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University of Plymouth

Higher National Certificate

Marine Engineering

CORF153 Marine Control and Instrumentation

Assignment 1

Control Systems

Word Count: 1,886

1. Executive summary

1.1 It is essential in the Ministry of Defence that weapon systems perform to the highest standard possible. In order for the torpedo to do this, it must have a fully functioning, automatic steering mechanism. This system will be partitioned into sub systems and explained in detail in order to demonstrate the way the mechanism works and controls the torpedo missile. Research, both online and through text books, was conducted in order to understand the details of each of the four systems; the sensing part (given the set point), the detecting part (controller), the engine (transmitter) and the rudders (actuator). As this type of missile is dated; all of the controls are mechanic rather than electrical and communicate via valves and gears. These sub systems work together in order to keep the torpedo on the desired route to target. It was found that overshooting plays a large part in keeping the torpedo on route and due to the mechanism being so accurate and sensitive the weapon only slightly oscillates even when taking into the account such outreach. For these reasons it is known that without fault with said system; the weapon is very accurate. Overall, the report has given the apprentice a wider understanding of control systems and will allow them to apply this theory to other mechanisms.

2. Contents

1. Executive summary…………………………………………..….1

2. Contents…………………………………………………………..……1

3. Introduction…………………………………………………….……1

4. The mechanism……………………………………………….…..2

5. The gyroscope……………………………………………………..3

6. The pallet mechanism………………………………………….4

7. The engine and rudders……………………………………….5

8. Overshot……………………………………………………………….6

9. Conclusion…………………………………………………………….6

10. Evaluation……………………………………………………………6

11. References…………………………………………………….……6

3. Introduction

3.1 The mark 15 torpedo replaced the mark 11/12 as a weapon with a larger range as well as a larger warhead. The standard destroyer-deployed weapon was in service during World War II and was first launched in 1938 (Detailed description of torpedoes, 1978). The torpedo has two sets of rudders; the horizontal rudders keep it to its preset depth and the vertical rudders keep it on its directional course to reach its set point. The system that controls the movement of the vertical rudders will be laid out in detail in the following paper. This paper is being written in order for the author to gain a wider understanding of the weapons’ mechanism and apply the same systematic logic to other control/actuator circuits. In order of appearance, topics discussed are; The mechanism, the gyroscope, the pallet mechanism, the engine and rudders and overshot.

4. The mechanism

4.1 The steering mechanism of a torpedo functions by keeping the weapon on its preset course after it has been fired. If the torpedo veers off in any one direction (left or right), the mechanism changes the vertical rudders position in order to correct the error. This system is made up of four different sub systems. These are; the sensing part (set point), the detecting part (controller), the engines (transmitter) and the rudders (actuator).

4.2 Within the mechanism the sensor system is the gyroscope. This is continuously aimed in the same direction regardless of the direction of the rest of the torpedo.

4.3 The controller of this system is the pallet mechanism. This part of the steering mechanism sends correcting orders to the steering engine dependant on the gyro position detected.

4.4 The transmitter (steering engine), throws the actuators (rudders) when it receives instruction from the pallet mechanism. Changing the position of the rudders brings the torpedo back onto its desired course. The engines are powered through pressured air and this compressed air is what turns the rudders into the required position.

4.5 The figure (1) below shows the assembly that was present on the Mark 15 torpedo, viewed from the starboard side. The housing of the gyroscope is seen in the centre (labelled gyro pot), the top plate seen on the ‘gyro pot’ is where low pressure air passes through a fitting in order to keep the gyro spin constant. To gain its initial speed, the gyro spinning and unlocking mechanism labelled engages the spur gear when the weapon is fired. On top of this is the pallet mechanism aforementioned which detects the gyroscope axis whereabouts. This mechanism is connected to the steering engine via a valve which is how the communication of the gyroscopes movements is transmitted. Due to the depth and steering controls being part of the same mechanism, this image also shows the depth systems.

Gyro and depth mechanism (Department of ordnance and gunnery, 1957, chapter 12)

5. The gyroscope (containing set point)

5.1 Throughout the journey of the weapon, the axis of the gyroscope remains facing the correct direction of the set point (target). The figure (2) below shows how the gyroscope in the mechanism is configured. The first on the images is of the lone gyro wheel, when the torpedo is fired, the starting valve opens and low pressure air from the gyro reducer strikes the outer rim of the wheel in order to maintain its speed. Where one of the spur gears is engaged by the spinning mechanism to introduce its speed (the gyro reaches full speed in just over half a second), once the gyroscope reaches full speed a valve in the spinning and locking mechanism closes in order to shut off the supply of pressured air. The other is to provide balance and stability to the gyro and the other gear.

5.2 The fourth image (from left to right) shows the wheel mounted within both of its gimbals (inner and outer), this allows the gyroscope to spin on a horizontal axis. The inner gimbal is also allowed to spin at a right angle horizontal to the gyroscope wheel, within the outer gimbal. When the torpedo is fired, the centring pin (from the spinning and unlocking mechanism) is engaged in a cavity, located below the opening for the spur gear, within the inner gimbal. This means the gyro is locked at an axis parallel to the axis of the intended torpedo direction. The pin is retracted from the cavity when the gyro has reached its full speed, this unlocks the gyroscopes movements and allows it to lead the steering of the torpedo (Roberts, 1942).

5.3 The outer gimbal can also rotate, however this can rotate only vertically on bearings within the gyro pot. This means the gyroscope is free to turn along with the torpedo in all directions without making issue with the axis. The cam plate, shown atop the third, fourth and fifth image is attached into place on the outer gimbal. As it is not a moveable part, when the outer gimbal moves, the cam plate moves along with it.

Gyro-general arrangement (Roberts, 1942, p.15)

6. The pallet mechanism (control)

6.1 As previously stated, the cam plate connected to the outer gimbal of the gyroscope is rigid and moves on a vertical axis relevant to it. The pallet mechanism detects the position and movement of the cam on the cam plate, by doing this it can identify when the torpedo has gone off course/deterred from the set point and which direction it has done so.

6.2 The figure (3) below shows the pallet mechanism, which is positioned on the gyro top plate.

Pallet mechanism (Roberts, 1942, p.26)

6.3 Figure (4) shows the operations of the pallet mechanism. On the port side of the cam a groove is cut in the upper area, whereas on the starboard side a groove is cut in the lower domain. If the torpedo is on the correct course when the pallet shaft (shown below in the lower images) is moving forward; the two cam pawls will enter the grooves on the cam. This means the pallet shaft will not be rotated, therefore the torpedo will stay on its current course. When the torpedo is off its desired course, the cam pawl relevant to the torpedo’s error will hit the cam and therefore rotate the shaft. Then when the pallet shaft is moving backwards it will make contact with the pallet pawls and through this link a valve in the steering engine will move.

Pallet mechanism operation (Department of ordnance and gunnery, 1957, chapter 12)

7. The engine (transmitter) and rudders (actuator)

7.1 The steering engine is shown in figure (3) along with the valves and adjusting arm. When the valve connected to the steering engine is moved in a direction according to the pallet shaft/pawls movements, its piston moves fully in the opposite direction. When the piston moves forward, the the piston fork and therefore the rudder rod move with it, moving the starboard rudder accordingly. If the piston is moved in the opposite direction, the consequence is thereafter opposite moving the port rudder accordingly. The rudder valves are shown in the figure (5) below.

Rudder engine (Roberts, 1942, p.26)

8. Overshot

8.1 When the rudders are moved in order to put the torpedo back onto the correct course, the weapon does not immediately stop turning when it is in the correct position. In fact it is overshot and wings off of its course in the opposite direction. The rudder stays in this position until the engine gets the new instruction for the opposite movement. This does not happen until the torpedo has been steered past its course in order for the cam to strike the pallet shaft and move it the other way.

8.2 The rudders are constantly changing direction, ergo; the torpedo’s path is not a straight line. Although the weapon weaves in its path, the overshoot is small because of the pallet mechanisms sensitivity (Department of ordnance and gunnery, 1957).

9. Conclusion

9.1 To summarise; the system is fully mechanical. The gyro remains on its set course in order to remind the mechanism of the intended direction, while the cam plate remains stationary with the outer gimbal (the direction the weapon is headed). This communicates with the pallet mechanism which acts as the control system. The pallet pawls then, through mechanical linkage, communicate the correcting orders to the engine valve (transmitter). The steering engine will then throw the corresponding rudders in the direction to correct the error of the torpedo. This movement causes overshot, this means the process will start again with he opposite outcome. This happens repeatedly until the weapon reaches its target. Therefore, this mechanical system follows the method of the required control system - measure, compare and respond.

10. Evaluation

10.1 The completed research that led to the report above gives the author a wider understanding of how control systems work as well as how they are applied to the business area. Through distinguishing how the mechanism works ‘step-by-step’, the apprentice can then apply the same idea to other systems and break them down into functions; both making it easier to apprehend and fix issues.

11. References

F. H. Roberts, W. E. Follin, R. S. Carr, TG 1942. U.S. Navy Torpedo Gyroscopes Non-Tumble Types. Newport - Rhode Island.

Department of ordnance and gunnery, NO&G 1957. Naval ordnance and gunnery Vol 1. Washington.

E. W. Jolie, 1978, BHOT. A brief history of U.S. Navy Torpedo Development [online]. Available at: https://www.history.navy.mil/museums/keyport/html/part2.htm [17.02.17]

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