Five-cavity series-connection symmetrical single-rod electro-hydrostatic actuator and working method thereof

Abstract

The invention discloses a five-cavity series-connection symmetrical single-rod electro-hydrostatic actuator and a working method thereof. The actuator comprises double controllers, double motor pump units and a five-cavity series-connection symmetrical single-rod actuating cylinder. The actuator cylinder body is provided with a port A, a port B, a port C, a port D and two ports T, the port A and the port B are connected with the hydraulic pump of the first motor pump unit, the port C and the port D are connected with the hydraulic pump of the second motor pump unit, and the two ports T are communicated with external air. The piston forms three equal-area working cavities and two air cavities between the cylinder body and the inner cylinder barrel, so that the stress area is the same when the piston stretches out (oil enters from an A / C port) and retracts (oil enters from a B / D port). The double-motor pump unit comprises a bypass valve, when a single unit breaks down, the bypass valve isolates a fault oil way, and the other unit maintains driving. The axial size is compressed through the five-cavity symmetrical single-rod structure, the reliability is improved by combining the double-unit redundancy design, and the electro-hydrostatic actuator is suitable for electro-hydrostatic actuator occasions with strict requirements for the size and the service life.

Description

Technical Field

[0001] The present invention relates to the technical field of hydraulic transmission and control, in particular to the technical field of a five-chamber series symmetric single-rod electro-hydrostatic actuator and its working method. Background Art

[0002] As a distributed drive device integrating a servo motor and a hydraulic pump, the electro-hydrostatic actuator (EHA) has been widely used in the flight control systems of high-end aircraft such as large airliners and advanced fighter jets in recent years. Compared with the traditional centralized hydraulic system, the electro-hydrostatic actuator cancels the long-distance hydraulic pipeline by means of local drive and accumulator energy supply, significantly improving the safety, reliability and maintainability of the system, and effectively reducing the weight of the whole machine. It has become an important development direction of the aircraft rudder surface control system.

[0003] However, the existing electro-hydrostatic actuator technology still has the following deficiencies: 1. Contradiction between structural size and power density: The current mainstream solution adopts a symmetric double-rod actuator cylinder structure. Although it can keep the piston in two-way force balance, its axial size is relatively large, resulting in a low power density and being difficult to meet the application scenarios with higher requirements for compactness.

[0004] 2. Flow imbalance problem of the single-rod solution: Some solutions using a single-rod actuator cylinder can reduce the volume, but due to the unequal effective acting areas on both sides of the piston, it is necessary to additionally configure a complex flow compensation mechanism, which not only increases the system complexity, but also easily leads to a decrease in control accuracy and response asymmetry.

[0005] 3. Insufficient reliability redundancy: The existing systems mostly rely on a single motor-pump unit for driving. Once this unit fails, it will directly cause the entire actuator to paralyze, and it is difficult to meet the fault tolerance requirements of high-safety occasions such as flight control systems.

[0006] Therefore, it is urgent to develop a new type of electro-hydrostatic actuator, which can significantly improve the power density and fault tolerance ability on the premise of ensuring the two-way thrust symmetry and control accuracy, so as to meet the increasingly stringent requirements for the compactness and reliability of actuators in the fields of aviation, heavy machinery, etc. Summary of the Invention

[0007] The object of the present invention is to solve the problems of insufficient power density and low reliability in the existing electro-hydrostatic actuator system, and to propose a five-chamber series symmetric single-rod electro-hydrostatic actuator and its working method, which can significantly shorten the axial dimension of the actuator cylinder while ensuring the same piston bi-directional force-bearing area, and at the same time achieve fault isolation and redundant control through the cooperation of the dual motor pump unit and the bypass valve, improving the system integration and reliability.

[0008] To achieve the above object, the present invention proposes a five-chamber series symmetric single-rod electro-hydrostatic actuator, including a dual controller, a dual motor pump unit and a five-chamber series symmetric single-rod actuator cylinder; The dual motor pump unit is a first motor pump unit and a second motor pump unit, and each motor pump unit includes a motor, a hydraulic pump, an accumulator, two one-way valves, two overflow valves and a bypass valve; The five-chamber series symmetric single-rod actuator cylinder includes a cylinder block, an inner cylinder, an intermediate seat, a cylinder end cover and a piston; Wherein: The dual controller is electrically connected to the two motors respectively; The cylinder block is hermetically connected to the cylinder end cover, and the inner cylinder and the intermediate seat are fixed at the center inside the cylinder block; The piston is located between the cylinder block and the inner cylinder, and forms a sealed sliding fit with the cylinder block, the inner cylinder, the intermediate seat and the cylinder end cover; The cylinder block is provided with an A port, a B port, a C port, a D port and two T ports; The A port and the B port are respectively connected to the suction and discharge oil ports of the hydraulic pump of the first motor pump unit, and the C port and the D port are respectively connected to the suction and discharge oil ports of the hydraulic pump of the second motor pump unit; The two T ports are connected to the external air, used to compensate for the volume change caused by the piston movement and balance the internal volume difference of the actuator cylinder; When the A port and the C port are simultaneously supplied with oil, the piston extends; when the B port and the D port are simultaneously supplied with oil, the piston retracts, and the force-bearing areas of the piston during extension and retraction are the same.

[0009] Preferably, the dual motor pump unit is two units with the same structure; the motor is a permanent magnet synchronous motor; the hydraulic pump is a bidirectional hydraulic pump with a suction port and a discharge port.

[0010] Preferably, in each motor pump unit, the inlet ports of the two one-way valves are both connected to the accumulator, and the outlet ports are respectively connected to the suction port and the discharge port of the hydraulic pump.

[0011] Preferably, in each motor pump unit, the two ends of the two overflow valves are respectively connected to the suction and discharge oil ports of the hydraulic pump, and the conduction directions of the two overflow valves are opposite.

[0012] Preferably, in each motor pump unit, two ends of the bypass valve are respectively connected to the oil suction port and the oil discharge port of the hydraulic pump.

[0013] Preferably, five chambers are formed inside the actuating cylinder: The first working chamber is formed by the inner cylinder barrel and the piston, and communicates with port A and port C; the second working chamber is formed by the cylinder block, the inner cylinder barrel, the intermediate seat and the piston, and communicates with port B; the third working chamber is formed by the cylinder block and the piston, and communicates with port D; the two air chambers are formed by the cylinder block, the inner cylinder barrel, the intermediate seat and the piston, and communicate with the two T ports respectively; the effective acting area of the first working chamber is I, the effective acting area of the second working chamber is II, and the effective acting area of the third working chamber is III.

[0014] Furthermore, the area I of the hydraulic oil acting on the first working chamber, the area II of the second working chamber, and the area III of the third working chamber satisfy: area I = area II = area III.

[0015] Preferably, a bushing is installed at one end of the piston, a plug rod is connected to the center of the inner end of the bushing, and a magnetic ring is fixed to the plug rod; a displacement sensor is installed on the cylinder block, and the magnetic ring is sleeved outside the displacement sensor to form a non-contact detection structure.

[0016] Preferably, a damping hole is provided on the cylinder block end cover, which communicates port D with the third working chamber; when the piston extends to the limit position, the piston blocks port D, and the hydraulic oil flows into the third working chamber through the damping hole to push the piston to retract.

[0017] The cylinder block is provided with port A, port B, port C, port D and two T ports. Port A and port B are respectively connected to the oil suction and discharge ports of the hydraulic pump 1, and port C and port D are respectively connected to the oil suction and discharge ports of the hydraulic pump 2. The two T ports communicate with the air chambers inside the cylinder block, and are used to balance the pressure difference caused by the change of the internal volume during the actuating process. Among them, the hydraulic oil at port A and port C enters the first working chamber and acts on the area I; the hydraulic oil at port B and port D enters the second and third working chambers and acts on the area II and area III respectively. When port A and port C are feeding oil and port B and port D are returning oil, the high-pressure oil acts on the area I, and the low-pressure oil acts on the area II and III, so that the piston extends to move and the extending action of the actuating cylinder is realized; on the contrary, when port B and port D are feeding oil and port A and port C are returning oil, the hydraulic oil pushes the piston to retract and move, and the retracting action of the actuating cylinder is realized. Due to the five-chamber design and symmetrical arrangement, the effective acting areas of the piston in the extending and retracting directions are the same, realizing symmetrical thrust and smooth movement.

[0018] A working method of an electro-hydrostatic actuator based on any one of the above, comprising: Normal driving mode: When the piston extends, the dual controller controls the first and second motor pump units to supply oil to ports A and C respectively, with ports B and D returning oil, and the two T ports inhaling air; when the piston retracts, the dual controller controls the first and second motor pump units to supply oil to ports B and D respectively, with ports A and C returning oil, and the two T ports discharging air. Fault-tolerant mode: When a single motor pump unit fails, its bypass valve opens to isolate the faulty oil circuit, and the other motor pump unit independently drives the actuator to move.

[0019] Advantages of the present invention: 1. Significantly improved fault redundancy ability: Through the collaborative design of the dual motor pump units and the bypass valve, when a single unit fails, the bypass valve automatically isolates the faulty oil circuit, and the remaining unit can independently drive the piston to move, greatly improving the system reliability and meeting the fault-tolerant requirements of high-safety scenarios such as aviation flight control.

[0020] 2. Effectively compressed axial dimension: Adopting a five-chamber series symmetric single-rod actuator structure, under the condition of ensuring the same bidirectional stress area of the piston, the axial length is only half of that of the traditional symmetric double-rod structure, significantly improving the power density and solving the installation problem in a compact space.

[0021] 3. Optimized motion smoothness: Damping holes are provided on the cylinder end covers, and a buffer oil circuit is formed when the piston moves to the limit position, avoiding jamming or reverse failure caused by rigid impact, and enhancing the end positioning accuracy and service life.

[0022] 4. Compatible with standard hydraulic components: The actuator has good symmetry in the inlet and outlet oil flow, and can be directly matched with industrial standard bidirectional hydraulic pumps without customized design, reducing the system implementation difficulty and cost.

[0023] 5. Strong application adaptability: It has the comprehensive advantages of a compact structure, symmetric thrust, and high fault tolerance, and is particularly suitable for fields such as aircraft rudder surface control and heavy machinery actuation that have strict requirements for volume and reliability.

[0024] The features and advantages of the present invention will be described in detail through embodiments in combination with the accompanying drawings. Brief description of the drawings

[0025] Figure 1 is the schematic diagram of the principle of a five-chamber series symmetric single-rod electro-hydrostatic actuator and its working method of the present invention; Figure 2 is the schematic diagram of the oil circuit when the piston of the actuator of a five-chamber series symmetric single-rod electro-hydrostatic actuator and its working method of the present invention extends; Figure 3It is a schematic diagram of the oil circuit when the piston of the actuator cylinder of a five-chamber series-connected symmetric single-rod electro-hydrostatic actuator and its working method in the present invention retracts; Figure 4 It is a schematic structural diagram of a chamber series-connected symmetric single-rod actuator cylinder of a five-chamber series-connected symmetric single-rod electro-hydrostatic actuator and its working method in the present invention.

[0026] In the figure: 1 - Controller, 2 - Motor, 3 - Pump, 4 - Accumulator, 5 - Check valve, 6 - Relief valve, 7 - Bypass valve, 8 - Five-chamber series-connected symmetric single-rod actuator cylinder, 81 - Displacement sensor, 82 - Cylinder block, 83 - Plug rod, 84 - Piston, 85 - Cylinder block end cover, 86 - Piston end cover, 87 - Inner cylinder barrel, 88 - Intermediate seat, 9 - Air. Specific embodiments

[0027] Refer to Figure 1 、 Figure 2 、 Figure 3 and Figure 4 The electro-hydrostatic actuator of the present invention includes a dual controller, a dual motor-pump unit and a five-chamber series-connected symmetric single-rod actuator cylinder; the dual motor-pump unit is a first motor-pump unit and a second motor-pump unit, and each motor-pump unit includes a motor, a hydraulic pump, an accumulator, two check valves, two relief valves and a bypass valve; the five-chamber series-connected symmetric single-rod actuator cylinder includes a cylinder block, an inner cylinder barrel, an intermediate seat, a cylinder block end cover and a piston; wherein: the dual controller is electrically connected to the two motors respectively; the cylinder block is hermetically connected to the cylinder block end cover, and the inner cylinder barrel and the intermediate seat are fixed at the center inside the cylinder block; the piston is located between the cylinder block and the inner cylinder barrel, and forms a sealed sliding fit with the cylinder block, the inner cylinder barrel, the intermediate seat and the cylinder block end cover; the cylinder block is provided with port A, port B, port C, port D and two T ports; port A and port B are respectively connected to the suction and discharge oil ports of the hydraulic pump of the first motor-pump unit, and port C and port D are respectively connected to the suction and discharge oil ports of the hydraulic pump of the second motor-pump unit; the two T ports are communicated with the external air to compensate for the volume change caused by the piston movement and balance the internal volume difference of the actuator cylinder; when port A and port C are simultaneously supplied with oil, the piston extends; when port B and port D are simultaneously supplied with oil, the piston retracts, and the force-bearing areas when the piston extends and retracts are the same.

[0028] The working method of the present invention includes: Normal driving mode: When the piston extends, the dual controller controls the first and second motor-pump units to supply oil to port A and port C respectively, and port B and port D return oil, and the two T ports suck in air; when the piston retracts, the dual controller controls the first and second motor-pump units to supply oil to port B and port D respectively, and port A and port C return oil, and the two T ports discharge air; Fault tolerance mode: When a single motor-pump unit fails, its bypass valve opens to isolate the faulty oil circuit, and the other motor-pump unit independently drives the actuator cylinder to move.

[0029] Working process of the present invention: During the working process of a five - chamber series - connected symmetric single - rod electro - hydrostatic actuator and its working method of the present invention, it will be described with reference to the accompanying drawings.

[0030] Embodiment 1: As Figures 1-4 shown, this embodiment provides a five - chamber series - connected symmetric single - rod electro - hydrostatic actuator, which mainly includes a dual - controller, a dual - motor pump unit, and a five - chamber series - connected symmetric single - rod actuator cylinder. The motor pump unit is composed of a motor, a hydraulic pump, a check valve, a relief valve, and a bypass valve. The controller is electrically connected to the motor, and the output shaft of the motor drives the hydraulic pump to work through a coupling.

[0031] The motor is a permanent - magnet synchronous motor, and the hydraulic pump is a bidirectional reversible hydraulic pump, which has an oil - suction port and an oil - discharge port. The inlet ports of the two check valves are both connected to the accumulator, and the outlet ports are respectively connected to the oil - suction port and the oil - discharge port of the hydraulic pump. Both ends of each relief valve are respectively connected to the oil - suction and oil - discharge ports of the hydraulic pump. The two relief valves have opposite conduction directions, and the set opening pressure is higher than the maximum working pressure of the system, which is used to release excessive pressure in case of abnormal conditions and play a role in protecting the system. The a - port and b - port of the hydraulic pump can be alternately used as the oil - suction port or the oil - discharge port according to the operating conditions.

[0032] The working principle of the motor - pump hydraulic system is: the motor converts electrical energy into mechanical energy, drives the hydraulic pump to rotate, and the hydraulic pump converts mechanical energy into hydraulic energy, and delivers the pressurized liquid to the actuator cylinder to realize the conversion of hydraulic energy into mechanical output.

[0033] The bypass valve is arranged between the oil - suction and oil - discharge passages of the hydraulic pump, and is used to conduct the passage when a certain motor - pump unit fails, isolate the faulty system, so as to ensure that the other motor - pump unit can independently drive the actuator cylinder and enhance the redundancy of the system.

[0034] To achieve high - precision control, the system is equipped with multiple sensors: pressure sensors are respectively installed in the pipelines of port A and port B, the motor is equipped with a resolver and a current sensor, and a displacement sensor is arranged inside the actuator cylinder. The controller performs operations according to the position command signal, adjusts the motor speed to control the flow rate of the hydraulic pump, and further controls the piston position. The displacement, current, pressure, and motor - angle signals fed back by the sensors are used for real - time closed - loop control.

[0035] The above configurations and principle introductions are for a single - set motor - pump unit configuration. The configurations and principles of the two - set motor - pump units are the same, and will not be elaborated here.

[0036] The five-chamber series-connected symmetric single-rod actuator includes a cylinder block, an inner cylinder barrel, an intermediate seat, a cylinder head cover, and a piston assembly. The cylinder block is hermetically connected to the cylinder head cover, and the inner cylinder barrel is installed on the central axis inside the cylinder block. The piston is located between the cylinder block and the inner cylinder barrel and forms a sealed sliding fit structure with the cylinder block, the inner cylinder barrel, and the end cover.

[0037] The cylinder block is provided with port A, port B, port C, port D, and two port Ts, which are respectively used for the intake and exhaust of hydraulic oil and air. Ports A and B are connected to the suction and discharge oil ports of the first motor pump unit, and ports C and D are connected to the second motor pump unit. When oil enters ports A and C simultaneously, the piston moves to achieve extension, and when oil enters ports B and D, the piston moves to achieve retraction.

[0038] There are three working chambers and two air chambers arranged inside the five-chamber actuator. The first working chamber is formed by the inner cylinder barrel and the piston, and the hydraulic oil acts on area Ⅰ; the second working chamber is jointly formed by the inner cylinder barrel, the piston, the cylinder block, and the intermediate seat, with an acting area of Ⅱ; the third working chamber is formed by the cylinder block and the piston, corresponding to area Ⅲ. The air chamber communicates with the outside through port T to balance the pressure fluctuation in the chamber and maintain the stable operation of the system.

[0039] One end of the piston is provided with a bushing, an external spherical hinge is connected, the center of the inner end of the bushing is connected to an insertion rod through a thread, the insertion rod is connected to a magnetic ring, and a displacement sensor is inserted through the magnetic ring to achieve non-contact displacement detection. A connecting head is installed at one end of the cylinder block, and a spherical plain bearing is built in for docking with an external mechanism.

[0040] The piston includes a piston rod and a piston body. The piston body forms a sealed sliding fit with the annular inner wall of the cylinder block and the annular outer wall of the inner cylinder barrel to ensure that the hydraulic oil does not leak. The magnetic ring is provided with a through central hole, and the annular outer wall forms a clearance fit with the annular inner wall of the inner cylinder barrel to ensure smooth movement.

[0041] The cylinder head cover of the actuator is provided with a damping hole, which is used to form a damping oil return path when the piston moves to the limit position, relieve the end impact and prevent overshoot. When oil enters port D, the oil acts on the bottom surface of the piston body through the damping hole, prompting the piston to move reversely to achieve return.

[0042] During the movement of the actuator: when oil is supplied to ports A and C and ports B and D return oil, high-pressure hydraulic oil enters the first working chamber, pushing the piston to achieve extension, low-pressure oil flows back from the second and third working chambers, and air enters through port T to supplement the internal space; when oil is supplied to ports B and D and ports A and C return oil, the piston achieves retraction.

[0043] The three working areas Ⅰ, Ⅱ, and Ⅲ formed by the inner cylinder, piston, intermediate seat, and cylinder block are respectively equal. The cylinder block is provided with ports A, B, C, D, and two T ports. Port A and port B are respectively connected to the suction and discharge ports of the first motor pump unit, and port C and port D are respectively connected to the suction and discharge ports of the second motor pump unit. The two T ports communicate with the internal air cavity of the cylinder block and are used to balance the pressure difference caused by the change in internal volume during the actuation process. Among them, the hydraulic oil at port A and port C enters the first working chamber and acts on area Ⅰ; the hydraulic oil at port B and port D enters the second and third working chambers and acts on area Ⅱ and area Ⅲ respectively.

[0044] Through the structural design with three hydraulic acting areas Ⅰ = Ⅱ = Ⅲ, the thrust symmetry and action stability during the entire actuation process are ensured, meeting the strict requirements of high-performance flight control systems for actuator response consistency and structural compactness.

[0045] The working principle of the five-chamber series symmetric single-rod actuator of the present invention is as follows: When the actuator extends, the oil enters the first working chamber from port A and port C and acts on area Ⅰ to extend the piston. The oil in the second and third working chambers is discharged from port B and port D, and the two air chambers inhale air from the two T ports. When the actuator retracts, the oil enters the second and third working chambers from port B and port D and retracts the piston by acting on area Ⅱ and area Ⅲ. The oil in the first working chamber is discharged from port A and port C, and the two air chambers discharge air from the two T ports.

[0046] The above embodiments are illustrative of the present invention and not restrictive thereof. Any scheme obtained by simply transforming the present invention belongs to the protection scope of the present invention.

Claims

1. A five-chamber series-connected symmetric single-rod electro-hydrostatic actuator, characterized in that: It includes a dual controller, a dual motor pump unit, and a five-chamber series-connected symmetric single-rod actuator cylinder; The dual motor pump unit consists of a first motor pump unit and a second motor pump unit; each motor pump unit includes a motor, a hydraulic pump, an accumulator, two one-way valves, two overflow valves, and a bypass valve; The five-chamber series-connected symmetric single-rod actuator cylinder includes a cylinder block, an inner cylinder barrel, an intermediate seat, a cylinder block end cover, and a piston; Among them: The dual controllers are electrically connected to the two motors respectively; The cylinder block is hermetically connected to the cylinder block end cover, and the inner cylinder barrel and the intermediate seat are fixed at the center inside the cylinder block; The piston is located between the cylinder block and the inner cylinder barrel, and forms a sealed sliding fit with the cylinder block, the inner cylinder barrel, the intermediate seat, and the cylinder block end cover; The cylinder block is provided with port A, port B, port C, port D, and two port Ts; Port A and port B are respectively connected to the suction and discharge oil ports of the hydraulic pump of the first motor pump unit, and port C and port D are respectively connected to the suction and discharge oil ports of the hydraulic pump of the second motor pump unit; The two port Ts are connected to the external air to balance the internal volume difference of the actuator cylinder; When port A and port C are simultaneously supplied with oil, the piston extends; when port B and port D are simultaneously supplied with oil, the piston retracts, and the force-bearing areas during the piston extension and retraction are the same.

2. The electro-hydrostatic actuator according to claim 1, characterized in that: The dual motor pump unit is two units with the same structure; the motor is a permanent magnet synchronous motor; the hydraulic pump is a two-way hydraulic pump with a suction port and a discharge port.

3. The electro-hydrostatic actuator according to claim 1, wherein: In each motor pump unit, the inlet ports of the two one-way valves are both connected to the accumulator, and the outlet ports are respectively connected to the suction port and the discharge port of the hydraulic pump.

4. The electro-hydrostatic actuator according to claim 1, wherein: In each motor pump unit, the two ends of the two overflow valves are respectively connected to the suction and discharge oil ports of the hydraulic pump, and the conduction directions of the two overflow valves are opposite.

5. The electro-hydrostatic actuator according to claim 1, characterized in that: In each motor pump unit, the two ends of the bypass valve are respectively connected to the suction port and the discharge port of the hydraulic pump.

6. The electro-hydrostatic actuator according to claim 1, wherein: Five chambers are formed inside the actuator cylinder: The first working chamber is surrounded by the inner cylinder barrel and the piston, and is connected to port A and port C; the second working chamber is surrounded by the cylinder block, the inner cylinder barrel, the intermediate seat, and the piston, and is connected to port B; the third working chamber is surrounded by the cylinder block and the piston, and is connected to port D; the two air chambers are surrounded by the cylinder block, the inner cylinder barrel, the intermediate seat, and the piston, and are respectively connected to the two port Ts; the effective acting area of the first working chamber is I, the effective acting area of the second working chamber is II, and the effective acting area of the third working chamber is III.

7. The electro-hydrostatic actuator according to claim 6, characterized in that: The areas I of the hydraulic oil acting on the first working chamber, area II of the second working chamber, and area III of the third working chamber satisfy: area I, area II, and area III are equal.

8. The electro-hydrostatic actuator according to claim 1, characterized in that: One end of the piston is installed with a bushing, the center of the inner end of the bushing is connected with an insertion rod, and the insertion rod is fixed with a magnetic ring; a displacement sensor is installed on the cylinder block, and the magnetic ring is sleeved outside the displacement sensor to form a non-contact detection structure.

9. The electro-hydrostatic actuator according to claim 1, characterized in that: The cylinder block end cover is provided with a damping hole, which communicates port D with the third working chamber; when the piston extends to the limit position, the piston blocks port D, and the hydraulic oil flows into the third working chamber through the damping hole to push the piston to retract.

10. A working method of the electro-hydrostatic actuator according to any one of claims 1-9, characterized in that It includes: Normal driving mode: When the piston extends, the dual controllers control the first and second motor pump units to supply oil to port A and port C respectively, port B and port D return oil, and the two port Ts inhale air; when the piston retracts, the dual controllers control the first and second motor pump units to supply oil to port B and port D respectively, port A and port C return oil, and the two port Ts discharge air; Fault tolerance mode: When a single motor pump unit fails, its bypass valve opens to isolate the faulty oil circuit, and the other motor pump unit independently drives the actuator to move.

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