Accurate Tracking: A Look at Position and Distance Sensors
Engineering processes like robotics, feedback control, and security systems rely on position and distance sensors to provide real-time information that enables machines to operate accurately and safely.
See Our Previous Articles on Industrial Sensors:
- Magnetic and Inductive Sensing
- Pressure Sensor Fundamentals and Types
- Level Sensors For Use in Level Measurement
- Flow Rate Sensors for Fluid System Measurement
- Microelectromechanical System (MEMS) Sensors
Knowing the position and displacement of an object is vital in many applications. Engineering processes that rely on this information include robotics, feedback control, and security systems. The position of an object is given by its coordinates relative to a known reference. Displacement is the movement of an object from one location to another with a determined distance and angle.
The history of position sensors dates back to the nineteenth century with the invention of the potentiometer by Johann Poggendorf in 1841. The potentiometer measures the position of a movable contact on a resistive track. Later in the early twentieth century, magnetoresistive sensors further advanced the field of position sensing. These sensors can measure a material’s resistance change due to a magnetic field.
Figure 1. A standard potentiometer. Image used courtesy of Adafruit
Solid-state electronics developed in the sixties created new position sensors, including linear variable differential transformers (LVDTs). Digital position sensors were first developed two decades later, providing high-resolution measurements that could be integrated into computer systems. Recent trends continue to push toward the miniaturization of position and distance sensors.
Position and distance sensors provide real-time information that enables machines to operate accurately and safely. These sensors work by detecting changes in physical properties such as displacement, capacitance, inductance, and magnetic fields. This article will discuss various position and distance sensors, including potentiometric, gravitational, capacitive, radar, and thickness and level sensors.
Potentiometric sensors measure linear or angular displacement by employing a resistive element known as a potentiometer. In its basic construction, a potentiometer consists of a thin wire or film wrapped around a resistive element such as a ceramic rod. When the ceramic rod moves, the wire or film slides, changing the resistance. The resistance is quantified using an electronic circuit that produces a voltage output indicating the position or displacement of the measured object.
Potentiometric sensors are considered highly accurate and with a high degree of resolution. At the same time, they are relatively inexpensive and simple to build compared to other types of sensors. On the other hand, they can be affected by wear and tear over time due to the constant friction between moving parts.
Figure 2. Servos and robot applications sometimes employ strategically placed potentiometric sensors for position measurement.
These sensors can be found in various commercial and industrial applications. For example, they are commonly used in robotic arms to measure the position and orientation of the robot’s end effector. They also help determine the position of conveyor belts, robotic welding arms, and hydraulic cylinders. In the automotive industry, they are used for measuring the position of the accelerator, brake, and clutch pedals. Potentiometric sensors are also popular in medical devices and consumer electronics such as smartphones and tablets.
Gravity-based position sensors measure changes in gravitational forces caused by the movement of objects. These sensors use a pendulum or mass suspended from a spring, which moves when an object moves nearby. The pendulum's displacement generates a voltage signal proportional to the gravitational force.
Figure 3. Satellites rely on gravitational sensors for measuring position. Image used courtesy of Helmholtz Centre Potsdam
The accelerometer discussed in a previous installment is one of the most common examples of gravitational sensors. Other applications include aerospace, rocket science, satellites, geophysical surveys, and seismology.
Gravitational sensors are regarded as highly accurate and with minimum power consumption. However, they are relatively expensive to fabricate and require precise calibration. They are also sensitive to external variables such as temperature changes and vibration.
Capacitive sensors measure the change in capacitance caused by the proximity of objects. They consist of two conductive plates separated by a dielectric material. When an object moves close to the plates, the capacitance changes, generating a voltage signal proportional to the distance between the plates.
Figure 4. A capacitive sensor commonly used in industrial automation. Image used courtesy of Circuitrocks
The accuracy and resolution of capacitive sensors are formidable. They can measure displacement in the nanometric range. They are also resilient in harsh environments. On the other hand, they can be affected by outside interference from magnetic fields and humidity.
Radio detection and ranging (Radar) technology uses radio waves to locate objects. A radar system transmits radio waves at a specific frequency and listens for echoes of the signal when they bounce back from objects. Analysis of the returned signal can determine an object's location, speed, and even size.
Figure 5. Radar sensors are used in protection zone monitoring and collision avoidance. Image used courtesy of Pilz
Radar can be found in multiple applications, including weather forecasting, aviation, and maritime navigation. Industrial applications include level measurement and monitoring and collision avoidance.
Thickness and Level Sensor
These sensors can measure the thickness and level of liquids, solids, and powders. They employ many technologies discussed in this series, including ultrasonic, laser, and optical. For example, the thickness of a material is calculated based on distance measurement between the sensor and an object within a referenced space. Similarly, level sensors measure the material height in a container to produce a level reading.
Thickness and level sensors are used in food processing, chemical processing, and pharmaceutical manufacturing, to name a few.
Industrial Torque Sensors
In our next series installment, we will look at torque sensors and the common types used in industrial applications.