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D(r)ive into Deep Water

While deep sea exploration technology is revolutionary, it comes with numerous motion challenges that underwater remotely operated vehicles (ROV) need to overcome. Elmo Motion Control addresses each motion challenge of this ever-advancing world and offers the ultimate solution.

The ROV

A Remotely Operated Vehicle (ROV) is an unmanned underwater robot connected to an operator ship via a series of cables, referred to as an “umbilical cord.” These underwater vehicles often include robotic manipulators, sensors, and cameras that, together, allow them to interact with their environment for tasks such as opening and closing valves, cutting, cleaning, clearing, screwing, and welding. With these sophisticated sensing and manipulating capabilities, ROVs can undertake many tasks that are difficult or dangerous for humans to carry out, such as taking samples or monitoring and repairing underwater equipment – often several kilometers below sea level, under extreme pressures and temperatures.

elmo Drive into Deep Water

Challenges Posed by Underwater Operation

The electrical and electronic components that control ROVs and AUVs (Autonomous Underwater Vehicles) and their robotic manipulators must be robust – capable of withstanding the pressures, temperatures, and vibrations that result from deep-water operation. Elmo`s servo drives are unique in their ability to operate in these conditions, suitable for temperatures from -40 to 70 degrees C and pressures up to 700 bar (the equivalent of 7000 m below sea level).

In an ROV, space is a precious commodity, as the ROV must be lightweight and small enough to maneuver through and around underwater equipment and natural barriers. Here, too, Elmo drives are perfectly suited to meet the requirements of high power in the smallest, lightest possible package. For example, ROVs use multi-directional thrusters to propel, maneuver, and stabilize them in the water. In addition to the extreme subsea conditions they must operate in, these thrusters can require very high currents and voltages. Elmo meets these challenges with its high voltage Gold and SimplIQ drives, which can deliver up to 800 VDC in a 111 x 76 x 30 mm footprint. Despite the need for high power, heat generation must be kept to a minimum inside the small, sealed environment of an ROV. Whether the thrusters require only a few watts for steering or several kilowatts for propulsion, Elmo drives can supply the required power without generating significant heat, which prolongs the life of the drive and other ROV components.

Another challenge posed by the thrusters is the need to start up when thruster motion has already been induced by water currents. Elmo drives address this situation with advanced servo algorithms such as “Hot Plugging,” in which the drive compensates for existing motion to avoid harmful current spikes.

Delivering Power 6 km Under the Sea

With extremely high speed thrusters that work to maintain stability against high water currents, ROVs demand significant power. This power, along with commands and other data, is transmitted from the operator to the ROV via a series of cables, referred to as an “umbilical cord.” The core of the umbilical cord is built up by combining copper conductors for power and signaling with fiber optic elements for communication. The length of the umbilical cord – as much as 6 km long – makes it prone to significant voltage drops. But as ROVs are designed to operate in deeper and deeper waters, manufacturers face the challenge of increasing power transmission while reducing weight.

To reduce the stress on umbilical cord cables and the ROV due to the weight of the cord, a Tethering Management System (TMS) is typically installed between the ship and the ROV. The TMS is only a few hundred meters away from the ROV, while the umbilical cord makes up as much as several kilometers of the transmission between the boat and the TMS. The TMS contains a step-down transformer that reduces and rectifies the high AC voltage from the operator ship to a lower, usable DC voltage for the ROV. Whether the ROV system requires high voltage AC, stepped-down AC, or already rectified DC power, drives from Elmo can handle any range of power delivery requirements.

Servo Drives for Extreme Subsea Conditions

As the market for ROVs and AUVs grows, and as these underwater vehicles evolve from hydraulic to electronic and electromechanical systems, the physical and operational requirements for servo drives will become more demanding. Elmo is already meeting and exceeding these requirements with its SimplIQ and Gold servo drives, capable of operating under extreme subsea temperatures and pressures with advanced servo capabilities for high-speed thrusters and precise, multi-axis manipulators.

Case Study: 6000 Meters Below Sea Level

A European-based remotely operated vehicle (ROV) manufacturer is determined to go all-electric with its fleet of ROV models, which are sold worldwide. These underwater crafts are used for a variety of applications – from servicing deep sea oil rigs to performing important marine life research. With more than 800 ROVs in operation worldwide, the manufacturer is not just a leader in its field, but also a recognized innovator of underwater technologies In its question to go all-electric, the ROV manufacturer needed drive technologies to power the thrusters that propel, maneuver, and stabilize the ROV.

While some ROV models require just 5 kW of power for the thrusters, other, heavier-duty models require nearly 50 kW of power for thrusting. However, a wide power range wasn’t the only requirement for the drives – the manufacturer also needed a drive solution that could address several control-related issues, size constraints, and the extreme underwater environment.

After extensive research and product testing, the manufacturer found that Elmo’s Trombone servo drive was the ideal solution. The Trombone’s compact form and high voltage capability met the manufacturer’s seemingly conflicting requirements. Each thruster is welded to a small, thick-walled pressurized tube, and the drive is mounted inside the tube. The Trombone servo drive from Elmo was the only drive powerful enough to meet the thruster’s voltage requirements, yet small enough to fit inside this pressurized tube. And since the tube is completely sealed, with no air flow to aid cooling, the Trombone’s high efficiency (greater than 98 percent) was also an essential feature, ensuring that minimal heat is generated inside the sealed tube.

Another significant challenge faced by ROV manufacturers is the on-and-off switching of thruster rotation, especially when water forces are also a factor. The manufacturer’s largest ROV navigates through the water efficiently using 11 different thrusters offering full, six-axis control. But for most ROV movements, not all thrusters need to be operated simultaneously. An issue arises when a thruster that is “off” is commanded to turn on but is already rotating due to the water currents.

The process of turning a motor on when it is already rotating (i.e. “on-the-fly”) is not straightforward. Typically, when the motor drive is enabled, it is set with an initial 0 V command. However, since the motor is already rotating with some back-EMF (which could be high or low, depending on the motor’s speed), the voltage drop between the zero command and the potentially high back-EMF may be dangerous. A resulting current spike would be harmful to the drive electronics and could ultimately lead to complete failure of the thruster. Elmo’s advanced Hot-Plugging motion algorithm (Figure 1) protects the system by pre-determining the existing back-EMF and compensating the initial voltage command to account for the back-EMF, eliminating the potential for harmful current spikes.

elmo Drive into Deep Water

Figure 1: High current spike without Hot Plugging (left). Avoidance of dangerous current spike with the use of Hot Plugging (right).

Another reason the manufacturer chose Elmo’s Trombone servo drive is for its high voltage capacity. Since the ROV is operating at a distance of up to 6 km from its power source on the ship above, power transmission a complex operation.

Aboard the ship is a power generator. Power from the generator is further stepped up to an astonishing 3000 V 3-phase at 800 Hz. This extremely high power travels down the umbilical cord – a series of cables connecting the ROV to the operator ship – for several kilometers, to the Tethering Management System (TMS). The stepping up is necessary due to the immense voltage drop that occurs along the length of the umbilical cord. At the TMS station – only a few hundred meters from the ROV itself – the voltage is rectified and stepped down. A DC bus of 500 to 800 V is then available for the ROV, typically 6 km from the ship. Elmo’s Trombone drive is one of few drives on the market that is capable of supplying up to 800 VDC to any motor.

elmo Drive into Deep Water

Elmo’s off-the-shelf drives met the various, and sometimes conflicting, requirements of the ROV manufacturer, from handling the harsh subsea environment to overcoming the challenges of operating against unknown water currents. Elmo’s compact, highly advanced servo drives provided the footprint, robustness, and reliability that are critical for vehicles that operate remotely in otherwise inaccessible locations.

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