MA732 filter window settings consideration for ABZ

Motor Drivers/Sensors
1
  • MA732 has default filter window of 119, resulting 11.5 effective resolution and 370hz bandwidth.
  • If I need to increase bandwidth (say 3000 hz),
    ** What are the things to be considered at ABZ interface?
    ** How do I determine what PPT value to select.
    **And for hysteresis, datasheet says 13 times the 1σ Noise, but does that change with different PPT setting other than default value?
2

Hi Bijendra, sorry for the delayed response.

  1. Filter Window setting allows the user to adjust the sensor accuracy. By decreasing this setting the noise will increase, which could cause false signal toggles. To prevent this, HYS field must be adjusted (information on how to set it can be found on “ABZ Hysteresis” section at page 24 in the Datasheet).

  2. Depends on your setup and required application. For instance, if your setup only requires four points (0º, 90º, 180º and 270º), PPT value will be set to 1 (4 edges per revolution).
    The “angle step” can be reduced down to ~0.09º by increasing PPT value (angle_step = 360/edges_per_revolution). You can find more information on Tables 19 and 20 on the Datasheet.

  3. Hysteresis value is independent from PPT setting. This field is used to prevent false signal toggles due to noise.

I hope this helps.

1 Like
3

Hi Diego,

Thanks for explaining ABZ and hysteresis setting.
Would you mind explaining a bit more regarding filter window setting and PPT setting with following example?

  1. So, If I set MA732 resolution to 10bit changing filter window to FW[68],
    does PPT value also needs to match this resolution setting of 10bit and set such that edges per revolution is 1024.
  2. Would it make any sense to leave PPT value to default of 1024 i.e 4096 edges even though filter window setting has been set to restrict sensor resolution at 10bits.
  3. or would it be better to keep the PPT value at lower than 256 (i.e 1024 edges). This one I’m asking because in MA704 with fixed resolution of 10 bit, PPT for ABZ is set to equivalent of 8bit i.e 256 edges per revolution.

Thanks,
Bijendra

4

It is recommended to set PPT, at least, one bit below your desired FW. If FW provides a resolution of 10 bits, then the recommended pulses per revolution will be 2^(10-1) = 512 or below.

Here’s the process you can follow to set these values:

  1. Set desired FW value: FW = 68 accomplishes 10 bits of resolution.
  2. Measure noise: σ = ~0.06.
  3. Set HYS: 12·σ = 0.72 → HYS = ~116
  4. Set PPT: With resolution = 10 bits, PPT = 512 - 1 = 511
5

Thanks Diego for keeping the steps together for setting FW and related registers in systematic steps. It’s clearer to me now.

Regarding setting Hysteresis, I’m bit confused if it is ‘12·σ’ or ‘±12·σ’?
One of the application note ‘Understanding the MagAlpha Digital Filter’ says as follows:

image
image1314×132 16.2 KB

Thanks again,
Bijendra

6

Hysteresis value is chosen as twice the resolution, since resolution is ±3σ, the hysteresis will be ±6σ (also defined as 12σ).

The article is mistaken on at this sentence, thanks for pointing that out! We will correct it as soon as possible.

1 Like
7

Thanks a lot for walkthrough explanation.
It makes sense and is pretty clear now regarding FW setting, hysteresis setting and PPT setting for ABZ.