Robert D Christ and Robert L. Wernli, Sr - March 03, 2009
As discussed in the prior chapter on ROV design, the design team must consider the overall system. To reinforce the importance of this point, a few additional comments about the design process are warranted.
An ROV is essentially a robot. What differentiates a robot from its immovable counterparts is its ability to move under its own power. Along with that power of locomotion comes the ability to navigate the robot, with ever increasing levels of autonomy to achieve some set goal. While the ROV system, by its nature, is one of the simplest robotic designs, complex assignments can be accomplished with a variety of closed-loop aids to navigation.
Some ROV manufacturers are aggressively embracing the open source computer-based control models, allowing users to design their own navigation and control matrix. This is an exciting development in the field of subsea robotics and will allow development of new techniques, which will only be limited by the user's imagination.
This concept takes the control of the development of navigation capabilities (which is the mission) from the hands of the design engineer (who may or may not understand the user's needs) into the hands of the end user (who does understand the needs). Designing efficient and cost-effective systems with the user in mind is critical to the success of the product and ultimately the mission.
Do not over-design the system. The old saying goes that a chain is only as strong as its weakest link. Accordingly, all components of an ROV system should be rated to the maximum operating depth of the underwater environment anticipated, including safety factors. However, they should not be over-designed.
Figure 3.1 ROV submersible components.
As the operating depth proceeds into deeper water, larger component wall thicknesses will be required for the air-filled spaces (pressure-resistant housings) on the vehicle. This increased wall thickness results in an increased vehicle weight, which requires a larger floatation system to counter the additional weight. This causes an increase in drag due to a larger cross-section, which requires more power. More power drives the cable to become larger, which increases drag, etc. It quickly becomes a vicious design spiral.
Careful consideration should be given during the design phase of any ROV system to avoid over-engineering the vehicle. By saving weight and cost during the design process, the user will receive an ROV that has the capability of providing a cost-effective operation. This is easier said than done, as 'bells and whistles' are often added during the process, or the 'latest and greatest' components are chosen without regard to the impact on the overall system. Keep these ideas in mind as the various component choices are presented in the remainder of this chapter.
Mechanical and electromechanical systems
3.1 MECHANICAL AND ELECTRO/MECHANICAL SYSTEMS
Since weight is one of the most critical design factors, the components/subsystems having a significant impact in this area are discussed first.
The frame of the ROV provides a firm platform for mounting, or attaching, the necessary mechanical, electrical, and propulsion components. This includes special tooling/instruments such as sonar, cameras, lighting, manipulator, scientific sensor, and sampling equipment. ROV frames have been made of materials ranging from plastic composites to aluminum tubing. In general, the materials used are chosen to give the maximum strength with the minimum weight. Since weight has to be offset with buoyancy, this is critical.
The ROV frame must also comply with regulations concerning load and lift path strength. The frame can range in size from 6 in. × 6 in. to 20 ft × 20 ft. The size of the frame is dependent upon the following criteria:
Archimedes' principle states: An object immersed in a fluid experiences a buoyant force that is equal in magnitude to the force of gravity on the displaced fluid. Thus, the objective of underwater vehicle flotation systems is to counteract the negative buoyancy effect of heavier than water materials on the submersible (frame, pressure housings, etc.) with lighter than water materials; A near neutrally buoyant state is the goal.
The flotation foam should maintain its form and resistance to water pressure at the anticipated operating depth. The most common underwater vehicle flotation materials encompass two broad categories: Rigid polyurethane foam and syntactic foam.
The term 'rigid polyurethane foam' comprises two polymer types: Polyisocyanurate formulations and polyurethane formulas. There are distinct differences between the two, both in the manner in which they are produced and in their ultimate performance.
Polyisocyanurate foams (or 'trimer foams') are generally low-density, insulation-grade foams, usually made in large blocks via a continuous extrusion process. These blocks are then put through cutting machines to make sheets and other shapes.
ROV manufacturers generally cut, shape, and sand these inexpensive foams, then coat them with either a fiberglass covering or a thick layer of paint to help with abrasion and water intrusion resistance. These resilient foam blocks have been tested to depths of 1000 feet of seawater (fsw) and have proven to be an inexpensive and effective flotation system for shallow water applications (Figure 3.2).
Figure 3.2 Polyurethane fiberglass encased and simple painted float blocks.
Polyisocyanurate foams have excellent insulating value, good compressive-strength properties, and temperature resistance up to 300°F. They are made in high volumes at densities between 1.8 and 6 lb per cubic foot, and are reasonably inexpensive. Their stiff, brittle consistency and their propensity to shed dust (friability) when abraded can serve to identify these foams.
For deep-water applications, syntactic foam has been the foam of choice. Syntactic foam is simply an air/microballoon structure encased within a resin body. The amount of trapped air within the resin structure will determine the density as well as the durability of the foam at deeper depths. The technology, however, is quite costly and is normally saved for the larger deep-diving ROV systems.
Propulsion and Thrust
3.1.3 Propulsion and Thrust
The propulsion system significantly impacts the vehicle design. The type of thrusters, their configuration, and the power source to drive them usually take priority over many of the other components.
126.96.36.199 Propulsion systems
ROV propulsion systems come in three different types: Electrical, hydraulic, and ducted jet propulsion. These different types have been developed to suit the size of vehicle and anticipated type of work. In some cases, the actual location of the work task has dictated the type of propulsion used.
For example, if the vehicle is operated in the vicinity of loosely consolidated debris, which could be pulled into rotating thrusters, ducted jet thruster systems could be used. If the vehicle requires heavy duty tooling for intervention, the vehicle could be operated with hydraulics (including thruster power).
Hydraulic pump systems are driven by an electrical motor on the vehicle, requiring a change in energy from electrical to mechanical to hydraulic " a process that is quite energy inefficient. A definite need for high mechanical force is required to justify such an energy loss and corresponding costs.
The main goal for the design of ROV propulsion systems is to have high thrust-to-physical size/drag and power-input ratios. The driving force in the area of propulsion systems is the desire of ROV operators to extend the equipment's operating envelope. The more powerful the propulsion of the ROV, the stronger the sea current in which the vehicle can operate. Consequently, this extends the system's performance envelope.
Another concern is the reliability of the propulsion system and its associated subcomponents. In the early development of the ROV, a general practice was to replace and refit electric motor units every 50-100 hours of operation. This increased the inventory of parts required and the possibilities of errors by the technicians in reassembling the motors. Thus, investing in a reliable design from the beginning can save both time and money.
The propulsion system has to be a trade-off between what the ROV requires for the performance of a work task and the practical dimensions of the ROV. Typically, the more thruster power required, the heavier the equipment on the ROV. All parts of the ROV system will grow exponentially larger with the power requirement continuing to increase. Thus, observation ROVs are normally restricted to a few minor work tasks without major modifications that would move them to the next heavier class.
188.8.131.52 Thruster basics
The ROV's propulsion system is made up of two or more thrusters that propel the vehicle in a manner that allows navigation to the work site. Thrusters must be positioned on the vehicle so that the moment arm of their thrust force, relative to the central mass of the vehicle, allows a proper amount of maneuverability and controllability.
Thrust vectoring is the only means of locomotion for an ROV. There are numerous placement options for thrusters to allow varying degrees of maneuverability. Maneuvering is achieved through asymmetrical thrusting based upon thruster placement as well as varying thruster output.
The three-thruster arrangement (Figure 3.3) allows only fore/aft/yaw, while the fourth thruster also allows lateral translation. The five-thruster variation allows all four horizontal thrusters to thrust in any horizontal direction simultaneously.
Figure 3.3 Thruster arrangement.
Also, placing the thruster off the longitudinal axis of the vehicle (Figure 3.4) will allow a better turning moment, while still providing the vehicle with strong longitudinal stability.
Figure 3.4 Thruster aligned off the longitudinal axis.
One problem with multiple horizontal thrusters along the same axis, without counter-rotating propellers, is the torque steer issue (Figure 3.5). With two or more thrusters operating on the same plane of motion, a counter-reaction to this turning moment will result.
Figure 3.5 Thruster rotational effect upon vehicle.
Just as the propeller of a helicopter must be countered by the tail rotor or a counter-rotating main rotor, the ROV must have counter-rotating thruster propellers in order to avoid the torque of the thrusters rolling the vehicle counter to the direction of propeller rotation. If this roll does occur, the resulting asymmetrical thrust and drag loading could give rise to course deviations " the effect of which is known as 'torque steering'.
184.108.40.206 Thruster design
Underwater electrical thrusters are composed of the following major components:
The most critical of these components will be discussed in more detail in the following sections.
On a surface-powered ROV system, power arrives to the vehicle from a surface power source. The power can be in any form from basic shore power (e.g. 110 VAC 60 Hz or 220 VAC 50 Hz " which is standard for most consumer electrical power delivery worldwide) to a DC battery source.
For an observation-class ROV system running on DC power, the AC source is first rectified to DC on the surface, then sent to the submersible for distribution to the thrusters. The driver and distribution system location will vary between manufacturers and may be anywhere from on the surface control station, within the electronics bottle of the submersible, to within the actual thruster unit. The purpose of this power source is the delivery of sufficient power to drive the thruster through its work task.
Electric motors come in many shapes, sizes, and technologies, each designed for different functions. By far the most common thruster motor on observation-class ROV systems is the DC motor, due to its power, availability, variety, reliability, and ease of interface. The DC motor, however, has some difficult design and operational characteristics. Factors that make it less than perfect for this application include:
Permanent magnet DC motors
Per Clark and Owings (2003) the permanent magnet DC motor has, within the mechanism, two permanent magnets that provide a magnetic field within which the armature rotates. The rotating center portion of the motor (the armature) has an odd number of poles " each of which has its own winding (Figure 3.6).
Figure 3.6 Commutator and brushes.
The winding is connected to a contact pad on the center shaft called the commutator. Brushes attached to the (+) and (-) wires of the motor provide power to the windings in such a fashion that one pole will be repelled from the permanent magnet nearest it and another winding will be attracted to another pole. As the armature rotates, the commutator changes, determining which winding gets which polarity of the magnetic field. An armature always has an odd number of poles, and this ensures that the poles of the armature can never line up with their opposite magnet in the motor, which would stop all motion.
Near the center shaft of the armature are three plates attached to their respective windings (A, B, and C) around the poles. The brushes that feed power to the motor will be exactly opposite each other, which enables the magnetic fields in the armature to forever trail the static magnetic fields of the magnets. This causes the motor to turn. The more current that flows in the windings, the stronger the magnetic field in the armature, and the faster the motor turns.
Even as the current flowing in the windings creates an electromagnetic field that causes the motor to turn, the act of the windings moving through the static magnetic field of the motor causes a current in the windings. This current is opposite in polarity to the current the motor is drawing from the power source. The end result of this current, and the countercurrent (CEMF " counter electromotive force), is that as the motor turns faster it actually draws less current.
This is important because the armature will eventually reach a point where the CEMF and the draw current balance out at the load placed on the motor and the motor attains a steady state. The point where the motor has no load is the point where it is most efficient. It is also the point where the motor is the weakest in its working range.
The point where the motor is strongest is when there is no CEMF; All current flowing is causing the motor to try to move. This state is when the armature is not turning at all. This is called the stall or startup current and is when the motor's torque will be the strongest. This point is at the opposite end of the motor speed range from the steady-state velocity (Figure 3.7).
Figure 3.7 Motor velocity versus torque and power.
While the maximum efficiency of an electric motor is at the no-load point, the purpose of having the motor in the first place is to do work (i.e. produce mechanical rotary motion that may be converted into linear motion or some other type of work). The degree of turning force delivered to the drive shaft is known as motor torque.
Motor torque is defined as the angular force the motor can deliver at a given distance from the shaft. If a motor can lift 1 kilogram from a pulley with a radius of 1 m it would have a torque of 1 newton-meter. One newton equals 1 kilogram-meter/second2, which is equal to 0.225 pounds (1 inch equals 2.54 centimeters, 100 centimeters are in a meter, and there are 2π'radians in one revolution).
The formula for mechanical power in watts is equal to torque times the angular velocity in radians/second. This formula is used to describe the power of a motor at any point in its working range. A DC motor's maximum power is at half its maximum torque and half its maximum rotational velocity (also known as the no-load velocity). This is simple to visualize from the discussion of the motor working range: Where the maximum angular velocity (highest revolutions per minute (RPM)) has the lowest torque, and where the torque is the highest, the angular velocity is zero (i.e. motor stall or start torque).
Note that as motor velocity (RPM) increases, the torque decreases; At some point the power stops rising and starts to fall, which is the point of maximum power.
When sizing a DC motor for an ROV, the motor should be running near its highest efficiency speed, rather than its highest power, in order to get the longest running time. In most DC motors this will be at about 10 percent of its stall torque, which will be less than its torque at maximum power. So, if the maximum power needed for the operation is determined, the motor can be properly sized. A measure of efficiency and operational life can then be obtained by oversizing the motor for the task at hand.
Brushless DC motors
For brushless DC motors, a sensor is used to determine the armature position. The input from the sensor triggers an external circuit that reverses the feed current polarity appropriately. Brushless motors have a number of advantages that include longer service life, less operating noise (from an electrical standpoint), and, in some cases, greater efficiency.
DC motors can run from 8000 to 20,000 RPM and higher. Clearly, this is far too fast for ROV applications if vehicle control is to be maintained. Thus, to match the efficient operational speed of the motor with the efficient speed of the thruster's propeller, the motor will require gearing.
Gearing allows two distinctive benefits " the power delivered to the propeller is both slower and more powerful. Further, with the proper selection of a gearbox with a proper reduction ratio, the maximum efficiency speed of the motor can match the maximum efficiency of the thruster's propeller/kort nozzle combination.
Drive shafts, seals, and couplings
The shafts, seals, and couplings for an ROV thruster are much like those for a motorboat. The shaft is designed to provide torque to the propeller while the seal maintains a watertight barrier that prevents water ingress into the motor mechanism.
Drive shafts and couplings vary with the type of propeller driving mechanism. Direct drive shafts, magnetic couplings, and mechanical (i.e. geared) couplings are all used to drive the propeller.
Technology advances are being exploited in attempts to miniaturize thrusters. In one case, a new type of thruster housing places the drive mechanism on the hub of the propeller instead of at the drive shaft, allowing better torque and more efficient propeller tip flow management. Others are developing miniature electric ring thrusters, where the propeller, which can be hubless, is driven by an external 'ring' motor built into the surrounding nozzle. Such a design eliminates the need for shafts, and sometimes seals, altogether.
There are various methods for sealing underwater thrusters. Some manufactures use fluid-filled thruster housings to lower the difference in pressure between the sea water and the internal thruster housing pressure by simply matching the two pressures (internal and external). Still others use a lubricant bath between the air-filled spaces and the outside water (Figure 3.8).
Figure 3.8 Fluid sealing of direct dive thruster coupling.
A common and highly reliable technique is the use of a magnetically coupled shaft, which allows the air-filled housing to remain sealed (Figure 3.9).
Figure 3.9 Magnetically coupled thruster diagram.
The propeller is a turning lifting body designed to move and vector water opposite to the direction of motion. Many thruster propellers are designed so their efficiency is much higher in one direction (most often in the forward and the down directions) than in the other.
Propellers have a nominal speed of maximum efficiency, which is hopefully near the vehicle's normal operating speed. Some propellers are designed for speed and others are designed for power. When selecting a propeller, choose one with the ROV's operating envelope in mind. The desired operating objectives should be achieved through efficient propeller output below the normal operating envelope.
A kort nozzle is common on most underwater thruster models. The efficacy of a kort nozzle is the mechanism's help in reducing the amount of propeller vortices generated as the propeller turns at high speeds. The nozzle, which surrounds the propeller blades, also helps with reducing the incidence of foreign object ingestion into the thruster propeller. Also, stators help reduce the tendency of rotating propellers' swirling discharge, which tends to lower propeller efficiency and cause unwanted thruster torque acting upon the entire vehicle.
Coming up in Part 2: ROV primary subsystems, electrical considerations, and control systems.
Printed with permission from Butterworth Heinemann, a division of Elsevier. Copyright 2007. "The ROV Manual: A User Guide for Observation Class Remotely Operated Vehicles" by Robert D Christ and Robert L. Wernli, Sr. For more information about this title and other similar books, please visit www.elsevierdirect.com.
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