A brief discussion on the optical sensor technology behind sweeping robots

I sprained my back the other day and had to sleep on the floor for a few nights. The first night I was woken by the vacuum. Not only was it noisy, (although very quiet), but it kept circling me, bumping into me, backing up, moving a few inches to the left or right, and then trying to go around me again. I was not part of the normal layout of the floor. The vacuums use or cameras to navigate the room. The camera-guided vacuums didn't bump into me, but I had to pay a lot of money. The navigational vacuums depend on contact between the bumper and an object like a table leg or me.

There is a gap between the bumper and the body of the robot. Imagine the ring of light created by the gap. The ring of light breaks when the bumper hits the table leg because the bumper is recessed enough to block the light. “No light” means it hits an object. The robot stops, turns an angle, and then cleans in a straight line again. The ring of light is created using discrete emitters and detectors, reflective sensors, or integrated digital light sensors such as the VCNL36687S VCSEL-based proximity sensor.

There are several possible configurations for discrete infrared emitters and detectors. A single high-powered emitter connects a flexible light pipe to a photodiode at the other end of the light pipe. This is similar to a fiber optic cable, but in this case the flexible light pipe is compressed when it hits an object, interrupting the light. The second configuration requires the continuous bumper strip to be divided into sections. The infrared emitters in each section are aimed at the photodiode parallel to the outer ring. When the bumper strip partially hits an object, it dents inward and blocks the light beam, again creating a "no light" condition. This configuration is called transmissive sensing. In both cases, the photodetector output can be amplified, converted and interpreted by a microcontroller.

Falling is always a big problem for robot vacuum cleaners. If there are stairs that extend downward, there is a possibility of falling. The robot will be damaged if it falls down the stairs; therefore, falls need to be avoided. In this case, a digital optical proximity sensor can be used, which is tilted at a certain angle towards the ground and located at the front edge of the robot. This proximity sensor integrates an infrared emitter, a photodiode, and processing circuitry in a single package. The output of a sensor such as the Vishay VCNL36687S is a number from 0 to 4095 because the resolution is 12 bits.

As it moves along the floor, the transmitter reflects light off the surface of wood, tile, or carpet to a photodiode, which the sensor reads as a "high" count and passes to the microcontroller via a HIGH pin. When the count drops to zero, indicating that there is no floor, the robot stops. To reduce the burden on the robot, instead of constantly polling for values, the sensor can react to a set threshold and pass the information to the microcontroller via a simple HIGH/LOW pin (interrupt pin). The sensor system must be good enough to know the difference between a reflective surface and being suspended, and from a reflective surface like wood to a dark shag carpet that can swallow infrared light, and thus decide whether to go ahead or stop.

One of the most important functions of a robot vacuum cleaner is to know the distance traveled. This can also be done using optical sensors, transmissive sensors, or interrupters with gaps, where the light from the infrared emitter passes through the gaps in the code disc to the phototransistor. In this case, the transmissive sensor is a single package rather than the discrete devices mentioned above. The code disc is attached to the drive shaft of the wheel and blocks the infrared light transmitted to the phototransistor. They are collectively called optical. Using a 3-channel transmissive sensor such as the TCUT1630X01, the robot can track the distance traveled, the forward and reverse direction, and the necessary number of turns.

Thank goodness all of these sensors use 940 nm infrared light. If it were visible light, I'd be screwed.

Editor: Huang Fei