FULL AUTHORITY SUBMARINE CONTROL CONCEPT DEVELOPMENT
5. DEVELOPMENT OF AN AUTOMATIC TRIM COMPENSATION ALGORITHM
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects and is now used as standard in aerospace by companies
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
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects The algorithm utilises boat sensor data and information on the autopilot operational mode to generate a compensation flow demand. The setpoint for the algorithm is always a neutral trim state demand. The first stage of the algorithm generates an estimate of the current trim state of the boat and compares this with the neutral trim demand. The estimate of the trim state is generated utilising an augmented Kalman filter and associated pre-filtering of sensor inputs. The Kalman filter states are estimated using the known control inputs to the system, the key sensor measurements, and a system dynamics model which accounts for changes in boat speed. State estimation filters of this type have previously been successfully deployed for out of trim estimation displays.
The trim state error is then translated into a compensation flow demand using a classical PID control algorithm.
The flow demand output is then shaped to ensure flows are not demanded which can cause both excessive plant wear and undesirable effects such as water hammer and cavitation.
Because of the limitations of the linear dynamics model used to predict the trim state, the estimated trim state becomes less accurate when rapid pitch transients are achieved, this typically occurs at the beginning and end of depth changes. To avoid erroneous compensation flow demands being generated, a state machine has been created which implements logic to ensure that out of trim compensation demands are suspended during transient manoeuvres where rapid pitch transients occur.
Algorithm development and testing has focussed on achieving performance increases in three areas. These areas have been targeted as a result of direct experience of systems operating in service:
x Operation in high sea state and swell conditions at periscope depth.
x Depth changes which generate an out of trim condition, due to a combination of vertical density gradients and boat compressibility effects.
x Depth keeping situations where a change of trim occurs, this could be due to traversing a density front or stores release.
5.1 OTC TESTING RESULTS 5.1(a) Sea State Performance
Figure 11 illustrates the depth control achieved in a sea state 5 with a 12s swell applied, both using OTC in concert with the autopilot, and using the autopilot alone.
The simulation has been run for an elapsed time of half an hour. The lowest subplot shows the delta change in compensation tank volume about the initial tank level.
Figure 11: OTC performance in sea state & swell
In this situation the application of the out of trim compensator improves depth and pitch control significantly.
The application of the sea state produces first order forces on the boat which reflect the energy spectra of the sea state. Depending on the sea state applied the peak period is in the region of 8 to 10 seconds. In addition, longer period second order suction forces are seen due to the periodic reinforcement of the different sea state frequency elements, and there is also a mean suction force on the boat when at periscope depth. The first order sea state forces applied are too short in period for the OTC to counteract, hence the controller has been tuned to counteract the longer period second order and steady state out of trim forces.
5.1(b) Disturbance Rejection in Depth Keeping
To illustrate the benefits of using OTC when depth keeping, Figure 12 illustrates the depth control achieved when the boat traverses through two successive density fronts. Each density front is a change of 2kg/m3, applied over a distance of 200m.
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects Figure 12: OTC performance through density fronts.
The first front is reached at approximately 100s, the second at approximately 500s. In pure AP control, the first density front is traversed successfully, with the transient and steady state disturbance counteracted by hydroplane deflections. When the second front is reached, there is insufficient remaining control authority on the bowplane to counteract the transient and depth is lost. With OTC active, the majority of the transient disturbance is still countered by hydroplane motion, but the steady state force is compensated for by OTC. This allows the hydroplanes to return to their neutral position and restores their control authority, allowing future control actions to either perform manoeuvres or to counter disturbances effectively.
5.1(c) Depth Change Performance.
Figure 13 illustrates the depth control achieved during a depth change from 50m to 110m in heave mode at 4kts.
A vertical density gradient has been applied which results in the boat becoming light as it descends.
Figure 13: OTC performance in heave depth change Under AP control only, the bowplane is saturated at its position limit, so no further control authority is available to perform the depth change and maintain zero pitch. As the boat gets deeper its descent rate slows. Stirling have experienced this situation in trials in areas of high density variation. This produces a perception of inconsistency and unpredictability in AP performance, both in terms of depth change time and also depth overshoot when achieving the new ordered value.
With OTC engaged, the boat is maintained in a neutrally buoyant state as she descends, providing a constant rate of descent, and more consistent overshoot and settling characteristics on the new depth. Quiescent periods of flow are due to the transient manoeuvre detection logic inhibiting the demand.
OTC has also been employed successfully during depth changes performed at a set pitch angle. Figures 14 and 15 show results for a long 200m depth change, performed at a pitch demand of 10 degrees, at a speed of 6kts.
In Figure 15 a density gradient has been applied which results in the boat becoming light, whilst in figure 16 the reverse has been applied, resulting in the boat becoming
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects
equally heavy. In only AP control, performance is markedly different for the two depth changes, both in terms of time and depth overshoot. With OTC engaged, the performance becomes consistent, and compares to that achieved in a neutral buoyancy situation.
Figure 14: OTC performance in pitch mode depth change
Figure 15: OTC Performance in pitch mode depth change
6. SUMMARY OF OTC DEVELOPMENT