An Aluminum-Nickel Micro-Heater-Sensor Device with PI-Compensation

P. DEEKLA(1), N. CHOMNAWANG(1), R. PHATHANAKUN*(2), S. SUJITJORN*(2)

(1) School of Electrical Engineering, Suranaree University of Technology, THAILAND (2) Synchrotron Light Research Institute (Public Organization), THAILAND

*correspondence: rungrueang@slri.or.th, sarawut.sujitjorn@slri.or.th http://www.slri.or.th Abstract: - A low-cost micro-heater-sensor device is proposed. Its fabrication uses micro-electro-mechanical systems (MEMS) technology based-on UV lithography technique available at Synchrotron Light Research Insitute, Thailand. The proposed device employs aluminum (Al) and nickel (Ni) fabricated on a glass substrate that render a rugged device with low investment cost. As a common case, the device being run open-loop responds slowly to command changes. PI compensation is used to enhance its performance in a closed-loop manner. Responses of the compensated device is about 60 times faster than an uncompensated one. Our design and practical results are illustrated. Key-Words: MEMS, micro-heater-sensor device, static calibration, dynamic response, PI-compensation.

1 Introduction Thermal cycling for DNA amplification [1-3] and gas sensing [4-6] are some real-world applications that require a micro-device capable of heating and measuring temperature simultaneously. Such a device normally comes as an IC fabricated by MEMS technology. These devices thus have their sizes ranging from 20 microns to millimeters. In practice, high cost is usually a serious problem due to complicated fabrication processes utilizing expensive chemicals and materials. A low-cost device suitable for 40 C to 120 C operating range has been proposed [7]. For microfluidic applications, it is commonly found that heating of the fluid is conducted in an open-loop manner [8-9], which is not quite accurate. Accuracy and dynamic response can be improved by using a closed-loop compensation. This work has proposed a low-cost micro-device for sensing and heating fluids, e.g. biological meterials etc. The device is suitable for a temperature range of 40 - 120 C. With this low temperature range, aluminum (Al) and nickel (Ni) are strong candidates for being used in fabrication. An open-loop device is expected to respond slowly to command changes as commonly found. Closed-loop control technique will be useful for response improvement. The device was fabricated using a lithography facility available at Synchrotron Light Research Insitute, Thailand, by micro-machining team.

This article contains 5 sections. Section 2 gives a brief description of our micro-device. Static calibration of the device appears in Section 3 with signal conditioning circuits presented. Responses of the device due to step commands are discussed in Section 4 together with a closed-loop compensation using a PI-compensator. Simulation and practical results are also illustrated. Section 5 concludes the paper.

2 An Overview of our Micro-Heater-Sensor Device Cost effectiveness is an important issue in a real production. Concerning an implementation of a micro-actuator-sensor, its fabrication process and materials used must be considered carefully. Materials must be chosen to suit physical parameter ranges, i.e. temperature in this context. Aluminum, copper, nickel, gold and platinum are common for use in heating and measuring elements. Orderly, they cover the lowest to the highest prices corresponding to the narrowest to the widest temperature ranges. As an approximation, the prices of gold and platinum are 25,000 times higher than those of aluminum (Al) and copper. For use as a heating element, materials with low temperature coefficients of resistance (TCR) are suitable. Materials with high TCR are appropriate for use as a heat measuring element. For fabrication, x-ray lithography process is more straightforward than

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UV lithography in an exchange of a considerably high cost.

Fig. 1 2D pattern of our Al-Ni micro-heater-sensor device.

Our micro-device proposed in [7] uses Al as its

heating element, and nickel (Ni) for sensing temperature. It is aimed for a temperature range of 40 - 120 C. To keep the cost low, Al micro-heater and Ni micro-sensor are fabricated onto a glass substrate of 1-mm thickness via UV lithography.

The device pattern is shown in Figure 1. The nominal resistances of the Al micro-heater and the Ni micro-sensor are 114 and 1,427 Ω, respectively. By applying voltages across the two terminals of the heater, heat is generated rapidly. The Ni RTD sensor placed inside the heater as shown can accurately sense temperature changes. Fig. 2 Al-Ni micro-heater-sensor with signal conditioning circuits.

3 Static Calibration Our micro-device is a resistive type. Their nominal resistances correspond to 25 C, i.e. Rheater = 114 Ω and Rsensor = 1,427 Ω. As a common practice, the sensor can be placed as a resistive arm of either a

voltage divider or a DC bridge. Static calibration provides a useful transfer curve mapping the temperature being measured and the output voltage of the signal conditioning circuit. The design of our signal conditioning circuits follows conventional procedures found in textbooks. The circuit diagram is shown in Figure 2. The micro-sensor with a few resistors form a voltage divider. The micro-heater is driven by a PN2222A transistor in turn controlled by an 8-bit AVR microcontroller. A zero-span buffer and a simple low-pass filter are incorporated into the circuits.Component list is as follows: Heater driver:RB= 330 Ω, Rheater = 114 Ω, VS= 9 V Voltage divider: Rd = 1.5 kΩ, Rvol = 253 Ω, Rsensor

= 1.427 kΩ,Vcc = 5 V Zero-span: R1 = 1.277 kΩ, R2 = 1.5 kΩ,RG = 30

kΩ, RF = 330 kΩ Low-pass filter:Rfil= 100 kΩ, Cfil = 0.1 µF With this circuit arrangement, the sensor resistance varies in the range of 0 ≤ R ≤ 500 Ω in accordance with the temperature range of 25 ≤ T ≤ 140 C. correspondingly, the zero-span circuit produces 0.2 ≤ Vout≤ 4.0 V Since the micro-device is so tiny (1.4 mm x 1.4 mm), measuring the temperature at this spot can be made easy via an FLIR-T360 infrared camera. During our experiments, the AVR processor produced decimal integers in the range of 20-210 for driving the micro-device. Temperature levels, output voltages and resistance changes were recorded. Fig. 3 Experimental equipment setup. Experiments were repeated 10 times to obtain averaged results. Figures 3 and 4 show our experimental setup and results, respectively.

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Fig. 4 Experimental results for static calibration: (a) Rsensor vs PWM code, (b) Vout vs Rsensor, (c) Rsensor vs temperature and (d) temperature vs Vout. The averaged results are displayed in Figures 4(a)-(d). Based-on the Figure 4(d), a linear fit is obtained as follows: T = 31.315Vout + 18.387 (1) where T is temperature in C, and Vout is output voltage of the zero-span circuit in volts. This linear

fit is applicable for 25 ≤ T ≤ 140 C Sensitivity of our micro-sensor is S = 0.032 V/C. Next, we consider dynamic response of the device, and present a way to improve it.

4 Dynamic Response and Compensa-tion Dynamic responses of the device are important for any designs surrounding it. We measured its step responses of various steady-state temperature levels based-on the measurement configuration shown in

Fig. 5 Measurement setup for step responses of the micro-device. Figure 5. The recorded temperature levels represent the heat generated by the micro-heater. The AVR processor introduces the following decimal codes: 19, 50, 84, 123, 163 and 214 for driving the transistor. These codes correspond to the step commands of 40, 60, 80, 100, 120 and 140 C, respectively. The recorded responses are illustrated in Figure 6. Notice that they reflect overdamped characteristic of the device with settling time as long as 100s. This naturally comes from the property of the materials used and the thickness of the glass substrate. Our device is low in price and rugged. Better responses can be achieved by using noble metals with high prices, and very thin glass substrate but fragile. Here, we offer an alternative to improve the performance of our device via a PI-compensator.

Fig. 6 Step responses of the micro-device without compensation.

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We use ARMAX technique with MATLAB for identification of the device model. To keep the problem simple, we consider to use first-order model representation. Equation (2) expresses the device model indicating a settling time of 40 s,

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while the practical one is about 100 s (see Figure 7(a)). This sluggish response can be improved by a closed-loop compensation. A PI-compensator is adopted to ensure a zero steady-state error, and a phase-margin of at least 50 deg. The design follows standard design procedures found in textbooks. We achieveKp = 30.00 and Ki = 10.65 such that the compensator is represented by

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(b) Fig. 7 (a) Model validation plot, (b) simulated responses with and without PI-compensation.

This design ensures GM of infinity and PM of 88 deg. With a unity-gain feedback, our closed-loop system is described by

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Figure 7(b) displays simulated step responses for comparison purposes. The PI-compensator improves the dynamic responses of our micro-device significantly. In this, an overshoot of less than 5% and a settling time of less than 10 s are observed. Figure 8 represents our closed-loop system. Fig. 8 Block diagram of our closed-loop system. Experimental results are displayed in Figure 9. It can be observed that our micro-heater without compensation settles to 100 C in 100 s, while it takes only 16 s with the proposed compensation to settle. Furthermore, the closed-loop system exhibits underdamped characteristic. The transient response possesses only 3% overshoot and rise-time of 1.7 s, approximately. Regarding these figures, our micro-device with the PI-compensator is 58.88 times faster than the open-loop one in terms of rise-time, and 6.25 times faster in terms of settling time. Beyond 150 s, the device cools down naturally.

5 Conclusion The article has proposed a micro-device composing of Al-heater and Ni-sensor fabricated on a glass substrate. The materials used are rugged and low-cost for a nominal operating temperature of 40-120 C. Fabrication technique utilizes UV lithography at Synchrotron Light Research Institute, Thailand. The results of static calibration has been presented. Dynamic responses of the micro-device according to step-temperature commands have been discussed with a proposal of PI-compensation. The proposed method enhances the performance of the device significantly. Practical results show that the compensated device responds about 60 times faster than the uncompensated one in terms of rise-time.

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Acknowledgements The first author's thanks are due to Synchrotron Light Research Institute, Thailand, for his doctoral scholarship.

Fig. 9 Experimental results. References: [1] C. Y. Lee,G. B. Lee, J. L. Lin and F. C.

Huang,Integrated microfluidic systems for cell lysis, mixing/pumping and DNA amplification,Journal of Micromechanics and Microengineering, Vol. 15, 2005, pp. 1215-1223.

[2] Z. Q. Niu,W. Y. Chen,S. Y. Shao, Y.Jiaand P. W.Zhang,DNA amplification on a PDMS-glass hybrid microchip, Journal of Micromechanics and Microengineering, Vol. 16, 2006, pp. 425-433.

[3] D. S. Lee, S. H. Park, K. H. Chung and H. B.Pyo,A disposable plastic-silicon micro PCR chip using flexible printed circuit board

protocols and its application to genomic DNA amplification,IEEE Sensors Journal, Vol. 8, 2008, pp. 558-564.

[4] G. S. Chung and J. M. Jeong, Fabrication of micro heaters on polycrystalline 3C-SiC suspended membranes for gas sensors and their characteristics,Microelectron Engineering, Vol. 87, 2010, pp. 2348-2352.

[5] J. H. Yoon and J. S. Kim, Study on the MEMS-type gas sensor for detecting a nitrogen oxide gas,Solid State Ion, Vol. 192, 2010,pp. 668-671.

[6] W. J. Hwang,K. S. Shin, J. H. Roh, D. S. Lee and S. H. Choa,Development of micro-heaters with optimized temperature compensation design for gas sensors,Sensors, Vol. 11, 2011, pp. 2580-2591.

[7] R. Phattanakun, P. Deekla, W. Pummara, C. Sriphung, C. Pantong and N. Chomnawang, Design and Fabrication of Thin-Film Aluminum Microheater and Nickel Temperature Sensor, In Proceedings of The 7th IEEE international conference on Nano/Micro Engineered and Molecular Systems (IEEE-NEMS 2012), Kyoto, Japan, 5-8 March, 2012, pp.159-162.

[8] T. Guan and R. Puers,Thermal Analysis of a Ag/Ti Based Microheater, InProceedings of EurosensorsXXIV, Linz, Austria, 5-8 September 2010, pp. 1356-1359.

[9] K. L. Zhang, S. K. Chou and S. S.Ang, Fabrication, modeling and testing of a thin film Au/Ti microheater, International Journal of Thermal Sciences,Vol. 46, 2007, pp. 580-588.

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