A wheel-legged robot

Abstract

The application discloses a wheel-foot robot, which comprises a robot body and a robot leg, and the robot leg comprises a thigh, a shank and a rotating wheel. The thigh comprises a thigh weak side surface on the same side of a hip joint and a thigh strong side surface away from the hip joint. The shank is rotationally connected to the thigh weak side surface through a knee joint, and comprises a shank strong side surface on the same side of the knee joint and a shank weak side surface away from the knee joint. The rotating wheel is rotationally connected to the end of the shank away from the thigh. The shank is provided with a first stop surface and a second stop surface. The shank can be rotated relative to the thigh to the first stop surface to abut against the hip joint and form a limit in a first direction, and the shank can be rotated relative to the thigh to the second stop surface to abut against the hip joint and form a limit in a second direction. The first direction and the second direction are opposite directions. The application can realize the rotation limit of the shank by abutting the hip joint against the first stop surface or the second stop surface, the overall structure is simplified, the size is reduced, and the activity range of the wheel-foot robot is larger.

Description

Technical Field

[0001] This invention belongs to the field of quadruped robot technology, and in particular relates to a wheeled robot. Background Technology

[0002] Quadruped robots can mimic the movements of quadruped animals to traverse terrains inaccessible to wheeled or tracked robots. They can perform emergency response and engineering tasks in complex environments, thus attracting widespread attention. Currently, the thighs and lower legs of quadruped robots are typically linked and constrained by linkages, as seen in Chinese patents CN118907264A ("A Quadruped Robot Leg Structure and Quadruped Robot") and CN119099754A ("An Energy Storage Robot Leg and Its Motion Control Method"). In these cases, the rotational range of the lower leg relative to the thigh is limited by the linkage structure. Furthermore, to prevent damage from excessive movement of the linkages and other structures, special constraining structures are usually required, making the overall structure more complex and increasing the overall size of the quadruped robot. Summary of the Invention

[0003] To overcome the shortcomings of the prior art, the present invention provides a wheel-legged robot that uses the hip joint to limit the rotation of the lower leg, with a simplified overall structure and reduced size.

[0004] The technical solution adopted by this invention to solve its technical problem is: a wheeled robot, comprising a robot body and four robot legs, each robot leg comprising:

[0005] The thigh, including the weak side of the thigh on the same side as the hip joint, and the strong side of the thigh on the side away from the hip joint.

[0006] The lower leg is connected to the weak side of the thigh via the knee joint, and includes the strong side of the lower leg on the same side as the knee joint and the weak side of the lower leg away from the knee joint.

[0007] Rotate the wheel to the end of the lower leg that is furthest from the thigh;

[0008] The lower leg is provided with a first stop surface and a second stop surface. The lower leg can rotate relative to the thigh in a first direction until the first stop surface abuts against the hip joint and forms a limit. The lower leg can also rotate relative to the thigh in a second direction until the second stop surface abuts against the hip joint and forms a limit. The first direction and the second direction are opposite to each other.

[0009] In this invention, the rotation of the lower leg is achieved by driving the knee joint motor, which eliminates the need for additional limiting structures. The rotation of the lower leg can be limited by the hip joint abutting against the first or second stop surface, simplifying the overall structure and allowing for a relatively smaller overall size of the wheeled robot. At the same time, since there is no need for linkages or other structures, the rotation angle between the lower leg and thigh is greatly increased by directly limiting the movement through the hip joint, resulting in a larger range of motion and more flexible movement for the wheeled robot. In addition, the above-mentioned structural design allows the thigh and lower leg to stand upright, or in other words, reduces the power consumption of walking in an upright state.

[0010] Furthermore, the hip joint has a hip joint shell, a hip joint motor rotor, and a hip joint motor stator. The hip joint motor stator is connected to the weak side of the thigh. The hip joint motor rotor is fixedly arranged. The hip joint shell is located around the hip joint motor rotor. The hip joint motor stator and the thigh rotate synchronously under the drive of the hip joint motor rotor. The first stop surface or the second stop surface can abut against the outer surface of the hip joint motor stator.

[0011] Because the stator of the hip joint motor rotates synchronously with the thigh, and the rotor of the hip joint motor is relatively fixed, the first winding reel located inside the hip joint housing will not rotate, and consequently the wire inside will not become entangled due to rotation.

[0012] Furthermore, a first winding disc is formed inside the hip joint shell, which has a first through hole for the cable to pass through. A cable routing cavity is formed inside the thigh. The weak side of the thigh has a second through hole corresponding to the first through hole, which communicates with the cable routing cavity. The end of the lower leg connected to the thigh has a knee joint shell, which has a notch communicating with the cable routing cavity. The entire cable routing process is simple and reasonable, and because it is located on the inside of the robot leg, the probability of it being affected by external impacts is small, thus improving the overall structural reliability.

[0013] Furthermore, the knee joint shell has an isolation portion, on both sides of which are formed winding areas; the isolation portion has an inclined guide surface. This ensures the orderly winding of the threads inside the knee joint, prevents the threads from winding randomly, provides more relatively independent space for the threads, and avoids mutual wear between the threads.

[0014] Furthermore, the lower leg rotates at an angle of 20-340° relative to the thigh. This gives the wheel-legged robot a wider range of motion and more flexible movement.

[0015] Furthermore, the weak side of the thigh extends obliquely such that the angle between it and the hip joint axis is 75-80°; the direction of the oblique extension of the weak side of the calf is the same as or tends to be the same as the direction of the oblique extension of the weak side of the thigh.

[0016] Furthermore, the maximum width of the robot body is L, and the distance between the planes containing the weak sides of the two lower legs is L1, so L1 / L is 0.6-0.9.

[0017] If the ratio of L1 to L exceeds 0.9, the robot's legs will retract too little, which will not only increase the overall size of the wheeled robot, but also make the robot's center of gravity not concentrated enough, which is not conducive to its rapid movement and rapid action feedback. If the ratio of L1 to L is less than 0.6, the robot's legs will retract too much, the lateral width of its contact with the ground will be too small, and the overall stability of the movement will not be high.

[0018] Furthermore, the knee joints of the front thigh and lower leg are located behind the hip joint, while the knee joints of the rear thigh and lower leg are located in front of the hip joint, creating an X-shape with the front thigh, front lower leg, rear thigh, and rear lower leg. This makes the robot legs symmetrical front and back, resulting in consistent force exertion and reduced overall energy consumption. Moreover, due to the mirrored arrangement of the front and rear robot legs, the wheeled robot's direction of travel is not restricted by its front-to-back orientation, allowing it to move forward or backward without needing to turn its head, thus increasing its mobility.

[0019] Furthermore, the knee joint has a knee joint shell, a knee joint motor, and a knee joint output end. The knee joint motor is fixedly mounted on the end of the thigh, and the knee joint output end is connected to the knee joint motor for driving the lower leg to rotate relative to the thigh. A first protective part is formed on the strong side of the thigh, and the first protective part is set at the position of the knee joint motor output connector.

[0020] The first protective section is an outward-facing triangular arch structure, which provides sufficient installation space for the cable connector and also provides good protection for the cable connector, preventing damage to the internal cable due to impact. In addition, when the thigh and calf are drawn inward, the strong side of the thigh generates an outward and downward force. The presence of this force can easily cause deformation and damage to the strong side of the thigh. Since the first protective section is located at the lower end of the strong side of the thigh, its triangular arch structure increases its strength, enabling it to withstand and resist the deformation caused by the above-mentioned forces, thus increasing the stability of the overall structure.

[0021] Furthermore, an actuation mechanism is provided between the rotating wheel and the lower leg, and a second protective part is formed on the strong side of the lower leg that partially covers the actuation mechanism. When the thigh and lower leg are drawn inward, the strong side of the lower leg generates an outward and downward force. The presence of this force can easily cause deformation and damage to the strong side of the lower leg. Since the second protective part is located at the lower end of the strong side of the lower leg, and the second protective part has a triangular arched structure, its strength is increased, which can withstand and resist the deformation caused by the above-mentioned force, thereby increasing the stability of the overall structure.

[0022] Furthermore, the width of the actuation mechanism is greater than the width of the side of the thigh connected to the hip joint. The lower leg can rotate smoothly until its first stop surface contacts the hip joint, and smoothly rotate until its second stop surface contacts the hip joint, avoiding interference during rotation and ensuring a large range of motion.

[0023] Furthermore, the first and second protective parts are outwardly triangular arched structures.

[0024] Furthermore, a third protective part is formed at the end of the knee joint motor. This provides protection for the knee joint, preventing it from being damaged by external impacts.

[0025] The beneficial effects of this invention are as follows: Lower leg rotation is achieved through a knee joint motor drive, eliminating the need for additional limiting structures. The lower leg rotation is limited by the hip joint abutting against either the first or second stop surface, simplifying the overall structure and allowing for a relatively smaller overall size of the wheeled robot. Simultaneously, since linkages and other structures are unnecessary, the rotation angle between the lower and upper legs is significantly increased through direct hip joint limiting, resulting in a wider range of motion and more flexible movement for the wheeled robot. Furthermore, the aforementioned structural design allows the thigh and lower leg to stand upright, or in other words, reduces power consumption during upright movement. Because the hip joint motor stator rotates synchronously with the thigh, and the hip joint motor rotor remains relatively fixed, the first winding disc inside the hip joint housing does not rotate, preventing the internal wires from tangling due to rotation. The entire wire routing process is simple and efficient, and being located on the inner side of the robot leg, it is less susceptible to external impacts, improving the overall structural reliability. Increased overall strength allows it to withstand and resist the deformation caused by the inward contraction force of the robot leg, enhancing the overall structural stability. Attached Figure Description

[0026] Figure 1 This is a perspective view of the present invention.

[0027] Figure 2 This is the front view of the invention, showing the thighs and lower legs being drawn inwards.

[0028] Figure 3 This is the front view of the invention, showing that the thighs and calves are not retracted inwards.

[0029] Figure 4 This is a side view of the present invention.

[0030] Figure 5 This is a cross-sectional view of the hip joint in this invention.

[0031] Figure 6 The three-dimensional representation of the robot leg in this invention Figure 1 .

[0032] Figure 7 The three-dimensional representation of the robot leg in this invention Figure 2 .

[0033] Figure 8 This is a partial three-dimensional representation of the robot leg in this invention. Figure 1 .

[0034] Figure 9 This is a partial three-dimensional representation of the robot leg in this invention. Figure 2 .

[0035] Figure 10 This is a schematic diagram of the structure connecting the hip joint and thigh in this invention.

[0036] Figure 11 This is a schematic diagram of the structure of the knee joint in this invention.

[0037] Figure 12 This is a schematic diagram of the connection between the thigh and lower leg in this invention.

[0038] Figure 13 This is a schematic diagram of the structure of the knee joint in this invention.

[0039] Among them, 1-robot body, 2-robot leg, 3-thigh, 31-weak side of thigh, 32-strong side of thigh, 321-first protective part, 33-wiring cavity, 34-second through hole, 4-lower leg, 41-weak side of lower leg, 42-strong side of lower leg, 421-second protective part, 43-first stop surface, 44-second stop surface, 5-hip joint, 51-hip joint shell, 511-first winding area, 52-hip joint motor rotor, 53-hip joint motor stator, 54-first winding disc, 541-first through hole, 6-knee joint, 61-knee joint shell, 62-notch, 621-notch, 63-isolation part, 631-tilted guide surface, 64-knee joint motor, 641-outlet connector, 642-third protective part, 7-rotating wheel, 71-actuation mechanism. Detailed Implementation

[0040] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0041] like Figures 1-4As shown, a wheeled robot includes a robot body 1 and four robot legs 2. Each robot leg 2 includes a thigh 3, a lower leg 4, and a rotating wheel 7. The thigh 3 includes a weak thigh side 31 on the same side as the hip joint 5 and a strong thigh side 32 on the opposite side of the hip joint 5. The lower leg 4 is rotatably connected to the weak thigh side 31 via a knee joint 6, and the lower leg 4 includes a strong lower leg side 42 on the same side as the knee joint 6 and a weak lower leg side 41 on the opposite side of the knee joint 6. The rotating wheel 7 is rotatably connected to the end of the lower leg 4 away from the thigh 3, specifically to the strong lower leg side 42. Figure 3 Taking the direction shown as an example, the strong side of the thigh 32 and the strong side of the lower leg 42 are located on the outer side of the wheeled robot, while the weak side of the thigh 31 and the weak side of the lower leg 41 are located on the inner side of the wheeled robot. Here, the inner side is the side where the robot body 1 is located.

[0042] like Figure 1 , Figure 6 , Figure 7 As shown, the lower leg 4 is provided with a first stop surface 43 and a second stop surface 44. The lower leg 4 can rotate relative to the thigh 3 in a first direction until the first stop surface 43 abuts against the hip joint 5 and forms a limit. The lower leg 4 can also rotate relative to the thigh 3 in a second direction until the second stop surface 43 abuts against the hip joint 5 and forms a limit. The first and second directions are opposite to each other. Figure 1 Taking the left front leg in the indicated direction as an example, the first direction is clockwise, and the second direction is counterclockwise.

[0043] In this invention, the rotation of the lower leg 4 is achieved through a motor-driven knee joint 6. This eliminates the need for additional limiting structures; the rotation of the lower leg 4 is limited by the hip joint 5 abutting against the first stop surface 43 or the second stop surface 44. This simplifies the overall structure and allows for a relatively smaller overall size of the wheeled robot. Furthermore, since there is no need for linkages or other structures, the rotation angle between the lower leg 4 and the thigh 3 is significantly increased by directly limiting the rotation through the hip joint 5, reaching 20-340°. This results in a wider range of motion and more flexible movement for the wheeled robot. Additionally, the aforementioned structural design allows the thigh 3 and lower leg 4 to stand upright, or in other words, reduces power consumption during upright movement.

[0044] like Figure 5As shown, the hip joint 5 has a hip joint housing 51, a hip joint motor rotor 52, and a hip joint motor stator 53. The hip joint motor stator 53 is connected to the weak side of the thigh 31, and the hip joint motor rotor 52 is relatively fixed. It should be noted that the definitions of stator and rotor here are not exactly the same as those in the prior art. The hip joint motor stator 53 and the thigh 3 rotate synchronously under the drive of the hip joint motor rotor 52. The hip joint housing 51 is located around the hip joint motor rotor 52. Because the hip joint motor stator 53 and the thigh 3 rotate synchronously, and the hip joint motor rotor 52 is relatively fixed, the first winding disc 54 located inside the hip joint housing 51 will not rotate, and consequently, the wire inside will not become entangled due to rotation. The aforementioned first stop surface 43 and second stop surface 44 abut against the hip joint 5, specifically against the outer surface of the hip joint motor stator 53.

[0045] like Figure 6 , Figures 8-11 As shown, a first winding disc 54 is formed inside the hip joint shell 51. The first winding disc 54 has a first through hole 541 for the cable (not shown) to pass through. The weak side of the thigh 31 has a second through hole 34 corresponding to the first through hole 541. A cable routing cavity 33 is formed inside the thigh 3. The second through hole 34 and the cable routing cavity 33 are connected. The knee joint 6, which connects the thigh 3 and the lower leg 4, has a knee joint shell 61. The knee joint shell 61 has a notch 62 that is connected to the cable routing cavity 33. The cable extends from the first through hole 541 of the first winding disc 54, then passes through the second through hole 34 into the cable routing cavity 33, and then passes through the notch 62 into the knee joint shell 61. After being electrically connected to the knee joint 6, it winds inside the knee joint shell 61, and finally passes through the lower leg 4 to the actuation mechanism 71 between the rotating wheel 7 and the lower leg 4. The entire cable routing process is simple and reasonable, and because it is located on the inside of the robot leg, the probability of it being affected by external impacts is small, thus improving the overall reliability of the structure.

[0046] To achieve orderly winding of the thread inside the knee joint shell 61, an isolation section 63 is formed inside the knee joint shell 51. Winding areas are formed on both sides of this isolation section 63. Specifically, a first winding area 511 is formed on the knee joint shell 51 itself, and a second winding area (not shown in the figure) is formed at the connection point between the knee joint shell 51 and the lower leg strong side 32. This second winding area and the first winding area 511 are arranged adjacent to each other but isolated from each other, and are connected by another notch 621 corresponding to the notch 62. After the thread is wound in the first winding area 511, it enters the second winding area through the notch 621. The isolation section 63 near the notch 621 has an inclined guiding surface 631, which guides and isolates the thread, preventing it from winding randomly, providing more relatively independent space for the thread routing, and preventing mutual wear between the threads.

[0047] like Figure 2 As shown, the weak side of the thigh 31 extends obliquely, making its angle with the axis of the hip joint 5 75-80°, i.e., α in the figure is 75-80°. Similarly, the oblique extension direction of the weak side of the calf 41 is the same as or close to the oblique extension direction of the weak side of the thigh 31, i.e., the angle between the weak side of the calf 41 and the axis of the hip joint 5 is 75-80°. With both the thigh 3 and calf 4 of the robot legs 2 extending obliquely, the distance between the planes containing the weak sides of the calf 41 is defined as L1, and the maximum width of the robot body 1 is defined as L. Then, L1 / L is 0.6-0.9. Once the ratio of L1 to L exceeds 0.9, the inward retraction of the robot legs 2 is too small, which will not only increase the overall volume of the wheeled robot, but also make the center of gravity of the wheeled robot not concentrated enough, which is not conducive to its rapid movement and rapid action feedback. If the ratio of L1 to L is less than 0.6, then the robot leg 2 retracts inward too much, its lateral width in contact with the ground is too small, and the overall stability of the movement is not high.

[0048] like Figure 4 As shown, the knee joints 6 of the front thigh 3 and lower leg 4 of the wheel-legged robot are located behind its hip joint 5, and the knee joints 6 of the rear thigh 3 and lower leg 4 are located in front of its hip joint 5, thus making the front thigh 3, front lower leg 4, rear thigh 3, and rear lower leg 4 form an X shape. At this time, the robot legs 2 are symmetrical front and back, with consistent force exertion, reducing overall energy consumption. Furthermore, because the front and rear robot legs are mirror images, the wheel-legged robot's direction of travel is not limited by its forward or backward movement, allowing it to move forward or backward without turning its head, making its movement more flexible. Of course, in other embodiments, the front thigh 3, front lower leg 4, rear thigh 3, and rear lower leg 4 can also be in a >> state, or a << state, or a <> state; the specific arrangement is not limited.

[0049] like Figure 6 , Figure 9As shown, the knee joint 6 has a knee joint shell 61, a knee joint motor 64, and a knee joint output end. The knee joint motor 64 is fixedly mounted at the end of the thigh 3, and the knee joint output end is connected to the knee joint motor 64 for driving the lower leg 4 to rotate relative to the thigh 3. A first protective part 321 is formed on the strong side 32 of the thigh, which is positioned corresponding to the output connector 641 of the knee joint motor 64. The first protective part 321 is an outward triangular arch structure, which provides sufficient installation space for the output connector 641 and also provides good protection for the output connector 641, preventing damage to the internal wires due to impact. In addition, when the thigh 3 and the lower leg 4 are brought inward, the strong side 32 of the thigh generates an outward and downward force. The presence of this force can easily cause the strong side 32 of the thigh to deform and be damaged. Since the first protective part 321 is located at the lower end of the strong side 32 of the thigh, its triangular arch structure increases its strength, enabling it to withstand and resist the deformation caused by the above-mentioned forces, thus increasing the stability of the overall structure.

[0050] An actuation mechanism 71 is provided between the rotating wheel 7 and the lower leg 4. The strong side 42 of the lower leg forms a second protective part 421 that partially covers the actuation mechanism. Similarly, the second protective part 421 corresponds to the outlet connector position of the actuation mechanism 71. Its shape is an outward triangular arch structure. When the thigh 3 and the lower leg 4 are retracted inward, the strong side 42 of the lower leg generates an outward and downward force. The presence of this force can easily cause the strong side 42 of the lower leg to deform and be damaged. Since the second protective part 421 is located at the lower end of the strong side 42 of the lower leg, the triangular arch structure of the second protective part 421 increases its strength and can withstand and resist the deformation of the above-mentioned force, thereby increasing the stability of the overall structure.

[0051] like Figure 13 As shown, a third protective part 642 is also formed at the end of the knee joint motor 64, which also protects the knee joint 6 from damage caused by external impact. Specifically, the third protective part 642 can be a protective cover structure, which is fixedly assembled with the knee joint shell 61 using screws.

[0052] To ensure the aforementioned limiting is achieved, the width of the actuation mechanism 71 is greater than the width of the side where the thigh 3 connects to the hip joint 5, i.e. Figure 3 S1 is greater than S2, so the lower leg 4 can rotate smoothly to its first stop surface 43 to contact the hip joint 6, and smoothly rotate to its second stop surface 44 to contact the hip joint 6, avoiding interference during rotation and ensuring a large range of motion.

[0053] The above specific embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A wheeled robot, comprising a robot body and four robot legs, characterized in that, Each robotic leg includes: The thigh, including the weak side of the thigh on the same side as the hip joint, and the strong side of the thigh on the side away from the hip joint. The lower leg is connected to the weak side of the thigh via the knee joint, and includes the strong side of the lower leg on the same side as the knee joint and the weak side of the lower leg away from the knee joint. Rotate the wheel to the end of the lower leg that is furthest from the thigh; The lower leg is provided with a first stop surface and a second stop surface. The lower leg can rotate relative to the thigh in a first direction until the first stop surface abuts against the hip joint and forms a limit. The lower leg can also rotate relative to the thigh in a second direction until the second stop surface abuts against the hip joint and forms a limit. The first direction and the second direction are opposite to each other.

2. The wheeled robot according to claim 1, characterized in that: The hip joint has a hip joint shell, a hip joint motor rotor, and a hip joint motor stator. The hip joint motor stator is connected to the weak side of the thigh. The hip joint motor rotor is fixedly arranged. The hip joint shell is located around the hip joint motor rotor. The hip joint motor stator and the thigh rotate synchronously under the drive of the hip joint motor rotor. The first stop surface or the second stop surface can abut against the outer surface of the hip joint motor stator.

3. The wheeled robot according to claim 2, characterized in that: The hip joint shell forms a first winding disc inside, which forms a first through hole for the cable to pass through. The thigh forms a cable routing cavity inside. The weak side of the thigh has a second through hole corresponding to the first through hole, which is connected to the cable routing cavity. The lower leg has a knee joint shell at the end connected to the thigh, which has a notch that is connected to the cable routing cavity.

4. The wheeled robot according to claim 3, characterized in that: The knee joint shell has an isolation portion, on both sides of which are formed a winding area; the isolation portion has an inclined guide surface.

5. The wheeled robot according to claim 1, characterized in that: The rotation angle of the lower leg relative to the thigh is 20-340°.

6. The wheeled robot according to claim 1, characterized in that: The weak side of the thigh extends obliquely such that the angle between it and the axis of the hip joint is 75-80°; the direction of the oblique extension of the weak side of the calf is the same as or tends to be the same as the direction of the oblique extension of the weak side of the thigh.

7. The wheeled robot according to claim 6, characterized in that: The maximum width of the robot body is L, and the distance between the planes containing the weak sides of the two lower legs is L1. Therefore, L1 / L is 0.6-0.

9.

8. The wheeled robot according to claim 1, characterized in that: The knee joints of the front thigh and lower leg are located behind the hip joints, while the knee joints of the back thigh and lower leg are located in front of the hip joints, so that the front thigh, front lower leg, back thigh, and back lower leg form an X shape.

9. The wheeled robot according to claim 4, characterized in that: The knee joint has a knee joint shell, a knee joint motor, and a knee joint output end. The knee joint motor is fixedly installed at the end of the thigh. The knee joint output end is connected to the knee joint motor for driving the lower leg to rotate relative to the thigh. A first protective part is formed on the strong side of the thigh, and the first protective part is set at the position of the knee joint motor output connector.

10. The wheeled robot according to claim 9, characterized in that: An actuation mechanism is provided between the rotating wheel and the lower leg, and a second protective part is formed on the strong side of the lower leg to cover the actuation mechanism.

11. The wheeled robot according to claim 10, characterized in that: The width of the actuation mechanism is greater than the width of the side where the thigh connects to the hip joint.

12. The wheeled robot according to claim 10, characterized in that: The first and second protective parts are outward-facing triangular arched structures.

13. The wheeled robot according to claim 9, characterized in that: The end of the knee joint motor forms a third protective part.

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