Rotation control system for Z-type propulsion apparatus

There is provided a system for controlling the rotation of a rotary housing mounting a propeller unit of a Z-type propulsion apparatus in which the rotary housing does not rotate in the direction opposite to that of rotation of a steering handle even when the steering handle for commanding the rotary housing to rotate is angularly moved by more than .+-.180.degree.. A vector calculation circuit either produces a sinusoidal signal representative of a sine of the difference of angle between the steering angle of the steering handle and the follow-up angle of the rotary housing and a cosine signal representative of a cosine of the difference of angle or produces the sinusoidal and cosine signals and a second sinusoidal signal representative of an angle obtained by adding 45.degree. to the difference of angle. And a signal processing circuit produces, in accordance with either the first and second sinusoidal signals and cosine signal or the first sinusoidal and cosine signals, a signal whose polarity is not reversed even when the steering angle exceeds .+-.180.degree.. The signal produced by the signal processing circuit is supplied to a drive unit for rotating the rotary housing to thereby control the rotation of the rotary housing.

FIELD OF THE INVENTION 
This invention relates to a rotation control system for a Z-type propulsion 
apparatus for a watercraft such as a tug boat. 
BACKGROUND ART 
In recent years, harbors have become much crowded with vessels of a large 
size. Therefore, it has been required that tug boats should operate to 
move the vessel toward and away from the shore in a safe and rapid manner. 
For this reason, there have now been extensively used tug boats equipped 
with a Z-type propulsion apparatus which can easily vary the direction of 
propulsion over the range of 360 degrees. There has been proposed a 
rotation control system for controlling the rotation of a rotary housing 
mounting a propeller unit of such a Z-type propulsion apparatus which 
comprises a steering handle for commanding the rotary housing to rotate by 
a desired amount of angle. The steering handle is provided with a 
potentiometer for detecting the angular movement thereof (steering angle 
.theta.1). There is provided in the system another potentiometer for 
detecting the angular movement of the rotary housing (follow-up angle 
.theta.2). And a vector calculation circuit is provided for outputting a 
sinusoidal signal sin(.theta.1-.theta.2) to a servo-control circuit, the 
signal sin(.theta.1-.theta.2) representing a sine of the difference of 
angle between the steering angle .theta.1 and the follow-up angle 
.theta.2. The servo control circuit produces a servo signal from the 
sinusoidal signal sin(.theta.1-.theta.2) and feeds it to a servomotor 
which in turn controls a hydraulic circuit in accordance with its 
rotational movement. The hydraulic circuit controls a hydraulic motor, and 
the hydraulic motor rotates the rotary housing via a gear mechanism. 
With the construction of the above-described conventional system however, 
the rotary housing rotates in the opposite direction to that of rotation 
of the steering handle when the difference of angle (.theta.1-.theta.2) 
exceeds 180.degree., since the sinusoidal signal sin(.theta.1-.theta.2), 
whose polarity is reversed when the difference of angle 
(.theta.1-.theta.2) exceeds 180.degree., is directly inputted to the servo 
control circuit. As shown in FIG. 2, when the steering handle is operated 
in such a manner that the steering angle .theta.1 is abruptly varied from 
0.degree. to 180.degree. clockwise, the rotary housing rotates not 
clockwise but counterclockwise by the follow-up angle .theta.2, as 
indicated by a broken line in the same figure. As a result, the tug boat 
mounting this system turns in the opposite direction, i.e., 
counterclockwise. As is described above, the conventional system has a 
deficiency that the rotary housing rotates in the opposite direction when 
the difference of angle (.theta.1-.theta.2) exceeds 180.degree.. 
It is therefore an object of the present invention to provide a rotation 
control system for controlling the rotation of a rotary housing mounting a 
propeller unit of a Z-type propulsion apparatus in which the rotary 
housing can be rotated in the selected direction even when the rotary 
housing is commanded to rotate by more than 180.degree., thereby a safe 
and good steerability being obtained. 
It is another object of the invention to provide such a system which is 
simple in construction and can be manufactured at lower costs. 
DISCLOSURE OF THE INVENTION 
According to one aspect of the present invention, there is provided a 
rotation control system for controlling the rotation of a rotary housing 
mounting a propeller unit of a Z-type propulsion apparatus comprising a 
steering angle detector for detecting as a steering angle an angular 
position of a steering handle for commanding the rotary housing to rotate; 
a follow-up angle detector for detecting as a follow-up angle an angular 
position of the rotary housing; a vector calculation circuit responsive to 
outputs of the steering angle detector and the follow-up angle detector 
for outputting a first sinusoidal signal representative of a sine of the 
difference of angle between the steering angle of the steering handle and 
the follow-up angle of the rotary housing, a cosine signal representative 
of a cosine of the difference of angle, and a second sinusoidal signal 
representative of a sine of an angle obtained by adding +45.degree. to the 
difference of angle; a signal processing circuit responsive to the first 
and second sinusoidal signals and cosine signal for outputting a positive 
constant value when the difference of angle, which varies within 
-360.degree. and +360.degree., is greater than +180.degree. and when the 
second sinusoidal signal is negative, the signal processing circuit 
outputting a negative constant value when the difference of angle is less 
than -180.degree. and when the second sinusoidal signal is negative, the 
signal processing circuit outputting the first sinusoidal signal when the 
difference of angle is between -180.degree. and +180.degree. or when the 
second sinusoidal signal is greater than or equal to 0; and a drive unit 
responsive to output of the signal processing circuit for rotating the 
rotary housing. 
According to another aspect of the present invention, there is provided a 
rotation control system for controlling the rotation of a rotary housing 
mounting a propeller unit of a Z-type propulsion apparatus comprising: a 
steering angle detector for detecting as a steering angle an angular 
position of a steering handle for commanding the rotary housing to rotate; 
a follow-up angle detector for detecting as a follow-up angle an angular 
position of the rotary housing; a vector calculation circuit responsive to 
outputs of the steering angle and follow-up angle detectors for outputting 
a sinusoidal signal representative of a sine of the difference of angle 
between the steering angle of the steering handle and the follow-up angle 
of the rotary housing and a cosine signal representative of a cosine of 
the difference of angle; a signal processing circuit responsive to the 
sinusoidal and cosine signals for outputting the sinusoidal signal when 
the cosine signal is greater than a predetermined value, the signal 
processing circuit outputting a positive constant value when the 
difference of angle is positive and when the cosine signal is less than 
the predetermined value, the signal processing circuit outputting a 
negative constant value when the difference of angle is negative and when 
the cosine signal is less than the predetermined value; and drive unit 
responsive to output of the signal processing circuit for rotating the 
rotary housing. In this system, the signal processing circuit may be 
constructed so that it holds and outputs the value of the sinusoidal 
signal obtained at the time when the cosine signal becomes less than the 
predetermined value if the cosine signal is less than the predetermined 
value. And in this system, the signal processing circuit may further 
comprise a circuit for comparing the value of the sinusoidal signal held 
by itself with the sinusoidal signal outputted from the vector calculation 
circuit to release the holding of the value of the sinusoidal signal when 
the polarity of the value and the polarity of the sinusoidal signal 
outputted from the vector calculation circuit coincide with each other and 
when the cosine signal becomes greater than the predetermined value. 
Alternatively, the signal processing circuit may further comprises a 
circuit for comparing the value of the sinusoidal signal held by itself 
with the sinusoidal signal outputted from the vector calculation circuit 
to release the holding of the value of the sinusoidal signal when the 
cosine signal becomes greater than the predetermined value and when the 
difference between the value held by itself and the sinusoidal signal 
outputted from the vector calculating circuit is less than a second 
predetermined value. 
According to a further aspect of the present invention, there is provided a 
rotation control system in which the signal processing circuit comprises a 
counter circuit for performing one of incremental and decremental counting 
operations which is identical to the preceding counting operation each 
time the polarity of the cosine signal is changed and when the polarity of 
the sinusoidal signal at the time of changing of the polarity of the 
cosine signal is different from the polarity of the sinusoidal signal at 
the time of the preceding changing of the polarity of the cosine signal, 
the initial count value of the incremental and decremental counting 
operations being 0, the counter circuit performing one of the incremental 
and decremental counting operations which is of the different kind from 
that of the preceding counting operation each time the polarity of the 
cosine signal is changed and when the polarity of the sinusoidal signal at 
the time of changing of the polarity of the cosine signal is identical to 
the polarity of the sinusoidal signal at the time of the preceding 
changing of the polarity of the cosine signal; and a signal hold circuit 
for outputting the sinusoidal signal to the drive unit when the count 
value outputted from the counter circuit is 0, the signal hold circuit 
holding and outputting the value of the sinusoidal signal obtained at the 
moment when the count value becomes other than 0 if the count value is 
other than 0. And in the systems provided in accordance with the present 
invention, the steering angle detector and the follow-up angle detector 
may comprise potentiometers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
FIGS. 1, 3 and 5 show a system for controlling the rotation of a rotary 
housing mounting a propeller unit of a Z-type propulsion apparatus 
according to the present invention. In the figures, reference numeral 1 
denotes a steering handle for commanding a rotary housing 2 mounting a 
propeller unit of a Z-type propulsion apparatus to rotate, the rotary 
housing 2 being rotatably mounted on a hull. The steering handle 1 is 
connected to an angular position detector 3 comprising a well-known 
potentiometer or the like which outputs a sinusoidal voltage sin .theta.1 
and a cosine voltage cos .theta.1 in accordance with the angular position 
.theta.1 (steering angle) of the steering handle 1. Another angular 
position detector 4 such as a potentiometer is connected to a worm shaft 5 
through a gear mechanism, the potentiometer outputting a sinusoidal 
voltage sin .theta.2 and a cosine voltage cos .theta.2 (follow-up angle) 
in accordance with the angular position .theta.2 of the rotary housing 2. 
The output voltages sin .theta.1 and cos .theta.1 of the angular position 
detector 3 and the output voltages sin .theta.2 and cos .theta.2 of the 
angular position detector 4 are inputted to the calculation circuit 6 
(vector calculation circuit). The calculation circuit 6 produces a first 
sinusoidal signal sin(.theta.1-.theta.2) and a cosine signal 
cos(.theta.1-.theta.2) of an angle (.theta.1-.theta.2) representative of 
the difference of angle between the steering angle .theta.1 and the 
follow-up angle .theta.2, and also produces a second sinusoidal signal 
sin(.theta.1-.theta.2+45.degree.) of an angle 
(.theta.1-.theta.2+45.degree.) which is obtained by adding 45.degree. to 
the difference of angle (.theta.1-.theta.2). These signals 
sin(.theta.1-.theta.2), cos(.theta.1-.theta.2) and 
sin(.theta.1-.theta.2+45.degree.) are inputted to a signal processing 
circuit 7. 
As shown in FIG. 4, the calculation circuit 6 comprises multipliers MUL1 to 
MUL4, a subtracter SUB1 and adders ADD1 and ADD2. These operational 
devices may be constituted of analog circuits including operational 
amplifiers or of digital circuits including a microprocessor. The signal 
processing circuit 7 is connected to a limiter 8, and this limiter 8 is 
connected to a servo control circuit 9 for controlling a servomotor 10. 
The servomotor 10 is connected to a hydraulic pump 11 which is coupled to a 
hydraulic cylinder 14 through connecting tubes 12 and 13. The piston 14a 
of the hydraulic cylinder 14 is linked to a control lever 15a of a 
hydraulic pump 15 to control the direction and amount of the oil passing 
therethrough, the hydraulic pump 15 being connected to an electric motor 
16. The hydraulic pump 15 is coupled through connecting tubes 17 and 18 to 
a hydraulic motor 19 which is linked to a worm wheel 21 through a worm 20 
formed on a shaft 5, and the worm wheel 21 is connected to the rotary 
housing 2. 
The sinusoidal signal sin(.theta.1-.theta.2) inputted to the signal 
processing circuit 7 is supplied through a square wave circuit 22 to a 
+180.degree. crossover detection circuit 23 and to a -180.degree. 
crossover detection circuit 24, the square wave circuit 22 shaping the 
waveform of the signals inputted thereto into square waves. The 
+180.degree. crossover detection circuit 23 detects the positive to 
negative change of the polarity of the sinusoidal signal 
sin(.theta.1-.theta.2) when the difference of angle (.theta.1-.theta.2) 
becomes 180.degree. while the -180.degree. crossover detection circuit 24 
detects the negative to positive change of the polarity of the sinusoidal 
signal sin(.theta.1-.theta.2) when the difference of angle 
(.theta.1-.theta.2) becomes -180.degree.. The sinusoidal signal 
sin(.theta.1-.theta.2) is also supplied through contacts 25a of a signal 
switching circuit 25 to the limiter 8 which limits the amplitude of the 
inputted signal to a predetermined range. An output terminal of the 
+180.degree. crossover detection circuit 23 is connected to one input 
terminal of an AND circuit 26, while an output terminal of the 
-180.degree. crossover detection circuit 24 is connected to one input 
terminal of another AND circuit 27. And the cosine signal 
cos(.theta.1-.theta.2) inputted to the signal processing circuit 7 is 
supplied through a square wave circuit 28 to a polarity decision circuit 
29 which outputs a H level signal when the cosine signal 
cos(.theta.1-.theta.2) is negative. An output terminal of the polarity 
decision circuit 29 is connected to the other input terminal of the AND 
circuit 26 and to the other input terminal of the AND circuit 27. An 
output terminal of the AND circuit 26 is connected to a SET terminal of a 
flip-flop 30, while an output terminal of the AND circuit 27 is connected 
to a SET terminal of another flip-flop 31. The sinusoidal signal 
sin(.theta.1-.theta.2+45.degree.) inputted to the signal processing 
circuit 7 is supplied through a square wave circuit 32 to a flip-flop 
reset circuit 33. This flip-flop reset circuit 33 is connected to a RESET 
terminal of the flip-flop 30 and to a RESET terminal of the flip-flop 31. 
And an output terminal of the flip-flop 30 is connected to a + input 
terminal of an operational amplifier 34 and to one input terminal of an OR 
circuit 35. An output terminal of the flip-flop 31 is connected to a - 
input terminal of the operational amplifier 34 and to the other input 
terminal of the OR circuit 35. An output terminal of the operational 
amplifier 34 is connected through contacts 25b of the signal switching 
circuit 25 to the input terminal of the limiter 8, and an output terminal 
of the OR circuit 35 is connected to the .+-.180.degree. crossover 
switching circuit 36. This .+-.180.degree. crossover switching circuit 36 
closes the contacts 25b of the signal switching circuit 25 when a H level 
signal is inputted thereto, and closes the contacts 25a when a L level 
signal is inputted thereto. 
The operation of this system will now be described with reference to FIG. 
6. 
It is assumed that the steering handle 1 begins to be pivotally moved when 
the steering angle .theta.1 of the steering handle 1 and the follow-up 
angle .theta.2 of the rotary housing 2 are equal to each other. In this 
case, a difference of angle appears between the steering angle .theta.1 
and the follow-up angle .theta.2. It is also assumed that the difference 
of angle (.theta.1-.theta.2) becomes positive when the steering handle 1 
is pivoted clockwise and that the difference of angle (.theta.1-.theta.2) 
becomes negative when the steering handle 1 is pivoted counterclockwise. 
The output signals sin .theta.1 and cos .theta.1 of the angular position 
detector 3 and the output signals sin .theta.2 and cos .theta.2 of the 
angular position detector 4 are inputted to the calculation circuit 6 
which in turn executes a calculation using these signals to form three 
kinds of signals, i.e., the sinusoidal signals sin(.theta.1-.theta.2) and 
sin(.theta.1-.theta.2+45.degree.) and the cosine signal 
cos(.theta.1-.theta.2) (see FIG. 6). The waveforms of the signals 
sin(.theta.1-.theta.2), sin(.theta.1-.theta. +45.degree.) and 
cos(.theta.1-.theta.2) outputted from the calculation circuit 6 are shaped 
into square waves by the square wave circuits 22, 23 and 28, respectively. 
And in the case where the difference of angle (.theta.1-.theta.2) is 
within the range of between -180.degree. and +180.degree., i.e., 
-180.degree..ltoreq.(.theta.1-.theta.2).ltoreq.+180.degree., both of the 
+180.degree. crossover detection circuit 23 and -180.degree. crossover 
detection circuit 24 do not operate, so that the .+-.180.degree. crossover 
switching circuit 36 opens the contacts 25b of the signal switching 
circuit 36 and closes the contacts 25a of the same. As a result, the 
sinusoidal signal sin(.theta.1-.theta.2) is inputted to the limiter 8 
intactly. The output signal Sl of the limiter 8 is therefore a signal 
derived from the sinusoidal signal sin(.theta.1-.theta.2) with its 
amplitude limited to the values determined by the limiter 8. 
Next, when the difference of angle (.theta.1-.theta.2) exceeds 180.degree., 
the both output signals of the +180.degree. crossover detection circuit 23 
and the polarity decision circuit 29 go to H level, so that the flip-flop 
30 is brought into a set state. As a result, .+-.180.degree. crossover 
switching circuit 36 opens the contacts 25a of the signal switching 
circuit 25 and closes the contacts 25b of the same, so that a positive 
voltage obtained by amplifying the output of the flip-flop 30 in a 
non-inverting fashion by the operational amplifier 34 is fed to the input 
terminal of the limiter 8. And during this operation, the signal Sl is 
kept to a constant voltage level. Thus, the rotary housing 2 is rotated in 
accordance with the output signal Sl of the limiter 8. And when the 
difference of angle (.theta.1-.theta.2) is decreased to an extent that the 
polarity of the sinusoidal signal sin(.theta.1-.theta.2+45.degree.) is 
changed, that is to say, the difference of angle (.theta.1-.theta.2) 
becomes less than + 135.degree., the flip-flop 30 is brought into a reset 
state (see the waveform indicated by a dot and dash line in FIG. 6). 
Consequently, .+-.180.degree. crossover switching circuit 36 opens the 
contacts 25b of the signal switching circuit 25 and closes the contacts 
25a of the same, so that the sinusoidal signal sin(.theta.1-.theta.2) is 
intactly supplied to the input terminal of the limiter 8. 
When the difference of angle (.theta.1-.theta.2) becomes less than 
-180.degree., the both outputs of the -180.degree. crossover detection 
circuit 24 and the polarity decision circuit 29 go to H level, so that the 
flip-flop 31 is brought into a set state. As a result, .+-.180.degree. 
crossover switching circuit 36 opens the contacts 25a of the signal 
switching circuit 25 and closes the contacts 25b of the same, so that a 
negative voltage obtained by inversely amplifying the output of the 
flip-flop 31 by the operational amplifier 34 is applied to the input 
terminal of the limiter 8. Thus the rotary housing 2 is rotated in 
accordance with the signal Sl outputted from the limiter 8, the signal Sl 
being at a negative constant voltage. And when the difference of angle 
(.theta.1-.theta.2) is increased to an extent that the polarity of the 
sinusoidal signal sin(.theta.1-.theta.2+45.degree.) is changed, that is to 
say, the difference of angle (.theta.1-.theta.2) becomes greater than 
-45.degree., the flip-flop reset circuit 33 resets the flip-flop 31 (see 
the waveform indicated by the dot and dash line in FIG. 6), so that the 
.+-.180.degree. crossover switching circuit 36 opens the contacts 25b of 
the signal switching circuit 25 and closes contacts 25a of the same. As a 
result, the sinusoidal signal sin(.theta.1-.theta.2) is intactly supplied 
to the input terminal of the limiter 8. 
The signal thus inputted to the limiter 8 is outputted to the servo control 
circuit 9 as the signal Sl whose amplitude has been limited to 
predetermined levels by the limiter 8, as shown in FIG. 6. 
The servo control circuit 9 controls the servomotor 9 in accordance with 
the signal Sl, and the servomotor 9 controls the hydraulic motor 19 
through the hydraulic pump 11, the hydraulic cylinder 14 and the hydraulic 
pump 15 to rotate the worm 20 of the shaft 5 in accordance with its 
rotational movement. As a result, the worm wheel 21 rotates so that the 
rotary housing 2 of the Z-type propulsion apparatus begins to rotate in 
unison with the steering handle 1 in the direction of rotation thereof. 
And when the difference of angle (.theta.1-.theta.2) reaches 0 the rotary 
housing 2 stops. Thus, with the construction of this system, the signal 
processing circuit 7 can control the rotary housing 2 so as to rotate in 
unison with the steering handle 1 in the direction of rotation thereof on 
condition that the difference of angle (.theta.1-.theta.2) is within the 
range of between -225.degree. and +315.degree., i.e., 
-225.degree.&lt;(.theta.1-.theta.2)&lt;+315.degree.. 
With the construction of this system, if the steering handle is operated to 
command such a rapid rotation of the rotary housing 2, which is beyond the 
response characteristic of the mechanical system, that the difference of 
angle (.theta.1-.theta.2) becomes less than -225.degree. 
(-360.degree..ltoreq.(.theta.1-.theta.2).ltoreq.-225.degree.) or more than 
+315.degree. 
(+315.degree..ltoreq.(.theta.1-.theta.2).ltoreq.+360.degree.), the rotary 
housing rotates in the direction opposite to that of rotation of the 
steering handle 2. However, when it is desired to rotate the rotary 
housing by more than 225.degree., the steering handle is usually operated 
so that it rotates not in the forward direction beyond 180.degree. but in 
the reverse direction. Actually, it is rare to command the rotary housing 
to rotate by more than 225.degree., so that any significant problem will 
not be encountered. 
The sinusoidal signals sin(.theta.1-.theta.2) and 
sin(.theta.1-.theta.2+45.degree.) and the cosine signal 
cos(.theta.1-.theta.2) may alternatively be obtained by employing synchros 
and Scott transformers and removing the ac components from its output 
signals. These sinusoidal and cosine signals may also be obtained by 
employing resolvers and differential transformers or by employing rotary 
encoders or the like. 
FIG. 7 shows a second embodiment of the present invention in which like 
references denote same parts of the first embodiment of the invention. A 
calculation circuit 6a of this system is so constructed as to supply the 
sinusoidal signal sin(.theta.1-.theta.2) representing a sine of the 
difference of angle (.theta.1-.theta.2) between the angular position 
.theta.1 of the steering handle 1 and the angular position .theta.2 of the 
rotary housing 2 to a sample and hold circuit 38 of a signal processing 
circuit 7a and to supply the cosine signal cos(.theta.1-.theta.2) 
representing a cosine of the difference of angle (.theta.1-.theta.2) to a 
polarity decision circuit (comparator) 39 of the signal processing circuit 
7a. The polarity decision circuit 39 detects the polarity of the cosine 
signal cos(.theta.1-.theta.2) and feeds the detection result to the sample 
and hold circuit 38. The sample and hold circuit 38 is so constructed as 
to hold the inputted signal when the polarity decision circuit 39 detects 
a negative signal, and an output terminal of this sample and hold circuit 
38 is connected to the limiter 8. The limiter 8 is connected to the servo 
control circuit 9 for controlling the servomotor 10. 
The operation of this system will now be described with reference to FIG. 
8. 
When the steering handle 1 is pivotally moved, the calculation circuit 6a 
executes a calculation using the output signals sin .theta.1 and cos 
.theta.1 of the angular position detector 3 and the output signals sin 
.theta.2 and cos .theta.2 of the angular position detector 4 to form two 
kinds of signals, i.e., the sinusoidal signal sin(.theta.1-.theta.2) and 
the cosine signal cos(.theta.1-.theta.2), (see FIG. 8). The polarity 
decision circuit 39 detects the polarity of the cosine signal 
cos(.theta.1-.theta.2) outputted from the calculation circuit 6a and 
brings the sample and hold circuit 38 into a sample mode A when the cosine 
signal cos(.theta.1-.theta.2) is equal to or more than a predetermined 
value 0 (cos(.theta.1-.theta.2).gtoreq.0). As a result, the sinusoidal 
signal sin(.theta.1-.theta.2) outputted from the calculation circuit 6a to 
the sample and hold circuit 38 is intactly supplied to the limiter 8. The 
polarity decision circuit 39 brings the sample and hold circuit 38 into a 
hold mode B when the cosine signal cos(.theta.1-.theta.2) is less than 0 
(cos(.theta.1-.theta.2)&lt;0). As a result, a signal representative of the 
value sin(+90.degree.) is inputted to the limiter 8 when the difference of 
angle (.theta.1-.theta.2) is greater than 90.degree. 
((.theta.1-.theta.2)&gt;90.degree.), while a signal representative of the 
value sin(-90.degree.) is inputted to the limiter 8 when the difference of 
angle (.theta.1-.theta.2) is less than 90.degree. 
((.theta.1-.theta.2)&lt;90.degree.). An output signal Ss of the sample and 
hold circuit 38 is the sinusoidal signal sin(.theta.1-.theta.2) itself 
outputted from the calculation circuit 6a when the difference of angle 
(.theta.1-.theta.2) is between -90.degree. and +90.degree. 
(-90.degree..ltoreq.(.theta.1-.theta.2).ltoreq.+90.degree.). The signal Ss 
is a signal representative of the value sin(-90.degree.) when the 
difference of angle (.theta.1-.theta.2) is between -270.degree. and 
-90.degree. (-270.degree.&lt;(.theta.1-.theta.2)&lt;-90.degree.) and is a signal 
representative of the value sin(+90.degree.) when the difference of angle 
(.theta.1-.theta.2) is between +90.degree. and +270.degree. 
(+90.degree.&lt;(.theta.1-.theta.2)&lt;+270.degree.). The limiter 8 limits the 
amplitude of the output signal of the sample and hold circuit 38 to form a 
signal Sl shown in FIG. 8 and feeds it to the servo control circuit 9. The 
servo control circuit 9 controls the servomotor 10 in accordance with the 
signal Sl, and the servomotor 10 controls the hydraulic motor 19 through 
the hydraulic pump 11, the hydraulic cylinder 14 and the hydraulic pump 15 
to rotate the worm 20 of the shaft 5. As a result, the worm wheel 21 
rotates so that the rotary housing 2 of the Z-type propulsion apparatus 
begins to rotate in unison with the steering handle 1 in the direction of 
the rotation thereof. And when the difference of angle (.theta.1-.theta.2) 
reaches 0 the rotary housing 2 stops. Thus, the signal processing circuit 
7a can control the rotary housing 2 to rotate in unison with the steering 
handle 1 in the direction of rotation thereof on condition that the 
difference of angle (.theta.1-.theta.2) is within the range of between 
-270.degree. and +270.degree., i.e., 
-270.degree.&lt;(.theta.1-.theta.2)&lt;+270.degree.. 
In the above-described system, the polarity decision circuit 39 is so 
constructed as to hold and output the sinusoidal signal 
sin(.theta.1-.theta.2) when the cosine signal cos(.theta.1-.theta.2) is 
negative, however the circuit 39 may be modified so as to hold the 
sinusoidal signal sin(.theta.1-.theta.2) when the cosine signal 
cos(.theta.1-.theta.2) is less than a predetermined value other than 0. 
With the construction of this system, the sample and hold circuit 38 does 
not hold the sinusoidal signal sin(.theta.1-.theta.2) when the difference 
of angle (.theta.1-.theta.2) is between -360.degree. and -270.degree. 
(-360.degree..ltoreq.(.theta.1-.theta.2).ltoreq.-270.degree.) or when the 
difference of angle (.theta.1-.theta.2) is between +270.degree. and 
+360.degree. 
(+270.degree..ltoreq.(.theta.1-.theta.2).ltoreq.+360.degree.). Therefore, 
if the steering handle is operated to command such a rapid rotation of the 
rotary housing 2, which is beyond the response characteristic of the 
mechanical system, that the absolute value of the difference of angle 
(.theta.1-.theta.2) becomes greater than .+-.270.degree., the rotary 
housing 2 rotates in the direction opposite to that of rotation of the 
steering handle 1. However, when it is desired to rotate the rotary 
housing by more than .+-.270.degree., the steering handle is usually 
operated in such a manner that the rotary housing rotates not in the 
forward direction beyond 180.degree. but in the reverse direction. 
Actually, it is rare to command the rotary housing to rotate by more than 
.+-.270.degree., so that any significant problem will not be encountered. 
FIG. 9 shows a third embodiment of the invention in which like references 
denote same parts of the second invention. 
In the figure, the sinusoidal signal sin(.theta.1-.theta.2) outputted from 
the calculation circuit 6a is supplied to the sample and hold circuit 38 
of a signal processing circuit 7b and to one input terminal of a polarity 
comparator 40. The cosine signal cos(.theta.1-.theta.2) outputted from the 
circuit 6a is fed to the polarity decision circuit 39 of the signal 
processing circuit 7b. An output signal of the sample and hold circuit 38 
is supplied to the input terminal of the servo control circuit 9 and to 
the other input terminal of the polarity comparator 40. The polarity 
decision circuit 39 outputs a true signal when the cosine signal 
cos(.theta.1-.theta.2) is negative, and the polarity comparator 40 outputs 
a true signal when the two input signals are different in polarity to each 
other. And the output signals of the polarity decision circuit 39 and the 
polarity comparator 40 are logically added by an OR circuit 41, and the 
resultant signal is fed to the sample and hold circuit 38. 
When the output signal S1 of the OR circuit 41 is true, the sample and hold 
circuit 38 holds its input signal. And when the signal S1 is false, the 
circuit 38 samples its input and outputs it. 
The operation of this system will now be described with reference to FIG. 
12. When the difference of angle (.theta.1-.theta.2) is between 
-90.degree. and +90.degree. 
(-90.degree..ltoreq.(.theta.1-.theta.2).ltoreq.+90.degree.), the output of 
the polarity decision circuit 39 is not true. And in this case, the output 
of the polarity comparator 40 is also not true, so that the sample and 
hold circuit 38 is brought into the sample mode A to output the sinusoidal 
signal sin(.theta.1-.theta.2) intactly. 
When the difference of angle (.theta.1-.theta.2) exceeds +90.degree., the 
cosine signal cos(.theta.1-.theta.2) becomes less than 0, so that a true 
signal is outputted from the polarity decision circuit 39. As a result, 
the OR circuit 41 outputs the signal S1 (hold signal) to bring the sample 
and hold circuit 38 into the hold mode B, so that the circuit 38 outputs a 
signal representative of the value sin(+90.degree.), i.e., the sinusoidal 
signal sin(.theta.1-.theta.2) at the moment when the difference of angle 
(.theta.1-.theta.2) is equal to +90.degree.. The hold mode B is maintained 
so long as the difference of angle (.theta.1-.theta.2) is between 
+90.degree. and +270.degree. 
(+90.degree.&lt;(.theta.1-.theta.2)&lt;+270.degree.). If the difference of angle 
exceeds +270.degree., the cosine signal cos(.theta.1-.theta.2) becomes 
positive. In this case however, the polarities of the input and output 
signals of the sample and hold circuit 38 differs from each other, so that 
the polarity comparator 40 outputs a true signal, thereby the hold signal 
S1 being outputted from the OR circuit 41. As a result, the sample and 
hold circuit 38 is brought into the hold mode C to output the positive 
value sin(+90.degree.). The hold mode C is maintained so long as the 
difference of angle (.theta.1-.theta.2) is between +270.degree. and 
+360.degree. (+270.degree..ltoreq.(.theta.1-.theta.2)&lt;+360.degree.), since 
the polarity of the output of the sample and hold circuit 38 differs from 
that of the sinusoidal signal sin(.theta.1-.theta.2). 
If the difference of angle becomes less than -90.degree., the cosine signal 
cos(.theta.1-.theta.2) becomes less than 0 so that the polarity decision 
circuit 39 outputs a true signal. As a result, the OR circuit 41 outputs 
the hold signal S1, so that the sample and hold circuit 38 is brought into 
the hold mode B to output sin(-90.degree.), i.e., the value of the 
sinusoidal signal sin(.theta.1-.theta.2) at the moment when the difference 
of angle (.theta.1-.theta.2) is equal to -90.degree.. The hold mode B is 
maintained so long as the difference of angle (.theta.1-.theta.2) is 
between -270.degree. and -90.degree. 
(-270.degree.&lt;(.theta.1-.theta.2)&lt;-90.degree.). If the difference of angle 
(.theta.1-.theta.2) becomes less than -270.degree., the cosine signal 
cos(.theta.1-.theta.2) becomes positive. In this case however, the 
polarity comparator 40 outputs a true signal, since the polarities of the 
input and output of the sample and hold circuit 38 are different from each 
other. As a result, the OR circuit 41 outputs the signal S1, so that the 
sample and hold circuit 38 is brought into the hold mode C to output the 
negative value sin(-90.degree.). The hold mode is maintained so long as 
the difference of angle (.theta.1-.theta.2) is between -360.degree. and 
-270.degree. (-360.degree.&lt;(.theta.1-.theta.2).ltoreq.-270.degree.), since 
the polarities of the input and output of the sample and hold circuit 38 
differ from each other. 
Thus, with the construction of this system, the rotary housing 2 is 
controlled to rotate in unison with the steering handle 1 in the direction 
of rotation thereof on condition that the difference of angle 
(.theta.1-.theta.2) is within the range of .+-.360.degree.. In this 
system, the polarity decision circuit 39 may be modified so as to output a 
true signal when the cosine signal cos(.theta.1-.theta.2) is less than a 
predetermined value other than 0. 
FIG. 10 shows a fourth embodiment of the present invention. 
This system differs from the system shown in FIG. 9 in the following 
respects. In a signal processing circuit 7c of this system, the both 
signals at the input and output terminals of the sample and hold circuit 
38 are inputted to both input terminals of an error detection circuit 42. 
The error detection circuit 42 detects the difference (error) between the 
two signals inputted thereto and outputs the error to an error comparator 
43. The error comparator 43 is constructed in such a manner that it 
outputs a true signal when the output of the error detection circuit 42 is 
greater than a predetermined value (for example, 1 corresponding to the 
value sin 90.degree.). The output of the error comparator 43 is supplied 
to the other input terminal of the OR circuit 41. 
The operation of this system will now be described with reference to FIG. 
12. 
When the difference of angle (.theta.1-.theta.2) is within .+-.90.degree., 
the output of the polarity decision circuit 39 is not true. And in this 
case, the output of the error comparator 43 is also not true since the 
output of the error detector 42 is 0, so that the sample and hold circuit 
38 is brought into the sample mode A to output the sinusoidal signal 
sin(.theta.1-.theta.2) intactly. 
When the difference of angle (.theta.1-.theta.2) exceeds +90.degree., the 
cosine signal cos(.theta.1-.theta.2) becomes less than 0, so that a true 
signal is outputted from the polarity decision circuit 39. As a result, 
the OR circuit 41 outputs the signal S1 to bring the sample and hold 
circuit 38 into the hold mode B, so that the circuit 38 outputs a signal 
representing the value sin(+90.degree.), i.e., the sinusoidal signal 
sin(.theta.1-.theta.2) at the moment when the difference of angle 
(.theta.1-.theta.2) is equal to +90.degree.. The hold mode B is maintained 
so long as the difference of angle (.theta.1-.theta.2) is between 
+90.degree. and +270.degree. 
(+90.degree.&lt;(.theta.1-.theta.2)&lt;+270.degree.). If the difference of angle 
(.theta.1-.theta.2) exceeds +270.degree., the cosine signal 
cos(.theta.1-.theta.2) becomes positive. In this case however, the output 
(error) of the error detection circuit 42 is greater than the value 
predetermined at the error comparator 43 (it is assumed herein that the 
predetermined value is 1), so that the error comparator 43 outputs a true 
signal, thereby the hold signal S1 being outputted from the OR circuit 41. 
As a result, the sample and hold circuit 38 is brought into the hold mode 
C to output the positive value sin(+90.degree.). The hold mode C is 
maintained so long as the difference of angle (.theta.1-.theta.2) is 
between +270.degree. and +360.degree. 
(+270.degree..ltoreq.(.theta.1-.theta.2)&lt;+360.degree.), since the 
difference between the output and input of the sample and hold circuit 38 
is greater than the predetermined value (=1). 
If the difference of angle (.theta.1-.theta.2) becomes less than 
-90.degree., the cosine signal cos(.theta.1-.theta.2) becomes less than 0 
so that the polarity decision circuit 39 outputs a true signal. As a 
result, the OR circuit 41 outputs the hold signal S1, so that the sample 
and hold circuit 38 is brought into the hold mode B to output 
sin(-90.degree.), i.e., the value of the sinusoidal signal 
sin(.theta.1-.theta.2) at the moment when the difference of angle 
(.theta.1-.theta.2) is equal to -90.degree.. The hold mode B is maintained 
so long as the difference of angle (.theta.1-.theta.2) is between 
-270.degree. and -90.degree. 
(-270.degree.&lt;(.theta.1-.theta.2)&lt;-90.degree.). If the difference of angle 
(.theta.1-.theta.2) becomes less than -270.degree., the cosine signal 
cos(.theta.1-.theta.2) becomes positive. In this case however, the error 
comparator 43 outputs a true signal, since the output (error) of the error 
detection circuit 42 is greater than the value (=1) predetermined at the 
error comparator 43. As a result, the OR circuit 41 outputs the signal S1, 
so that the sample and hold circuit 38 is brought into the hold mode C to 
output the negative value sin(-90.degree.). The hold mode C is maintained 
so long as the difference of angle (.theta.1-.theta.2) is between 
-360.degree. and -270.degree. 
(-360.degree.&lt;(.theta.1-.theta.2).ltoreq.-270.degree.), since the 
difference between the output and input of the sample and hold circuit 38 
is greater than the predetermined value. 
Thus, with the construction of this system, the rotary housing 2 is 
controlled to rotate in unison with the steering handle 1 in the direction 
of rotation thereof on condition that the difference of angle 
(.theta.1-.theta.2) is within the range of .+-.360.degree.. 
Incidentally, a limiter circuit may be provided at the input terminal of 
the servo control circuit 9 to limit the amplitude of the output signal of 
the sample and hold circuit 38 to appropriate values. In the system 
described above, it is assumed that the value predetermined at the error 
comparator 43 is 1, however, the value may be increased or decreased from 
1 to expand or reduce the hold mode range. Further, the polarity decision 
circuit 39 may be modified so as to output a true signal when its input 
becomes less than a predetermined value other than 0. 
FIG. 11 shows a fifth embodiment of the present invention. 
A signal processing circuit 7d of this system shown in this figure differs 
from those of the second to fourth embodiments in the following respects. 
The sinusoidal signal sin(.theta.1-.theta.2) outputted from the calculation 
circuit 6a is supplied to the input terminal of the sample and hold 
circuit 38 and to an input terminal of a polarity decision circuit 44. The 
polarity decision circuit 44 outputs a signal S2 indicating whether the 
sinusoidal signal sin(.theta.1-.theta.2) is positive or negative. The 
cosine signal cos(.theta.1-.theta.2) outputted from the calculation 
circuit 6a is supplied to a polarity-inversion detection circuit 45. The 
polarity-inversion detection circuit 45 outputs a pulse signal P when a 
positive to negative or a negative to positive change of the polarity of 
the cosine signal cos(.theta.1-.theta.2) is detected. The pulse signal P 
is fed to a pulse generating circuit 46. When the pulse signal P is 
inputted to the the pulse generating circuit 46 with a signal S2 
representing a polarity of the sinusoidal signal sin(.theta.1-.theta.2) 
different from that represented by it when the preceding pulse signal P is 
generated, the circuit 46 outputs one of an increment pulse P1 and a 
decrement pulse P2 which is the same as the pulse precedingly outputted 
therefrom. The pulse generating circuit 46 outputs one of the increment 
and decrement pulses which is of the type different from that of the pulse 
precedingly outputted therefrom, when the pulse signal P is inputted 
thereto with the signal S2 representing the same polarity of the signal 
sin(.theta.1-.theta.2) as that represented by it when the preceding pulse 
signal P is generated. The increment and decrement pulses are fed to a 
counter circuit 47. The count value outputted from the counter circuit 47 
is supplied to a zero detection circuit 48 to decide whether the value is 
zero or not. This zero detection circuit 48 brings the sample and hold 
circuit 38 into a sample mode when the count value of the counter circuit 
47 is zero, while it brings the sample and hold circuit into a hold mode 
when the count value is other than 0. 
The operation of the above system will now be described with reference to 
FIG. 12. 
When the difference of angle (.theta.1-.theta.2) is within .+-.90.degree. 
the polarity of the cosine signal cos(.theta.1-.theta.2) does not change, 
so that the pulse signal P is not outputted. In this case, the count value 
of the counter circuit 47 is 0. And therefore, the sample and hold circuit 
38 is kept in the sample mode by the output of the zero detection circuit 
48 and outputs the sinusoidal signal intactly. 
Next, when the difference of angle (.theta.1-.theta.2) exceeds +90.degree., 
the cosine signal (.theta.1-.theta.2) changes its polarity from a positive 
to a negative state, as indicated by an arrow a in FIG. 12, so that pulse 
signal P is outputted from the polarity-inversion detection circuit 45. At 
this moment, the polarity decision circuit 44 outputs to the pulse 
generating circuit 46 signal S2 indicating that the sinusoidal signal 
sin(.theta.1-.theta.2) is positive. The pulse generating circuit 46 stores 
the state indicating that the sinusoidal signal sin(.theta.1-.theta.2) is 
positive and at the same time outputs increment pulse P1 to the counter 
circuit 47 since the contents of the counter circuit 47 is 0. As a result, 
the count value of the counter circuit 47 is incremented from 0 to +1, 
which causes the zero detection circuit 48 to output the hold signal to 
the sample and hold circuit 38. The sample and hold circuit 38 therefore 
holds and outputs (hold mode) sin(-90.degree.), i.e., the value of the 
sinusoidal signal sin(.theta.1-.theta.2) at the moment when the difference 
of angle (.theta.1-.theta.2) is equal to -90.degree.. Each time when the 
polarity of the cosine signal cos(.theta.1-.theta.2) changes, the pulse 
generating circuit 46 compares the polarity of the sinusoidal signal 
sin(.theta.1-.theta.2) at that moment with the polarity of the sinusoidal 
signal sin(.theta.1-.theta.2) stored by itself at the moment of the 
preceding change of the polarity of the cosine signal 
cos(.theta.1-.theta.2). And if the both polarities differ from each other, 
the pulse generating circuit 46 outputs to the counter circuit 47 the same 
kind of pulse signal (pulse signal in the same count direction) as that 
precedingly outputted therefrom, the pulse signal being an incremental 
pulse signal or a decremental pulse signal. On the other hand, if the both 
polarities coincide with each other, the pulse generating circuit 46 
outputs a pulse signal in the different count direction to the counter 
circuit 47. As a result, the contents of the counter circuit 47 is 
incremented or decremented by the pulse signal. 
When the difference of angle (.theta.1-.theta.2) becomes less than 
-90.degree., the polarity of the cosine signal cos(.theta.1-.theta.2) 
changes from positive to negative as indicated by an arrow c in FIG. 12, 
so that the polarity-inversion circuit 45 outputs the pulse signal P. At 
this time, the polarity decision circuit 44 outputs a signal indicating 
that the sinusoidal signal sin(.theta.1-.theta.2) is negative. The pulse 
generating circuit 46 stores the negative polarity of the sinusoidal 
signal sin(.theta.1-.theta.2), and outputs the increment pulse P1 to the 
counter circuit 47 since the contents of the counter circuit 47 is zero. 
Consequently, the contents of the counter circuit 47 is incremented from 0 
to +1, so that the zero detection circuit 48 outputs the holding signal to 
the sample and hold circuit 38. As a result, the sample and hold circuit 
38 holds and outputs (hold mode) sin(-90.degree.), i.e., the value of the 
sinusoidal signal sin(.theta.1-.theta.2) at the moment when the difference 
of angle (.theta.1-.theta.2) is equal to -90.degree.. In this embodiment, 
although the counter circuit 47 is supplied with an increment pulse to 
increment the contents thereof from 0 to 1 when the difference of angle 
(.theta.1-.theta.2) becomes less than -90.degree., this system may be 
modified so that when the difference of angle (.theta.1-.theta.2) becomes 
less than -90.degree. decrement pulse P2 is supplied to the counter 
circuit 47 to decrement its contents from 0 to -1. 
Next, the case where the steering handle is abruptly rotated clockwise by 
+300.degree. will be described in detail. 
In the beginning of a clockwise rotation of the steering handle 1, the 
follow-up angle .theta.2 remains 0 since the follow-up operation has not 
yet been commenced, so that only the steering angle .theta.1 begins to 
increase. And when the steering angle .theta.1 exceeds +90.degree. the 
difference of angle (.theta.1-.theta.2) exceeds +90.degree., so that the 
polarity of the cosine signal cos(.theta.1-.theta.2) is changed from a 
positive to a negative state. At this moment, the polarity of the 
sinusoidal signal sin(.theta.1-.theta.2) is positive, and the contents of 
the counter circuit 47 is 0. The pulse generator 46 therefore memorizes 
the positive polarity of the sinusoidal signal sin(.theta.1-.theta.2) and 
outputs increment pulse P1 to increment the contents of the counter 
circuit 47 from 0 to +1. As a result, the sample and hold circuit 38, 
which has outputted the sinusoidal signal sin(.theta.1-.theta.2) intactly 
(sample mode) until then, holds and outputs (hold mode) the value 
sin(+90.degree.). And when the steering angle .theta.1 exceeds 
+270.degree., the difference of angle (.theta.1-.theta.2) also exceeds 
+270.degree., so that the polarity of the cosine signal 
cos(.theta.1-.theta.2) is changed from a negative to a positive state. At 
this moment, the polarity of the sinusoidal signal sin(.theta.1-.theta.2) 
is negative, and the pulse generating circuit 46 compares this polarity 
with the polarity of the signal sin(.theta.1-.theta.2) precedingly stored 
thereinto. The polarity precedingly stored is positive and the present 
polarity is negative, so that the pulse generating circuit 46 stores the 
present polarity thereinto and outputs the same kind of pulse signal 
(increment pulse P1) as that previously outputted therefrom. As a result, 
the count value of the counter circuit 47 is incremented from +1 to +2. 
The resultant count value of the counter circuit 47 is not 0, so that the 
sample and hold circuit 38 continues to output the value sin(+90.degree.). 
When the steering angle .theta.1 becomes +300.degree., the follow-up 
operation is commenced, so that the follow-up angle .theta.2 begins to 
increase, thereby the difference of angle (.theta.1-.theta.2) being 
decreased. And when the difference of angle (.theta.1-.theta.2) becomes 
less than +270.degree., the polarity of the cosine signal 
cos(.theta.1-.theta.2) is changed from a positive to a negative state. At 
this time, the polarity of the sinusoidal signal sin(.theta.1-.theta.2) is 
negative, so that the pulse generating circuit 46 stores the present 
polarity of the signal sin(.theta.1- .theta.2) and outputs a pulse signal 
(decrement pulse P2) which is of the kind different from that of the pulse 
signal precedingly outputted therefrom. As a result, the count value of 
the counter circuit 47 is decremented from +2 to +1. The sample and hold 
circuit 38 therefore continues to output the value sin(+90.degree.). When 
the difference of angle (.theta.1-.theta.2) is decreased to less than 
+90.degree., the polarity of the cosine signal cos(.theta.1-.theta.2) is 
changed from a negative to a positive state. The polarity of the 
sinusoidal signal sin(.theta.1-.theta.2) at this time is positive and the 
polarity of the signal sin(.theta.1-.theta.2) precedingly stored is 
negative, the pulse generating circuit 46 therefore outputs the same kind 
of pulse signal (decrement pulse P2) as that precedingly outputted 
therefrom. As a result, the count value of the counter circuit 47 is 
decreased from +1 to 0, so that the sample and hold circuit 38 is released 
from its hold mode and outputs the sinusoidal signal 
sin(.theta.1-.theta.2) intactly (sample mode). And when the difference of 
angle (.theta.1-.theta.2) reaches 0, the follow-up operation is completed. 
With the construction of the above-described system, even when the 
difference of angle (.theta.1-.theta.2) is varied in any way, for example, 
even when the difference of angle (.theta.1-.theta.2) exceeds 
+360.degree., the rotary housing 2 is controlled to rotate in unison with 
the steering handle in the direction of rotation thereof. And if the 
counting capacity of the counter circuit 47 is increased, the rotary 
housing 2 can be controlled to rotate a plurality of revolutions in unison 
with the steering handle. The system may also be modified by setting the 
counting capacity to an appropriate value so as to restrict the maximum 
number of its revolutions to a specific value. In addition, the ratio of 
the steering angle .theta.1 to the follow-up angle .theta.2 is not limited 
to 1:1 but can be changed to 1:n or n:1 to enhance the accuracy of the 
operation. Further, the construction for effecting the counting operation 
at the counter circuit 47 may be modified so that the polarity of the 
cosine signal cos(.theta.1-.theta.2) immediately after a change of the 
polarity of the same is compared with that of the sinusoidal signal 
sin(.theta.1-.theta.2) to decide which pulse should be generated an 
increment pulse or a decrement pulse. In this case, the pulse generating 
circuit 46 is modified so that it outputs a decrement pulse to the counter 
circuit 47 to decrement its contents by one when the two polarities 
coincide with each other and that the circuit 46 outputs an increment 
pulse to the counter circuit 47 to increment its contents by one when the 
two polarities are different from each other. 
APPLICABILITY TO INDUSTRIES 
The rotation control system according to the present invention is 
particularly suitable for controlling the rotation of a rotary housing 
mounting a propeller unit of a Z-type propulsion apparatus mounted on a 
vessel such as a tug boat which is required to be steered in a rapid 
manner and to make a small turn.