IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006

New Non-contacting Linear Displacement InductiveSensors for Industrial Automation

Dr. Manfred JagiellaVP Sensors

Balluff GmbHNeuhausen a.d.F., Germanymanfred.jagiellagballuff. de

Dr. Sorin Fericean, IEEE Senior MemberCorporate Innovation Management

Balluff GmbHNeuhausen a.d.F., Germanysorin.fericeangballuff. de

Reinhard DroxlerElectronic Sensors Development

Balluff GmbHNeuhausen a.d.F., Germanyreinhard.droxlergballuff. de

Abstract-Non-contacting inductive sensors are applicable on alarge scale for position detection or travel measurement inindustrial applications due to their wear-free sensing of thetarget (any metal object), reliability, robustness, resistance tofouling, water tightness and compact size. Mostly these sensorsare realized as inductive proximity sensors (IPS), which convertthe distance between the target and sensor active face into anelectrical analog or binary signal. Because of physically givenfactors, the resulting measuring ranges are relatively limited.There are basically two approaches to convert an analog IPSinto a displacement sensor with a large measuring range: ei-ther mechanical means or modification of the sensor primarytransducer. The first approach is implemented in a popularmechanical method, which converts the linear travel into adistance change with a metal incline. Unfortunately, thelinearity, accuracy and repeatability of the IPS aredowngraded by the incline's conversion ratio. In order torender the mechanical conversion unnecessary whilepreserving the qualities of the IPS, we developed a new non-contacting linear displacement inductive sensor (LDIS) family.Due to its flexibility and compact construction, it is ideal fornumerous industrial applications and even opens up areas fornew implementations.

I. INTRODUCTION

In modem industrial production processes the position ofmoving metal objects is ideally detected without mechanicalcontact. The requirements for reliability, robustness, resis-tance to fouling, water tightness, etc. are exigent. Electricalparameters such as temperature and supply voltage ranges,repeatability, accuracy, and EMC requirements are firmlydefined in standards [1]. These requirements are completelyfulfilled by inductive proximity sensors (IPS), which convertthe distance between the metallic target and sensor activeface into an electrical analog or binary signal. The funda-mentals of the IPS, the technological progress and theachieved status today have been presented in [2, 3]. Inparallel to a great deal of positive features, the IPS presentsome restrictions as a consequence of physical given factors.

By nature, the IPS are distance sensors with small measuringranges that are heavily dependent on the sensor size.

To overcome this obstacle, ingenious users usually com-plete the system with a metal incline, which is rigidly cou-pled with the object. The large displacement is convertedmechanically into a small change of the distance between theincline and the active face of the IPS. This method enablessignificant increases in the sensing range and is the basis forthe use of the analog IPS for clamping distance monitoringon tool spindles (Fig. 1) and work-piece clamping cylinders.In the former case the task is to monitor whether the clamp-ing device is open or closed with or without the tool, and inthe latter case whether the work-piece is correctly clamped.The function of the incline is performed by a cone located onthe device shaft. As a result of the linear clamping motionand during the continuous rotation, the peripheral surface ofthe cone is moved towards the sensor.

Figure 1. Photo of analog IPS use for clamping distance monitoring in amilling machine and symbolic representation ofthe conversion: travel of a

rotating cone into distance to a cylindrical IPS.

Unfortunately, this method has a negative influence onthe measurement parameters. As a result, the positive fea-tures of the IPS, namely linearity, accuracy, repeatability,etc. will be compromised by the incline's conversion ratio(usual values: 5 to 10).

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IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006

A better approach to convert the IPS function into a dis-placement sensor is the implementation of the travel infor-mation into the sensing element. The magneto-inductive sen-sor described in [4] is a displacement sensor which detectsthe translatory movement of an object with the aid of a per-manent magnet (auxiliary target). In order to achieve a directtravel measurement, which renders both the mechanical con-version but also the auxiliary magnet target unnecessary andpreserves the qualities of the IPS, we developed a new non-contacting linear displacement inductive sensor (LDIS)family.

II. FIRST IMPLEMENTATION

A. Measuring principle ofthe new LDISThe new LDIS is a fully integrated, small sized, single

unit part. It operates without contact and detects the positionof a slender metallic target performing a translatory move-ment in parallel to the sensor active face. The target acts lo-cally on the sensor's primary transducer (PT) causing a po-sition-dependent change in its Q-factor which is processedby the sensor electronics (SE).

The basics ofthe inductive interaction between target andPT are presented in detail in [2, 3] and can be summarized bya position-dependent reduction of the Q-factor. Basically,this reduction is proportional to the target projection on thePT active face. If the geometry of PT active face changesalong the measuring path, the result is a univocal correlationbetween the Q-factor and the displacement of the target.

B. Primary TransducerThe radical change in the structure of the PT of LDIS in

contrast to IPS is the reshaping of the classical wire-woundcoil with round core [2] into a planar structured coil on di-electric substrate with flat core on its rear surface.

Figure 3. Quality factor characteristic of the single coil PT.

The formula of the Q-factor expressed in terms of fieldquantities [3]

17

P ji (i "IP=f, (1)

allows a rapid and exact evaluation of the results provided byCAD simulation [5]. In (1) the vectors H and J represent themagnetic field strength and conduction current density re-spectively as functions ofthe position vector p. The scalars [iand a are the magnetic permeability and electrical conduc-tivity. The index i represents the current number of the nsystem components in the simulation model (Fig. 4).

Displacementdirection x

""li_

00

Figure 2. Drawing of one coil of the PT for the first LDIS version(dimensions 30 mm x 10 mm).

The target acts on the planar sensor coil excited by theSE with a high-frequency current and causes a position-de-pendent change in the Q-factor [2], which is continuouslymeasured. Fig. 2 illustrates our final topology with 10windings shaped as interleaved isosceles triangles. We usean 8-layer planar structure with 8 such coils connected inseries. The high mutual coupling between the layers resultsin a total inductivity of around 100 pfH and high Q-factorvalues. Fig. 3 shows the measured Q-factor characteristicwith a monotonic range from -17 mm to 0 mm, in which asensing operation can be implemented.

Figure 4. Magnetic field of the coil illustreted in Fig. 2 with target.Simplified simulation model (one layer) and Cartesian ZY-representationfor a cross section in the coil middle and perpendicular to the x-direction.Visible are six windings, the ferrite foil and a segment ofthe round target.

C. Sensor ElectronicsThe sensor electronics can be basically divided into three

units (Fig. 5). The front end comprises an oscillator, whichprovides the excitation of the PT coil in order to measure theQ-factor, a trimming circuit and a first temperature compen-sation stage TC1. We preferred a proven, very feasible os-cillator with negative resistance [3, 6, 7], integrated in a bi-polar ASIC [3]. The interface performs further signal condi-tioning with a second temperature compensation stage TC2and two adjustable amplifiers for offset and gain adjustment.These amplifiers are digitally controlled by digital potenti-

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IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006

ometers and a microcontroller whereas the signal path stillremains pure analog. The standardized sensor output signalis generated by the output stage in the back end. This stagealso offers the required protection functions [1].

Figure 5. Block diagram of the first LDIS version.

III. ADVANCED IMPLEMENTATION

A. Improved evaluation ofthe qualityfactorThe evaluation of the quality factor using a single coil

has several disadvantages due to the strong temperature-de-pendence of the quality factor and linearity error. There is aproven balancing technique, which uses two coils and ex-ploits the properties of a detuned oscillator bridge. The PTconsists of two identical triangular coils reversely placed ona PCB. It provides an analog, time-continuous voltage whichdepends only on the ratio oftwo inductances [8].

VIN

R3

Figure 6. Double resonant oscillator bridge.

For use in the advanced LDIS, a new double resonantbridge was developed. The capacitors of the oscillator bridgewere replaced by resistors and two additional capacitors wereconnected in parallel to the coils (Fig. 6). The oscillatorbridge comprises two separate resonant circuits. Assumingthat the resonant frequencies are equal and the bridge is ex-cited with this resonant frequency co0, the resonant circuitscan be replaced by parallel resistances Rpl, RP2 [3]. With thequality factors Q1 = Rp1 I ooL1 and Q2= RP2 1 wo0L2 and theresistances R3, R4 the transfer function can be written as

pXT' IS R; -R,=_= __

ZV 2Q.+Q:+ R +R (2)

which - apart from a constant term - depends only on theratio of the two quality factors Q1 and Q2. A very simpleapproach to evaluate the transfer function is to keep VIN con-stant by using feedback. Then VOUT is proportional to thetransfer function.

B. OscillatorThe oscillator bridge can be excited using an external os-

cillator. It is however preferable to use the bridge itself as theresonant circuit of an oscillator. The oscillator shown sche-matically in Fig. 7 is implemented as a push-pull oscillatorwith output voltage VOUT referenced to ground. The oscillatoramplitude VIN is kept constant by means of a loop consistingof a subtractor, a peak detector and a PI controller.

Figure 7. Push-Pull oscillator with resonant LR-bridge and amplituderegulation.

C. Transferfunction characteristicFig. 8 shows the characteristic of the transfer function

corresponding to (2) obtained with a prototype of the doublecoil PT described above. It has a very large monotonic rangeof more than 70 mm - compared to 92 mm PT length - witha very good linearity. Temperature drift and sample-to-sam-ple tolerances are strongly reduced, because both coils areplaced on a common PCB. The oscillator output signal isfurthermore processed by an interface and a back end similarto Fig. 5 (TC2 no longer required).

Figure 8. Transfer function characteristic (measuring range 10 mm to80 mm, PT dimensions 92 mm x 12 mm).

IV. REALIZATION AND FEATURES

The first LDIS version is currently in field testing at ref-erence customers. Two mechanical designs have been fol-lowed, namely a robust cylindrical version (30 mm diameterand 40 mm total length) in metal housing (Fig. 9 left) and asmaller cubical version (30 mm x 30mm x 15mm) in plastichousing (Fig. 9 right). The sensors are fully potted, meet therequirements of IP67 and are useful even in harsh environ-mental conditions.

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smaller influence of the target material inhomogeneity andquality of its surface roughness.

Figure 9. Robust LDIS version assembled into a metal housing with amounting flange and a plastic cap as active face (left). Miniaturized LDIS

assembled into a plastic housing without mounting flange (right).

Precise setting with low sample variations is achieved bya two-point teach-in procedure after the sensors have beenfully assembled. Communication between the sensor andcalibration device takes place over the supply lines of thesensor (no additional connections required) [9]. Once thecalibration is finished, the calibration values are stored in anon-volatile memory and the communication interface isdeactivated. This teach-in procedure takes into account alltolerances which can affect the output signal, including theproperties of the potting material or the installation positionof the sensing element in the sensor housing.

Figure 10. Sensor output characteristic and linearity error curve measuredwith a steel reference target (displacement range in 14 mm).

Fig. 10 shows the output characteristic (blue) ofthe LDISin Fig. 9 (left) and its non-linearity (red) in the range: 0 V @10 mm to 1OV ) 24 mm. The limits refer to a referencehousing edge. With a linearity error of less than ± 3 % and aresolution limit of 0.1 0% (full scale), the LDIS offers the bestcost/benefit ratio for many measuring or positioning tasks.

V. APPLICATIONS

The standard application of the LDIS is given by a ma-chine part transporting a metallic target that performs a lineardisplacement. The sensor delivers an output signal which isproportional to the displacement. The target can also takecomplex movements (Fig. 1). In Fig. lIthe cone is replacedwith a slender metal disk. The main advantages are higheraccuracy, linearity and repeatability. Also important for theuser are: the ease of alignment, the simple, smaller and lightconstitution of the target, reduced vibrations as well as

Figure 1 1. LDIS as a replacement of an IPS in an application similarto Fig. 1.

VI. CONCLUSIONS

After a description of the traditional solutions for the in-ductive non-contacting sensing of linear travels, a first ver-sion of the new LDIS [10], which is in usage at referencecustomers, was presented. The description of an advancedversion, which is in the design validation phase followed.Finally the existing embodiments were specified, showingthe features and the advantages in the field applications.

ACKNOWLEDGMENT

The authors would like to thank all co-workers in thesensors development department, especially Eberhard Stabeland Dieter Rauschenberger, for their cooperation, andBalluff for providing the working conditions and support.

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[3] M. Jagiella, S. Fericean, A. Dorneich, and M. Eggimann, "Theoreticalprogress and recent realizations of miniaturized inductive sensors forautomation," Proceedings of IEEE Conference on Sensors, IrvineCA, pp. 488-491, 2005.

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[5] Ansoft Corp., Technical manual: Maxwell-Simulator.[6] A. A. Andronov, A. A. Vitt, and S. E. Khaikin, Theory of oszillators,

New York: Dover, 1987.[7] s. Fericean, M. Friedrich, M. Fritton and T. Reider, "Moderne

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[8] R. Droxler, "Bertihrungslos arbeitender Naherungsschalter," BalluffPatent, DE 19611810 C2, 2000.

[9] s. Fericean, and H. Kammerer, "Proximity switch operating in a non-contacting manner, "Balluff Patent, USA 5,408,132, 1995.

[10] M. Jagiella, "Inductive position measuring systems," Balluff Patent,US 6,714,004 B2, 2004.

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