Indian Journal of Geo Marine Sciences Vol. 46 (12), December 2017, pp. 2519-2526
Design and development of a remotely operated vehicle with new maneuvering method
Zainah Md. Zain*, Maziyah Mat Noh, Khairil Ashraf Ab Rahim & Nurfadzillah Harun
Instrument & Control Engineering (ICE) Cluster, Universiti Malaysia Pahang, Pekan Branch, 26600 Pekan, Pahang, Malaysia.
[zainah@ump.edu.my, maziyah@ump.edu.my]
Received 16 April 2017 ; revised 29 October 2017
In this paper, a remotely operated vehicle (ROV) consisting of four thrusters is designed and developed called X4-ROV.
X4-ROV is a micro observation class ROV to be used mainly for visual observation of underwater structure or environment by utilizing a high definition web camera. The designed vehicle structure was aims towards portability and maneuverability in attitude motions of roll, pitch, and yaw, and the translational motion forward/reverse/lateral. This work explains the use and modification of an open-source platform (OpenROV) into X4-ROV system.
[Keywords: Underwater vehicle, X4-ROV, OpenROV, thruster’s configuration]
Introduction
Remotely operated vehicle (ROV) is a type of the unmanned underwater vehicle (UUV) use in underwater exploration for carrying out the hazardous task in challenging environment1. The demands for these vehicles become significantly high during 1980 and were made by oil and gas industry2. In a few years later, an extensive research and development for UUV are done for deployment in many areas of interests. Currently, UUV are used in research and deployment for many fields of operations. In the maritime sector they are used to inspect a ships' hull condition3. Use in the oceanographic discovery and water pollution research in the science field4-5 and most ROV is deployed for commercial undersea operation as seen in oil and gas or telecommunication industry6.
Another class of UUV mostly used for underwater study is autonomous underwater vehicles (AUV). The difference of AUVs and ROVs is that AUVs are controlled automatically by on-board computers and can work independently without connecting to the surface.
ROVs, on the other hand, are controlled or remotely controlled by the human operator from a
cable or wireless communication on the ship or on the ground7.
An observation class UUV priority is real- time data telemetry between vehicles and operator for a successful mission, the presence of human operator makes complex multi-objective underwater missions possible: humans can react to sudden changes in a mission plan caused by the unpredictable nature of the ocean environment8. AUV, however, are more suitable for a pre- determined mission where data collection is the main goal and operator intervention is unnecessary9. Furthermore, due to the limitation of the advance technology AUV is still limited in both autonomy and capabilities. For this purpose, a ROV system is a definite choice for a given task. Commercially modern ROV systems can be categorized by size, depth capability, onboard horsepower, and whether they are all-electric or electro-hydraulic.
Generally, commercial ROV is group into four categories small, medium, heavy, and seabed class. The small class consists of micro and mini ROV with power less than 5hp use for the shallow underwater observation that carrying a camera for observation or inspection. The medium class ROV is that power up to 50 hp. They are the
larger size and durable construction to handle more pressure in the water, step up in propulsion size and increased payload, and usually fitted with manipulators to give them ability in handling some objects. The heavy class ROV is the vehicle with typical power less than 220 hp, they are heavy duty vehicle use to perform more challenging tasks, and typically fitted with multiple tools and manipulators to carry out specialize tasks. Seabed class is highly specialized vehicles use to lay undersea pipes and cable on seabed power at least 200 hp.
The paper is organized as follows. In section 2, the overall system design for X4-ROV is presented followed by coordinate system of an AUV, hull design, mass and added mass and electronics system. The basic concept of X4-ROV propulsion system is discussed in Section 3. In Section 4, we present the X4- ROV thrusters based-on OpenROV thrusters configuration to control the attitude and position of X4- ROV. The results and discussions are given in Section 5.
Section 6 concludes the paper.
X4-ROV Overall System
ROV system performance is a delicate balance between design and operational characteristic trade-offs. They are formed by a highly interrelated group of the subsystem to provide impressive subsea capabilities. The design of X4- ROV is based on a few operational goals: low cost, high mobility/portability, and live data streaming (video feed). The project is divided into two major parts which are mechanical design, and electronic/software development. The overall vehicle system is shown in Fig. 1.
Fig. 1 X4-ROV system
Definition of Coordinate System
In order to describe the underwater vehicle's motion, a special reference frame must be
established. There have two coordinate systems:
i.e., inertial coordinate system (or fixed coordinate system) and motion coordinate system (or body-fixed coordinate system). The coordinate frame {E} is composed of the orthogonal axes {Ex
Ey Ez} and is called as an inertial frame. This frame is commonly placed at a fixed place on Earth. The axes Ex and Ey form a horizontal plane and Ez has the direction of the gravity field. The body fixed frame {B} is composed of the orthonormal axes {X, Y, Z} and attached to the vehicle.
The body axes, two of which coincide with principle axes of inertia of the vehicles, are defined in Fossen10 as follows:
X is the longitudinal axis (directed from aft to fore)
Y is the transverse axis (directed to starboard) Z is the normal axis (directed from top to bottom)
Fig. 2 shows the coordinate systems of underwater vehicle, which consist of a right-hand inertial frame {E} in which the downward vertical direction is to be positive and right-hand body frame {B}.
Fig. 2 Coordinate systems of underwater vehicle
Letting
x y z
T denote the mass center of the body in the inertial frame, defining the rotational angles of X-, Y- and Z-axis as
T , the rotational matrix R from the body frame {B} to the inertial frame {E} can be reduced to:
c c c
s s
c s s s c c c s s s s c
s s c s c s c c s s c c
R (1)
where cα denotes cos α and sα is sin α.
Hull Design
X4-ROV is a type of ROV with a torpedo hull shape and is driven by four thrusters allocated on the side of the fuselage at equal intervals.
Assigning the thrusters on the side of the fuselage is a key point, when controlling the motion and attitude of the fuselage in 6-degree of freedom (DOF) as in Fig. 3.
Fig. 3 X4-ROV
X4-ROV has four thrusters arranged vertically and horizontally to control the position by itself using the difference in thrust generated by the thrusters. The X4-ROV hull in Fig. 4 is designed using SolidWorks.
To simplify the development process, a prototype of X4-ROV shown in Fig. 5 is constructed using a PVC pipe, laser cut acrylic, and a few 3D printed parts for testing of system operation and to produce the platform so that further testing and experiments can be conducted.
Mass and Added Mass
A phenomenon that affects underwater vehicles is added mass. When a body moves underwater, the immediate surrounding fluid is accelerated along with the body. This affects the dynamics of the vehicle in such a way that the force required to accelerate in the water can be modelled as added mass. The added mass is added to the vehicle model to produces the equivalent effect. This added mass depends on the X4-ROV shape and the fluid density. From the characteristic of added mass, the total mass matrix M of the body can be written as
f
bI M
M
M (2)
Fig. 4 SolidWorks drawing for X4-ROV
(a)
(b) (c)
Fig. 5(a) Cylinder hull from PVC; (b) laser cut acrylic as watertinght flange; (c) 3D printed part for thruster mounting
where Mb is a mass of the vehicle, Mf is an added mass matrix, I is a 33identity matrix. The additional mass matrix Mf can be written as
ABC
Mf diag , , (3)
These additional masses can be determined, as explained by Leonard11, such as
2 2 2
C B A
(4)
in which the α, β and γ, the elements of Mf are reduced to
2 3 2 2 2 1
0 2 3 3 2 1
0 2 2 3 2 1
0 2 1 3 2 1
r r r
r r d r r
r r d r r
r r d r r
(5)
where ri (i = 1, 2, 3) is a semi axis of the ellipsoidal body along the axis, is an eccentricity of an ellipse. Calculating and ellipsoidal vehicle with r1 = 5r and r2 = r3 = r.
Setting that 1 2 3
3 4 rrr
V is a volume of the vehicle and is a density of the fluid.
3 3
96 . 5 0894 . 0
94 . 3 0591 . 0
r V
C B
r V
A
(6)
The added mass of the cylinder in water is calculated using added mass of the cylinder formula12
L R
Mf 2 (7) Here is a density of the fluid, R is the radius and L is the length. Using the cylindrical body specification in Table 1, the required thrust for 0.5m/s2 acceleration is
a M M
F( b f) (8) By assuming, = 1000 kg/m3 gives Mf = 4.086 kg, Mb (body included electronic parts) = 3.4 kg and F = 3.743 N. The thrust power from the thrusters is measured by using simple test tank setup in Fig. 6. When the motor thrust forward direction a force in opposite direction is produced and can be read on the electronic scale.
Electronic System
OpenROV13 is a mini observation class of ROV which has the advantage of compact size and weight as shown in Fig. 7(a).
(a)
(b)
Fig. 6Test tank setup for thrust measurement Table 1 Cylindrical body specification
Radius Length Mass
55mm 430mm ~1.3 kg
It could stream HD video to a laptop at the ground station over an ultra-thin two-wire tether.
Besides that, OpenROV can dive to a depth of 100 m (328 ft) and the intuitive control system allows for smooth movement13. OpenROV is based on open source platform where a user can work collaboratively with each other to improve user experience and push the ROV system beyond its limitation. Although a user could purchase a fully functional ROV with the do-it-yourself (DIY) style kit. There is a few modified version of OpenROV platforms have been developed such as Kevin_K ROV in Fig. 7(b)14.
(a) (b)
Fig. 7 (a) OpenROV; (b)Kevin K Workclass ROV
X4-ROV is controlled from the ground station by using a laptop connected to the vehicle using umbilical cable. Fig. 8 shows the block diagram of the X4-ROV electronic system.
Fig. 8 Electronic system block diagram
The onboard controller consists of two main electronic systems working together as a brain, and nerve system of the X4-ROV. The microcomputer, Beaglebone handles higher level tasks, such as running the webserver that hosts the cockpit page/software, acquiring the video stream from the webcam over USB. The controller board (based on ATMega2560) however is a host and linked to many different electronic parts, such as sensors, power circuitry, and the electronic speed controller (ESC). The OpenROV controller board is modified by attaching an extra electronic speed controller to the board before implementing on the X4-ROV. The modification of the current OpenROV software is made by changing the operator control interface to accommodate the changes made in hardware. Fig. 9 shows the system interfacing, and it is control through cockpit window open in the laptop. The resulting commands are read and convert to appropriate drive signal on the Arduino board.
Fig. 9 The ROV system architecture
Basic Concepts of X4-ROV Propulsion System The propulsion system for an X4-ROV is very well modeled with four thrusters in a cross configuration, by applying conventional quadrotor design concepts. The control of the X4-ROV motion can be achieved by varying the speed of each thruster to change the thrust and torque produced by them. Each thruster produces both thrust and torque about its center of rotation, as well as a drag force opposite to the vehicle’s direction of travel.
Driving the two pairs of thrusters in opposite directions removes the need for tail rudders.
Consequently, longitudinal rotation is achieved by creating an angular speed difference between the two pairs of thrusters. Increasing or decreasing the speed of the four thrusters simultaneously permits forward acceleration. Rotation about the vertical and the lateral axis and consequently horizontal or vertical motion is achieved by tilting the vehicle.
This is possible by conversely changing the thruster speed of one pair of thrusters as described in Fig. 10.
In spite of the four thrusters, an X4-ROV remains an underactuated and dynamically unstable system. This concept offers better payload and is simpler to build and control, which is a decisive advantage15. If all thrusters are spinning at the same speed, with thrusters 1 and 3 rotating counter clockwise and thrusters 2 and 4 clockwise, the net hydrodynamic torque, and hence the angular acceleration about the roll axis is exactly zero, which implies that any roll stabilizing rudders of conventional vehicles are not needed. Angular accelerations about the pitch and yaw axis can be caused separately without impacting the roll axis. Each pair of thrusters rotating in the same direction controls one axis, either yaw or pitch, and increasing thrust for one thruster while decreasing thrust for the other will maintain the torque balance needed for roll
stability and induce a net torque about the yaw or pitch axis.
Fig. 10 X4-ROV thrusters (Back view)
Attitude and Position Control for X4-ROV
ROV operates in environment with 3D space, it require the vehicle to position itself defined by 6- DOF of motion. The notation of motion for a marine vehicle is shown in Table 2. An observation class ROV requires more agility and manoevrebility in both translational and rotational motion in comparison to work class ROV which emphasize on stability in translational motion.
Among the problems for observation or inspection is that the water condition or environment causing a poor visibility at a distance. Hence, inspection of larger object become a complex task.
Table 2 Cylindrical body specification
DOF Motion Notation
1 Translational motion in x-direction Surge 2 Translational motion in y-direction Sway 3 Translational motion in z-direction Heave 4 Rotational motion in in x-direction Roll 5 Rotational motion in in y-direction Pitch 6 Rotational motion in in z-direction Yaw
Some modification are made to the OpenROV default controller board program (which is default to control three thrusters only) to suit with the X4- ROV thrusters configuration which is has four thrusters to control. The thruster program of the default system as shown in Fig. 11 is used to control two thrusters for surge and yaw motion while the vertical mounted thruster use for ROV heave motion. Thus, this configuration allow for 3-DOF of motion. Meanwhile X4-ROV has four
thrusters and configured in ‘X4’ configuration, a typical four thruster configuration of a ROV as shown in Fig. 12(a) allow for a 3 or 4-DOF. Fig.
12(b) shows an extra ESC is added to the electronic system to control four thrusters.
(a) Top view (b) Rear view Fig. 11 OpenROV default hardware configuration
(a)
(b)
Fig. 12 (a) Typical 4-thusters configuration;
(b) Modification to controller board
Theoretically, our ‘X4’ configuration allows 6-DOF of motion for the vehicle. For rotational and translational surge motion the program control four thrusters speed and direction. While to achive heave and sway motion only two thrusters are used as shown in Fig. 13. Later modification to user control are made on the cockpit interface ROVs locomotive are different than other marine vehicle that they are using
multiple thrusters to position itself in underwater environment.
Translational motion in y (sway) and z (heave) direction is not directly possible with current X4 thruster configuration, thus a vector thrust strategy are adapt on the X4-ROV as shown in Fig. 14 and Fig.14. However it is still to be proves wether the sway and heave motion will be possible with this approach. A modification to some cockpit interface of OpenROV is done as more control keys are needed to reflect the addtional DOF of the vehicle from 3-DOF to 6- DOF.
Fig. 13 Rotational motion of X4-ROV
Fig. 14 Translational motion of X4-ROV
Results and Analysis
To demonstrate the pressure impact on the X4- ROV body, the SolidWorks software is used by simulating the water flow (pressure) in forward direction. The Fig. 15 shows the cut plot to demonstrate the body reaction which focused at the tip of the X4-ROV body.
A surface plot shows the pressure acted on ROV body as in Fig. 16. The precaution should be taken since the region with higher pressure distribution can cause break or leakage on the body. This can be avoided by thickening the body wall. The water flow trajectory in forward motion is shown in Fig. 17. It is necessary to study the turbulence while moving in the fluid so that the thrust are used efficiently. From the test, the
maximum thrust obtain is 1.25kg. Test result is shown in Fig. 18.
Fig. 15 Translational motion of X4-ROV
Fig. 16 Translational motion of X4-ROV
Fig. 17 Translational motion of X4-ROV
Fig. 18 Translational motion of X4-ROV
Conclusion and Future Work
The X4-ROV is successfully designed and developed. The initial thrust test result can be further improved. In future, a full testing will be
0 2
10 20 30 40 50 60 70 80 90 100
Thrust (kg)
Thrust factor (%)
Thrust allocation
carried out after finalizing the hardware and software integration and the analysis on the sensor output and required motor speed can be conducted.
Acknowledgement
The authors would like to thank for the support given to this research by Universiti Malaysia Pahang (UMP) under grant RDU170366.
References
1. Aras, M. S. M., Kamarudin, M. N., Nor, A. S. M., Jaafar, H. I., Shah, H. N. M., Kassim, A. M., Rashid, M.
Z. A., Development and Modelling of Unmanned Underwater Glider Using The System Identification Method, Journal of Engineering and Technology, Vol.
56 (2013) 136-145.
2. The Remotely Operated Vehicles Committee of the Marine Technology Society (MTS ROV), Citing Internet sources URL http://www.rov.org/rov
3. Azis, F. A., Aras, M. S. M., Rashid, M. Z. A., Othman, M. N. and Abdullah, S. S, Problem Identification for Underwater Remotely Operated Vehicle (ROV): A Case Study, Procedia Engineering, Vol. 41 (2012) 554-560.
4. Lygouras, J. N., Lalakos, K. A., and Tsalides. P. G., THETIS: An Underwater Remotely Operated Vehicle for Water Pollution Measurements, Microprocessors and Microsystems, 22(5) (1998) 227-237.
5. Bachmayer, R., Humphris, S., Fornari, D. J. and Van Dover, C. L., Oceanographic Research Using Remotely
Operated Underwater Robotic Vehicles: Exploration of Hydrothermal Vent Sites on the Mid-Atlantic Ridge at 37 (degrees) North 32 (degrees) West, Marine Technology Society Journal, 32(3) (1998) 37.
6. Zingaretti, P. and Zanoli, S. M., Robust Real-time Detection of an Underwater Pipeline, Engineering Applications of Artificial Intelligence, 11(2) (1998) 257-268.
7. Soylu, S., Proctor, A. A., Podhorodeski, R. P., Bradley, C., and Buckham, B. J., Precise Trajectory Control for an Inspection Class ROV, Ocean Engineering, 111 (2016) 508-523.
8. Christ, R. D., and Wernli Sr, R. L., The ROV Manual: A User Guide for Observation Class Remotely Operated Vehicles, Chapter 3 (2007) 46-80.
9. Citing Internet sources URL OpenRov, Openrov.com 10. Fossen T.I, Guidance and Control of Ocean Vehicles,
John Wiley & Sons Ltd., 1994.
11. Leonardo, N. E., Stability of a Bottom-heavy Underwater Vehicle, Automatica, Vol. 33, no. 1 (1997) 331-346.
12. MIT EDU Website,
http://web.mit.edu/2.016/www/handouts/
13. Work Class Open ROV, Citing Internet sources URL https://forum.openrov.com/t/work-class-openrov/674.
14. Le Tu, G. R., Tobita, S., Watanabe, K., and Nagai, I., The Design and Production of an X4-AUV, 54th Annual Conference of the Society of Instrument and Control Engineers of Japan (SICE) (2015) 1118-1121.
15. Bouabdallah, S., Murrieri, P., and Siegwart, R., Towards Autonomous Indoor Micro VTOL, Autonomous Robots, vol. 18 (2005) 171–183.
