1496 IEEE SENSORS JOURNAL, VOL. 12, NO. 5, MAY 2012

Capacitive Pressure Sensing Based Key in PCBTechnology for Industrial Applications

Ljubomir Vracar, Member, IEEE, Aneta Prijic, Member, IEEE, Dusan Vuckovic, Student Member, IEEE, andZoran Prijic, Member, IEEE

Abstract—This paper reports the design and manufacturing ofa device suitable for use in command panels of industrial equip-ment in the place of mechanical buttons and switches. The deviceconsists of a capacitive pressure sensor, low-cost microcontroller,current driver, and state indication LEDs. All components are em-bedded into multilayer printed circuit board using standard manu-facturing steps. The device has user-programmable properties andit can behave either as a key or as a switch. An interconnection fea-ture is provided in a sense that multiple devices can be organizedinto an array thus forming a keyboard. A simple two-wire commu-nication interface for controlling the keyboard is described.

Index Terms—Capacitive-pressure sensor, keyboard, PCB tech-nology, two-wire communication.

I. INTRODUCTION

D EVELOPMENT of versatile human interfaces for variouselectronic devices has attracted much attention in recent

years [1]. Although efforts are mainly directed towards con-sumer devices, there are requirements from industrial, medicaland other more specialized electronic branches. Manufacturersof such equipment are looking for a substitution of mechanicalpush buttons and switches used in command panels with moresophisticated and customizable components.

Capacitive sensors, either touch or pressure, are found to be agood alternative to mechanical devices, mainly due to their im-plementation flexibility, reliability and overall durability. Thesesensors are used in conjunction with microcontrollers to provideaccurate detection of the change in capacitance due to a touch ora pressure of a human finger. Touch sensors have limited usagein the industrial environment due to their susceptibility to falsereadings induced by humidity and inability to implement a singleconductive touching surface over several devices. Pressure sen-sors are more appropriate for the implementation in the equip-ment designed for harsh working conditions. For the pressurerange of interest these sensors are designed as MEMS devicesusually using fabrication processes from the printed circuit board

Manuscript received September 05, 2011; accepted October 09, 2011. Dateof publication October 25, 2011; date of current version April 13, 2012. Thiswork was supported in part by the Serbian Ministry of Education and Scienceunder Grant TR32026 and in part by Ei PCB Factory, Nis. The associate editorcoordinating the review of this paper and approving it for publication was Prof.Kiseon Kim.

Lj. Vracar, A. Prijic and Z. Prijic are with the University of Nis, Fac-ulty of Electronic Engineering, 18000 Nis, Serbia (e-mail: ljubomir.vracar@elfak.ni.ac.rs; aneta.prijic@elfak.ni.ac.rs; zoran.prijic@elfak.ni.ac.rs).

D. Vuckovic is with DELTA–IdemoLAB, 2970 Hørsholm, Denmark (e-mail:duv@delta.dk).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2011.2173483

Fig. 1. Isometric view and cross–section of the capacitive pressure sensor (one-quarter shown due to the symmetry of the structure; not to scale).

(PCB) technology [2]–[4], while for different applications othertechniques have been reported [5]–[10]. Many details of sensorsdesign and usage are also given in the documents issued by sev-eral major semiconductor device manufacturers [11]–[16].

The objective of this paper is to present a composite devicewhich consists of the capacitive pressure sensor, microcontrollerand the peripherals, and can act either as a key or as a switch.The device is realized in a conventional PCB technology andit has properties customizable by the user. The sensor designwith double layered dielectric and a self-calibrating procedureto improve sensitivity and reliability of the device are proposed.Apart from the standalone use, an interconnection capability isintroduced to make the devices suitable for organizing into anarray which behaves like a keyboard. Its flexible configuration,compact design and the ability to be used with gloves make thedevice suitable for the command panels in industrial equipment.

In the following sections, the capacitive pressure sensor de-sign is presented, and the device construction is described in de-tail. The application interface and the communication betweendevices and controller logic, including protocol details, are alsopresented and discussed.

II. PRESSURE SENSOR

The device is designed using a square-shaped geometry suit-able for production in conventional PCB technology. The pres-sure sensor is a circular parallel plate capacitor occupying a partof the device volume, as shown in Fig. 1. On the square shaped

1530-437X/$26.00 © 2011 IEEE

VRACAR et al.: CAPACITIVE PRESSURE SENSING BASED KEY IN PCB TECHNOLOGY FOR INDUSTRIAL APPLICATIONS 1497

TABLE IDIMENSIONS OF THE STRUCTURE FROM FIG. 1

Fig. 2. Illustration of the working principle of the capacitive pressure sensor.

core, made of the Flame Retardant Class 4 (FR4) woven glassreinforced epoxy resin, the bottom Cu electrode of a circulargeometry is printed. The top electrode is realized as Cu layeron the bottom side of the top square FR4 layer, which seals theelement. An appropriately shaped spacer, also from FR4, sep-arates core and top layer and forms cylindrical cavity partiallyfilled with solid dielectric (in this case prepreg). The actual di-mensions of the structure from Fig. 1 are listed in Table I.

Elasticity of FR4 material enables the top layer to deflectunder pressure applied to its surface at the area above the cavity,as illustrated in Fig. 2. In this way the top electrode is pushedtoward the bottom one and the capacitance of the sensor ischanged. The capacitance of the sensor should be as high aspossible in order to minimize influence of parasitic capacitancesdue to the rest of the device structure. Therefore, larger area ofthe bottom electrode, thinner spacer and higher permittivity ofthe dielectric between the electrodes are preferable. Diameterof the bottom electrode and the spacer thickness are deter-mined on the basis of the overall device dimensions and bythe limits of the used PCB technology. Apart from the designdescribed in [12], dielectric between the electrodes is doublelayered consisting of the thin prepreg and air, thus formingtwo capacitors in series [17]. The capacitor with prepreg hasa constant capacitance, whereas another capacitance is deter-mined by the air gap between the top electrode and the surfaceof the prepreg. In this way the relative change of the equivalentcapacitance is increased as the air gap thickness is decreased.As a result, sensitivity of the element is considerably improvedfor the larger pressures comparing to the sensor with the samegeometry having only one dielectric. Also, existence of thesolid dielectric layer make it impossible for the top and bottomelectrodes to be shortened and it maintains the sensor’s oper-ability even when the top layer is considerably damaged. Notethat, apart from the reported designs involving other polymers[2], [10], in this construction only FR4 is used.

Mechanical properties and sensitivity of the capacitive pres-sure sensor from Fig. 1 are analyzed by numerical simulation in

TABLE IIMECHANICAL AND ELECTRICAL PARAMETERS OF THE

MATERIALS USED FOR THE SIMULATION

Fig. 3. (left) Total deformation and (right) equivalent stress values in the sensorunder applied pressure of 115 kPa.

ANSYS 12 Workbench and APDL Software [18]. Relevant me-chanical and electrical properties of the materials used for thesimulation are given in Table II. Simulation geometry was sim-plified in comparison to the actual one (see Section III) by as-suming that the core extends up to the bottom of the device. Me-chanical constrains were set by fixing support at the bottom sideof the structure and by setting the symmetry boundary conditiononto the vertically halving plane. Loads are defined by applyingpressure up to 115 kPa onto the circular pressing area at the topsurface of the top layer. This area is assumed to have a diam-eter of an average human fingerpad (1.2 cm) [19]. A mesh forstructural analysis was the one primarily generated by ANSYSMechanical Workbench with subsequent refinement of the ele-ments on the top surface of the top layer. Simulation results ofthe total deformation and equivalent stress values in the sensorunder applied pressure of 115 kPa are shown in Fig. 3. In thiscase the top layer is deflected to the limit when the top elec-trode touches the surface of the prepreg. The maximum equiv-alent stress in the layer has the value of 43.9 MPa at the centerof the pressing area. This value is well below the yield strengthof FR4 material (Table II) and reliable mechanical properties ofthe sensing element should be expected.

The capacitance of the sensor for different pressure values isdetermined through CMATRIX macro of ANSYS APDL soft-ware by implementing SOLID123 type of 3D tetrahedral elec-trostatic elements. It was assumed that Cu electrodes are isolatedconducting surfaces onto the FR4 dielectric areas. A nonuni-form, free meshing strategy with an initial smart sizing option

1498 IEEE SENSORS JOURNAL, VOL. 12, NO. 5, MAY 2012

Fig. 4. Simulated relative change of the sensor capacitance versus appliedpressure.

was used for mesh generation of the solid model. The subse-quent refinement of the volume elements was conducted to ob-tain solution independence on the mesh density. Electrostaticconstrains set the bottom electrode at the high potential and thetop electrode grounded. Also, the bottom surface of the core wasgrounded as a part of the shield used to improve noise immunityin the real sensor.

The capacitance formed by the bottom electrode and theshield is connected in parallel with the sensing capacitance.Therefore, as being built–in and constant, its value of 4.23 pFcontributes to the total simulated capacitance. The relativechange of the sensor capacitance is defined as:

(1)

where is the capacitance in the pressed and in the un-pressed state. Simulation results for as a function of the ap-plied pressure are given in Fig. 4, for single and double layereddielectrics. It is evident that is increased by the introductionof a double layered dielectric. It should be emphasized that forthe case of a single layered dielectric relative change of the ca-pacitance does not depend on its permittivity. Simulated valuesof the total sensor capacitance are in the range 8.6 – 12.7 pF.However, since the sensor is embedded in the much complexstructure, correlation between the simulation and experiment isperformed on a fabricated device, as described in Section III.

III. DEVICE DESCRIPTION

Functional block diagram of a single key device incorporatingdescribed pressure sensor is shown in Fig. 5. The blocks arearranged using conventional multilayer PCB technology withembedded electronic components, as shown in Fig. 6. The com-ponents are laid out in a manner that enhances noise immunity[20]. Note that the bottom side of the top layer and the top sideof the bottom layer are used as ground planes.

Voltage changes caused by pressing the sensor are acquiredby a 10–bit ADC converter of a low–cost PIC12F683 microcon-troller [21] using a Capacitive Voltage Divider (CVD) principledescribed in [22] and the corresponding hardware and softwareguidelines [23]. In essence, changes of the ADC readings areinversely proportional to the changes of the sensor capacitance.

Fig. 5. Functional block diagram of a single key device.

Fig. 6. Exploded cross section of a single key device (not to scale).

Fig. 7. Waveforms measured on a voltage divider showing voltage changedue to the pressure applied on the sensor. ADC readings start when themicrocontroller opens an internal switch to measure voltage on its sample andhold capacitor.

Waveforms captured on a voltage divider are shown in Fig. 7.Threshold values of the ADC readings, which define pressedand released states of the key (Fig. 9), are set utilizing a SlewRate Limiter filter [24]. Waveforms illustrating the behaviourof the whole device are shown in Fig. 8. Output delay time is

VRACAR et al.: CAPACITIVE PRESSURE SENSING BASED KEY IN PCB TECHNOLOGY FOR INDUSTRIAL APPLICATIONS 1499

Fig. 8. Waveforms measured on the device: the upper waveform shows ADCreadings on the voltage divider; the lower waveform shows output from the de-vice. Crosshairs (a) and (b) mark the moments when the sensor is pressed andoutput is produced, respectively, thus defining output delay time.

Fig. 9. Threshold values adjustment used to compensate the effect of a contin-uous pressure.

dependent on the filter adjustments [24], and debounce timewhich can be prolonged as a customizable parameter. Reliableoperation of the device is achieved by using the minimal valueof 50 ms.

Moreover, an additional functional problem is particularlyhandled. If a high pressure is applied on the key either for along time (e.g., using some object as a load rather than a finger)or by a hit, slowly reversible or permanent deformation of thetop layer may occur. When the load is released, the key may be-have as still being pressed. A software watchdog timer whichmonitors duration of the applied pressure is introduced. Whenthe monitored value reaches the predefined timeout (10 s inthis case), the key is set to the released state and the thresholdvalues are set below the current ADC readings, as illustratedin Fig. 9. Therefore, the key will be activated again only if ahigher pressure is applied. If the top layer’s deformation fullyrecovers after the pressure has been removed, Slew Rate Limiterwill set the thresholds to their initial values. The described read-justment of the threshold values may be qualified as a self–cal-ibrating feature. The device also has an ability to detect its stateon power–up. This is achieved by saving the reading of an un-pressed state into microcontroller’s EEPROM during regular

Fig. 10. Photograph and the pinout of a fabricated single key device (sidelength 35 mm).

operation of the device. Then, within the startup sequence, thesaved value is compared to the current reading so the device can”decide” whether or not it is being pressed (i.e., held) during thepower–up.

The device is designed to be used either in a single key(standalone) mode or in an array (keyboard) mode. When in thesingle key mode, pin (Fig. 5) is short-circuitedto the supply voltage and pin is used toprovide high, low or high impedance state to the host applica-tion. Optionally, pin may be short-circuited tothe gate of the built–in MOSFET to extend the output currentcapability of the device up to 0.5 A. Open–drain configurationof the MOSFET is for driving relays which is often requiredin industrial equipment. In a keyboard mode these pins areused to receive clock and send/receive data, as described inSection IV. The LED out pin should be short-circuited to theLED in pin to control state indication LEDs. Optionally, thispin may be connected to the other type of indication control. Inorder to achieve interconnection accessibility, pins are arrangedon all four sides of the device [25], as shown in Fig. 10. Theconnection pads are realized to accommodate standard one-rowheaders having 2.54 mm pitch spacing. When the headers aresoldered, the device can be inserted into a main command panelboard like any other through–hole component. The connectionexamples are given in Section IV.

Sensitivity of a single key device from Fig. 10 is experimen-tally determined using force transducer as a load and recordingcorresponding ADC readings. The obtained results are shownin Fig. 11 and compared to the simulation. The measured valuesare averaged for randomly selected devices from various PCBpanels to obtain a typical curve. Normalized ADC readings arecalculated as:

(2)

1500 IEEE SENSORS JOURNAL, VOL. 12, NO. 5, MAY 2012

Fig. 11. Normalized ADC readings vs. applied pressure for a single key device.��� refers to the reading in the unpressed state.

Fig. 12. Photographs of a fabricated device: (a) top layer—top view; (b) bottomlayer—bottom view; (c) core—bottom view; (d) bottom layer—top view; (sidelength 25 mm).

where is the reading in the unpressed state,sample and hold capacitance of the AD converter and

is the capacitance of the microcontroller’s input pin towhich the sensor is connected. A fairly good agreement is foundbetween the simulated and measured values, whereas the dis-crepancies are due to the simplified simulation geometry andtechnology tolerances.

On the basis of the presented simulating technique, a pa-rameter–driven modelling is employed to design a device withsmaller lateral dimensions by keeping the same sensitivity. Thephotographs of the realized device, including the inner layersdepicted in Fig. 6, are shown in Fig. 12. The smaller devicesmight be more convenient for a keyboard arrangement, whereasthe larger ones are for a standalone use, especially for operatorswearing industrial gloves.

A decorative and protective fascia of a user’s choice shouldbe mounted over the top of the keys. It can be made even of

Fig. 13. Top and side view of the prototype command panel for the torch con-trol in an industrial process.

metal with the nonconductive foil attached beneath, as shownin Fig. 13. It should be pointed out that any possible tensioneffect caused by the mask fixture on the keys is compensated bytheir self–calibrating feature. In order to ensure waterproofing,the top pin headers can be sealed using epoxy resin.

Reliability of the device was successfully tested up toactivations with 2 s period using pressure of 30 kPa as twice thevalue assumed to be most commonly used in practice.

IV. APPLICATION INTERFACE

The described device can be customized in terms of adjust-ment of its behaviour and properties. This is accomplished byusing the custom controller board and hardware interface shownin Fig. 14. The controller board consists of the microcontroller(PIC18F4520 in this case), common interfaces for the connec-tion with the host application, and the two wire interface forthe communication with attached devices. Each device can beconfigured to act either as a standalone or as a part of a key-board using the same controller board. For devices intended tobe used as standalone, the controller board is needed only forthe configuration of their properties and not for the applicationimplementation, as illustrated in Fig. 15.

Communication between the controller board and the devicesis designed as a serial–based, using clock and data lines. Theclock line is driven by the controller. The data line is bidirec-tional, so it can be used for sending and receiving data. In theidle state the clock line is at the low level and the data line is atthe high level via a pull–up resistor to the supply voltage. Bytetransfer resembles SPI 0 mode with an additional clock pulseat the end of the transmission to release the data line. It shouldbe noted that neither the controller nor any of the attached de-vices actually pulls the data line to the high level, instead thehigh impedance state is used on their data pins. During the ini-tial phase of communication, where the controller board sendsthe packet, all devices in the array receive the packet. After re-ceiving the packet all devices set the data line to low level andas every single one finishes with processing the packet it sets itsdata line pin to high impedance. In this way the data line is goingto return to high level only when all devices have finished withprocessing the data. When such an event occurs, the controllerboard initiates reading of the first byte of the reply. Dependingon the value of the received byte, which can vary from the error

VRACAR et al.: CAPACITIVE PRESSURE SENSING BASED KEY IN PCB TECHNOLOGY FOR INDUSTRIAL APPLICATIONS 1501

Fig. 14. Block diagram of the controller board and connection interface for anarray configuration. In the inset, a photograph of the controller board is shown(dimensions are 25 mm� 40 mm).

code to a beginning of the configuration readout from the device,the controller board finishes the communication or continues toreceive additional bytes from the device that has sent the reply.

A developed communication protocol consists of a set ofcommands with the following syntax:

where is a byte containing the command identi-fication, is a byte containing the number of bytes tofollow, is a byte containing the particular device’saddress, and are up to 7 bytes containing configura-tion parameters. An address is required for the device operatingin the keyboard mode and may take value from 1 to 127. Theaddressing procedure starts when the controller issues a com-mand to set an address and as a part of the command it sendsthe desired address value. All keys in the keyboard receive thecommand and get into the addressing mode. While in the ad-dressing mode, every pressed key will assign to itself the ad-dress it has received. The keys remain in the addressing modeuntil the command for ending the address procedure has been is-sued by the controller. Note that it is possible to assign the sameaddress to multiple keys, e.g., in the case when the same keyrole is required in different places on the keyboard. Althoughthe designed address space allows up to 127 keys, in order tomaintain signal integrity practical usage should be restricted toa few tenths of keys.

Fig. 15. Examples of the keys arrangement in an application environment:(a) standalone mode, (b) keyboard mode. Black jumpers indicate short–circuitedpads.

Once the keyboard has been configured, the role of the con-troller is to decode the address of a pressed key and to pass it tothe host application. When a key is pressed it signals its state tothe controller by pulling the data line low. The controller thenstarts to acquire the address of a pressed key by using the searchprocedure according to the flowchart given in Fig. 16. The pro-cedure is designed to speed up an overall keyboard response andit has been inspired by the search method used in 1–Wire devices[26], [27]. The address required is obtained in 10 ms. Note thatwithout using the described procedure search time would be or-ders of magnitude longer (depending on the number of keys),which would result in severe degradation of keyboard respon-siveness. Communication protocol and the search procedure canalso be built in the host application, thus eliminating the need forthe controller board.

A PC–based utility with a graphical user interface is devel-oped to simplify the configuration of the devices. When the con-

1502 IEEE SENSORS JOURNAL, VOL. 12, NO. 5, MAY 2012

Fig. 16. Flowchart of the address search procedure (Unified Modeling Lan-guage notation used).

troller board is connected to PC using USB interface (Fig. 14),the following device properties are configurable:

• function (key or switch)• retainability of the previous state on start–up, if the device

is configured as a switch• on/off LED state indication (red, green or none)• on/off output state (high, low or high impedance), if the

device is configured as a single key• press threshold• prolonged input (how long the device should be pressed

before activation)• prolonged output (how long the device should stay active

after it has been pressed)• address, if the device is configured in a keyboard mode.

The configuration parameters are non–volatile. Multiple devicescan be configured at once with the same parameters by usingreserved address value of 255 in the configuration command.

V. CONCLUSION

The new pressure sensing–based device was developed andrealized in PCB technology. The pressure sensor using doubledielectric for improved sensitivity and reliability has been de-signed, simulated and experimentally verified. The built–in mi-crocontroller was used not only to acquire sensors’ data but alsoto drive integrated peripherals. A self–calibrating technique to

cancel the effect of undesirable working conditions was devel-oped. An all–side input/output design was proposed as a wayfor convenient interconnection between the devices. The abilityof the device to be configured either as a single key or withina keyboard was demonstrated. A simple and reliable two–wirecommunication protocol between the controller board and thedevices arranged in a keyboard was developed.

Although the device is designed to be used in industrial equip-ment, its robustness and flexibility make it suitable for other ap-plications like those in medical equipment, home appliances,etc. Because its design is based on a widely available technologyand comprises low–cost electronic components it is also appro-priate for a volume production.

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Ljubomir Vracar (M’99) received the B.S. and M.S.degrees in electrical engineering from the Universityof Nis, Nis, Serbia, in 1999 and 2009, respectively.

He has been a member of the Academic Staffwith the Department of Microelectronics, Facultyof Electronic Engineering, University of Nis, since2002. He gained his vast pedagogical experienceby performing a large number of lab experimentsduring his studies and was engaged in developmentand modernization of teaching material. From2001–2007, he was the head of the Applied Physics

and Electronics Department, Petnica Science Centre , Valjevo, Serbia. Duringthis time, he worked on establishing international cooperation with the WeizmanInstitute of Science in Israel in the field of communications and education ofyoung talents. He has authored or coauthored over 30 papers published in thenational and international journals and conference proceedings, and is currentlyworking on embedded controller systems and smart sensors design, especiallyin field of home automation and energy harvesting.

Aneta Prijic (M’91) received the B.S., M.S. andPh.D. degrees in electrical engineering from theUniversity of Nis, Nis, Serbia, in 1993, 1996 and2007, respectively.

She has been a member of the Academic Staffwith the Faculty of Electronic Engineering, De-partment of Microelectronics, University of Nis,since 1995. She has authored or coauthored over 30papers published in the international journals andconference proceedings. Her main research areas aremodelling and simulation of electrical and electronic

devices, micro-electromechanical and energy harvesting systems.

Dusan Vuckovic (M’05) received the M.S. degreein electrical engineering from the University of Nis,Nis, Serbia, in 2009. He is currently working towardthe Ph.D. degree at the Danish Technical University(DTU), Lyngby, Denmark.

He is employed as a Specialist at a test andconsultancy company DELTA, Hørsholm, Denmark.His research interests are in the areas of embeddedsystems, energy harvesting, wireless sensor networksand power management.

Zoran Prijic (M’91) received the B.S., M.S. andPh.D. degrees in electrical engineering from theUniversity of Nis, Nis, Serbia, in 1987, 1990, and1993, respectively.

He joined Ei-Microelectronics, Nis, in 1987,working initially on CMOS integrated circuitsand then on power MOS transistors technologydevelopment. In 1990, he joined the Academic Staff,Faculty of Electronic Engineering, University ofNis, where he is currently Head of the Departmentof Microelectronics. He was Head of the Laboratory

for Microelectronics and Head of the Computer Center. From 2001 to 2006, hewas a Research and Development Vice-President of Ei Holding Co, Nis. He hasauthored or coauthored over 50 papers in the international technical literature.His area of research is modelling and simulation of electronic componentsand micro-electromechanical systems, while his technical interest includesindustrial informatics.