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Authors of various books published with Artech House Publishing blog about current topics in microwave technology that are covered in their recent books.

How do inductive sensors work? A look into their technical inner life and their evolution over the last decennia with Sorin Fericean

July 15, 2020

Sorin Fericean’s book is titled Inductive Sensors for Industrial Applications and primarily deals with sensors containing inductive primary sensing elements, and that is because the “Inductive” is one of most widely used in the market and also due to the fact that the author acted mainly in the R & D, manufacturing and tests and marketing fields for inductive sensors for more than 25 years [1].

However, it is absolutely necessary to underline the universality of this technical reference book. The volume implicitly addresses all sensors experts having industrial sensor preoccupations, either through industrial applications or due to the activity in sensor design and validation, manufacturing and tests, sales and marketing, etc., at different levels.

The Artech House Insider Blog excellently distinguishes some main attributes of the volume in a previously published article and also provides an extensive write-up of the book titled: “How the book “Inductive Sensors for Industrial Applications” helps to develop the understanding of all sensors for industrial applications” .

You can review and order the book here.

In addition to this 360 degree-view reading, in this post, the author would like to give a general overview of the functional structure of an inductive sensor in particular, as well as of miscellaneous industrial sensors like capacitive, ultrasonic, photoelectric, magnetic, microwave based, etc., and also of the complexity evolution of these kinds of sensors over the last decennia.

Fundamental Parts of a Sensor

The technical structure of a sensor can primarily be divided into two fundamental parts: the sensing element SE and the evaluation electronics EE.

Specific to the inductive sensors ISs is the fact that the SE has an inductive behavior. Th

is inductive sensing element ISE is placed in front of the sensor embodiment, sometimes called active face, and is a determining element for the sensor operation and features. General block representations, provided by professional publications [2], present the ISE as a coil (inductor) supplied with high-frequency HF current and forming the active face.

Functional block schematic of an inductive sensor

More exactly, the representation in the figure above corresponds to a contact-less proximity sensor, which can detect metal targets approaching the sensor, without physical contact with the object to be detected, namely the target. As a result, the sensor provides a final digital or analog output signal that is dependent on the target distance/position.

The coil supplied with HF-current plays a double role. Primarily, the inductor generates an HF magnetic field spread in a front-end domain of the sensor called sensing zone. When the target approaches the magnetic field, an induction current (Eddy current) flows in the target due to electromagnetic induction.

In general, the inductor performs the sensing task in parallel. As the target approaches the coil, the Eddy current flow raises and causes a higher power dissipation in the target. In turn, this effect increases the load on the electronic circuit, which supplies the coil with HF-current, namely the oscillator circuit. Accordingly, oscillation changes or stops. The sensor electronics detects this change (attenuation, frequency sweep, impedance variation, etc.) and outputs a detection signal, finally. The change is strongly dependent not only on the distance between the target and the sensing face – sensing distance/position – but also on the target properties: dimensions, material (ferromagnetic, diamagnetic), etc. These undesired influences have to be eliminated or reduced.

Two well-known companies give a blow-by-blow description of the inductive sensors inner life [3], [4]. The parts in sight are already mentioned and the operation is similar. A first coil generates an electromagnetic field that radiates out from the active face, inducing eddy currents in the target. If the sensor has an additional receiving coil, it can operate on the basis of Faraday’s Law, which means a change in the magnetic flux in a wired coil induces a voltage in a nearby coil. This voltage is the primary quantity provided by the ISE

Although ISs have been uninterruptedly produced for fifty years, year in and year out, in enormous quantities, the knowledge about their current topology is not very different from the aforementioned information. The reasons are the high number of low-performance & low-cost devices but also the confidentiality of the company knowhow. However, their complexity and intelligence have made significant improvements.

The task of the book is to raise the top cover of this field and to give the reader up-to-date information, as far as possible. For this purpose, in Chapter 7, the book author proposes a generic functional diagram (see the following figure), which should universally be valid for the very large diversity of ISs. The functional blocks and their composing electronic stages are solely defined by showing their functionality. The implementation and integration levels of this functional diagram starting with historical discrete circuitries on transistor level and ending with future-oriented software defined sensors SDS are subjects of the following Chapters, 8 to 13.

Review and order the book Inductive Sensors for Industrial Applications

Inductive Sensing Element and Front-end Electronics – complex and challenging Core Parts of an Inductive Sensor

Generic functional diagram, universally valid for the very large diversity of inductive sensors IS

The first sensor fundamental part, namely the inductive sensing element ISE, is elaborately presented in book Chapters 4 to 6. The ISE is now figuratively depicted in the generic functional diagram by the entity of three inductors (see below). In the large majority of cases, the ISE contains a resonance circuit, which consists of the sensing coil and of a resonance capacitor, like in the generic diagram.

The second sensor part i.e. the evaluation electronics EE can essentially be divided in four basic functional units. The first, namely the front-end electronics FEE, marks the low signal and high sensitivity unit of every sensor. Furthermore, the FEE usually consists of the following electronic stages:

  • Oscillator OSC, which is the core unit of each IS and has the task to supply the ISE. Moreover, for some systems, it is also involved in the evaluation of the physical measurand.
  • Local Trimming LTM provides an adjustment to the FEE – basically to the OSC – to set the main IS features related to the sensing range. The popular teach-in sensor calibration can be manually made on the sensor assembling line or automatically at the end of the production line (testing and final check) with the support of digital central unit
  • Amplifier AMP picks up the weak signals coming from ISE (range between a few tens and few hundreds of millivolts) resulted by the primary evaluation of the measurand in the ISE and prepares them to be suitable for the next stage.
  • Detector DET converts the AC signals originated from ISE into signals, which are to be facilely processed in a digital or analogue way.

Generic functional diagram, universally valid for the very large diversity of inductive sensors IS

  • Temperature compensation TCP is a fundamental item of the ISs and solves the conflict between the negative inf
  • luence of the temperature on the operating mode, on the features of components and materials belonging to the ISE, etc. and the very tough, specific demands of the industrial applications sensors for stability over temperature in a large temperature range.

The Link with the external World, namely the Industrial Application, is provided by the Back-end Electronics of the Sensor

The back-end electronics BEE follows the FEE and continues the signal processing from a primary status offered by FEE to the status of sensor final output signals. To maintain the generic character, the BEE structure above contains two parallel signal paths: an analogue one and a digital one, which can coexist. However, usual sensors contain either of them.

  • Signal conditioning SCD opens the analogue signal path and keeps on doing a signal pre-conditioning.
  • Linearization LIN carries on with the preparation of the required or of the standard compliant sensor characteristic curve. The linearity of the IS represents a second main demand beside the temperature stability. The number of IS users, which are pleased with a monotonic sensor characteristic curve without high linearity requirements appears rather manageable.
  • Analog output stage The analogue signal path ends with this stage, where a final amplification occurs, so that the output signals V/I-OUT (voltage or current) are standard compliant.
  • Threshold comparator CMP opens the digital signal path and is often a regenerative comparator with hysteresis (Schmitt trigger) implemented by applying positive feedback to the noninverting input of a comparator or a differential amplifier. The circuit is named a "trigger" because the output retains its value until the input changes sufficiently to trigger a change in the output. CMP converts the analogue signal coming from DET to a digital output signal. The trigger operation is required by the standard demand for an implemented hysteresis H of sensor binary outputs. The trigger threshold and the hysteresis H can be fixed or programmed via DCU.
  • Combinational logic unit CLU symbolizes the logical links through different stages and resulting signals, which provide the sensor specific logical functions, e.g. the switching functions and switching output types. These various logical operations are internally hard-wired, can be ex-factory hard-coded or are programmable again via DCU.
  • Switching Output driver(s) SOD provides the switching output signal(s), which are symbolized by the binary signal line OUT. It is a very important stage, which complies with a wide number of standard demands regarding the sensor switching capability, output types: high-side driver HSD, low-side driver LSD, and push-pull, normally-open NO or normally-closed NC, output current efficiency, switching dynamic performances, etc.
  • Short-circuit protection SCP shares a circuitry for both outputs (analogue and binary), which continuously monitors the output currents and immediately acts to protect the outputs if the output current IOUT exceeds a maximal value ISC (over-current) or if a short-circuit occurs at the output.
  • Display / Optical Indicator D/OI is realized with a lot of possible implementations: binary indicators with light-emitting diodes LED or liquid crystal display LCD, analogue bargraph indicators, numerical (7-segments) or alpha-numerical (16-segments or 35-dot matrix), as well as display screens. The minimum configuration is given by LED-indicator(s).

The Power Supply and Protections Sensor Chain performs Robustness of the Industrial Sensor

The power supply and protections PSP is an essential functional unit of any sensor installed in

industrial plants. In the figure above, this block PSP is a placeholder not only for sensor internal supplying but also for sensor internal protection functions, which are mandatory for industrial sensor implementations. The inventory list of these tasks contains:

  • Power management;
  • Generation of the sensor internal supply voltage(s): +VCC, +VDD, +VSS based on the usually singular unipolar supply voltage +VB ;
  • High-voltage protection;
  • SURGE protection against high-energetic pulses on the supply voltage line +VB ;
  • ESD protection against electrostatic discharges;
  • EFT protection against electric fast transients of the supply voltage.

Up-to-Date Inductive Sensors become more and more Digital Control Units  employing Systems

The digital control unit DCU is an advanced functional unit, which belongs to modern, state-of-the-art IS. As a result of sensor specific constraints (low costs, limited free space inside the sensor housing, low current consumption, etc.), the DCU has a moderate extent commonly consisting of:

  • Finite-state machine FSM, which is defined in the automata theory as a mathematical model of computation and represents the second class of automata after the combinational logic. It is generally an abstract machine that can be in exactly one of a finite number of states (equivalent to the sensor states) at any given time. The FSM can change from one state to another in response to a change of some inputs or conditions. Such changes are called transitions. An FSM is formally defined by the list of its states, its initial state, and the conditions for each transition.
  • Memory MEM that normally consists of a volatile memory to store temporary data and a non-volatile part to store sensor parameter and/or programs.
  • Clock generator CLG principally acts as a clock signal maker for the FSM but also as a time base for generation of specific sensor times, time constants.
  • Programming and control PGC is responsible for the execution of several programming subroutines (sensor calibration, teaching of different parameters, etc.).
  • Fieldbus interface Up-to-date inductive sensors unconditionally have at least a serial bidirectional communication channel I/O-BUS, which is either a serial interface or a fieldbus.

From the discrete Sensor Electronics with distinct Transistors in the seventies up to the Large Scale Integrated Sensor Electronics today

Due to the wide spread of the inductive proximity sensors IPSs for industrial automation applications over the last 50 years, their evaluation electronics was continuously developed and integrated on a larger and larger scale. In addition, the involved silicon foundries and also their applicants, namely the sensor manufacturers, willingly published these technological steps. Correspondingly, the evolution was partially open.

With this rich volume of information, it was possible to elaborate in the book a morphological map (Figure 7.

2), which shows the evolution steps of the EE of IPS. This morphological map begins with the lowest technological level of discrete designs and passes more phases of integration, starting with the integrated circuit IC levels, going through application specific integrated circuit ASIC levels and ending with the top level of systems-on-chip SOC.

These evolution steps preferentially refer to commercial parts, provided by silicon foundries. In addition, there is a significant number of proprietary ICs and ASICs, which were developed by sensor manufacturers at their own expense. They are exclusively used for the in-house production and are obviously confidential.

The next figure shows the layout picture of a real ASIC, which includes the entire generic diagram described above on a silicon area of above 6 square millimeters. The connections of the ASIC metal pads with the electronic periphery occur in this case by means of face-up chip on board with wire bonding connections.

Microscope photography of the active face of a planar-integrated ASIC, fabricated in silicon by means of complementar metal-oxide CMOS technology

Making the next technological step and replacing the bonding wires by solder bumps directly attached on the ASIC pads, it was possible to implement the “flip-chip” technology [5] and to produce highly miniaturized inductive sensors with a metal housing external diameter of only 3 millimeters.

Assembled 3 mm - inductive sensor and its components (see the photo top): the ISE containing a wired coil on a drum ferrite core and the carrier FR4 board with “flip-chip” face-down attached ASICs [5]

 

For purchasing the book Inductive Sensors for Industrial Applications  please go to our US website  or our UK website.

References

[1]     https://www.balluff.com

[2]     https://www.keyence.co.uk/ss/products/sensor/sensorbasics/proximity/info/

[3]     https://www.baumer.com/gb/en/service-support/know-how/function-principle/functionality-and-technology-of-inductive-sensors/a/Know-how_Function_Inductive-sensors

[4]     https://www.motioncontroltips.com/what-are-inductive-proximity-sensors/

[5]     M. Jagiella, S. Fericean, and A. Dorneich, Progress and Recent Realizations of Miniaturized Inductive Proximity Sensors for Automation, IEEE Sensors Journal, Vol. 6, pp. 1734-1741, December 2006.

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