Analyses / Blog
ST Micro 53LCX Analysis
Rev 0.1 from DPRG list post 12/15/21
We had a discussion about the VL53 family of IR proximity sensors at RBNV last night, and we were unsure of the differences between e.g. VL53L1X,VL53L1 (no X) etc. This st page has a family decoder chart:
https://www.st.com/en/imaging-and-photonics-solutions/time-of-flight-sensors.html
In a nutshell, it looks like:
VL53L0CX is 200cm single zone
VL53L1CX is a 400cm single-zone programmable FoV
VL53L1CB is 800cm multi-zone/multi-target programmable FoV (looks like best
of all - what's the downside?) 27 degree max FoV
VL53L3CX is 500cm multi-zone/multi-target with close distance detection
VL53L5CX is 400cm multi-zone/multi-target Wide FoV: 64 degrees
Sparkfun sells the VL53L3CX, VL53L1X and VL53L5CX, and some have arduino libraries, some don't. They refer to the VL53L1X as "venerable", aka old, single-zone. I only find the VL53L0X and VL53L1X (single-zone) sensors on Amazon.
It seems to me the VL53L1CB is also known as VL53L1 (no X). Someone at ST ought to be shot for defining VL53L1 and VL53L1X part numbers that have such different characteristics but only differ in part number by inclusion or not of X.
It looked like David had the VL53L1 version, so presumably VL53L1CB. Not sure about Pat.
The diagram below decodes the part numbers.
The diagram below is also helpful - it shows that single-zone vs. multi-zone are really for different applications.
Single-zone ranging is good for e.g. liquid levels, but programmable FoV gives flexibility by narrowing the FoV used to report a single zone range, Multi-zone & multi-target give more XYZ detail. Multi-zone refers to multiple regions of interest in the XY plane. Multi-target seems to refer to objects at different depths within a single region of interest - if they're separated in Z by more than 80cm it will report up to 3 of them - it histograms the arrival times and bucket-izes range readings.
These are pretty sophisticated sensors! The driver even loads the device firmware into the chip! Hats off to Pat for getting it to do ranging. As illustrated by the discussion last night, there's a lot to learn about these things.
Broadcom AFBR-S50LV85D
Rev 0.1 from DPRG list post 3/23/22
I found an excellent application note here:
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 makes 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.