Many engineering processes like security systems, feedback control, and robotics rely on position and distance sensors for machines to operate safely and accurately. These sensors provide vital information in real-time about the position and displacement of an object. The coordinates of an object relative to a known reference give a measure of its position. The movement of the object from one location to another with a determined angle and distance, is its displacement.
The history of position sensors begins with the potentiometer. Johann Poggendorf invented the potentiometer in 1841. With a change in resistance, it measures the position of a movable contact on a resistive track. Later, the field of position sensing was taken over by magnetoresistive sensors, which measured the change in a material’s resistance due to a magnetic field.
Soon, new position sensors appeared. These were based on solid-state electronics and included LVDTs or Linear Variable Differential Transformers. Later came digital position sensors, which offered high-resolution measurements suitable for integrating into computer systems. The latest trend is to miniaturize position and distance sensors.
Position and distance sensors provide real-time information by detecting changes in physical properties like magnetic field, inductance, capacitance, and displacement. There are various position and distance sensors, including level, thickness, radar, capacitive, gravitational, and potentiometric sensors.
Thickness and level sensors are useful in measuring the thickness and level of powders, solids, and liquids. Primary technologies they employ include optical, laser, and ultrasonic. For instance, they calculate the thickness of a material based on distance measurement between an object and the sensor within a referenced space. In the same way, level sensors measure the height of the liquid in a container to produce a level reading. Industries using thickness and level sensors include manufacturing, pharmaceutical, chemical, and food processing.
RADAR or Radio Detection and Ranging technology locates objects using radio waves. They detect objects by transmitting radio waves at specific frequencies and listening for echoes of the signals as they bounce back from objects. By analyzing the time and frequency of the returned signal, radar can determine the object’s size, speed, and location.
Capacitive sensors measure distance by measuring the change in capacitance due to the proximity of objects to the sensor. The basic sensor has two conductive plates with a dielectric material separating them. As an object moves closer to one of the plates, the capacitance of the sensor changes. This generates a voltage signal proportional to the distance of the object from the plate.
The advantage of capacitive sensors is their formidable accuracy and resolution. For instance, capacitive sensors can measure displacement in the nanometric range. Harsh environments do not affect these sensors. However, outside interference in the form of humidity and magnetic fields can affect their performance.
Potentiometric sensors typically measure linear or angular displacement. For this, the sensor employs a resistive element. The basic construction consists of a thin resistive wire of film wound around an insulating element like a ceramic rod. A metallic wiper moves on the resistive element. As the wiper moves, its resistance changes with reference to one end of the resistive wire. An electronic circuit quantifies the resistance to produce a voltage output, indicating the displacement of the wiper attached to the measured object.
