Proximity sensing generally refers to detecting:
a. The presence or absence of an object.
b. The size or simple shape of an object.
Proximity sensors can be further categorized in operation as contact or non-contact, as well as analog or digital. The choice of sensor depends on physical, environmental, and control conditions. These include:
Mechanical:
Any suitable mechanical/electrical switch can be employed, but due to the force required to operate mechanical switches, microswitches are typically used.
Pneumatic:
These proximity sensors operate by disrupting or disturbing an air flow. Pneumatic proximity sensors are an example of contact sensors. However, they are not suitable for lightweight components that may be blown away.
Optical:
In its simplest form, an optical proximity sensor operates by interrupting a beam of light falling on a photosensitive device such as a photocell. These are examples of non-contact sensors.
It is noteworthy that the lighting environment for these sensors must be carefully considered, as optical sensors can be obscured by, for instance, flashes during arc welding, and dust and smoke in the air may hinder light transmission.
Electrical:
Electrical proximity sensors can be either contact or non-contact. Simple contact sensors operate by completing a circuit between the sensor and the component. Non-contact electrical proximity sensors rely on inductive principles to detect metals or on capacitance to detect non-metals.
Range Sensing:
Distance sensing involves detecting how close or far an object is from the sensing position, although they can also function as proximity sensors. Distance or range sensors employ non-contact analog technology. Short-range sensing, from a few millimeters to a few hundred millimeters, is achieved using capacitive, inductive, and magnetic techniques. For longer-range sensing, various types of emitted energy waves (e.g., radio waves, sound waves, and lasers) are utilized.
There are six types of forces that may require sensing. In each case, the application of force can be either static (stationary) or dynamic. Force is a vector because it must be specified in both magnitude and direction. Consequently, force sensors operate analogically and are sensitive to the direction of their action. The six types of forces are:
Tensile Force
Compressive Force
Shearing Force
Torque (Twisting Force)
Bending Force
Frictional Force
Multiple techniques exist for sensing forces, some direct and some indirect.
Tensile Force:
It can be determined by strain gauges, which exhibit a change in their resistance when subjected to an increase in length. The change in resistance measured by these gauges can be converted into a force measurement, thus making them indirect devices.
Compressive Force:
It can be determined by devices known as load cells, which operate by "detecting changes in the dimensions of the cell under compressive load, or by detecting an increase in pressure within the cell under load, or by changes in resistance under compressive load."
Torque (Twisting Force):
It can be considered a combination of tensile and compressive forces, thus utilizing a combination of the aforementioned techniques.
Frictional Force:
These involve situations where motion is to be restricted, hence frictional forces are indirectly detected "by using a combination of force and motion sensors. For instance:"
Tactile sensing refers to the act of sensing through touch. The simplest type of tactile sensor utilizes a simple touch sensor array arranged in rows and columns, commonly known as a matrix sensor.
Each individual sensor becomes activated upon contact with an object. By detecting which sensors are active (digital) or the magnitude of the output signal (analog), the impression of the component can be determined. This impression is then compared against previously stored impression information to ascertain the size or shape of the component.
Mechanical, optical, and electronic tactile sensors have been implemented.
As part of process control or as a means of safety control, thermal sensing may be necessary. Multiple methods are available, and the choice of method primarily depends on the temperature to be detected.
Some common methods include: bimetallic strips, thermocouples, resistance thermometers, or thermistors. For more complex systems involving low-level heat sources, infrared thermography can be employed.
Sound Sensing (Auditory)
Acoustic sensors are capable of detecting and, in some cases, distinguishing between different sounds. They can be utilized for speech recognition to issue verbal commands or identify abnormal sounds such as explosions. The most common acoustic sensor is a microphone.
In industrial environments, a notable challenge with acoustic sensors is the substantial background noise.
Certainly, acoustic sensors can be adjusted to respond only to certain frequencies, enabling them to differentiate between various noises.
Gas or smoke sensors that are sensitive to specific gases rely on chemical changes in the material contained within the sensor. These chemical changes can result in physical expansion or generate sufficient heat to trigger switching devices.
Robotic Vision (Sight)
Vision is arguably the most active area of research in robotic sensory feedback at present.
Robotic vision refers to the process of capturing images in real-time through some form of camera and converting those images into a form that can be analyzed by a computer system. This conversion typically involves transforming the image into a digital field that the computer can comprehend. The entire process of image capture, digitization, and data analysis must be sufficiently rapid to enable the robotic system to respond to the analyzed images and take appropriate actions during the execution of its task set.
The refinement of robotic vision will unlock the full potential of artificial intelligence in industrial robots. Its applications include detecting presence, location, and movement, as well as identifying and recognizing different components, patterns, and features.