Direct-acting actuator

Abstract

To provide a direct-acting actuator that enables extended service life, increased design flexibility around a motor, and reduced installation space.SOLUTION: A direct-acting actuator 1 includes an electric motor 4, a motion conversion mechanism 5 that converts the rotational motion of the electric motor 4 into linear motion, a stroke sensor 6 that detects the linear displacement of the motion conversion mechanism 5, and a control device 3 that controls the electric motor 4 using the linear displacement detected by the stroke sensor 6. The electric motor 4 is a brushless motor. The control device 3 has an angle converter 16 that converts the linear displacement detected by the stroke sensor 6 into a rotational angle for the electric motor 4. The control device 3 performs follow-up control of the rotational angle converted by the angle converter 16 to match a rotational angle target value provided from an upper ECU 19.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a linear actuator, and to a technique that can achieve a longer life, improved design freedom, and a reduction in installation space. [Background technology]

[0002] Linear actuators incorporating ball screws play an important role in the automotive industry, replacing hydraulic drives in applications such as automobile brakes. Traditional linear actuator designs typically combine a brushed motor with a stroke sensor, which is easy to control. This is because brushed motors utilize a mechanical commutation system, where the brushes and commutator automatically energize the appropriate coil based on the rotor position, eliminating the need for external angle recognition. However, brushed motors have a short lifespan due to mechanical wear of the brushes, which are in constant contact with the rotating part of the motor.

[0003] Brushless motors, on the other hand, have a long lifespan due to the lack of brushes and mechanical contacts. However, brushless motors require an angle sensor because they rely on knowing the rotor angle to properly power the associated windings. This presents a significant challenge in applications where installation space for an angle sensor is limited or where the use of an angle sensor is not possible.

[0004] As shown in Figure 6, the prior art proposes feedback control using a stroke sensor 50 to control the output position of an actuator (Patent Document 1). The prior art describes control that combines the stroke sensor 50 with a brushless motor, but does not disclose the control method. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-74788 Summary of the Invention [Problem to be solved by the invention]

[0006] Generally, vector control is used to control brushless motors. A rotation angle sensor is required for vector control of brushless motors. Rotation angle sensors are generally installed on or near the outer periphery of the motor shaft, which can increase the overall length of the system. This poses a major challenge in applications where installation space for a rotation angle sensor is limited, the cost of a rotation angle sensor is high, or the use of a rotation angle sensor is not possible.

[0007] An object of the present invention is to provide a linear actuator that enables control through a combination of a stroke sensor and a brushless motor, thereby extending the life of the actuator, improving the degree of freedom in designing the motor periphery, and reducing installation space. [Means for solving the problem]

[0008] A linear motion actuator 1 of the present invention is a linear motion actuator including an electric motor 4, a motion conversion mechanism 5 that converts the rotational motion of the electric motor 4 into linear motion, a stroke sensor 6 that detects linear displacement of the motion conversion mechanism 5, and a control device 3 that controls the electric motor 4 using the linear displacement detected by the stroke sensor 6, The electric motor 4 is a brushless motor, The control device 3 has an angle converter 16 that converts the linear displacement detected by the stroke sensor 6 into a rotation angle of the electric motor 4, and controls the rotation angle converted by this angle converter 16 to follow a given rotation angle target value.

[0009] With this configuration, a brushless motor is used as the electric motor 4, which is the drive source of the linear motion actuator 1, and this allows for a longer lifespan than a linear motion actuator that uses a brushed motor. Instead of using a rotation angle sensor, the linear motion actuator 1 can indirectly determine the rotation angle of the electric motor 4 by using a stroke sensor 6 that can be built into the actuator body 2 of the linear motion actuator 1. This enables highly accurate motor control that controls this rotation angle to follow a target rotation angle value.

[0010] Therefore, it is possible to realize a linear motion actuator 1 that requires less installation space around the motor than a linear motion actuator that uses a rotation angle sensor, and that enables highly accurate motor control. Furthermore, because the rotation angle can be determined indirectly without using a rotation angle sensor, it is possible to increase the degree of freedom in the design of the linear motion actuator 1. By avoiding the use of a rotation angle sensor, it becomes possible to use an existing system by simply replacing the electric motor 4.

[0011] The motion conversion mechanism 5 may include a reducer 11 that reduces the rotation speed of the electric motor 4, and a linear motion mechanism 9 that converts the rotation output of the reducer 11 into linear motion. In this case, by interposing the reducer 11 between the electric motor 4 and the linear motion mechanism 9, it becomes possible to output a desired torque even with a relatively small electric motor 4. With this configuration, the electric motor 4 can be made smaller than a linear motion actuator that does not have a reducer, and therefore the entire linear motion actuator can be made smaller and lighter.

[0012] The control device 3 may perform feedback control of the motor current with respect to a given current target value. In this case, for example, by performing current feedback control on the motor current obtained by measuring the current of the electric motor 4, the responsiveness of the controlled object can be improved, enabling highly accurate motor control.

[0013] The control device 3 may perform feedback control of the motor speed with respect to a given speed target value. In this case, the control device 3 can estimate the motor speed, for example, by time-differentiating the rotation angle output from the angle converter 16. By performing speed feedback control of this motor speed, it is possible to improve responsiveness to the speed target value and enable highly accurate motor control. [Effects of the Invention]

[0014] The linear motion actuator of the present invention is a linear motion actuator comprising an electric motor, a motion conversion mechanism that converts the rotational motion of the electric motor into linear motion, a stroke sensor that detects the linear displacement of the motion conversion mechanism, and a control device that controls the electric motor using the linear displacement, wherein the electric motor is a brushless motor, and the control device has an angle converter that converts the linear displacement detected by the stroke sensor into a rotational angle of the electric motor, and controls the rotational angle converted by the angle converter to follow a given target rotational angle value, thereby achieving a longer life, greater freedom in designing the motor and its surroundings, and a reduction in installation space. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a conceptual diagram showing a configuration of a linear actuator according to an embodiment of the present invention. [Figure 2] FIG. 2 is a perspective view of an actuator body of the linear motion actuator. [Figure 3] 4 is a diagram showing the relationship between rotation of an electric motor of the linear actuator and linear displacement. FIG. [Figure 4] FIG. 2 is a block diagram of a control system for the linear actuator. [Figure 5] 10 is a flowchart showing the control of the linear actuator in stages. [Figure 6] FIG. 10 is a block diagram of a control system for a linear motion actuator according to a conventional example. DETAILED DESCRIPTION OF THE INVENTION

[0016] [First embodiment] A linear motion actuator according to an embodiment of the present invention will be described with reference to Figures 1 to 5. The linear motion actuator is used, for example, in electric brake devices mounted on vehicles, linear motion devices mounted on ships and aircraft, and other industrial machinery. <Actuator body> As shown in Figure 1, a linear motion actuator 1 includes an actuator body 2 and a control device 3, which will be described later. The actuator body 2 has an electric motor 4, a motion conversion mechanism 5, and a stroke sensor 6. As shown in Figure 2, the actuator body 2 is assembled by connecting predetermined components, and can be traded independently on the market.

[0017] <Electric motor> The electric motor 4 is a three-phase brushless motor, and is detachably attached to, for example, an actuator case 7. As shown in FIG. 1, a motor shaft 4a of the electric motor 4 is supported so as to be rotatable forward and backward by a rolling bearing (not shown) fitted inside a motor case 8. The electric motor 4 is supported by the actuator case 7 (FIG. 2) so that the motor shaft 4a is parallel to an axial direction C1 of a screw shaft 10 in a linear motion mechanism 9 (described later).

[0018] <Movement conversion mechanism> The motion conversion mechanism 5 converts the rotational motion of the electric motor 4 into linear motion. This motion conversion mechanism 5 has a reducer 11 that reduces the rotation speed of the electric motor 4, and a linear motion mechanism 9 that converts the rotation output of this reducer 11 into linear motion. As the reducer 11, for example, a parallel gear having a drive gear 12 connected to the motor rotation shaft 4a and a driven gear 13 that meshes with the drive gear 12 is applied. Alternatively, a worm gear, a planetary gear, or the like may be applied as the reducer.

[0019] The linear motion mechanism 9 is a ball screw mechanism including a ball screw nut 9a and a ball screw shaft 9b. The ball screw nut 9a is rotatably supported by a rolling bearing (not shown) attached to the actuator case 7 (FIG. 2). A driven gear 13 is fixed to the outer periphery of the ball screw nut 9a. The rotation of the ball screw shaft 9b is restricted, for example, by a rotation restricting member provided at the axial base end of the ball screw shaft 9b being inserted into a guide groove in a shaft case 14 (FIG. 2). Although the rotation of the ball screw shaft 9b is restricted by the rotation restricting member, it is still movable in the axial direction C1. An output shaft 15 is provided at the axial tip of the ball screw shaft 9b. This output shaft 15 functions as an operating unit that operates the device to be operated. The shaft case 14 shown in FIG. 2 is fixed to the actuator case 7.

[0020] <Operating principle> As shown in Figure 1, when the electric motor 4 rotates, its rotational motion is transmitted to the driven gear 13 via the drive gear 12, and the driven gear 13 amplifies the rotational torque at a set reduction ratio. The torque of the driven gear 13 rotates the ball screw nut 9a. The rotational output of the ball screw nut 9a is converted into linear motion in the axial direction C1 of the ball screw shaft 9b. As shown in Figure 3, the rotation of the electric motor, shown on the horizontal axis, is linearly proportional to the linear displacement per rotation of the ball screw (shown on the vertical axis in Figure 3).

[0021] <Stroke sensor> The stroke sensor 6 in Fig. 1 detects linear displacement of the motion converting mechanism 5. Specifically, the stroke sensor 6 detects the amount of movement of the ball screw shaft 9b in the axial direction C1 and can output the result as a position sensor value. The stroke sensor 6 can be any of a variety of sensors, including magnetic, optical, and capacitance types, but in this embodiment, a magnetic sensor is used. The stroke sensor 6 has, for example, a sensor main body fixed inside the actuator case and a sensor measurement target fixed to the output shaft 15.

[0022] A permanent magnet, for example, is used as the sensor measurement target. The sensor body faces the path along which the sensor measurement target moves forward and backward. When the ball screw shaft 9b and the output shaft 15 are at a predetermined axial position (advance / retract position), the sensor body and the sensor measurement target face each other across a predetermined gap. The sensor body reads changes in the magnetic field of the permanent magnet that accompany the advance / retraction of the ball screw shaft 9b, and detects the advance / retract position of the ball screw shaft 9b. The predetermined gap is a gap that is arbitrarily determined by design or the like, and is determined by, for example, determining an appropriate gap through testing and / or simulation.

[0023] Based on the aforementioned operating principle, the rotation angle of the output shaft is calculated from the sensor output of the stroke sensor 6. Specifically, a characteristic conversion factor is determined based on the design parameters of the linear actuator 1, particularly the linear displacement achieved per complete rotation. Once this conversion factor is determined, the exact rotation angle of the linear actuator 1 can be calculated from the linear displacement. This is because there is a linear proportional relationship between the linear displacement of the linear actuator 1 and the rotation speed of the output shaft, as follows: Linear displacement = Number of rotations of the output shaft x Distance traveled by the ball screw shaft during one rotation After the rotation angle is calculated, it is utilized to control the system inputs, aiming to minimize the discrepancy between the actual and target values ​​of the rotation angle.

[0024] As shown in FIGS. 3 and 4, for example, it is assumed that the output shaft 15 moves linearly by 5 mm for each rotation of the electric motor 4. Here, as an example, it is assumed that the linear displacement detected by the stroke sensor 6 is 3 [mm]. The rotation angle can be calculated as follows: Conversion factor = 1 / 5 [revolutions / mm] = 0.2 [revolutions / mm] Number of revolutions of the electric motor = 3 [mm] x 0.2 [revolutions / mm] = 0.6 Rotation angle = 0.6 x 360 = 216 degrees In other words, a linear displacement of 3 mm corresponds to a rotation angle of 216 degrees of this ball screw type linear actuator 1. In this case, to simplify the calculation, the reduction ratio of the reducer is set to "1".

[0025] <Control device> The control device 3 in FIG. 4 controls the electric motor 4 using the linear displacement. The control device 3 includes an angle converter 16, a speed calculation unit 17, and a vector controller 18. The angle converter 16 converts the linear displacement detected by the stroke sensor 6 into a rotation angle of the electric motor 4. The control device 3 controls the rotation angle converted by the angle converter 16 to follow a target rotation angle value provided by a host ECU 19. Specifically, a comparison unit 20 of the control device 3 compares the converted rotation angle with the target rotation angle value. An error, which is the difference between the rotation angle (actual measurement value) and the target rotation angle value, is supplied to the vector controller 18, which controls the system input to reduce the system error.

[0026] When the linear motion actuator 1 is applied to, for example, an electric brake device mounted on a vehicle, a vehicle control unit (VCU) that controls the entire vehicle is applied as the host ECU 19. The host ECU 19 includes a command means 19a, and the command means 19a provides the output of a sensor that changes depending on, for example, the amount of operation of an operating means by a driver of the vehicle, to the control device 3 as command values ​​(target rotation angle value, target current value, target speed value).

[0027] For example, a battery of the vehicle on which the linear actuator 1 is mounted can be used as the power source for supplying power to the electric motor 4 and the control device 3. The command means 19a may be the operating means itself, or, as in an autonomous vehicle, it may automatically determine and output a command value from the vehicle state and information from various sensors, without relying on the operation of the operating means.

[0028] The speed calculation unit 17 can estimate the motor speed (angular speed), for example, by time differentiating the rotation angle output from the angle converter 16. The vector controller 18 of the control device 3 feedback controls the motor speed with respect to a speed target value given by the upper ECU 19.

[0029] The vector controller 18 of the control device 3 performs feedback control of the motor current with respect to a current target value given by the host ECU 19. For example, the actual motor current can be detected by a current sensor 21 that measures the U, V, and W three-phase current of the electric motor 4. It is also possible to detect the current of any two of the three phases by the current sensor 21 and determine the current of the remaining phase by calculation.

[0030] <Flowchart> FIG. 5 is a flowchart showing the step-by-step control of the linear actuator. The following description will also refer to FIG. 4 as appropriate. After starting this process, the control device 3 acquires the linear displacement of the motion conversion mechanism 5 from the stroke sensor 6 (step S1), and converts the linear displacement into a rotation angle of the electric motor 4 using the angle converter 16 (step S2). The rotation angle is used in steps S3, S5, and S8. As shown within the dashed-dotted line frame in FIG. 5, the vector control system, which is the control device 3, performs vector control using the rotation angle. The speed calculation unit 17 calculates the motor speed from the rotation angle (step S3), and the vector controller 18 feedback-controls the motor speed relative to a speed target value (step S6).

[0031] The vector controller 18 performs coordinate conversion (step S5) on the three-phase current detected by the current sensor 21 (step S4) into two-phase, and then feedback controls the motor current with respect to the current target value (step S7). After that, the vector controller 18 performs inverse coordinate conversion from two-phase to three-phase (step S8) and outputs the motor current (step S9). Then, the process ends.

[0032] <Action and effect> According to the linear motion actuator 1 described above, a brushless motor is used as the electric motor 4, which is the drive source of the linear motion actuator 1, thereby achieving a longer lifespan than linear motion actuators that use brushed motors. Instead of using a rotation angle sensor, the linear motion actuator 1 can indirectly determine the rotation angle of the electric motor 4 by using a stroke sensor 6 that can be built into the actuator body 2 of the linear motion actuator 1. High-precision motor control is possible by controlling this rotation angle to follow a target rotation angle value. Unlike conventional feedback control methods, the vector control system makes it possible to independently control the torque and magnetic flux of the electric motor 4. This enables smoother and more precise motor control.

[0033] Therefore, it is possible to realize a linear motion actuator 1 that requires less installation space around the motor than a linear motion actuator that uses a rotation angle sensor, and that enables highly accurate motor control. Furthermore, because the rotation angle can be determined indirectly without using a rotation angle sensor, it is possible to increase the degree of freedom in the design of the linear motion actuator 1. By avoiding the use of a rotation angle sensor, it becomes possible to use an existing system by simply replacing the electric motor 4.

[0034] As shown in Figure 1, the motion conversion mechanism 5 has a reducer 11 that reduces the rotation of the electric motor 4, and a linear motion mechanism 9 that converts the rotation output of this reducer 11 into linear motion. In this way, by interposing the reducer 11 between the electric motor 4 and the linear motion mechanism 9, it becomes possible to output a desired torque even with a relatively small electric motor 4. With this configuration, the electric motor 4 can be made smaller than a linear motion actuator that does not have a reducer, and therefore the entire linear motion actuator can be made smaller and lighter.

[0035] The control device 3 performs feedback control of the motor current with respect to a given current target value. In this case, by performing current feedback control of the motor current obtained by measuring the current of the electric motor 4, the responsiveness of the controlled object can be improved, enabling highly accurate motor control. The control device 3 performs feedback control of the motor speed with respect to a given speed target value. In this case, as shown in Figure 4, the control device 3 can estimate the motor speed by time-differentiating the rotation angle output from the angle converter 16. By performing speed feedback control of this motor speed, it is possible to improve responsiveness to the speed target value and enable highly accurate motor control.

[0036] <Other embodiments> The linear motion mechanism is not limited to a ball screw mechanism, and a sliding screw type feed screw mechanism such as a trapezoidal screw or a triangular screw may also be applied. As another linear motion mechanism, a rack and pinion mechanism may also be applied. It is also possible to omit the reducer and directly convert the rotational output of the electric motor into linear motion using a linear motion mechanism.

[0037] Although the embodiments of the present invention have been described above, the disclosed embodiments are illustrative in all respects and are not limiting. The scope of the present invention is defined by the claims rather than the above description, and it is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0038] 1... Linear motion actuator, 3... Control device, 4... Electric motor, 5... Motion conversion mechanism, 6... Stroke sensor, 9... Linear motion mechanism, 11... Reducer, 16... Angle converter

Claims

1. A linear motion actuator including an electric motor, a motion conversion mechanism that converts rotational motion of the electric motor into linear motion, a stroke sensor that detects linear displacement of the motion conversion mechanism, and a control device that controls the electric motor using the linear displacement detected by the stroke sensor, the electric motor is a brushless motor, The control device is a linear actuator having an angle converter that converts the linear displacement detected by the stroke sensor into a rotation angle of the electric motor, and controls the rotation angle converted by this angle converter to follow a given rotation angle target value.

2. 2. The linear motion actuator according to claim 1, wherein the motion conversion mechanism comprises a reducer that reduces the rotation speed of the electric motor, and a linear motion mechanism that converts the rotation output of the reducer into linear motion.

3. 3. The linear actuator according to claim 1, wherein the control device feedback-controls the motor current with respect to a given current target value.

4. 3. The linear actuator according to claim 1, wherein the control device feedback-controls the motor speed with respect to a given speed target value.

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