Smart Pressure Transducer Resistive to Aggressive Media

Smart Pressure Transducer Resistive to Aggressive Media RADIMIR VRBA - MIROSLAV SVEDA Department of Microelectronics - Department of Information Systems Brno University of Technology CZ-60200 Brno, Udolni 53 CZECH REPUBLIC

Abstract: - The pressure transducer was researched to reach the IEEE 1451 functionality, better pressure ranges and lower price. It is typically prepared for paper and chemical industry, medicine appliances and pharmacy: designed to 0 ... 10 kPa up to 0 ... 6 MPa pressure ranges, accuracy according to IEC 60770 0.5% FSO. The ceramic diaphragm for pressure sensing is applied and a new principle of special ASIC with multiplexed frequency transmitter for conditioning of fX and fR frequencies controlled by CX and CR sensor capacitances is exploit. An efficient protection against the moist and dusty ambient air and surrounding electromagnetic parasitic fields is achieved. Key-Words: - Smart pressure sensor, embedded microcomputer, IEEE 1451 functionality, aggressive media resistivity

1 Introduction Sensitive pressure transducer with Al2O3 ceramic diaphragm and capacitive read-out system with a special ASIC is described. Measured pressure is transformed into the deformation of a thick layer ceramic diaphragm. Ceramic diaphragm creates a measuring capacitance, which changes with varying pressure. Pressure dependent capacitance CX is evaluated by a frequency fX of a special multiplexed transmitter (oscillator) controlled by the capacitance CX. Due to necessary temperature and supply voltage compensation, a capacity CR of the reference capacitor (located in the close position to the pressure sensed capacitor) is measured in original multiplexed mode of the oscillator. Both frequencies fX and fR are processed by an embedded microcontroller and corrected according to measured temperatures of the ceramic diaphragm and electronics. The range of the measurement can be selected in a factory by switching the microcontroller counter input to one of prepared outputs of the frequency divider of the universal ASIC. Typical areas of use: paper industry, chemical industry, medicine appliances and pharmacy. Designed to 0 ... 100 kPa up to 0 ... 6 MPa pressure ranges. Presented work guarantees pressure measurement accuracy according to IEC 60770 better than 0.5 % FSO. Accuracy respects nonlinearity, hysteresis and repeatability errors. Time stability of internal multiplexed oscillator is better than 0.05 % FSO. Previous works [1] were aimed especially to research and to develop new types of pressure sensors and appropriate smart electronics with embedded intelligence for capacitive measurements exploiting a new sophisticated ceramic diaphragm system [3] and a method of fX/fR frequency ratio measurement [4]. Originally two independent oscillators sensed by CX

resp. CR were exploit, but the time and temperature stabilities were not sufficient [2].

2 Accuracy and stability analysis Researched frequency transmitter (oscillator) exploits the only one channel of originally two-channel oscillator as a part of thick film hybrid integrated circuit (HIC) with embedded chip of an CMOS ASIC. Periods of oscillating are determined by capacities of two multiplexed capacitors CX and CR. Several types of electronic multiplexers were tested. Only originally reference oscillator of two-channel transmitter ASIC is used due to its smaller parasitic capacitance. Original measuring oscillator channel is stopped. CR pin of reference channel is connected to simple multiplexer formed by two NMOS transistors. Both switching transistors are controlled by the embedded microcontroller. Detailed analysis was done concerning connecting of substrates of both NMOS transistors: a) grounded substrate and b) substrate connected to source. Advantages and disadvantages are as follows: a) Grounded substrate Leakage current of the internal suppressor, connected between the gate and the substrate, flows innocuously outside the signal path to the ground. Substrate grounding increases parasitic capacitances of the switches. Substrate grounding causes lower stimulating of the channel and increasing of switched transistor resistance. b) Substrate connected to source Leakage current of the internal suppressor flows into the signal path and influence loading of measuring, resp. reference, capacitor. Substrate connection to source causes decreasing of parasitic capacitances of the switch.

Substrate connection to source eliminates the effect of lower stimulating of the channel. Initial testing of both possibilities proved exigency not to connect the substrate to the source. This connection provides time instability: switching from measuring to reference capacitor and vice versa causes different frequency, varying at about 25 Hz from the initial and its slow resetting to the correct original value within up to one minute. Heating to +40 °C results in strengthening this phenomenon, by heating up to +80 °C it is weakened. It does not mean simple leakage current of the internal suppressor of the transistor. It is rather the latch-up of multi-layer semiconductor structure of double suppressor and substrate. Final solution of the multiplexer design contains two NMOS transistors with grounded substrate. Mathematical model. Model of the analog part of the frequency transmitter contains:

RSX, RSR CSX, CSR

fR =

1 1 , ≈ 2 ln2 × RR C R 1,386 294 RR C R

XP =

RX C X . RR C R

Frequencies fX, fR and measuring pressure factor XP of the ideal frequency transmitter with the stable elements RX, CX, RR, CR are independent on supply voltage and ambient temperature. Real frequency transmitter brings the certain voltage, temperature and time sensitivities, which were analyzed and tested. PRESSURE CAPACITANCE SENSOR

HYBRID IC

CONTROL X CONTROL R TEMPERATURE

resistances in the HIC, nominally 200 kΩ, capacitances of measuring and reference sensor capacitors, serial output resistances of the ASIC gates, parallel input capacitances of the ASIC comparators and parasitic dissipation capacitances of the HIC,

OSCILLA TOR

FREQUENCY DIVIDER

CX

MULTIPLEXER

PRESSURE

nominally 15 kΩ, CX, CR

1 1 , ≈ 2 ln2 × RX C X 1,386 294 RX C X

TEMPERATURE SENSOR 1

VN supply voltage, RA, RB, RC resistances of the voltage divider in the ASIC, RX, RR

fX =

FREQUENCY

CR

Fig. 1: Simplified block diagram of the special multiplexed oscillator for capacitive pressure sensor with ceramic diaphragm

EHX, EHR, input offsets of the ASIC comparators. ELX, ELR

Mathematical model of the frequency transmitter is formed by a system of transfer equations, defining relations between CX, resp. CR capacitances and frequency of the output rectangular signal of the frequency transmitters. Circuit analysis of the model results in transfer equations of both frequency transmitters  K H + EHX / VN 1 − K L + ELX / VN 1 = ( RX + RSX ) (C X + CSX ) ln  fX  1 − K H − EHX / VN K L − ELX / VN

  

 K H + EHR / VN 1 − K L + ELR / VN 1 = ( RR + RSR ) (CR + CSR ) ln  fR  1 − K H − EHR / VN K L − ELR / VN

  

where KH and KL stand for dividing ratios of the ASIC resistive divider, KH = (RB + RC) / (RA + RB + RC), KL = RC / (RA + RB + RC). Frequency output of the transmitter is derived using output frequencies fX a fR as a measuring pressure factor f XP = R . fX

For ideal zero parasitic elements RS, CS, EL, EH and nominal values KH = 2/3, KL = 1/3 result transfer equations and measuring pressure factor into simple forms

3 Smart system design New sensing device with the Al2O3 ceramic diaphragm shown in Fig. 4a was fabricated. The sensor contains measuring capacitor with CX capacitance (dependent on external pressure bending the diaphragm) and reference capacitor with CR capacitance (only for temperature and supply voltage compensation). A unique multiplexed transmitter according to Fig. 1 measures both capacitances. The NMOS transistors for switching CX and CR capacitors are controlled directly by the embedded microcontroller via signals CONTROL X and CONTROL R. A frequency divider with several outputs counts the output transmitter signal. The appropriate divider output FREQUENCY for following processing can be selected in the factory. For temperature compensation of transmitter (oscillator) output frequency a simple temperature sensor with a semiconductor diode is applied. Its TEMPERATURE voltage signal is digitized in the embedded microcomputer on electronics SMT part of the transducer. The transmitter output frequency divided rectangular signal is counted by the microcontroller internal programmable counter and digitally processed. Digital data derived from signals mirroring CX and CR capacitances are re-calculated exploiting digitized

temperature of the ceramic diaphragm (temperature sensor 1) and digitized temperature of the microcontroller itself (temperature sensor 2). Temperature errors of the frequencies fX and fR and of the internal analog-digital converter inside the microcontroller are digitally corrected. Temperature 2 is then used to correct digital data sent to the output DAC digital-analog converter for actuating the output amplifier generating the output current or voltage signal.

principle of special ASIC with multiplexed frequency transmitter for conditioning of fX and fR frequencies controlled by CX and CR sensor capacitances. Several technology goals were also reached, especially to prevent the influence of the moist and dusty ambient air and surrounding electromagnetic parasitic fields. The new pressure transducer is typically prepared for paper industry, chemical industry, medicine appliances and pharmacy: designed to 0 ... 10 kPa up to 0 ... 6 MPa pressure ranges,

CURRENT OUTPUT ELECTRONICS + VS

TEMPERA TURE SENSOR 2

TEMPERA TURE SENSOR 2

DA C

LOOP CURRENT CONTROL & SUPPLY

+SUPPLY (+I LOOP ) -SUPPLY (-ILOOP )

SHIELD

CONTROL R

TEMPERA TURE

HA RT MODEM FREQUENCY

EEPROM

There are two versions of the output circuitry design. The first one (see Fig. 2) generates the 4 … 20 mA loop current for 2 or 3 wire connection, it provides also the HART communication for initial and additional parameter settings. The second one provides optional 2-wire 4 … 20 mA or 3-wire 0 … 20 mA loop current or the 3-wire 0 … 10 V output voltage (see Fig. 3). It guarantees also the RS 232 communication for initial and additional parameter settings. System compatibility. The pressure transducer is designed to fulfill all requirements of industry and Fig. 4: Pressure transducer final assembly relative standards. The main problems were to suppress the influence of the ambient air humidity and ambient electromagnetic parasitic fields to total accuracy of the transducer. There are main technological hints illustrated in Fig. 4.

4 Conclusions The new device for precise and reliable pressure measurement is described and first results are presented. The main aim was to prepare a new pressure sensor with ceramic diaphragm and to find a new

DA C

+SUPPLY (+I LOOP ) -SUPPLY (-ILOOP ) SHIELD

V OLTA GE OUTPUT

RS 232

FREQUENCY

Fig. 2: Block diagram of the transducer electronics for 4 ... 20 mA output current range and HART communication for parameters setting

LOOP CURRENT CONTROL & SUPPLY

+ VS

CONTROL X MICROCONTROLLER

TEMPERA TURE

MICROCONTROLLER

CONTROL X CONTROL R

CURRENT/V OLTAGE OUTPUT ELECTRONICS

+ V OUT

RS 232

EEPROM

Fig. 3: Block diagram of the transducer electronics for 4 ... 20 / 0 ... 20 mA output current range or 0 ... 10 V output voltage range and RS 232 communication for parameters setting

accuracy according to IEC 60770 0,5% FSO (BFSL: 0,25% FSO), ceramic sensor (without oil-filling) with high resistance against aggressive medias (e.g. acids and lye), durable to mechanical contact, small thermal effect, excellent long term stability, high resistance against electrical faults caused by incorrect wiring, short-circuit and over voltage. Full IEEE 1451 functionality is enabled by the embedded microcontroller. Acknowledgements This research has been supported by the Czech Grant Agency as the project GA102/03/0619, by the Ministry of Industry as the projects FT-TA/050 and FF-P/112, and by the Czech Ministry of Education as a CZ262200022 Research Project. References: [1] Ristic L.: Sensor Technology and Devices. Artech

House, Boston, London 94, ISBN 0-89006-532-2 [2] Pressure Technology. Data Catalog. Buchlovice,

BD SENSORS 2003 [3] Kapazitiver keramischer Drucksensor ME 703.

Technische Blatt. BD SENSORS GmbH, METALLUX ELECTRONIC, SRN, Thierstein 2002 [4] New Generation Pressure Sensor FF-P/112 Research and Development Project, Czech Ministry of Industry. Praha 2003