[Dprglist] Broadcom AFBR-S50LV85D vs. ST Micro 53LCX

[Paul Bouchier]

At last night's RBNV, Doug P introduced us to the Broadcom AFBR-S50LV85D
TOF range sensor. It looked interesting, with a max range of 30m, and I
wondered how it compared to the ST Micro 53LCX, which I'd analyzed
previously and reviewed in a DPRG list post on Dec 15 2021. That review is
now on my site at
https://sites.google.com/site/paulbouchier/home/analyses-blog.

I found an excellent application note here:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiv2qCiudz2AhWGl2oFHR7yAq0QFnoECAcQAQ&url=https%3A%2F%2Fwww.avnet.com%2Fwps%2Fwcm%2Fconnect%2Fonesite%2F217f3b0e-788e-44c0-a63a-9595debc8370%2FApplication%2BNote%2BAFBR-S50%2BToF%2BBasics.pdf%3FMOD%3DAJPERES%26CVID%3DnuRdfj6%26CVID%3DnuRdfj6%26CVID%3DnuRdfj6%26CVID%3DnuRdfj6&usg=AOvVaw1D41jdve_frPH2eVGMTVF2

Like the ST 53LCX family of sensors, Broadcom offers a family of sensors
under the AFBR-S50LV, and the 85D version is optimized for long range. Like
the 53LCX family, they use "Inferred Time-of-Flight" (iTOF) for distance
measurement, which as John Swindle observes, isn't direct time-of-flight
measurement, but is phase measurement of the return of the modulated
transmit signal cross-correlated with the received signal.

Someone last night observed that the Broadcom sensors have a linear (8 x 4)
array of sensors and wondered if this is a linear array sensor like the old
Sharp sensors, which used parallax to measure distance as indicated by
which sensor in a linear array received the reflected light. Not really.
The linear array enables the sensor to detect nearby and distant objects,
whose reflections would fall in different places, so it accommodates
parallax, and it  looks at several factors including amplitude to choose
which pixels to use for distance reporting, but it doesn't use parallax for
distance measurement.

The Broadcom sensor family varies the diameter of the transmit spot in
different models. The 85D is long range because it uses a very narrow beam
that consequently produces a stronger reflection from distant objects than
the ST senstors and has better range in bright sunlight. By contrast, the
ST sensors don't seem to have such narrow beams - they illuminate the whole
of the sensed region. This lets the ST sensors offer selectable areas of
the array for sensing, or to sense multiple areas, producing a multi-zone
sensing. It's not clear to me whether the Broadcom 85I version can also
produce multiple sense zones.

Unlike the ST sensors, the Broadcom ones don't appear to offer multi-target
capability (i.e. multiple returns at different objects reported as
different objects within the same zone.

The broadcom application note brought up an interesting facet of
phase-difference distance measurement which didn't appear in the ST
documentation: Range Ambiguity. There is a range, dependent on the
modulation frequency, beyond which a reflection will appear to be much
closer than it is. This is because the phase angle of the return exceeds
TWO_PI, so it thinks the object is a phase closer. This distance is 6m or
12m depending on the short or long range setting of the sensor. Maybe the
ST sensors, being shorter range, don't suffer from this problem because
they can't detect anything further away - I don't know. Broadcom has some
techniques to mitigate this.

In summary, they're similar kinds of sensors, both sophisticated, and with
versions optimized around various applications. Which is better is
application-dependent, and ya pays ya money an' ya takes ya choice. 😁

I would have thought that for obstacle sensing for small, slow robots, a
short-range multi-zone wide-angle sensor would be better, whereas for an
automobile, a single-zone long-range sensor would be better. But there
might be other considerations - feel free to chime in with your thoughts.

I hope this was useful.

Regards

Paul
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.dprg.org/pipermail/dprglist-dprg.org/attachments/20220323/fa73fa63/attachment.html>