Abstract:
A trolling motor having an integral sonar transducer including: a submerged motor housing; an electric motor housed in the housing; a sonar transducer housed in the housing; a motor controller having a pulse width modulated output and a shielded electrical cable connecting the sonar transducer to a sonar device. Noise from all sources (i.e., electrical noise from ground loops, radio frequency interference, and magnetic interference) is suppressed in the output signal of the transducer through a number of techniques. The electrical cable includes an outer shield connected to the chassis ground of the trolling motor; an inner shield connected to the circuit ground of the sonar device; and a pair of signal carrying conductors comprising a twisted pair. In addition, the resonant frequency of the transducer is selected such that it is not a harmonic of the operating frequency of the pulse width modulated output of the motor controller.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority from copending U.S. provisional patent application Ser. No. 60/218,850, filed Jul. 13, 2000, the disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to a trolling motor for a fishing boat. More particularly, but not by way of limitation, the present invention relates to a trolling motor for a fishing boat with an integral sonar transducer for use with a fish locator, depth finder, or the like.  
           [0004]    2. Background  
           [0005]    Trolling motors are well known in the art as are sonar devices. Generally speaking, a trolling motor is a small electric motor coupled to a propeller for quietly adjusting the position of a fishing boat at relatively low speeds. Trolling motors are available with a variety of features such as variable speed, electric steering, power mounts, etc. Some models connect to a sonar device such as a depth finder to provide the user with navigational options, for example following a straight-ahead course, following a bottom contour, etc.  
           [0006]    Sonar devices are also popular equipment for the sport fisherman. Typically, a sonar transducer is placed in the water facing generally downward. A control unit, preferably mounted within view of the fisherman, causes the transducer to emit a pulse of acoustical ultrasonic energy. Upon completion of the pulse, the control unit uses the transducer to “listen” for return echos. By measuring the time period to a particular echo, the control unit can determine the distance between the transducer and an object. By measuring the amplitude of the echo, the control unit may predict the size of the object. Many contemporary control units employ complex computer software and additional sensors to provide information to the fisherman regarding a host of underwater factors and navigational information.  
           [0007]    In a conventional installation, an on-board sonar device utilizes a transducer that has been mounted to the hull of the boat. This sort of mounting arrangement, however, is not without its problems. For example, if the transducer is attached to the boat with screws, the screws must necessarily penetrate the outer surface of the hull at a point that may be at least occasionally beneath the water line. Obviously, this can lead to leaks and associated problems. Alternatively, if the transducer is adhesively mounted, the adhesive must be capable of continuous contact with water without becoming compromised. Mounting a transducer to a bracket which is attached to the boat above the waterline can mitigate these concerns to a certain extent, but that solution is not always available.  
           [0008]    Since a trolling motor must also be mounted to the boat and since the motor and propeller must be submerged, it would thus appear, at least at first blush, that a trolling motor would provide an ideal platform for placement of an ultrasonic transducer. The prospect of placing a sonar transducer in a trolling motor is even more appealing in light of the fact that, at least with high-end equipment, an ever increasing number of trolling motor features are being coupled to display devices such as sonar control units so that an operator may be provided a visual indication of operating parameters and conditions. Placing a sonar transducer in a trolling motor also simplifies cable routing since the sonar cable may easily be routed through the trolling motor support column.  
           [0009]    Unfortunately, prior attempts to locate a sonar transducer in a trolling motor have been hindered by the amount of electrical and electromagnetic noise present in the general vicinity of the trolling motor. Trolling motors generate such noise in at least four areas.  
           [0010]    First, by the very nature of the control system that is typically used in combination with a trolling motor. Electronic control of trolling motors and electronic steering systems have almost universally employed pulse width modulation (PWM) schemes, as opposed to linear drive circuitry. PWM control systems operate more efficiently than linear systems which result in components that operate at a substantially lower temperature, utilize less power, and need less complex drive circuitry. However, the general nature of a PWM control system requires production of a substantially rectangular waveform at the output of the driver. Such a waveform inherently contains substantial harmonic content many times over the fundamental frequency of the PWM signal. When a PWM signal is used to drive the motor, the electrical currents are relatively high, thus generating nontrivial amounts of high frequency electrical and electromagnetic noise. If this signal has harmonic content approximately equal to the frequency of the signal produced by the sonar transducer, the received transducer signal may be completely obscured by the noise thus produced. Further, if the transducer cable and the power supply cable for the motor both run through the support column, the opportunity for cross talk from the power cable to the transducer cable is greatly enhanced. Of course, such cross talk poses still another opportunity for the motor drive signal to find its way into the received sonar signal.  
           [0011]    Second, noise generally referred to as “brush noise” is created by the motor. As the armature rotates in the motor, the brushes ride on the commutator, cyclically energizing the windings located on the armature. As a particular winding is de-energized during this process, arcing is likely to occur between the brush and the contact on the commutator resulting in electric and electromagnetic noise.  
           [0012]    Third, substantial amounts of noise may also be introduced into the received sonar signal through common circuit conductors. Commonly known as ground loops, unwanted electrical currents often flow through conductors when multiple electrical paths are created through the interconnection of multiple circuit elements, e.g., the trolling motor and the sonar control unit. For example, the control unit may be connected to the boat&#39;s electrical system, the negative lead of which is normally connected to chassis ground. The trolling motor may be connected instead to a spare battery located in the boat. The negative power supply lead from the spare battery may also be connected to trolling motor chassis components. Once the trolling motor is mounted to the boat, or when placed in the water, an electrical connection is made between the boat electrical system and the trolling motor electrical system. In this case, no currents will flow because there is a single common connection. On the other hand, if, for example, a shield within a signal cable is then connected between the trolling motor and the control unit, unwanted electrical current will likely flow when either an accessories attached on the boat is powered, or when the trolling motor is operational. The resistance of the conductors causes voltage losses when such currents flow which appear as noise to the receiver circuitry. The reactive characteristics of the conductors will likewise induce noise from these currents.  
           [0013]    Finally, the spinning armature in the motor produces an external magnetic field which varies over time at a frequency proportional to the rate of rotation of the motor. This varying magnetic field will induce a voltage in nearby conductors. The wires connected to a sonar transducer mounted in a trolling motor will necessarily be subjected to such a magnetic field which will result in additional unwanted noise in the received sonar signal.  
           [0014]    The ability of an electronic device to resolve meaningful information from a received signal is determined, in part, by the signal to noise ratio (often given in dB) present at a receiver. Thus, as is well known to those skilled in the art, placing the sonar transducer near the trolling motor will cause the introduction of a substantial degree of noise into the receiver both from noise resulting from the motor drive and possibly from ground loops, thereby reducing the circuit&#39;s ability to resolve meaningful information from a received pulse.  
           [0015]    Trolling motor models are available which include an integral sonar transducer. However, these trolling motors and the integral sensor only operate with a specific control unit which is properly configured for such operation. These systems are designed such that electrical noise that is created by the drive circuitry for the trolling motor and noise induced from ground loops will appear as common mode noise relative to the transducer output and may be subtracted out of the sonar signal by the receiver circuitry. The circuitry incorporated into the vast majority of trolling motors which are currently available does not allow for this sort of solution. An example of such a combination is disclosed in U.S. Pat. No. 5,525,081 issued to Mardesich, et al. which is hereby incorporated by reference.  
           [0016]    It is thus an object of the present invention to provide a trolling motor with integral sonar transducer which will operate with an existing sonar control unit. It is also a related object of the present invention to provide a trolling motor with integral sonar transducer wherein there is substantial improvement in the signal to noise ratio of the signal presented to the sonar receiver from the transducer so incorporated into the trolling motor.  
           [0017]    It is a further object of the present invention to provide a cable for connection between a sonar transducer and a sonar control unit which provides improved shielding from sources of high frequency noise in close proximity to the cable.  
           [0018]    It is yet another object of the present invention to provide a method for electrically connecting a trolling motor with integral sonar transducer to a power source and to a sonar control unit so as to eliminate multiple ground paths on the sonar signal or reduce the effect thereof.  
         SUMMARY OF THE INVENTION  
         [0019]    The present invention provides a trolling motor for a fishing boat with an integral sonar transducer for connection to a sonar control unit. In one embodiment of the present invention, an inventive cable for connecting the sonar transducer to a control head provides improved shielding from unwanted noise emanating from nearby sources.  
           [0020]    In yet another embodiment of the present invention, there is provided a method for connecting the trolling motor to an electrical system and the sonar control unit to the transducer which will avoid the creation of ground loops.  
           [0021]    Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 shows the inventive trolling motor with integral sonar transducer in its general environment.  
         [0023]    [0023]FIG. 2 provides a side view of the inventive trolling motor with integral sonar transducer.  
         [0024]    [0024]FIG. 3 provides a cutaway side view of the inventive trolling motor with integral sonar transducer.  
         [0025]    [0025]FIG. 4 provides a representative wiring diagram for connection of the inventive trolling motor to a power supply and for connecting the integral sonar transducer to a sonar control unit.  
         [0026]    [0026]FIG. 5 provides a block diagram of a typical controller for a trolling motor which provides pulse width modulated drive signals.  
         [0027]    [0027]FIG. 6 provides a detail view of a stripped end of a sonar transducer cable incorporated in the inventive trolling motor with integral sonar transducer.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    A preferred embodiment of the inventive trolling motor with integral sonar transducer  10  is shown in its general environment in FIG. 1. Typically, trolling motor  10  is removably attached to a fishing boat  18 . To fully utilize the inventive aspects of trolling motor  10 , boat  18  is most preferably equipped with sonar control unit  28 . In operation, trolling motor  10  is preferably controlled by foot pedal  24  connected to trolling motor  10  by control cable  22 . Sonar transducer  30  incorporated in trolling motor  10  is connected to sonar control unit  28  with cable  32 .  
         [0029]    Referring next to FIGS. 2 and 3, trolling motor  10  preferably comprises: motor housing  14  containing motor  34  which is drivingly coupled to propeller  12 ; support column  16  supporting motor housing  14  from mounting bracket  20 ; control head  26  mounted to the upper end of support column  16  housing motor controller  42  (FIG. 5); and cables  22  and  32  (FIG. 1) passing through support column  16  thereby reducing exposure to water.  
         [0030]    Sonar transducer  30  is mounted to trolling motor  10  with shield  40  to protect transducer  30  from electromagnetic fields produced by motor  34 . Cable  32  preferably contains a pair of twisted conductors  44   a  and  44   b  (FIGS. 4 and 6) which connect transducer  30  to sonar control unit  28 .  
         [0031]    Referring next to FIG. 6, wherein is shown the construction of cable  32 , conductors  44   a  and  44   b  are housed within an inner dielectric jacket  46 . Inner jacket  46  is next surrounded by in inner shield  48 . Preferably, inner shield  48  is a metal foil, however, it will be apparent to one skilled in the art that braided shield may also perform satisfactorily with the present invention. Shield  48  is then surrounded by an intermediate dielectric jacket  50  which is next wrapped in outer shield  52 . Preferably, outer shield  52  is likewise metal foil, however once again, braided shield may also perform with acceptable results. Finally, outer jacket  54  surrounds and protects outer shield  52 . It should be noted that shielded cable with twisted pair signal conductors is known in the art. However, unlike prior art cable used with sonar devices, the inventive cable provides a second layer of shielding  52  separated from inner shield  48  by tubular dielectric  50 . As shown in FIG. 4 and discussed further hereinbelow, inner shield  48 , as incorporated in the inventive trolling motor, is referenced to circuit ground  100  (FIG. 4) of sonar control unit  28  while outer shield  52  is referenced to the negative lead  104  of trolling motor power supply  106 .  
         [0032]    A cable constructed in accordance with the present invention will reduce the effects of nearby motor-produced magnetic fields on the received sonar signal. As is known in the art, a varying magnetic field will induce a voltage in nearby conductors. In the inventive cable however, outer shield  50  shields the inner conductors from such magnetic fields. Since outer shield  50  is preferably connected at only one point, voltages induced in outer shield  50  will not result in any unwanted electrical currents and will not affect the received sonar signal. In addition, the effects of any magnetic field which does in fact reach conductors  44   a  and  44   b  will be minimized by the twisting of conductors  44   a  and  44   b  in cable  32 . In a similar manner, the outer shield  50  will shield signal carrying conductors  44   a  and  44   b  from the effects of electrical noise (i.e., radio frequency interference, crosstalk from adjacent conductors, and the like), of particular importance is protection from the noise produced in driving the motor with a pulse width modulated signal. Again, the twisting of conductors  44   a  and  44   b  will also reduce the effects of such noise.  
         [0033]    Referring to FIG. 6, a motor controller  42  for use with the inventive trolling motor  10  would typically include pulse width modulator  62 , reversing relay  60  energized by relay driver  64 , motor driver  66  protected by freewheeling diode  74 , and current sense circuitry  68  which conditions the output of current sense resistor  70 . Preferably, pulse width modulator  62  is implemented using a microcontroller  72 . However, it will be apparent to those skilled in the art that pulse width modulation may be accomplished by any one of a variety of known methods for performing such modulation. In operation, the controller receives a speed command from an user operated input device, preferably a foot pedal  24 . The controller then calculates a duty cycle corresponding to the commanded motor speed and provides motor driver  66  with the appropriate waveform.  
         [0034]    As previously discussed, the waveform produced by a PWM controller is typically rectangular in nature and thus a substantial amount of energy is produced at harmonic frequencies of the PWM waveform. An RC network comprising resistors  74  and  76 , and capacitor  78  provides low pass filtering of the waveform driving motor driver  66 . This filtering tends to somewhat “round” the waveform driving the motor and thus reduces the harmonic content of the signal so produced. As will be apparent to one skilled in the art, this increases the switching time of driver  66  thereby increasing the power dissipated in driver  66 . Hence it can be seen that a tradeoff exists between the efficiency of the motor driver and reduced high frequency noise produced by the driver. For example, it was discovered that selecting a value of ten ohms for resistor  74 , a value of fifteen ohms for resistor  76 , and a value of 0.01 microfarads for capacitor  78  produced a noticeable reduction in harmonic noise with a minimal increase in the power dissipated in driver  66 .  
         [0035]    Generally speaking, a sonar transducer of the type incorporated in the present invention will preferably have a resonant frequency which coincides with the frequency at which the transducer is intended to operate. In the preferred embodiment, sonar transducer  30  is intended to operate at approximately  200  kilohertz. It will be apparent to one skilled in the art that such transducers are available for operation at a number of different frequencies and the selection of a particular frequency is not critical for operation of the present invention.  
         [0036]    According to a preferred aspect of the instant embodiment, the frequency of the pulse width modulator  62  is selected such that the resonant frequency of the transducer  30  is not an exact harmonic of the pulse width modulation frequency, thereby minimizing or preventing direct excitation of sonar transducer  30  by the motor controller  42  or the motor  34 . It should be noted that at particular frequencies, a typical sonar transducer will exhibit a relatively low impedance. Accordingly, it is even more preferable to “tune” the pulse width modulation frequency to a particular transducer, or select a transducer for a particular pulse width modulation frequency, such that the transducer crystal will exhibit a low impedance at the frequency of the pulse width modulation thereby effectively shunting any signal induced into the received sonar signal by the motor circuit.  
         [0037]    It will be further apparent to one skilled in the art that, except as to frequency as discussed hereinbefore, operation of the motor controller is not a part of the present invention. Accordingly the discussion of the motor controller in reference to the preferred embodiment is given only by way of example and not limitation.  
         [0038]    Referring next to FIG. 4, interconnection between the various components is constrained to a large degree by the existing interface to the sonar control unit  28 . Typically, the sonar control unit  28  will receive power from the boat&#39;s primary electrical system. Therefore, the signal from the sonar transducer  30  will ultimately be referenced to chassis ground of the boat. It should also be noted that in a typical configuration, one lead connected to transducer  30  is also connected to circuit ground within control unit  28  and thus, conductor  44   b  of cable  32  connects the transducer  30  to circuit ground at sonar control unit  28 . The other conductors of cable  32  are preferably connected as follows: twisted pair conductor  44   a  provides the signal path to and from transducer  30 ; inner shield  48  is connected to the negative lead  100  of the boat&#39;s electrical power supply  102  at sonar control unit  28  (circuit ground) and preferably to shield  40  (FIGS. 2, 3, and  4 ) over transducer  30  at motor housing  14  (FIGS. 2, 3, and  4 ); and preferably, outer shield  50  is connects to the negative power supply lead  104  of the trolling motor power supply  106  at control head  26 . Optionally, conductor  44   b  is also connected to inner shield  48  at both control unit  28  and at transducer  30 .  
         [0039]    It should be noted that it is common in the art to switch the negative side of the trolling motor with motor driver  66  (FIG. 4). In addition, reversing relay  60  reverses the polarity of the conductors which supply power to the motor. As a result, when trolling motor  10  is connected to a battery independent of the boat&#39;s electrical system, there is not a chassis ground, as such, connected to the motor. In fact, there may not be a continuous connection of the negative lead  104  of the trolling motor power supply  106  to the motor  34  or the chassis. Optionally the support column  16  may be connected to the negative power supply lead  104  of trolling motor battery  106  thereby providing a chassis ground relative to the trolling motor power supply  106 .  
         [0040]    Preferably, the trolling motor controller  42  derives its primary power from an external battery independent of the electrical system of the boat. Hence, a fisherman will not have to worry about retaining sufficient charge in the boat&#39;s battery to restart the boat&#39;s engine. If the external components of the trolling motor are not connected to the negative power supply of the battery, inner shield  48  comprises a single ground point between the two electrical systems at the transducer shield  40 . On the other hand, if support column  16  is connected to the negative power supply lead for the trolling motor  10 , there will exist the potential for two common ground connections between the two electrical systems, the second one being through the support components of the trolling motor. While such a connection may result in some unwanted electrical current flowing through inner shield  48 , since motor  34  does not draw power through a chassis ground, no trolling motor currents will flow through inner shield  48  or conductor  44   b  and therefore, the impact on the received sonar signal will be minimal.  
         [0041]    As will be understood by those skilled in the art, although the above preferred embodiment of the inventive apparatus has been discussed in regard two independent electrical systems, the inventive apparatus would perform acceptably when operated from a single power supply common to both the trolling motor  10  and sonar control unit  28 .  
         [0042]    It should be noted that while it may be possible to obtain an acceptable signal to noise ratio with either the cable disclosed herein or appropriate matching of the pulse width modulation frequency and the transducer, preferably a trolling motor constructed in accordance with the present invention would incorporate both the cable and matching of the pulse width modulation frequency and the sonar transducer.  
         [0043]    Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the appended claims.