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What exactly are the advantages of a triple-sensor action over its dual-sensor counterpart? For that matter, would a quadruple-sensor or quintuple-sensor ad infinitum be superior to a triple-sensor?

tl;dr - https://youtu.be/OWJOoBaFhYs?t=78
short text explanation:
1. Single sensor just reads when a key is pressed - so no way to read how fast you pressed it
2. Double-sensor action detects two positions - when key is pressed, and when key is somewhere half the way - and by time delta it can detect how fast it was pressed. But it introduced an issue now that key pressing detection requires to pass both sensors, so to repeat the note - you have to release the key to the point where second sensor kicks in.
3. Triple-sensor action reduces the distance of the issue mentioned in #2, to the point where it almost never appears, as well as allow more precise calculations (you have three timestamps now to calculate how strong was the press).

Having more sensors would probably increase precision even more, allowing better control over dynamic, but that will be in the area of so highly trained touch softness where question if you should play on DP or acoustic is not anymore.

Dual-sensor: The sound needs to cut off before it can re-triggered i.e. the "note-off" sensor is also the initial trigger sensor that sees "now the key started its travel towards the bottom".

Triple-sensor: The sound doesn't need to be cut off to be retriggered as there's a "retrigger" sensor between the initial trigger and the bottom trigger.

Triple-sensor also allows to detect the key release velocity.

For some official text, images and audio demos you can turn to e.g. Kawai and go to: http://www.kawaimp.com/mp7se/detail/touch/

So it seems like at least theoretically 3 sensors is better than 2, and 4 is better than 3, and so on.....

So I ask, how is an optical sensor better? Is it because it’s continuous rather than on/off, or am I missing something here?

Also, Alpha piano claims that their sensor tech is the best because it uses ‘strip-like’ sensors that act more like strings, and incidentally are struck by real hammers. Will this approach really work better or is it simply marketing jibber-jabber, and might end up being worse than optical sensors due to hammers/strip sensors wearing off, and the system needing more overall maintenance?

I feel this is mild, but if it’s too spicy for some please feel free to go to the organic-kumbaya, healthy-vegetarian, yoga 🧘‍♂️ centered chi energy section!

I feel like this has been discussed before. But good luck trying to find the right topic. 😄

"PW Digital Piano topics" in the "sticky thread" might help: http://forum.pianoworld.com/ubbthreads.php/topics/2752919/digital-piano-master-sticky-thread.html

Or we can repeat everything once more.

Namaste😔

4 better than 3 ? The advantage of 3 sensors over 2 is that each sensor has one function and can be placed ideally. With 2 sensors, the same sensor cut off the sound when releasing the key and start the timer when pressing the key. Then its place is a compromise.

To get an advantage of the 4th sensor, we have to introduce a new function for the new one. The only one I see is the acceleration measure like in CLP7xx.

With a continuous sensor (typically optical), the only advantage is 1/ measuring the acceleration (like CLP7xx... I don’t know if the AdvantGrand take care of it), 2/ having an half release if we release the key slowly.

I would say that in theory a single pressure sensitive solid state sensor would be the best.
In practice though they are not easy to calibrate precisely. Yet. I hope this will change in few years.
I believe there is one manufacturer already offering action with solid state sensors, but don't remember the brand.

Alpha pianos use a pressure sensor (and more likely an other sensor for the note-off event).

But if the pressure is more likely to be proportional to the velocity. I a not sure it should be far better. Perhaps the acceleration measure of CLP7xx makes the tone control more accurate.

Yep, I thought about Alpha; it looks like it uses more complicated technology: http://www.printedelectronicsworld.com/articles/9859/printed-sensor-paste-improves-pianos

I am not that convinced “ The big advantage of a strain gage versus most other sensor technologies is that the strain gage is variable. The harder the key hits the gage surface, the bigger the electrical signal that is transmitted to the instrument's audio processor brain which converts the various current levels to the appropriate notes.”... What ?! we have a dynamic keyboard : the harder we press, the louder is the sound (like most 200€+ keyboard...).

The later part of the article tells about low pressure measure (something a 2 sensors has no trouble to measure).

Three sensors are not required to calulate release velocity. Just as, on the way down, you can measure the time it takes to travel from hitting sensor point 1 to sensor point 2, you can measure the time on the way up in moving from sensor point 2 to sensor point 1. Korg Kronos has RV on its pianos, and it uses a 2-sensor action.

On a real piano the damper is released halfway through the hammer travel, i.e. 5mm out of a total of 10mm. That means that you should put the first sensor there in order to be able to generate a note-off event realistically. This however means that if you try to repeat the key if you haven't released the key above that note-off point, you won't be able to retrigger the key. So, you allow for a realistic release but inability to retrigger correctly. On a grand piano you can retrigger just 1mm above the bottom. Which is why the use three sensors now, the first on at 5mm to detect release (and release velocity), the other two strictly for velocity measurement.