sensor, accelerometer and microphone), processor, RF transceiver, solar cell and battery can be packaged as a button-like autonomous microsystem. Related ...
Pervasive Computing: Proceedings of the 2nd Int'l Conference, pages 225-237, Vienna, Austria,
18.-23. April 2004
Towards Wearable Autonomous Microsystems Nagendra B. Bharatula, Stijn Ossevoort, Mathias St¨ ager, Gerhard Tr¨ oster Wearable Computing Laboratory ETH Z¨ urich, Switzerland {bharatula, ossevoort, staeger, troester}@ife.ee.ethz.ch http://www.wearable.ethz.ch
Abstract. This paper presents our work towards a wearable autonomous microsystem for context recognition. The design process needs to take into account the properties of a wearable environment in terms of sensor placement for data extraction, energy harvesting, comfort and easy integration into clothes and accessories. We suggest to encapsulate the system in an embroidery or a button. The study of a microsystem consisting of a light sensor, a microphone, an accelerometer, a microprocessor and a RF transceiver shows that it is feasible to integrate such a system in a button-like form of 12 mm diameter and 4 mm thickness. We discuss packaging and assembly aspects of such a system. Additionally, we argue that a solar cell on top of the button – together with a lithium polymer battery as energy storage – is capable to power the system even for a user who works predominantly indoors.
1
Introduction
Ubiquitous computing [1] or Wearable computing [2] in combination with an intelligent environment are often cited to change our future significantly by supporting its user and enhancing everyday life. Applications range from tourist guides [3], health monitoring [4] to remembrance agents [5]. A system that can perform all these tasks needs to be context aware. Context awareness can be described as the ability of a system to model and recognize what the user is doing and what happens around him, and to use this information to automatically adjust its configuration and functionality [6]. While some of the context information can be retrieved from sensors not carried by the user (e.g. beacons in the environment for location information), many applications require sensors to be placed on the user’s body or in his clothes (e.g. accelerometers for motion recognition or sensors to monitor body function like pulse and blood pressure). In this paper we show how recent advances in CMOS, RF frontend design and microsystems packaging technologies can be used to combine sensors, computing elements, power generation and ultra low power wireless transceiver to develop autonomous microsystems suitable for embedding in clothes. We will discuss the feasibility of a wearable sensor button. Moreover, We will address power generation and power consumption issues to show that even with a few
microwatts of power available, such a small system can be built for useful context recognition. Furthermore, we will discuss wearability and fashion aspects. As an example, we will show how a sensor node with three different sensors (light sensor, accelerometer and microphone), processor, RF transceiver, solar cell and battery can be packaged as a button-like autonomous microsystem. Related Work Some projects like the Smart-Its [7], the Active Badge [8], the Smart Badge [9] or the TEA Device [10] project explore the possibility of wearable systems for context recognition but don’t focus on small and ultra low-power hardware. Till today few research groups tried to develop autonomous microsystems which are of several cubic millimeter in total size. The ‘Smart Dust’ project [11, 12] at Berkeley and the ‘eGrains’ project [13] at IZM Fraunhofer are focussed on building the system in a volume of a few cubic millimeters. For the operation of such small electronic devices often power sources of a few microwatts to a milliwatt are sufficient. Attempts to use human body heat based on the seebeck effect in the form of micro thermoelectric generators [14, 15] or vibration based energy generation [16–18] are elsewhere reported. These power sources based on MEMS (MicroElecto Mechanical System) technology are advantageous in terms of cost, performance and batch fabrication. Paper Contributions and Organization The main aspect which sets us aside from the work done by other groups in the field of autonomous microsystems is our focus on wearable, autonomous systems. The paper addresses the main issues in the development of an autonomous sensor node with respect to wearable systems. This means that the sensor node, if rightly placed, is allowed to have a volume of a cubic centimeter and can exploit the human body as a power source. However, the means of communication between nodes is limited. Due to a missing line-of-sight, optical communication is out of question. Instead, wireless RF communication will be discussed. The remaining paper is organized as follows: Chapter 2 will show which components are required for such a system. In Chapter 3 we will discuss the human body in terms of its possibility to act as a power source. We will discuss the location of the sensor nodes on the human body with respect to wearability, context recognition and power generation. In Chapter 4 an example system for system validation will be presented. We will show how the intended system can be packaged to fulfill the energy and size requirements. Finally, Chapter 5 will give a conclusion and an outlook.
2
Overall System Layout
A context aware system needs to gather sufficiently detailed personal information about the wearer and his/her environment. One widely pursued approach to build context aware systems is via arrays of simple ultra low power sensors distributed over the user’s body [19]. The sensors provide context information
to a central wearable computer. Examples of sensors that have been shown to provide useful context information are accelerometers for motion analysis and gesture recognition as in [20, 21], galvanic skin response sensors, blood volume pulse sensors and respiration sensors for recognizing the user’s state as in [22] or a collection of other sensors like light, humidity, temperature and atmospheric pressure sensors for various context information as in [19]. Even sound has proven to be a useful source of information that can be handled with low power resources [23, 24]. The chief requirements for such sensor nodes can be summarized as follows: – Low Power Consumption: Since context monitoring has to be performed continuously during the whole day it only makes sense if it can be accomplished with minimal power consumption. Ideally, the sensor nodes should be fully autonomous, operating for months or years on a miniature battery or even better, extracting the energy from the environment. – Small Size: The sensor nodes should be small in size (a few mm3 to one cm3 ) to ensure good wearability and possible integration with clothes and accessories. – Wireless Transmission: To guarantee flexibility, the sensor nodes should transmit their results wirelessly to a central wearable computer. The wireless connection is essentially important if the sensors are located in different pieces of clothing that can not be connected by wires (e.g. shoes, trousers, jackets or glasses). – Local Processing Capabilities: The sensor nodes may need to perform certain amount of local processing, to reduce the amount of data that needs to be transmitted because wireless transmission consumes more energy than computation.
Fig. 1. Overall system architecture of an autonomous microsystem
From the above requirements a rough system architecture as shown in Fig. 1 can be derived. A sensor node that represents an autonomous microsystem contains the sensors itself, an AD converter, a computational unit (microprocessor µP,
DSP, or custom made ASIC), a small memory, a RF transceiver including an antenna, a power generator and an energy storage device, for example a solar cell and a battery.
3
The Human Body Aspect: Placement of Sensor Nodes
When designing a wearable autonomous microsystem one has to take into account the limitations imposed by the human body. One key aspect of such a system is wearability. Wearability [25] is described as the interaction between the human body and the wearable object. As optimal examples of wearability can be found amongst our clothes, we propose to integrate autonomous microsystems in our daily clothing. Since clothes are produced according to well established production methods, it would be best to create autonomous microsystems that can be seamlessly integrated into the existing production processes (see Section 3.3). Each sensor node could be part of a sensor network linked via data lines woven into the fabric [26], however if its placement is arbitrary, the system could best communicate via a RF wireless system. Autonomous microsystems can not just be placed anywhere on our clothing, each position offers different (im)possibilities. Some places diminish our comfort, some places do not give adequate sensor feedback while some places can not be reached from the production point of view. In the following sections we will discuss these placement issues, that pose constraints to our system. 3.1
Data extraction possibilities
The type of sensors and its optimal position highly depend on the tasks that need to be recognized. Generally the placement of sensors is simply defined by the type (e.g. light sensors on top surfaces, acceleration sensors for each limb). 3.2
Energy harvesting possibilities
A true autonomous microsystem should generate its own energy extracted from the environment. This means, both energy directly derived from our body and energy from our environment are useful to power the system. Starner [27] mentions energy from breathing, body heat, blood transport and various types of motion. Because we focus on a system that can be integrated to our clothing, we are limited to motion and body heat. The possibility to use motion and body heat is highly dependent on the placement of the system. The dark shades in Fig. 2 indicate where most of the energy from motion (left) and heat (right) can be extracted to generate power. Kinetic generators can be best placed on body parts that are subject to high acceleration or move frequently, preferably limbs. To harvest energy from heat, one would think that the torso is the best place since its temperature is kept constant. However, our clothing prevents a temperature gradient between the sensor and its surrounding. So, only body surfaces exposed to air can be used
max
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Fig. 2. Places where energy from motion and heat can be extracted
for harvesting energy from body heat. Therefore thermoelectric generators need to be placed in close contact to the body, if possible integrated with clothes that are usually worn in a single layer. An important source of energy that is available from our environment is solar energy. Solar cells offer a greater degree of freedom, they can be placed at all points that are frequently exposed to light. Another useful source of energy could be electro-magnetic radiation. The energy could be provided by inductive coupling by a coil integrated in the fabric, placed underneath the microsystem. Tab. 1 gives a comparison of these options. Table 1. A comparison of power generators useful for wearable microsystems Power Generator Power Remarks 2 150µW/mm direct sun Solar cell (outdoors) [28] 1.5µW/mm2 cloudy day 5.7µW/mm2 desk lamp Solar cell (indoors) [28] 0.06µW/mm2 standard desk 1-10mW piezoelectric Motion (shoe generator) [29] 50-250mW electro-magnetic (rotary) 200µW for legs when walking Motion (inertial generator) [18] 50µW for torso when walking Thermoelectric generator [30] 0.2µW/(cm2 K)
3.3
Comfort and integration
We realize that, although autonomous microsystems can be very small, their placement is still subject to comfort [31]. Areas of our body that are often in contact with various objects (feet, back, bottom, underside of arms and hands) should be free from any electronic system unless the system can be seamlessly integrated within the thickness of the fabric. Elsewhere, the microsystem could be an element of embroidery, integrated in a tuck, part of a label or encapsulated in a button, as shown in Fig. 3, to allow easy integration in the production process.
Button
Label
Embroidery
Tuck/Seam
Fig. 3. Possible placement of microsystems in a fabric
4
Feasibility Study
To show that it is feasible to build a wearable autonomous microsystem, we will consider one specific example. This study uses latest research results in CMOS, RF frontend and MEMS technology to create an ultra-low power microsystem in the size and shape of a button. Our system consists of three different sensors: a differential MEMS microphone [32], a photo diode as a light sensor (SFH 3410 from Opto Semiconductors or PDCA12-68 from Optospeed Ticino SA), and an accelerometer (3-axis accelerometers from [33] or [34], or micro accelerometers such as [35]). Signal processing is handled by a low power microprocessor (CoolRisc 88 or 816 core [36]. To AD convert the analog signal of the photo diode and the accelerometer an ultra-low power successive approximation ADC similar to the one in [37] is choosen. A MEMS microphone similar to [38] with an ADC can be used. A memory in the range of 16-32 KByte supports the microprocessor. An ultra-low power direct conversion RF transceiver similar to [39], operating in the ISM band at 868 MHz provides the communication capabilities to the microsystem. The RF transceiver also needs some external components. A solar cell and a lithium polymer battery as an energy storage completes the system. In the following section packaging and assembly of the system in the form of a button is discussed. Size and energy consumption will be addressed, before analyzing the power budget for a typical scenario of an office worker. 4.1
Packaging and Assembly
The packaging and assembly of different modules (communication, memory, power and sensor) places a restriction on the selection of standard solutions. This is due to the differences in the characteristic behavior and technology used in the development of individual modules. Packaging is one of the important factors which determines the overall size and total cost of the microsystem. The sensors chosen for our system place additional constraints to the packaging technology. Solar cells, the photo diode and the microphone need to be on the surface of the system. The RF chip should be shielded from the other chips. Antenna placement turns out to be difficult. We suggest a helix antenna embedded in the side walls of the plastic housing. A different solution would be a patch antenna beneath the solar cell.
Plastic Encapsulation
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Fig. 4. Button-sized microsystem: side view
Opening for Microphone
Solar Cell
Holes for Button Opening for Photo Diode
Fig. 5. Button-sized microsystem: top view, top level (substrate 2)
Fig. 4, 5 and 6 show the packaged and assembled system. The sequential buildup starts with wirebonding the accelerometer, the microphone, the RF transceiver and the microprocessor to a FR-4 or liquid crystal polymer (LCP) substrate 1 by copper wires (see Fig 6). This setup is connected to the battery compartment with vias. The thin film solarcells and the photodiode need to be wirebonded to a separate substrate 2 (see Fig 5). The two packaged parts are attached to each other. This entire part can be placed in a injection moulded plastic housing, which is designed to provide sufficient mechanical stability, stiffness and easy assembly. The side walls of the housing encapsulate the helix antenna. The whole system is sealed off by a translucent resin.
MEMS Microphone ADC Cool Risc 88 Processor Memory Antenna
Plastic Housing External Components RF Transceiver
Micro Accelerometer Holes for Button Photodiode
Fig. 6. Button-sized microsystem: top view, bottom level (substrate 1)
4.2
Size of the System
The overall size of the system is limited by its components and the used packaging and assembly technique. Bare dies are recommended in order to reduce the size of the system [40]. The area requirements of the individual components are summarized in Tab. 2. Table 2. Area requirements of the individual components Component Area [mm2 ] ADC (active die area) 0.06 CoolRisc 88 processor 1.5 Memory 2.5-3 MEMS microphone (baredie) 1.69 - 2.0 Micro accelerometer 6.6 Photo diode 1.5 RF Transceiver 10.5 External components 4 2 Solarcells 34-38
The analog and digital parts, including the RF components altogether occupy an estimated area of 28 mm2 . The total area of substrate 1 measures 44 mm2 (after deducting 6.3 mm2 for the 2 holes), which leaves enough remaining space for the wirebonding. The available space on substrate 2 is 72 mm2 which allows the two solar cells to occupy a maximum area of 37 mm2 After carefully
considering the dimensions of battery(s) and the plastic housing with integrated antenna we believe that it is possible to design this system in a button-like form with 12 mm diameter and 4.2 mm thickness. 4.3
Power Consumption of the System
Tab. 3 shows the power consumption of the major components when continuous operation is assumed. For many applications it is possible to reduce the duty cycle whereby reducing the power consumption. The value for wireless data transmission at a rate of 1kBit/s includes analog RF frontend and baseband processing in the microprocessor. The power consumption value of the accelerometer, the microphone and the light sensor include signal conditioning and analog to digital conversion. Tab. 3 also lists the sampling frequencies fs for the sensors. These values have proven to be sufficient to create accurate results for context recognition [5, 19, 23]. The Cool Risc 88 processor is running at 20 MHz. The power consumption values for the microprocessor include memory operations. Table 3. Power consumption for continuous operation Functional Unit Accelerometer MEMS microphone Light sensor Microprocessor Data transmission Total
fs [Hz] 50 5000 2 (20 MHz) (1 kBit/s)
Power [µW] 150–200 50–80 50–100 140–170 100–150 490–700
In total the energy consumption is in the range of 490-700µW. We conclude that such a microsystem can be realized with a power consumption of 1 mW or less. 4.4
Energy production
Our proposed system uses solar energy to collect, process and forward sensory data to a central wearable computational device. Under continuous lit circumstances solar cells can provide between 0.06 to 150 µW/mm2 (see Tab. 1). However, mounted on a wearable device, lighting conditions vary strongly depending on the user’s activities, his environment and the placement of the solar cell. For any solar cell applied to clothing it would be more appropriate to consider the energy harvested during a daily activity. To illustrate this, we estimated the total amount of energy a solar cell could harvest in a scarce lit environment like an office. We created a scenario of a north European office worker and estimated the amount of solar power this person could collect during 24 hours (see Fig. 7a). Our fictive person wakes up before 7.00h and walks to his office between 8.00h
a Generated Power [ mW/mm 2 ]
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Fig. 7. Power and accumulated energy collected with a microsystem worn by an office worker
and 9.00h. Between 13.00h and 14.00h he is outdoor for lunch and has a meeting in a brighter space between 14.00h and 16.00h. Between 18.00h and 19:00h he walks back home. The difference between indoor and outdoor activity is clearly visible in Fig. 7a. The continuous line in Fig. 7b shows the accumulated energy derived by integrating the values of Fig. 7a over time. The final amount of energy (707 mJ/mm2 ) collected within 24 hours would be adequate to provide our system (solar cell area of 37 mm2 ) continuously with 37 · (707/(24 · 60 · 60))mJ/sec = 300µW. Assuming that our system is deactivated during night time, it will only consume energy between 6.20h and 20.00h. If our system would consume about 1 mW (line A) it would largely exceed the collected energy which fails the system to be autonomous. However, if our system would consume 0.5 mW during it’s on-time it could be autonomous (line B). In this case: (1) At 6.20h the system activates. Because no solar energy is available, the system has to use previously stored energy from the battery. (2) At 13.10h the difference between the collected and used energy reaches a minimum value of −125 mJ/mm2 . (3) At 14.05h the difference between the collected and used energy reaches a maximum value of about 200 mJ/mm2 .
(4) At 20.00h the system deactivates. The surplus of collected energy is stored in the battery. To overlap the periods of energy shortage or surplus the system should include a small battery. If we would like the system to run according to this scenario, the 2 minimum storage capacity of the battery should be 200mJ/mm ·37mm2 = 7.4J, which is well feasible with a small lithium polymer battery. We conclude, that if our proposed system with a power consumption of 0.5 to 0.7 mW (see Section 4.3) is running on duty cycle below 70% the solar cells and the battery are well capable to power the system in the given scenario.
5
Conclusions
This paper has shown that the design process for a wearable autonomous microsystem needs to take into account aspects of the human body. While the placement of the microsystems can pose problems in terms of comfort and wearability, it can also offer possibilities in terms of energy harvesting through motion or body heat. The wearable environment also offers the possibility to integrate the microsystem into clothes and accessories. We suggest to encapsulate the system in an embroidery or a button. A study of a microsystem consisting of a light sensor, a microphone, an accelerometer, microprocessor and a RF transceiver shows that it is feasible to integrate such a system in a button-like form of 12mm diameter and 4mm thickness. In continuous operation the system consumes less then 700µW. We argue that if the system is running on a 70% duty cycle, which is acceptable for context recognition tasks, it can be made autonomous: A solar cell of 37mm2 size on top of the button – together with a lithium polymer battery as energy storage – is well capable to power the system even for a user who works predominantly indoors. However, here we presented an idealized scenario and effects such as occlusion of solarcells, various indoor environments and light absorbing interiors were not considered. Therefore our future work is focussed on extending the scenarios mentioned in section 4.4 to various users with different daily activities in order to verify the implications on the energy production. We are also investigating the use of alternative power sources, such as thermoelectric and inertial generators, to support the solar cell. Furthermore, in our future work we will address issues of power consumption, reliability, alternative packaging designs and fabrication of a prototype.
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