculation is that I'm too near the region of the magnet that is "brackish" between the two poles....I'm essentially getting a square wave from the sensor.

    I'm going to test going further out to see if I can solve this issue.

  • 0 in reply to AL
    TI__Genius 17990 points

    Ok, let me know how that goes.  Also you may want to try the TO-92 package version outside of the magnet.  For positioning these, you have two main options:

    1) You can place the package so that the front of the package is facing the magnet.  This will allow the sensors to see the strongest magnetic field that the magnet produces (ie, at the north and south poles).  If you do it this way, based on the sensor sensitivity and magnet strength, then the sensors will be a little farther from the magnet. This positioning can also allow you to use a weaker magnet than option 2.

    2) You can place the package so that the front of the package is perpendicular to the magnet.  This causes the strongest field that the sensors see to be the field in between the north and south pole, which will not be as strong as the magnetic field at the poles.  If you do it this way, you will generally be able to put the sensors closer to the magnet than in the first option (for the same sensitivity version).