With certain black floor materials carpet and hard floor, the drop or cliff sensors on vacuums detect false drops and fail to clean. An experiment was performed with a possible alternative to the standard proximity sensors which use changing intensity of IR light reflection with distance.
[edit] Maybe robots are just using the wrong parts. One sensor 22-70mm from Omron described as "convergent reflective" specifies
"Robust performance with little influence from sensing object color or material.
Detects objects with mirror reflective surfaces and transparent objects."
Omron B5W-LA01 data sheet
https://www.omron.com/ecb/products/pdf/en-b5w_la01.pdfActually cheaper than the Neato Robotics Sharp sensor because lacks processing of the pulse signal, and just includes a power transistor for the emitter and output from the detector -- four wire instead of three.
Description at
https://www.components.omron.com/products/photo/special/b5w-la01/indexWith spec for a particular distance, looks like the parallax method described in this post below.
(the only other model seems to be for 5mm, too close for cliffs)

In this test a parallax method was used less dependent on the intensity of reflection to measure distance. When the emitter and detector spreads cross there should be no reflection detected. The standard method relies upon the narrower spread of the focus of either detector or emitter resulting in less intense reflection over distance with beam spread, allowing calibration of distance. This method is more vulnerable to distortion from variations in floor material properties.
[edit] Note that if intensity variation from material property exceeds that of distance variation, increasing sensitivity to see black material would ruin normal material distance measurements. Only a parallax method could use higher sensitivity.
A fairly high powered emitter tightly focussed was used. [edit] Apparently not compared to the over-driven TV remote methods mentioned later below are used, with short duty cycles up to half amp. What is in the standard proximity components is not clear, trade secrets not in data sheets.
[edit] A Sharp sensor advises a large 10uf capacitor 5v power connection support, maybe for the emitter pulses.
The components worked well on light colored material and some black materials such as luggage nylon, and borderline on a black textured vinyl cell phone cover, still workable.
However, black knit cloth completely defeats detection, at least in IR. This probably means optical methods of cliff detection will not work on the problem floors reported, even with enhancements. Sonar would be needed or some mechanical feeler. Unless there are UV emitters and detectors? [edit] Some higher power emitter and other strategies mentioned later below might still make optical methods usable.
[edit] Samsung Powerbot sensors detected black knit cloth, so the problem material remains hard to find.
[edit] More sensitive TV remote IR receivers do work on the black knit fabric. See later post.
I did not have an IR laser to test, just a laser level in visible light not detected in IR. Powerful lasers present safety hazards, with lidars low powered accordingly so probably not useful.
[edit] PC laser mouse works on the black fabric. See later post.
Perhaps some camera sensor would be more sensitive than single phototransistors but likely too expensive. Such sensors are used in the vacuum lidars employing parallax for full range finder operation, where the return is indeed weak, so maybe in principle they would work aside from the economics. Yet with high enough sensitivity, emitter reflections between the floor and bottom of the robot could present a problem achieving the necessary discrimination.
Since vacuums move on rollers positioning the cleaning head and robot stance, I wonder if some mechanical sensing could be attached to those. However, they cannot easily distinguish navigable short drops from dangerous cliffs.
Samsung precautions in their user guides the robots will not work on black floors. At least they are up front with disclosure.
The emitter shows a useful focus in the picture but the detector, specified 20 degrees, was picking up returns far outside the supposed focus. There was still a marked voltage variation of 2v in the output over a half inch range at the cone crossing. Background without the emitter output virtually zero, along with any black knit cloth reflection. An additional lens would be needed to get proper detector focus.