A novel MEMS based batteryless active RFID system

Abstract

In this paper, a novel, vibration based micro energy harvester system has been proposed. The modeling, design and fabrication of the devices have been carried out. There are several propositions in this work. The biggest claim is to achieve high power density levels with a mm3 device by using a novel vibration based mechanical structure. The operating regime is nonlinear unlike its conventional counterparts, which yields to generate more electrical energy. The proposed packaging method of the device is very simple and easy to realize. Furthermore, a wireless communication scheme based on the well-known RFID (Radio Frequency Identification) technology has also been proposed. The mechanical structure of the device is based on a clamped-clamped beam where four tethers share a thick proof mass. The proof mass is used in order to bring the first eigenfrequency of the system below 50Hz which enlarges the operating frequency range. The surface of the proof mass also acts as a platform for the sensor circuits. Vibrations or sudden movements cause the mechanical structure do not only bend but also stretch, thus working in non-linear regime. The piezoelectric thin film layer on the silicon tethers converts the mechanical stress into the electrical energy. Microwatts of power can be achieved with a mm3 device which yields a high power density levels on the order of the solar panels. This device is named “smart sand”, because it has sensor capabilities that can store information, its size is almost a sand grain and the main material used for the fabrication is silicon. The modeling and design of the mechanical structure has been developed by using COMSOL™ (a finite element analysis (FEA) tool) and SIMULINK™ (an extension tool of MATLAB™). Firstly, the stress levels have been calculated and the transient response of the structure has been investigated. Then, the theoretical analysis has been combined with these two tools and expected power density levels have been found. The fabrication of the “smart sand” has also been developed in this work. Bulk micromachining techniques have been used in order to form the proof mass. The piezoelectric thin film layer has been deposited onto the tethers by sol-gel method. Electrodes of the piezoelectric layer have been fabricated by the lift-off process. The packaging has been embedded into the fabrication process which reduced the handling issues during the process. An elastomer (poly dimethyl siloxane, PDMS) which is very easy to pattern, handle and cheap has been used for the packaging which has not been tried before. The “smart sand” serves as a typical accelerometer which senses the continuous vibrations or sudden accelerations. In order to further develop the sensor capabilities of the system, a new approach has been proposed which is called the “smart RFID” platform where the device has been combined with the RFID concept. In this context, an RFID transponder has been designed. The circuitry has the capabilities of sending and receiving 64 bits of data. The 64-bit memory has been constructed with the magnetic random access memory (MRAM) cells. Few bits have been reserved for the acceleration or temperature data. When a sudden or continuous acceleration happen, the “smart sand” generates voltage and this voltage is used to write the memory bits without the need of a battery. Therefore, the “smart RFID” platform behaves like a batteryless active RFID transponder. In order to determine whether the adequate voltage level is reached or not, a mechanical relay has been proposed and its design has been investigated. Other bits are used for standard identification and they are written or read via an external reader. The frequency of operation has been chosen 2.45GHz. Several applications have been proposed that is possible to use the “smart RFID” platform; such as continuous acceleration monitoring in package delivery, self-powered sensors for homeland security, temperature monitoring of the perishable food item delivery and a batteryless heart rate sensor. Their feasibilities have been investigated and discussed. The applications can be further enhanced and new application alternatives can be developed. The proposed RFID platform has been designed and simulated. All sub-blocks have been designed and the results have been given. In addition to the standard circuit designs, a new modulation technique has been proposed and simulated. This technique eases the data and clock recovery circuits.

Özet

Bu makalede yeni bir titreşim temelli mikro enerji harmanlayıcı sistemi önerilmiştir. Titreşimler ve ani hareketlerin, mekanik yapının sadece eğilmesine değil aynı zamanda gerilmesine yol açması prensibine dayanarak, sistemin doğrusal olmayan bölgede çalışması sağlanmıştır. Tasarlanan ve modellemesi yapılan mekanik yapının üzeri ince bir piezoelektrik film tabakası ile kaplanmış ve bu tabaka üzerinde oluşan mekanik stres elektrik enerjisine çevrilerek devreleri beslemek için kullanılmıştır. Doğrusal olmayan bölgede çalışmanın, mikrowatt mertebesindeki güç seviyelerini mm3’lük aletlerle elde edebileceği göz önüne alındığında, güneş panellerinde elde edilen güç yoğunlukları kadar yüksek enerjilerin elde edilebileceği görülmüştür. Algılayıcı kabiliyeti sayesinde bilgi depolayabilen, kum tanesi büyüklüğünde olan ve üretiminde kullanılan temel malzeme silikon olan bu aletler “zeki kum” olarak isimlendirilmiştir. Mekanik yapının modellenmesi ve tasarımı geliştirilmiş ve üretim sonuçları da ayrıca verilmiştir. Sistemin bilgi gönderebilmesi ve alabilmesi amacıyla iyi bilinen RFID teknolojisi tabanlı bir kablosuz haberleşme yöntemi önerilmiştir. Bu bağlamda, paket taşımacılığında sürekli ivme denetleme, sınır güvenliği için kendinden beslemeli algılayıcılar, çabuk bozulan yiyeceklerin taşımacılığında sıcaklık denetleme ve pilsiz kalp atışı algılayıcı gibi birçok uygulama önerilmiştir. RFID devrelerinin tasarımı yapılmış ve benzetim sonuçları elde edilmiştir. Oku-yaz bellek olarak manyetik belleklerden yararlanılmıştır. Bu sayede, pil kullanılmadan aktif bir şekilde veri yazabilen bir sistem yaratılmıştır. “Zeki RFID” olarak isimlendirilen bu sistemde ayrıca bilginin iletimi için de yeni bir modülasyon yöntemi önerilmiştir.