Patent Publication Number: US-6704643-B1

Title: Adaptive calibration strategy for a manually controlled throttle system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to a manually controlled throttle system and, more particularly, to a calibration strategy that minimizes potential errors that could otherwise result from the buildup of manufacturing and assembly tolerances within the throttle control system. 
     2. Description of the Prior Art 
     Manually controlled throttle systems, used in conjunction with marine vessels, are well known to those skilled in the art. In many different types of pleasure craft, the operator of the marine vessel is provided with a manually movable hand lever or levers which can be used by the operator to select both engine speed and gear choice. With regard to engine speed, the operator is typically provided with a choice from idle speed to wide open throttle (WOT). With regard to gear selection, the operator is typically provided with choices of forward, neutral, or reverse gear positions. In drive-by-wire systems, the position of the manually operated handle, or lever, is sensed by an appropriate sensor, such as a potentiometer, and a signal is provided to a microprocessor. That signal is representative of the position of the manually movable throttle handle. The microprocessor then interprets the desired engine speed from the received signal and controls the actual throttle and/or fuel injectors of the engine to obtain the desired speed as requested by the operator of the marine vessel. 
     U.S. Pat. No. 6,414,607, which issued to Gonring et al on Jul. 2, 2002, discloses a throttle position sensor with improved redundancy and high resolution. A throttle position sensor is provided with a plurality of sensing elements which allow the throttle position sensor to provide a high resolution output to measure the physical position of a manually movable member, such as a throttle handle, more accurately than would otherwise be possible. The plurality of sensors significantly increases the redundancy of the sensor and allows its operation even if one of the sensing elements is disabled. 
     U.S. Pat. No. 6,273,771, which issued to Buckley et al on Aug. 14, 2001, discloses a control system for a marine vessel. The control system incorporates a marine propulsion system that can be attached to a marine vessel and connected in signal communication with a serial communication bus and a controller. A plurality of input devices and output devices are also connected in signal communication with the bus and a bus access manager, such as a CAN Kingdom network, is connected in signal communication with the controller to regulate the incorporation of additional devices to the plurality of devices in signal communication with the bus, whereby the controller is connected in signal communication with each of the plurality of devices on the communication bus. The input and output devices can each transmit messages to the serial communication bus for receipt by other devices. 
     U.S. Pat. No. 5,664,542, which issued to Kanazawa et al on Sep. 9, 1997, describes an electronic throttle system. On one side of a valve shaft, there are provided an accelerator drum connected to an accelerator pedal by an accelerator wire, a return spring for urging the accelerator drum in a valve closing direction, and an accelerator sensor for detecting rotation of the accelerator drum and transmitting a detected signal to a host system. On the other side of the valve shaft, there are provided a large-diameter gear and an opening sensor. An armature of a solenoid clutch is attached to the gear and held on a motor shaft via a slide bearing. Thus, the motor, the solenoid clutch, and the throttle valve are arranged in a U-shape form for interconnection through four gears. 
     U.S. Pat. No. 6,095,488, which issued to Semeyn, Jr. et al on Aug. 1, 2000, describes an electronic throttle control with adjustable default mechanism. The system has a housing with a motor, throttle valve, gear mechanism, and fail-safe mechanism. A spring member attached to a gear member and default lever, and which is biased when the throttle valve is in its fully open and closed positions, operates to open the throttle valve in the event of an electric failure, thus allowing the vehicle to limp home. An adjustable pin member is used to adjust the position of the default lever and thus the throttle valve in a fail-safe situation. 
     U.S. Pat. No. 5,381,769, which issued to Nishigaki et al on Jan. 17, 1995, describes a throttle valve drive apparatus. It comprises an actuator which serves to mechanically drive a throttle valve disposed in an intake passage of an internal combustion engine and is controlled in accordance with an instruction from a control unit, an accelerator lever which serves to mechanically drive the throttle valve and to adjust the opening degree of the throttle valve in accordance with an amount of operation performed by an operator. A first clutch disposed between rotary shafts of the actuator and the throttle valve and serving to transmit a turning force from the actuator to the throttle valve, and a second clutch disposed between rotary shafts of the accelerator lever and the throttle valve and serving to transmit the turning force from the accelerator lever to the throttle valve. An engaging force of the first clutch is discriminated from that of the second clutch, one of the first and second clutches having a greater engaging force comprising an on-off constant engagement type clutch, the other clutch having a smaller engaging force comprising a constant engagement type clutch, and the on-off type clutch is switched on and off so as to transmit the turning force to the throttle valve selectively from the actuator or the accelerator lever. 
     The patents described above are hereby expressly incorporated by reference in the description of the present invention. 
     Unlike known throttle control systems for marine vessels, in which push-pull cables connect a manually movable lever to the actual throttle control linkage system of the marine propulsion device, drive-by-wire throttle control systems provide electrical signals between a manually controllable throttle lever mechanism and an engine control unit of the marine propulsion device. A sensor is provided to detect the physical position of the manually movable handle and the sensor provides electrical signals, on the signal wires, to the engine control unit (ECU) associated with the one or more engines of a marine propulsion system. This type of system requires that the sensor be sufficiently accurate to measure and provide appropriate signals representing the physical position of the manual movable handle. Because of the potential buildup of tolerances during the manufacture of the manually controllable lever and associated equipment, the signal provided by the position sensor may not be completely reliable with regard to the precise position of the handle. 
     It would therefore be significantly beneficial if a system could be provided that accurately and efficiently allows the calibration of a drive-by-wire throttle control system. 
     SUMMARY OF THE INVENTION 
     A method for operating a throttle control system, in accordance with the preferred embodiment of the present invention, comprises the steps of providing a manually operated throttle controller, providing a sensor connected to the manually operated throttle controller and having an output which is representative of the position of the manually operated throttle controller, and providing a microprocessor connected in signal communication with the sensor and having an input connected in signal communication with the output. The present invention further comprises the steps of receiving a first position indicating signal from the sensor which is representative of a first known position of the manually operated throttle controller and receiving a second position indicating signal from the sensor which is representative of a second known position of the manually operated throttle controller. 
     In its most basic application, the present invention reads the first and second position indicating signals and stores those indicating signals for later use during the operation of a marine vessel. After the calibration is complete, the method for operating the throttle control system of the present invention further comprises the steps of receiving a subsequent position indicating signal from the sensor which is representative of a subsequent position of the manually operated throttle controller and then calculating the subsequent position of the manually operated throttle controller as a function of the subsequent position indicating signal and the first and second position indicating signals. 
     As will be described in greater detail below, each of the position indicating signals received by the microprocessor actually comprise three distinct magnitudes of three signals. The three signals are intended to be generally redundant to each other and are provided for purposes of accuracy and redundancy in the event that one or more of the three signals are unavailable to the system. 
     In a particularly preferred embodiment of the present invention, the method comprises the steps of receiving first, second, third, fourth, and fifth position indicating signals from the sensor which are representative, respectively, of first, second, third, fourth, and fifth known positions of the manually operated throttle controller, or lever handle. The receipt and storage of five position indicating signals allows the present invention to determine whether the throttle handle is in a neutral position range, whether the throttle handle is in a forward gear selection position or reverse gear selection position, and also allows the present invention to determine the percentage of wide open throttle engine speed that is being currently requested by the operator of the marine vessel. In the particularly preferred embodiment of the present invention, the first and fifth known positions correspond to reverse and forward maximum throttle positions. The second and fourth known positions correspond to the reverse and forward shift detent positions that signify the transition location between the neutral gear position and both reverse and forward gear positions. The fifth known position is a detent location that is generally in the center portion of the neutral gear selection range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully and completely understood from a reading of the description of the preferred embodiment in conjunction with the drawings, in which: 
     FIG. 1 is a simplified representation of some of the components used in association with the present invention; 
     FIG. 2 is a graphical representation of the various signals provided by a throttle control device; 
     FIG. 3 is a graphical representation of various signal magnitudes used by the present invention; 
     FIG. 4 is similar to FIG. 3, but with certain signal ranges accentuated for purposes of the exemplary discussion; 
     FIG. 5 is generally similar to FIG. 4, but with additional information provided to facilitate a description of the mathematical calculation performed by the present invention; and 
     FIG. 6 is a simplified flow chart of the method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the description of the preferred embodiment of the present invention, like components will be identified by like reference numerals. 
     The present invention will be described in terms of a throttle control system that incorporates a throttle position sensor generally similar to the sensor disclosed in U.S. Pat. No. 6,414,607 which is described above. That particular throttle position sensor provides three signals, each of which represents a position of a throttle handle. The three signals are intended to be generally redundant with respect to each other, but are used cooperatively to determine the position of the throttle handle with respect to forward, neutral, or reverse gear selection positions and, in addition, with respect to the relative position of the handle with regard to a percentage of wide open throttle engine speed that is being requested by the operator. 
     It should be understood that the method of the present invention does not require the use of the throttle position sensor disclosed in U.S. Pat. No. 6,414,607. Alternatively, it can be used in conjunction with a throttle position sensor that only provides a single signal. However, for purposes of the description of the preferred embodiment of the present invention below, a three-potentiometer system such as that described in U.S. Pat. No. 6,414,607, will be used. 
     FIG. 1 is a highly schematic representation of a throttle control system of a marine vessel incorporating a manually operated throttle control device  10  which incorporates a manually movable handle  12 , or lever. The manually movable handle  12  is rotatable about a pivot  14  to allow the operator of a marine vessel to select an engine speed and a gear position. In FIG. 1, five dashed lines are shown to represent five possible positions of the throttle handle  12 . For example, dashed line  21  can represent a position of the throttle handle  12  that represents a command, by the operator of the marine vessel, for a wide open throttle (WOT) engine speed in combination with a forward gear selection. Position  22  represents a forward gear selection at approximately idle engine speed which can be, for example, about 20% of WOT. Position  23  is at a location which is near a midpoint of a neutral gear selection range. Position  24  is the location at which the throttle control system switches from neutral gear position to reverse gear position at approximately idle engine speed. Position  25  is a demand for wide open throttle engine speed in reverse gear. The range between positions  22  and  24  represent a neutral gear range. The range between positions  21  and  22  represent the forward gear selection range and the range between positions  24  and  25  represent a reverse gear selection range. 
     Typically, a potentiometer or similar device is provided in the throttle selection apparatus  10  to provide a signal, on line  28 , to microprocessor controlled device, such as the monitor  50 . The microprocessor of the helm control unit  30 , as will be described in greater detail below, determines the actual engine speed demand, as represented by the throttle handle  12 , and provides appropriate signals, on line  32 , to the throttle mechanism  36  which can include a control motor for manipulating the position of a throttle plate and/or a control unit for determining the appropriate operation of a plurality of fuel injectors to achieve the engine speed demanded by the position of the throttle handle  12 . The result of the action of the throttle mechanism  36 , under control of the helm control unit  30 , is that the operating speed of the engine  40  is maintained at the magnitude requested by the operator of the marine vessel. 
     With continued reference to FIG. 1, the display monitor  50  can be provided with a screen  52  on which information can be displayed to the operator of, the marine vessel or, during calibration procedures, to a person (e.g. a boat builder) who is installing the marine propulsion system in the marine vessel. In other words, a boat builder would likely be the initial person who utilizes the calibration procedures of the present invention. 
     Requests are provided by the helm control unit  30  and transmitted to the monitor  50 , on line  56 , with instructions to be followed by the person calibrating the system. At least one switch, such as a push button, is provided to allow the person calibrating the system to communicate with the helm control unit  30 , on line  58 . Lines  56  and  58  in FIG. 1 functionally represent the directions of information exchange. It should be understood, however, that the exchange of information is physically made on a CAN bus in a preferred embodiment of the present invention. A system which utilizes a CAN bus in conjunction with a marine vessel is disclosed in U.S. Pat. No. 6,273,771 which is described above. The procedures used during the calibration process of the present invention will be described in greater detail below, but it essentially comprises a step-by-step process during which the helm control unit  30  communicates requests to the person calibrating the system, that person performs the requested action, and then a push button is actuated to inform the helm control unit  30  that the action has been completed. A series of actions are performed by the person doing the calibrating of the system and a series of signals are received by the helm control unit  30  in order to properly calibrate the positions of the throttle handle  12  with respect to the signals provided by the sensor on line  28 . This allows the helm control unit  30  to determine the accurate meaning of the signals received on line  28  when the operator of the marine vessel is using the vessel. In a preferred embodiment of the present invention, the monitor  50  is provided with a microprocessor to control the displays and to perform certain mathematical computations. It should also be understood that most of the individual components and devices are connected directly to the CAN bus and this arrangement eliminates the need for them to be hard wired to each other although they communicate information to each other. 
     It should be understood that one potential problem of a throttle control system  10 , such as that described above, is that the position of the sensor within the throttle control system  10  may not be precisely associated with the physical positions in which the throttle control handle can be placed. In other words, when the throttle handle  12  is moved to its maximum wide open throttle position, in forward gear, the sensor position may be such that it does not provide a maximum possible signal on line  28  to the helm control unit  30 . Similarly, when the throttle handle  12  is moved to its maximum wide open throttle position in reverse gear, the magnitude of the signal provided by the sensor may not be in its maximum position. Mechanical tolerances associated with the throttle control system  10  may cause the signal magnitudes associated with positions  21 - 25  to be other than would normally be expected if all of the components of the throttle control mechanism  10  were perfectly and precisely assembled. 
     FIG. 2 is a graphical representation which shows the variable signals provided by three potentiometers associated with a throttle position sensor as a function of actual handle position. For purposes of reference, the graphical representation in FIG. 2 is generally of the type that would result from the use of a throttle position sensor similar to that disclosed in U.S. Pat. No. 6,414,607, but without a pronounced dead band in the central region of the first signal  61  as discussed in that patent. In FIG. 2, the first signal  61  is a generally V-shaped signal that represents the output magnitudes from a sensor of the throttle control system  10  which represent positions of the throttle handle  12  from a full wide open throttle position in reverse gear, at −80°, to a full wide open throttle position in forward gear at +80°. A second signal  62  and a third signal  63  are provided by two other potentiometers associated with the sensor in a throttle control system  10  similar to that described in U.S. Pat. No. 6,414,607. Because of normal assembly tolerances, the minimum signal magnitude of the first signal  61  may not coincide precisely with the zero degrees rotation position of the throttle handle  12 , as represented by position  23  in FIG.  1 . Similarly, the second and third signals,  62  and  63 , may not cross at precisely the center position  23 . These misalignments are exaggerated in the Figures for purposes of illustration. 
     With reference to FIG. 3, the calibration process of the present invention first requests that the person doing the calibrating procedure place the throttle handle  12  in position  25  and, more specifically, in a position at which a mechanical detent is provided so that the person calibrating the system can select position  25  with greater assurance. When the calibrator responds, by actuating a push button, that the throttle handle  12  is in position  25 , the microprocessor of the helm control unit  30  receives a first position indicating signal. Since three potentiometers are used by the sensor of the throttle control device  10 , three signal magnitudes are received by the helm control unit  30 . These three values are identified by reference numerals  71 A,  71 B, and  71 C, for the first, second, and third signals from the three potentiometers described above. The person calibrating the system is then requested to move the throttle position handle  12  to position  24 . When this action is acknowledged by push buttons, the microprocessor of the helm control unit  30  receives signals  72 A,  72 B, and  72 C. This procedure is repeated for positions  23 ,  22 , and  21 . When completed, the microprocessor of the helm control unit  30  has received and stored  15  signal values, as represented in FIG. 3, with three signal values being associated with each of the five positions,  21 - 25 . Signal magnitudes  71 A- 75 A being associated with the first signal  61 , signal magnitudes  71  B- 75 B being associated with the second signal  62 , and signal magnitudes  71 C- 75 C being associated with the third signal  63 . 
     FIG. 4 is generally similar to FIG. 3, but with certain portions of the first, second, and third signals,  61 - 63 , darkened to represent certain ranges of the signal magnitudes that will be discussed. Also, dashed vertical lines are provided in FIG. 4 to connect associated magnitudes of the three signals at each of the five positions. For purposes of the description of the preferred embodiment of the present invention, the range of travel of the throttle handle  12  will be divided into three ranges. A central range  80  represents the range of travel of the throttle handle  12  between positions  22  and  24 . This range corresponds to a neutral gear position selection between the associated detent locations at positions  22  and  24 . The reverse gear range  82  includes all of the engine speed selection positions between idle speed, at position  24 , and wide open throttle (WOT) in reverse gear at position  25 . Range  84  represents the range of forward engine speeds between positions  22  and  21 . At position  22 , the gear selection transitions from neutral gear to forward gear and the engine speed increases as the throttle handle  12  is moved from position  22  to position  21 . 
     Also shown in FIG. 4 is an additional vertical axis that provides the digital value of the signal provided by the sensor of the throttle control mechanism  10  to the helm control unit  30 . These digital signals can be provided by an 10-bit analog-to-digital converter associated with the zero to 5 volt signal represented on the left vertical axis in FIG.  4 . 
     FIG. 5 is generally similar to FIG. 4, but with only selected portions of the first, second, and third signals being shown. The portions of these signals that are located in the neutral range  80  have been removed in order to simplify the illustration for the purpose of describing how the information described above in conjunction with FIG. 4 is used to determine a precise position of the throttle handle  12  when the operator of the marine vessel selects a subsequent throttle position in forward gear after the calibration process has been completed. 
     The following description of the use of the present invention will relate to its use after the calibration procedure has been completed. More particularly, the sensor magnitudes identified by reference numerals  74 A- 74 C and  75 A- 75 C are known by the microprocessor of the helm control unit  30  and have been stored during the most recent calibration procedure. It should also be understood that the magnitudes identified by reference numerals  71 A- 71 C and  72 A- 72 C are also known by the helm control unit  30 , but do not directly relate to the example that will be discussed below. 
     With reference to FIGS. 1 and 5, the microprocessor of the helm control unit  30  continuously monitors the signals received on line  28  from the sensor of the throttle control device  10  which represent the current position of the throttle handle  12 . The signals received on line  28  by the helm control unit  30  typically comprise the three magnitudes associated with the first, second, and third signals,  61 - 63 , described above. For purposes of this example, the three signals received on line  28  are associated with dashed line  90  in FIG.  5  and are identified by reference numerals  91 A- 91 C. As can be recognized by one skilled in the art, the absolute value of the signal magnitude  91 A is not, by itself, sufficiently informative to determine whether or not the throttle handle  12  is in a forward gear position or a reverse gear position. This results from the fact that the first signal  61  is V-shaped and the value of signal magnitude  91 A could possible be associated with either side of the V-shaped signal. However, the second and third signals,  62  and  63 , allow the helm control unit  30  to determine that the throttle handle  12  is in the forward gear range  84 . With this known, the precise position of the throttle handle  12 , as represented by dashed line  90  in FIG. 5, can be calculated. 
     With continued reference to FIG. 5, and more particularly to signal magnitude  91 A, its position can be calculated with respect to the stored position indicating signal magnitudes  74 A and  75 A. First, the total signal range between these two stored points can be calculated and is represented by arrow  101  in FIG.  5 . The difference between signal magnitudes  91  and  74 A can also be easily determined mathematically and is represented by arrow  102 . The ratio of the magnitudes represented by arrows  102  and  101  is equivalent to the ratio between arrows  110  and  112  which are associated with the actual angular travel of the throttle handle  12 , as represented by degrees of rotation of the throttle handle about point  14  in FIG.  1 . 
     In some systems, the use of a single sensor signal could suffice. However, for purposes of redundancy, the present invention utilizes three potentiometers. 
     A similar calculation can be made with respect to magnitudes  74 B,  91 B, and  75 B. The appropriate subtractions can be made and the magnitudes represented by arrows  120  and  121  can be compared to determine their ratio. The resulting ratio is equivalent to the ratio of arrows  110  and  112 . Similarly, signal magnitudes represented by reference numerals  74 C,  91 C, and  75 C can be compared to determine the ratio of arrows  130  and  131  which, when compared, result in a ratio that is equivalent to the ratio of arrows  110  and  112 . All three of the above described calculations should result in the ratio of arrows  10  to  112 . These three calculations provide a degree of redundancy and error checking that can detect faults that may occur in the mechanical and electrical system. If any one signal significantly differs from the others, it can be ignored and an alarm message can inform the vessel operator. 
     With continued reference to FIG. 5, the above described example related to a throttle handle position  90  in the forward gear selection range  84 . One skilled in the art will readily appreciate that the same calculation procedures could be used to determine the throttle handle position within the reverse gear selection range  82 . For either of these two gear selection ranges,  82  or  84 , the present invention uses three pairs of signal magnitudes which are stored by the microprocessor in comparison with three subsequent position indicating signals,  91 A- 91 C, to mathematically determine the position of the throttle handle  12  relative to the range represented by the three pairs of stored signals. 
     In a particularly preferred embodiment, the current throttle handle position can be monitored as the operator moves the throttle handle  12  within the neutral gear selection range  80 . This procedure is not required in all embodiments of the present invention, but can be useful in anticipating the movement of the throttle handle  12  from the neutral gear position  80  into either the forward or reverse ranges,  84  or  82 . In a manner generally similar to the determination of the ratios of arrows  110  to  112 , the microprocessor determines the position of the throttle handle with respect to stored position indicating signals  72 A- 72 C,  74 A- 74 C, and  73 A- 73 C as illustrated in FIG.  4 . Even though the gear selector would remain in neutral gear as the throttle handle  12  is moved between positions  22  and  24 , the microprocessor could be programmed to anticipate an imminent movement of the throttle handle into either the forward gear range  84  or the reverse gear range  82 . This anticipation could be used to assure that the engine speed begins to be increased immediately as the gear selector is moved from neutral to either forward or reverse gears. 
     FIG. 6 is a simplified flow chart of the process of the present invention. From a starting point  200  the microprocessor of the helm control unit  30  determines whether or not a calibration procedure is requested. This is represented by functional block  201 . If the procedure is requested, typically by a boat builder, the person performing the calibration procedure is instructed to place the throttle handle  12  in a first position, as represented by functional block  202 . This could be position  21  in FIG.  1 . Then the signal on line  28  is read and stored as represented by functional block  203 . Depending on how many positions of the throttle handle are required to achieve the desired calibration accuracy, this process is repeated for each throttle handle position. This is represented by functional blocks  204  and  205  for the second position, functional blocks  206  and  207  for the third throttle position, functional blocks  208  and  209  for the fourth throttle position, and functional blocks  210  and  211  for the fifth throttle position. The program then returns to the start position  200  as represented by reference numeral  220 . 
     With continued reference to FIG. 6, if the calibration procedure is not requested, at functional block  201 , the microprocessor of the helm control unit  30  reads the current position signal at functional block  230 . This process involves the receipt, on line  28 , of the three signal magnitudes which are representative of the current position of the throttle handle  12 . In the example described above in conjunction with FIG. 5, these three signal magnitudes are identified by reference numerals  91 A- 91 C and their throttle handle position is represented by dashed line  90 . As represented by functional block  231  in FIG. 6, the present invention then determines the gear selection position by calculating the differences of the current throttle position signals with the stored signal magnitudes. These difference are represented by the vertical arrows on the right side of FIG.  5 . The ratios provided by the comparison of these signal magnitudes allows the helm control unit  30  to determine the ratio of arrows  110  to  112 . This, in turn, allows a precise determination of the position of the throttle handle  12 . The calculation of the ratio between arrows  110  and  112 , in FIG. 5, allows the microprocessor to calculate the percentage of wide open throttle represented by the position of throttle handle  12  as described in functional block  232  in FIG.  6 . Then the microprocessor returns to the start position  200  as indicated by reference numeral  240  in FIG.  6 . 
     As described above, the method of the present invention receives a plurality of position indicating signals that each represent known positions of the manually operated throttle controller which are determined during the calibration procedure. These positions indicating signals are stored for future reference. In a preferred embodiment, five position indicating signals are received and stored and are each representative of an associated known position of the throttle handle  12 . Subsequently, after the calibration procedure is completed, a subsequent position indicating signal is received from the sensor of the throttle control system. The subsequent position indicating signal is representative of a subsequent position of the manually operated throttle controller. The method of the present invention then calculates the subsequent position of the throttle handle as a function of the subsequent position indicating signal and the previously received and stored position indicating signals. The present invention provides a simple, but accurate calibrating process that allows the throttle control system to accurately determine the throttle handle position regardless of the mechanical tolerances relating to the assembly of the throttle handle and its associated throttle control mechanism and sensor. It should be understood that other calculation methods, other than determining the ratios, are also within the scope of the present invention. This also includes predetermined look-up tables which are based on the magnitudes determined during the calibration procedure. 
     Although the present invention has been described with particular specificity and illustrated to show a preferred embodiment, it should be understood that alternative embodiments are also within its scope.