underwater ISSN 0141 0814. International Journal of the Society for Underwater Technology, Vol 26, No 3, pp ??-??, 2005 TECHNOLOGY

Research on the compensation for the underwater hydraulic motor Technical Paper Technical Y LI and Q WANG The State Key Laboratory of Fluid Power Transmission and Control, Zhejiang University, Hangzhou, PR China

Abstract The properties of UAP can be considered incompressible at a moder- Considering the influence of underwater ambient pres- ate depth (down to about one thousand meters). The sure (UAP) on underwater hydraulic motors (UHMs), and pressure at the moderate depth h below the sur- utilising the state-of-the-art of pressure compensation of face is underwater hydraulic systems (UHS), this paper propos- 1 ⎡ ⎛ v ⎞ 2 ⎤ ρ 2 ⎢ ⎥ es a pressure compensation technique for the UHS psea =p a + gh+ V1-⎜ ⎟ (1) 2ρ ⎣⎢ ⎝ V ⎠ ⎦⎥ where the hydraulic power unit is installed inside a or a submersible drilling platform with where Psea is the ambient hydrostatic pressure Pa is the , some actuators of which are atmospheric pressure, ρ is the density, g the gravi- directly surrounded by seawater and others are installed tational acceleration, h the depth of the point, v the inside it. An underwater ambient-pressure-compensation local , and V the velocity of the body through (UAPCV) has been developed. The pressure in the the water. return line of the external sub-circuits of the UHS is com- The first two items on the right of equation (1) is a pensated by this UAPCV, but not that of the internal sub- seawater static pressure. Moreover, there exists a seawa- circuits. Theoretical analysis, simulation and experimental ter dynamic pressure, which is the last item on the left results show that reliable pressure compensation can be of equation (1). ensured with a small flowrate by the UAPCV. The opera- The currents result from the superposition of tional performance of the UHM is further improved after a great number of disturbances or external . its leakage pressure and return pressure are compensat- Basically they include atmospheric-pressure variations, ed. forces acting on the free surface, seismic distur- bances acting on the bottom of the and the Keywords: Underwater hydraulic system, pressure combined attraction of the and acting on the compensated valve, underwater seawater.1 When the undercurrent dynamic variation acts on the surface of underwater equipments, the 1. Introduction undercurrent flow variation is turned into pressure vari- ation acting on the external surface of underwater Underwater vehicles, such as a remotely operated vehicle equipments. (ROV) or autonomous underwater vehicle (AUV), and The speed of underwater equipments, such as machinery, such as submarine and ascent, causes a hydrodynamic pressure. cable trenchers, deep diving , manned sub- The vertical speed causes a variation of seawater depth. mersibles, submersible drilling platform etc, play an The external ambient pressure of equipment changes important role in ocean exploitation research. Hydraulic with seawater depth; the horizontal speed of underwa- systems, due to their remarkable advantages, are widely ter equipment creates a hydrodynamic pressure because used to control these underwater equipments. When of a relative kinematical velocity to the seawater. The working in a certain depth, the underwater ambient pres- UAP variation affects a UHM in many respects when sure (UAP) will considerably affect the performance of the UHM is underwater. some hydraulic control components and actuators of an underwater hydraulic system (UHS). Hence, one must 2. Influence of the UAP on UHM take into account the influence of the UAP on the hydraulic components and try to reduce or remove its Influence on output torque of UHM influence by adopting some suitable techniques. The The operating schematic of a radial piston hydraulic main goal of this paper is to explore and describe an motor is shown in Fig 1.2 The UAP directly acts on an available pressure compensation method for some special external end face of a shaft 6 when it is working in the sea- underwater hydraulic motors. water environment. The other end face of shaft 6 is in

51 Li & Wang. Research on the pressure compensation for the underwater hydraulic motor

sary that some suitable methods must be adopted in order that conventional hydraulic motors can directly be used in the sea- water pressure environment.

Necessity of the ambient pressure compensation Traditionally, a method to solve the above problems is adopting Fig 1: Operating schematic of radial piston hydraulic motor pressure compensation for underwater hydraulic systems. housing 1 and 2. The internal chamber of the housing is That is to say, a UAP is added to hydraulic systems. connected with a leakage line or a return line of an Pressure in the leakage line of UHM and the return line underwater hydraulic system. The pressure of the inter- of UHS is slightly larger than the UAP by a constant. nal end face of shaft 6 is approximately atmospheric. Balance between the internal and external pressure of Shaft 6 is subject to an unbalanced axial . The unbal- the UHS can be reached after being compensated. Like anced axial force acts on its main bearings as well. The this, the unbalanced axial force caused by the UAP can unbalanced axial force is variable with the seawater be removed. Any elastic deformations of the UHM depth, which causes an increase of the output torque. The shell will disappear; those sealing components prevent- output torque caused by the UAP reduces the mechanical ing contamination from external leakage can work nor- efficiency of the hydraulic motor, leads to an increased mally as well. requirement for input pressure, increases internal leakage On the other hand, using a cam lobe hydraulic and reduces volumetric efficiency. Additionally, when the motor as an example, the leakage consists of seawater depth is deep enough, the UAP can affect main three parts; the leakage in the clearance of the port bearing life and even the UHM life. plate, the external leakage in the clearance between pis- tons and the cylinder hole and the volume loss caused by Influence on pressure proof capacity of the the elastic compression of the fluid at the bottom of pis- UHM shell tons. Each parameter related with the structure of the The inner chamber of a conventional hydraulic motor hydraulic motor cannot change with the operating envi- is connected with the leakage line or the return line of ronment after the type of a hydraulic motor has been hydraulic systems. The hydraulic motor shell can only determined. These above-mentioned volume losses are suffer from a lower pressure than ambient hydrostatic only related to the supply pressure and the return pres- pressure. So the hydraulic motor effectively runs in a sure of the hydraulic motor.3 The volume loss caused by shallower depth. the UAP can be removed or greatly reduced if the pres- A mechanical element shaped as a vessel, such as a sure difference between the input port and the output hydraulic motor, hydraulic cylinder, control valve and so port of the UHM can be kept constant or only slightly on, can inevitably suffer an elastic deformation due to the changed after the pressure has been compensated. variation of the external UAP. The holding force of some connecting pieces can become larger or smaller due to State-of-the-art research the elastic deformation. Bolts of the shell perhaps may be Hydraulic systems of underwater machinery can be loose and lead to failure of the underwater hydraulic classified into two kinds according to their layout. In the component and seawater invasion into the UHS. first type, the whole hydraulic system is directly under- water, such as hydraulic systems of underwater vehicles Influence on sealing components (ROV/AUV), underwater working tools, underwater Some conventional hydraulic sealing components are construction machinery, such as a subsea cable trencher, used to act in only one direction. The function of the and so on (shown in Fig 2(a)). The second type is that seal is to leak-proof in one direction. A UHM is subject the hydraulic power unit is installed in the atmospheric to bi-directional pressure when it is underwater. These circumstance, some hydraulic actuators of the system sealing components will lose their functionality when are surrounded by seawater, and other actuators are in the UAP is bigger than the inner pressure, and the sea- the same environment as the hydraulic power unit; for water may invade the hydraulic system and contami- example, external hydraulic sub-circuits of manned nate the fluid. submersibles or submersible drilling platforms, etc, The UAP can affect efficiency, mechanical perform- (shown in Fig 2 (b)). ance and sealing components of the UHM to some Presently many researchers have been carrying out extent as illustrated by the above analysis. So it is neces- research on conventional pressure compensators. A blad-

52 underwater TECHNOLOGY Vol 26, No 3, 2005

hydraulic system utilises seawater as its pressure medium, which is com- patible with the oceanic working environment. This type of system can use direct seawater suction and dis- charge. The pressure of the hydraulic system is automatically compensated, which is suitable for any seawater depth. The system structure is sim- pler, has less and a higher effi- ciency. Response frequency and posi- Fig 2: Layout of underwater hydraulic systems tion control precision are largely enhanced but the physicochemical property of seawa- der-type pressure compensator can be installed directly on ter is different to mineral oil. The properties of seawa- the UHP reservoir to compensate for pressure, where the ter are lower viscosity, poor lubricating ability, higher hydraulic power unit is directly in the underwater pressure conductance, higher vaporisation pressure, and so on. ambience.4-7 This kind of reservoir is watertight. The Some problems, such as corrosion, abrasion, cavita- UAP is transferred into the reservoir by the pressure tion erosion, insulation, seal related, freeze, and so on, compensator. The pressure in the UHS oil tank is have to be solved when hydraulic equipments and increased to a pressure slightly exceeding the UAP. components that are compatible with seawater are The compensated pressure is added into the supply developed. As a research direction, seawater hydraulic pressure of the UHS as an increase in the suction systems are facing severe challenges because of the pressure of the hydraulic pump. The goal of the pres- above disadvantages.12-14 sure compensation is reached. But the hydraulic At present there is no suitable pressure compensa- power source of an underwater hydraulic system tion for external hydraulic sub-circuits whose hydraulic whose oil tank is at atmospheric pressure can supply power unit is in the atmospheric circumstance. This pressure fluid to sub-circuits both in the atmospheric paper describes a new method of pressure compensa- circumstance and in the . In tion of these hydraulic systems and a key pressure-com- this circumstance, the reservoir is not watertight and pensated hydraulic component — a valve meeting the open to the atmospheric pressure. So the convention- required underwater ambient pressure compensation al pressure compensation method cannot be used in structure is proposed. Moreover, the performance of an this type of system with the hydraulic power unit underwater hydraulic motor is investigated after the being under atmospheric pressure. A patent applied new pressure compensation measure has been used. for by Bodycoat and DiNola (USA 1994), stated that a rubber diagram could be used as the pressure-sensing 3. Operating mechanism of pressure element to transmit underwater ambient pressure compensation with UAPCV (UAP) to an external hydraulic system on a submarine8 but it hardly has any practical values because the This new pressure compensation method of an UHS, working principle is similar to the underwater whose hydraulic power unit is in the atmospheric cir- hydraulic power source and the working volume is lim- cumstance, is to install a pressure control valve with ited by rubber diaphragms. Another patent proposed underwater ambient pressure compensation on the by Moody (USA 1993), stated that a submarine exter- return line of the UHS, as shown in Fig 3. The UAPCV nal hydraulic fluid-isolated system can isolate an exter- is inserted into the return line of the UHS. The nal hydraulic system from the main system to prevent UAPCV senses a UAP with a direct measurement and the seawater invasion into the main hydraulic system, transfers the UAP into the return line of the UHS. A but the pressure compensation measure was not taken UAP is forced into the return pipe of the UHS by this into account.9 A traditional handling method is to put valve. This arrangement will raise the pressure in the hydraulic control components and actuators into a return line from the atmospheric pressure to a slightly pressure container subject to underwater pressure. But higher pressure above the UAP. The pressure in the it makes the equipment’s structure more complicated return pipe can vary with the depth of water and and the weight heavier. And it also needs a reliable always maintains a constant increment above the UAP. dynamic and static seal in the interfaces between the A method of pressure compensation for an UHM, pressure container and the hydraulic motor shaft and which has a leakage port, has to be carried out depend- between the pressure container and the cylinder rod. ing on the operating property of the hydraulic motor. On the other hand, there are more researches10,11 The return pressure must be somewhat higher than the on types of seawater hydraulic systems. The seawater leakage pressure at a certain speed to prevent rollers 3

53 Li & Wang. Research on the pressure compensation for the underwater hydraulic motor

Fig 3: Operating principle of UAPCV continuity equation are given as follows, respectively dx K (x -x)-K (x +x)+A (p -p )-K + sc sc0 r r0 c sea c u dt dx2 2C C A(x)p cosθ =m dv c tdt 2 (2) 2 Q=CA(x) p Ldρ c (3)

2 Vdp dx Q=CA(x) p+ c c -(A -A ) +C p (4) Ldρβ c cv lc e dt dt

Where ksc is the stiffness coefficient of the preloading

spring, xsc0 is the precompressed length of the preload- ing spring, x the axial displacement of the spool and the rolling diaphragm, (that is the opening length of the

valve), kr is the stiffness coefficient of the back-moving

spring, xr0 is the precompressed length of the back-mov-

inside the hydraulic motor (shown in Fig 1) from leaving ing spring, Ac is the effective area of the pressure com- the track 5. UAPCVs must be inserted into the return pensated chamber, A(x) the opening area of the valve,

line and the leakage line of the UHM. and it is a function of the spool displacement x, psea is the

UAP, pc is the pressure compensated, Cd is the discharge

Operating principle of UAPCV coefficient, Cv is the velocity coefficient, mt is the equiv- The operating principle of the UAPCV (shown in Fig 3) alent mass of the spool and the rolling diaphragm unit, ρ is: a rolling diaphragm senses UAP psea. The rolling Ku is the viscous damping coefficient, is the fluid mass

diaphragm transfers the UAP psea into a pressure compen- density, QL is the flow in return line of UHM, Vc the vol-

sated chamber Vc. Pressure compensated pc in chamber Vc ume of the closed chamber between the valve port and β slightly exceeds UAP psea by a constant due to a force the actuator, e is the effective bulk modulus, AV is the

exerted by a preloading spring. Making use of direct feed- end area of the spool, and CL is the leakage coefficient. back of the compensated pressure enhances the response Only the steady state flow force is considered. The and the accuracy of the pressure compensation. transient flow force, motion resistance force and In practice, the UAPCV is a reactive mechanism. coulomb friction are all neglected because they are The position of the rolling diaphragm is dependent on small in equation (2). the working position of the spool. Movement of the In making a dynamic analysis it is necessary that the rolling diaphragm depending on a pressure difference nonlinear algebraic equations be linearised. The lin-

between pc and psea manipulates movement of a control earised equations of (2), (3), (4), Laplace transformed, . The control bar manipulates the valve spool move- and rearranged, are ment. The spool position controls the pressure compen- ⎡ 2 ⎤ ⎣mt s +Kuscrsxcseacsp s+(K +K -K )⎦ x=A (p -p )+K pc (5) sation. There are three operating positions of the spool: V pressure compensated working position, mid-position ⎡(A -A )s-K⎤ x+Q =( c s+K +C )p ⎣ cv qx⎦ L β qp l c (6) and refilling working position. When the valve is at the e refill working position, fluid in the supply line of the 2 hydraulic system refills the pressure compensated where K=2C p x (d-x ) is the flow gain, qx dρ c0 0 0 chamber preventing fluid scarcity caused by leakage. The pressure compensation can also be carried out 1 even though the actuator has not operated for a long K=CA(x)qp d 0 is the flow pressure coefficient, 2pρ period. When the valve is at mid-position, each port is c0 blocked. When the actuator is operating, fluid runs K =4C C cosθ p x (d-x ) through the return line of the hydraulic system. The sx d v c0 0 0 is the spring gradient of θ valve port connected with oil tank is opening. The valve steady flow force, Ksp =4C d C v cos A(x 0 ) is the steady is at the pressure compensated operating position. flow force pressure coefficient. A diagram of the transfer function of the 4. Modelling and simulation of UAPCV UAPCV is showed in Fig 4 according to equations (5) and (6). The input signals are a UAP and a return flow

Modelling of the UHS or a leakage flow of the UHM QL. Fig 4 When the UAPCV is at the pressure compensated work- shows that the open loop transfer function of the system ing position, its kinematic equation, flow equation and contains a one second-order oscillating section, one

54 underwater TECHNOLOGY Vol 26, No 3, 2005

Fig 4: The transfer function diagram of a UAPCV delay section and one differential section. The main coefficient, the leakage coefficient and so on. All the section influencing the dynamic performance of the above-mentioned methods can improve the pressure UAPCV is the second-order oscillating unit. compensation precision of the UAPCV.

Setting QL=constant, if only the UAP would change, the steady error of the pressure compensated Simulation caused by the UAP variation would be Simulation of the above nonlinear mathematical model equations (2), (3), (4)) has been carried out by MAT- Pc = 1 = ++ LAB. Simulation parameters are given by Psea s 0 (-)()KKKKC K sc r sx qp l + sp -1 (7) ρ=850kg/m 3, C =0.65,C =0.96, C 1×10-14, θ=690, AKcqx Ac d v l d=4mm, dc=55mm, dv=12mm, ksc=2820N/m, Setting psea = constant, if only the flow would β × 9 kr=3130N/m, xsc0=30mm, xr0=7mm, e=1 10 , change, the steady error of pressure compensated V =1l, k =60, m =0.2kg. caused by the flow variation would be c u t A response curve of the pressure compensation for the Pc 1 = UAP is illustrated in Fig 5. The response curve shows that Q s=0 K(A-K)qx c sp (8) L (K +C )- the response speed is faster than that of a conventional qp l K+K-K sc r sx valve. But the pressure compensated overshoot is somewhat Equations (7) and (8) indicate that the flow, the UAP large. A fluctuation of the pressure compensation occurs and some main geometric parameters of the UAPCV when the flow rate changes. The resulting pressure com- are the main influencing factors of the accuracy of the pensation caused by either UAP variation or flow rate pressure compensation. change is always slightly higher than the UAP, and cannot (1) Influence of the flow: the sum of spring forces of affect normal operation of the UHS. Moreover the steady the spool end, and steady state flow force will increase error of the pressure compensation is small. with the flow increasing when the UAPCV is at the The statistic data of the UAP, the flow and the com- pressure compensated position and also the compensat- pensated pressure during each time slice are listed in ed pressure compensated will increase. The steady error Table 1. It indicates the variation of the compensated of the compensated pressure will increase.

(2) Influence of the UAP: the UAP will increase with Fig 5: Response curve of pc to psea variation and flow increasing seawater depth which causes large pressure change compensation. The deeper the depth the smaller is the valve opening area. When the flow is constant the sum of spring forces will reduce with increasing seawater depth. Eventually steady error of the compensated pressure will reduce. (3) Influence of some structural parameters: reduc- ing the spring stiffness can reduce the variation of spring forces caused by variation of the opening length. Increasing the effective working area of the pressure compensated chamber if the spring force is constant, increases the flow gain, increasing the flow pressure

Table 1: Simulation data

t (s) Psea (MPa) QL(litre/min) Pc(Pa) Pc-Psea(Pa)

t<0.1 0 0 25 900 25 900 0.1 ≤ t<0.2 1 0 1025 300 25 300 0.2 ≤ t<0.3 1 3 1026 300 26 300 0.3 ≤ t<0.4 1 8 1026 700 26 700

55 Li & Wang. Research on the pressure compensation for the underwater hydraulic motor

pressure caused by the UAP variation and flow change. The above simulation results reveal that the pressure The difference between the compensated pressure and compensation performance of the UAPCV can be the UAP reduces with the UAP increasing when the entirely ensured when the UAP rapidly increases or flow is zero. The steady error of the compensated pres- reduces, whether the flow rate changes or not (whether sure reduces; the difference between the compensated the UHM is working or not); the follow-up performance pressure and the UAP increases with the flow increasing of the UAPCV is also better. when the UAP remains constant. The steady error of the compensated pressure rises. These results are the 5. Experimental research same as the previous theoretical analytical results. Fig 6 shows the simulation result of the compensat- Establishment of an experimental rig ed pressure increasing and reducing with the UAP vari- An established experimental rig is illustrated in Fig 7. A ation when the UAP is a triangle wave. Function of pres- high- imitates the UAP. A UAPCV is sure compensation can also be completed during t<0.6s installed in the leakage line of a UHM. The UAPCV when the flow rate passing through the UAPCV is zero can sense and transfer the UAP into the leakage line of (which is the UAPCV is working at mid-position). A fluc- the UHM in order to compensate for a pressure in the tuation of the compensated pressure caused by the flow leakage line of the UHM. A proportional pressure con- rate change is very small at t=0.6s and t=1.1s when the trol valve, which is installed in the return line of the flow rate is 3litre/min and 6litre/min respectively. The UHM, is used as pressure compensation for the return follow-up performance of the compensated pressure is line of the UHM. The UAPCV and the UHM are better, too, when the UAP rapidly reduces. installed in the high-pressure vessel. Other components are all installed in the atmospheric environment. Some real time experimental data, such as the UAP, the com- pensated pressure, the supply pressure and the return pressure, are collected by a computer.

Experimental analysis of the UAPCV

Fig 8 and Fig 9 indicate that compensated pressure pc is

always higher than UAP psea by a constant whether UAP

psea increases or decreases, and follows up UAP psea vari- ation in time. Even if UAP psea rapidly changes, the

compensated pressure pc can change with UAP psea. The dynamic performance of the UAPCV is good. Fig 10 shows that the fluctuation of compensated pressure caused by a flowrate change cannot be detect-

Fig 6: Response curve of pc to psea ascent and ed when UAP psea doesn’t change but the flowrate pass- descent and flow change ing through the UAPCV changes (the leakage flow rate changes when the return pressure of the UHM changes). This phenomenon can be explained in that the variation of the compensated pressure is rather small. It is also found in Fig 8 and Fig 9 that the difference

between pc and psea is somewhat higher in the lower psea

Fig 7: Schematic of the experimental rig Fig 8: Variation of supply pressure with psea ascent

56 underwater TECHNOLOGY Vol 26, No 3, 2005

Fig 9: Variation of supply pressure with psea descent Fig 11: Experimental curve in the atmospheric environ- ment when the UHM runs at a certain rotational speed

Fig 10: Response curve of pc with psea when the return Fig 12: Experimental curve in the underwater environ- pressure changes ment when UHM runs at a certain rotational speed

during than that in the higher psea. Thus the steady under two different working environments with the return error of compensated pressure pc is smaller. This result pressure increasing. The experimental curves, for the is the same as the analytical result. hydraulic motor running at the same rotational speed Based on the experimental results, we make the under different operating environments, are shown in Figs following remarks: 11 and 12. Theses two figures show that the difference Firstly, let us compare the influences of the UAP between the supply pressure and the return pressure is variation and the compensated pressure on the per- enhanced slightly when the return pressure increases, but formance of UHM. It can be seen from Fig 8 and Fig the difference between the supply pressure and the return 9 that the difference between the supply pressure and pressure in the atmospheric environment is greater than the return pressure of the UHM decreases as the UAP that in the underwater ambience if the return pressure is increases and increases as the UAP decreases. Both the kept the same as in these two situations. When the difference between the supply pressure and pc (the pres- hydraulic motor is underwater the ambient pressure leads sure in the UHM shell) and the difference between the to increased leakage pressure. The performances of the return pressure and pc decrease because pc increases as hydraulic motor in the two different environments are the

UAP psea increases. This leads to a small force by which same if the influence of the underwater ambient pressure rollers of the hydraulic motor press onto the track. and water viscosity is not taken into consideration. Mechanical efficiency will increase. The difference Thirdly Figs 13 and 14 show the experimental curves for between the supply pressure and the return pressure the hydraulic motor running at a gradually increasing rota- and the power loss with lower compensated pressure are tional speed under different operating environments. It can larger than that with higher compensated pressure. be seen that the differences between the supply pressure and Therefore the efficiency of the UHM can be enhanced the return pressure are roughly the same if the hydraulic when the compensated pressure approaches the return motor runs at the same speed and the difference between pressure under normal working conditions; the efficien- the return pressure and the leakage pressure is kept constant cy is lowest without pressure compensation. under the two different operating environments. That is to Secondly we compare the performance of the UHM say, the performance of the UHM utilising the pressure

57 Li & Wang. Research on the pressure compensation for the underwater hydraulic motor

Acknowledgement

The authors acknowledge the financial support provid- ed by the National Foundation of China (No. 50475105).

References

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