Abstract:
An energy storage module for use in an electric vehicle, such as a lift truck, is disclosed. The energy storage module includes a bank of super capacitors or ultra-capacitors which are connected between the battery and the load. In operation, the energy storage module charges the capacitors, and uses the charged capacitors to level the load on the battery, limiting spikes in current draw, and assuring a substantially smooth discharge profile, wherein the battery discharge is substantially steady state. The energy storage module further includes sensors for determining when the battery and load are connected.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Not applicable. 
     STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     The present invention relates to load leveling of an electrical vehicle using super-capacitors or ultra-capacitors, and more particularly to battery-powered lift trucks that include capacitive elements for leveling the electrical load. 
     BACKGROUND OF THE INVENTION 
     Lift trucks, material handling and industrial vehicles, and other types of vehicles are frequently constructed using electric motors. To power these vehicles, electric storage batteries, typically lead-acid batteries, are used. These batteries are typically sized to provide sufficient charge for a work shift or other defined period, so that the vehicle remains sufficiently charged during the entire work shift, and can be recharged when the shift is complete. 
     It is desirable, however, to extend the useful charge life of the battery between off-duty charging cycles. To meet this need a number of different systems have been developed. One method for extending the charge of the battery is known as opportunity charging. In opportunity charging, the truck is plugged into a charger during breaks or other non-work periods, to allow for recharging of the battery. This method is helpful in increasing the efficiency of the vehicle, but requires the installation of high kilowatt electrical service throughout a facility, which is both expensive and space-intensive. Moreover, opportunity charging returns only between about five and ten percent of battery charge during an operator break, thereby providing a relatively low return on a significant investment. Additionally, the charging is relatively slow, requiring the vehicle to sit for a significant period of time. 
     Another method for extending the life of a battery during use is known as fast charging. In fast charging, a large battery charger that operates at two to three times the output of the current as a regular battery charger is used. These devices can deliver, for example, four hundred to six hundred amperes during work breaks, and therefore deliver a significantly higher amount of energy to the battery in a reduced time as compared to regular chargers. Fast charging, however, requires alteration to the vehicle to permit the use of special connectors installed on the battery. As with opportunity charging, significant and expensive alterations must be made to the facility to enable charging. Furthermore, the concentrated application of charge to the battery results in significant heating of the battery, and the elevated temperature can be detrimental to the life of the battery. 
     There remains a need, therefore, for an inexpensive, and efficient method for maintaining the charge on a battery during use. The present invention addresses these issues. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a method for maintaining a level of charge between a battery and a load during operation of an electrical vehicle. The method comprises the steps of connecting a bank of super capacitors between an electrical load and a battery supplying power to the load, sensing a connection between the bank of super capacitors and the battery and between the bank of super capacitors and the load, isolating the super capacitors from the battery, and measuring a battery voltage at the battery and a capacitor voltage at the bank of super capacitors. A voltage difference is calculated between the battery voltage and the capacitor voltage, and the capacitor bank is charged when the voltage difference is greater than a predetermined minimum voltage level. When the super capacitors are charged, they supplement the current draw by the load, thereby reducing the rate of discharge from the battery to the electrical load. 
     In another aspect of the invention, an energy storage module for providing load leveling between a battery and an electrical load is provided. The energy storage module includes a battery connector for providing an electrical connection to a battery, and a load connector for connection to the electrical load. The battery connector and the load connector each including corresponding sensing device for sensing a connection. A bank of super capacitors are connected between the electrical connector to the battery and the electrical connector to the load, and a controller is connected to the battery connector, the load connector, and the bank of super capacitors. The controller is programmed to monitor the battery connector sensing device and the load connector sensing device, sense a battery voltage level and a capacitor voltage level when the battery connector sensing device and the load connector sensing device indicate that the battery connector and the load connector are connected, and to calculate a voltage difference between the battery voltage level and the capacitor voltage level. The controller compares the voltage difference to a predetermined minimum voltage value, and charges the super capacitor bank when the voltage difference is greater than the predetermined minimum. Therefore, when the bank of super capacitors is charged, the charge stored in the bank of super capacitors can be drawn by the electrical load, thereby limiting the current draw from the battery, and increasing the usable battery time between charges. 
     These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a lift truck that can include the pallet counter system constructed in accordance with the present invention. 
         FIG. 2  is a block diagram of a control system of the lift truck of  FIG. 1 . 
         FIGS. 3A and 3B  are a simplified circuit diagram of the energy storage module of  FIG. 2 . 
         FIG. 4  is a graph correlating a pulse width modulation duty cycle to a voltage difference level for charging the capacitors of the energy storage module of  FIG. 2 . 
         FIGS. 5A and 5B  are a flow chart illustrating one embodiment of a charging sequence for use in the lift truck of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a method and apparatus for leveling an electrical load in an electrical vehicle supplied by, for example, a battery, a fuel cell, or a combination of these and other types of power supplies. Generally, an energy storage module  57  ( FIG. 2 ) is connected between an electrical power supply  37  and an electrical load  70 , as described more fully below. The energy storage module  57  includes a bank of ultra or super-capacitors. As is known in the art, these ultracapacitors or super capacitors are electrochemical capacitors characterized by a much greater energy density and power per pound than typical electrostatic and electrolytic capacitors, typically on the order of thousands of times greater than a high-capacity electrolytic capacitor. 
     Referring now to the Figures, and more particularly to  FIG. 1 , one embodiment of a material handling vehicle or lift truck  10  which incorporates the present invention is shown. The material handling vehicle  10  includes an operator compartment  11  comprising a body  17  with an opening  19  for entry and exit of the operator. The compartment  11  includes a control handle  14  which is mounted to the body  17  at the front of the operator compartment  11  proximate the forks  31 , and a floor switch  20  positioned on the floor  21  of the compartment  11 . A steering wheel  16  is also provided in the compartment  11 . Although the material handling vehicle  10  as shown by way of example as a standing, fore-aft stance operator configuration lift truck, it will be apparent to those of skill in the art that the present invention is not limited to vehicles of this type, and can also be provided in various other types of material handling and lift truck configurations. Furthermore, although the charging device of the present invention is described and shown in conjunction with a reach truck, it will be apparent that the present invention can be implemented on any lift truck vehicle that includes a fork intended for moving pallets and loads of material, and can also be implemented in other types of electrical vehicles including without limitation electrical cars, golf carts, wheel chairs, and other devices. 
     Referring now to  FIG. 2 , a block diagram of a control system for the lift truck  10  which incorporates the present invention is illustrated. Generally, the electrical load  70  of the lift truck  10  includes a vehicle control system  12  and associated actuators and motors. These are powered by a power supply which, as shown here, can include one or more battery  37 , or a battery in combination with a fuel cell or other power supply devices. The battery  37  is connected to the electrical load  70  through the energy storage module  57  which, as discussed above, includes a plurality of ultra-capacitors or super-capacitors which are used to level the draw of current from the battery  37  to the vehicle electrical load  70 . 
     Referring still to  FIG. 2 , the electrical load  70  of the lift truck  10  includes the vehicle control system  12 , which receives operator input signals from electrical components including the operator control handle  14 , the steering wheel  16 , a key switch  18 , and the floor switch  20  and, based on the received signals, provides command signals to each of a lift motor control  23  and a drive system  25 . The vehicle control  12  can also provide data to a display  55  for providing information to the operator. 
     Referring still to  FIG. 2 , the drive system  25  includes a traction motor control  27  and a steer motor control  29 . The traction motor control  27  drives one or more traction motor  43  which is connected to a wheel  45  to provide motive force to the lift truck  10 . The speed and direction of the traction motor  43  and associated wheel is selected by the operator from the operator control handle  14 , and is typically monitored and controlled through feedback provided by a speed/distance sensor  44  which can be an encoder or other feedback device coupled to the traction motor  43 . The wheel  45  is also connected to friction brake  22  through the traction motor  43 , to provide both a service and parking brake function for the lift truck  10 . The traction motor  43  is typically an electric motor, and the associated friction brakes  22  can also be electrically operated devices. The steer motor control  29  is connected to drive a steer motor  47  and associated steerable wheel  49  in a direction selected by the operator by rotating the steering wheel  16 , described above. 
     The lift motor control  23  provides command signals to control a lift motor  51  which is connected to a hydraulic circuit  53  for driving the forks  31  along the mast  33 , thereby moving the load  35  up or down, depending on the direction selected at the control handle  14 . The drive system  25  provides a motive force for driving the lift truck  10  in a selected direction. 
     The electrical load  70  of the lift truck  10  is, as described above, powered by one or more battery  37 , typically connected to the load  70  through a bank of fuses or circuit breakers  39 . The battery  37  includes a battery connector  59  that mates to a mating connector  65  connecting to the load  70 . The energy storage module  57  is connected between the battery  37  and the electrical load  70 , and includes a first connector  61  that mates to the battery connector  59  and a second connector  63  that connects to the mating connector  65 . The energy storage module  57 , therefore, can be connected with an existing electrical system, and when removed, the load connector  65  can be reconnected directly to the battery connector  59  to provide a direct connection between the battery  37  and the electrical load  70 . Because of these connections, the energy storage module  57  can be selectively connected or removed from the truck circuitry. 
     Referring now to  FIGS. 3A and 3B , a block diagram illustrating the components of the energy storage module  57  of the present invention is shown. The energy storage module  57  generally includes a controller or control circuit  76 , including a central processing unit  78  which can be, for example, a microprocessor or microcontroller. The controller  76  further includes a battery voltage sensing circuit  84 , a capacitor switch circuit  90 , a pulse width modulation circuit  88 , a capacitor voltage sensing circuit  86 , and a light indicator circuit  82 . The controller  76  controls the charging of a capacitor bank  98 , as described below, and monitors feedback from connectors  61  and  63 , which connect the energy storage module  57  between the electrical load  70  and the battery  37 , as described above. 
     Referring still to  FIGS. 3A and 3B , each of the pairs of connectors to the energy storage module,  63  and  65  and  61  and  59 , include a sensor device and associated reader which can be read by the controller  76  of the energy storage module  57  to sense the presence of a connection to each of the battery  37  and the electrical load  70 , or to identify the connected battery  37  or the lift truck  10 . For example, an RFID tag can be provided on the battery connector  65  and load connector  59 , and associated RFID readers  72  and  74  on the connectors  63  and  61  to the energy storage module  57 , respectively. Similarly, a magnet can be coupled to the connectors  65  and  59 , and the reader devices  72  and  74  can comprise Hall sensing devices connected to each of the connectors  63  and  61 . Proximity sensor, or other types of identifying or sensing components can be similarly provided on the connectors  65  and  59 , and an associated reader or sensor  72  and  74  provided on the mating connectors  63  and  61 , or otherwise connected to the controller  76 . Irrespective of the type of device used, the controller reads signals from the sensing devices  72  and  74  to determine when the battery  37  and electrical load  70  are connected to the energy storage module  57 , as discussed more fully below. When the sensing devices are capable of identifying the battery  37  or vehicle  10 , the controller can store the identity data, and this data can be retrieved, for example, for maintenance analysis, or other reasons. 
     Referring still to  FIGS. 3A and 3B , as described above, the controller  76  is further connected to a bank of ultra or super capacitors  98  connected in series between the positive and negative terminals of the battery  37  through the capacitor switch  102 . As shown here, depending on the voltage and current requirements of the battery  37  and associated electrical load  70 , the bank of capacitors  98  can also optionally include multiple banks of series-connected capacitors connected in parallel. 
     Referring still to  FIGS. 3A and 3B , the bank of ultra or super capacitors  98  are connected between the positive and negative terminals of battery  37  through a charging circuit  103 , here shown generally as a resistor  101  and MOSFET switching device  105  controlled by the pulse width modulation circuit  88  in controller  76 . A discharge circuit  104  including both a discharge resistor  107  and manual discharge switch  109  are connected in parallel with the capacitor bank  98 . When activated, the switch is closed to provide a current path from the capacitor bank  98 , through the associated discharge resistor, to ground, thereby allowing discharge of the capacitor bank  98 , particularly, for example, when maintenance is required and it is necessary to discharge the capacitors. A capacitor switch which, as shown here, can be a semiconductor device such as a MOSFET  102 , is selectively activated to connect the bank of capacitors  98  between the positive and negative terminals of the battery  37 . The control circuit  76  further includes a capacitor voltage sensing circuit  86  and a battery voltage sensing circuit  84 , for sensing the voltage in each of the battery  37  and capacitor bank  98 . A bank of indicator lights, here shown as light emitting diodes  92 ,  94  and  96 , is selectively activated by the controller  76  to provide an indicator when the battery  37  is connected, when capacitor bank  98  is charged, and when the electrical load  70  is connected to the energy storage module  67 , respectively. 
     Referring now to  FIGS. 5A and 5B , a flow chart illustrating the operation of the energy storage module  57  as controlled by controller  76  is shown. Initially, in process step  110 , the controller  76  reads the voltage on the capacitors  98  by referencing the voltage at the capacitor sense circuit  86 . At step  111  the controller  76  determines whether the capacitor bank  98  is charged. If the capacitor bank is charged, at process step  113 , the “capacitor charged” indicator  94  is illuminated. If the capacitor is not charged, the indicator  94  is deactivated at process step  136 . 
     In either case, at step  112 , the controller  76  reads the sensor  72  associated with the plug  65  connected to the battery  37  and in step  114  determines whether the battery  37  is connected to the energy storage module  57 . If not, the battery indicator light  92  is deactivated in step  116 , the capacitor switch  102  is held open to isolate the capacitors  98  from the battery  37  and to preserve charge on the capacitors  98 , and the process returns to step  110 . If a connection to the battery  37  is found, the indicator  96  is turned on in step  118 , and the process moves on to step  120 , where the controller  76  reads sensor  74  to determine whether the load connector  61  is connected to the energy storage module  57 . If the load connector  61  is not connected, the load connector indicator  96  is turned off in step  126 , the capacitor switch  102  is held open, and the controller  76  loops back to step  112 . The processor  76  therefore continues to read the inputs at sensors  72  and  74  until both the battery connector  65  and load connector  61  are connected to the controller  76 . 
     If the load connector  61  is connected, the load connector indicator  96  is activated in step  128 . At this point, the sensors  72  and  74  indicate that the electrical load  70  and the battery  37  are connected, and therefore that the energy storage module  57  is connected to the truck electrical system. The controller  76 , therefore, advances to step  140 , where a voltage difference is calculated as the difference between the battery voltage detected at process step  142  and the capacitor voltage as determined at process step  110 . At step  144 , the voltage difference calculated from step  140  is compared to a minimum predetermined voltage, calculated based on the wattage capacity of the components used in the capacitor circuit. Particularly the voltage difference value is determined to be small enough such that, when the capacitor switch is closed, the peak current flowing between the battery  37  and the capacitor will be at a level that will not stress the components. For example, if the predetermined minimum voltage is half a volt, and the internal resistance of the capacitors is 0.001 ohms, the peak current will be 500 amps when the capacitor switch closes, and will decrease rapidly until the capacitor voltage and battery voltage are equal. 
     If the difference is greater than the minimum voltage value, charging is required and the process moves to step  148 , where the difference is correlated to a pulse width modulation duty cycle as shown in the chart of  FIG. 4 . This duty cycle, in step  150  is applied to the pulse width modulated charge circuit  88  at controller  76 , which controls the switch  105  associated with charge circuit  103 . In step  152 , the capacitor voltage is monitored as the pulse width modulated charging sequence is applied. At step  156 , the capacitor voltage is once again read, as described above with respect to step  110 , and the process loops back to step  148  to continue charging the capacitor bank  98  until the voltage difference is less than the predetermined voltage minimum. When the capacitor  98  is charged, in process step  146 , the controller  76  ends the charging process by opening the switch in charge circuit  103 , and closing the capacitor switch  102 . The capacitors  98  are therefore connected in parallel across the battery  37 , between the positive and negative terminals, and can be used to level the current draw from the load  70 , as described below. 
     In operation, when the capacitors  98  are charged, the energy storage module  57  provides leveling of the draw of the electrical load  70  to enhance energy delivery from the battery  37 . Depending on the application, the energy storage module  57  can further absorb transient energy from the vehicle load  70  during regenerative braking or regenerative lowering, and quickly discharges the battery when high instantaneous current is required, reducing the stresses that would otherwise be imposed on the battery  37 . By reducing the rate of discharge of the battery  37  and smoothing out the discharge profile, the battery  37  is discharged essentially in a steady state, thereby reducing spikes in current that would otherwise heat the battery, and allowing the battery to run cooler. As a result, the length of usable time per battery charge is increased, and the overall life of the battery is increased. 
     Although preferred embodiments have been shown and described, it will be apparent that various modifications can be made to the features described above. For example, although the energy storage module is described herein for use with a lift truck, it will be apparent to those of ordinary skill in the art that the storage module of the present invention can be used in other types of battery powered electrical vehicles. Additionally, although the power supply shown here is a battery  37 , it will be apparent that power supplies that include fuel cells and regenerative power as, for example, by recovering energy from lifting and lowering the forks  31 , can be used in the present invention in addition to a battery alone system. 
     To apprise the public of the scope of this invention, the following claims are made: