Linear motion actuator

Abstract

To provide a linear motion actuator that can achieve reduction of an installation space and improvement of flexibility in design, achieve cost reduction, and increase the accuracy of a control system.SOLUTION: A linear motion actuator 1 includes: an electric motor 4; a motion conversion mechanism 5 that converts a rotational motion of this electric motor 4 into a linear motion; a rotation angle sensor Sa that detects a rotation angle of the electric motor 4; and a control device 3 that controls the electric motor 4. The control device 3 has a linear displacement converter Lc that converts the rotation angle detected by the rotation angle sensor Sa into a linear displacement of the motion conversion mechanism 5 and a zero point correction mechanism Zc that corrects a zero point of an initial position of the linear displacement. The control device 3 controls the electric motor 4 using the linear displacement converted by the linear displacement converter Lc.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a linear actuator, and to a technique that can reduce installation space, improve design freedom, reduce costs, and increase the accuracy of a control system. [Background technology]

[0002] Linear actuators incorporating ball screws play an important role as an alternative to hydraulic drives in the automotive industry, for example in automobile brakes. Conventional linear actuator designs typically combine a brushed motor, which is easy to control, with a stroke sensor. However, the stroke sensor must be attached to the linear actuator, which is a limitation. This requires the housing of the linear actuator to be enlarged, increasing the dimensions of the linear actuator. This poses a major challenge in applications where the installation space for a stroke sensor is limited or where the use of a stroke sensor is not possible.

[0003] As shown in Figure 8, the prior art proposes feedback control using both a stroke sensor 60 and a rotation angle sensor 61 to control the rotation of the motor and the output position of the actuator (Patent Document 1), but does not disclose the control method. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-207182 Summary of the Invention [Problem to be solved by the invention]

[0005] Generally, a stroke sensor is used in feedback control systems that control the output shaft position of linear actuators that use brushed motors. This poses a major challenge in applications where the installation space for a stroke sensor is limited or where the use of a stroke sensor is not possible. Furthermore, achieving fine position control requires a high-resolution stroke sensor, which leads to a significant increase in overall costs.

[0006] An object of the present invention is to provide a linear actuator that can reduce installation space, improve design freedom, reduce costs, and increase the accuracy of the control system. [Means for solving the problem]

[0007] 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 rotation angle sensor Sa that detects the rotation angle of the electric motor 4, and a control device 3 that controls the electric motor 4, The control device 3 is a linear displacement converter Lc that converts the rotation angle detected by the rotation angle sensor Sa into a linear displacement of the motion converting mechanism 5; a zero point correction mechanism Zc that corrects a zero point that is an initial position of the linear motion displacement, The electric motor 4 is controlled using the linear displacement converted by the linear displacement converter Lc.

[0008] With this configuration, the rotation angle sensor Sa can usually be directly incorporated onto or near the motor's rotating shaft, thereby reducing installation space compared to conventional linear actuators. Providing an alternative method for indirectly determining linear displacement without a stroke sensor increases the design freedom of the linear actuator 1. Rotation angle sensors Sa, which are in the same cost range as stroke sensors, have higher resolution than stroke sensors, resulting in superior control system accuracy. Therefore, by using only the rotation angle sensor Sa, the high resolution allows for increased control system accuracy while maintaining low costs.

[0009] 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.

[0010] The control device 3 may perform feedback control of the linear displacement relative to a given target position, in which case it is possible to minimize the discrepancy between the linear displacement, which is an actual measurement value, and the target position.

[0011] When the zero point is corrected by the zero point correction mechanism Zc, the control device 3 may limit at least one of the speed, motor current, and motor voltage of the linear motion part 15 of the linear motion mechanism 9. In this case, it is possible to prevent the linear motion part 15 from undesirably colliding with a housing or the like when correcting the zero point.

[0012] The electric motor may be a brushed motor. [Effects of the Invention]

[0013] 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 rotation angle sensor that detects the rotation angle of the electric motor, and a control device that controls the electric motor, wherein the control device has a linear motion displacement converter that converts the rotation angle detected by the rotation angle sensor into linear motion of the motion conversion mechanism, and a zero point correction mechanism that corrects the zero point, which is the initial position of the linear motion displacement, and controls the electric motor using the linear motion converted by the linear motion displacement converter, thereby reducing installation space and improving design freedom, reducing costs, and increasing the accuracy of the control system. [Brief explanation of the drawings]

[0014] [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 the rotation angle of the ball screw and linear displacement of the linear actuator. FIG. [Figure 4] FIG. 2 is a block diagram of a control system for the linear actuator. [Figure 5] 4 is a diagram showing the relationship between a ball screw shaft and a housing of the linear actuator. FIG. [Figure 6] 10 is a flowchart showing a step-by-step process of zero point correction of the linear actuator. [Figure 7] 10 is a flowchart showing the control of the linear actuator in stages. [Figure 8] FIG. 10 is a block diagram of a control system for a conventional linear actuator. DETAILED DESCRIPTION OF THE INVENTION

[0015] [First embodiment] A linear motion actuator according to an embodiment of the present invention will be described with reference to Figures 1 to 7. 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, the 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 rotation angle sensor Sa. As shown in Figure 2, the actuator body 2 is assembled by connecting predetermined components, and can be traded independently on the market.

[0016] <Electric motor> The electric motor 4 is a brushed 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 in a motor case 8 (Fig. 2). 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).

[0017] <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.

[0018] 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 restriction 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), which serves as a housing.

[0019] The ball screw shaft 9b is restricted in rotation by the rotation restricting member, but 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, the 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 torque of the ball screw nut 9a is converted into linear motion in the axial direction C1 of the ball screw shaft 9b, and as shown in Figure 3, the linear motion is linearly proportional to the linear displacement per rotation of the ball screw mechanism.

[0021] <Rotation angle sensor> The rotation angle sensor Sa shown in FIG. 1 detects the rotation angle of the electric motor 4. For example, a resolver or a magnetic encoder is preferably used as the rotation angle sensor Sa, as it provides high accuracy and reliability. The rotation angle sensor Sa has a detection target and a sensor unit (not shown), and is incorporated on or near the motor rotation shaft. The detection target is fixed, for example, to one axial end of the motor rotation shaft 4a. The sensor unit is fixed to the motor case 8 (FIG. 2) across a predetermined radial or axial gap from the detection target, and detects the detection target. This sensor unit is electrically connected to the control device 3. Note that the rotation angle sensor Sa can also use various sensors, such as an optical encoder.

[0022] Based on the aforementioned operating principle, the linear displacement of the output shaft is calculated from the sensor output of the rotation angle sensor Sa attached to the electric motor 4. Specifically, a characteristic conversion coefficient is first determined based on the design parameters of the linear actuator 1, particularly the linear displacement achieved per complete rotation. Once this conversion coefficient is identified, the accurate linear displacement of the output shaft of the linear actuator 1 can be calculated from the rotation angle. 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 travel distance of the output shaft during one rotation After the linear displacement is calculated, it is utilized to control the system input, aiming to minimize the discrepancy between the actual value of the linear displacement and the target position.

[0023] 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 rotation angle detected by the rotation angle sensor Sa is 576 degrees. The rotation angle can be calculated as follows: Conversion factor = 5 [mm / rotation] Number of revolutions of the electric motor = 576 / 360 = 1.6 [rpm] Displacement of linear actuator = 1.6 [rotations] x 5 [mm / rotation] = 8 [mm] In other words, the rotation angle of 576 degrees of the motor rotating shaft 4a (Fig. 1) in this ball screw linear motion actuator 1 corresponds to a linear motion displacement of 8 mm of the output shaft. In this case, to simplify the calculation, the reduction ratio of the reducer is set to "1".

[0024] <Control device> The control device 3 in Figure 4 controls the electric motor 4. The control device 3 includes a linear displacement converter Lc, a zero point correction mechanism Zc, and a feedback controller 18. The linear displacement converter Lc converts the rotation angle detected by the rotation angle sensor Sa into a linear displacement of the motion conversion mechanism 5. The zero point correction mechanism Zc corrects the zero point, which is the initial position of the linear displacement, when the electric motor 4 is started. The process of correcting the zero point, which is the initial position of the linear displacement, is called "zero point correction." The zero point, which is the initial position, is set as the point at which the ball screw shaft is driven to the stall (end stop) position. For example, as shown in Figure 5, the zero point is the point of contact between the axial base end 9ba of the ball screw shaft 9b and the bottom 14a of the shaft case 14, which is the housing.

[0025] As shown in Fig. 4, the control device 3 controls the electric motor 4 using the linear displacement converted by the linear displacement converter Lc. The control device 3 feedback-controls the linear displacement with respect to a target position provided by a host ECU 19. Specifically, a comparison unit 20 of the control device 3 compares the converted linear displacement with the target position. An error, which is the difference between the current linear displacement (actual measurement value) and the target position, is supplied to a feedback 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 position, target speed value, target current value, target voltage value), etc.

[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] Incidentally, when the linear motion mechanism 9 is driven by a normal driving operation during zero point correction, there is a risk that the ball screw shaft 9b in FIG. 5 may undesirably collide with the shaft case 14, which is the housing. Therefore, in order to prevent the above-mentioned collisions, the control device 3 in FIG. 4 limits at least one of the speed, motor current, and motor voltage of the output shaft (linear part) 15 of the linear motion mechanism 9 when the zero point is corrected by the zero point correction mechanism Zc.

[0029] The control device 3 can estimate the speed by, for example, using the speed calculation unit 17 to time-differentiate the linear displacement output from the linear displacement transducer Lc. The actual motor current can be detected by a current sensor 21 that measures the current of the electric motor 4. The motor voltage can be a value output from a feedback controller 18. The upper and lower limits of the speed, motor current, and motor voltage are appropriately set, for example, through testing or simulation.

[0030] <Flowchart> <Zero point correction flow> Fig. 6 is a flowchart showing the zero point correction of the linear actuator in stages. During this zero point correction, at least one of the speed, motor current, and motor voltage is limited. The following explanation will be made with reference to Figs. 4 and 5 as needed. After starting this process, the zero point correction mechanism Zc drives the ball screw shaft to the stall position (step a1) and checks whether the axial base end 9ba of the ball screw shaft 9b is in contact with the bottom 14a of the shaft case 14, which is the housing (step a2).

[0031] For example, if the ball screw shaft 9b is driven toward the stall position and there is a change in the current linear displacement (actual measurement value), the zero point correction mechanism Zc determines that the axial base end 9ba of the ball screw shaft 9b is not in contact with the housing 14 (step a2: No) and returns to step a1. When there is no change in the actual measurement value, the zero point correction mechanism Zc determines that the axial base end 9ba of the ball screw shaft 9b has come into contact with the housing 14 (step a2: Yes) and proceeds to step a3. Thereafter, the zero point correction mechanism Zc sets the current position of the ball screw shaft 9b as the zero point (step a3) and ends this process. In determining whether the axial base end 9ba of the ball screw shaft 9b has come into contact with the housing 14 in step a2 above, when there is no change in the actual measurement value, this refers to when there is no displacement in the rotation angle detected by the rotation angle sensor Sa.

[0032] <Position feedback control flow> The control flow in Fig. 7 is always executed. The zero point correction flow in Fig. 6 is executed, for example, when the electric motor 4 is started and a predetermined target position is given from the host ECU 19. As shown in Figures 4 and 7, after starting the processing of the position feedback control flow, the control device 3 acquires the rotation angle of the electric motor 4 from the rotation angle sensor Sa (step b1), and converts the rotation angle into a linear displacement in the linear displacement converter Lc (step b2).

[0033] The feedback control system, which is the control device 3, feedback controls the linear displacement with respect to the target position given by the host ECU 19 (step b3). That is, the comparison unit 20 of the control device 3 compares the converted linear displacement with the target position. The error, which is the difference between the current linear displacement (actual measurement value) and the target position, is supplied to the feedback controller 18, which controls the system input to reduce the system error. Thereafter, the feedback controller 18 outputs the motor voltage (step b4). Then, this process ends.

[0034] Although the linear motion actuator 1 shows an example of position feedback control, the present invention is not limited to position feedback control and can also be adapted to feedback control of the torque, current, or speed of the electric motor 4. Specifically, the feedback controller 18 of the control device 3 can also feedback control the motor torque with respect to a torque target value given by the host ECU 19. The feedback controller 18 can also feedback control the motor current with respect to a current target value given by the host ECU 19. The feedback controller 18 can also feedback control the speed with respect to a speed target value given by the host ECU 19. By combining these various types of feedback control, the control system can be made more accurate.

[0035] <Action and effect> According to the linear motion actuator 1 described above, the rotation angle sensor Sa can be directly incorporated onto or near the motor rotation shaft, thereby reducing installation space compared to conventional linear motion actuators. By providing an alternative method for indirectly determining linear motion displacement without a stroke sensor, the design freedom of the linear motion actuator 1 can be increased. A rotation angle sensor Sa, which is in the same cost range as a stroke sensor, has higher resolution than a stroke sensor, resulting in superior control system accuracy. Therefore, by using only the rotation angle sensor Sa, the high resolution allows for improved control system accuracy while maintaining low costs.

[0036] 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.

[0037] As shown in Figure 4, the control device 3 feedback-controls the linear displacement relative to a given target position, making it possible to minimize the discrepancy between the actual linear displacement and the target position, thereby improving the accuracy of the control system.

[0038] The control device 3 limits at least one of the speed, motor current, and motor voltage of the linear motion part of the linear motion mechanism 9 when the zero point correction mechanism Zc corrects the zero point. This makes it possible to prevent the output shaft 15, which is the linear motion part, from undesirably colliding with the shaft case 14 (FIG. 5) when correcting the zero point. This makes it possible to prevent damage to the shaft case 14 (FIG. 5), etc., and to extend the product life. It also makes it possible to prevent causes of abnormalities in the linear motion actuator 1.

[0039] <Other embodiments> The electric motor 4 may be a brushless motor, a stepping motor, or another type of motor. 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.

[0040] 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]

[0041] 1... Linear motion actuator, 3... Control device, 4... Electric motor, 5... Motion conversion mechanism, 9... Linear motion mechanism, 11... Reducer, 15... Output shaft (linear motion part), Sa... Rotation angle sensor, Lc... Linear motion displacement converter, Zc... Zero point correction mechanism

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 rotation angle sensor that detects a rotation angle of the electric motor, and a control device that controls the electric motor, The control device a linear displacement converter that converts the rotation angle detected by the rotation angle sensor into a linear displacement of the motion converting mechanism; a zero point correction mechanism that corrects a zero point that is an initial position of the linear motion displacement, A linear actuator that controls the electric motor using the linear displacement converted by the linear displacement converter.

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 motion actuator according to claim 1, wherein the control device feedback-controls the linear motion displacement with respect to a given target position.

4. 3. The linear actuator according to claim 2, wherein the control device limits at least one of a speed of a linear part of the linear motion mechanism, a motor current, and a motor voltage when the zero point is corrected by the zero point correction mechanism.

5. 2. The linear actuator according to claim 1, wherein the electric motor is a brushed motor.

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