CN218816730U

CN218816730U - Wave energy power generation device based on constant pressure type hydraulic PTO

Info

Publication number: CN218816730U
Application number: CN202222424466.7U
Authority: CN
Inventors: 岳旭辉; 温斌斌; 陈骏; 徐军杨; 童舟波; 张奇伟; 杨俊杰
Original assignee: Hangzhou Huachen Electric Power Control Engineering Co ltd; PowerChina Huadong Engineering Corp Ltd
Current assignee: Hangzhou Huachen Electric Power Control Engineering Co ltd; PowerChina Huadong Engineering Corp Ltd
Priority date: 2022-09-14
Filing date: 2022-09-14
Publication date: 2023-04-07
Anticipated expiration: 2032-09-14

Abstract

The utility model provides a buoyancy pendulum type wave energy power generation device based on a constant pressure type hydraulic PTO, which comprises a buoyancy pendulum body, wherein the buoyancy pendulum body is completely submerged in the sea level and converts wave energy into mechanical energy; a hydraulic PTO system to receive mechanical energy and generate electrical energy; the transmission system converts wave energy into mechanical energy and transmits the mechanical energy to the hydraulic PTO system, the hydraulic PTO system comprises a main oil way, a pressure transmitter, a variable displacement hydraulic motor and a three-phase permanent magnet synchronous generator are arranged between an oil inlet and an oil outlet of the main oil way, a rotating speed torque instrument is arranged between the variable displacement hydraulic motor and the three-phase permanent magnet synchronous generator, the pressure transmitter is used for acquiring system oil pressure of the main oil way, and the rotating speed torque instrument is used for measuring torque signals and rotating speed signals of an input shaft of the three-phase permanent magnet synchronous generator. The utility model discloses utilize generator vector control to stabilize the rotational speed, begin to implement motor displacement control after the rotational speed deviation is less than certain threshold value in order to stabilize the oil pressure, can realize the synchronous invariant of the rotational speed under the actual irregular ripples sea state and the system oil pressure.

Description

Wave energy power generation device based on constant pressure type hydraulic PTO

Technical Field

The utility model belongs to wave energy power generation field especially relates to a wave energy power generation facility based on constant pressure formula hydraulic pressure PTO.

Background

Constant-pressure hydraulic PTO (CPHPTO) is a common transmission form in a wave power generation device. The CPHPTO is additionally provided with the high-pressure energy accumulator between the hydraulic cylinder and the motor, so that the fluctuation of the internal oil pressure, flow and the rotating speed of the motor is greatly reduced, and the energy conversion efficiency is improved. A disadvantage of CPHPTO is that it is difficult to achieve rapid adjustment of the PTO force (torque) by changing the motor displacement. CPHPTO is currently widely used in sea snake type devices, wavestar devices, raft type devices, heaving float devices, and various pendulum type devices, such as SEAREV devices, eagle type devices, buoyancy pendulum devices, floating pendulum devices, and resonance type devices.

Although the CPHPTO has high energy conversion efficiency and stable working point, the internal nonlinear energy loss can not be ignored, and the working point of the system still fluctuates slightly under the complex irregular wave sea state and the limited nominal volume of the high-pressure accumulator. Therefore, it is still necessary to fully examine various possible operating points and to operate CPHPTO in a high-efficiency stable region by operation control, so as to avoid excessive transmission loss and severe fluctuation of operating parameters.

The document "Integrated characteristics currents of the constant-pressure water Power-off wave Energy conversion" (Journal article, journal name: international Journal of electric Power and Energy Systems, published year: 2019, volume number: 117, article number: 105730) indicates that the operation curve in the CPHPTO comprehensive characteristic curve can be used to guide the efficient and stable operation of CPHPTO. The operating curves include the constant flow line, the force limit line and the constant efficiency line. Wherein the output limiting line passes through the high-efficiency stable regions at different flow rates. The output limiting line comprises two sections, wherein the first section is limited by the designed output (namely rated power of the three-phase permanent magnet synchronous generator), and the second section is limited by the designed rotating speed (namely rated rotating speed of the three-phase permanent magnet synchronous generator) and the designed system oil pressure (namely designed hydraulic motor inlet oil pressure, and because the outlet of the hydraulic motor is connected with a low-pressure oil tank, the outlet oil pressure is far lower than the inlet oil pressure, the designed system oil pressure can also be regarded as the designed hydraulic motor inlet-outlet pressure difference). When the instantaneous output exceeds the design output, the system flow exceeds the design flow, the flow is limited to the design flow through the speed regulating valve, the output is limited to the design output, when the instantaneous output is smaller than the design output, the system operates according to a second section of output limiting line, and the system oil pressure and the system rotating speed are respectively stabilized at the design system oil pressure and the design rotating speed through constant-speed and constant-pressure control.

The invention discloses a comprehensive characteristic curve acquisition method based on a wave power generation hydraulic PTO system (patent number ZL 201710346136.1, granted bulletin date: 6/19/2020), and further indicates that the rotation speed of an output shaft of a hydraulic motor can be maintained to be the rated rotation speed of a three-phase permanent magnet synchronous generator by adjusting the displacement of the hydraulic motor, and the mechanical torque of a main shaft is adjusted by vector control of the three-phase permanent magnet synchronous generator, so that the pressure difference of an inlet and an outlet of the hydraulic motor is stabilized, and finally constant-speed constant-pressure control is realized. However, the structural design and control flow of the controller of the constant-speed and constant-pressure control strategy are not elaborated in detail in the patent.

At present, scholars propose constant-speed constant-pressure control technologies based on CPHPTO and carry out simulation research. The document, "research on key technologies of buoyancy pendulum type wave energy power generation devices" (doctor's paper, published unit: zhejiang university, published time: 2011) indicates that constant speed and constant voltage of CPHPTO and stability of output power under the action of ideal periodic step wave force can be realized through variable motor displacement control or variable electrical load control, wherein the variable electrical load control has better operation stability on the CPHPTO. The CPHPTO system oil pressure is limited by overflow valve overflow aiming at ideal sinusoidal reciprocating motion and alternating current resistive load of a hydraulic cylinder piston rod, and the stability of the rotating speed and the output power is realized based on motor displacement PID Control in the literature "Operation characteristics and methods of the hydraulic power take-off system" (journal articles, journal names: transactions of the Institute of Measurement and Control, published year: 2020, volume number: 43, term number: 5, article number: 014233122093435).

The constant-speed constant-pressure control technology has the following two defects:

1) The input of CPHPTO is excessively simplified in the research process, so that the simulation result is ideal, and the difference from the actual situation is large, therefore, the constant-speed constant-voltage control technology has low actual operability and cannot be used for guiding the actual operation process. The document 'buoyancy pendulum type wave energy power generation device key technology research' simplifies input wave force into periodic step force, and the document 'Operation characteristics and methods for the hydraulic power tab-offset' simplifies hydraulic cylinder input displacement into sinusoidal reciprocating motion, and both fail to effectively research input conditions under actual irregular wave sea conditions.

2) The oil pressure and the rotating speed of the system cannot be synchronously constant. In the literature, "research on key technologies of buoyancy pendulum wave power generation devices" only uses single variable motor displacement control or variable electrical load control, and the single control can only realize the stability of a single working condition parameter (namely, system oil pressure or rotating speed), while the other working condition parameter tends to fluctuate under actual irregular wave sea conditions. In contrast, the literature, "Operation characteristics and methods for hydraulic power take-offset" only limits the upper limit of the system oil pressure of CPHPTO through an overflow valve, and does not implement effective feedback control on the system oil pressure, so that only the constant rotation speed can be achieved, the pressure inevitably fluctuates in real time under the actual irregular wave sea condition, and the system oil pressure is stabilized at the overflow pressure only when the overflow occurs under the sea condition.

The inventor finds that the fundamental problem of the prior art is that the system oil pressure can fluctuate violently without considering actual irregular wave sea conditions and limited accumulator volume when the CPHPTO constant-speed constant-pressure control technology is researched, for example, when the sea conditions are small and the system oil pressure is lower than the charging pressure of the accumulator, the accumulator cannot work normally, and the system oil pressure fluctuates violently; when the sea condition is large and the oil pressure of the system is far higher than the design pressure, the pressure stabilizing capacity of the energy accumulator is reduced, and the amplitude of the oil pressure of the system is increased under the same volume of liquid charging and discharging. The CPHPTO has unique pressure-flow-rotating speed coupling characteristics, so that the fluctuation of the system oil pressure and the fluctuation of the rotating speed are related but not mutually determined, for example, when the system oil pressure is constant through control, the fluctuation of the hydraulic power and the flow is generated due to the fluctuation of the input power under irregular wave sea conditions and the limited action of an energy accumulator, and the flow fluctuation is transmitted to the output shaft of the hydraulic motor and the three-phase permanent magnet synchronous generator shaft synchronously rotating with the output shaft of the hydraulic motor through a relational expression (without efficiency loss) of flow = rotating speed and hydraulic motor displacement, so that the rotating speed cannot be constant; when the rotation speed is controlled to be constant, but the input power fluctuates under irregular wave sea conditions and the limited action of the accumulator causes that the load torque of the hydraulic motor must synchronously fluctuate to maintain the input and output power balance, the system oil pressure also fluctuates and cannot be constant according to a relational expression of system oil pressure = hydraulic motor load torque × 2 × pi ÷ hydraulic motor displacement (without considering efficiency loss). Therefore, it is not practical to achieve the constancy of the system oil pressure and the rotational speed at the same time by a single control.

Disclosure of Invention

An object of the utility model is to provide a wave energy power generation facility based on constant pressure formula hydraulic pressure PTO solves the unable invariable problem of system oil pressure and rotational speed synchronization under the actual irregular ripples sea condition of realization of current CPHPTO constant speed constant voltage control technique.

In order to achieve the above purpose, the utility model provides a following technical scheme:

the utility model provides a buoyancy pendulum type wave energy power generation device based on a constant pressure type hydraulic PTO, which comprises a buoyancy pendulum body, wherein the buoyancy pendulum body is completely submerged in the sea level and converts wave energy into mechanical energy; a hydraulic PTO system to receive mechanical energy and generate electrical energy; the transmission system converts wave energy into mechanical energy and transmits the mechanical energy to the hydraulic PTO system, the hydraulic PTO system comprises a main oil way, a pressure transmitter, a variable displacement hydraulic motor and a three-phase permanent magnet synchronous generator are arranged between an inlet and an outlet of the main oil way, a rotating speed torque instrument is arranged between the variable displacement hydraulic motor and the three-phase permanent magnet synchronous generator and is used for acquiring system oil pressure of the main oil way, and the rotating speed torque instrument is used for measuring torque signals and rotating speed signals of an input shaft of the three-phase permanent magnet synchronous generator.

The utility model discloses further set up, transmission system includes guide bar group, rack, gear, main shaft, buoyancy pendulum body sets up on the main shaft, buoyancy pendulum body drive main shaft drives the epaxial gear of owner and the synchronous rotation of pendulum body, the gear set up in on the main shaft, the gear with rack toothing, the rack with guide bar group link, the removal that hydraulic pressure PTO system passes through the rack obtains hydromechanical energy.

The utility model discloses further set up, hydraulic pressure PTO system still includes pneumatic cylinder one, pneumatic cylinder two, a plurality of check valve, high-pressure accumulator, governing valve, overflow valve, low-pressure oil tank, pneumatic cylinder one, pneumatic cylinder two are located the both sides of guide bar group, the guide arm fixed connection with the guide bar group of pneumatic cylinder one, pneumatic cylinder two, the rack is formed with the moving member in the guide bar group, the moving member promotes the piston rod of pneumatic cylinder one, pneumatic cylinder two and removes for the cylinder body, wherein pneumatic cylinder two, check valve one, main oil circuit oil inlet, high-pressure accumulator, governing valve, pressure transmitter, variable displacement hydraulic motor, main oil circuit oil-out, check valve four, pneumatic cylinder one constitute return circuit one; the hydraulic cylinder I, the check valve II, the main oil way oil inlet, the high-pressure energy accumulator, the speed regulating valve, the pressure transmitter, the variable displacement hydraulic motor, the main oil way oil outlet, the check valve III and the hydraulic cylinder II form a loop II.

The utility model has the advantages that: the generator vector is used for controlling the stable rotating speed, and the motor displacement control is started to stabilize the oil pressure when the rotating speed deviation is smaller than a certain threshold value, so that the synchronous constancy of the rotating speed and the system oil pressure under the actual irregular wave sea condition can be realized, and the real constant speed and constant pressure control effect is achieved.

Drawings

The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the application and together with the description serve to explain the application and not to limit the application. In the drawings:

FIG. 1 is a mechanical structure diagram of a buoyancy pendulum type wave energy power generation device based on a constant pressure type hydraulic PTO; reference numerals are as follows: 1-a buoyancy pendulum body; 2-a guide rod group; 3-a rack; 4-a gear; 5-a main shaft; 6-1-a first hydraulic cylinder; 6-2-a second hydraulic cylinder; 7-1-a one-way valve I; 7-2-a second one-way valve; 7-3-one-way valve III; 7-4-check valve four; 8-a high pressure accumulator; 9-a speed regulating valve; 10-variable displacement hydraulic motor; 11-a three-phase permanent magnet synchronous generator; 12-an overflow valve; 13-a low-pressure oil tank; 14-a pressure transmitter; 15-rotational speed torquemeter;

Detailed Description

Embodiments of the present application will be described in detail with reference to the drawings and examples, so that how to implement technical means to solve technical problems and achieve technical effects of the present application can be fully understood and implemented.

The inventor finds that the CPHPTO irregular wave simulation research firstly utilizes the generator vector to control the stable rotating speed, and starts to implement the motor displacement control to stabilize the oil pressure when the rotating speed deviation is less than a certain threshold value, so that the synchronization and constancy of the rotating speed and the system oil pressure under the actual irregular wave sea condition can be realized, and the real constant speed and constant pressure control effect can be achieved.

The utility model discloses do not consider to adopt the constant speed constant voltage control strategy of stable oil pressure of generator vector control and the stable rotational speed of motor displacement control of invention patent comprehensive characteristic curve acquisition method based on wave power generation hydraulic pressure PTO system, the reason lies in: the physical limitation of the maximum and minimum displacement of the motor enables the rotating speed stability of the motor displacement control to be limited under the condition that the flow is changed in a large range, and the indirect control of the oil pressure through vector control easily causes the motor to reversely pump oil.

Therefore, the following technical scheme is proposed:

the utility model relates to a wave energy electricity generation hydraulic pressure PTO constant speed constant voltage control method based on two parameter allies oneself with transfers is to the wave energy power generation facility who adopts constant pressure formula hydraulic pressure PTO structure.

The embodiment considers a buoyancy pendulum type wave energy power generation device based on a constant pressure type hydraulic PTO shown in figure 1, and the mechanical structure comprises: a buoyancy pendulum body 1; a guide rod group 2; a rack 3; a gear 4; a main shaft 5 and a constant pressure hydraulic PTO. The constant pressure hydraulic PTO includes: 6-1 of a hydraulic cylinder; 6-2 of a hydraulic cylinder II; 7-1 of a one-way valve; 7-2 of a second one-way valve; 7-3 of a one-way valve; 7-4 parts of a check valve; a high-pressure accumulator 8; a speed regulating valve 9; a variable displacement hydraulic motor 10; a three-phase permanent magnet synchronous generator 11; an overflow valve 12; a low-pressure oil tank 13; and a main oil passage 23. The buoyancy pendulum body 1 is mounted on the main shaft 5 and can rotate relative to the main shaft. The cylinder body of the first hydraulic cylinder 6-1, the cylinder body of the second hydraulic cylinder 6-2, the one-way valve I7-1, the one-way valve II 7-2, the one-way valve III 7-3, the one-way valve IV 7-4, the high-pressure energy accumulator 8, the speed regulating valve 9, the variable displacement hydraulic motor 10, the three-phase permanent magnet synchronous generator 11, the overflow valve 12, the low-pressure oil tank 13, the pressure transmitter 14, the rotating speed and torque meter 15 and the main oil way 23 are all fixed on the inner side wall of the buoyancy pendulum body 1. The gear 4 is fixed on the main shaft 5 and meshed with the rack 3. A ribbed plate is welded in the middle of the back of the rack 3, and two sides of the ribbed plate are fixedly connected with a piston rod of the first hydraulic cylinder 6-1 and a piston rod of the second hydraulic cylinder 6-2 respectively. The guide rod group 2 is divided into an upper guide rod and a lower guide rod, and two ends of the guide rods are fixedly connected with the cylinder body of the first hydraulic cylinder 6-1 and the cylinder body of the second hydraulic cylinder 6-2 respectively. The rib plate of the rack 3 is provided with a through hole, and the guide rod group 2 passes through the through hole and forms a moving pair with the rack 3 through a linear bearing. An oil port of the first hydraulic cylinder 6-1 is connected with an oil inlet of the first check valve 7-1 and an oil outlet of the third check valve 7-3, an oil port of the second hydraulic cylinder 6-2 is connected with an oil inlet of the second check valve 7-2 and an oil outlet of the fourth check valve 7-4, an oil outlet of the first check valve 7-1 and an oil outlet of the second check valve 7-2 are connected with an oil inlet of the main oil path 23, and an oil inlet of the third check valve 7-3 and an oil inlet of the fourth check valve 7-4 are connected with an oil outlet of the main oil path 23. The oil inlet of the main oil path 23 is also divided into one path of oil path to be connected to the oil inlet of the overflow valve 12, and the oil outlet of the overflow valve 12 is connected to the low-pressure oil tank 13. The oil outlet of the main oil path 23 is also divided into one oil path to be connected to the low-pressure oil tank 13. A high-pressure accumulator 8, a speed regulating valve 9, a pressure transmitter 14 and a variable displacement hydraulic motor 10 are sequentially arranged between an oil inlet and an oil outlet of the main oil way 23. The output shaft of the variable displacement hydraulic motor 10 is connected with the input shaft of the rotating speed and torque instrument 15, and the output shaft of the rotating speed and torque instrument 15 is connected with the input shaft of the three-phase permanent magnet synchronous generator 11 through couplings.

The working principle of the buoyancy pendulum wave power generation device based on the constant pressure type hydraulic PTO is as follows: the buoyancy pendulum body 1 is completely immersed in seawater, stands on the main shaft 5 under the action of buoyancy moment, and when incident waves act on the front surface of the buoyancy pendulum body 1, the buoyancy pendulum body 1 swings around the main shaft 5. The swinging of the buoyancy pendulum body 1 relative to the main shaft 5 is converted into the translation of the rack 3 relative to the guide rod group 2 through the meshed gear 4 and the rack 3. The rack 3 further pushes the piston rods of the first hydraulic cylinder 6-1 and the second hydraulic cylinder 6-2 to move relative to the cylinder body through the back rib plate, so that the oil cavity expands to absorb oil or is compressed to generate high-pressure oil. When the rib plate 3 of the rack pushes a piston rod of the hydraulic cylinder II 6-2 to compress an oil cavity of the hydraulic cylinder II 6-2, the generated high-pressure oil enters an oil inlet of the main oil way 23 through the check valve I7-1, sequentially passes through the high-pressure energy accumulator 8, the speed regulating valve 9, the pressure transmitter 14 and the variable displacement hydraulic motor 10, then reaches an oil outlet of the main oil way 23, and enters the oil cavity of the hydraulic cylinder I6-1 through the check valve IV 7-4. Similarly, when the rib plate 3 of the rack pushes the piston rod of the hydraulic cylinder I6-1 to compress the oil cavity of the hydraulic cylinder I6-1, the generated high-pressure oil enters the oil inlet of the main oil path 23 through the second one-way valve 7-2, sequentially passes through the high-pressure energy accumulator 8, the speed regulating valve 9, the pressure transmitter 14 and the variable displacement hydraulic motor 10, then reaches the oil outlet of the main oil path 23, and enters the oil cavity of the hydraulic cylinder II 6-2 through the third one-way valve 7-3. The 4 one-way valves play a role in rectification and convert the two-way flow of high-pressure oil into one-way flow on the main oil way 23. The relief valve 12 is used to prevent the constant pressure hydraulic PTO from being over-pressurized, and opens the relief when the inlet oil pressure of the main oil passage 23 reaches the upper limit of the working oil pressure. The high-pressure accumulator 8 is used for stabilizing the oil pressure of an oil inlet of the main oil way 23, and further stabilizing the flow of the main oil way 23. The speed regulating valve 9 is used for flow regulation of the main oil way 23 and participates in power generation power control, and meanwhile when the flow of the main oil way 23 reaches the upper flow limit, the flow can be limited through the speed regulating valve 9 in order to avoid over-speed of the rotating speed of the three-phase permanent magnet synchronous generator 11. The variable displacement hydraulic motor 10 is used for converting hydraulic energy into rotary mechanical energy, and the three-phase permanent magnet synchronous generator 11 is used for converting the rotary mechanical energy into electric energy to be output. The low-pressure oil tank 13 is used for stabilizing the oil pressure of an oil outlet of the main oil path 23, supplementing system leakage and eliminating short-time negative pressure.

As used in the specification and in the claims, certain terms are used to refer to particular components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. This specification and claims do not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms "include" and "comprise" are used in an open-ended fashion, and thus should be interpreted to mean "include, but not limited to. "substantially" means within an acceptable error range, that a person skilled in the art can solve the technical problem within a certain error range to substantially achieve the technical effect.

It should be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a good or system that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such good or system. Without further limitation, an element defined by the phrases "comprising one of \8230;" does not exclude the presence of additional like elements in an article or system comprising the element.

While the foregoing description shows and describes several preferred embodiments of the invention, it is to be understood, as noted above, that the invention is not limited to the forms disclosed herein, but is not intended to be exhaustive of other embodiments, and is capable of use in various other combinations, modifications, and environments and is capable of changes within the scope of the inventive concept as expressed above, or as otherwise known in the relevant art. But that modifications and variations may be effected by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A buoyancy pendulum type wave energy power generation device based on a constant pressure type hydraulic PTO is characterized by comprising a buoyancy pendulum body, a power generation device and a power generation device, wherein the buoyancy pendulum body is completely submerged in the sea level and converts wave energy into mechanical energy; a hydraulic PTO system to receive mechanical energy and generate electrical energy; the transmission system converts wave energy into mechanical energy and transmits the mechanical energy to the hydraulic PTO system, the hydraulic PTO system comprises a main oil way, a pressure transmitter, a variable displacement hydraulic motor and a three-phase permanent magnet synchronous generator are arranged between an inlet and an outlet of the main oil way, a rotating speed torque instrument is arranged between the variable displacement hydraulic motor and the three-phase permanent magnet synchronous generator and is used for acquiring system oil pressure of the main oil way, and the rotating speed torque instrument is used for measuring torque signals and rotating speed signals of an input shaft of the three-phase permanent magnet synchronous generator.

2. The floating pendulum wave power generation device based on the constant pressure hydraulic PTO, as claimed in claim 1, wherein the transmission system comprises a guide bar set, a rack, a gear, and a main shaft, the floating pendulum is disposed on the main shaft, the gear is fixedly connected to the main shaft, the pendulum rotates relative to the main shaft to drive the guide bar set and the rack to rotate synchronously, and the rack translates reversely along the guide bar set under the reverse action of the gear.

3. The floating pendulum wave power generation device based on the constant-pressure hydraulic PTO as claimed in claim 2, wherein the hydraulic PTO system further comprises a first hydraulic cylinder, a second hydraulic cylinder, a plurality of check valves, a high-pressure accumulator, a speed regulation valve, an overflow valve and a low-pressure oil tank, the first hydraulic cylinder and the second hydraulic cylinder are positioned at two sides of the guide rod group, the first hydraulic cylinder and the second hydraulic cylinder are fixedly connected with the guide rod of the guide rod group, the rack is provided with a moving member in the guide rod group, the moving member pushes piston rods of the first hydraulic cylinder and the second hydraulic cylinder to move relative to the cylinder body, wherein the first hydraulic cylinder, the first check valve, the main oil way oil inlet, the high-pressure accumulator, the speed regulation valve, the pressure transmitter, the variable displacement hydraulic motor, the main oil way oil outlet, the fourth check valve and the first hydraulic cylinder form a first loop; the hydraulic cylinder I, the check valve II, the main oil way oil inlet, the high-pressure energy accumulator, the speed regulating valve, the pressure transmitter, the variable displacement hydraulic motor, the main oil way oil outlet, the check valve III and the hydraulic cylinder II form a loop II.