A mixed-mode temperature control circuit for gas sensors

A MIXED-MODE TEMPERATURE CONTROL CIRCUIT FOR GAS SENSORS R. Casanova1, J.L.Merino1, A. Dieguez1, S.A. Bota2, J. Samitier1. 1

Sistemes d’Instrumentació i Comunicacions (SIC), Departament d’Electrònica, Universitat de Barcelona. C/. Martí Franqès 1, E-08028, Barcelona. Spain 2 Grup de Tecnologia Electrónica, Universitat de les Illes Balears. Carretera de Valldemossa, km 7.2 E-07122 Palma de Mallorca. Spain ABSTRACT

It is described in this work a mixed-mode temperature control system for gas sensors. It has been designed to be capable to control different gas sensor structures. The range of temperature and accuracy depends on the gas sensor structure but are usually in the range from Room Temperature up to 600ºC within 1ºC accuracy. The circuit has been designed with 0.8um HV CMOS AMS technology in order to support the 15V biasing voltage that is necessary to reach these high temperatures. A serial peripheral interface communication port allows real time monitoring of the heater resistance. It allows the setting of the parameters of the control and temperature reference also. 1. INTRODUCTION During the last years, semiconductor gas sensors research has experienced a great advance because of the interest by the scientific and the industrial communities. The multiple applications of these devices in environmental control, security at closed places and home, military applications and electronic noses suppose a wide market to be exploited. These applications require new gas sensors with portability, high performance, reliability, endurance, and high yield characteristics. Semiconductor gas sensing is based on a change of resistance of the sensing element under the presence of gases. The response of the gas sensors is dependent on the ambient but also on the active material and on its structural properties. Other main factors affecting sensitivity are the temperature of operation and humidity. Because of the lack of selectivity of this type of sensors, the usual is to use an array of gas sensors to determine the presence of a certain gas or its concentration. In this way, selectivity is improved by: (i) Using sensors made of the same sensing material x working at different temperatures [1] x impregnated with different catalytic materials

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x using sensing layers of different thickness x using different electrode configuration x using several types of catalytic filters (ii) Using sensors made with different materials or even different types of sensors. In the case of semiconductor gas sensors, the usual range of working temperature is between 200ºC and 400ºC for high sensitivity. These temperatures are usually reached with a heater integrated in the gas sensor. So the control of the temperature in solid gas sensors is important to obtain the high sensitivity required and to play with selectivity. In this work, we present a general purpose mixedmode temperature control system for gas sensors. This system has been designed to be capable to control different type of heaters. Particularly two different heaters have been tested, which characteristics are shown in table 1.

Maximum temperature Maximum current Dissipated power Resistance range

CNM

EADS

400ºC

550ºC

50mA 250mW 50: - 100:

12.5mA 125mW 310: - 800:

Table 1. Characteristics of the European Agency Defence Space (EADS) and Centro Nacional de MicroelectrónicA (CNM) heaters. The maximum current and dissipated power correspond to the highest temperature.

2. CONTROL OF TEMPERATURE Control circuits of heaters for flow and gas sensors have been reported in several ways in the literature [2-8]. Most of the employed methods are usually based on a constant power heating rather than on the direct control of the temperature. Nevertheless, because of the dependence of the heater temperature on external factors such as humidity ambient temperature, airflow…, methods based on constant heating power have in general to be avoided. The

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temperature is thus controlled by directly measuring the heater resistance. In reference [2] a Pulse Width Modulation (PWM) approach is used to directly control the heater resistance (and hence temperature). Nevertheless such approach has the disadvantage of high injected noise and high ripple for accurate control (accuracy lower than 1ºC). A simple Proportional Integral Derivate (PID) mixed control avoids all these problems. Compared with the PWM solution, with a digital PID control the actuation variable can achieve more than the two values of the PWM, exactly 2N-1 different values if a N bits D/A converter is used. The result is that the actuation variable changes more smoothly and the ripple can be removed if the PID parameters, proportional, derivative or integral constants (Kp, KD and KI) are adjusted correctly. In addition, turning on and off the actuation variable (PWM) produces a large power consumption because of the continuous charging and discharging of parasitic capacitances. This is also reduced with the PID control. 3. DESIGNED SYSTEM The system presented in this work has been designed in order to command different types of gas sensors (figure 1). The major limitations is that the maximum operating voltages and currents of the heaters can never exceed 13V and 400mA, respectively. At the left of figure 1 there is a microcontroller. It has a Serial Peripheral Interface (SPI) communication port that allows the programmability of the PID parameters and the resistance (temperature) reference. The two external reference voltages VRN and VRP of the D/A converter set the minimum and the maximum heater temperatures achievable. This D/A converter drives an

operational amplifier in a non inverting configuration that controls the drop voltage at the heater Vh. The accuracy achievable in temperature is proportional to the difference between VRP and VRN. Nevertheless, decreasing accuracy, the temperature operation is reduced also. So it is necessary to find a compromise between the temperature operating range and the accuracy. The system works as follows: initially the PID parameters and the reference resistance (temperature) are set externally. Then the microcontroller starts the control. The heater drop voltage Vh and the current Ih flowing thorough the heater are measured with a voltage follower and a differential amplifier. A 10 bits A/D converter transforms these analogue values to digital. To reduce the area only one A/D converter is used and the measures are multiplexed in time. Then the digital controller calculates the heater resistance and compares it with a heater resistance reference, Rref. According to the result of the comparison, the controller drives the heater using a PID algorithm. The dynamic response of the heater is controlled via the PID parameters. The control loop can be closed in several ways. For example, it could be used a DSP or a microcontroller. Nevertheless, as it has been decided to integrate the whole system, minimization of the whole area is a must. So, it seems a best option to design a dedicated microcontroller with only the indispensable elements to make the PID control, thus reducing area. To design this part of the system the Verilog” hardware description language has been used. The reusability of the design is assured in this way. For test proposes it has been implemented initially in a FPGA FLEX10K30 of Altera making use of about an 80% of the resources.

Figure 1. Scheme of the mixed-mode temperature control system.

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accuracies of the driver are summarized in table 2. The resolution used internally for calculations is 16b. The 600 500

Temperature(ºC)

In this scheme, the analogue parts have been biased at 15V in order to supply the necessary power to reach the desired working temperatures. So, a high voltage technology is needed. In this work the 0.8Pm High Voltage (HV) CMOS Austria MicroSystems (AMS) technology has been used to design the system. The DAC, ADC and the digital part are biased at 5V to reduce power consumption. The resulting system is a mixed-mode circuit formed by three different domains: x Analogue high voltage (15V): heater driver and voltage and current measurement circuits. x Analogue low voltage (5V): D/A and A/D converters x Digital low-voltage (5V): programmable block for evaluation of the heater resistance and realization of the PID control. To communicate the two analogue domains, the signals that are shared must be transformed from LV to HV and viceversa: x The operational amplifier in a non inverting configuration transforms the LV control signal VC in a HV signal Vk, (figure 1). As the maximum output current of the amplifier is not enough to achieve the desired maximum temperatures, an external NPN bipolar transistor BC107A is used to buffer the output. It generates heater currents up to 400mA for loads of 30:. x The heater current Ih is measured with an external shunt resistance RS. In order to keep the output voltage of the differential amplifier below 5V to avoid burning the 10 bits A/D converter, the value of RS must be adjusted as a function of the characteristics of the heater x At the output of the voltage follower a voltage divider composed by two resistors has been added to transform a 15V signal to a 5V signal.

EADS heater CNM heater

400

300 200

100 0 0.5

1.0

1.5

2.0

2.5

3.5

4.0

Figure 2. THEATER as a function of control signal Vc.

ADC and DAC have 10b resolution. It is worth to comment that larger temperatures are achievable by incrementing VC. To test the closed loop response of the system we have applied to the EADS heater different reference temperatures as shown in figure 3. Every input pulse has duration of 10ms to assure that the heater can achieve the stationary state. The maximum frequency of the input signal is limited by the thermal response of the heater (WEADS~1ms). The PID constants Kp, Ki, and Kd have been set to 150, 10 and 5 respectively. These constants have been tuned by a prove-error method. As the heater is a non linear system, the analytic methods to tune the PID constants are not applicable. It can be seen that the control system is able to cover all the whole temperature range required for the EADS heater, i.e., from RT up to 550ºC. Greater temperatures than 550ºC are reachable. The stationary error is practically 0.

4. RESULTS Our system has been verified by simulation with two different gas sensors, one from the EADS and the other from CNM. The VRP and VRN voltages have been set to 4V and 500mV in order to obtain an accuracy in temperature less than 1ºC and achieve the desired temperature range in both cases. As shown in figure 2, in the case of the CNM heater, the working temperature range goes from 50ºC to 400ºC. For the EADS heater the range goes from 25ºC up to 550ºC. The external shunt resistance RS has been set to 2: for the CNM heater and 12: for the EADS heater. The maximum temperatures of the CNM and EADS heaters have been achieved with 1.8V and 3.25V control voltages VC, respectively. These values and the obtainable

3.0

Vc(V)

Tmin(ºC) for VC=500mV Tmax(ºC) for VC=1.8V (CNM) and VC=3.25V (EADS) Worst accuracy (ºC/bit) Best accuracy (ºC/bit)

CNM 53.5

EADS 28.4

400

550

0.71 0.31

1.1 0.36

Table 2. Operating temperatures an accuracies for the CNM and EADS heaters.

Figure 4 shows the response of the EADS heater to a periodic reference waveform (sinusoidal) for different frequencies. Such input signal would correspond to those used in some type of measurement mode of semiconductor gas sensors in which it is analyzed the response to gases

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600

596.9ºC

25Hz

300

EADS heater

200

154.8ºC

100 0 0.04 300

500

Tem perature(ºC)

Temperature(ºC)

440.4ºC 400 300

283.8ºC

200 100

0.01

0.02

0.03

0.04

0.08

0.10

50Hz 154.0ºC

0 0.04 300

0.06

Time(s)

0.08

0.10

200

100Hz 149.4ºC

100 0.06

Time(s)

0.08

0.10

200

28.4ºC 0 0.00

Time(s)

100

0 0.04 300

127.3ºC

0.06

200

500Hz 103.4ºC

100 0 0.04

0.05

Time(s)

Figure 3. Response of the system for different references of heater resistance for Kp=150, Kd=10, Ki=5. The control system covers the whole temperature work range (28ºC to 550ºC).

under periodic temperature oscillations. It should be taken into account that the only result of importance here is that the electronic system is capable to drive the heater for such type of temperature waveforms, but the response of the sensor is not of interest. As corresponds to the thermal response of the EADS heater, the maximum frequency of a sinusoidal waveform is limited to frequencies near 25 Hz. For larger frequencies the response of the heater is distorted. The same analysis in the CNM heater gives maximum frequencies for which the heater can follow the input waveform about 1.25 Hz. In spite of these results, as the chemical response of gas sensors in time has typical values of several seconds, these cut-off frequencies do not have any effect on the normal operation of the system. 5. CONCLUSIONS A flexible driver system for the control of temperature in semiconductor gas sensors has been designed. The system actuates by directly measuring the resistance of the heater, thus being independent of ambient fluctuations. It is flexible in the sense that can directly actuate over different types of heaters by only modifying the different programmable parameters of the PID algorithm and the external polarization voltages VRP and VRN and the external shunt resistance RS. The system is able to drive any heater with the sole limitation of maximum operating voltage of 13V and maximum current of 400mA. The system has been verified by using two heaters, one from EADS and the other from CNM. In the two cases, the maximum temperatures have been set to 600ºC and the accuracies obtained are under ~1ºC. The speed of the system is limited by the heater and not by the own system.

0.06

Time(s)

0.08

0.10

Figure 4. Thermal response of the EADS heater. Frequencies larger than 25Hz are attenuated.

6. ACKNOWLEDGES This work is funded by the European Commission (IST99-19003) and by the Spanish Science Commission CICYT (TIC-2000-1852-CE). 7. REFERENCES [1] J.W. Gardner, and P.N. Bartlett, “A brief history of electronic noses”, Sensors and Actuators B 18-19, 1994, pp. 211-220. [2] A. Diéguez, etc., “A CMOS monolithically integrated gas sensor array with electronics for temperature control and signal interfacing”, 28th Annual Conference of IEEE Industrial Electronics Society, IECON02 (Sevilla), 2002, pp 2727-2732. [3] Q. Huang, C. Menolfi, and H. Baltes, “Temperature and supply voltage stabilized power driver for flow sensors”, Proc. of the 8th International Conference on Solid-State Sensors and Actuators, Stockholm (Sweden), 1995, pp. 440-442. [4] S.S.W. Chan, and P.C.H. Chan, “A resistance variation tolerant constant power heating circuit for integrated sensor application”, 23rd European Solid-State Circuits Conference, Southampton (UK), 1997, pp. 5-8. [5] P.F. Rüedi, etc., “Interface circuit for metal-oxide gas sensor”, IEEE 2001 Custom Integrated Circuits Conference, San Diego (USA), 2001, pp. 109-112. [6] C. Hagleitner, etc., “Smart single-chip gas sensor microsystem”, Nature, vol 414, 2001, 293-296. [7] J.W. Gardner, M. Cole, and F.Udrea, “CMOS gas sensors and smart devices”, Sensors, 2002. Proceedings of IEEE, 12-14 June 2002, 721 -726 vol.1. [8] G.C. Cardinali, etc., “An integrated microstructure with temperature control for gas sensors”, Proceedings of International Conference on Microelectronics , Nis, Serbia, pp. 515-518, 1997.

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