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Topic: How the TFMINI and other sensors actually work (Read 211 times) previous topic - next topic

dudrick

Hi, I'm doing a project for collage and I'm writing about measuring distance with difference types of waves so I got a TFMINI near infrared sensor that works off the TOF principle which I understand but I was bit confused the process that they use for measuring phase difference as a part of their main distance equation, like does it simply just read the modulated wave with one pixel or does an array of pixels read the reflected wave? If anyone has any knowledge on this than that we be greatly appreciated. Also does anyone know the microprocessor or any circuitry onboard the TFMINI.
Thanks

jremington

#1
Aug 31, 2019, 08:19 pm Last Edit: Aug 31, 2019, 08:21 pm by jremington
Carefully study the TFmini product manual. That is all the information you are likely to find.

MarkT

It has a high-speed IR emitter, which is modulated at some frequency (lets guess its 100MHz).
It has a high-speed photo diode.  Both have lens focussing tightly straight ahead.

It operates in bursts (given the power consumption parameters).  During a burst it will be correlating the
transmitted drive level against the amplified received level.  It might use time domain (square waves) or a linear phase detector (sine wave modulation).  In practice the emitter and photodiode's bandwidth limitations may mean
the signals are basically sine waves anyway, so I'd guess a linear phase detector is used.

However if 100MHz is used the phase repeats every 1.5m of distance, so some extra modulation is used
to count the multiples of 1.5m (10ns duration).  So the modulation could be trains of pulses with gaps,
the gaps allow cycle counting, and during the train the phase-detector determines the phase difference.

During the burst the circuitry will have to set the receiver gain dependent on the signal level coming in,
so there will be a lot going on.

I may be completely wrong, but these are the sorts of tricks you have to pull with modulated time of flight
tech (as used in GPS too, for instance).

The reason it uses short bursts is to save power (the emitter has to be very bright to get reflections at range),
and to allow interworking with other similar detectors in the vicinity - hopefully the bursts don't clash in time too often.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

dudrick

Thanks for the response that @MarkT that really helps me get my head around this subject. I'm assuming because of the TFmini being a proprietary product then no circuit diagrams will be available, unlike HC-SR04 which has a lot of documentation supporting it. Does anyone know the chip used on the TFMini I can't seem to find it anywhere?
Thanks again

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