EP2310796B1

EP2310796B1 - Rudder machinery

Info

Publication number: EP2310796B1
Application number: EP08876552.4A
Authority: EP
Inventors: Johan Jarnhamn; Lars Ahlgren
Original assignee: Saab AB
Current assignee: Saab AB
Priority date: 2008-07-07
Filing date: 2008-07-07
Publication date: 2014-04-16
Anticipated expiration: 2028-07-07
Also published as: EP2310796A4; EP2310796A1; WO2010005350A1; ES2464717T3

Description

The present invention concerns machinery intended for actuating and controlling a rudder on a missile.

TECHNICAL FIELD

One specific area of concern in the construction of a portable missile is the operation of the rudders. Missile control rudders are commonly positioned by rudder machinery mounted within the missile body. The machinery exerts appropriate rotational torque and control on the rudder in response to commands from the guidance control system to steer the missile. In a portable missile, there exists for a rudder machinery stringent requirements of size, weight, torque and angle of deflection delivered to the rudder shaft, backlash, cost, and ease of construction. The small body diameter requirement intensifies the mechanical problem of converting rotary motion from a motor into torque to be applied to a rudder shaft. In addition, all of the missile components must withstand great temperature variations due to their shipment and use in many different climates.
The rudder machinery is a very costly part of traditional missile design due to its very high demands for mechanically precision and low tolerances.

BACKGROUND ART

Missile rudders are normally controlled by pneumatic or electro-mechanical means.
The invention disclosed in US 6,827,310 is an example of the electro-mechanical principle. The patent concerns a fin actuator for a portable missile and a method of using the same. An electric motor converts a rotating force to a linear force and thereafter to a new rotating force on the fin shaft using a lead screw fixedly coupled to the power shaft of the motor, the lead screw having a lead nut threadingly engaged and moving linearly along the lead screw in relation to the direction of rotation of the power shaft. The means for converting the linear movement of the lead nut to rotational movement of the fin shaft includes the lead nut operatively coupled to a crank arm including slots to allow freedom of movement, the crank arm being fixedly coupled to and effecting the rotation of the fin shaft.
This type of machinery is very expensive. Such high costs are problematic and especially not justified in small, less expensive missiles.
In EP0628783A1 , an actuation system is described for bringing an aerodynamic control surface into one or other of two active stable positions opposite each other relative to a neutral position. The actuation system includes two electromagnetic coils arranged on a fixed support, one facing the other; a moving blade, one of the ends of which is fixed elastically to the said fixed support, and the other end of which is arranged between the said coils and is capable of being attracted by each of the said coils; and a moving element secured to said blade and carrying said aerodynamic control surface. This type of actuation system is adapted for moving the aerodynamic control surface into one of two stable positions opposite one another in relation to a neutral position and quickly switching from one stable position to the other, however it lacks the ability to achieve arbitrary deflection angles and control the actual movements of the aerodynamic control surface.

SUMMARY OF THE INVENTION

The object of the invention is to achieve a means for actuating and controlling a missile rudder which means is less expensive than the known type, but yet gives as high precision.
A further object is to achieve a means for actuating and controlling a missile rudder which means has less mechanical play between the motor and the rudder.
Another objective is to achieve a means for actuating and controlling a missile rudder which means is light and small enough to be built into a portable missile.
This problem is solved by the invention through the use of a machinery comprising a linear electric motor as means for actuating the rudder.
Thus, the invention concerns a rudder machinery for actuating and controlling a rudder in a missile, wherein the rudder comprises a shaft and the machinery comprises means for rotating the shaft, which means comprises at least one linear electric motor, each motor having a magnetic circuit fixedly mounted on the missile structure and a mobile coil connected to the shaft, such that a current passing through the coil will result in a force acting on the coil, the coil moving to push or pull the shaft, the direction and magnitude of the force being determined by the magnitude and direction of the electrical current in the coil.
Preferably the electric motor of the machinery is controlled by commands from a guidance control system.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic cross-section through the type of motor used in the machinery of the invention.
Fig. 2 is a schematic view of a rudder with a motor attached, seen from above.
Fig 3 shows the principle of the present machinery.
Figs. 4a and b show two schematic cross sectional views, from the rear and from the side, through a rudder having two motors attached.
Figs. 5a, b and c show schematically two missiles having the rudders arranged at the end and at the front, respectively, and also a cross section through the missiles showing four rudders with motors attached.
Fig. 6 is a flow chart showing how the motor of the machinery is controlled.
Fig. 7 is circuit diagram showing the guidance control system.

DETAILED DESCRIPTION OF THE INVENTION

To position and actuate the rudder on a missile there is need for a motor. According to the present invention a dc linear motor is used. To have accurate control of the rudder there is need of a feedback in the system. The entire rudder machinery of the invention contains: regulator, drive, DC linear motor, and position potentiometer for regulator feedback.
To manufacture the rudder machineries of the missiles of today is costly and constitutes a significant part of the complete missile cost. The rudder machinery must be able to deliver a certain torque to the rudder. The weight of the rudder machinery has to be as low as possible. The rudder machinery shall have high bandwidth, to be able to move from one end position to the other several times per second. Further the rudder machinery has to use as little energy as possible in view of minimizing the size of the batteries.
The idea of the invention is to use a linear force acting directly on the rudder. Such a solution has the advantage that the need of mechanical components decreases.
Fig 1 shows schematically and in cross section a dc linear motor that may be used in the present machinery. The motor comprises a yoke 9 of a magnetic material having a high permeability, coil 8, ring 10 of a magnetic material having a high permeability and magnet 1 of a magnetic material having high magnetic saturation level, resulting in magnetic field 11 and force 12 working on the coil 8.
Figs 1 and 2 illustrate how the torque 17 induced by the force 12 from the motor 13 actuates the rudder 2. The electrical current in the coil 8 induces the force 12. As the magnet 1 of the motor 13 is fastened to the missile structure 5 (figs 4a and b) and therefore stationary, the force 12 will work on the coil 8, pushing it out of or pulling it into the magnetic field 11 of the motor 13. The coil 8 is fastened to the rudder shaft 3 of the rudder 2. Thus, the current in the coil 8 will generate a force 12 pushing or pulling the rudder. The direction of the current will decide the direction in which the rudder 2 will move. The average magnitude of the current will decide the magnitude of the force 12, and therefore of the torque 17 working on the rudder shaft 3. For the force 12 and torque 17 to be linear throughout the movement there should preferably be two motors 13, one assembled on each side of the rudder 2 as shown in Fig 2.
The idea of the invention is that the rudder is positioned by a linear force that acts directly on the rudder. Such a solution has the advantages that the need of mechanical components decreases. To be usable, the rudder machinery has to deliver sufficient high torque to keep the rudder in the required position. The rudder machinery should further have a high bandwidth.
The principle of the rudder machinery is showed in Fig. 3. Two motors 13, one on each side of the rudder 2, will push and pull the rudder into the desired position. The voltage cross the motors 13 is supplied by a drive 15. The rudder 2 is deflected by the angle ϕ. The new position of the rudder 13 is measured by a position potentiometer 7 and given as feedback. A regulator 16 receives the signal from the potentiometer 7 and regulates the position of the rudder 2 via the drive 15. On this figure the magnetic field is denoted by R and the force by F.
Figs 4a,b show more clearly the principle of how the motor pushes, pulls the rudder 2 into the right position. The force of the coil 8 is transferred to the rudder 2 via a lever 6 on the rudder shaft 3.
Thus, the magnetic part 1 of the motor is fastened to the missile structure 5. The coil 8 is fastened to the lever 6 which is fastened to the rudder shaft passing through the bearings 4. For each rudder 2 there are two motors 1, 8, mounted on either side of the plane of the rudder 2. The force acting on the coil 8 will move the coil in a direction toward or away from the lever, pushing, pulling the end of the lever 6 in the same direction. The movement of the coil end of the lever 6 will rotate the rudder shaft 3 and thus the rudder. The position of the rudder and rudder shaft is detected by the position potentiometer 7 on the shaft.
Since the rudder and lever are moving in a circular movement, but the motor and coil is moving in linear movement, the design requires a bearing point between the coil and lever and the motor and missile structure, respectively. This bearing point must be designed depending on specific system requirement such as folded missile rudders. It could in its simplest form be the usage of ball bearings or ball joints.
Figs. 5a and b show different locations for the rudders on the missile. Fig 5a shows the rudders 2 on the tail part with two motors 13 as dotted lines under the upper rudder 2. Fig 5b illustrates a missile with tail fins 14 and canard rudders 2 near the missile nose. Fig 5c is a schematic cross section through the missile on Fig 5a or 5b.
The machinery for one rudder is described above as two linear motors and a potentiometer mounted on the rudder shaft for detecting the position of the rudder.
The machinery is controlled by a guidance control system sending a signal to the machinery which adjusts the position of the rudder.
A guidance control system is schematically shown on Fig. 6. It is fed with the actual value in the form of a voltage signal from the position potentiometer and with the desired value, also in the form of a voltage signal defining the desired position of the rudder and obtained form the missile's guidance system.
In the system shown on Fig 6 the actual value, e.g. 2V, and the desired value, e.g. 3V, are compared in a comparator resulting in a signal showing the voltage difference, in this case 1V. The signal is delivered to two operational amplifiers (OP-amplifiers) in parallel in conjuction with a triangular puls (wave), resulting in two mirrored "pulse width modulated" (PWM) signals in the form of square waves where the pulse length is proportional to the voltage difference.
The PWM signal generation is shown on Fig 7 together with a full-bridge dc-dc-converter and the dc motor.
The triangular wave and thus the PWM-signal has a high frequency, about 200 kHz. This is very much higher than the band width of the rudder machinery. Therefore the mechanical inertia of the rudder result in the rudder not reacting in the form of twitches but with a smooth movement based on the average polarity voltage level.

Example:

In a laboratory test machinery of a size suitable for a portable missile was constructed in accordance with Fig 3 using a drive and a regulator as shown on Figs 6 and 7. This machinery had the following parameters:
- The weight of each motor 0.940 kg.
- The size of each motor, diameter 40 mm, length 30 mm.
- The length of the moment arm about 50 mm.
- The torque acting on the rudder 2-3 Nm.
- The maximum deflection of the rudder ±9 degrees.
- The stroke of the coil 16 mm (±8mm).

This performance of a linear DC motor in actuating and controlling a rudder was achieved with parts and materials easily accessible at low cost. A person skilled in the art can without problem find components and materials for optimizing the performance and for adjusting the machinery to a larger missile.

Claims

Rudder machinery for actuating and controlling a rudder (2) in a missile, wherein the rudder (2) comprises a shaft (3) and the machinery comprises means for rotating the shaft (3), which means comprises at least one linear electric motor (13), characterized in that each motor (13) having a magnetic circuit fixedly mounted on the missile structure (5) and a mobile coil (8) connected to the shaft (3) via a lever (6), such that a current passing through the coil (8) will result in a force (12) acting on the coil (8), the coil (8) moving to push or pull the rudder (2), the direction and magnitude of the force (12) being determined by the magnitude and direction of the electrical current in the coil (8), and wherein the lever (6) is connected between the shaft (3) and the motor coil (8) whereby the coil will work on the lever (6) so that the shaft (3) will rotate the rudder (2) in the desired direction wherein the rudder machinery further comprises a guidance control system comprising a regulator (16), a drive (15) and a position potentiometer (7) on the rudder (2) which determines the position of the rudder (2) and sends a signal to the regulator (16), where the actual position is compared with the desired position and the necessary current for adjusting the rudder (2) is determined and delivered to the motor (13) or motors through the drive (15) and wherein the regulator (16) and drive (15) comprise a mirrored pulse width modulated signal controlling a full-bridge dc-dc converter.
Machinery according to claim 1, comprising two motors (13) per rudder (2).
Machinery according to any of claims 1-2, where the machinery actuates and controls a canard rudder (2).
Machinery according to any of claims 1-3, where the machinery actuates and controls a rudder (2) on the tail part of the missile.