Abstract:
A marine sensor device mounts in a single opening in a hull of a marine vessel. The sensor includes a housing secured in the opening. Positioned within the housing is a body containing at least two sensors. The body is removable from the housing. A magnetized paddlewheel can be disposed in a first cavity formed on a first half of the body, the paddlewheel has a plurality of paddles extending from a circular central hub and rotatably mounted on an axle extending transverse a fore and aft direction of travel of the vessel. A magnetic sensor can be located adjacent the paddles, the magnetic sensor senses the rotation of the paddles and provides speed indications. The magnetic sensor can be a Hall-effect device. A sonic transducer for depth detection can be disposed within a second cavity formed on a second half of the body. A thermal sensor for sensing water temperature can be disposed in a well formed in the body.

Description:
RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/429,514, filed Nov. 26, 2002 and U.S. Provisional Application No. 60/402,493, filed Aug. 8, 2002 and includes the disclosures discussed in U.S. Pat. No. 4,898,029, issued Feb. 6, 1990, which is a continuation of U.S. Pat. No. 4,836,020, issued Jun. 6, 1989, which reissued on Jul. 7, 1992 as Re. 33,982, and which is a continuation of U.S. application Ser. No. 7,527, filed Jan. 28, 1987, now abandoned, the entire teachings of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   There are many types of marine sensors available for commercial and pleasure craft today. Some of them include instruments for measuring water depth, boat speed, water temperature, as well as, locating fish. Certain depth measuring devices employ an ultrasonic transducer that emits an acoustic beam downwardly from the boat. When the beam strikes something, such as the bottom, the beam reflects an echo back to the transducer. This is converted into electrical energy, amplified and displayed as information on a screen. The information can be displayed on a paper graph, flashing device, or video display. 
   For the most part, speed, depth, and temperature measuring instruments were three separate devices that required drilling three holes in the hull. Today, these measuring sensors have been combined into a single instrument which provides information with respect to all three parameters of speed, temperature, and depth. However, these single instruments do not allow the sensors to be readily removable from within the hull. In particular, the depth sensor cannot be removed while the vessel is afloat because the size of the transducer element is greater than the opening in the hull. 
   Despite the above efforts, and that of other workers in the art, a need exists for a through-hull device with that allows the sensing components to be removable from within the hull while the vessel is afloat. 
   SUMMARY OF THE INVENTION 
   A marine sensor device mounts in a single opening in a hull of a marine vessel. The sensor includes a housing secured in the opening. Positioned within the housing is a body containing at least two sensors. The body is removable from the housing. A magnetized paddlewheel can be disposed in a first cavity formed on a first half of the body, the paddlewheel has a plurality of paddles extending from a circular central hub and rotatably mounted on an axle extending transverse a fore and aft direction of travel of the vessel. A magnetic sensor can be located adjacent the paddles, the magnetic sensor senses the rotation of the paddles and provides speed indications. The magnetic sensor can be a Hall-effect device. A sonic transducer for depth detection can be disposed within a second cavity formed on a second half of the body. A thermal sensor for sensing water temperature can be disposed in a well formed in the body. 
   In some embodiments, the cross-sectional area of the paddles in a plane transverse a direction of flow of water being traversed by the marine vessel and in a plane parallel the direction of flow versus the available cross sectional area in the respective planes define a Cross-Sectional ratio having a range between about 0.25 and 0.5. Moreover, the lowest point on the periphery of the hub can be located tangentially adjacent to or vertically above the lowest point in the first cavity. It is important to maintain the Cross-Sectional ratio is preferably maintained between 0.25 and 0.5 to reduce the build-up of negative cavity pressure and thus minimize the tendency to cavitate within the cavity at high speeds. 
   In some embodiments, the Cross-Sectional ratio in the plane parallel to the direction of flow is different than the Cross-Sectional ratio in the plane transverse the direction of flow. In other embodiments, the Cross-Sectional ratio in the plane parallel to the direction of flow is about equal the Cross-Sectional ratio in the plane transverse the direction of flow. The first cavity can be an asymmetric cavity 
   The transducer can be a piezoelectric element having an aspect ratio defined in terms of the length, width, and height of the piezoelectric element. The aspect ratio being optimized such that the maximum acoustic energy of the element is produced when the element vibrates with a frequency of about 150 kHz to about 250 kHz. The maximum acoustic energy of the piezoelectric element is produced when the element vibrates with a frequency of about 235 kHz. In some embodiments, the piezoelectric element can be made of PZT. 
   In one embodiment, the length of the piezoelectric element can be about 1.0 to 1.3 inches in length, about 0.1 to 0.5 inches in height, and about 0.1 to 0.5 inches in width. In another embodiment, the length of the piezoelectric element can be about 1.25 inches, the height is about 0.23 inch, and the width is about 0.22 inch. The transducer can have a beamwidth of about 11°×38° at about −3 dB. 
   The sensor body is disposed in a housing that fits into a single circular opening through a hull of the vessel. The housing contained at least two removable sensors, a speed sensor and a depth sensor. Optionally, a temperature sensor may be disposed in the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  is a perspective view of a high speed through-hull speed sensor of one embodiment of the invention. 
       FIG. 2  is a front view of the sensor of FIG.  1 . 
       FIG. 2A  is a top view of the sensor along the line  2 A— 2 A of FIG.  2 . 
       FIG. 2B  is a bottom view of the sensor along the line  2 B— 2 B of  FIG. 2   
       FIG. 2C  is a side view of the sensor along the line  2 C— 2 C of FIG.  2 . 
       FIG. 3  is an exploded view of the sensor of FIG.  1 . 
       FIG. 4  is a perspective view of an inner tubular body of the sensor of FIG.  1 . 
       FIG. 5  is a front view of the tubular body of FIG.  4 . 
       FIG. 5A  is a top view of the tubular body along the line  5 A— 5 A of FIG.  5 . 
       FIG. 5B  is a bottom view of the tubular body along the line  5 B— 5 B of FIG.  5 . 
       FIG. 5B   a  is another bottom view of the tubular body illustrating the cross sectional area of a cavity of the body as occupied by a paddle of a paddlewheel speed sensor. 
       FIG. 5B   b  is a detailed view of the cross sectional area of the cavity identified in  FIG. 5B   a  as occupied by the paddle. 
       FIG. 5B   c  is a detailed view of the cross sectional area of the cavity identified in  FIG. 5B   a  when not occupied by the paddle. 
       FIG. 5C  is a side view of the tubular body. 
       FIG. 5D  is a partial cutaway side view of the tubular body shown as shown in  FIG. 5C , the partial view being taken along the line  5 D— 5 D with the paddlewheel fully shown illustrating the cross sectional area of the cavity of the body as occupied by a paddle of a paddlewheel speed sensor. 
       FIG. 5D   a  is a detailed view of the cross sectional area of the cavity identified in  FIG. 5D  as occupied by the paddle. 
       FIG. 5D   b  is a detailed view of the cross sectional area of the cavity identified in  FIG. 5D  when not occupied by the paddle. 
       FIG. 6  is an exploded view of the tubular body of FIG.  4 . 
       FIG. 6A  is a detailed view of a support board and a lower tube portion of the tubular body of  FIG. 4   
       FIG. 7  is a perspective view of the sensor with a single center skeg in accordance with the invention. 
       FIG. 8  is a perspective view of the sensor with no skeg in accordance with the invention. 
       FIG. 9  is a perspective view of the sensor with two outer skegs in accordance with the invention. 
       FIG. 10  is a perspective view of the sensor with a center and two outer skegs in accordance with the invention. 
       FIG. 11  is a perspective view of the ultrasonic transducer of the sensor of FIG.  1 . 
       FIG. 12  is a plot of the athwartships beam pattern of the transducer of FIG.  11 . 
       FIG. 13  is a plot of the fore aft beam pattern of the transducer of FIG.  11 . 
       FIG. 14  is a perspective view of the ultrasonic transducer of the sensor of  FIG. 1  with dimensions. 
       FIG. 15A  is a 3-dimensional displacement map exhibiting a “good” coupling mode. 
       FIG. 15B  is a 3-dimensional displacement map exhibiting a “poor” coupling mode. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIGS. 1-3 , there is shown a high speed through-hull tri-sensor device  10  for marine vessels. The device  10  can be mounted through a hull of the marine vessel. The device  10  has a housing designated, generally, by the numeral  12 , the housing  12  having a vertically extending threaded cylinder  12 S, and lower flared portion  12 L, respectively. The illustrated sensor device  10  is shown by way of example only. The invention is not limited for use in such a device, the present invention can be used in any other suitable sensor device, such as, for example, a transom mounted sensor. 
   The threaded cylinder  12 S is positioned in an opening through the hull. A hull nut  13  with internal threads engage a set of external threads  21  on the exterior periphery of the threaded cylinder  12 S. When the device  10  is positioned in the hull, the hull is wedged between the hull nut  13  and the flared portion  12 L. 
   Referring also to  FIGS. 4-6 , a tubular body  52  is shown having an upper portion  52   a  and a lower portion  52   b , the lower portion  52   b  forming an asymmetric paddlewheel cavity  30 . The paddlewheel cavity  30  is positioned on one side of the lower portion  52   b  of the body and is considered to be asymmetric as contrasted to the paddlewheel cavity of prior art devices which used the entire lower portion of the circular tube. The body  52  is adapted to slide into the threaded cylinder  12 S. O-rings  32  and  34 , respectively, are positioned in grooves on the periphery of the body  52  and form a fluid-tight seal between the body  52  and threaded cylinder  12 S. The body  52  is of tubular shape and is formed of metal or plastic. A cap  14  with internal threads engage with the threads  21  so that the cap  14  can be hand tightened onto the threaded cylinder  12 S to seal the tubular body  52  within the threaded cylinder  12 S. 
   A cable  16  containing wires  19  ( FIGS. 1 and 6 ) is coupled through a bored hole  27  formed in an enlarged wall of the threaded cylinder  12 S. The wires  19  provide electrical connection to components, such as a Hall-effect device  56 , a transducer  60 , and a thermistor  62  (FIG.  5 C), each of which are positioned within the lower tube portion  52   b.    
   Referring in particular to  FIGS. 3 and 6 , a paddlewheel  26  is mounted on axle  28  within the asymmetric cavity  30  formed in the lower tube portion  52   b . The paddlewheel  26  is an integral structure having a hub  38  from which four symmetric shaped paddles  42  extend about the periphery thereof. The axle  28  rotates within bearings  37  disposed in a bore  39  which extends through a central opening in the hub  38 , and into opposing recessed holes in the cavity side walls formed in the lower tube portion  52   b . The paddlewheel  26  is thereby rotatably suspended within the cavity  30 . In some embodiments, the paddlewheel  26  is formed of amorphous magnetized material, such as barium ferrite. The paddles  42  can be polarized with respect to the hub  38 , or with respect to each other. As the paddles  42  rotate about the axle  28  when the vessel traverses the water, the variation in magnetic field is sensed by the Hall-effect device  56  mounted on a support board  57  positioned in the lower tube portion  52   b.    
   The flared portion  12 L is mounted flush against the outer surface of the hull to prevent impact with objects as the marine vessel moves through the water. A skeg  15  extends from the lower tube portion  52   b  of the body  52  to straighten the flow of water past the paddlewheel  26 , for example, when the vessel, such as sail boat heels over as it moves through the water. 
   Also, a preferred location for the Hall-effect device  56  is within the lower tube portion  52   b  adjacent the cavity  30 , where it can be encapsulated and protected from the water. Therefore, the tubular body  52  in some embodiments is constructed of material that is permeable to the magnetic field emanating from the paddlewheel. 
   The thermal sensing device  62  and Hall-effect device  56 , can be of the types as described in U.S. Pat. No. 4,555,938, the entire contents of which are incorporated herein by reference, and are electrically coupled respectively via wires  19  to a temperature display  80  and a speed display  70  by way of terminals  100   a  and  100   b  extending from the board  57  (see, in particular,  FIGS. 1 ,  3 ,  6 , and  6 A). The thermal sensing device  62  and the Hall-effect device  56  are contained within the lower tube portion  52   b  of the tubular body  52 . The thermal sensing device  62  extends from the board  57  and is positioned within a well  62   w  ( FIG. 6A ) of the lower tube portion  52   b.    
   The upper wall  44  ( FIGS. 3 and 6 ) of cavity  30  is an arched surface, closely spaced from the tip T of the four paddles  42  as they rotate about axle  28 . The periphery P of the hub  38  is approximately flush with the bottom surface B of the cavity. The types of paddlewheels that can be used in the present invention are described in detail in U.S. Pat. Nos. 4,898,029, 4,836,020, and Re. 33,982, all of the contents of which are incorporated herein by reference. 
   Note however that to further minimize build-up of negative cavity pressure and thus reduce the tendency to cavitate within the cavity at higher speeds, the cross-sectional shape  80  of the paddles  42 , in the present embodiment is changed from the above-referenced art. The cross-sectional shape taken in a plane parallel the direction of flow versus the available cross-sectioned space  82  in the cavity  30  in the same plane, i.e., Cross-Sectional Ratio, is preferably and optionally between about 0.25 to 0.5. This ratio is in the order of 0.30. This ratio is achieved by using a relatively thin cross-sectional paddle and by symmetrically rounding off the side walls Z of the paddles at the tips and symmetrically removing material from the sidewalls as they join the hub. This reduces the numerator of the ratio, i.e., paddle cross-section. 
   Note also that the cavity  30  has an asymmetric shape, which results from positioning the paddlewheel  26  off-center relative to a center plane  84  extending along the length of the tubular body  52 , that is, extending into the page. This arrangement as well as the small size of the transducer  60  has the particular advantage of placing the transducer  60 , thermal sensing  62 , and Hall-effect device  56 , as well as the paddlewheel  26 , all within a single structure, namely, the tubular body  52 , and more particularly, the lower portion  52   b  of the tubular body  52 . 
   Also note that the Cross-Sectional Ratio may vary within the cavity  30 , as illustrated in  FIGS. 5D ,  5 D a , and  5 D b . Although the cross-sectional shape  80  of the paddles  42  is the same when taken in a plane transverse the direction of flow (FIG.  5 D a ), the available cross-sectioned space  92  ( FIG. 5D   b ) in the cavity in the same plane can be different than the available cross-sectioned space  82  shown in  FIG. 5B   c . As such, the Cross-Sectional Ratio for the plane of  FIG. 5D  is about 0.34. However, in some embodiments the Cross-Sectional Ratio is the same for both planes, even though the shape of the cross-sectioned shapes  82  and  92  are different. 
     FIGS. 8-10  show alternate embodiments of the device discussed above. Recall that a single skeg  15  was positioned at the center of lower portion  12 L of the sleeve  12  (shown again in  FIG. 7  for reference). However, there can be more than one skeg located at different positions on the lower portion  12 L. For example, as shown in  FIG. 9 , there are a pair of parallel skegs  15   a  and  15   b  located near respective outer regions of the lower portion  12 L. These can also be combined with the center skeg  15  as shown in FIG.  10 . Optionally, in some implementations, the device  10  does not include a skeg as illustrated in FIG.  8 . 
   Referring to  FIG. 11 , the transducer  60  is formed of a piezoelectric element  102  coated with an upper and lower layer of silver  104 ,  106  that provides an electrical connection with terminals  100   a  and  100   b  on board  57  ( FIGS. 3 ,  6 ) to an electronic driver assembly  90  ( FIG. 1 ) associated with a depth indicator  90  (FIG.  1 ). The transducer  60  is made to fit within the lower tube portion  52   b  of the body  52  such that the body can be removed from the housing  12  while the vessel is afloat. Other than the radiating face (not shown), the piezoelectric element  102  is enclosed in a resilient backing member  61  ( FIGS. 3 ,  6 ), preferably consisting of cork material or an equivalent material. That is, the backing member  61  encloses the piezoelectric element  102  at the top and sides of the generally rectangular piezoelectric crystal  102 . The piezoelectric element  102  can be made from well-know lead zirconate titanate material, barium titanate material or other equivalent material. 
   The purpose of the backing member  61  enclosing the top and side walls of the piezoelectric element  102  is to provide a barrier against unwanted transmission of acoustic waves toward the top of the enclosure, rather than in the preferred direction, out the bottom. Hence, the backing member  61  acts as a pressure release material as if the top and sides of the piezoelectric element are pushing against air, while the bottom, or radiating face effectively vibrates or pushes against the water. 
   The entire inner portion of the housing is encapsulated in potting material  64  ( FIGS. 1 ,  4 ), such as polyurethane to ensure water-tight encapsulation and at the same time, provide a path for acoustic energy from the piezoelectric element  102  to travel unimpeded out the bottom of the housing. 
   In a typical operation, a drive voltage from the drive assembly  90  is applied across the upper and lower layers  104 ,  106  of the transducer  60  at a frequency of about 235 kHz for about 100 to 1000 μsec. After which, the drive voltage stops, such that the transducer  60  waits for the echo to be reflected from the bottom of the body of water back to the transducer  60 . By determining the time difference between the transmission of the ultrasonic signal and the detection of the echo, the depth indicator  91  ( FIG. 1 ) calculates the depth of the water. 
   A particular feature of the sensor  10  is that the speed sensor, depth sensor, and temperature sensor all fit within the housing or tubular body  52  that fits through a single opening in the hull of the vessel. Hence, the relationship between the height (h), width (w), and length (l) of the piezoelectric element  102 , or aspect ratio, is optimized such that the ultrasonic transducer  60  fits within the lower tube portion  52   b  in one orientation while the acoustic energy from the transducer  60  is maximized at a particular frequency, in this case, 235 kHz. In this way, the transducer  60  provides broad beam coverage athwartships, as shown in  FIG. 12 , so that depth reading is substantially immune to the rolling of the boat and substantially immune to the dead rise angle where the transducer is mounted. The longitudinal beam is optimized for high acoustic signal by making its length as large as possible (within the space available), consistent with good piezoelectric coupling to the water as described in the next paragraph. The word “coupling” means how effectively an electrical signal can be converted to an acoustic signal in water (as well as the reverse, how effectively an acoustic signal can be converted into an electrical signal). More explicitly, these interactions are called ‘transmit’ and ‘receive’ as specific functions. The objective of the FEA analysis is to design a transducer so that it is efficient in both functions. In other words, the objective is to design transducer dimensions to produce strong ‘coupling’ between electrical and acoustical signals. 
   The dimensions of the piezoelectric element  102  are critical to the device&#39;s performance because these dimensions greatly influence how effective the piezoelectric element is in transmitting and receiving acoustic signals. Selection of these dimensions is based on detailed modeling, which is used to predict which vibration modes of the element (among a multiplicity of vibration modes) can optimally transmit and receiver acoustic signals in the water. [Some (undesired) vibration modes couple poorly to water even though these modes have strong vibrations.] Optimization of acoustic coupling is based on modeling by a mathematical technique known as finite element analysis (FEA). Airmar has refined the FEA technique so that the model can predict several important features: the frequency of the vibration, the shape of the vibration, the strength of acoustic signals, the acoustic beam pattern, and electrical impedance of the element. Each of these features is important in the transducer&#39;s overall performance. Transducer designers iterate the dimensions until they have achieved the best performance. Examples of Airmar&#39;s modeling process are described in detail in “Proceedings of the 1994 IEEE Ultrasonics Symposium,” Cannes, France, pp. 999-1003, catalog 94CH3468-6; “Proceedings of the 1998 IEEE Ultrasonics Symposium,” Sendai, Japan, pp. 1051-1055, catalog 98CH36102; and “Proceedings of the 2001 IEEE Ultrasonics Symposium,” Atlanta, Ga., pp. 467-470, catalog 01CH37263, each of which is incorporated by reference in its entirety. Other commercially available FEA software are applicable to modeling, such as ANSYS Inc., Canonsburg, Pa., which analyzes in the frequency domain, and Weidlinger Associates, Inc., Los Altos, Calif., which analyzes in the time domain. 
   Material properties of the piezoelectric material are provided as input parameters to the FEA model. The material properties of the piezoelectric element  102  for PZT4 are shown in Table 1, listing both “book” values commonly found in textbooks and “model” values. 
                                                                                                                                                         TABLE 1                   Material parameters for PZT4, Poled direction is along Z(or “3”)                        Dielectric loss   Maximum           Density kg/m 3     Mechanical Q   tangent   stress kg/cm 2                 Book   7500   500   0.005   —       Model   7650   750   0.002   300                    Temperature Expansion Coefficients (10 −6 /degree C)                Non-pole               direction   Pole direction                       3.8   1.7                        Piezoelectric Constants (10 −12  Coulomb/Newton)                    d 13     d 33     d 15                         Book   −123   289   496           Model   −123   335   525                       The “d” matrix is the charge sensitivity, related by Q = d T, where Q is the charge vector and T us the mechanical stress vector.             Dielectric Constants (ε 0  = 8.854 * 10 −12  Farad/meter)                    K 11 /ε 0     K 33 /ε 0                         Book   1475   1300           Model   1490   1325                        Elastic Constants (10 −12  m 2 /Newton)                    S 11     S 12     S 13     S 33     S 44     S 66                         Book   12.3   −4.05   −5.31   15.5   39.0   33           Model   12.2   −3.66   −5.25   15.0   40.5   32                       The “s” matrix is the compliance of the material, related by the following coupled equation. By symmetry, S ij  = S ji .            ε i  = s ij T j  + d im E m              D n  = K nm E m  + D nk T k              where            ε is vector of mechanical strain            T is vector of mechanical stress            E is vector of electric field            D is vector of dielectric displacement             
The model values have been refined for the material properties (of PZT, for example) by iteratively adjusting the values to obtain the best match between experiment and predictions of the FEA over a number of trials. The values, deduced in this way, are typically more precise than those provided by the manufacturer, mainly because the model values are measured at the operating (ultrasonic) frequency whereas the manufacturer&#39;s values are typically measured at low frequency (below 1 kHz). When comparing theory and experiment for the “best match” of a fully assembled transducer, the resonance frequency and the acoustic sensitivity (in both transmission and reception of ultrasonic acoustic waves) are considered. In summary, the ability of FEA modeling to predict transducer performance depends of the values of the material properties used at the outset of the FEA calculation—the more accurate the material values, the more accurate are the FEA predictions.
 
   Referring to TABLE 1, the temperature expansion coefficient is listed in the second set of numbers, and the charge sensitivity [d], listed in the third set of numbers, relates the electrical charge produced in PZT4 to the stress applied to the material. (The subscript notation is standard to those skilled in the art.) The relative dielectric constant [k] is listed in the fourth set of numbers, and the elastic compliance [s], listed in the fifth set of numbers, primarily determines the mechanical resonance frequencies of the transducer  60 . Finally, the two constitutive equations shown at the end of TABLE 1 couple the mechanical and piezoelectric variables. 
   With the above discussed FEA model, the aspect ratio of the piezoelectric element  102  is optimized for the specific material properties listed in TABLE 1. That is, the transducer designer chose dimensions and iterated these dimensions until a “high coupling” mode ( FIG. 15A ) was achieved. (In  FIGS. 14 and 15A , only one quadrant is modeled as shown, since the other quadrants can be deduced on the basis of symmetry of element  102 .) When “high coupling” occurs, most of the vibration is uniform so that a well-defined acoustic beam is formed. In comparison, “poor coupling” ( FIG. 15B ) occurs when the vibration is non-uniform and portions of the element are expanding while other portions are contracting (as in the undulations exhibited in FIG.  15 B). In this “poor” case, only a weak acoustic beam is produced. A further requirement for an optimized mode is that no (undesired) vibration modes are found at nearby frequencies. Otherwise, the undesired modes can be inadvertently excited, and thereby weaken the desired mode. Optimized dimensions for element  102  were observed when a desired vibration mode ( FIG. 15A ) and desired beam patterns, as shown in  FIGS. 12 and 13 , were found between about 150 and 250 kHz. In a particular embodiment, the maximum acoustic signal occurred at about 235 kHz, for an element  62  having a length of about 1.25 inches, a height of about 0.23 inches and width of about 0.22 inches. For other embodiments, the length, height, and width of the element  62  can vary between 1.0 to 1.3 inches in length, 0.1 to 0.5 inches in height, and 0.1 to 0.5 inches in width depending upon the desired frequency. 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the scope of the invention encompassed by the appended claims.