Distance Sensors

Distance sensors are electronic components that are commonly used in many industries. First of all, they are used in industrial automation and robotics, but we often encounter them in everyday life as well. Many devices and machines, such as cars and elevators, contain distance sensors that make it possible to implement safety features and functions, such as a parking assistant or adaptive cruise control. In industrial conditions, distance sensors allow to detect the position of an object on the production/transmission line, position the object placed in the machine, determine its size, etc. Constructors and designers select the appropriate type of component strictly depending on the characteristics of the application, required accuracy, operating speed and the system in which the sensor will perform its role. Let us take a look at the basic parameters of sensors that should be considered when looking for the right solution.

How distance sensors work – technology and classification

Distance sensors most often consist of three elements: a controller, an emitter and a receiver. The controller uses the transmitter to send a signal (acoustic, electromagnetic) and then "waits" for the wave to bounce off the nearest surface and be registered by the receiver. The measurement can be made in two ways. (1) Given the time and speed of wave propagation, the controller calculates the distance travelled by the signal, and then sends this information to the master device. (2) Another method is to register the intensity of the reflected light (the weaker the returning wave, the greater the distance the sensor registers). The basic division of sensors is based on the type of the emitted signal.

Laser distance sensor

The undoubted advantage of laser sensors is their accuracy and range: the emitted beam is sent to a precisely defined point, and its appropriate propagation allows for the detection of objects located at a distance of even several dozen meters. At the same time, sampling can be performed with a frequency of several hundred hertz. In projects where the price of the component does not play a key role, a laser distance sensor will usually be the best possible solution.

Infrared and reflective distance sensors

“Reflective” sensors have a somewhat misleading name because, as described above, distance detection is usually done by reflecting some type of a signal. Elements of these type use IR (i.e. infrared) emitting diodes and a detector, e.g. a phototransistor. Compared to a laser beam, the signal is weaker and scattered. On the one hand, it limits the precision of the measurement and its maximum distance, and on the other hand, it allows the construction of sensors that detect objects present in a much wider field (they can be conventionally called the "sensor's field of view"). These types of sensors are used, among others as "cliff sensors" that prevent the machine from falling – this role is found, for example, in automatic vacuum cleaners. The range and precision are improved by using a series of sensor arrays in a single module. Equipped with many outputs, the circuit allows, for example, to detect movement parallel to the sensor plane (speed measurement). It should be noted here that the reflection sensors that record the intensity of the returning signal are perfect for detecting dark, light-absorbing surfaces. They are therefore used for detecting printed lines or counting pulses in encoders that control the speed of rotation and axis position (slot sensors).

Ultrasonic distance sensor

Ultrasonic sensors, unlike the photodetectors described above, use the propagation of acoustic waves. This solution has several advantages. First of all, the sensor can detect objects made of transparent materials as well as liquids. An example of a daily use is the circuit monitoring the water level in the tank. As the piezoelectric emitter vibrates, a (desirable) side effect of its operation is cleaning the sensor, especially of dust and tarnish. In the case of a laser sensor, similar contamination could lead to erroneous readings. Ultrasonic sensors are easy to implement and affordable, making them popular among amateur builders, especially in the construction of self-propelled robots (obstacle detection).

Distance sensor interface

Another key issue to consider when choosing a distance sensor is the type of interface. The basic division is, of course, the division into analogue and digital interfaces.

Analogue interface

Thanks to the analogue interface (current or voltage), the control system (e.g. a microcontroller) has direct access to the sensor readings and can analyze, correct, calibrate and compensate the measurement against environmental factors. It also facilitates software calibration of the sensor. Moreover, the constructor can use the analogue signal to generate interrupts, so exceeding the set value will result in an immediate reaction of the control system. The most common application of analogue interfaces is in the case of reflection sensors – the interface can be, for example, a phototransistor collector.

Digital interface

The undoubted advantage of digital interfaces is their resistance to electromagnetic radiation and the limitation of the number of wires required. In the case of the analogue interface, each sensor takes one line of the controller (or multiplexer, A/C converter). The use of such a transmission method as, for example, I₂C allows (theoretically) to connect literally hundreds of sensors to one line. Digital interfaces are a desirable solution in industrial settings, as they allow easy integration with existing control systems.