Abstract:
A fuel cell is integrated with an optical navigation device to extend operational lifetime. A water recycling fuel cell is used to reduce fuel requirements to allow operation of a remote optical navigation device together with a computer on the order of six months or more.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the use of micro fuel cells to power wireless computer pointing devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     Wireless optical navigation devices such as battery operated optical mice with radio frequency or infrared transmitters are presently available based on sensors such as Agilent Technology&#39;s ADNS-2030 or ADNS-2020. Typically, a light emitting diode (LED) light source illuminates the surface under the mouse as the mouse is moved. Battery life is limited by the system&#39;s total power consumption including the optical light source, such as the LED, the optical sensor, processing electronics and the radio frequency or infrared transmitter. Depending on the amount of use, typical intervals for battery changes for wireless, battery operated optical mice is in the range from 1 to 3 months. Additionally, batteries add considerable weight to the optical mouse interfering with ease of use.  
       SUMMARY OF THE INVENTION  
       [0003]     In accordance with the invention, an attractive application for micro fuel cells is to provide a lightweight power source for wireless optical navigation devices such as wireless optical mice used as a pointing device in conjunction with computers such as personal computers and workstations. In particular, micro fuel cells offer an environmentally friendly source of power allowing six months or more of typical use for wireless optical navigation devices for computers before refueling is required.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  shows a micro fuel cell layout suitable for use with a wireless optical navigation device in accordance with the invention.  
         [0005]      FIG. 2   a  shows an embodiment in accordance with the invention.  
         [0006]      FIG. 2   b  is a simplified diagram showing the electrical connections in accordance with an embodiment of the invention.  
         [0007]      FIG. 3  shows a micro fuel cell configuration in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]     In accordance with an embodiment of the invention,  FIG. 1  shows water recycling micro fuel cell system  100  suitable for integration with a wireless optical navigation device, such as a wireless optical mouse, to allow extended operation. Micro fuel cell system  100  includes fuel cell stack  120 , fuel source  140  coupled to anode side  160  of fuel stack  120  and oxidant source  185  coupled to cathode side  180  of fuel cell stack  120 . A water recovery mechanism  122  separates and collects water from the cathode exit stream and feeds water from cathode side  180  of fuel cell stack  120  to mixing chamber  124  where fuel is hydrated prior to delivery to anode side  160  of fuel cell stack  120 . Mixing chamber  124  includes water input port  126  and fuel input port  128 . Fuel recovery mechanism  129  recycles hydrated fuel from anode side  160  of fuel cell stack  120  and discharges anode gas products (e.g., carbon dioxide) into anode gas exit stream.  
         [0009]     In micro-fuel cell system  100 , selectively permeable membrane  130  is positioned upstream of water input port  126 . Membrane  130  is permeable to water but largely impermeable to fuel. Hence, selectively permeable membrane  130  allows water to enter mixing chamber  124  while largely preventing outflow of fuel to cathode side  120  of fuel cell stack  120 . Selectively permeable membrane  130  inhibits fuel from mixing with oxidant at cathode side  180  of fuel cell stack  120  to prevent contamination of the oxidant and reduced cathode performance.  
         [0010]     Selectively permeable membrane  132  is located upstream of fuel input port  128 . Selectively permeable membrane  132  is permeable to fuel and largely impermeable to water. Selectively permeable membrane  132  prevents the outflow of water from mixing chamber  124  into fuel source  140 . Thicknesses and cross-sectional areas of selectively permeable membranes  130 ,  132  are selected to achieve a target mixing ratio of fuel and recycled water, typically in the range of about 0.5/99.5 to about 4/96. To achieve the desired flow rates of recycled water and fuel, some embodiments in accordance with the invention may include multiple channels to supply recovered water from cathode side  180  of fuel stack  120  into mixing chamber  124  and multiple channels to supply fuel from fuel source  140  into mixing chamber  124 .  
         [0011]     Selectively permeable membrane  130  is permeable to water and largely impermeable to fuel. Exemplary materials for selectively permeable membrane  130  for a direct methanol micro fuel cell include hydrophilic material such as mordenite or perfluorosulfonic acid polymer such as NAFION® available from E. I. Du Pont de Nemours Company. Selectively permeable membrane  132  is permeable to fuel and largely impermeable to water. Exemplary materials for selectively permeable membrane  132  for a direct methanol micro fuel cell include hydrophobic material such as polyolefins and rubbery polymers such as neoprene.  
         [0012]     For embodiments in accordance with the invention, fuel cell stack  120  may be implemented using any one of a wide variety of different fuel cell technologies such as low-temperature polymer electrolyte fuel cell technology. The micro fuel cell may use liquid or gas reactants. For liquid based micro fuel cells, the recycled water may serve as a dilutent. In these systems, osmosis through selectively permeable membrane  130  accomplishes the dilution. For feed gas based micro fuel cells, product water at cathode side  180  of fuel cell stack  120  may be used for humidification. In feed gas based systems, diffusion from wet cathode side  180  to dry feed gas provides water transport. In one embodiment in accordance with the invention, micro fuel cells in fuel cell stack  120  are implemented as direct methanol micro fuel cells which each include a membrane electrode assembly that is formed from a thin, proton transmissive solid polymer membrane-electrolyte or ion-exchange membrane positioned between an anode layer and a cathode layer. The membrane electrode assembly is typically sandwiched between a pair of electrically conductive anode and cathode current collectors and typically contains channels for distributing hydrated methanol from mixing chamber  124  over the anode and distributing air from oxidant source  185  over the cathode.  
         [0013]     Water recovery mechanism  122  recovers water from cathode side  180  of fuel cell stack  120  and may be a passive water recovery mechanism such as a membrane selectively permeable to cathode gas products and impermeable to water. Fuel recovery mechanism  129  recovers hydrated fuel from anode side  160  of fuel cell stack  120  and may include a membrane that is selectively permeable to anode gas products and impermeable to the hydrated fuel received from anode side  160  of fuel cell stack  120 .  
         [0014]      FIG. 2   a  shows a simplified side view of the layout of major components for wireless optical mouse  200  in accordance with an embodiment of the invention using micro fuel cell system  100  described in  FIG. 1 . Fuel cartridge  210  is positioned over mixing and osmotic membrane region  225  and air vents  230 . Typical dimensions for fuel cartridge  210  are determined by the need to store the desired amount of methanol fuel. Fuel cartridge  210  is typically replaceable and incorporates selectively permeable membrane  132  (see  FIG. 1 ). It is typically convenient to have selectively permeable membrane  132  be part of the replaceable fuel cartridge as the membrane is degraded by contaminants during use leading to clogging.  
         [0015]     Rechargeable battery  245 , such as a 20 g lithium polymer battery operating at 3 volt, provides about 3 W hours of power. Rechargeable battery  245  directly powers wireless optical mouse  200  and is recharged by fuel cell stack  270  from which power is preferentially drawn at a constant rate. Fuel cell output is limited by the exchange membrane ionic resistance, by the crossover time of the reactants across the ion exchange membrane positioned between cathode side  180  and anode side  160  of fuel cell stack  120  and mass transport of reactants to the electrodes. A narrow peak power operating window results. Therefore, a fuel cell is typically not good for applications requiring burst power. Rechargeable battery  245  is typically used in embodiments requiring high frame rates (see discussion below) as is the case for an optical mouse for videogame applications. Here it is important for wireless optical mouse  200  to respond to rapid movement. For lower frame rates, a capacitor (not shown) may be substituted for rechargeable battery  245 . For example, if wireless optical mouse  200  has a burst power requirement of 100 mW but burst power is required for only 10% of the operating time of wireless mouse  200 , typical for a low power optical mouse with relatively low frame rate, the use of a capacitor on the order of 1 μF allows fuel cell stack  120  to operate at a constant power outpt that is about 10% of the burst power requirement of 100 mW.  
         [0016]     The air intake portion of air vents  230  typically includes two filters. A first filter is relatively coarse and used to keep particulate contaminants out of fuel cell stack  270 . A typical material for the first filter may be the porous foam used in personal computer cooling fan applications. A second filter is used to keep water from passing out of wireless optical mouse  200  while allowing air into fuel cell stack  270 ; filter materials such as GORTEX® may be used for the second filter. Fuel cell stack block  270  includes fuel recovery mechanism  129  and mixing chamber  124 . Typical dimensions for fuel cell stack  270  are about 5 cm 2  area with an approximate thickness on the order of 5 mm. The anode vent portion of air vents  230  is an exhaust to allow reaction by-products to exit fuel cell stack  270 . For an embodiment using a methanol fuel cell, the anode exhaust gas is carbon dioxide. Cathode vent portion of air vents  230  includes water recovery mechanism  122  and allows the venting of water vapor.  
         [0017]     Printed circuit board block  240  may be an existing wireless optical mouse control board with area dimensions of about 5 cm by 8 cm. The optical navigation system includes optical source  290  which may be a low powered VCSEL (vertical cavity surface emitting laser) based optical mouse, an edge emitting low powered laser based optical mouse or an LED (light emitting diode) based optical mouse. Additionally the optical navigation system includes imager  285 , imaging lens  216  and lens  215  as well as the relevant navigation electronics in mouse control electronics  235 . An important feature for optical navigation systems is the frame rate, defined as the number of images obtained at the navigation surface per unit time. In an embodiment in accordance with the invention shown in  FIG. 2   a,  light from optical source  290  passes through lens  215  to a surface and returns to imager  285  via imaging lens  216 . Lens  215  functions to adjust collimation and beam size. Mouse control electronics  235  typically includes the radio frequency or infrared transmitter that allows wireless optical mouse  200  to communicate with a computing device having a video interface. Considerations for the optical system typically include the desire to have compact optics to reduce package size and reducing distances lowers the collimation requirements. Additionally, large diameter lenses are expensive and bulky. Compact design allows the use of most of the light by the imager while reducing the problem of stray light from divergent beams over larger distances. Typical choices for imager  285  are CMOS or CCD detectors in the range of 17 by 17 pixels to 33 by 33 pixels The use of compact optics also provides for more design freedom for the layout of fuel cell system  100  or similar systems.  
         [0018]      FIG. 2   b  is a simplified block diagram showing the electrical layout of an embodiment in accordance with the invention for optical wireless mouse  200 . Fuel cell stack is connected to battery  245 , so that anode  160  is electrically connected to the negative battery terminal  299  of battery  245  and cathode  180  is electrically connected to positive battery terminal  298 . Battery  245  is electrically connected in parallel to imager  285 , optical source  290  and control electronics  235 .  
         [0019]     An important consideration for a wireless optical navigation device such as wireless optical mouse  200  is the operation time available between refueling. A reasonable interval between refueling which provides an advantage over conventional battery operated wireless optical mice is on the order of six months. To estimate the amount of fuel required for a typical six months of operation, a power budget must be obtained. A typical low power VCSEL source requires approximately 5 mW of power, a typical CMOS imager together with the processor requires approximately 30 mW and a radio frequency transmitter having the desired ranges requires about 20-40 mW (Bluetooth) yielding a total power requirement of 55-75 mW. Assuming an average use of six days per week, eight hours per day for six months gives 1152 hours of use. Assuming a user interaction time of about 10% during which time wireless optical mouse  200  operates at full power and otherwise is in sleep mode where there is no power draw results in about 115 hours of actual operation. Hence, the total power budget for six months of use is approximately 35 W hours. Assuming a micro fuel cell efficiency of between about 20-30% and using methanol, with a thermal energy of 5600 W hours/kg, results in the need for about 4-6 g of methanol to achieve the six month operation requirement. Power requirements may be further reduced if an infrared link is used in place of the radio frequency as power consumption is typically less than 5 mW. An infrared link typically requires a line of sight to the computer.  
         [0020]     The operation time between refueling may be improved by higher efficiency fuel cells, use of fuels with higher thermal energies and system level power management improvements. System level improvements include using low power electronics, simplified navigation algorithms requiring less processor overhead, smaller CMOS imagers and use of pulsed LEDs or lasers.  
         [0021]      FIG. 3  shows simplified a side view of fuel cell portion  300  of an embodiment in accordance with the invention. Fuel cartridge  345  is typically located above mixing chamber  310  and next to anode region  375 . Fuel cartridge  345  includes fuel membrane  350  which is typically part of fuel cartridge  345 . Mixing chamber  310  houses water membrane  312  and includes selectively permeable membrane  130 . A microelectromechanical system (MEMS) pump (not shown) may be used to facilitate mixing at about 0.05 cc/hour if diffusive mixing is inadequate. Typical MEMS pumps have power requirements much less than 1 mW. Insulation layer  320  located between fuel cartridge  345  and anode region  375  acts to thermally insulate fuel cartridge block  345  from anode region  375 . Anode region  375  includes anode side  160  of fuel stack  120  shown in  FIG. 1 . Air vent  316  vents anode region  375  and air vent  315  vents cathode region  380 . Typically, two layers of filter material cover air vents  315  to prevent entry of contaminants-a first outer layer that is coarse to keep out particulates and a second layer underneath that is finer, made from material such as, for example, GORTEX®. Membrane  325  separates anode region  375  from cathode region  380  and contains the fuel cell membrane electrode assembly. Cathode region  380  includes cathode side  180  of fuel cell stack  120  shown in  FIG. 1 . Cathode region  380  is adjacent to water recovery region  330  which includes an array of microchannels of water recovery mechanism  122  (see  FIG. 1 ). The microchannels are typically about 1-5 mm in length with a width and depth of about 100 μm. The number of microchannels needed in water recovery region  330  is determined by the power output and the resulting flow rate. Given a micro fuel cell operating at about 20-30% efficiency, the methanol flow rate is about 0.04 cc/hour. Water production from a 1 W fuel cell is approximately on the order of magnitude of the perspiration of a human which is about 6.1×10 −4  g/hour and scales linearly with the power of the fuel cell.  
         [0022]     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description.  
         [0023]     Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.