Patent Publication Number: US-2013241272-A1

Title: Integrated electronic hydraulic brake system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 2012-0025408, filed on Mar. 13, 2012 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments of the present invention relate to an integrated electronic hydraulic brake system including an actuator having a master cylinder and a pedal simulator, electronic stability control (ESC), and hydraulic power unit (HPU) which are configured as a single unit. 
     2. Description of the Related Art 
     Recently, development of hybrid vehicles, fuel cell vehicles and electric vehicles has been vigorously carried out in order to improve fuel efficiency and reduce exhaust gas. A brake device, i.e., a brake device of a brake system for vehicles, which functions to decelerate or stop a driving vehicle, is essentially installed in such vehicles. 
     In general, brake devices of brake systems for vehicles include a vacuum brake to generate braking force using suction pressure of an engine, and a hydraulic brake to generate braking force using hydraulic pressure. 
     The vacuum brake exhibits large braking force at a small force through a vacuum booster using a difference between suction pressure of a vehicle engine and atmospheric pressure. That is, the vacuum brake generates output greater than force applied to a brake pedal when a driver pushes the brake pedal. 
     In case of such a conventional vacuum brake, suction pressure of the vehicle engine is supplied to the vacuum booster to form a vacuum, and therefore, fuel efficiency is lowered. Further, the engine is driven at all times to form the vacuum even when the vehicle is stopped. 
     Furthermore, a fuel cell vehicle and an electric vehicle have no engine and thus application of the conventional vacuum brake boosting the driver&#39;s pedal force during braking to the fuel cell vehicle and the electric vehicle may be impossible, and a hybrid vehicle implements an idle stoppage function during stopping to improve fuel efficiency and requires introduction of a hydraulic brake. 
     That is, since implementation of a regenerative braking function is required to improve fuel efficiency in all vehicles, the regenerative braking function is easily implemented by employing a hydraulic brake. 
     In case of an electronic hydraulic brake system which is a kind of hydraulic brake, when a driver pushes a pedal, an electronic control unit senses pushing of the pedal and supplies hydraulic pressure to a master cylinder, thereby transmitting hydraulic pressure for braking to a wheel cylinder (not shown) of each wheel to generate braking force. 
     In order to control hydraulic pressure transmitted to wheel cylinders  20 , the electronic hydraulic brake system includes, as shown in  FIG. 1 , an actuator  1  including a master cylinder  1   a , booster  1   b , reservoir  1   c  and pedal simulator  1   d , an electronic stability control (ESC)  2  to independently control braking force to each wheel, and a hydraulic power unit (HPU)  3  including a motor, pump, accumulator and control valve, which are respectively configured as a unit. 
     The above units  1 ,  2  and  3  constituting the electronic hydraulic brake system are separately provided and installed. As a result, it may be necessary to secure a space to install the electronic hydraulic brake system. Due to the limited installation space of a vehicle. In addition, the weight of the electronic hydraulic brake system is increased. For these reasons, an advanced electronic hydraulic brake system which secures safety of a vehicle during braking, improves fuel efficiency, and provides proper pedal feel has been required. 
     Therefore, according to the above requirements, research and development of an electronic hydraulic brake system which has a simple configuration, exhibits normal braking force even when a failure occurs and is easily controlled are underway. 
     SUMMARY 
     Therefore, it is an aspect of the present invention to provide an integrated electronic hydraulic brake system which has a simple configuration to improve braking safety and installation efficiency in a vehicle, thereby providing stable pedal feel during braking, and which supports regenerative braking, thereby improving fuel efficiency. 
     Additional aspects of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned from practice of the invention. 
     In accordance with an aspect of the present invention, an integrated electronic hydraulic brake system includes an integrated hydraulic control device comprising a master cylinder to generate hydraulic pressure based on pedal force of a brake pedal, a reservoir coupled to an upper portion of the master cylinder to store oil, two hydraulic circuits, each of the hydraulic circuits being connected to two wheels of the vehicle, an accumulator to store a predetermined level of pressure, a flow control valve and pressure reducing valve connected to one of the two hydraulic circuits to control pressure transmitted from the accumulator to wheel cylinders installed at the wheels, a first shut off valve and second shut off valve installed between the master cylinder and the two hydraulic circuits to shut off the hydraulic pressure generated based on the pedal force from the driver, a balance valve to connect the two hydraulic circuits, a pedal simulator connected to the master cylinder to provide reaction force of the brake pedal, and a simulation valve to control connection between the master cylinder and the pedal simulator, and a power source unit comprising a pump to suction oil from the reservoir through a hydraulic pipe and discharge the suctioned oil to the accumulator to generate pressure in the accumulator and a motor to drive the pump, wherein the power source unit is configured as a separate unit to isolate noise generated from the power source unit, and the integrated hydraulic control device and the power source unit are connected to each other via an external pipe, wherein a check valve is further provided in a channel connecting the master cylinder to the pedal simulator such that the pressure according to the pedal force of the brake pedal is transmitted to the pedal simulator only through the simulation valve. 
     The flow control valve and the pressure reducing valve may be single high capacity valves, wherein the flow control valve and the pressure reducing valve are Normally Closed type solenoid valves which remain closed in normal times. 
     The balance valve may be a Normally Closed type solenoid valve which is closed in normal times and opened based on pressure information. 
     The accumulator and the pump may be connected to each other via the external pipe, and the external pipe may have a check valve installed therein to prevent backward flow of pressure of the accumulator. 
     Each of the hydraulic circuits may include a Normally Open type solenoid valve disposed upstream of the wheel cylinders to control transmission of the hydraulic pressure to the wheel cylinders, a Normally Closed type solenoid valve disposed downstream of the wheel cylinders to control discharge of the hydraulic pressure from the wheel cylinders, and a return channel to connect the Normally Closed solenoid valve to the hydraulic pipe. 
     The first shut off valve and the second shut off valve may be Normally Open type solenoid valves which remain open in normal times and are closed during normal operation of braking. 
     The simulation check valve may be a pipe check valve having no spring to return the remaining pressure of the pedal simulator when the pedal force of the brake pedal is released. 
     A pulsation attenuator may be provided in a channel connecting the flow control valve and pressure reducing valve to the one of the hydraulic circuits to minimize pressure pulsation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a view schematically illustrating a configuration of a conventional electronic hydraulic brake system; 
         FIG. 2  is a hydraulic circuit diagram showing a state in which an integrated electronic hydraulic brake system according to an embodiment of the present invention is not operated; 
         FIG. 3  is a hydraulic circuit diagram showing a state in which the integrated electronic hydraulic brake system according to the embodiment of the present invention is normally operated; and 
         FIG. 4  is a hydraulic circuit diagram showing a state in which the integrated electronic hydraulic brake system according to the embodiment of the present invention is abnormally operated. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The terms used in the following description are defined taking into consideration the functions obtained in accordance with the embodiments, and the definitions of these terms should be determined based on the overall content of this specification. Therefore, the configurations disclosed in the embodiments and the drawings of the present invention are only exemplary and do not encompass the full technical spirit of the invention, and thus it will be appreciated that the embodiments may be variously modified and changed. 
       FIG. 2  is a hydraulic circuit diagram showing a state in which an integrated electronic hydraulic brake system according to an embodiment of the present invention is not operated. 
     The integrated electronic hydraulic brake system may include two units. Referring to  FIG. 2 , the integrated electronic hydraulic brake system may include an integrated hydraulic control device  100  including a brake pedal  30  manipulated by a driver during braking, a master cylinder  110 , force from the brake pedal  30  being transmitted thereto, a reservoir  115  coupled to the upper portion of the master cylinder  110  to store oil, two hydraulic circuits HC 1  and HC 2  connected respectively to two wheels RR, RL, FR and FL, a first and second shutoff valve  173  and  174  installed between the two hydraulic circuits HC 1  and HC 2  to shut off hydraulic pressure according to a driver&#39;s pedal force, an accumulator  120  to store a predetermined level of pressure, a pedal simulator  180  connected to the master cylinder  110  to provide reaction force of the brake pedal  30 , and a simulation valve  186  installed in a channel  188  connecting the pedal simulator  180  with the reservoir  115 , and a power source unit  200  including a pump  210  to suction oil from the reservoir  115  through a hydraulic pipe  116  and discharge the suctioned oil to the accumulator  120  to generate pressure in the accumulator  120  and a motor  220  to drive the pump  210 . 
     Also, the integrated hydraulic control device  100  may further include a flow control valve  141  and pressure reducing valve  142  connected to one of the two hydraulic circuits HC 1  and HC 2  to control pressure transmitted from the accumulator  120  to wheel cylinders  20  installed at wheels FL, FR, RL and RR of a vehicle, a balance valve  190  to connect the two hydraulic circuits HC 1  and HC 2  to each other, and pressure sensors  101 ,  102  and  103  to measure pressure. 
     The integrated hydraulic control device  100  and the power source unit  200  are connected to each other via an external pipe  10 . That is, the pump  210  of the power source unit  200  and the accumulator  120  of the integrated hydraulic control device  100  are connected to each other via the external pipe  10 . The power source unit  200  including the pump  210  and the motor  220  is separated from the integrated hydraulic control device  100  to isolate operational noise. Also, the master cylinder  110 , the reservoir  115 , and the pedal simulator  180  are incorporated into the integrated hydraulic control device  100  as a single unit, and functions of an ESC and HPU are included in the integrated hydraulic control device  100 , to reduce the weight of the integrated electronic hydraulic brake system and improve installation space of the integrated electronic hydraulic brake system. 
     Hereinafter, structures and functions of the components constituting the integrated electronic hydraulic brake system will be described in more detail. First, the master cylinder  110 , which may have at least one chamber to generate hydraulic pressure, has two chambers respectively having a first piston  111  and a second piston  112  therein as shown in  FIG. 2 . Thereby, the master cylinder  110  generates hydraulic pressure according to pedal force of the brake pedal  30  and is connected to the two hydraulic circuits HC 1  and HC 2 . The reservoir  115  containing oil is installed at the upper side of the master cylinder  110 , and the master cylinder  110  is provided with an outlet at the lower portion thereof to allow oil discharged from the outlet to be transmitted to the wheel cylinders  20  installed at the wheels RR, RL, FR and FL via first and second backup channels  171  and  172 . 
     The two chambers of the master cylinder  110  are connected to the two hydraulic circuits HC 1  and HC 2  to secure safety when a failure occurs. As shown in  FIG. 2 , the first hydraulic circuit HC 1  is connected to the right front wheel FR and left rear wheel RL, and the second hydraulic circuit HC 2  is connected to the left front wheel FL and right rear wheel RR. Alternatively, the first hydraulic circuit HC 1  may be connected to the two front wheels FL and FR, and the second hydraulic circuit HC 2  may be connected to the two rear wheels RL and RR. The two hydraulic circuits HC 1  and HC 2  are independently provided as above in order that braking of the vehicle is performed even if one of the circuits malfunctions. 
     Each of the hydraulic circuits HC 1  and HC 2  includes a channel connected to the wheel cylinders  20 , a plurality of valves  151  and  161  is installed in the channel. In  FIG. 2 , the valves  151  and  161  are divided into a Normally Open type (hereinafter, “NO type”) solenoid valve  151  disposed upstream of the wheel cylinders  20  to control transmission of hydraulic pressure to the wheel cylinders  20 , and a Normally Closed type (hereinafter, “NC type”) solenoid valve  161  disposed downstream of the wheel cylinders  20  to control the hydraulic pressure leaving the wheel cylinders  20 . Opening and closing the solenoid valves  151  and  161  may be controlled by an electronic control unit (not shown) that is commonly used. 
     Also, each of the hydraulic circuits HC 1  and HC 2  includes a return channel  160  to connect the NC type solenoid valve  161  with the hydraulic pipe  116 . The return channel  160  allows the hydraulic pressure transmitted to the wheel cylinder  20  to be discharged and transmitted to the reservoir  115  or to the accumulator  120  by pumping from the pump  210 . 
     A balance valve  190  is installed between the two hydraulic circuits HC 1  and HC 2  to control connection between the hydraulic circuits HC 1  and HC 2 . The balance valve  190  is an NC type solenoid valve which is closed in normal times and opened based on pressure information. The balance valve  190  connects the hydraulic circuits HC 1  and HC 2  to each other to supply hydraulic pressure to the wheel cylinders  20  provided in the hydraulic circuits HC 1  and HC 2 , and operation thereof will be described later. 
     Meanwhile, unexplained reference numeral  31  indicates an input rod installed at the brake pedal  30  to transmit pedal force to the master cylinder  110 . 
     At least one pump  210  is provided to pump the oil introduced from the reservoir  115  with high pressure to generate braking pressure. At one side of the pump  210  is provided the motor  220  to provide driving force to the pump  210 . The motor  220  may be driven, receiving the driver&#39;s intension to brake the vehicle according to the pedal force of the brake pedal  30  from the first pressure sensor  101 , which will be described later, or pedal displacement sensor. 
     The accumulator  120  is provided at the outlet of the pump  210  to temporarily store high-pressure oil generated by driving the pump  210 . That is, as previously described, the accumulator  120  is connected to the pump  210  via the external pipe  10 . In the external pipe  10  is installed a check valve  135  to prevent backward flow of the high-pressure oil stored in the accumulator  120 . 
     At the outlet of the accumulator  120  is provided the second pressure sensor  102  to measure oil pressure of the accumulator  120 . The oil pressure measured by the second pressure sensor  102  is compared with a pressure set by the electronic control unit (not shown). If the measured oil pressure is low, the pump  210  is driven to suction oil from the reservoir  115  and to supply the suctioned oil to the accumulator  120 . 
     To supply braking oil stored in the accumulator  120  to the wheel cylinders  20  through the pump  210  and the motor  220 , a connection channel  130  connected to the external pipe  10  is provided. The connection channel  130  is connected to one of the two hydraulic circuits HC 1  and HC 2 . In  FIG. 2 , the connection channel  130  is connected to the first hydraulic circuit HC 1 . In the connection channel  130  are provided the flow control valve  141  and the pressure reducing valve  142  to control braking oil stored in the accumulator  120 . 
     The control valve  141  and the pressure reducing valve  142  are NC type solenoid valves which remain closed in normal times. Thus, when the driver applies force to the brake pedal  30 , the flow control valve  141  opens and transfers the braking oil stored in the accumulator  120  to the wheel cylinders  20 . The braking oil transferred via the flow control valve  141  is transferred to the first hydraulic circuit HC 1  connected to the connection channel  130 , and at the same time the balance valve  190  connecting the two hydraulic circuits HC 1  and HC 2  opens to allow the braking oil to be transferred to the second hydraulic circuit HC 2  as well. That is, the braking oil in the accumulator  120  is transferred to the wheel cylinders  20  as the flow control valve  141  and the balance valve  190  open. 
     Since the flow control valve  141  and the pressure reducing valve  142  are provided as single valves to produce hydraulic pressure for braking, they may be high capacity valves. Also, the flow control valve  141  and the pressure reducing valve  142  shown in  FIG. 2  are single valves, but embodiments of the present invention are not limited thereto. If more capacity is needed, they may be configured with a combination of two or more valves. 
     In addition, the integrated hydraulic control device  100  may further include a pulsation attenuator  145  provided in the connection channel  130  to minimize pressure pulsation. The pulsation attenuator  145  is a device that temporarily stores oil to attenuate pulsation generated between the flow control valve  141  and pressure reducing valve  142  and the NO type solenoid valve  151 . The pulsation attenuator  145  is well known in the art to which the present invention pertains, and therefore a detailed description thereof will be omitted. 
     In addition, the connection channel may further include a third pressure sensor  103  to sense pressure transmitted to the hydraulic circuit HC 1 . Thereby, the third pressure sensor  103  controls the pulsation attenuator  145  so that pulsation is attenuated according to the sensed pressure of the braking oil. 
     According to the embodiment of the present invention, a first backup channel  171  and a second backup channel  172  may be provided to connect the master cylinder  110  with the hydraulic circuits HC 1  and HC 2  when the integrated electronic hydraulic brake system fails. In the first backup channel  171  is provided a first shut off valve  173  to shut off pressure of the master cylinder  110  according to the driver&#39;s pedal force. In the second backup channel  172  is provided a second shut off valve  174 . The first and second shut off valves  173  and  174  are NC type solenoid valves which are open in normal times and closed in normal operation of braking. In addition, the first backup channel  171  is connected to the first hydraulic circuit HC 1  and connection channel  130  via the first shut off valve  173 . The second backup channel  172  is connected to the second hydraulic circuit HC 2  via the second shut off valve  174 . Particularly, the first backup channel  171  may be provided with the first pressure sensor  101  to measure oil pressure of the master cylinder  110 . When braking is normally performed, therefore, the backup channels  171  and  172  are shut off by the first shut off valve  173  and the second shut off valve  174 , and the driver&#39;s intention of braking is determined by the first pressure sensor  101 . If braking is abnormal, the braking pressure generated by the master cylinder  110  is allowed to be directly transmitted to the wheel cylinders  20  by the opened first and second shut off valves  173  and  174 . 
     According to the embodiment of the present invention, the pedal simulator  180  to generate pedal force of the brake pedal  30  is provided between the first pressure sensor  101  and the master cylinder  110 . 
     The pedal simulator  180  includes a simulation chamber  182  to store oil discharged from the outlet of the master cylinder  110  and a simulation valve  186  provided at the inlet of the simulation chamber  182 . The simulation chamber  182 , which includes a piston  183  and an elastic member  184 , is adapted to have a predetermined range of displacement based on oil introduced into the simulation chamber  182 . The simulation valve  186  is an NC type solenoid valve which remains closed in normal times. When the driver pushes the brake pedal  30 , the simulation valve  186  is opened to supply the braking oil to the simulation chamber  182 . 
     Also, a simulation check valve  185  is provided between the pedal simulator  180  and the master cylinder  110 , i.e. between the pedal simulator  180  and the simulation valve  186 . The simulation check valve  185  is connected to the master cylinder  110 . The simulation check valve  185  is adapted to transmit pressure generated by pedal force of the brake pedal  30  to the pedal simulator  180  only through the simulation valve  186 . The simulation check valve  185  may be a pipe check valve having no spring to return the remaining pressure of the pedal simulator  180  when the pedal force of the brake pedal  30  is released. 
     The integrated hydraulic control device  100  is provided as a block including an electronic control unit (ECU) (not shown) electrically connected to the valves and the sensors to control the valves and the sensors, and therefore the integrated electronic hydraulic brake system is allowed to have a compact structure. That is, the integrated electronic hydraulic brake system according to the embodiment of the present invention is divided into the power source unit  200  including the motor  220  and the pump  210  and the integrated hydraulic control device  100  including the accumulator  120 , the valves, the sensors, and the pedal simulator  180  to generate pedal force of the brake pedal  30  configured as a single block. Consequently, installation space of the integrated electronic hydraulic brake system is easily secured, and the weight of the integrated electronic hydraulic brake system is reduced. 
     Hereinafter, operation of the integrated electronic hydraulic brake system according to the embodiment of the present invention will be described in detail. 
       FIG. 3  is a hydraulic circuit diagram showing a state in which the integrated electronic hydraulic brake system is normally operated. 
     Referring to  FIG. 3 , when braking is commenced by a driver, the amount of braking required by the driver is sensed based on information on pressure to the brake pedal  30  pushed by the driver, measured by the first pressure sensor  101  or pedal displacement sensor. The ECU (not shown) may receive the amount of regenerative braking, calculate the amount of frictional braking based on the difference between the amount of braking required by the driver and the amount of regenerative braking, and thereby detect the magnitude of increase or decrease of pressure at the wheels. 
     Specifically, when the driver pushes the brake pedal  30  at the initial stage of braking, the vehicle is sufficiently braked by the regenerative braking, and therefore control may be performed not to generate the amount of frictional braking. Consequently, it may be necessary to reduce the pressure of braking oil so that hydraulic pressure applied from the brake pedal  30  to the master cylinder  10  is not transmitted to the wheel cylinders  20 . At this time, the pressure reducing valve  142  is opened to discharge hydraulic pressure generated in the connection channel  130  to the reservoir  115  such that no pressure is generated at the wheels RR, RL, FR, and FL, and the pressure of the brake pedal is maintained. 
     Thereafter, an operation of adjusting the amount of frictional braking based on the change in the amount of regenerative braking may be performed. The amount of regenerative braking, which varies depending on the battery charge level or velocity of the vehicle, is drastically reduced when the velocity of the vehicle is a predetermined value or less. To control hydraulic pressure of the wheel cylinders  20  to cope with such situation, the flow control valve  141  may control the flow rate of the braking oil supplied from the accumulator  120  to the connection channel  130 . 
     Afterward, there is no amount of regenerative braking, and thus braking may be performed based on a normal braking condition. 
     Meanwhile, as the connection channel  130  is connected with the first hydraulic circuit HC 1 , pressure is transmitted to the two hydraulic circuits HC 1  and HC 2  in braking by opening the NC type balance valve  190  adapted to control the connection between hydraulic circuits HC 1  and HC 2 . 
     Also, the pressure generated by the master cylinder  110  according to the pedal force of the brake pedal  30  is transmitted to the pedal simulator  180  connected to the master cylinder  110 . At this time, the simulation valve  186  disposed between the master cylinder  110  and the simulation chamber  182  is opened to supply hydraulic pressure to the simulation chamber  182 , and thereby the piston  183  moves and pressure corresponding to the weight of the spring  184  supporting the piston  183  is generated in the simulation chamber  182 , providing proper pedal feel to the driver. 
       FIG. 4  is a hydraulic circuit diagram showing a state in which the integrated electronic hydraulic brake system is abnormally operated. 
     Referring to  FIG. 4 , for backup braking in a case in which the integrated electronic hydraulic brake system is not normally operated, braking oil is supplied to the wheel cylinders  20  via the first and second backup channels  171  and  172  to generate braking force. Here, since the first and second shut off valves  173  and  174  installed in the two backup channels  171  and  172  and the solenoid valves  151  of the hydraulic circuits HC 1  and HC 2  are NO type solenoid valves in an open state, and the flow control valve  141 , pressure reducing valve  142  and balance valve  190  are NC type solenoid valves in a closed state, hydraulic pressure is directly transmitted to the wheel cylinders  20 . Thereby, stable braking may be performed and braking stability may be enhanced. 
     The master cylinder  110  may have a smaller inner diameter than a conventional master cylinder to maximize the performance of mechanical braking based on pedal force of the brake pedal  30 . That is, the master cylinder may provide sufficient braking force through braking oil stored in the master cylinder although the master cylinder  110  has a smaller inner diameter than the conventional master cylinder. 
     As is apparent from the above description, the integrated electronic hydraulic brake system according to the embodiment of the present invention has effects as follows. 
     First, the integrated electronic hydraulic brake system is divided into the power source unit including the pump and the motor and the integrated hydraulic control device including the accumulator, the valves, the sensors and the pedal simulator to generate pedal force of the brake pedal configured as a single block. Consequently, installation space of the integrated electronic hydraulic brake system may be easily secured, and the weight of the integrated electronic hydraulic brake system may be reduced. Also, the integrated electronic hydraulic brake system may be easily assembled. 
     Second, as pressure is supplied to and released from the two hydraulic circuits via the single flow control valve and pressure reducing valve with the two hydraulic circuits connected by the balance valve, easy and good control is secured. 
     Third, braking of a vehicle is achieved when the brake system malfunctions, and therefore the integrated electronic hydraulic brake system is easily applied to electric vehicles, fuel cell vehicles and hybrid vehicles. 
     Fourth, the remaining pressure is minimized by the simulation check valve having no spring, and pedal feel delivered to a driver may be stably maintained even when pressure is arbitrarily adjusted during braking. 
     Fifth, the integrated electronic hydraulic brake system generates braking force required by a user regardless of whether an engine is present and whether the engine is operated, thereby contributing to improvement of fuel efficiency. 
     Sixth, the integrated electronic hydraulic brake system has a simpler configuration than a conventional negative pressure type booster, and does not use suction pressure of an engine unlike a vacuum brake, thereby improving fuel efficiency of a vehicle. Furthermore, due to its simple configuration, the integrated electronic hydraulic brake system may be easily applied to a small vehicle. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.