EP2905219B1

EP2905219B1 - Turning control device for ship propulsion device

Info

Publication number: EP2905219B1
Application number: EP13844375.9A
Authority: EP
Inventors: Masanori Kodera; Shinichi Tanaka; Koji Takei; Koichi Shiraishi; Kazuhisa Higuchi
Original assignee: Niigata Power Systems Co Ltd
Current assignee: Niigata Power Systems Co Ltd
Priority date: 2012-10-05
Filing date: 2013-04-10
Publication date: 2017-09-27
Anticipated expiration: 2033-04-10
Also published as: EP2905219A4; NO2905219T3; EP2905219A1; ES2642405T3; JP6395996B2; WO2014054304A1; JP2014073783A

Description

The present invention relates to a turning control unit as a steering system providing turning control of a marine propulsion machine which is equipped with propulsion function and steering function and known collectively as an azimuth thruster or the like, such as Z-type propulsion system, L-type propulsion system and pod propulsion system. Particularly, the present invention relates to an electric turning control unit which includes a plurality of AC servo motors sharing the load equally and which can keep operating smoothly in the event of failure of any of the AC servo motors.

BACKGROUND ART

Heretofore, a hydraulic turning apparatus has been commonly used in turning maneuver by way of the azimuth thruster such as the Z-type propulsion system or pod propulsion system. The conventional hydraulic turning apparatus has a complicated structure where hydraulic devices including a hydraulic pump, a servo valve, a suction filter, an oil tank and the like are interconnected by means of piping. The hydraulic turning apparatus also suffers fouling and the like due to oil leakage. Whenever failure occurs, the apparatus requires repair or maintenance work for de-airing of filling oil and the like, which may sometimes pose an obstacle to stable operation of the devices.
In order to eliminate the complexity of the structure and the complication of the maintenance work, the present applicants have worked with utmost effort to invent the turning control unit which employs an electric motor instead of hydraulic pressure for turning the azimuth thruster. The following Patent Literature 1 discloses a turning control unit for azimuth thruster proposed by the present applicants. An object of the disclosed invention is to provide the turning control unit which is less likely to lose control by way of the AC servomotors and has high following accuracy. According to this invention, a turning control board calculates a motor speed command based on a deviation between a handle signal from an operating handle and a feedback signal from a sensor and simultaneously transmits a calculation result as the same digital signal to a plurality of AC servo amplifiers via digital communications.
Fig. 5 is a block diagram showing a configuration of this servo amplifier 100. A motor-speed reading transmitted from the preceding turning control board is transmitted to a subsequent stage via a limiter 101 supplying a speed limit. The AC servo motor 102 is provided with a sensor 103 for measuring the motor speed. The feedback signal from this sensor 103 is converted to a voltage signal by a frequency to voltage converter 104. A comparator 105 calculates a deviation between the motor-speed reading supplied via the limiter 101 and the feedback signal converted to the voltage signal. A control unit (PID regulator) 106 calculates the motor speed command value according to the deviation. This motor speed command value is supplied to a current amplifier 108 via a limiter 107 and amplified by the current amplifier 108. The resultant command value is supplied to the AC servo motor 102.
Even though assembled with plural pairs of AC servo amplifiers 100 and AC servo motors 102, this turning control unit is adapted to control each pair of AC servo amplifier and AC servo motor independently. If any of the AC servo amplifiers 100 should fail, the control unit as a whole does not lose control.

Citation List

Patent Literature

Patent Literature 1: JP-A No. 2010-58741
WO 02/47973 relates to a steering propeller which can be rotated via two electric control motors.

SUMMARY OF THE INVENTION

Technical Problem

According to the invention proposed by the present applicants in the above Patent Literature 1, respective pinions P (P1, P2) of the plural AC servo motors mesh with a circumferential traversing gear G from inside, as shown in Fig. 6. The traversing gear G is disposed in a turning drive mechanism of the azimuth thruster. The turning drive mechanism is constructed such that a driving force of each AC servo motor 102 is transmitted to the traversing gear G via each pinion P so as to turn the azimuth thruster. As shown in Fig. 6, however, backlash exists between the pinion P driven by the AC servo motor and the traversing gear G of the azimuth thruster. Furthermore, each of the plural AC servo motors 102 is independently subjected to speed control. In some cases, therefore, much of the load may be applied to a particular one of the AC servo motors depending upon whether an external water flow exerts a back force or a following force on the azimuth thruster. This is an undesirable matter. In Fig. 6, the leftward pinion P2 has backlash and hence, is practically in an unloaded condition.
The present invention addresses the above-described problem of the prior art and has an object to provide an electric turning control unit including a plurality of AC servo motors which allows the individual AC servo motors to share the load equally and can continue smooth operation if any of the AC servo motors should fail.

Solution to Problem

In a turning control unit for marine propulsion machine according to claim 1, a turning control unit for controlling a marine propulsion machine which includes a rotatably driven propeller and is turnably mounted to a marine vessel for setting a propulsive direction, includes:

an operating handle arranged to output a handle signal indicative of a turning position of the marine propulsion machine by setting the turning position thereof;
a sensor arranged to detect the turning position of the marine propulsion machine and output a feedback signal;
control means arranged to calculate a deviation between the handle signal outputted from the operating handle and the feedback signal outputted from the sensor, the control means further arranged to calculate a motor-speed reading according to the deviation, and simultaneously transmit the motor-speed reading as the same digital signal to a plurality of objects via digital communications;
a plurality of AC servo amplifiers arranged to each receive the motor-speed reading simultaneously transmitted as the same digital signal from the control means via the digital communications and output a motor speed command value according to the motor-speed reading; and
a plurality of AC servo motors each of which are arranged to be driven in response to the motor speed command value from each of the AC servo amplifiers so as to turn the marine propulsion machine, characterized in that each AC servo amplifier includes load calculating means for calculating a load amount from a value of current through the AC servo motor;
wherein each AC servo amplifier is arranged to calculate and output the motor speed command value by correcting the motor-speed reading transmitted from the control means according to a load amount of the corresponding AC servo motor;
wherein each AC servo amplifier is arranged to perform a calculation for reducing the motor speed command value according to the load amount when the load of the AC servo motor is characteristic of power running; and wherein each AC server amplifier is arranged to perform a calculation for increasing the motor speed command value according to the load amount when the load of the AC servo motor is characteristic of regenerative running.

In the turning control unit for marine propulsion machine according to claim 1, a turning control unit for marine propulsion machine according to claim 2 has a structure wherein when the load amount of the AC servo motor is equal to or less than a predetermined fixed value; and wherein each AC servo amplifier is arranged to calculate and output the motor speed command value without correcting the motor-speed reading according the load amount of the AC servo motor.
In the turning control unit for marine propulsion machine according to any one of claims 1 to 2, a turning control unit for marine propulsion machine according to claim 3 has a structure wherein each AC servo amplifier includes:

a correction calculating part arranged to perform a necessary calculation on the motor-speed reading transmitted from the control means according to the load amount of the AC servo motor calculated by the load calculating means, and arranged to output the calculation result as a corrected value; and
second control means which are arranged to calculate a deviation between the corrected value outputted from the correction calculating part and the feedback signal outputted from the sensor for measuring the motor speed of the AC servo motor, the arrangement being such that it calculates the motor speed command value according to the deviation, and outputs the calculation result.

Effect of the Invention

According to the turning control unit for marine propulsion machine according to claim 1, the control means calculates the motor-speed reading based on the deviation between the handle signal from the operating handle and the feedback signal from the sensor and simultaneously transmits the motor-speed reading as the same digital signal to the plural AC servo amplifiers via digital communications. The AC servo amplifier corrects this motor-speed reading according to the load amount of the corresponding AC servo motor and thereby outputs the corrected value as the motor speed command value to the AC servo motor. Therefore, the respective loads on the plural AC servo motors can be made uniform to a value given by dividing the total load by the number of AC servo motors. Further, each and every one of the plural AC servo motors can be independently controlled, negating the need for communications between the motors as performed in a master-slave system control. If any of the AC servo amplifiers should fail, load balance can be kept by cutting off the faulty motor followed by driving the machine with the remaining normal motor(s). Hence, the control unit as a whole is less likely to lose control and can ensure stable operation. Providing control based on the plural AC servo motors, the control unit can be widely applied to large and small turning-type marine propulsion machines. The control unit also features high following accuracy without influence of offset such as suffered by an analog type controller.
According to the turning control unit for marine propulsion machine according to claim 1, when the external water flow exerts the back force on the turnably driven marine propulsion machine, a condition called power running where the marine propulsion machine is turned by the driving force of the AC servo motor is the primary state concerning the drive of the AC servo motor against the external load. In this case, the faster AC servo motor bringing the pinion into contact with the traversing gear first is subjected to the greater load. In order to disperse the load, therefore, correction is made such that the speed of the AC servo motor under the greater load is reduced in proportion to the load. This allows for the increase in the load on the other AC servo motor(s) so that the loads of the plural AC servo motors become uniform as a whole.
When the external water flow exerts the following force on the turnably driven marine propulsion machine, a condition called regenerative running where the marine propulsion machine is turned by the external water flow is the primary state concerning the drive of the AC servo motor against the external load. In this case, the slower AC servo motor bringing the pinion into contact with the traversing gear last is subjected to the greater load. In order to disperse the load, therefore, correction is made such that the speed of this AC servo motor under the greater load is increased in proportion to the load. This allows for the increase in the load on the other AC servo motor(s) so that the loads of the plural AC servo motors become uniform as a whole.
According to the turning control unit for marine propulsion machine according to claim 2, when the load of the AC servo motor is equal to or less than the predetermined fixed value negating the need for the correction, the AC servo amplifier does not correct the motor-speed reading transmitted from the control means, but can calculate the motor speed command value using the speed feedback signal and output the motor speed command value to the AC servo motor.
According to the turning control unit for marine propulsion machine according to claim 3, the correction calculating part outputs the corrected value by correcting the motor-speed reading from the control means with the load amount of the AC servo motor acquired by the load calculating means. The second control means calculates the deviation between the feedback signal from the motor speed sensor and the corrected value, calculates the motor speed command value according to the deviation and outputs the calculation result to each AC servo motor. Therefore, the control unit can provide control to make the loads of the plural AC servo motors uniform, thus reliably achieving the effect offered by the invention according to claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

[Fig. 1] Fig. 1 is a block flow diagram of a turning control unit for a marine propulsion machine according to an embodiment of the present invention, showing an outline of the control structure.
[Fig. 2] Fig. 2 is the block flow diagram of the turning control unit of the above embodiment, showing the outline of the control structure in combination with a turning mechanism and the like of an azimuth thruster as a controlled object.
[Fig. 3] Fig. 3 is a block flow diagram showing detailed structures of an AC servo amplifier and the like in the turning control unit of the above embodiment.
[Fig. 4] Fig. 4 is a group of graphs each showing a characteristic for correction of the motor-speed reading by the turning control unit of the above embodiment in terms of relation between the load amount of motor and the rotation speed of motor.
[Fig. 5] Fig. 5 is a block flow diagram showing detailed structures of an AC servo amplifier and the like according to the prior art.
[Fig. 6] Fig. 6 is a diagram showing a turning drive mechanism of an electric turning control unit employing a plurality of motors, and particularly showing a state where there is backlash between a pinion driven by the motor and a traversing gear of the azimuth thruster.

MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention is described with reference to Fig. 1 to Fig. 4.
Fig. 1 is a block flow diagram of a turning control unit 2 for a marine propulsion machine 1 according to one embodiment of the present invention, showing an outline of the control structure.
As shown in Fig. 1, the marine propulsion machine 1 includes: a casing 3 turnably projecting from an outside of a bottom of an unillustrated marine vessel; and a propeller 4 mounted in the casing 3 and rotatably driven as connected to an unillustrated main engine. The turning control unit 2 of the present embodiment is used for setting the marine propulsion machine 1 to a desired turning position by turning the marine propulsion machine by a desired angle, so as to arbitrarily set a propulsive direction of the marine propulsion machine 1. The following description is made on each of the components of this turning control unit 2.
An operating handle 5 disposed at a wheel house of the marine vessel is a device that a member of the vessel's crew operates so as to set the turning position of the marine propulsion machine 1 as a target. Outputted from the operating handle 5 so operated is a handle signal indicating a to-be-set turning position of the marine propulsion machine 1.
On the other hand, the marine propulsion machine 1 is provided with an angle sensor 6 which outputs a feedback signal by detecting an actual turning position of the marine propulsion machine 1.
In a turning control board 7 as first control means, the handle signal outputted from the operating handle 5 and the feedback signal from the angle sensor 6 are digitized by an A/D converter 8, and a deviation between these signals is calculated by a CPU 9.
The CPU 9 includes: a ROM 10 storing a control program and a variety of data items necessary for the control, or specifically data indicating relation between the above-described deviation and motor-speed reading and the like; and a RAM 11 allowing a variety of data items to be read therefrom or written thereto on an as-needed basis. It is noted that the motor-speed reading is a numerical value (signal) indicating the motor rotation direction and the rotational speed of the motor. The motor-speed reading is maintained constant in case that the rotational speed does not exceed the maximum capacity of AC servo motors M1, M2 (hereinafter, also represented or referred to as "motor" in some cases) when the above-described deviation in either of the polarities of "+" and "-" increases above a certain limit.
Using the calculated deviation and the data stored in the ROM 10, the CPU 9 calculates the motor-speed reading according the control program stored in the ROM 10. This motor-speed reading is converted to a digital signal by a serial signal generating part 12 (SIO) as a communication IC. The resultant signal is outputted to each of the plural external (two in this embodiment) AC servo amplifiers A1, A2 (hereinafter, also represented or referred to as "servo amplifier" in some cases) via two drivers 13a, 13b connected in parallel to the serial signal generating part 12 (SIO) based on a digital communications system conforming to RS-422 Standard.
The digital motor-speed reading outputted from the turning control board 7 is inputted to each of the two AC servo amplifiers A1, A2 disposed externally of the turning control board 7. These two AC servo amplifiers A1, A2 are independent from each other. Upon receiving the motor-speed reading, the two AC servo amplifiers A1, A2 provide control by sending the motor-speed reading to the respectively corresponding AC servo motors M1, M2.
The AC servo amplifiers A1, A2 are each provided with an alarm switch 14 as alarm means for alerting malfunction of the relevant AC servo amplifier A1, A2 to the turning control board 7. In the event of a malfunction of the AC servo amplifier A1, A2, either of the alarm switches 14, 14 that corresponds to the relevant AC servo amplifier A1, A2 sends alarm (contact signal) to the turning control board 7. In response to this, the turning control board 7 turns OFF either of the servo ON/OFF switches 15, 15 disposed in one-on-one correspondence to the AC servo amplifiers A1, A2. Thus, the turning control board 7 places the servo amplifier A1 or A2 sending the alarm in the servo OFF state and thereby excludes the AC servo amplifier A1 or A2 from the controlled object.
Fig. 2 is a block flow diagram of the turning control unit 2 according to the present embodiment, showing an outline of the control structure previously described with reference to Fig. 1 as well as a turning mechanism and the like of an azimuth thruster as the controlled object.
A traversing gear G shown in Fig. 2 is coaxially fixed to an upper end of the above-described casing 3 turnably mounted to the outside of the bottom of the unillustrated marine vessel. Pinions P1, P2 mounted to respective drive shafts of the above-described AC servo motors M1, M2 are meshed with internal teeth of the traversing gear G. Another pinion P3 spaced from the respective pinions P1, P2 of these AC servo motors A1, A2 is mounted to an input shaft of the above-described angle sensor 6. The pinion P3 follows the turning motion of the traversing gear G so that the feedback signal indicative of the actual turning position of the marine propulsion machine 1 is outputted from the angle sensor 6 and inputted to the turning control board 7.
Fig. 3 is a block flow diagram showing detailed structures of the AC servo amplifier A and the AC servo motor M in the turning control unit 2 of this embodiment.
The AC servo amplifier A is provided with a correction calculating part 20 for correcting the motor-speed reading transmitted from the turning control board 7. This correction calculating part 20 has a function to perform a calculation for correcting the motor-speed reading from the turning control board 7 according to the load amount of the AC servo motor M and to output the correction result as the motor speed command value. Therefore, the servo amplifier A includes load calculating means 22 which calculates the load amount of the relevant motor M by acquiring the value of current through the AC servo motor M from a current amplifier 21 outputting the motor speed command value finally supplied to the motor M. This load calculating means 22 is connected to the correction calculating part 20. The details of the correction calculation performed by the correction calculating part 20 will be described hereinafter with reference to Fig. 4.
The motor speed command value outputted from the correction calculating part 20 is transmitted to a subsequent comparator 24 via a limiter 23 applying a speed limit. On the other hand, the AC servo motor M is provided with a sensor S for measuring the motor speed. A speed feedback signal from this sensor S is converted to a voltage signal by a frequency to voltage converter 25 before it is supplied to the above-described comparator 24. The comparator 24 in turn calculates a deviation between the motor speed command value supplied via the limiter 23 and the feedback signal converted to the voltage signal. A control part 26 (PID regulator) as second control means calculates the motor speed command value according to the resultant deviation. After going through a process by a limiter 27, this motor speed command value is amplified by the above-described current amplifier 21 and then is supplied to the AC servo motor M.
The details of the correction calculation by the above-described correction calculating part 20 are described with reference to Fig. 4. Fig. 4 is a group of graphs each showing a characteristic for correction of the motor speed command value (the characteristic in correcting the motor-speed reading to the motor correction command value according to the load amount of the motor M) in terms of relation between the load amount of the motor M (abscissa) and rotation speed of the motor (ordinate). The correction calculating part 20 of the servo amplifier A has the correction characteristic.
In a case where an external water flow exerts a back force on the marine propulsion machine 1, a condition called power running where the marine propulsion machine 1 is turned by the driving force of the AC servo motor M is the primary state. In this case, as shown in Fig. 6, the rightward pinion P1, as seen in the figure, which is the first to come into contact with the traversing gear G and the faster AC servo motor M allowing this pinion P1 to exert the driving force are subjected to the greater load. The way the load is applied to the leftward pinion P2, as seen in the figure, and its relevant AC servo motor M varies according to the condition of contact with the gear. In order to disperse and make the load on the individual motors M uniform, correction is made such that the speed of the AC servo motor subjected to the greater load is reduced in proportion to the load as illustrated by Fig. 4(a) (characteristic during power running). The characteristic of the motor speed command value during the power running as shown in the graph of Fig. 4(a) can be expressed in the following equation (1): $Motor speed command value = motor - speed reading \times \{1 - A \times (load amount / rated load)\}$
where 'A' denotes the constant during power running; the motor speed command value is an arithmetic instruction in the AC servo amplifier; and the motor-speed reading is the motor-speed reading outputted from the turning control board 7.
In this manner, the load of the other AC servo motor M is increased so that the loads on the plural AC servo motors M are made uniform as a whole.
In a case where the external water flow exerts a following force on the marine propulsion machine, a condition called regenerative running where the marine propulsion machine 1 is turned by the external water flow is the primary state. In this case, as shown in Fig. 6, the leftward pinion P2, as seen in the figure, which is the last to come into contact with the traversing gear G and the slower AC servo motor M driving this pinion P2 are subjected to the greater load. In order to disperse the load, correction is made such that the speed of this AC servo motor subjected to the greater load is increased in proportion to the load as illustrated by Fig. 4(b) (characteristic during regenerative running). The characteristic of the motor speed command value during the regenerative running as shown in the graph of Fig. 4(b) can be expressed in the following equation (2): $Motor speed command value = motor - speed reading \times \{1 + B \times (load amount / rated load)\}$
where B denotes the constant during regenerative running; the motor speed command value is an arithmetic instruction in the AC servo amplifier; and the motor-speed reading is the motor-speed reading outputted from the turning control board 7.
In this manner, the load of the other AC servo motor M is increased so that the loads on the plural AC servo motors M are made uniform as a whole.
In a case where the load of the AC servo motor M is equal to or less than a predetermined fixed value negating the need for the correction, the AC servo amplifier A does not commit the motor-speed reading transmitted from the turning control board 7 to the correction calculating part 20 for correction but can calculate the motor speed command value by directly applying the speed feedback signal from the sensor S to the control part 26 so as to output the resultant motor speed command value to the AC servo motor M via the current amplifier 21 as illustrated by Fig. 4C (characteristic during running with load of fixed value or less). The motor speed command value during running with load of fixed value or less as shown in the graph of Fig. 4C can be expressed in the following equation (3): $Motor speed command value = motor - speed reading$
where the motor speed command value is an arithmetic instruction in the AC servo amplifier; and the motor-speed reading is the motor-speed reading outputted from the turning control board 7.
According to the turning control unit 2 of this embodiment, as described above, the correction calculation is performed for correcting the motor-speed reading outputted from the turning control board 7 as a host controller by increasing or reducing the motor-speed reading according to the value proportional to the actual amount of load on each motor M and then, the resultant value is used as the motor speed command value to be supplied to the motor M. Specifically, control logic to reduce the motor speed command value in proportion to the load on the motor M when the load is characteristic of the power running, to increase the motor speed command value in proportion to the load on the motor M when the load is characteristic of the regenerative running, or to directly output the speed reading from the turning control board 7 as the speed command value when the load amount is equal to or less than the fixed value and the load is characteristic of the power running or regenerative running is incorporated in each of the AC servo amplifiers A. Therefore, the respective loads on the plural AC servo motors M can be made uniform to the value given by dividing the total load by the number of the AC servo motors M.
The plural AC servo amplifiers A are each independently connected to the turning control board 7 and simultaneously receive the motor-speed reading as the same digital signal from the turning control board 7 via digital communications. The plural AC servo amplifiers A are not in a master-slave relationship and each and every one of them is adapted for independent control. Unlike a control based on the master-slave system, therefore, the control unit does not require communications between the AC servo amplifiers. If any of the AC servo amplifiers A should fail, the load balance can be kept by cutting off the faulty motor M by means of the alarm switch 14 and servo ON/OFF switch 15 followed by driving the machine with the remaining normal motor M. Hence, the control unit as a whole is less likely to lose control and can ensure stable operation.
Providing control based on the plural AC servo motors, the control unit can be widely applied to large and small turning-type marine propulsion machines. The control unit also features high following accuracy without influence of offset such as suffered by an analog type controller.

Reference Signs list

1: MARINE PROPULSION MACHINE
2: TURNING CONTROL UNIT
5: OPERATING HANDLE
6: ANGLE SENSOR
7: TURNING CONTROL BOARD AS FIRST CONTROL MEANS
13a,b: DRIVER
20: CORRECTION CALCULATING PART
22: LOAD CALCULATING MEANS
26: CONTROL PART AS SECOND CONTROL MEANS
M(M1,M2): AC SERVO MOTOR (MOTOR)
A(A1,A2): AC SERVO AMPLIFIER (AMPLIFIER)
S: SPEED DETECTION SENSOR MOUNTED TO MOTOR

Claims

A turning control unit (2) for controlling a marine propulsion machine (1) which includes a rotatably driven propeller (4) and is turnably mounted to a marine vessel for setting a propulsive direction, the unit comprising:
an operating handle (5) arranged to output a handle signal indicative of a turning position of the marine propulsion machine (1) by setting the turning position thereof;

a sensor (6) arranged to detect the turning position of the marine propulsion machine (1) and output a feedback signal;

control means (7) arranged to calculate a deviation between the handle signal outputted from the operating handle (5) and the feedback signal outputted from the sensor (6), the control means (7) further arranged to calculate a motor-speed reading according to the deviation, and simultaneously transmit the motor-speed reading as the same digital signal to a plurality of objects via digital communications;

a plurality of AC servo amplifiers (A1, A2) arranged to each receive the motor-speed reading simultaneously transmitted as the same digital signal from the control means (7) via the digital communications and output a motor speed command value according to the motor-speed reading;

a plurality of AC servo motors (M) each of which are arranged to be driven in response to the motor speed command value from each of the AC servo amplifiers (A1, A2) so as to turn the marine propulsion machine (1);

characterized in that each AC servo amplifier (A1, A2) includes load calculating means (22) for calculating a load amount from a value of current through the AC servo motor (M);

wherein each AC servo amplifier (A1, A2) is arranged to calculate and output the motor speed command value by correcting the motor-speed reading transmitted from the control means (7) according to a load amount of the corresponding AC servo motor (M);

wherein each AC servo amplifier (A1, A2) is arranged to perform a calculation for reducing the motor speed command value according to the load amount when the load of the AC servo motor (M) is characteristic of power running; and

wherein each AC servo amplifier (A1, A2) is arranged to perform a calculation for increasing the motor speed command value according to the load amount when the load of the AC servo motor (M) is characteristic of regenerative running.
The turning control unit (2) for a marine propulsion machine (1) according to claim 1, wherein when the load amount of the AC servo motor (M) is equal to or less than a predetermined fixed value; and
wherein each AC servo amplifier (A1, A2) is arranged to calculate and output the motor speed command value without correcting the motor-speed reading according the load amount of the AC servo motor (M).
The turning control unit (2) for a marine propulsion machine (1) according to any one of claims 1 to 2, wherein each AC servo amplifier (A1, A2) includes:
a correction calculating part (20) which is arranged to perform a necessary calculation on the motor-speed reading transmitted from the control means (7) according to the load amount of the AC servo motor (M) calculated by the load calculating means (22), and arranged to output the calculation result as a corrected value; and

second control means (26) which are arranged to calculate a deviation between the corrected value outputted from the correction calculating part (20) and the feedback signal outputted from the sensor (S) for measuring the motor speed of the AC servo motor (M), the arrangement being such that it calculates the motor speed command value according to the deviation, and outputs the calculation result.