How the sensors of robotic vacuum cleaners work

Watching the robot vacuum cleaner work is quite meditative and peaceful. But from time to time, inquiring minds have a question: “How does a robot manage to navigate in space and overcome obstacles on its way?” Let’s figure it out!

Despite the huge number of myths about the work of a robot vacuum cleaner, this device rightfully takes its place in our homes, and all thanks to the ease and speed with which it performs cleaning . Its effectiveness largely depends on the number and type of electronic sensors installed on board. Depending on the model, the robot vacuum cleaner uses from 6 to 15 sensors included in various systems.

The purpose of the sensors is to build a map of the object, orientate in space and ensure the safety of the device. The data received from the sensors is processed by the control program. Based on the obtained parameter values, certain scenarios are used that directly affect the actions of the cleaning robot.

Only the well-coordinated work of all systems ensures the operation of the vacuum cleaner.

Positioning system
The main system of any robot vacuum cleaner, responsible for building a map of the area to be cleaned and determining the exact location of an electronic cleaner inside the room.

The system is based on the SLAM (Simultaneous Localization And Mapping) method, the main idea of ​​which is the construction of a situational map and the localization of an object in space. It happens in the following way. The scanner installed on the object checks the area around and, based on the response of its sensors, makes a map of the area.

In the segment of household appliances, the invention came from the field of space exploration and nearby planets: one of the first such radars (more precisely, lidars) were received by lunar rovers and rovers.

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In robotic vacuum cleaners, building a map is necessary to determine the optimal cleaning algorithm. After drawing up the map, the control program develops and sends for execution the optimal route of the robot’s movement. A mobile vacuum cleaner must look into even the most distant corner!

In modern robotic vacuum cleaners, the construction of a map of the surrounding space is performed by one of two types of sensors.

Laser scanning of space
Space is scanned using a lidar (or, as it is also called, an LDS sensor), a device used for accurate measurements in a gaseous medium. Recognizing an LDS sensor is quite simple: it is a small washer-shaped protrusion located on the upper plane of the device. The sensor contains a source and a receiver of a laser or light beam (in low-power devices, LEDs are used that emit light beams in the infrared range). To provide a circular view, the LDS sensor rotates around its axis at a fairly high frequency.
The emitted light beam, meeting with obstacles in its path (walls, large furniture, etc.), is reflected from them and is captured by the lidar receiver. The distance to the obstacle is calculated from the time delay between the generation and reception of the laser beam. In most models of robotic vacuum cleaners, the rotation speed of the sensor, as a rule, is 5 rps, which is quite enough for building a map and fairly accurate calculation of the position of the vacuum cleaner in the room.

Sensors working on the same principle can be found on prototypes of unmanned vehicles.

The LDS sensor allows you to accurately determine the distance to walls, large objects and other obstacles. As a rule, sensors are used in robotic vacuum cleaners that allow you to confidently scan an area at a distance of up to 6 meters.

The main disadvantage of this design is that the sensor protrudes above the level of the upper plane, and adds a few centimeters to the height of the robot vacuum cleaner. In some cases, this can be critical, since the vacuum cleaner simply cannot physically drive under a low shelf or space under a bed or closet.

Visual navigation system
Another way of navigation is the so-called lidless system based on a wide-angle camera.

In stores: in 2 stores
Here are just a special camera that allows you to create volumetric images of space. In other words, such cameras are called “depth cameras” or ToF-cameras (Time of Flight, which literally means “flight time”).

ToF cameras are a new trend in the field of mobile gadgets. Many flagship smartphones are equipped with them. With the help of such a camera, a face recognition mechanism is easily and rather inexpensively implemented; it is impossible to deceive it with a photograph of a person.

A ToF camera is an infrared light source and a light sensor that captures the intensity of the reflected light. Their principle of operation is similar to laser distance detection. The camera calculates the time from the moment the light beam is emitted to the moment it is fixed on the photosensitive matrix, calculates the distance to the object in accordance with the time delay and makes a volumetric map of the room.

This method has several advantages. First, the lighting level does not play a decisive role. Even in the twilight, the sensor is able to “draw” the boundaries of the space to be cleaned. Secondly, the camera is built in flush with the top surface of the robot, which makes it more compact, and, therefore, open its way to hard-to-reach places.

Orientation system in space
The aim of the orientation system is to minimize collisions with obstacles in the path of the robot vacuum cleaner.

Unlike the positioning system, which scans the area around the vacuum cleaner for several meters, the orientation sensors are able to detect an obstacle within one meter. Typically, two types of sensors are used to detect obstacles: ultrasonic and infrared.

Their principle of operation is similar. Both designs have a signal transmitter and receiver. As the signal itself, either sound waves inaudible to the human ear (with a frequency of over 20 kHz) or light rays of the infrared range are used.

When obstacles are detected, the control program makes adjustments to the trajectory of the robot vacuum cleaner and takes it to the side.

Infrared sensors are placed on the lateral surfaces of the robot around its perimeter. They complement the main sensor, giving the vacuum cleaner the ability to track obstacles in an all-round way.

Side sensors serve another function. They provide movement of the robot along the wall when you need to clean around the perimeter of the room. As a rule, the sensors allow maintaining the distance from the wall at the level of 10-15 mm. This is quite enough for removing debris with the movable brushes of the robot vacuum cleaner.

In the case when the obstacle did not fall into the coverage area of ​​any of the listed sensors and the collision with the surface nevertheless occurred, the third group of sensors, installed in the movable bumper of the robot vacuum cleaner, comes into operation – touch sensors. When triggered, they send a signal to the central processor, which in turn promptly corrects the trajectory of the robot. The touch sensors are made either in the form of conventional limit switches, or in the form of an optocoupler, in which the light beam is interrupted by a movable “flag” at the moment the front bumper is pressed.

Fall protection from a height is provided by a group of sensors installed in the lower part along the perimeter of the device.

These are already familiar infrared sensors, with the same principle of operation, but the logic of their operation is significantly different. The sensor constantly monitors the presence of a hard surface under the wheels of the robot vacuum cleaner. As soon as it disappears (the robot has approached the edge of a step or is trying to move off a high threshold), the central processor receives an alarm signal from the sensor and changes the trajectory of the robot cleaner.