Case study for Submarine Technology Limited

What is Submarine Technology Ltd?

STL’s main office is located at the Isle of Wight and there is another office in Cornwall. STL Research

has been involved in many leading edge projects within the sub-sea and offshore arena – including

diving systems, ROV/ diving bell launch and recovery systems, hyperbaric lifeboats, HP/HT valves,

ship-borne space stabilized crew transfer solutions, well head control & monitoring systems,

automated ultrasonic NDT and many more. STL are involved with innovative research &

development projects which are currently pre-commercial.

What are their products?

1. Motion Platform

Description

• The Motion Platform top frame may be moved in a way that simulates the motion of the

deck of a vessel at sea. Equipment intended for marine use may be mounted on the top

frame of the platform in order to evaluate it’s performance when subjected to the pitch, roll,

yaw, heave surge and sway motions normally experienced at sea.

• The top and bottom frames of the Motion Platform are mechanically connected by six ‘legs’.

Each ‘leg’ consists of a hydraulic cylinder with an associated position sensor attached. The

position sensors monitor the extension of the cylinder rods.

• Pressurised oil is supplied to the hydraulic cylinders via dedicated Digital Servo-proportional

Directional Valves. In this way, the extension of the cylinders may be independently

controlled.

Control methods

• Digital Hand Control (Open Loop). Digital position demands are provided by ‘extend/retract’

toggle switches mounted on a simple switch panel known as the ‘Digital Hand Controller’

(DHC). The directional valves are operated in an open loop ’Jog’ mode.

• Analogue Hand Control (Closed Loop). Analogue position demands are provided by an

Analogue Hand Controller (AHC). Control is further enhanced using position feedback

derived from the position sensors mounted on each hydraulic cylinder. The directional

valves are operated in a closed loop ‘Normal’ mode.

• Computer Control (Closed Loop). Analogue position demands are provided by a Digital-to-

Analogue Converter (DAC) connected to a PC running STL’s Motion platform Control

software. The directional valves are operated in a closed loop mode.

2. Neptune Offshore Access System

The Neptune system enables transfer of personnel or cargo from a vessel to a fixed offshore

structure with full motion-compensation so that all wave-induced motions are removed and the

payload arrives at the target structure with no relative movement between them.

Principle

• A stand-alone piece of equipment that can be installed on a suitable vessel and operates

without requiring any vessel services or data, except for the optional use of ship-generated

electrical power.

Description

• An articulated two-section arm is mounted on a foundation containing a slew-ring and a

gimbal-base. It carries a gondola for personnel transfer or a container for cargo. The slew-

ring, gimbal, and arm sections are moved under computer control to remove all wave-

induced motions from the gondola or cargo.

3. Autonomous Synchronised Stabilised Platform (ASSP) Project

Description

• STL are developing a ship-based multi-axis robotic arm that will form part of a new

Autonomous Synchronised Stabilised Platform (ASSP) to enable ASVs to execute intervention

tasks – e.g. equipment transfers, survey and inspection, or launch and recovery operations.

Space-stabilisation technology as used in STL’s Neptune personnel access system will be

further developed to permit synchronous-stabilisation between two moving platforms, such

as an ASV and another vessel, a floating wind-turbine, a wave-energy converter, or other

target with wave-induced motion.

Application

• A stabilised robotic arm will also find applications on-board manned vessels. For example,

launch and recovery of underwater ROVs and AUVs is labour-intensive and potentially

hazardous to personnel and the equipment itself. A robotic arm could increase efficiency,

safety, availability, and expand the weather window for operations.

What tasks was I given?

My task is to model ASSP RDU hydraulic system using Matlab/Simulink (SimHydraulics) for

incorporation into larger simulation of overall ASSP RDU system.

The first step is to model the hydraulic circuit in SimHydraulics as shown below in Figure 1. The

model is built by putting in the blocks that will represent each of the components in the hydraulic

circuit. Then, the next step is to set the parameters of each component. The parameters of the

components are at default values so the actual values have to be obtained from the datasheets.

However, some of the parameters cannot be obtained directly from the datasheets. These

parameters can be acquired from the graphs given in the datasheet such as flow rate graph and time

response graph. In order to do that, we have to build a separate test model as shown in Figure 2 to

perform an optimization on the 4-way directional valve to match the simulation results with the

reference graphs as shown in Figure 3 and 4. The parameters obtained from the optimization will

then go into the 4-way directional valve in the main hydraulic circuit model to ensure that all the

components behave similarly to the real system.

Figure 1: Hydraulic Circuit Model

Figure 2: Test Model

Figure 3: Before Optimization Figure 4: After Optimization

Next, we have to perform another optimization to obtain the parameters of the proportional valve

actuator which is connected to the signal port of the 4-way directional valve. Another separate

model is used to perform this optimization which is shown in Figure 5. For this optimization, it is

crucial to find the right initial values. The before and end results are shown in Figure 6 and Figure 7.

Figure 5: Actuator Optimization Model

Figure 6: Before tuning the initial values

Figure 7: After tuning the initial values

Then, a PID controller is added into the system to control the hydraulic cylinder displacement. A

control system is built as shown in Figure 8. Simulink Design Optimization Tool is used to auto-tune

the PID parameters as shown in Figure 9 and 10.

Figure 8: Control System

Figure 9: Before tuning the PID parameters

Figure 10: After tuning the PID parameters

What I have learnt from this internship

As STL is a small company, my role and responsibility on the ongoing project is rather big. However, I

believe that this great responsibility has a good impact as it pushes myself to achieve results

everyday. Furthermore, this is a work from home internship so time management is very important.

Throughout this internship, I have greatly enhanced my decision making and time management

skills. I have also learnt a lot in working with Matlab Simulink in terms of optimizations, hydraulic

circuits and control system designs. These are the valuable skillsets which will greatly benefit me in

my future endeavors.