CA1060972A - Automatic piloting system - Google Patents

Abstract

ABSTRACT
An automatic piloting system for the automatic steering of a marine or other vessel along an intended course in which a digital signal representing an intended course is processed to provide rudder commands for steering the desired course. The system includes a rudder angle indicator having a differential capacitor having first and second capacitances. The capacitances are equal for a zero rudder position and change in opposite directions when the rudder position changes. The indicator provides a pulse width modulated signal of average amplitude representative of rudder position.

Description

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This is a division of our co-pending Canadian patent ~pplication No. 265 272 filed 9th November, 1976.
This invention relates to automatic piloting systems and more par-ticularly to a rudder angle indicator for use in an electronic system for the steering of a marine or other vessel along an intended course.
In most automatic piloting systems known in the art, a course transmitter is employed to provide a signal indication of an intended course to be steered and to provide steering command signals for rudder control to maintain the intended course. A feedback signal representing rudder position is provided by a rudder angle sensor, this feedback signal being applied to the course transmitker which will be rotated or otherwise adjusted to seek a null condition. Although such systems perform adequately for many purposes, their implementation is usually quite complex, requiring analog servomechani-cal apparatus including a specialized course transmitter for indicating an intended course and for generating steering commands. Moreover, being null type serve systems, known automatic piloting systems are most sensitive at the null point and are of reduced accuracy for increasing course error.
The following disclosure describes a relatively simple automatic piloting system in which an intended course is denoted by a digital signal and processing accomplished by relatively simple electronics without need for complex servo-mechanisms.
~he invention provides for use in an automatic piloting system in which rudder control signals are provided to control rudder position, a rudder angle indicator comprising: a differential capacitor coupled to said rudder and including a first ana a second capacitor, respectively, providing .. 1 ,. ~ .
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a first capacitance and a second capacitance; said first and second capa tances being equal for a zero rudder position, said first capacitance in-creasing and said second capacitance correspondingly decreasing for rudder positions of one sense, said first capacitance decreasing and said second capacitance correspondingly increasing for rudder positions of opposite sense; and means operative in response to said first and second capacitances for providing a pulse width modulated signal of average amplitude representa-tive of rudder position.
In preferred implementation, a remote reading digital magnetic compass is empl~yed which provides digital signals representative of compass heading. With a vessel being steered along a desired course, these digital signals provide an input to the automatic piloting system, the course being maintained by relatively simple electronic circuitry. The system requires no specialized course transmitter to introduce a signal representing desired heading, but rather can be employed with any input source which provides a digital number representing the intended course, such as a digital magnetic compass. The system exhibits wide dynamic range and provides high accuracy control over the entire range of course errors encountered.
The invention will be more fully understood from the following detailed description of a preferred embodiment thereof, given by way of example only, taken in conjunction with the accompanying drawings in which:
Figure 1 is a diagrammatic representation of an automatic piloting system including a rudder angle indicator according to the invention;
Figure 2 is a diagrammatic representation of the course error cir-cuitry of Figure 1.

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Figure 3 is a schematic representation of a capacitive rudder angle sensor; and Figure 4 is a schematic representation of the capacitive rudder angle sensing circuitry.
Referring to Figure 1, there is shown a digital magnetic compass 10 providing digital output signals representative of compass heading which are applied to a course error circuit 12 which, in turn, provides a digital signal representative of the magnitude of course error and a digital signal indication of whether the error is to the right or left of intended course.
The digital compass is the subject of United States Patent 3,833,901 assigned to the assignee of this invention. This compass provides serial output - pulses of a number representative of compass heading as derived from an electro-optically sensed compass card. The intended course to be maintained is provided in the course error circuit 12 typically by entering the reading from digital compass 10 when the vessel is on an intended course. The sig-nals from circuit 12 are applied to a digital-to-analog converter 14 which provides an analog output voltage representative of the magnitude and sense of course error. This signal is coupled via a capacitor Cl to a differen-tiator 16 which is typically implemented by an operational amplifier 18 hav-ing an adjustable resistor 20 in feedback connection between the output and an input thereof. The other input of operational amplifier 18 is coupled to a reference voltage source 22. The output of differentiator 16 is coupled via a resistor Rl to the negative input of a summing amplifier 24 which in-cludes a feedback resistor R2 therearound. The output signal from converter 14 is also applied to the negative input of amplifier 24 by means of a 18~4)97Z

resistor R3. The output ~oltage from converter 14 is also applied via a gain control 26 to an integrator 28 composed of an operational amplifier 30 having a capacitor C2 in feedback connection therewith. The second input to ampli-fier 30 is coupled to a reference voltage (V f), such as from source 22.
The output signal from integrator 28 is coupled by way of a resistor R4 to the negative input of amplifier 24.
A rudder angle indicator 32 provides a pulse width modulated signal representative of measured rudder angle to a pulse width demodulator 34 which provides an analog output signal representative of angular rudder position, and which signal is applied via a buffer amplifier 36 and a resistor R5 to the positive input of amplifier 24. A gain control 37 is provided in feedback connection around amplifier 36. The output of amplifier 24 is applied to an input of respective comparators 38 and 400 The comparators 38 and 40 are connected to respective reference sources 42 and 44 which provide respective reference signals Vref 1 and Vref 2 to the corresponding comparators. One of the reference sources provides a relatively positive threshold level with respect to a zero course error reference value, while the other reference source provides a relatively negative reference level with respect to this reference value. me reference sources 42 and 44 pro-vide bipolar threshold levels defining a range within which no error cor-rection is performed, and these reference sources are adjustable to control the error range within which no rudder control is provided. Such adjustment serves as a weather control, the adjustable range usually being selected in accordance with sea and weather conditions.
me output signals from comparators 38 and 40 are applied to '~

1~60972 respective AND gates 46 and 48, the respective outputs of which are applied to respective AND gates 50 and 52. The output signal from comparators 3~ -and 40 are also applied via an OR gate 54 to a multivibrator circuit 56, the output of which is applied to the second inputs of gates 46 and 4~ The multivibrator circuit 56 is operative in response to input signls less than a predetermined value to provide output pulses representative of the input signal level. For course error signals of magnitude above the predetermined value, circuit 56 is operative to provide a DC level as a gating signal for gates 46 and 48. Pulse signals are provided for course error signals which are relatively small, typically less than + 2 error. A control 58 is pro-vided for circuit 56 to adjust the output pulses thereof to produce intended rudder movement in response to a corresponding error signal. This control 58 in effect adjusts the pulse output in accordance with the dynamics of the particular rudder to be steered. For larger error signals, typically greater than + 2, a continuous, rather than a pulsed control signal is produced for rudder drive.
The output signal from amplifier 36 is applied to respective inputs of comparators 60 and 62. These comparators receive respective reference signals Vref 3 and Vref 4 from reference sources 64 and 66. The output signal from comparators 60 and 62 are applied to respective inputs of AND
gates 50 and 52. The gates 50 and 52 prov:~de respective right rudder and le~t rudder control signals to rudder drive apparatus 68 to cause movement of the rudder in the desired direction and by the desired amount. The ref-erence sources 64 and 66 are adjustable to provide a rudder limit control.
One of the reference sources 64 and 66 provides a relatively positive refer-ence level with respect to the zero reference position, while the other source provides a relatively negative reference level. A range is thus de-fined within which rudder control is performed and outside of which rudder control is inhibited. The comparators 60 and 62 provide a first logic level in the presence of input signals within the threshold range which serves as a gating signal for gates 50 and 52 to permit provision of rudder control signals to drive apparatus 68. In the event that a rudder angle signal from amplifier 36 is greater than the corresponding reference level, the associated one of comparators 60 and 62 provides an opposite ]ogic level which inhibits the associated AND gate. As a result, rudder control is in-hibited for an indicated rudder angle greater than the selected amount.
The digital-to-analog converter 14 provides an analog signal having an intermediate value when no course error is sensed, and having a more posi-tive or a more negative valu~ in response to course errors of corresponding sense. For example, positive analog output signals can denote course errors to the right, while negative output signals represent course errors to the left of desired course. Typically, the output voltage from converter 14 varies between 0 and 5 volts with a ~level of 2.5 volts being provided in response to a zero error condition. This output voltage typically varies by 0.1 volt for each degree of course error. Similarly, the analog output sig-nal from demodulator 34 representing indicated rudder angle can vary by 0.1 volt for each degree of rudder angle with an intermediate level of 2.5 volts for a zero or neutral rudder position. More positive and more negative volt-age levels are provided for respective ri~ht and left rudder angles.
The differentiator 16 provides a signal representing the rate of .. . :.............. .
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10~09~72 change of course error and which is subtracted from the error signal from converter 14. As the course error approaches zero, the derivative of the error also approaches zero, and, therefore, differentiator 16 is effective to da~pen the error signal and minimize overshoot. The integrator 28 is operative to avera~e the error signal from converter 14 and in effect pro-vides a system bias level about which the cQurseerror varies.
The rudder angle indicator is shown more particularly in Figures 2 and 3, As seen in Figure 2, a capacitive sensor 70 is coupled to a shaft 72 of a rudder 74. The sensor 70 includes first and second generally semi-circular capacitor plates 76 and 78 disposed in a common plane and having a third capacitor plate 80, also of generally semicircular configuration, -~is-posed in spaced relationship with respect to plates 76 and 78 and relatively movable with respect to plates 76 and 78 about a shaft 82. me capacitor structure serves as a differential capacitor and is itself known in the electronics art. Upon rotation of plate 80 with respect to plates 76 and 78, the capacitance of plates 76 and 80 will vary in a first s~se, while the capacitance of plates 78 and 80~will vary in an opposite sense, the total capacitance remaining substantially constant. m e capacitive sensor is particularly advantageous for rudder angle indication as there is no physical contact between the movable and non-movable capacitor plates. Typically, plate 80 is lir~ed to rudder shaft 72 for movement in association therewith.
Rotation of the rudder causes corresponding rotation of plate 80 relative to plates 76 and 78 to produce a differential capacitance representative of rudder angle.
Referring to Figure 3, the capacitive sensor 70 is shown schemati -:~0~97Z

cally, capacitor C3 depicting the capacitor formed by plates 76 and 80, while the capaeitor provided by plates 78 and 80 is depicted as capacitor C4. The common plate of the two capacitors is connected to ground, the respeetive plates of capaeitors C3 and C4 being connected to an input of respective comparators 84 and 86. ~ reference source 88 provides a voltage reference to the second input of eaeh eomparator 84 and 86. The output of eomparator 84 is coupled to the set input of a flip-flop 90, while the reset input of this flip-flop is couplèd to the output of eomparator 86. The Q
output of flip-flop gO is eoupled via a resistor R10 having a diode Dl in shunt therewith to eapaeitor C3. The Q output of flip-flop 90 is eoupled to the eapaeitor C4 by way of resistor Rll and shunt diode D2. The circuit ":.
output is provided by the Q terminal of flip-flop 90.
With the rudder at a zero position, the eapacitive sensor provides two equal eapaeitanees, eausing an output signal at the Q terminal of flip-flop 90 whieh is a square wave of zero average amplitude. When the rudder is moved to an angular position other than zero, the eapaeitanee of one of the eapacitors C3 and C4 will be eorrespondingly inereased, while the eapaei-tanee of the other will be eorrespondingly deereased, in aeeordance with the sense of the rudder position to the right or the left of its zero position.
The eapaeitor having inereased eapacitance will provide a longer ehargi~g time than the capacitor of deereased eapaeitanee sueh that the eomparator 84 or 86 assoeiated with the eapaeitor of lesser eapaeitanee ~ill be triggered more often to provide switehing of flip-flop 90. A pulse width modulated signal is provided by flip-flop 90 which will be of positive average ampli-tude for rudder angles of one sense and negative average amplitude for ~_ `': , ~

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rudder angles of opposite sense. This pulse width modulated signal is rela-tively immune to noise and can be provided on a two wire cable to the demod-ulator. For example, if the rudder is at an angular position at which capacitor C3 is of increased capacitance and capacitor C4 of corresponding decreased capacitance, the comparator 86 will be rapidly triggered by reason of the faster charging time of capacitor C4. As a result, flip-flop 90 is reset by the more rapid signals from comparator 86, in relation to the signals from comparator 84, such that an output signal is provided of a negative average amplitude representative of the angular position of the rudder.
The course error circuit 12 is shown more particularly in Figure 4 and is operative to provide digital outpu~ signals representative of the mag-nitude of course error and the sense of error to the right or left of an intended course. This circuit is similar to the compass averaging circuit which is the subject of United States Patent 3,975,621 of the same assignee as herein. The digital signal representa~ive of compass heading and pro-vided by digital compass 10 is, upon closure of switch 100, applied to a desired heading register 102 and to one input of a subtraction circuit 104.
Circuit 104 also receives an input from register 102 and provides output signals representative of the magnitude and sense of the differene between the digital signals provided by compass 10 and register 102. The register 102 is initially loaded with a representation of the compass heading sensed by digital compass 10. This heading stored by register 102 can be employed as the intended course indication, or the intended course can be altered by trimming the stored heading in register 102 by means of trim control 103.
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This trim control is operative to increment or decrement the digital count stored in register 102 to provide a representation of intended heading to be maintained.
The digital error signal from subtracter 102 is applied to a detec-tor circuit 106 operative to detect whether the error signal represents a heading difference greater than 180 degrees. If the heading error is greater than 180 degrees, detector 106 provides an output signal to circuit 108 which is operative to subtract a representation of 360 degrees from the heading error to provide an output signal representative of the magnitude of the course error. This output signal from subtracter 108 is provided via gate 110 as an output for application to converter 14. In the event that the course difference is 180 degrees or less, detector 106 provides a direct output indication of the magnitude of the heading error via gate 110.
The sense of the course error is provided by sign correction circuit 112.
For a course error greater than 180 degrees, the signal from detector 106 applied to circuit 112 causes inversion of the output signal from circuit 112 to denote an opposite sense.
It will be appreciated that the system described herein can be readily implemented in either discrete or integrated circuit form~add can be readily installed aboard a vessel without need for a specialized analog course transmitter or servomechanical apparatus. The particular implementation of the invention can vary to suit intended performance requirements. According-ly, the invention is not to be limited by what has been particularly shown and described except as indicated in the appended claims.

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Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH IN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. For use in an automatic piloting system in which rudder control signals are provided to control rudder position, a rudder angle indicator comprising:
a differential capacitor coupled to said rudder and including a first and a second capacitor, respectively, providing a first capacitance and a second capacitance;
said first and second capacitances being equal for a zero rudder position, said first capacitance increasing and said second capacitance correspondingly decreasing for rudder positions of one sense, said first capacitance decreasing and said second capacitance correspondingly increasing for rudder positions of opposite sense; and means operative in response to said first and second capacitances for providing a pulse width modulated signal of average amplitude representa-tive of rudder position.

2. The rudder angle indicator of claim 1 wherein said pulse width modulated signal means includes:
means for deriving first and second voltages representative of the charging time of said respective capacitors;
comparator means operative to provide first and second signals upon exceedance of a threshold level by said first and second voltages; and gate means operative in response to said signals to provide a pulse width modulated signal representative of rudder position.