A functionality focused hybrid design of an observation class bio-inspired underwater remotely operated vehicle

FIELD OF THE INVENTION

The disclosed invention falls under the field of underwater robotics. BACKGROUND AND PROBLEMS IN THE PRIOR ART

Underwater remotely operated vehicles (ROVs) are used in a variety of applications making their global market worth $1.2 billion dollars in 2014. They continue to find new applications which make their compound annual growth rate (CAGR) estimated to be 20.11% in 2019. Thus there is a growing demand for rugged underwater robotized vehicles with more efficiency and maneuverability.

Most current ROV designs in the market use inefficient conventional (rotary) propellers consuming more power to obtain the required maneuverability. However, there are evidences in experimental biology that biological underwater species have higher propulsive efficiencies and faster than conventional rotary propulsion systems. This led to bio-inspired underwater vehicle designs that mimic or use the principles of biological thrust generation for propulsion, yet they suffer from disadvantages of a complex control system design. Therefore, conventional underwater vehicle designs with rotary propellers suffer disadvantages related to low propulsive efficiency and bio-inspired designs experience problems with a complex control system design, which leads to a requirement of an underwater vehicle design that is both efficient and simpler in control system design. For an autonomous underwater vehicle on observation tasks, propulsive efficiency and maneuverability are vital as such vehicles would only be able to carry a limited number of batteries. The proposed invention is motivated by these requirements and has addressed them with a novel hybrid design which combines both an efficient

l bio-inspired propulsion system and conventional rotary propellers to simplify control and improve maneuverability.

OBJECTS OF THE INVENTION:

A mere combination of the two types of propulsion systems will not produce satisfactory results in maneuverability and propulsive efficiency. The placement of the propulsion systems, body design, flapping axis orientation and tail shape have a direct effect on the performance of such a vehicle. Therefore, the objects of the invention are: a. To successfully integrate bio-inspired propulsion system to an observation class underwater vehicle with conventional rotary thrusters for better performance through appropriate vehicle body design, configuration and design of bio-inspired flapping mechanism.

b. To focus the design on the functionality of efficient bio-inspired propulsion for straight long distance navigations and the inefficient rotary thrusters only for maneuvering. This makes the vehicle more efficient for an overall mission as most underwater missions involve long straight line navigations with occasional maneuvering.

c. To reduce the complexities in control system design without compromising on the maneuverability of the vehicle.

d. To improve stability of the vehicle by learning about effects of flapping axis orientation for such observation class underwater bio-inspired vehicle designs.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT AND ITS WORKING:

The proposed hybrid design consists of two conventional rotary thrusters pivoted to the body in addition to a bio-inspired caudal fin type thruster, thus making it a hybrid design (see Fig. 1 in sheet no. 1). This design will enable the operator to use the more efficient bio-inspired thruster for long distance straight line navigations and the conventional rotary thrusters for complex maneuvering operations, therefore making the vehicle perform well in both efficiency and maneuverability. Maneuverability is improved with this addition of rotary propellers as a similar bio-inspired vehicle without a rotary propeller would struggle or require complex control to make a sharp turn, whereas the disclosed invention will be able to achieve it without such complexities. The design is modular with separate modules for different components of the vehicle, thereby extending its possibilities. For instance, the current design allows the replacement of the bio-inspired thruster with a conventional rotary thruster depending on the specific requirements of the application. This replacement of the bio-inspired module with a third rotary thruster will give additional degree of freedom in movement, thereby improving maneuverability. With these capabilities, this design could potentially find applications in scientific exploration missions and subsea oil and gas observation missions. The design is also ideal for monitoring operations in aquaculture and many such recent evolutions of the underwater vehicles market with the modular design allowing expansion to many such new market segments with only minor modifications.

The design consists of three key components - hull/body, pivoted rotary thrusters and bio-mimetic caudal fin propulsion mechanism (see Fig. 1). The hull, with a particular weight distribution and shape, is adapted for such hybrid vehicle designs. The rotary thrusters are placed at a location anterior to centroidal axis of the top projected area of the body. The biomimetic flapping of the caudal fin is achieved with a mechanism consisting of a slotted lever to flap a flexible tail. The bio-mimetic flapping fin is horizontal to reduce amplitude of oscillation of the hull during its operation. Each of these key components, their brief description and function are elaborated in this section (Please refer next section for more detailed explanations).

The shape of the hull, the position of rotary thrusters and weight distribution are crucial for performance of the vehicle. The rotary thrusters are mounted on a rotary mount whose axis is in the same vertical plane as the position of center of gravity and center of buoyancy of the vehicle. This rotary mount can rotate the thrusters in this axis from 0 to 180 degrees. The rotary mount can also make a full rotation of 0 to 360 degrees, but only the first half rotation is useful as the other half is redundant as the thruster can run in reverse. At 0 degrees, the thruster will be facing up and at 180 degrees, the thruster will be facing down. So, the thruster can be oriented in any direction within this range to provide manoeuvrability for the vehicle with 4 degrees of freedom. The hull has a streamlined shape with more top projected area posterior to the center of gravity. This allows for natural pitching of the vehicle while moving in the heave direction. For instance, when the thrusters are switched on in the 180 degrees position, vehicle moves down while also pitching the vehicle to face downward because of the additional moment created by differential drag on the top and bottom surface areas. This is highly advantageous as the operator can see downward as the vehicle dives down, without the need for additional actuators or camera. The degree of pitching can be controlled by adjusting this differential value of top projected area. More projected area on the posterior regions would improve the amount of pitching and vice versa. This is a unique and unprecedented feature for such a vehicle design. Other vehicle designs would need an extra propeller or other actuator to achieve this motion.

The other considerable aspect of the design is the flapping axis orientation. Through their analysis, inventors have found that a horizontal axis of flapping (dorsoventral flapping) will be beneficial for the proposed design. The inventors find that such orientation will reduce the amplitude of oscillations of the vehicle during flapping cycles. By placing the center of buoyancy above (and in the same vertical line) the center of gravity, a harmonic restoring torque proportional to the vertical distance between the center of gravity and center of buoyancy is generated during oscillations, thus reducing the amplitude of oscillation. Such a condition is only possible with horizontal axis of flapping. Reduced amplitude of flapping implies that the stability of the vehicle is improved while flapping, thereby improving the quality of image recordings by visual sensors in the vehicle. This feature has also not been reported in literature and is unique to the invention described here.

Another novel aspect of the invention is the bio-inspired propulsion system design. The design incorporates a slotted lever mechanism with a flexible tail, actuated by an electric motor. The tail is made of polypropylene material with a shape close to that of a dolphin. A dolphin tail shape has been adapted based on computational fluid dynamics studies. A tail with this shape, 200 mm chord, 400 mm span and 2 mm thickness is made with polypropylene. Larger tails can also be used for this design to improve thrust. Polypropylene, with a density of 910 kg/m ³ has been chosen as it is found (from experiments) to have the optimum flexibility for the given thickness to give the tail a phase lead of about 90 degrees on heaving over pitching. It has been observed from biology that such phase difference between heaving and pitching is best for good propulsive performance. Thickness, span and chord can be altered based on the thrust requirements/This design of the bio-inspired propulsive system with a slotted lever and flexible tail is also modular. The tail can be easily manufactured and replaced in a short interval of time, allowing easy replacement for different applications. For instance, a tuna-like shape can be used for applications demanding high speed and a dolphin-like tail can be used for applications demanding high propulsive efficiency. Other bio- inspired flapping propulsion system would require two or more actuator or a plurality of links to achieve the desired phase relation between heaving and pitching. However, the disclosed design of the bio-inspired flapping mechanism only uses minimal links and a single actuator to achieve the desired phase difference by manipulating the fin flexibility and kinematic parameters. A similar design for bio-inspired propulsion system with a slotted link has been used in US Patent no. 6,997,765 issued to McGuinness on Feb. 14, 2006. However, his design does not incorporate the effects of a flexible tail fin. It is known from literature that flexibility of the fin can improve the thrust performance of flapping fins. Also, the optimum phase difference between heaving and pitching as obtained in the proposed mechanism with flexible tail fin has not been achieved in McGuinness's design. In this regard, the propulsive system design described in the present invention will perform better in terms of the amount of thrust and efficiency.

The key parameters involved in the performance analysis of inventor's design are cost of transport, propulsive efficiency and manoeuvrability. It is known from published literature that bio-inspired flapping fins are capable of achieving high propulsive efficiency than conventional underwater thrusters. Propulsive efficiency as high as 70% has been shown experimentally for flapping fins in prior art. Therefore, propulsive efficiency of a least of 50% is achievable in the disclosed design, which is higher than most other commercial rotary propellers. High propulsive efficiency also implies low cost of transport. In this regard, a bio-inspired flapping system such as the one in the proposed design wilt have higher propulsive efficiency and low cost of transport compared to conventional underwater vehicles. Inefficient conventional thrusters on the vehicle can be used occasionally for manoeuvring operations which is minimal in underwater robotic missions. Therefore, using the bio-inspired flapping system for straight long distance navigations will lead to lower power consumption without compromising on the manoeuvrability. Other bio-inspired robotic designs try to achieve high propulsive efficiency at the cost of a complex control system leading to limited manoeuvrability. This is important especially in autonomous underwater vehicles with limited on-board power supplies.

A similar combination of propulsion systems has been said possible in US Patent 9,090,320 issued to Rufo et al. on Jul 18, 2015. However, their design incorporates a complex biomimetic actuation mechanism for propulsion with thrusters attached on the biomimetic fin flapping on a vertical axis. The thruster in their design can be actuated only in the vertical axis, thus requiring an additional actuator for heave control. Moreover, the purpose of using a thruster is also not clear in Rufo et al. It can be perceived that their intention is to use either the rotary thruster or a biomimetic foil for its operation, but not both. Thus, our invention which beneficially combines the positive aspects of both types of propulsion systems for simultaneous operation would perform better and does not conflict with this filing.

The main features of the invention are:

• Shape of the hull is streamlined and such that the vehicle has natural pitching for the vehicle to lean towards the direction of heaving.

• The flapping axis orientation and weight distribution is such that the amplitude of body oscillations while flapping is minimal. • The bio-inspired flapping mechanism has a slotted lever and flexible fin, with the flexibility of the fin chosen such that the heave leads the pitch by about 90 degrees.

• The design is capable of moving straight long distances propelled by the efficient bio-inspired fin and manoeuvring with rotary thrusters. This gives better overall efficiency for a given mission.

Distinction between the described invention and prior art:

achieve the desired phase difference between heaving and pitching, leading to high propulsive efficiency.

Actively controlled curvature * Patent 9045211 describes the design of an robotic pectoral fin ROV with bio-inspired flexible pectoral flexible

US Patent US 9G4521 issued to fins. However, it involves a complex design The United States Of America, As with a plurality of ribs, and does not describe Represented By The Secretary Of the phase relation between heaving and The Navy pitching. The invention disclosed here describes a design with flexible firts capable of achieving a phase difference of about 90 degrees between heaving and pitching with only a single degree of freedom actuator - an electric motor. In addition, the disclosed invention uses a caudal fin whereas patent 9045211 uses a pectoral fin.

Modeling and Control of a • This write-up describes biomimetic modelling Bio ^' inspired Robotic Fish for underwater robots. Similar art on bio- Underwater Vehicle and its inspired robots are plenty. However, they do Propulsion Mechanism not report the features disclosed here.

A write-up by Abhra Roy

Chowdhury

Underwater fish-type robot • This invention by Gao and Shan has two

GAO J and SHAN B horizontally oriented screw propellers and two Patent application number: fish-like fins. They have used the fins to steer CN201310615433A the vehicle and have used screw propellers for propulsion. In comparison with the invention disclosed here, Gao and Shan's design would have lesser manoeuvrability i.e. greater turning radius and lesser degrees of freedom. BRIEF DESCRIPTION OF FIGURES HEREIN:

Fig. 1: Illustration of the current invention showing the key components of the system Fig. 2: Exploded view of the vehicle assembly to show some of the key components of the vehicle

Fig. 3: Illustration of the positions of center of gravity and center of buoyancy, (a) when the body is horizontal (b) when the body is oscillating in the pitching axis

Fig. 4: Illustration of the bio-inspired thruster mechanism for a flexible flapping foil shaped like a fish tail

All the examples and embodiments are merely for explanation and understanding and do not limit the scope. All variations known to skilled persons is within the scope of the invention.

DETAILED DESCRIPTIO OF THE INVENTION;

The vehicle design and its parts are illustrated in Figures 1-4. The vehicle consists of three major components as mentioned in the previous section: the body/hull, bio- inspired propulsion module and rotary thrusters module (see Fig. 1 and Fig. 2 for vehicle design). These components are explained in detail in this section.

Hull/body:

• The body is streamlined in the XY and YZ plane (see Fig. 1) to have a very low drag coefficient. With a size of 394 mm (L) x 260 mm (W) x 90 mm (H), embodiment of the concept, through experiments, has been found to have a drag coefficient of about 0.05. The hull is made with polypropylene with density of 9 0 kg/m ³ . Polypropylene is chosen because of its lower density than water, providing higher buoyancy. The vehicle is designed to have an excess of 8 N of buoyancy fore© over weight and in the absence of payload, deadweights are added to make it neutrally buoyant. Higher buoyancy is preferred as it increases the payload capacity of the vehicle. Part 8 in Fig. 1 is provided to facilitate addition of dead weights. The front end of the vehicle is chosen for addition of deadweights as the body is tail heavy - the vehicle pitches backwards or clockwise when seen from the position side of z axis (Fig. 1). The front end is also the farthest usable end from the center of gravity and thus allows better pitch control with fewer dead weights (because of a larger moment about the center of gravity). Additional weights or buoyancy foams can be attached at locations 9-12 as shown in Fig. 1. Tapped holes for M10 bolts have been provided at these locations to control the weight balance of the vehicle. The center of gravity (28) and center of buoyancy (27) lie on the same vertical plane - parallel to the xz plane (Fig. 1) which is positioned at a distance of 225 mm from the front end. The center of buoyancy is designed to be vertically above the center of gravity by at least 6 mm to ensure stability of the vehicle in the pitch and roll axis. The body design is adapted for horizontal axis of flapping and also exhibits an advantage with this type of weight distribution. The amplitude of oscillation due to flapping of the bio-inspired propulsion system is reduced. During body oscillations, the displacements of center of buoyancy and center of gravity (29) leads to a restoring couple which tends to oppose the oscillating force, attempting to bring the body to its stable position. This restoring couple increases with the amplitude of oscillation to further limit the amplitude of oscillation. The restoring couple can be quantified and expressed with the following expression,

T = Mgdsm0 (1) Where d is the vertical distance between the center of gravity and center of buoyancy, M is the mass of the vehicle, g is acceleration due to gravity and Θ is the pitching angle measured from a vertical plane which contains th center of gravity and center of buoyancy. It can be observed from equation 1 that the restoring couple increases with oscillation amplitude Θ within the operational range. It can also be increased by increasing the distance d, which further improves stability of the vehicle with lesser amplitude of oscillation during flapping. • The hull comprises of different compartments, one main compartment (26) for the housing of electronics and batteries (if needed) and two other compartments, each on left and right sides of the vehicle (left side compartment shown as 20 in Fig. 2) for housing servo motors to control rotary thruster orientations: The main compartment is sealed with a cap (13) on an o-ring which is seated in a groove in the hull (18). The other two compartments are sealed with face seals in a similar fashion. Further details on this will be disclosed in the following parts of this section.

• The hull has a hole with a transparent casing of 32 mm diameter at the front face to allow visual feed for the operator or for image processing in the on-board computer. The opening diameter is chosen to be 32 mm diameter based on the field of view (FOV) of a particular camera which has about 49° and 37 ^° of horizontal and vertical FOV respectively.

Rotary thruster assembly

A pair of rotary thruster assembly (here BlueRobotics T100 rotary thruster (4) is used to illustrate the concept, but the disclosed invention is not limited to this thruster make), face plate (15), bearing (25), rotary shaft seal and a servo motor (19) is partly enclosed in the body's compartments and partly exposed in water. The parts inside the compartment are sealed with the face plat and an o-ring seated in a groove (21). The rotary shaft for the actuation of the thruster orientation is sealed with a rotary seal mounted on the face plate (15). The motor used is a brushed DC electric motor with quadrature encoders. The thrusters are positively coupled to the DC motor with a stainless steel shaft (14) supported on a rotary ball bearing. The DC motor is directly mounted on the face plate with a customized holder (16) which fully constrains the motor casing.

The servo motor controls the orientation of the rotary thrusters in an axis parallel to the z axis of the vehicle (5 in Fig. 1). Therefore, the orientation of the rotary thruster can be controlled in this axis in a range of 0 to 180 degrees to obtain thrust within this range of angle. This gives the vehicle 4 degrees of freedom: • Surge when the thrusters face forwards pointing in the negative y direction (see Fig. 1 for co-ordinate system orientations).

• Heave when the thrusters face upwards or downwards producing thrust parallel to the x axis.

• Roll when the two rotary thrusters face upwards or downwards, one producing thrust in the opposite direction to the other, but parallel to the x axis.

• Yaw when the two rotary thrusters face forward pointing in the negative y direction, with one generating thrust in the opposite direction to the other, but parallel to the y axis.

In this invention, only two rotary thrusters are used to obtain 4 degrees of freedom where other commercial vehicle designs mostly use 3 or more rotary thrusters for the same.

Bio-inspired propulsion system:

• The bio-inspired propulsion system is shown in Fig. 4. It consists of a pair of brushed DC electric motor (23), crank shaft (31), and a single slotted lever (34) which holds the biomimetic tail fin.

• The brushed DC electric motor is enclosed in a tubular casing (24) sealed with a cap (22) and o-ring. A rotary shaft seal, mounted on the tubular casing, is used to seal the rotating shaft. This shaft is coupled to a crank shaft which drives the slotted lever. The slotted lever is hinged at 30 as shown in Fig. 4. The tail is attached to the slotted lever at location 35 as shown in Fig. 4. Therefore, when the crank shaft rotates, the slotted lever oscillates, producing flapping motion of the tail. The crank shaft material is chosen to be stainless steel to withstand the torsion and bending stresses during operation. Crank radius has been chosen to be 24 mm, although the crank radius can be increased to up to 30 mm if higher amplitude of flapping is needed. The slotted lever material is chosen to be polyoxymethylene (delrin) due to its high strength to weight ratio and lower coefficient of friction - as it involves a sliding motion. • Shape of the tail has been researched with computational fluid dynamic (CFD) analysis. A set of 8 different tail shapes were chosen to find the shape with highest propulsive efficiency. We have found that a tail shape similar to that of a dolphin is optimum, with the highest propulsive efficiency. The optimum tail shape, according to our analysis should have a span of about 1.7 times the chord and that the tail should have a medium crescent moon shaped cut at the posterior end with the shortest point to the leading edge being 0.75 times the chord. Tail material is to chosen to be polypropylene because of its low stiffness and density. As mentioned in the previous section, the said tail dimensions and Shape is found to exhibit the desired phase difference between heaving and pitching because of its low stiffness.

• This module is fastened to the rear end of the body and it is completely detachable (see Fig. 2). It is fastened to the body at location 7 as shown in Fig.1. This type of modular design allows the replacement of the bio-inspired propulsion system with a conventional rotary thruster if efficiency is not of concern. Such a replacement with the third thruster in a horizontal orientation will result in 5 degrees of freedom for the vehicle - surge, heave, yaw, roll and pitch, thereby improving maneuverability.

In one aspect the invention has disclosed an underwater remotely operated vehicle with: a. a plurality of conventional rotary thrusters with blade propellers mounted on a rotary mount in the anterior end along with a bio-inspired propulsion system on the posterior end of the vehicle.

b. Bio-inspired propulsion system with a horizontal flapping axis and flexible tail with the heave leading pitching by 90 degrees.

c. Body with center of buoyancy vertically above and in the same plane as the center of gravity for improved stability during flapping d. Body with a streamlined shape to reduce drag and with more top projected area to the posterior of the vehicle relative to the anterior side to provide natural pitching while moving in the heave direction

In another aspect the invention teaches that the second thruster may also be a conventional rotary thruster, if required for the application. The third thruster will increase the total number of degrees of freedom of the robot from 4 to 5, resulting in improved manoeuvrability.

In another aspect the invention allows the change of tail fin design, wherein the flexible tail fin may also be replaced with different shapes and sizes depending on the application.

All the embodiments and variations are provided for the purpose of understanding the invention and do hot limit the scope of the invention. All the variations as obvious to the skilled persons are well within the scope and spirit of the invention. The applicant also relies upon the provisional specification and drawings for the purpose of disclosure of this complete specification.