Contactless Detection and Measurement (Part I)

The Simpler it is, the Better it is

The necessity to measure precisely, together with the development of complex surfaces, or of surfaces that cannot be touched during the process, trigger the development that the use of contactless technologies for detection and measurement is currently living through. Sensors of multiple shapes and sizes, based on different functioning principles, are being launched on the market.

By combining different optical techniques with modern computer systems, artificial vision systems are one of the most comprehensive tools for contactless detection and measurement. They are flexible, versatile and powerful, and their price is usually of several thousands of euros for the simplest projects.

The possibilities brought by vision systems are fascinating for any engineer, but if the matter to be solved is simple, and does not require an excessive flexibility, it is interesting to review the different specific sensors existing on the market, in order to determine whether one of the technologies developped could provide a quick and successful solution to the matter.

The classification of sensors is complex, since they are classified under several points of view: their functioning principle, the technology they use, the type of electrical signal they generate (analogical or digital) and the range of value they provide. The first classification this article is about is the one referring to the range of values a sensor can offer. If the sensor only has in output two possible states “on/off” separated by a threshhold value, we talk about a detector, whereas if it offers a range of value proportional to the dimension it measures, we talk about a transducer or measuring sensor.

Proximity Sensors and Distance Measurement

Based on different technologies, the first group of sensors I am about to present use different physical properties of the materials enabling, in the simplest case, not only to detect the presence of a part (detector), but also to measure the distance from the sensor to this part (measuring sensor), if the sensors are equipped with more sophisticated circuits for the signal treatment.

Proximity and distance sensors are usually made of a cylindrical-shaped threaded head – although there are also rectangle-shaped heads, as well as other special shapes -, and in many cases of an amplifier of the electrical signal. As for measuring sensors, this amplifier integrates the necessary electronic components as well as a digital display. If they are only used for part detection, the display is not necessary and the sensors have one or two leds enabling to visualize their possible states “on/off”.

The characteristics of the material to sense as well as the specific requirements of the application will indicate us which sensing technology is the most adapted. Bellow are presented the existing technologies for proximity sensors and/or measurement of distances, emphasizing only their most significative characteristics that I will complement with advice for their use as well as more detailed examples in other specific articles for each type.

Inductive sensors

The simplest ones work like detectors and the most sophisticated ones are able to measure distances. In both cases, their functioning is based on the change of inductance triggered by a metal object (ferric or not) in a magnetic field. By principle, they are suitable for the detection of all electricity-conducting metals, though they are not magnetic.

Their characteristics are:

• Low cost: from around 300€
• Quick response: they can work on up to several thousands of Hz.
• High precision: resolution depending on the total scale and up to the order of μm for the most expensive models
• Small size: take into account the fact that its cable is short, only a few metres.
• Immunity against adverse environments (dampness, dust, oils and temperature)

It is necessary to take into account the restrictions inherent to their own technology:

• The output linearity depends on the electrical properties of the material to detect, which is why an individual calibration is necessary for each case.
• The size of the object to detect has to be greater than the sensor diameter; if not the case, the detection distance – which usually ranges from some fractions of millimeter to about 80mm – is significantly reduced.

Capacitive Sensors

With a shape and a functioning similar to inductive sensors, the functioning of capacitive sensors is based on the change of capacity of the sensor triggered by a surface close to it. The main advantage of this technology is that it can detect the proximity of objects of any type (metal, liquid, solid, insulating). They detect all types of materials, as long as the constant dielectric of these objects is clearly superior to the air dielectric.

Their characteristics are:

• High precision: resolution depending on the total scale and up to the order of μm for the most expensive models
• Small size: take into account the fact that its cable is short, only a few metres
• Immunity against high temperatures

It is necessary to take into account the restrictions inherent to their own technology:

• They work on short distances, since their sensitivity disminishes clearly when the distance is superior to a few millimetres.
• They depend on the constant dielectric of the material to sense, and thus require a previous adjusting of their sensitivity, they are also very sensitive and unstable according to the dampness and dirt level.

Ultrasonic Sensors

Their heads emit ultrasonic waves and receive them back when they are reflected on an object. They detect the position of an object by measuring the time spent between the emission and the reception of ultrasounds. They are frequently used for the detection of liquids. Compared with the previous sensors, they provide a major detection distance but with less speed and a clearly inferior resolution.

Their characteristics are:

• They can work on long distances: up to 10m.
• Immunity against air impurities.

and they have the following limitations:

• Low resolution: : their maximal resolution is limited to 0.1mm
• Low response speed: they work at about 8MHz, a low speed compared to the other technologies
• Sensitivity to temperature, since it is affected by the sound speed, though some of them integrate temperature sensors able to compensate automatically the variations.