UDT 2016 - Further Developments in Full Authority Submarine Control

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© Stirling Dynamics 2016 stirling-dynamics.com

Dr. Nathan Thomas, Senior Engineer

Further Developments in Full Authority Submarine Control

02 June 2016

© Stirling Dynamics 2016

STIRLING DYNAMICS

● Stirling Dynamics provides technical engineering services and control system technology into the aerospace, marine, energy and training & simulation sectors

● Since the early 1990s, Stirling has supported 10 worldwide navies in the design and test of a wide range of submarines, from the German Type U214 and other diesel electric boats, through to the UK’s Trafalgar class and the latest generation of nuclear-powered submarine, the Astute class

● Stirling’s specialism lies in safety-critical steering, diving control systems and state-of-the-art autopilot and hover solutions, which deliver enhanced control and performance

● Stirling has successfully completed a number of high profile UK and overseas programmes and is currently engaged in several on-going projects including development of the safety critical software for the Astute Autopilot, Hover System and Platform Management System Central Computer

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● We believe that Full Authority Submarine Control (FASC) is the next generation of submarine steering, diving and hover control

● This technology combines Stirling’s active control technology with our expertise in submarine steering, diving and hover control systems

● FASC aims to: o Simplify submarine control by tying together all aspects of the

hover, trim, ballasting, and steering & diving control systems

o To control all these systems through a single interface, allowing more optimised control of a submarine or submersible

o To reduce the control complexity for the operator through the use of software control algorithms and tactile Human Machine Interface (HMI) technology

WHAT IS FULL AUTHORITY SUBMARINE CONTROL?

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Key Benefits of FASC

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Key benefits of this new technology:

● Reduced control complexity, leading to reduced operator workload, manning and training requirements.

● Optimised performance in a wider range of conditions due to integrated hydroplane and ballast control.

● Improved safety by integration of the safety envelope into the control algorithm and active control laws.

● Lower development and through-life costs – a single solution reduces management and integration overheads.


FASC Components

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● Integration of autopilot, hover and automatic ballast control into a single control algorithm.

● Active stick technology and touch screen displays.

● Integration of the Safety Manoeuvring Envelope (SME) into FASC operation


Integration of autopilot, hover and automatic ballast control into a single control algorithm

FASC Components

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● A fundamental component of the FASC solution is an integrated algorithm to manage automatic hydroplane depth and heading control, low-speed hover control and automatic ballast (i.e. trim and compensation) control. At the heart of this algorithm is a Multiple-Input Multiple-Output (MIMO) control strategy .

● The initial development of FASC used a more traditional control approach based around LQG and classical PID. In the latest update, a Model Predictive Control (MPC) algorithm has been implemented.



FASC Components

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● Model Predictive Control is a modern control technique with the key advantage that constraints can be accounted for in the formulation. Further advantages of MPC for submarine control applications include:o The ability to apply constraints to input and output variables (e.g. submarine

depth) allows for many of the scenarios which traditionally require complicated mode switching logic to be accommodated simply.

o The algorithms used for the MPC optimisation problem can also be used to generate feasible reference trajectories to be tracked.



FASC Components

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o Linear MPC has the advantage that when the constraints are not active, the algorithm is equivalent to a state feedback linear controller. This means that techniques available for analysing linear controllers can be applied to situations in which actuator constraints can be ignored. Furthermore, MPC can also be designed to be equivalent to robust linear modern control techniques.

o MPC is an inherently MIMO technique so interaction between the effect of hydroplane and hydrostatic actuators control is built into the algorithm.



FASC Components

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● The FASC algorithm controls the following actuators: rudder, fwd and aft hydroplane, compensation tank flow rate, fwd/aft Trim tank flow rate. These actuators are constrained by limits on plane angles, plane rates flow rates and tank volumes.

● Since some of the FASC control modes require an estimation of the Out of Trim (OOT) forces and moments on the boat, an Out-of-Trim Estimator (OTE) is incorporated into the algorithm.


Active stick technology and touch screen displays

FASC Components

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● The active stick is a key element of the FASC solution, generating a range of tactile cues and providing significant flexibility in how the operator interacts with the system. Stirling’s proven active stick technology is used by both military and civil simulator manufacturers around the world for flight control and R&D development programmes that require maximum force and position fidelity.

● Active sticks are self-contained systems that enhance the operator-boat interface with variable tactile feedback in real time. The control systems reduce operator’s workload whilst ensuring the operator remains the ultimate decision-making authority for the boat’s control



FASC Components

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● There are many options as to how these cues are exploited within FASC, which will be determined according to operator preference. Some examples of the options are :o Hard stops can be used to prevent hydroplane or pitch limits being

exceeded or, in general, to prevent the operator deviating from the Safety Envelope when controlling the boat through the stick. Alternatively, a soft stop can replace the hard stop, to indicate to the operator that the envelope limit is attained, while allowing it to be exceeded by pushing through the soft stop, should an emergency require it. The stick shaker could also be activated in these cases, or if certain warnings are raised by the PMS.

o Detents can be placed on the stick at positions corresponding to common operator demands to make it easier for the helmsman to order these values.



FASC Components

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o Active stick grip switches could be used to lock either the horizontal or vertical stick axis, to prevent accidental coupling during manoeuvres. The grip switches can also allow the helmsman to lock-in the current depth as the depth order, or to nudge the depth order by a predefined amount.

o The stick ‘feel’ is very important to the operator, and so this is also highly configurable through force-deflection characteristics, effective control grip mass and damping values.



FASC Components

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● FASC also features touch screen displays which enable easy viewing and input of information, to further aid in reducing operator workload


Integration of the Safety Manoeuvring Envelope (SME) into FASC operation

FASC Components

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● Integration of the SME (or Manoeuvring Limitation Diagram) into FASC provides a method for ensuring the safety limits are adhered to through tactile feedback and control algorithms that incorporate SME information.

● Recommended limitations to the boat operation (speed, hydroplane or pitch angles) are conventionally 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.


Integration of the Safety Manoeuvring Envelope (SME) into FASC operation

FASC Components

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● The safety manoeuvring envelope for the boat is integrated within the FASC concept; the 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 ‘carefree’ handling.

● We envisage that the system will incorporate the following key features: 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. As operator demands are interpreted by the algorithms, actuator demands are automatically adjusted to comply with SME limits.


FASC Modes

● There are two main operating modes of FASC, a hands-on-stick Manual mode and a hands off Automatic mode. In both modes the same integrated algorithm processes input demands to generate the most appropriate control orders :o FASC Manual Mode. In this mode, the operator uses the active stick to order

a pitch angle or depth rate (in heave mode), depending on the speed and selected sub-mode. The FASC controller calculates suitable forward and aft plane angles to achieve the operator order and actively controls the trim and ballast. A neutral stick position causes the controller to maintain depth in the presence of wave forces or other disturbances. This ‘carefree’ handling functionality reduces operator workload compared to traditional manual modes, while still allowing responsive stick controlled manoeuvres.

Functionality

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FASC Modes

o FASC Automatic Mode. Here the system acts as an autopilot combining automatic hydroplane, hover, trim and compensation functionality. In this mode, the operator enters setpoint orders via the touch screen. Optionally the stick can also be used to specify certain orders. When performing depth changes in this mode, the stick moves to follow the order and the operator can switch to FASC Manual Mode at any time by taking hold of the stick.

Functionality

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Transitioning to Hover

Functionality

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FASC Modes

● In either FASC mode, the integrated control of hydroplane and hydrostatic actuators is used in the following speed-dependent manner:

● Low Speed. Hover control only is applied to maintain or change depth. Forward and aft hydroplanes have little effect and are set to balance angles by the algorithm.

● Moderate/High Speed. At these speeds, hydroplanes alone are used to control depth/pitch, while the trim and ballast pumps are used to perform automatic Out-of-Trim Correction (OTC).

● Transition Speed. For speeds between the above cases, both hydroplane and hydrostatic actuators will influence the behaviour in the vertical plane. These speeds will be encountered when slowing the boat to transition into hover only control, and it is important that the algorithm retains adequate depth control during this process.

Functionality

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● The latest FASC development has resulted in the creation of a new real-time demonstrator, incorporating the FASC system and a simulated non-linear boat and environment model.

● The simulation environment includes a 6 DOF boat dynamics model (with hydrodynamic, hydrostatic and propulsion effects) hover and trim/compensation plant model, and sea state model.

● Implementation of the full FASC functionality is still ongoing, but experience of the system as it currently stands, both by Stirling personnel and experienced submarine helmsmen, has yielded useful observations on its operation and potential benefits.

FASC Real Time Demonstrator

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● Implementation of the full FASC functionality is still ongoing, but experience of the system as it currently stands, both by Stirling personnel and experienced submarine helmsmen, has yielded useful observations on its operation and potential benefits.


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● FASC Manual mode, with the pitch or depth rate ordered using the stick, is seen as a big improvement over conventional manual control. Carefree control in this mode was perceived as very important, particularly at periscope depth in a sea-state, where a substantial reduction in workload was noted.

● The ability to make small adjustments to the set depth using the stick grip switches has also been identified as being useful.

● One feature that is currently not implemented that experience suggests would be helpful would be a steering help indicator specifically for the FASC Manual mode Such a device would further reduce the effort required to accurately change depth.

● The reference trajectory generation functionality of the algorithm was found to be a surprisingly powerful technique. The predicted trajectory could be displayed in advance to the operator, allowing the choice from a number of options of how aggressively a depth change in automatic mode should be performed.


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● Further development of FASC is ongoing and will focus next on implementing and testing more of the lower speed functionality – in particular the transition into hover.

● It is also the intention to perform a like-for-like performance comparison of FASC against a conventional autopilot/hover system, to provide a quantitative demonstration of the stated performance benefits.

Next Steps

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End slide

Engineering

UDT 2016 - Further Developments in Full Authority Submarine Control