A Structural Design of Underwater Live Plug and Pull Connector

1 Overview

As underwater observation stations expand their range of underwater exploration and provide more and more data, this directly drives the continuous innovation of underwater plug-in connectors. Nowadays, the United States, the United Kingdom, and Japan all have their own underwater observation stations, and underwater plug-in connectors are one of the conditions for building new types of underwater observation stations. At present, overseas underwater plug-in connectors have developed from a single variety to various powerful improved products, such as all optical and optoelectronic hybrid connectors; Developing from low-power connectors to high-power, high-voltage, multi-core connectors.

There is already a considerable technical foundation in the field of watertight connectors in China, but most of them are dry plug underwater connectors, which are limited in application environments. At present, the field of underwater insertion and extraction technology in China is still blank and in the initial stage of development. Key technologies related to underwater insertion and extraction still need to be broken through. The underwater plug-in connectors in domestic marine engineering are all imported products, with high costs and long procurement cycles, which restrict the development of marine engineering.

There are significant differences between underwater plug-in connectors and traditional watertight connectors, in terms of principle, structural design, and operational performance. This component can play a crucial role in connecting equipment systems and weapons, and most importantly, it has an instant on/off function, greatly improving the mobility, safety, and adaptability of the equipment system. It can be widely used in various fields such as underwater oil platforms, mining engineering, underwater search and rescue, and submarines for power transmission, electrical/hydraulic control, signal transmission, etc. It is of great significance for China's marine exploration and development, as well as the development of naval equipment and weapon systems.

With the support of the scientific research project of the Ministry of Industry and Information Technology - the development of the docking plate and locking mechanism for underwater control systems, an underwater live plug and unplug connector has been developed.

Composition of underwater plug-in connector structure

The underwater plug-in connector consists of two parts: a socket and a plug.

The socket includes: pin assembly, base, metal shell, flange, locating pin, sealing ring, protective cover assembly, etc. The appearance of the socket is shown in Figure 1, and the structural diagram is shown in Figure 2.

A Structural Design of Underwater Live Plug and Pull Connector (Figure 1)

Figure 1 External view of socket

A Structural Design of Underwater Live Plug and Pull Connector (Figure 2)

Figure 2 Socket Structure (2D)

The plug includes: socket component, inner shell component for adjusting pressure balance oil chamber, ROV wet insertion and extraction device, locking mechanism, matching cable connection component, etc. The appearance of the plug is shown in Figure 3, and the structural diagram is shown in Figure 4.

A Structural Design of Underwater Live Plug and Pull Connector (Figure 3)

Figure 3 Plug Appearance

A Structural Design of Underwater Live Plug and Pull Connector (Figure 4)

Figure 4 Plug Structure (2D)

Working principle of underwater plug-in connectors

This product is an underwater live plug-in connector that adopts technologies such as dynamic sealing, pressure compensation, electrical contact flushing, cable sealing, and flexible elbow joints. The connection method is ROV wet insertion and extraction, which can be used for live insertion and extraction in marine environments through frogmans, remote-controlled submersibles (ROVs), or underwater robots (AUVs). It has the characteristics of fast, efficient, and reliable connection.

The plug design includes an electrical contact system, pressure balancing oil chamber, ROV wet insertion and extraction device, locking mechanism, and cable connection sealing chamber. ROV operation plug in seawater environment, completing the insertion and separation with the socket. When inserting and disconnecting plugs and sockets, the conductive parts of the pins and sockets are connected and disconnected in insulating oil, which can effectively extinguish the arc generated during connection and separation, and achieve live insertion and extraction. Its working principle is as follows:

3.1 Unplugged state

3.1.1 Plug

The sealing performance of the plug depends on the sealing effect of the inner shell component. The working principle of the inner shell component is shown in Figure 5.

A Structural Design of Underwater Live Plug and Pull Connector (Figure 5)

Figure 5 Inner shell components in non inserted state

One end of the socket component in the inner shell component is sealed and fixed in the front base hole, and the other end is sealed and fixed in the rear base hole; The sliding pin inside the socket assembly is made of PEEK high-strength insulation material, which is sealed with the internal cavity of the socket assembly through an 18mm long fluorosilicone rubber inlet sleeve.

The sealing chamber of the housing component inside the plug is filled with insulating oil, and the socket components are distributed in chamber 4 filled with insulating oil. When the plug enters the water, the external pressure is greater than the inside of the oil chamber, and seawater enters the outer chamber through the pressure balance hole 1. The seawater in the outer chamber causes membrane 1 to deform inward; The internal oil pressure of the buffer chamber increases, and the insulating oil flows and drives the pressure balance film 2 to move inward; The oil pressure in chamber 3 increases, and the insulating oil enters chamber 4 through hole 2, causing the pressure in chamber 4 to tend towards balance, ultimately achieving pressure balance inside and outside the plug oil chamber.

3.1.2 Sockets

The working principle diagram of the socket is shown in Figure 6. The pin assembly is fixed in the installation plate, and a sealing ring is used between the pin assembly, the installation plate, and the socket housing.

A Structural Design of Underwater Live Plug and Pull Connector (Figure 6)

Figure 6 socket not plugged in state

3.2 Underwater plugging and unplugging

3.2.1 Insertion process

The connector insertion status is shown in Figure 7. During insertion, the pin of the socket contacts the sliding pin of the plug, squeezing out the seawater in the groove of the sliding pin. The pin of the socket is inserted forward to push the sliding pin into the cavity of the socket component. The spring is compressed, and the conductive part of the pin contacts the slotted socket, completing the electrical connection.

A Structural Design of Underwater Live Plug and Pull Connector (Figure 7)

Figure 7 Plan view of connector insertion status

When the pin of the socket is inserted, the pressure in the cavity of the socket component increases, and the inlet valve of the socket component opens to communicate with cavity 4. The insulation oil inside the socket component flows towards the contact area of the socket under the pressure to flush the slotted socket and improve the reliability of electrical contact; At the same time, insulating oil enters chamber 4, causing an increase in pressure in chamber 4, forcing membrane 2 to deform outward. The oil pressure in the buffer chamber increases, causing membrane 1 to deform outward, achieving a balance between the pressure inside the oil chamber and the external seawater. During the insertion process, the pin component of the socket replaces the sliding pin and forms a dynamic seal with the inlet sleeve; After the insertion is completed, the insulation part of the pin assembly is tightly sealed with the inlet sleeve to seal the oil chamber.

3.2.2 Separation process

Pull the handle, and the locking mechanism and handle move backwards together. The locking hook on the inner shell assembly generates radial movement under the guidance of the unlocking sleeve contact slope on the locking mechanism. The locking hook opens and the plug and socket are unlocked; Continuing to pull the handle backwards, the inner shell will move together, and the socket and pin will separate to achieve electrical signal disconnection.

When the pin is pulled out, the pressure inside the cavity of the socket assembly decreases, and the insulating oil in cavity 4 enters the interior of the socket assembly. The buffer chamber, outer chamber, and seawater sequentially supplement the pressure and adjust the pressure balance. At the same time, the spring pushes the sliding pin and the pin assembly to move synchronously, and returns to their original position to seal the oil chamber again with the inlet sleeve.

4 Calculation analysis

This article combines the structural design of 4-core and 12 core underwater live plug and unplug connectors developed by the project, and provides the design and calculation method of underwater live plug and unplug connectors through theoretical calculation combined with experimental verification.

4.1 Calculation of frictional force for dynamic sealing

The dynamic sealing structure of the plug is shown in Figure 8:

A Structural Design of Underwater Live Plug and Pull Connector (Figure 8)

Figure 8 Dynamic Sealing Structure

For contact type dynamic seals, there is relative sliding between the sealing dynamic and static surfaces. In order to maintain sealing, a certain positive pressure must be applied between the sealing relative sliding surfaces. In the absence of a fit, the sealing friction force F is directly proportional to the effective contact pressure, i.e.:

A Structural Design of Underwater Live Plug and Pull Connector (Figure 9)

In the formula: μ- Friction coefficient;

PC - effective contact pressure;

A - Width of sealing contact surface;

B - The length of the sealing contact surface.

When the seal is a circular seal, b=π d, where d is the axial diameter of the seal.

In most cases, non-metallic sealing materials do not have the problem of surface fireworks film, and pollution has little impact on them. As for commonly used sealing rubber, it has the characteristics of being soft and elastic, and the friction coefficient varies with different sliding speeds. When the sliding speed is very low, the static friction coefficient is greater than the dynamic friction coefficient. When the sliding speed is high, the dynamic friction coefficient is very large. When dry friction occurs between rubber and structural components, the friction coefficient can reach around 0.8 to 1.0 at normal insertion and extraction speeds. The coefficient of friction is also a function of pressure, but its exact variation is influenced by various factors, and its approximate relationship is shown in Figure 9.

A Structural Design of Underwater Live Plug and Pull Connector (Figure 10)

Figure 9 Friction coefficient curve of rubber with pressure variation

Based on the mechanism analysis of dynamic sealing and combined with structural design, the inner cavity of the plug is sealed with oil filled pressure equalization, with a maximum pressure difference of 0.15MPa between the inside and outside. The corresponding sliding friction coefficient with oil or water as lubricant is 0.1, and the sealing shaft is Φ 3mm, the length of the inlet sleeve is 18mm, and the frictional force of the dynamic seal is determined as:

F1=0.1 × zero point one five × eighteen × three point one four × 3=2.54N.

4.2 Spring force of sliding pin spring

When the plug is pulled out of the socket, in order to ensure that the socket pin exits from the plug socket assembly, the sliding pin overcomes friction under the spring force and forms a seal with the inlet sleeve again, it is necessary to ensure that the spring has a certain spring force.

The stiffness of the spring causes the load required for unit axial deformation of the spring, i.e. Kp=the shear modulus of the material in the structural design of an underwater live plug connector (Figure 11);

D - Spring diameter;

D2- Spring diameter;

N - effective number of turns.

The spring designed according to structural requirements has a shear modulus of 81000MPa, a spring diameter of 0.8mm, a spring pitch diameter of 6.4mm, and an effective number of 26 coils. The calculated Kp is 0.6N/mm. The specific spring design parameters and spring stiffness in the application are shown in Figure 10.

A Structural Design of Underwater Live Plug and Pull Connector (Figure 12)

Figure 10 Spring design dimensions and characteristic curves

When a pin component of the socket is just inserted into the plug socket component, it needs to overcome a pre tightening spring force of 8N to finally achieve complete insertion of the pin component into the socket component, that is, the locking hook installed on the housing component of the plug and the circular groove on the socket are matched and locked. Inserting a pin component into the socket component requires overcoming a spring force of F2=32N.

4.3 Separation force of slotted socket

When the socket pin is inserted into the slotted socket of the plug socket component, the slotted socket is designed with a tapered closure to ensure full contact between the pin and the slotted socket, reducing contact resistance.

A Structural Design of Underwater Live Plug and Pull Connector (Figure 13)

Figure 11 Outline diagram of slotted socket

Design a slot width of 0.5mm and a slot length of 6mm, with four evenly distributed slots. Then, use a fixture to close the slot, with a taper of 2 °. After closing, perform an aging treatment of HV350-380, and finally coat with Cu/Ep Ni3Au1.27. Through verification experiments, standard pin diameters are used φ 2.98 mm, insertion depth of 5mm, separation force of the socket is about 200g~500g, which can ensure that the contact resistance between the pin and the slotted socket meets the requirements.

When the socket pin is inserted into the slotted socket, the maximum separation force is F3=500g, which is 4.9N.

4.4 Deformation force of locking hook and spring force of safety spring

The deformation force of the locking hook and the spring force of the safety spring mainly ensure that after the plug and socket are inserted, the plane of the locking hook reliably fits with the circular groove end face of the socket. In theory, under the load-bearing capacity of the locking hook, pulling the inner shell component in the direction shown in the diagram, the socket and plug cannot be unlocked. Only when the unlocking mechanism is pulled, the slope of the unlocking mechanism deforms the locking hook and causes the safety spring to move outward. When the locking hook completely exits from the socket ring groove, the socket and plug are unlocked and separated.

A Structural Design of Underwater Live Plug and Pull Connector (Figure 14)

Figure 12 Connector Locking Status

After the deformation force and spring force of the locking hook are verified through experiments to meet the locking requirements of the plug and plug, without installing a socket component in the housing component of the plug, the plug can be inserted into the socket and reliably locked. The deformation force of the locking hook and the spring force of the safety spring can be independently tested to be about 50N to 80N. When the socket is inserted or separated from the plug, the maximum deformation force of the locking hook and the spring force of the safety spring that needs to be overcome is F3=80N.

4.5 Insertion and separation forces between sockets and plugs

4-core plug and socket, insertion and separation force:

A Structural Design of Underwater Live Plug and Pull Connector (Figure 15)

12 core plug and socket, insertion and separation force

A Structural Design of Underwater Live Plug and Pull Connector (Figure 16)

Through theoretical calculations, the insertion and separation forces of the 4-core plug and socket can meet the required FM ax=250N. Due to differences in actual processing, assembly and debugging, as well as theoretical calculations, we obtained an actual insertion force of 245N and a separation force of 94N through multiple insertion and separation of the four core socket and plug, with a maximum insertion and extraction rate of 0.02m/s. This verified that the theoretical calculations are basically consistent with the actual situation.

The insertion and separation force of the 12 core plug and socket can meet the FMax requirement of 650N.

4.6 Necessity of pressure compensation system design

The pressure compensation of the plug is mainly achieved through a compensation film. The inner shell component of the plug is filled with oil. When the water depth changes, the skin bag type compensation film will change and stick to the inner liner. Based on the incompressibility of the oil, the internal compensation pressure will be equal to the external environmental pressure.

The pressure compensation system has three main functions in this electrical plug and unplug connector. Firstly, it reduces the difficulty of sealing. Secondly, the insertion and separation forces between the socket and the plug will not change with changes in water depth and pressure. Thirdly, the sliding pin of the socket component in the plug will not overcome the spring force and shrink back into the socket component due to changes in water depth, causing seawater to enter the plug.

If there is no pressure compensation system inside the plug, the pressure on the sliding pin in the deep sea increases with the increase of water depth, that is:

F5=P * S

In the formula: F5- pressure, N;

P-static water pressure, MPa;

Cross section area of S-sliding pin sealing shaft, mm2.

When the plug descends to a depth of 500m, the sliding pin sealing shaft is Φ 3mm, the sliding pin is subjected to a pressure of F5=35.3N, while the sliding pin overcomes the dynamic friction force F1 and the spring force entering the inlet sleeve, i.e. F2 ′=F1 ′+Kp * l.

In the formula: F2 '- Spring force required for the sliding pin to enter the inlet sleeve, N;

F1 '- Spring preload force, N;

Kp spring stiffness, N/mm.

L - length of inlet sleeve, mm.

F2 ′=F1 ′+Kp * l=8+0.6 × 18=18.8N.

Due to F5 ≥ F2 ′, the sliding pin of the plug will retract into the socket assembly when it reaches the working depth, resulting in seawater contamination of the plug and damage.

So the pressure compensation system is the key to the underwater live plug insertion and extraction. During the process of the plug entering and exiting the water, as well as in the non working state underwater, the pressure balance system constantly adjusts the internal pressure of the oil chamber to maintain consistency with the external seawater pressure, so that there is no pressure difference between the inside and outside of the oil chamber. Due to the pressure balance system designed inside the plug, the pressure inside the sealing chamber of the plug is consistent with the external seawater pressure, ensuring that the sliding pin in the socket assembly will not be pushed back into the socket assembly under seawater pressure in deep-sea environments, thus better ensuring the plug seal.

Conclusion

The technology of such connectors in our country is relatively backward, and foreign manufacturers have extremely high prices. Currently, they have optical and electrical connectors that can be plug and play underwater, and most of them are used in national key projects. Once the international situation changes, this connector becomes a prohibited item. Therefore, the successful development of underwater plug-in connectors has a very obvious effect and will be a huge promotion for the three-dimensional technology development in China's marine field.