THE FULL AUTHORITY SUBMARINE CONTROL CONCEPT

FULL AUTHORITY SUBMARINE CONTROL CONCEPT DEVELOPMENT

3. THE FULL AUTHORITY SUBMARINE CONTROL CONCEPT

The overall concept of FASC is to simplify control of the submarine by tying together all aspect of the hover, trim, ballasting, and steering & diving control systems.

Controlling all the aforementioned systems through a single interface allows for more optimised control of the submarine, while at the same time reducing the control complexity for the operator through the use of software control algorithms and tactile Human Machine Interface (HMI) technology.

The single control unit replaces all the separate steering and diving control functions; this unit controls both hydrodynamic and hydrostatic force actuators via a data bus. The control can therefore be optimised for performance across the entire speed and depth range.

FASC utilises Stirling’s active control stick and a touch screen to enable operator inputs for both manual and automatic control.

FASC operates in a similar manner to Fly-by-Wire Flight Control Systems on aircraft. Thus, it acts as an interface between manual inputs and the control surfaces. A change in perception is therefore required regarding the distinction between the traditional Manual and

Automatic modes. When operating with FASC active, using the stick to order, for example, a depth change, the operator no-longer orders the angle of the fwd and aft hydroplanes. With FASC, a movement of the control stick indicates a desired pitch angle or ordered depth rate when at slow speed. The plane angles are then calculated within the FASC algorithms to obtain the desired attitude. The trim of the boat is actively controlled through the manoeuvre to provide a consistent response.

The basic schematic for the FASC interface is shown in Figure 2.

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Fwd Hydroplanes Data Bus

Full Authority Submarine Controller

Hover Valve Bilge Pump

Trim Pump Ballast Pump Trim Pump

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Figure 2: FASC Schematic

There are four essential components to the FASC concept:

x Stirling’s active stick technology.

x Integration of autopilot, hover control and automatic ballast algorithms.

x Integration of the safety manoeuvring envelope into FASC operation.

x System design for portability.

These features are discussed further in the following subsections

3.1 ACTIVE STICK FUNCTIONALITY Active control technology is a key element within the FASC concept. The control stick used within FASC houses its own industrial single board computer, which runs a control algorithm at 1kHz in order to provide a smooth active feel with the effect of a mass spring damper. This technology is proven in use in both military and civil simulators and aircraft. Strain gauges in the unit detect the force being applied by the operator in each axis, motors in each axis then backdrive against the operator input to provide the required feel, including hard stops and axis lock features.

The current FASC control stick implementation has been met through iteration following a number of operator trials. The depth control axis while operating in FASC mode (Figure 3) has a series of soft-stops and detents

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

© 2011: The Royal Institution of Naval Architects programmed to indicate to the operator standard depth changes (for example, SLOW, NORMAL, FAST or 5°, 10°, 15°). However, selection of all angles between these stops is also possible. In this example, ordering a slow descent will send an order for 5° pitch-down to FASC. The algorithms within FASC handle the flare-in and ensure that the submarine achieves and maintains the desired pitch. When approaching the desired depth, the operator gradually moves the stick to its central position, once in this central position, the current depth is captured as the ordered depth.

Hard Stop Depth Hold

SLOW

NORMAL FAST Dive Rise

Figure 3: Active Stick Pitch Axis Demands

If the operator is at periscope depth, there is no requirement to attempt to counteract the sea state with the stick because this is carried out in FASC. Hence, to manually keep depth in a sea state, the stick is left at its central position. This borrows from ‘carefree’ control theories in the aerospace domain.

Superimposed onto the detents is a hard-stop beyond which the operator cannot move the stick. This hard stop is linked in real-time to limits imposed for safety or operational reasons, for instance by a Noise Reduction Mode or limits permitted by the SME. This gives the operator a tactile method for identifying the operating envelope. These hard stops can back-drive the control stick to prevent the boat from moving outside the safety manoeuvring envelope.

The heading control axis has a series of soft-stops and detents programmed to indicate to the operator steering rudder deflections (i.e. 10°, 20°, 30°) with hard-stops linked to the rudder limits (which may be speed- scheduled to limit depth excursions during a turn).

In addition to the detents and soft-stops the stick has the ability to modify a number of feel characteristics in real- time which can be configured during operator trials:

x Force – deflection characteristic.

x Detent and breakout characteristics.

x Hard stops.

x Dynamic and static friction.

x Stick shaker for warnings and alarms notifications.

x Axis locking and active trimming of neutral position through grip switches.

3.2 INTEGRATED SAFETY MANOEUVRING

ENVELOPE

Recommended limitations to the boat operation (speed, hydroplane plane angles) are historically presented in the form of a Safety Manoeuvring Envelope (SME) or Manoeuvring Limitation Diagram. These diagrams display a great amount of information which must be cross-referenced to key boat parameters and adhered to by the helmsman. An example SME is presented in Figure 4.

Figure 4: Example Safety Manoeuvring Envelope Stirling have integrated the safety manoeuvring envelope for the boat within the FASC concept, the FASC algorithms will ensure that the SME is adhered to, and provide visual and tactile feedback to the operator when limits are being approached and applied. This will enable the implementation of ‘carefree’ handling.

Stirling envisage that the system will incorporate the following key features:

x Operator pitch and depth demands can be automatically limited to the SME, tactile feedback can be provided immediately to the operator through inceptor hard stops or soft stops.

x As operator demands are interpreted by the algorithms, plane demands can be automatically adjusted to comply with SME limits.

In the future the estimated trim state of the boat could be used to optimise the SME in real-time, therefore enhancing the SME to the current state of the boat (as opposed to an assumed state from the conventional SME charts).

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

FASC control encompasses a number of automatic control strategies. Heading, depth and pitch setpoints are ordered through the HMI (Setpoint Mode), or through movement of the active inceptor (Inceptor Mode).

Inceptor inputs always take precedence.

All orders are interpreted by 3 control algorithms, incorporating a traditional steering and diving control algorithm, a hover control algorithm for depth control at low speed, and a new automatic trim and compensation control algorithm which works in concert with the steering and diving control algorithm. A central mode logic function ensures that the correct combination of hydroplane, trim and compensation and hover demands is ordered through the speed range. The FASC control algorithm elements are presented in Figure 5.

Figure 5: FASC Algorithm Components.

Given the performance requirements that are being placed on the latest generation of autopilots, it is physically impossible to design an autopilot algorithm which is fully optimised for performance when neutrally buoyant, and at the same time provides robustness against trim changes of the boat. For this reason the introduction of an automatic out of trim compensation algorithm is becoming ever more imperative.

The aim of the out of trim compensation (OTC) algorithm is to automatically compensate for trim and compensation changes due to sea-water density changes, boat compressibility or internal routines. This will Reduce levels of uncertainty and perceived unpredictability by the operator by ensuring a neutral trim state is maintained during evolutions.

Automatic control of trim and ballast will follow a strict set of rules to ensure that pumps and valves are not constantly moving. This will ensure that the submarine’s stealth capabilities are not compromised and that the trim and ballast system life is not reduced further than for a manually controlled system.

In order to maintain accurate depth control during low speed depth keeping and depth changing operations FASC also contains a hover control algorithm, currently this is configured to use a dedicated hover system to change and control the trim status of the boat through continuous flow demands. The specific arrangement of the hover system being controlled will eventually determine the control philosophy, but it is intended that FASC will control continuous hover flow rates in order to achieve the necessary performance.

The integrated approach used by FASC means the transitions and combinations between hydroplane control, automatic trim and compensation, and low speed hover is a seamless process, with the controller internally determining how to most effectively meet operator demands. This eliminates the potential for control algorithm conflict during the transition phase.

Due to the automatic handover between control algorithms and control effectors, operator actions and workload are reduced. This is illustrated Figure 6 and Figure 7, which describe a typical sequence of operations as speed is reduced to enter hover control, both under typical control systems found on the current generation of boats, and under FASC control.

Figure 6: Conventional Speed Reduction.

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

3.4 FASC PORTABILITY FEATURES

There are numerous FASC configurations envisaged by Stirling. Figure 8 presents an overview of the options identified.

Data Bus Master FASC (Control Room)

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Master FASC (Control Room)

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Auxiliary FASC (Bridge Fin)

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Figure 8: FASC Portability Options

The Master FASC referred to in Figure 8 is located in the Control Room, and is the default point from which FASC is operated. Additional active controllers could be connected into the data bus at any point on the submarine. Via the same data bus, the system can talk seamlessly with the steering and diving control plant.

The data bus does not necessarily have to be the ship’s primary system. The following options are currently apparent:

x When surfaced, the submarine can be controlled using a portable unit in the bridge.

x The system could be used to transmit control data to a UUV by plugging it into the UUV transmitter’s data bus.

x When alongside, the actual submarine system can be connected to shore-based simulators to provide alongside training facilities.

x Full platform control will be available from manoeuvring if the control room is unavailable, for example during fire or flood scenarios.

FASC’s portability also provides the following benefits:

x It is easy to implement redundancy in control through the use of multiple FASC units and data buses, this increases the system reliability.

x Maintenance is made simpler as due to their compact size, complete FASC units can be carried as lowest replaceable units in on-board spares.

x The ability to hot swap FASC units will improve system availability.

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STATUS OF FASC DEVELOPMENT

4.1 TRL LEVELS

The development of FASC as an integrated unit is currently at the initial demonstrator stage. The aim of the development program for FASC is to raise the technology to TRL 5/6 in readiness for the next generation of submarines.

FASC has been developed using the building blocks contained within Stirling Dynamics’ portfolio of submarine control algorithms. The technology readiness level of the elements used by Stirling Dynamics which are feeding into the FASC solution is summarised thus:

4.1(a) Automatic Out of Trim Compensation

Current TRL 3. Advisory ballast control algorithms have been developed and incorporated into two deployed systems which have both undergone successful sea trials.

No automated system has been developed, but the techniques matured during the advisory ballast system development are directly applicable to an automated control application.

4.1(b) Automatic Course and Depth Control Current TRL 9. Stirling autopilots, which feature automatic control of depth, pitch and heading, are now in service on several classes of boat. The latest generation of algorithms are currently undergoing first of class sea trials on a number of platforms.

4.1(c) Low Speed Hover Control

Current TRL 7/8. Stirling’s hover control algorithm, which provides continuous control of compensation at low speeds, is currently undergoing first of class sea trials.

4.1(d) Active Control Stick

Current TRL 8/9. Stirling Dynamics’ active control technology has been developed over the past ten years

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

such as NASA Langley and Lockheed Martin. Active control sticks have not yet been deployed in a submarine environment.

4.1(e) FASC

Current TRL 3. Recent system development has focussed on implementing a demonstrator, complete with an active controller, autopilot with rate control functions, user interface, and a full non-linear boat model. This has enabled initial functionality assessment by end users, the results of which are feeding into further development.

Given the maturity of Stirling’s existing autopilot and hover control algorithms, recent control development has focussed on automatic out of trim compensation algorithms that work in concert with the algorithm governing hydroplane demands. The findings of this work are presented in the next section.

Further development work will be targeted in the following areas:

x development of warnings, alarms and failure handling functionality.

x systems engineering for open architecture.

x full integration of all systems into FASC.

5. DEVELOPMENT OF AN AUTOMATIC

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