Abstract:
An ultracapacitor power supply for an electric vehicle is provided which employs an ultracapacitor as the primary power source and a battery as a supplemental power source. This vehicle is particularly effective in power-rail system having gaps in the power-rail or in non-power rail systems having recharging stations positioned along the track The ultracapacitor recharges quickly upon vehicle passage over a live power rail or entry into a recharging station. Optimum performance is achieved when the capacitor is allowed to fully recharge. The battery need only provide power when the ultracapacitor has been discharged or during acceleration or other periods of peak power consumption, thereby reducing the number of battery recharges required during any given operational period.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to provisional patent application serial No. 60/138,714 filed Jun. 11, 1999; the disclosure of which is incorporated by reference. 
    
    
     S 
     TATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT N/A 
     FIELD OF THE INVENTION 
     This invention relates to materials transport systems and, more particularly, to the use of ultracapacitors as the primary power source for electric vehicles used in such systems for moving semiconductor wafers or other materials to various locations along a track. 
     BACKGROUND OF THE INVENTION 
     Computer controlled materials transport systems are known for moving materials among various work stations of a facility. Such systems are employed, as an example, in semiconductor fabrication facilities for moving semiconductor wafers to successive work stations. In such a wafer transport system, a monorail track is routed past the various work stations and a plurality of electric vehicles are mounted on the track. The plurality of electric vehicles are moveable along the monorail track for delivering wafers to the successive work stations for processing, and for removing wafers therefrom after the requisite processing operations are completed. The track is composed of interconnected track sections which usually include one or more routing sections or modules which are operative to provide plural paths along the track. 
     Each vehicle includes one or more electric motors coupled to drive wheels which in turn engage the track for propelling the vehicle along the track. Electronic circuitry governed by an on board micro-controller controls operation of the vehicle in response to control signals provided from one or more central control points within the facility. It is not always possible, or desirable, to directly power the vehicle via a power rail or similar means. Therefore, a battery or battery pack is usually contained on board the vehicle for powering the motors and associated circuitry for at least a portion of vehicle operation. An ultracapacitor may also be contained on board as a supplemental power source to supply additional power during peak consumption periods. A materials transport system used for semiconductor wafer transport and other materials is described in U.S. Pat. No. 4,926,753, assigned to the same Assignee as the present invention. 
     Due to the power demands of an electric vehicle, the battery must be frequently recharged through either a mechanical electrical connection or inductive coupling of magnetic fields at recharging stations located at predetermined positions along the track as described in U.S. patent application and assigned to the same Assignee as the present invention. However, the batteries used to power these vehicles can be recharged only a specified number of times before they reach the end of their life cycle. In general, the life cycle of a rechargeable battery is less than that of the vehicle in which they are installed in. Because each battery replacement increases the cost of operating an electric vehicle, the cost associated with each vehicle must be increased to account for the eventual replacement of the rechargeable batteries used in it. In addition, certain types of batteries require periodic deep discharges to prevent the batteries from developing a charge “memory” that may limit the ability of the rechargeable battery to fully charge. Having to deeply discharge a battery in a vehicle being used runs the risk of stranding the vehicle without power, possibly resulting in a system blockage. This can result in a decrease in the efficient use of an electric vehicle as well. 
     Therefore, what is needed is a method and system that decreases the reliance of an electric vehicle on power that is supplied by a rechargeable battery by reducing the number of battery charge/discharge cycles required during a given period of vehicle operation, so as to extend the battery life. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the invention, an unltracapacitor power supply for an electric vehicle is provided which employs an ultracapacitor as the primary power source and a battery as a supplemental power source. This vehicle is particularly effective in power rail system having gaps in the power-rail or in non-power-rail systems having recharging stations positioned along the track The ultracapacitor recharges quickly upon vehicle passage over a live power rail or entry into a recharging station, and optimum performance is achieved when the capacitor is allowed to fully recharge. The battery then needs only provide power when the ultracapacitor has been discharged or during acceleration or other periods of peak power consumption, thereby reducing the number of battery recharges required during any given operational period. 
     A current surge limiter or constant current circuit may be inserted between the recharging power source and the ultracapacitor to avoid damage to the ultracapacitor or overload of the recharging power source. The current surge limiter may be an active current surge limiter. Furthermore, a cell voltage equalizer for limiting the charge voltage may be placed across each individual cell of an ultracapacitor assembly to extend the assembly lifetime. The cell voltage equalizer may include an overvoltage detector to detect any cell whose voltage has risen above a predetermined threshold level and a reporting mechanism to identify any such occurrence. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a pictorial view illustrating a monorail track and an electric vehicle incorporating the invention; 
     FIG. 2 is a block diagram of a power supply for an electrical vehicle having an ultracapacitor as a primary power source and a battery as a secondary power source; 
     FIG. 3 is a block diagram of the primary power source of the power supply of FIG. 2; 
     FIGS. 4-4C are schematic diagrams of three embodiments of the cell voltage equalizer circuit of the primary power source of FIG. 3; 
     FIG. 5 is a block diagram of the recharging system; 
     FIG. 6 is a block diagram of an inductively coupled power supply connection to the recharging system; 
     FIGS. 7A and 7B are schematic representations of the inductively coupled power supply connection of FIG. 6; and 
     FIG. 8 is a schematic diagram of the current limiting circuit for use in the recharging system of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An electric vehicle  10  incorporating the invention and a portion of a monorail track  12  upon which it rides is illustrated in FIG.  1 . The vehicle  10  may be used, for example, in a wafer handling system to transport wafer cassettes containing semiconductor wafers between stations in a semiconductor manufacturing facility. The track  12  is disposed along one or more predetermined pathways and may have several such vehicles riding on the track. The track  12  is usually composed of a plurality of modular sections which are interconnected by routing modules which are operative to interconnect track sections to provide flexible and efficient routing of the vehicles along desired paths. The track  12  may include a power rail (not shown) connected to a system power supply to directly power the vehicle  10 , except when gaps in the rail are encountered and the vehicle  10  must rely on internal power. In another embodiment the vehicle may be self-powered relying for power upon an onboard power supply, which preferably is rechargeable and contains a sufficiently high energy density to power the electric vehicle between recharging stations (not shown). 
     Referring to FIG. 2, a power supply  20  is disposed on, or within, the vehicle  10  and comprises a primary ultracapacitor power source  30  and a secondary power source  28  that may be a battery, or a second ultracapacitor coupled via a switching circuit  22  to an engine and drive unit  23 . Optionally, a DC-DC converter  24  may be used and disposed between the switching system  22  and the engine and drive unit  23 . A controller  14  is coupled to the engine and drive unit  23 , the primary and secondary power sources  30  and  28 , and switching system  22 . The controller  14  monitors the voltage and current from the primary and secondary power sources, and controls the switching device  22  to select either one, or both, of the primary and secondary power sources for supplying the necessary power for the vehicle  10  to operate. In addition, the controller is responsive to control signals provided by the track system controller  26  with regard to the control and motion of the vehicle  10 . The micro-controller  14  may be an embedded microprocessor device, such as the Pentium® III™ microprocessor available from Intel Corporation, of Santa Clara, California. The switching circuit  22  typically comprises an array of switching elements (not shown) controlled by the controller  14 . Recharging system  32  receives power from recharging system power supply  8  and provides the current necessary to recharge the primary and secondary power sources. As will be explained in more detail below, the recharging system  32  includes two parts: a recharging station part and an onboard vehicle part. The two parts of the recharging system  32  cooperate to transfer power from the recharging station to the onboard vehicle to recharge the primary and secondary power sources and, in one embodiment, power the electric vehicle. 
     The primary ultracapacitor power source  30  is illustrated in FIG.  3  and comprises an ultracapacitor assembly  32  having a plurality of ultracapacitor cells  34  electrically coupled together to supply sufficient voltage and current to operate the electric vehicle. Each ultracapacitor cell  34  may also includes a cell voltage equalizer and over-voltage detection system  50  connected across the voltage terminals of each ultracapacitor cell  34 . As will be described below, the cell voltage equalizer  50  prevents the recharging voltage applied to the ultracapacitor cells  34  from exceeding a specified maximum cell voltage during the recharging operation. In addition, each cell  34  may include an over-voltage detection system incorporated with the cell voltage equalizer to detect an over-voltage condition occurring on any of the ultracapacitor cells  34 . An over-voltage signal is generated by the over-voltage detection system and supplied to the over-voltage reporter system  52 . The over-voltage reporter system  52  provides a signal to the controller  14 . In one embodiment, the over-voltage signal may include an LED that illuminates when an over-voltage condition is detected the over-voltage detection system. A photo-sensor that can be monitored by the controller  14  may be used to detect the illumination from any of the plurality of LEDs. In this way, an over-voltage condition on any ultracapacitor cell  34  will trigger the photo-sensor and alert the controller  14 . In another embodiment the over-voltage detection system may provide a logic signal to an OR logic gate that can generate an output signal when any ultracapacitor cell  34  indicates that an over-voltage condition exists. The signal from the logic OR gate can be monitored by the controller  14 . Upon receipt of the overvoltage signal, the controller  14  can take appropriate action to prevent the over-voltage condition from damaging the ultracapacitor cells  34 . 
     Three different embodiments of the cell voltage equalizer  50  are illustrated in FIGS. 4A-4C. Referring to FIG. 4A, a voltage across the ultracapacitor cell  34  that is sampled by resistors  404  and  409  and that exceeds the turn-on voltage of NPN bipolar transistor  402 , switches the transistor  402  “on”. This allows current to pass through the transistor  402  and hence current to flow through a light emitting diode (LED)  408  causing it to illuminate. The voltage drop across the LED  408  and resistor  407  will turn “on” the PNP bipolar transistor  406 . This allows current to flow through transistor  406  discharging the ultracapacitor cell  34  through the transistor  406 . After the ultracapacitor cell  34  has discharged sufficiently such that the voltage across it falls below the turn “on” voltage of transistor  402 , transistor  402  will turn “off”, and not allow current to flow through it. This in turn will turn “off” the LED  408  and transistor  406  as well. 
     Similarly, in FIG. 4B, an over-voltage condition on an ultracapacitor cell will switch PNP transistor  414  “on”. This will allow current to flow through transistor  414  causing the LED  418  to illuminate and to turn “on” an n-channel field effect transistor  416 . Turing “on” transistor  416  allows current to flow through the transistor  416  discharging the ultracapacitor cell  34 . When the voltage across the ultracapacitor cell  34  falls below the over-voltage condition, transistor  414  turns “off” preventing current from flowing and turning off the illumination from LED  418  and turning off transistor  416 . 
     Finally, in FIG. 4C, a comparator  426  switches “on” when an over-voltage condition occurs. The comparator  426  compares a voltage sampled from the ultracapacitor cell  34  to a reference voltage set by resistor  427  and diode  429 . When the comparator  426  senses the measured voltage  422  exceeds the reference voltage  424 , the comparator switches to a high output. This allows current to flow through LED  430  causing it to illuminate, and enables a p-channel field effect transistor  428  to turn-on and discharge the ultracapacitor cell  34 . In all three embodiments, a photo sensor may be used to detect the illuminated LED. As discussed above, the photo sensor may be monitored by the controller  14  such that the over-voltage condition exists on an ultracapacitor cell  34  is thereby reported. Those skilled in the art will recognize that there are a variety of other mechanisms available for reporting a faulty cell to the micro-controller  14 , including the OR logic gate discussed above. 
     The secondary power source  28  can be a rechargeable battery pack. Preferably the battery pack is not of a type requiring a periodic deep discharge. As discussed above a battery that does not require a deep periodic discharge may be kept close to full charge and thereby reduce the possibility of the vehicle  10  becoming stranded on the track  12  without power. Alternatively, a second ultracapacitor may be used as the secondary power source  28   
     Both the ultracapacitor  30  and the rechargeable battery  28  are finite storage devices that discharge with vehicle use and, therefore, require periodic recharging to restore their charges. A recharging system  32  can be used to assure that both the ultracapacitor  30  and the rechargeable battery  28  are charged with sufficient power to move the electric vehicle to the next recharging station. 
     As discussed above, there are primarily two methods of providing power to an electric vehicle in a material handling system: a vehicle system that includes an onboard self-contained power supply, a “self-powered system”; and a vehicle system in which the vehicle receives power from a power rail system adjacent to the track, a “power rail system”. In a self powered system, one or more intermittent recharging stations may be positioned along the track  2  where, under predetermined conditions, an electric vehicle may stop to recharge the primary and secondary power sources. A recharging function may also be incorporated into a portion of, or all of, the work stations, where the electric vehicle may deliver or remove wafers. As used herein, a recharging station or a recharging function performed at a workstation are both referred to generically as a recharging system. 
     The intermittent recharging system  32  is powered by a recharging system power supply  8  and is preferably spaced close enough together so that the ultracapacitor assembly  32  does not completely discharge before reaching the next station. Should the ultracapacitor assembly fully discharge the power will be supplied by the secondary power source. The controller  14  may command a recharging as a regularly scheduled stop, at a work station or at a recharging station, and/or upon the detection of a low charge condition. Ideally, both the primary ultracapacitor power source  30  and the secondary rechargeable battery power source  28  are recharged to their full capacity before the vehicle  10  exits the station and continues on its route. 
     In a power rail system, the power rail directly powers the electric vehicle. In this system the primary ultracapacitor power source and the secondary rechargeable battery power source are utilized to provide power only where there are gaps in the power rail and the electric vehicle must rely on internal power. In this system, the recharging function occurs during the time that the electric vehicle is in contact with the power rail. Recharging is less frequent than in a rail powered system because the ultracapacitor assembly  32  need only provide power when the vehicle passes over gaps in the rail, which are preferably of short distance so that the ultracapacitor does not completely discharge. As used herein, the recharging function of the power rail system will also be subsumed within the term recharging system. 
     One embodiment of a recharging system  32  is illustrated in FIG.  5 . The power conditioning and conversion system  33  receives power from a recharging system power supply  8 . The reconditioned and converted power is provided to a recharging system coupler  16  that is configured and arranged to provide electrical power transfer to the motor vehicle coupler  6 . As will be explained below, there may be either a direct electrical connection between the two couplers or the couplers may be inductively coupled together. The power that is received by the motor vehicle coupler  6  is provided to an active current surge limiter  40  discussed in detail below, and may optionally include a second DC-to-DC converter  24  placed across the active current surge limiter  40 . In another embodiment, a passive current surge limiter may be used. This parallel combination may be used to maintain a constant charging current until the ultracapacitor is within approximately 1 or 2 volts of the full charge voltage. The charging current provided by the current limiting system and the DC-DC converter may be provided to the switching circuit  22  for distribution under the control of controller  14 . 
     The recharge system coupler  16  and electric vehicle coupler  6  are configured and arranged to provide a transfer of power from the recharging system to the electric vehicle that is sufficient at least recharge the primary and secondary power sources and in one embodiment, power the electric vehicle as well. The power transfer typically is either a direct electrical connection or may be inductively transferred. 
     In a direct electrical connection the motor vehicle coupler  6  may include one or more pair of electrically conductive brushes or connectors that are operative for maintaining a continuous electrical connection with a power rail or other electrical supply connector. 
     In an inductively coupled system, which may include both a power rail system or a recharging system, inductively coupled power is transferred between a primary coil located on the recharging system and a secondary coil located on the electric vehicle. One embodiment of an inductively coupled power transfer system is illustrated in FIG.  6 . The recharging system power supply  8  provides power to a voltage converter  604  and a power driver  612 . The voltage converter  604  provides voltage and current to the multi-vibrator that converts a direct current waveform into an alternating current waveform that is suitable for use in a transformer system. Primary coil  614  receives the alternating current waveform that has been amplified by power driver  606 . A detection means  610  and  620  cooperate to provide a signal indicating that the electric vehicle  10  is present to the controller  608 . The controller  608  controls the process of transferring power and prevents a loss of the track system efficiency by ensuring that power is supplied only when the electric car and is present. Primary coil  614  and secondary coil  616  are placed proximate to one another to maximize the inductive transfer of power therebetween. Secondary coil  616  provides the power received from the primary coil  614  to the recharging system  618  for application to the primary and secondary power sources. 
     One embodiment of the primary coil  614  and secondary coil  616  is illustrated in FIGS. 7A and 7B. The primary coil  614  may be U-shaped and is disposed along the longitudinal axis of the track. The secondary coil  616  is configured and arranged to pass through the center section  702  of the primary coil for the length of the primary coil allowing magnetic coupling to occur between the two coils. In one embodiment, the secondary coil  616  is E-shaped such that the secondary coil  616  is surrounded by the primary coil  614 . The secondary coil may be wound on a ferrite core to enhance the magnetic coupling between the magnetic filed of the primary coil and the secondary coil. Magnetic coupling between the primary and secondary coil induces a voltage and current in the secondary coil that is used to recharge the ultracapacitor and the rechargeable batteries. In addition, for the period of time that the electric vehicle is coupled to the recharging station, the induced secondary voltage and current may be used to operate the electric motor  12  as well. 
     During the process of recharging the ultracapacitor, the recharging current must be regulated to prevent the ultracapacitor from charging at too fast a rate that may damage the cells and overload the recharging station. Charging at a slow rate, however, may result in an unacceptably long charge period. This may result in an incomplete charge being placed on the ultracapacitor that may result in a loss of system performance or stranding the electric vehicle without power. Accordingly, the active current surge limiter  40 , or constant current circuit, is used within the recharging system  4  to provide the proper current to the ultracapacitor  32 . 
     One embodiment of the active current surge limiter  40  is illustrated in FIG.  8 . When a large potential exists between the input voltage, V in    802 , i.e., the recharge voltage, and the output voltage, V out    810 , i.e., the ultracapacitor voltage, a charge current flows through the low value series resistor, R s , and a field effect transistor  808 . However, if the charge current exceeds a predetermined level, in one embodiment, approximately 3 amps, a bipolar transistor  806  turns “on” to shunt the charging current away from transistor  808  and thereby shut “off” transistor  808 . This serves to reduce the charge current through the series resistor, R s , to a value sufficiently low to keep the voltage drop below the transistor  808  turn “on” voltage. 
     Preferably, the second DC/DC converter  24  is connected in parallel with the active current surge limiter. The DC/DC converter  24  initially converts the high voltage, low current input into a low voltage, high current output to rapidly charge the ultracapacitor at lower voltages when T 2  is “off” and the recharge current flowing through the active current surge limiter is low. As the DC/DC converter output voltage increases, the output current begins to fall off, however, as the voltage potential between the input voltage, V in , and the output voltage, V out , decreases, T 1  turns “off” and T 2  turns “on” and the active current surge limiter  40  provides the charge current. The charge current flowing into the ultracapacitor cell  34  thus remains essentially constant to within approximately one or two volts of the input voltage, V in . 
     The constant charge current is directed to the primary and secondary power sources via the switching device  22 . To achieve the maximum lifetime from an ultracapacitor assembly, the voltage on each and every cell must be maintained below a specified voltage during each recharge cycle. Resistors placed across the cells may equalize the cell voltages after several hours, but can not equalize the cell voltages for the first, or near first, recharge cycles. Working in conjunction with the above-described current surge current limiter  40 , the above-described cell voltage equalizer and over-voltage detector system  50  detects an overvoltage condition and reduces the voltage across an ultracapacitor cell  34  to a level below the specified voltage on every recharge cycle, regardless of the charge state of the cell  34 . Furthermore, because it is advisable to replace each faulty cell in the assembly, as described above, a mechanism is provided to report the over-voltage condition to the controller  14 . 
     While the invention has been described in relation to a semiconductor wafer handling system, it should be evident that the invention is also useful for other material handling systems in which a vehicle is moveable along a track. Accordingly, the invention is not to be limited by what has been particularly shown and described as variations and alternative implementations will occur to those versed in the art without departing from the spirit and true scope of the invention as represented by the appended claims.