Proximity Sensors Defined

Proximity sensors are used for switching and measuring.  Proximity sensors typically have either an analog or binary output signal that can be interfaced to a controller. 

Proximity Sensor Types

There are three different proximity sensing technologies: Inductive, Ultrasonic, and Capacitive.  Sensor technology tends to be well established and major changes in the sensing technology itself are rare.  Technological developments aim to reduce cost, enable faster measuring speed, increase measuring distances, etc.  However, even these incremental technological improvements can boost business, due to the highly competitive market.  Suppliers should closely follow trends to foresee upcoming changes in demand and stay ahead of their competitors.

Capacitive Sensors

Capacitive sensor technology is relatively mature and significant changes in the sensing technology itself are rare.  However, capacitive sensors follow other market trends in terms of sensing characteristics like distance, robustness, and flexibility.  New markets for capacitive sensors continue to emerge due to their high flexibility relative to both sensor selection and electronic parts.  Sensor suppliers should closely follow trends to foresee upcoming changes in demand, which is crucial in the competitive market for capacitive sensors. 

Capacitive sensors, which are nearly twice the price of inductive sensors, have some characteristics that could only be provided by this sensing technology.  These include:

• Theoretical resolution up to 1nm, practical resolution around 0.1mm
• High speed through a bandwidth in measuring up to 10 kHz
• Good stability
• Detection of non-metallic targets

The target characteristics are the most important difference in the application of capacitive sensors.  Capacitive sensors have wide applicability since they directly sense motion, chemical composition, and/or electric field; and indirectly sense many other variables, which can be converted into motion or a dielectric constant.  Independence from the material, small size, and low price make them attractive for many applications in both factory and process automation.  These include:

• Measuring angle or position (with high accuracy with digital output; or with less absolute accuracy, faster response, and simpler circuitry with analog output)
• Switches that detect the proximity of a metal machine component as an increase in capacitance, or the proximity of a plastic component by virtue of its increased dielectric constant over air
• Measuring liquid displacement in an orifice
• Measuring pressure by a diaphragm with stable deflection properties and spacing-sensitive detector
• Sensing liquid level in a reservoir by measuring changes in capacitance between conducting immersed plates, or applied to the outside of a non-conducting tank

The target could either be an object that is manufactured (object sensing) or a part of a machine (machine sensing).  For object sensing, it is important that the sensor has the flexibility to adjust to the new conditions and target characteristics.  The sensitivity of capacitive sensors makes this really crucial.

Inductive Sensors

Inductive sensors are typically used for switching and measuring in manufacturing and other applications.  These typically have a 24V output signal (analog or binary) that can be interfaced to a controller.  All inductive sensors use a magnetic field to detect the presence/absence of an object, the distance to an object, or different materials.  These measurements are based on changes in the magnetic field, which is created by a LC-Oscillator (see Inductive Sensor Model). 

The most important difference in the application of inductive sensors is the target characteristics (see Figure 2-5).  The target could either be an object that is manufactured (object sensing) or a part of a machine (machine sensing).  In the second case, the characteristics of the target will most probably never change in a sensor’s lifetime.  This is predominantly the case for inductive sensors, whereas photoelectric sensors dominate the market for object sensing (both markets are of similar size in terms of revenue).  In the first case, it is important that the sensor has the flexibility to adjust to the new conditions and target characteristics. 

Ultrasonic Sensors

 Ultrasonic sensors emit an ultrasonic pulse which is reflected by objects in its path and the reflected wave enters the sonic cone.  The time taken for this echo to return to the sensor is directly proportional to the distance of the object because sound has a nearly constant velocity (in air: 6m/s) - nearly constant because it depends on temperature and density.  To produce a sound wave, the sensor must first be "shocked" (this is done using piezo material).  The oscillations from this shock must die away before the sensor can accurately receive its echo pulse.  The important figure here is the cycle time.  This waiting time creates an unusable region that defines the minimum allowable distance for target detection.  Besides the dilemma of distance and measuring frequency, there is the dilemma of distance and accuracy.  To increase the distance, the sensor has to work at lower frequencies, but this also enlarges the switching distance and the angle of beam.

To address the problem of accuracy, cycle time, and switching distance, there are numerous designs of ultrasonic sensors (diffuse with one/two piezo crystals, retro reflective, through beam).  Compared to other sensing technologies, ultrasonic sensors are more robust in environments that contain dust, dirt, surface variations, color variations, and no uniform structure. However, ultrasonic sensors have some limitation in terms of technical specifications:

• Accuracy (around 0.5-2mm) depending on the wavelength
• Distortions with very hot or very cold objects

 Ultrasonic sensors are employed in a variety of non-contact material monitoring applications.  These include web loop control, level control, positioning, flow monitoring, and convey transfer.

For ultrasonic sensing, technological developments aim to reduce cost, enable faster measuring speed, increase measuring distances, etc.  However, even these incremental technological improvements can boost business, due to the highly competitive market.  Suppliers should closely follow trends to foresee upcoming changes in demand and stay ahead of their competitors

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