ROV TETHER MANAGEMENT

Description

TECHNICAL FIELD

This disclosure is related to tethered remotely operated vehicles and, in particular, to management of tethers associated with such vehicles.

BACKGROUND

Remotely operated underwater vehicles (ROUVs) are a class of remotely operated vehicles (ROVs) used to perform underwater observation, inspection, and/or physical tasks in the petroleum industry, scientific research, military, and other underwater applications. Such ROVs are typically in communication with an ROV control system via a tether, which allows an above-water crew to operate the ROV from an offshore vessel or, in some cases, from a remote land-based location. The tether is a sheathed cable including electrical conductors and/or fiber optics that carry electric power, video, and data signals between the control system and ROV. In some cases, the tether is load-carrying and can be used to lower the ROV from or hoist the ROV to an above-water base.

A tether management system (TMS) may be used to control the amount of tether extending between the ROV and the ROV base or control system. Such systems are intended to provide a sufficient length of tether to permit the ROV to reach a desired underwater distance while limiting the amount of tether that is in the water to avoid unwanted entanglements and/or excessive tether drag in the water. TMSs can be submerged systems that are part of a larger underwater base along the floor of the body of water, for example, or above-water systems that extend and retract the tether from a watercraft at the surface. Submerged TMSs are typically associated with larger-scale ROVs and may be housed in an ROV garage or part of a dock where the ROV returns for storage after completing desired tasks. These systems are only suitable for use at fixed underwater regions where power and communication lines along the floor of the body of water are continuously available and are not usually associated with mobile research vessels.

SUMMARY

Embodiments of tether manager include a tether source and are configured to change an amount of tether deployed from the tether source or an amount of force exerted on the tether in response to a signal received from a remotely operated vehicle (ROV) via the tether.

The tether manager may include any one or more of the following features in any technically feasible combination:

• • the tether source comprises a spool that rotates in a first direction to increase the amount of tether deployed from the tether source and in an opposite second direction to decrease the amount of tether deployed from the tether source;
  • a motor that rotates a spool in response to the signal received from the ROV via the tether;
  • a variable-torque motor that permits manual rotation of a spool;
  • a servomotor that rotates a spool;
  • a controller that receives the signal from the ROV and actuates the tether source to increase or decrease the amount of tether deployed from the tether source in response to the signal received from the ROV;
  • a motor drive that receives an actuation signal from a controller and operates a motor coupled with the tether source to increase or decrease the amount of tether deployed from the tether source;
  • an integrated servomotor that includes a driver and motor such that the integrated servomotor receives the signal directly from the ROV via the tether;
  • a programmable logic controller that receives the signal from the ROV and actuates the tether source;
  • a system including the tether manager and a remotely operated vehicle (ROV) in communication with the tether manager via the tether;
  • a submersible watercraft including a propulsion system configured to change the location of the ROV within a body of water;
  • a base such a as a mobile surface watercraft carrying the tether source;
  • a localization system and configured to determine a distance between the ROV and the tether source, the signal received by the tether manager being based at least in part on said distance;
  • the distance between the ROV and the tether source is determined at least in part using positional data received from a base of the system to which the ROV is tethered;
  • an ROV in two-way communication with the tether manager to receive information from the tether manager pertinent to the amount of tether deployed from the tether source or a torque on the tether source; or
  • a controller in direct communication with a driver of the tether manager via the tether such that the driver receives the signal from the controller and operates a motor coupled with the tether source to increase or decrease the amount of tether deployed from the tether source in response to the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of an embodiment of a tethered ROV system.

FIG. 2 is a schematic view of an example of a tether manager including a tether source.

FIG. 3 is a perspective view of an inboard side of a motor mount of the tether manager of FIG. 2.

FIG. 4 is a perspective view of a motor shaft coupling of the tether manager of FIG. 2.

FIG. 5 is a chart illustrated experimental results of a working example of the tether manager.

DESCRIPTION OF EMBODIMENTS

Described below is a tether manager for use with a remotely operated vehicle (ROV), a tethered-ROV system including such a tether manager, and various related methods. FIG. 1 schematically illustrates an embodiment of a tethered-ROV system 10. The illustrated system 10 includes an ROV 12, a base 14, and a tether manager 16 configured to control a deployed length L of a tether 18 extending between the base and the ROV. Alternatively or additionally, the tether manager 16 is configured to change a force exerted on the tether 18 in response to the signal received from the ROV via the tether. The ROV 12 in this example is a submersible watercraft in communication with an above-water control system 20 and portions of the tether manager 16 via the tether 18. The ROV 12 includes a propulsion system 22 operable to change the location and/or orientation of the ROV within the body of water and with respect to a datum, such as the base 14 or other reference point. The ROV 12 may additionally include other features not explicitly illustrated, such as one or more lights, sensors, controllers, hydraulic systems, manipulators, collectors, etc. The sensors may include a visual sensor (e.g., a camera), orientation sensors, such as an inertial measurement unit (IMU), pressure or depth sensors, etc. Notably, while the ROV 12 in the disclosed example is a remotely operated underwater vehicle (ROUV), the disclosed tether manager 16 can easily be adapted for use with other tethered devices, such as tethered aircraft, tethered land vehicles, or other above-water robots.

The tether 18 includes power and/or signal transmission lines in a protective sheath. A power line of the tether 18 may provide power to the ROV 12 and its respective systems from a power source at the base 14. Signal transmission lines of the tether 18 may include electrical conductors and/or fiber optics to carry control signals from the control system 20 to the ROV 12, control signals from the ROV to the tether manager 16, and other control or informational signals between the ROV and the control system 20 or tether manager 16.

The ROV control system 20 may include a controller and/or processor (e.g., a computer) that acts as an interface between a human user and the ROV 12 via the tether 18. A basic control system 20 may include a joystick- or toggle-type controller by which the user remotely navigates the ROV 12 based on a real-time visual feed from an ROV-mounted camera. Or a human user may interface with the control system from a remote location to navigate and/or control other ROV functions via radio signals, for example. The control system 20 can also be autonomous and configured to carry-out pre-programmed commands or a set of remotely received commands to control the ROV 12.

The base 14 can be any combination of: manned or unmanned; mobile, static, or tethered; and floatable or non-floatable. In this example, the base 14 is a floatable platform that supports the control system 20 and components of the tether manager 16 above and at the surface of the body of water. A floatable base 14 may be tethered to the floor of the body of water and/or be an unmanned base carrying wireless communications equipment to permit a land-based user to use the ROV system 10 to control the ROV 12. A static base 14 may be anchored to or otherwise maintain a fixed position with respect to the floor of the body of water, whether manned or unmanned, and need not be floatable. Any implementation of the base 14 may also carry GPS components, a power source for the ROV 12, the tether manager 16, the control system 20, communication systems, and/or a charging system (e.g., photovoltaic cells) for maintaining a charge on the power source. Examples of an unmanned mobile base 14 include an autonomous or remotely controlled surface watercraft. In other embodiments, the base 14 is a manned watercraft (e.g., a research vessel) carrying the tether manager 16 and the ROV control system 20, and the watercraft is configured to launch and retrieve the ROV 12 and to provide a physical base from which a crew can control the ROV in real time at any location reachable via watercraft.

The tether manager 16 may be referred to as a tether management system (TMS) and is operable to change the deployed length L of the tether 18 to maintain a desired amount of slack or tension in the tether. Conventional above-water TMSs of the type carried by mobile watercraft are largely manual systems requiring a human user to turn a reel in one direction to release more tether when the tether 18 is too taut and in an opposite direction to retract some of the tether when the tether has too much slack. The extent of assistance provided by a manually operated TMS is typically limited to a mechanism that helps organize the spooled tether 18 into evenly spaced windings as it is retracted onto the spool. While attempts have been made to motorize TMSs, such systems typically replace manual movement with motorized movement that still requires user attention and operation, or they use complex systems to try to approximate the amount of tether that should be extended or retracted.

The tether manager 16 disclosed herein includes a tether source 24 and is configured to change the amount of tether L deployed from the tether source and/or change the force exerted on the tether 18 in response to a signal received from the ROV 12 via a communication line of the tether 18. Stated differently, the ROV 12 itself is in control of changes in the deployed length L of and/or the force exerted on the tether 18, thus bypassing the need for any above-water user-supervision, processing power, or estimates of relative ROV 12 location upon which deployed tether length might otherwise be based. The ROV 12 may for example be equipped with a localization system and include a processor configured to determine a distance between the ROV and the tether source 24. The signal received by the tether manager 16 from the ROV 12 may be based at least in part on that determined distance. Suitable localization systems include Doppler velocity loggers (DVLs), ultra-short baseline systems (USBLs), simultaneous localization and mapping (SLAM), sonar arrays, or optical flow tracking. The ROV 12 may employ other methods of determining its distance from the tether source 24 by combining orientation and speed information obtained from an onboard IMU, for example. In some embodiments, the ROV 12 receives information pertinent to the location of the base 14, such as absolute location from a GPS system onboard the base 14, and uses that information in its determination of distance from the tether source. Such information may also be transmitted to the ROV 12 via the tether 18.

Notably, the distance between the ROV 12 and tether source 24 is related to but not necessarily the same as the required length L of deployed tether 18. The required length L is also a function of ROV depth and the mass of the tether 18 per unit length. The processor of the ROV 12 can be configured to take these and/or other parameters into account when controlling the tether manager 16, or a controller of the tether manager can adjust the control signal from the ROV 12.

FIG. 2 schematically depicts an example of the tether manager 16 of FIG. 1, which includes the tether source 24, an actuator 26, a controller 28, and an actuator drive 30. The tether manager 16 is configured to change an amount L of tether deployed from the tether source 24 in response to a signal received from the ROV 12 via the tether 18. Changing the amount of deployed tether 18 or force exerted on the tether is automated, or at least partially automated, based on the received signal. The tether source 24 provides a storage location for an undeployed portion of the tether 18 (omitted in FIG. 2) and is the source of additional tether when the deployed tether length L requires an increase. In this case, the tether source 24 includes a spool 32 that is rotatable about a spool axis A with respect to a stationary support frame 34. An axially central portion 36 of the spool 32, provided by axial rods in this example, radially supports the undeployed portion of the tether between a pair of radially extending discs at opposite axial ends of the central portion 36. The spool 32 rotates in a first direction (+) to increase the amount of tether 18 deployed from the tether source 24 and/or decrease an amount of force exerted on the tether and in an opposite second direction (−) to decrease the amount of tether deployed from the tether source and/or increase an amount of force exerted on the tether. While not shown in FIG. 2, the tether source 24 may additionally include a dynamic feeder that moves axially back-and-forth as the tether is extended or retracted from the spool 32 to control and/or minimize the resulting diameter of the undeployed layers of tether. Other sorts of tether sources 24 are possible such as those that store the tether 18 in a folded configuration, for example.

The illustrated actuator 26 is an electric motor that rotates the spool 32 in the necessary rotational direction (+, −) to increase or decrease the deployed length L of the tether 18 in response to the signal received from the ROV 12 via the tether. In this example, a housing of the motor 26 is affixed to the frame 34 of the tether source 24 via a motor mount 38 with an axially extending shaft of the motor rigidly coupled with the spool 32. Embodiments of the tether manager 16 include a variable-torque motor 26 that permits manual rotation of the spool 32 to extend or retract tether from the tether source 24 in cases where the automated system requires human intervention. A variable-torque motor 26 also permits changing the amount of force exerted on the tether 18 by changing the amount of torque applied to the spool 32 by the actuator 26. The actuator 26 may for example be a servomotor having an encoder-determined rotational position such that manual rotation of the spool 32 does not affect the ability of the system to track the deployed length L of the tether 18. Servomotors are often equipped with several customizable safety settings and may be equipped with a fieldbus interface to receive commands from the ROV 12, for example.

Other types of motors, such as stepper motors, usually rely on discrete, fixed amounts of rotation with no feedback loop such that manual rotation of the spool 32 would render the resulting length L of deployed tether unknown. However, it is possible to employ a stepper motor as a lower-cost solution by employing an addendum encoder. Such a stepper motor also provides a source of torque feedback. By setting maximum torque limits, the system can strictly adhere to max tension limits specified by the tether manufacturer. This reduces the risk of inadvertent damage to the tether caused by excessive tension. This type of damage can be difficult to diagnose because excessive tension may damage the conductors in the tether long before the tether sheath exhibits visible deformation.

Methods of using the system 10 or tether manager 16 may include applying torque limits to the motor 26, which can help limit or otherwise control the amount of tether slack. Considering forces applied to the tether 18 other than those caused by changing the amount L of deployed tether, such as ROV thrust, water currents, and wind at the base 14, such torque limits can provide additional utility. For example, increasing a holding torque—that is, a torque that resists spool rotation—while the ROV 12 is commanded to thrust away from the base can ensure that excessive slack in the tether is removed. In another example, the torque on the spool 32 is limited such that a maximum tension in the tether 18 is less than the rated max tension of the tether (e.g., 40% to 60% of the rated tension). Alternatively or additionally, the tether manager 16 is configured to increase the maximum amount of torque on the spool 32 to 100-150% of the rated max tension of the tether in an emergency situation to retrieve a disabled ROV. In another example, a holding force setpoint can be used to permit or prevent additional amounts of tether being deployed in cases where the ROV 12 is being pulled away from the base 14 by a water current.

The tether source 24 may also include one or more slip rings 40 along the rotational axis A to provide a continuous power and/or communications interface between the proximal end of the tether 18 and an ROV power source, the ROV control system 20, and/or the controller 28 or drive 30 of the tether manager 16 while the spool 32 is rotating. The slip ring 40 may be capable of transmitting Ethernet signals across the sliding interface. Such a slip joint can limit the bandwidth available for Ethernet communication—i.e., a typical Ethernet communication wire requires over 250 MHz of communication bandwidth, dramatically increasing the cost of Ethernet slip rings. As a low-cost solution, a power slip ring can be used to power an Ethernet-to-Wifi connection at roughly one tenth the cost. While this solution is not always preferred, it served well in the proof-of-concept experimental example discussed further below.

The controller 28 is configured to receive a signal from the ROV 12 via the tether 18 and to actuate the tether source 24 to increase or decrease the amount L of tether deployed from the tether source, change the amount of torque applied to the spool 32 by the actuator 26, and/or change the amount of force exerted on the tether 18 in response to the ROV-originated signal. In the illustrated example, the controller 28 of the tether manager 16 receives a command signal from the ROV 12 via the tether and operates the motor 26 to rotate the spool 32 an amount that will extend or retract an amount of tether consistent with the ROV signal. This operation of the motor 26 by the controller 28 is conducted through the motor drive 30, which interprets the controller's requested amount of rotation to provide the associated electrical energy to the motor to achieve that amount of rotation. The controller 28 may include or may be a programmable logic controller (PLC). In other examples, the controller 28 may be or may include software running on the ROV 12 or on hardware other than a PLC.

An estimated distance between the ROV 12—i.e., the distal end of the tether 18—and the tether source 24 can be generated by computing the Euclidean distance, which is the square root of the sum of the squares of the lateral distance, longitudinal distance, and depth. The primary inputs to the controller 28 may include the deployed tether length L, the current torque on the motor 26, and Euclidian distance estimation based on algorithms operating over onboard IMUs and/or other localization systems. Since slack mitigation in the tether 18 is an important aspect of ROV operation, part of the tuning for the tether manager 16 may include determining the minimum amount of slack that is required in the tether to minimize the risk of snapping the tether taut, along with determining the maximum amount of slack that should be permitted in the tether to minimize the risk of the tether becoming tangled. By accounting for uncertainty in localization due to DVL and GPS drift, the tension on the tether 18 can be dynamically adjusted to adhere to mission requirements while minimizing risk of tether damage.

In some embodiments, the tether manager 16 includes an integrated servomotor that includes a servo drive 30 as part of the servomotor assembly such that an external drive is not required. In that case, the controller 28 is in direct communication with the integrated servomotor to change the deployed length L of tether via rotation of the spool 34. In some embodiments, the above-water controller 28 can be omitted. For instance, the ROV 12 may house the controller 28 such that the ROV 12 is in direct communication with the above-water actuator drive 30, as indicated by the dashed line 18′ in FIG. 2. In that case, the ROV-provided controller 28 can be in direct communication with an external actuator drive 30 or the servo drive of an integrated servomotor 26.

The tether 18 may provide two-way communication between the ROV 12 and the tether manager 16. In other words, not only does the tether manager 16 receive signals from the ROV 12 to extend or retract the tether 18, but the ROV can also receive signals from the tether manager. The ROV 12 may for example receive information from the tether manager 16 pertinent to the amount L of tether deployed from the tether source 24. For example, the number of rotations and angular position of the shaft of the motor 26 and spool 32 is pertinent to the deployed length L of tether 18 at any given time. Such information may be used as additional data by the ROV's localization system, for instance. Other information may be passed between the tether manager 16 and the ROV 12 via the tether 18 such as sensed spool torque values, instantaneous base 14 location (e.g., GPS), power source state of charge, etc. The communications protocol between the tether manager 16 and ROV 12 may be any one of several known Ethernet protocols, such as Modbus TCP or MQTT. Alternatively, the ROV 12 and tether manager 16 are in two-way communication with each other and at least one of the two directions direction of communication is via a communication medium other than the tether 18. Examples of other suitable communication transmission types include acoustic, laser (both in fiber and out), visible light, and low-frequency wireless communication, including any of the aforementioned communication methods combined with other wireless technologies (e.g., Wi-Fi, cellular, satellite).

Experimental Example

A prototype tether manager controllable by a remotely operated vehicle as part of a tethered-ROV system has been developed and tested and is described below. An advantage of the disclosed tether manager and system is the provision of a reliably automated tether management system that is low-cost and safer than other state-of-the-art systems. It is also possible to build the system described below using commercially available components.

Acceptable tether speed performance characteristics were identified based on performance specifications of commercially available ROVs. One suitable ROV is available under the tradename BlueROV® (Blue Robotics Inc., St. Torrance, CA, USA), which has a maximum speed of 1.5 m/s. A safety factor of 2 was applied to the maximum speed to accommodate motion at the system base, as well. As the above-described tether source, a spool sold under the tradename “Fathom X” was selected. The Fathom spool has an estimated diameter between its axial ends of about 150 mm to provide a bending radius larger than the minimum required bend radius of Ethernet cable. This makes the minimum circumference of the spool just under 500 mm, providing the lower limit of the length of tether per revolution. The safety factor-adjusted max ROV speed of 3 m/s means the spool must be able to spin at approximately 720 rpm. Most stepper motors are easily capable of exceeding that speed, permitting the addition of a gearbox to trade speed for torque to minimize the power requirements of the tether manager. The selected motor was a VPL-A0633C-PK12AA servomotor having a top speed of 3000 rpm and enough torque to be directly mounted to the spool portion of the tether source, eliminating the need for a gearbox. An Allen-Bradley PLC and Kinetix servo drive platform were used. It was determined that a gearbox ratio of 10:1 is suitable for use with the anticipated max speed of the ROV. The resulting tether manager was capable of linear tether speeds in excess of 50 m/s but was capped at 1 m/s using the servomotor safety system parameters. Additionally, servomotor acceleration and peak torque input were reduced by a factor of 5 for the safety and convenience of the users. The controller and driver were powered using a 24 VDC power supply, and a transformer was used to generate the 220 VAC for the servomotor.

Conventional tether management systems often employ auxiliary systems for tether tension management and level-reeling, which serve to maximize the amount of tether on a spool while minimizing the risk of tangles caused by tethers winding outside of the spool and becoming tangled on the axle. In this example, an auxiliary tensioner was omitted by tightly coupling the tether length to the ROV localization system and by positioning the spool directly over the operating body of water, thereby allowing the tether weight in open-air to counteract the propensity to tangle the tether. A level-reeling device typically guides the tether wind point back and forth to ensure uniform tether layering while winding. Different winding strategies were evaluated, and it was determined that winding without a leveling device was reliable but required an oversized spool. Typically, non-layered winding also limits the amount of tether that can be reliably wound because of the variability in layer height across the width of the spool. However, an oversized spool also mitigates this issue by reducing the variation among unwinding revolutions. Elimination of the level-reeling system also reduces overall power requirements.

A control strategy and algorithm was also developed to automate the tether manager. The control strategy leverages measurements from onboard state estimation, which can be provided by a DVL localization system and IMU units, to estimate the distance of the ROV from the tether source and command the spool via fieldbus to wind and unwind based on that distance. For ease-of-use, the experimental tether manger had over-torque conditions handled by the controller, allowing for operators to manually feed out tether if necessary. The control commands for the system can use either Modbus/TCP or MQTT. These protocols were selected because of the native integration with ROS while also being designed to run over ethernet. This allows for the actual tether management processing to be executed on the ROV itself, further facilitating plug-and-play functionality with the BlueROV or any other ROS-based ROV.

The PLC used as the controller in these experiments was controlled directly via Matlab/Simulink. The code to communicate with the controller is shown below. The code was located in a subsystem block on a Simulink file connected to the existing location variables:

The MQ Telemetry Transport (MQTT) communication was established using Eclipse Mosquitto as the broker and then verifying communication using MQTT explorer. MQTT configuration is shown in the Matlab code, connecting the simulation to the broker and then writing the Euclidean distance to the spoolControl/distance topic. Using MQTT explorer, the data history confirmed that the Matlab simulation was successfully transmitting location data to the broker.

MQTT was chosen as a transport layer upon consultation such that the system could interact with both the driver of the tether manager and the tow rig, in addition to the Simulink model. Communication with the tow rig provided the opportunity to control the tether manager based on not only ROV motion, but also on the motion of the tow rig as to simulate motion of the base watercraft.

3D-printing was used to fabricate the motor mount 38 depicted in FIG. 3 for mounting the housing of the servomotor to the spool frame, along with a motor shaft coupling 42, which is depicted in FIG. 4, for coupling the motor shaft to the spool. FIG. 3 depicts the motor mount 38 from an inboard side—i.e., the side facing the spool frame. The motor mount 38 functions as an adapter and a spacer and includes a set of mounting holes 44 for attachment to the spool frame, a flange with a set of mounting holes 46 for attachment to the motor housing, and a tubular spacer 48 therebetween sized to accommodate the motor shaft coupling.

FIG. 4 depicts the motor shaft coupling 42 from the inboard side. The motor shaft coupling 42 functions as an adapter and includes a set of recesses or openings 50 sized and shaped to receive protrusions of the spool axle-which in this case are revealed when the manual hand crank is removed from the Fathom spool. The opposite side of the coupling 42 includes a recess sized and shaped to receives the keyed motor shaft. The outer diameter of the coupling 42 fits within the inner diameter of the tubular portion 48 of the motor mount 38.

TABLE I is a tether length chart listing the cumulative length of undeployed tether per layer based on the dimensions and specifications of the Fathom spool, with Layer 1 closest to the center of the spool.

The calculated lengths assume that the tether is layered sequentially. Layer 15 contains 76 feet of tether, and layer 14 contains 73 feet of tether. This roughly corresponds to the small discrepancy in the results discussed below. In these experiments, the tether as measured was not neatly layered onto the spool. Layering the cable neatly onto the spool requires an additional axis and a cable leveler to properly align each revolution onto the spool. The experiments showed that the best accuracy was obtained by manually tuning the tether parameters (e.g., effective diameter of the spooled tether) and checking the deployed tether length.

The resulting tether manager demonstrated acceptance of targeting (i.e., deployed length) commands over a variety of ubiquitous fieldbus protocols (MQTT, Modbus/TCP). For purposes of the experiments, the PLC programming software was used for setting the targets and tuning both the motor performance characteristics and physical parameters of the spool. To characterize performance, the servomotor was first zeroed and given a 4-foot offset to compensate for the distance between the floor and the spool. The tether manager was placed on top of a cart, and the ROV was placed on the bottom shelf of the same cart. The tether was pinned alongside the spool for purposes of measuring a consistent distance. The tether manager was commanded to adjust the deployed length of tether at each of several different lengths in a range from 3 to 50 meters. After each command, the tether was placed flat on the ground possible and measured.

The measurement results are illustrated in FIG. 5 and show that the tether manager can accurately adjust the deployed length of tether within 2 feet or less. Several different spool diameter tuning techniques were used, and it was found that the most consistent performance was obtained by assigning a static spool diameter rather than dynamically adjusting the diameter as the tether is deployed or retracted. The dynamic adjustments are believed to have been inconsistent due to the lack of a leveling mechanism—i.e., the lengths in TABLE I assume neatly layered tether on the spool, but this was not the case in the experiments.

Target-updating algorithms can present challenges with servomotor controllers because of the command processing schemes typically employed. The execution loop used in these experiments took that into account and dynamically adjusted the target lengths with update frequencies several orders of magnitude above the update frequency performance of the MQTT controller. Generally, the Modbus/TCP connections operated on a tighter, yet still non-deterministic, schedule slower than the servo manager as implemented in the code.

The use of a safety-rated controller and drives allowed for end-to-end SIL3 safety without the need for external torque monitoring. The tether manager was functionally safe without the need for external caging, but that could change if non-safety-rated drives or gearboxes capable of torque multiplication are used.

It is to be understood that the foregoing description is of one or more embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to the disclosed embodiment(s) and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art.

As used in this specification and claims, the terms “e.g.,” “for example,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.