Underwater Remotely Operated Vehicles (ROV)

Drive & Dive Motion Solutions

Deep sea exploration - where motion matters

Elmo’s motion solutions are ideal for the ever advancing world of underwater remotely operated
vehicles. Beyond the physical advantage, Elmo addresses the need for high speed underwater
thrusting with ultra-high voltage drives, thruster start up against water currents with advanced
servo algorithms, and mandatory robustness for deep water pressures and shocks.

Background
A Remotely Operated Vehicle (ROV) is an unmanned underwater robot that is connected to an
operator via a series of cables. Underwater robotic technology has been adapted for a wide range of
applications, from servicing deep water oil rigs, to environmental and scientific expedition.
Equipped with different sensors, cameras, and robotic manipulators, ROVs are used to monitor,
repair, weld, take samples and much more. These
highly dynamics vehicles have proven themselves to be
reliable, powerful and relatively simple to command,
and are the ideal solution for areas unreachable by
humans (sometimes several kilometers below sea
level). ROVs are exposed to extended subsea
temperature (sometimes operated in the arctic), and
extreme underwater pressures.
Another type of underwater robot is the AUV
(autonomous underwater vehicle). AUVs are most
commonly used for survey and mapping missions, and typically do not have an umbilical cord like
the ROV. AUVs are too equipped with thursters.

Mechanical Construction & Limitation

ROV’s are self-propelled with a varying number of multi-directional thrusters used to propel,
maneuver, and stabilize them in water. While some ROVs also simply used for taking photos or
monitoring, others also use robotics manipulators (typically 1, 2 or 3 per ROV depending on their
intended use) for a variety of applications. Most typically these manipulators are used for
interacting with its environment with tasks such gripping, sample taking, closing/opening of valves,
cutting, cleaning, clearing, screwing, welding, etc. As with most marine applications, the space
inside a vessel is generally limited. Since space is limited, each of the components onboard the ROV
must be as compact as possible

Power
With extremely high speed thrusters, needing to maintain stability against high water currents,
ROVs demand lots of power. A connecting umbilical cord, sometimes 4-6km longs, transmits power,
commands, and other data to and from the ROV. The cable core is built up by combining copper
conductors for power and signaling, and fiber optic elements for communication. The often very
long umbilical cord connecting the ROV to the ship can be several kilometers long, and is prone to
significant voltage drops. A big challenge for the industry, as ROVs dive deeper and deeper is to
increase power transmissions (while reducing weight). To reduce the strain on the cables and the
ROV, typically a Tethering Management System (TMS) is installed between the
boat on surface and the ROV itself. The TMS is only a few tens or hundreds of
meters away from the ROV, while there could be a few kilometers of heavy
 cords behind it to the boat. This reduces stress from the ROV significantly, and is a common industry
apparatus on the intermediate TMS station, is also where a step-down transformer reduces and
rectifies the high AC voltage from surface, to a lower and useable DC voltage needed by the ROV.
Industry Trend: ROVs Going Electric

Although traditionally hydraulic, there’s an industry wide trend for replacing hydraulic pumps of the
manipulators and thrusters for electromechanical and electrical components. Regulations dictate
that any leak that occur in the hydraulic mechanisms of the robot need to be reported and assessed
for environmental purposes. The fines payed as a result of these leaks are extremely heavy and thus
increase the cost of operation and insurance of these robots in the hydrocarbon exploitation and
exploration fields. This combined with the low efficiency of hydraulic systems, is the driving force
behind the need for a change in the ROV technology to one that is more efficient and
environmentally friendly.

(industry standard hydraulic robotic manipulator – Schilling Robotics)

Electric robotic systems will perform all tasks in the underwater domain,
including those now undertaken by hydraulic systems… ROVs and AUVs in
their present form will cease to exist and are replaced by transformative e‐
robotics that can roam, hover, reside and perform all underwater tasks…This
will bring further significant savings…savings that are already here today
as electric technologies bring considerable through‐life cost
economies…Electric is also more effective… as an all‐electric robotic
system is up to 50% more efficient, and typically has double the power
density of an electro-hydraulic system.

“Global Remotely Operated Vehicle Market Is

Expected to Reach $5.15 Billion by 2025” (SBWire)
D(r)ive into the deep end

High Speed/Power Thrusting Handle the Pressure –

Elmo’s wide range of drives can ultimate deep sea
provide ROVs with ultra-high robustness
currents and voltages to Elmo drives are designed to operate under
accommodate for any thruster speed extreme environmental conditions,
or acceleration. Thrusters of a dew whether in extended temperatures (-40 to
Watts or several Kilowatts are no 70°C), high pressures, shocks or vibrations
problem
Compact & Light ROV
Further contributing to the Umbilical Power Supply – No
miniaturization of ROVs, Elmo problem
drives are compact yet packed with Whether high voltage or stepped down
power. Extreme power density is from TMS, AC supply, or already
crucial for overall footprint and rectified DC, Elmo drives can handle
weight reduction any supply with a versatile family of
drives for any scenario
Thruster start-up against Ideal solution for the all-electric ROV
water currents
Advanced servo algorithms such as With extreme power density and
Hot Plugging allow ROV thrusters to unmatched efficiency, Elmo is the ideal
begin rotating regardless of any partner for the all-electric ROV, whether
existing thruster motion due to to precisely control the manipulators, or
water current smoothly rotate thrusters

High Voltage Gold and SimplIQ drives are often used in ROV
applications around the world. Typically, no advanced
positioning is used, but rather precise speed control is
essential. Hot Plugging, is available with the Gold (and also
with SimplIQ to some extend).
With the trend of ROVs going fully electric, manipulators will
begin requiring multi-axis control with full kinematic support,
and much more precise operation than required for thrusters.

Extreme temperatures? Keep in mind that ROV

manufactures often place drives against surfaces cooled by
surrounding water.
Extreme underwater pressure? Sure, drives may go down
several kilometers below surface where pressure could reach
600bar or more, but in such cases they are placed in
pressurized vessels and not actually exposed to such pressure.
6000 meters below sea level – case study

The European based ROV manufacturer

Sub Water Dynamics (SWD) is determined
to go all electric with their fleet of
models, sold worldwide. Their
underwater crafts are used for a variety
of professional applications, from
servicing deep sea oil rigs to performing
important marine life research. With 800
vehicles in operations worldwide, SWD is
not just a leader in its field but also a
recognized innovator of underwater
technologies.

SWD was looking for drive technologies to power their thrusters. While some of their models
require 5kW for their thrusters, others such as their heavyweight model (1.5 ton) draw nearly 50kW
just for thrusting. However, it wasn’t just power SWD was going after. Elmo came into the picture in
order to deal with various control related issues, size constraints, and the extended underwater
environment.

Elmo’s Trombone servo drive was selected to drive each of the ROV thrusters, propelling the
vehicles at 1100-1200 RPM. The Trombone’s compact form and high voltage capability made it an
ideal candidate for the task. Each Thruster is welded to a small pressurized tube (with thick walls), in
which the drive is meant to go. The Trombone was the only drive powerful enough capable of fitting
into this pressurized tube. Since completely sealed off with no airing, the Trombone’s high efficiency
(>98%) was also an essential factor ensuring minimal heat is generated within the enclosed tube.
“Technology breakthroughs are already bringing savings, with
reductions in vehicle size, weight, cable diameters, winch size
and other elements, leading to more compact systems that make
considerable economies for operators… the unprecedented ratio
of thrust to volume and speed through water provides the
payload, power and control stability needed to carry the range of
tooling and sensors usually associated with much larger
systems”, noted SWD Director of Sales.

One of the major challenges faced by SWD and all other ROV manufacturers, is the on and off
switching of thruster rotation, especially when water forces are also a factor. SWD’s largest ROV
navigates through the water efficiently using 11 different thrusters offering a full 6-axis control. To
move in a certain direction, not all thrusters need to be operating simultaneously. An issue arises
when a thruster which was just “off” is commanded to turn on, however it is already rotating due to
the water currents. The ability to turn a motor on-the-fly while it is already rotating is not
straightforward. When the motor drive is typically enables, it is set with an initial 0V command.
However since the motor is already rotating with some back-EMF (could be high or low depending
on speed), the voltage drop between the zero command and potentially high back EMF may be
dangerous. A resulting current spike may be harmful to the drive electronics, and could ultimately
lead to complete failure of the thruster. Elmo’s advanced Hot Plugging motion algorithm protects
the system by pre determining the existing back-EMF, and compensating the initial voltage
command eliminating the potential for harmful current spikes.

Figure 1: high current spike without Hot

Plugging (left), avoidance of dangerous
current spike with use of Hot Plugging
(right)

Another reason for using the Trombone is its high voltage capacity. Since the ROV is operating in a
distance of up to 6km from its power source, on the ship above, sending power down the umbilical
cord is a complex operation. Aboard the ship is a power generator, of which output is then further
stepped up to an astonishing 3000V 3-phase at 800Hz, which goes down couple of kilometers to the
Tethering Management System (TMS). The stepping up is necessary due to the immense voltage-
drop expected down the umbilical cord. At the TMS station, and only a
few hundred meters away from the ROV itself, the voltage is rectified
and stepped down. A DC bus of 500-800V is then available for the
ROV, 6km away from the ship. The Trombone is a high voltage drive,
capable of supplying up to 800VDC to any motor.

As the industry is moving away from the hydraulic systems, and as

international regulations on oil leaks are tightening (with a high price
tag), SWD realized that going electric on the robotic manipulators would be inevitable. After
utilizing industry standard hydraulic manipulators for years, the SWD engineers decided to develop
their own electric robotic manipulators. The 7-axis robotic arms (2 for each ROV) will have an
extended reach of 2meters, and will be tasked with assembling and disassembling of components,
opening and closing vales, etc. The company is now in design stages, and has selected Elmo’s ultra-
small servo drive – the Gold Twitter. The Gold Twitter is the world’s smallest most powerful servo
drive, and is the ideal drive to be mounted right within the joints of the manipulators.

Elmo’s off the shelf products answered all the needs

required by SWD demanding application, from handling
the harsh environment, to facing the challenges of
operating against unknown water currents. To top it all,
Elmo’s highly compact and highly advanced servo drives
provide the mandatory reliability crucial for the remote
operations of ROV.