JP2018024073A - Multiarticular flexible robot arm and brake - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multiarticular flexible robot arm with a low energy consumption and a simple control system.SOLUTION: A multiarticular flexible robot arm comprises: a drive part 1 for driving a plurality of push rods 11 to cause the same to make reciprocating linear motions; a rotational joint part including rotating joints 2 in the vertical direction and rotating joints 3 in the lateral direction, which are connected orthogonally and alternately, and in which the pivot axis of the rotating joint in the vertical direction is perpendicular to the pivot axis of the rotating joint in the lateral direction and the rotating joint in the vertical direction and the rotating joint in the lateral direction can rotate and stop around their respective pivot axes; a plurality of wires 5 to 8 which are connected at one ends to the rods, respectively, and sequentially pierce the respective rotating joints of the rotational joint part and then are fixed at the other ends while penetrating the most distal rotating joint; and brakes installed to the respective rotating joints. The multiarticular flexible robot arm controls rotation of the rotating joints by tightening or loosening the wires using the reciprocating linear motions of the push rods.SELECTED DRAWING: Figure 1

Description

  The present invention relates to the field of robots, and more particularly to an articulated flexible robot arm.

  The robot arm is one of the most commonly used robots in the industry, and the demand for the robot using the robot arm is increasing with the improvement of the industrial level and the requirements. A robot with a high degree of freedom has agility, and further improves the scope of application, work space, and remote processing capability, and can enter flaws in narrow labyrinth passages to detect flaws and avoid obstacles. However, these are difficult to achieve for general robots. In addition, it is not necessary to attach or detach the case, and it enters the inside of the peephole and checks the operation status of the equipment or the operation of the key member for cleanliness, maintenance, etc., sealing and detection of curved pipe connections, minimally invasive technology It can be used for Serpentine articulated flexible arms are typically attached to a moving carriage or automatic line for work, and at the end are attached various means such as cameras, searchlights, splitting devices, and brushes, A sealability test can be performed and a telescopic excavation type safety test can be performed in a limited space in the nacelle.

  Many current robot arms employ joint motors to drive the motion of each joint, so one joint motor is required for each joint, the mounting structure is complicated, and the failure rate is high. In addition, it is extremely complicated for a plurality of motors to perform coordinated control. There is still no good solution for the above issues.

  In the prior art, when a plurality of motors coordinately control a robot arm in the prior art, at least a mounting structure is complicated, energy consumption is large, a control system is complicated, and the like. A flexible robot arm is provided.

  The present invention includes a drive unit that drives a plurality of push rods to perform a reciprocating linear motion, and a vertical rotary joint and a horizontal rotary joint that are alternately connected in an orthogonal manner, and the vertical rotation The pivot of the joint is perpendicular to the pivot of the horizontal rotary joint, the rotary joint of the vertical direction and the rotary joint of the horizontal direction can rotate and stop about the respective pivot, and one end of each of the rotary joints. A plurality of wires that are connected to the push rod and sequentially penetrate the rotary joints of the rotary joint part, and then the other end penetrates and is fixed through the most advanced rotary joint, and a brake attached to each rotary joint The articulated flexible robot arm is characterized in that the rotation of the rotary joint is controlled by tightening or loosening a wire using a reciprocating linear motion of a push rod.

  On the other hand, the present invention is fixedly connected to the rotary joint coaxially and has a stop ring whose both end surfaces are tooth surfaces, and is slidably connected to both ends of the stop ring by a spline and a pivot, and is close to the stop ring. The end surface is also a tooth surface, and by having a plurality of springs on the outside in the axial direction, two movable rings that push out the movable ring and the stop ring to mesh the tooth surfaces, and electromagnetic installed on the outside of the movable ring in the axial direction A brake including a coil, and when the electromagnetic coil is energized, the movable ring is sucked and slid to the separated state from the stop ring, and the rotary joint is unbraking, and the electromagnetic coil is de-energized A brake applied to the above-described multi-joint flexible robot arm in which the movable ring is slid to a state where it meshes with the tooth surface of the stop ring by the elastic force of the spring, and the rotary joint is in a braking state. That.

  The articulated flexible robot arm according to the present invention moves a wire by a push rod, drives and rotates each joint by coupling with a brake, and shifts the push rod by installing a position sensor on the push rod. It senses the amount and precisely controls the rotation angle of each joint. Compared to conventional industrial robots, it has a modular design, has a compact structure and good compatibility, and reduces manufacturing and maintenance costs. In addition, it has a high degree of freedom and agility, can improve the application range and work space, and can adapt to different work duties and changes in the surrounding environment with an optimal posture by changing its joint angle.

FIG. 1 is a schematic perspective view showing an articulated flexible robot arm according to an embodiment of the present invention. FIG. 2 is a schematic perspective view showing the drive unit according to the embodiment of the present invention. FIG. 3 is a schematic perspective view showing the rotary joint according to the embodiment of the present invention. FIG. 4 is a cross-sectional view showing a rotary joint according to an embodiment of the present invention. FIG. 5 is an operation schematic diagram showing an articulated flexible robot arm according to an embodiment of the present invention.

As shown in FIG. 1, the rotary joint portion includes a vertical rotary joint 2 and a horizontal rotary joint 3 that are alternately connected orthogonally.
In the embodiment of the present invention, the direction in which the rotary joints are sequentially connected and extended from the drive unit 1 is set as the front end, and the opposite side of the front end is set as the rear end. The longitudinal rotary joint 2 is connected to the lateral rotary joint 3 at the rear end by a joint mount 4, and is connected to the lateral rotary joint 3 at the tip by another joint mount 4. The joint mount 4 has a flat plate shape, specifically, a flat plate shape having a circular outer peripheral surface and a square hole therein, and the joint control electric wire passes through the square hole inside. The rotary joint is connected to the joint mount 4 by a bolt. The vertical rotary joint 2 is rotated and braked about its vertical pivot 21, and the horizontal rotary joint 3 is rotated and braked about its horizontal pivot 31. The configuration of the brake will be described later.

  In the embodiment of the present invention, an example of four wires is given, but more wires may be used, for example, eight, twelve, or the like. The greater the number of wires, the greater the power and the greater the rotational moment. In addition, in the joint mount 4, wire holes are processed corresponding to the number of wires. The four wire holes are arranged at equal intervals along the circumferential direction of the joint mount. In addition, the connection at the center of the circle of the symmetrically installed wire holes is parallel to the axis of the pivot. Four wires are passed through each of the four wire holes, and the corresponding wire holes are sequentially penetrated so that the wires can connect the longitudinal rotary joint and the lateral rotary joint. By connecting with the above-described configuration, an operation part of the robot arm is configured, and by driving the wire by the driving unit 1, the robot arm can be driven to operate.

  In the embodiment of the present invention, 12 vertical rotational joints and 11 lateral rotational joints are adopted, and when the wire penetrates all the vertical rotational joints and the lateral rotational joints, Many complicated operations can be completed by bending the arm into an S shape. The rear end of the wire is connected to the push rod 11 by a connection joint 12, and the output end of the driving unit 1 is connected to the push rod 11 to provide power to the push rod 11, and perform reciprocating linear motion on the push rod 11. Make it. A circuit board 14 and a positioning ring 13 are installed at the tip of the push rod 11, and the positioning ring 13 determines the axial positions of the four electric push rods and the positions of the four wire holes of the joint mount. The axis position of the push rod and the position of the wire hole are precisely matched. The tips of the four electric push rods and the wires are connected by a connection joint 12. In the joint mount 4 of the tip of the wire, that is, the most advanced rotating joint, the wire is fixed by the connecting joint 12 and the loosening of the wire is controlled by the push rod 11 of the driving unit 1 to control the vertical rotating joint or the lateral joint. Rotate by controlling the rotating joint in the direction.

Next, the control process will be described in detail.
The wires are an upper wire 5, a lower wire 6, a left wire 7, and a right wire 8, respectively. As shown in FIG. 2, the push rod 11 includes an upper push rod 111, a lower push rod 112, a left push rod 113, and a right push rod 114, and corresponds to each wire.
The upper wire 5 and the lower wire 6 rotate up and down around the rotary joint by controlling the vertical rotary joint. The left wire 7 and the right wire 8 swing back and forth by controlling the lateral rotation joint.
Further, the wire performs a pulling motion by providing power by the push rod 11, and the amount of movement pushed out before the electric push rod or the amount of movement pulled backward controls the upper wire 5 and the lower wire 6 to tighten. It is then loosened and converted into the required joint rotation angle using the wire hole of the joint mount. For example, in order to swing the robot arm upward on a vertical plane, first all vertical and horizontal rotary joints are braked, and then the joints to be rotated are released. . At this time, the upper push rod 111 pulls the upper wire 5 backward, that is, the push rod 11 is tightened rearward, and the lower wire 6 is loosened forward, that is, the lower push rod 112 is pushed forward. To do.
Further, the left wire 7 and the right wire 8 do not move as they are. Since the upper wire 5 is tightened rearward, the upper wire 5 moves the joint mount 4 through the wire hole and pulls it upward, and the lower wire 6 is loosened forward, so that the upper wire 5 and the lower wire 6 are mixed together. , Pull the rotary joint up. The angle of swinging is controlled by tightening or loosening the wire, and when the swinging angle reaches a demand, the rotary joint portion is braked again. That is, it is possible to complete the operation of swinging upward on the vertical plane of the robot arm.
Further, when swinging downward, that is, the upper wire 5 is loosened, the lower wire 6 is tightened, and the left wire 7 and the right wire 8 are left as they are, and the robot arm swings downward in a vertical plane. Can be completed.

A general articulated robot arm has an angle detection device installed at each joint, but the configuration is complicated and the control design is complicated. On the other hand, the electric push rod of the articulated robot arm of the embodiment of the present invention is composed of a servo motor, a speed reducer, and a screw nut, and the electric push rod is provided with a position sensor 15 again. Yes. Therefore, it is possible to detect the amount of push rod expansion / contraction, that is, the amount of movement that pushes the push rod forward and the amount that it pulls backward, and the amount of movement that pulls behind the two upper and lower electric push rods is roughly the same as the amount of movement that pushes forward. is there.
Note that by installing an encoder on the electric push rod, it is possible to detect the rotation speed of the servo motor for detecting the accuracy of expansion and contraction of the push rod. Further, the amount of expansion and contraction of the push rod can be precisely controlled, and further, the rotation angle of the rotary joint can be precisely controlled. By employing an electric push rod having a position sensor to drive the movement of the wire, the number of joint sensors can be reduced.

  Also, for example, in order to swing the robot arm to the left on a horizontal plane (the direction toward the back of the drawing in the drawing), first all vertical and horizontal rotational joints are braked and then rotated. Release the braking of the rotating joints that require. The looseness of the upper wire 5 and the lower wire 6 is maintained, the left wire 7 is tightened by the left push rod 113 of the driving unit 1, and the right wire 8 is loosened by the right push rod 114, so that the robot arm is moved leftward in the horizontal plane. Swing to.

  The rotational joints in the same rotational direction have only one rotational joint that can rotate and operate at each control timing, and all other rotational joints in the same rotational direction are locked by a brake. The vertical rotary joint and the horizontal rotary joint can operate simultaneously, that is, the above-described upward swing and leftward swing can operate simultaneously. First, all the vertical and horizontal rotary joints are braked, and then the braking of one vertical rotary joint and one horizontal rotary joint is released. By tightening the upper wire 5 and the left wire 7 and loosening the lower wire 6 and the right wire 8, the swinging operation in the upward direction and the swinging operation in the left direction can be completed.

According to the robot arm of the present invention, the rotary motor is rotated by moving the rotary joint by canceling the joint motor and controlling the wire by using the electric push rod. Moreover, spatial operation can be completed by combining the brake and driving the robot arm.
The articulated robot arm according to the present invention has a unity design unity as compared with a conventional industrial robot, has a compact configuration and good compatibility, and reduces manufacturing and maintenance costs. In addition, it has a high degree of freedom and agility, can improve the application range and work space, and can adapt to different work duties and changes in the surrounding environment with an optimal posture by changing its joint angle. For example, the detection mission can be completed in a narrow space such as a nacelle, a vehicle cabin, and a curved pipe, but these are tasks that cannot be completed by a general robot. In addition, by installing an environmental sensor at the end of the arm, it is possible to avoid obstacles using its own memory function, and by exchanging the execution part at the end, gluing, discharging, and unnecessary to the airplane cavity It is possible to take out an object. In addition, when performing a long-distance operation work in an unknown or dangerous environment, it has a greater advantage. Rigidity can be improved by adopting a wire-type multi-joint arm.

  The vertical rotary joint and the horizontal rotary joint may have the same configuration as long as they are attached and their axes are perpendicular to each other. FIG. 3 is a configuration diagram of a rotary joint, and two joint mounts 4 are connected to the pivot 100 by connecting parts, respectively. The connecting component is configured such that the thin steel plate is bent vertically at the connection position with the joint mount 4. Specifically, the upper joint mount 41 and the upper connection component 411 are connected by bolts, and the upper connection component 411 is attached to the pivot 100 by a bearing to form a rotating pair, and the bearing mainly receives a radial force. A stop ring 103 is installed at the axial center of the pivot 100, and a brake case 107 is integrally connected to the stop ring 103 and the upper connecting part 411 by a rivet 105. In other words, the stop ring 103 does not rotate relative to the upper connection component 411. Since the inner diameter of the stop ring 103 is larger than the outer diameter of the pivot 100, the stop ring 103 and the pivot 100 have no positioning relationship by connection.

  The lower joint mount 42 and the lower connection component 421 are connected by bolts. The lower connection part 421 is fixedly connected to the pivot 100 by the connection part 102 and the conical pin 101. Therefore, the pivot 100 does not rotate relative to the lower joint mount and is fixedly connected. The braking structure inside the joint is symmetrical on both sides based on the stop ring 103, the two movable rings 104 and the middle stage of the pivot 100 are connected by a spline shift, and are arranged on both sides of the stop ring 103. That is, the movable ring and the pivot are positioned circumferentially and there is no relative rotation between them.

  Teeth 1031 are provided on both end faces of the retaining ring 103. The movable ring 104 has teeth 1041 blended with the retaining ring 103, and the movable ring and the retaining ring brake the joint by meshing the teeth on the tooth surface. A plurality of stepped holes are arranged at equal intervals along the circumferential direction of the case 107, and a carrier rod 1073, a spring 1072, and a biasing screw 1071 are attached to the inside of the holes. Coils 106 connected to external circuits are installed on both sides of the movable ring 104 in the axial direction.

  When braking, the coil 106 is cut off, and the pressure of the biasing screw 1071 of the spring 1072 moves the movable ring 104 along the axial direction of the pivot 100 toward the end surface of the stop ring 103, and further, the end surface teeth are moved. Engage with each other to urge the relative rotation of the movable ring and the stop ring, and the rotating joint cannot rotate and enters a braking state.

  When the brake is released, the coil 106 is energized, and the coil attracts the movable ring 104 and slides in the direction away from the stop ring 103 along the axial direction of the pivot 100, so that the teeth 1031 and the teeth 1041 are not engaged with each other. When the upper joint mount 41 is pulled up by the wire penetrating the wire hole 412, the upper connection component 411 and the stop ring 103 that are integrally connected to the upper joint mount 41 are simultaneously rotated.

  The electromagnetic brake according to the embodiment of the present invention employs non-energized braking to distinguish it from general brakes. Since there are many joints, only one rotation joint rotates in the same rotation direction (lateral direction or vertical direction), and all others are in a braking state. Therefore, the embodiment of the present invention can save power consumption by adopting de-energized braking. The above is a configuration of one type of brake. For example, various types of brakes such as a disc brake and a drum brake can be adopted and used in combination with the wire articulated robot arm of the present invention.

An environment detection sensor 9 is installed at the rotary joint at the tip, and position information of the object is fed back by the environment detection sensor. In addition, using an arm control system, the rotation of each joint and the movement of the entire robot arm are coordinated to enter a narrow space. For example, when the robot arm performs a passing motion around each of two points on the vertical plane, point B and point A in FIG. 5 are located on the same vertical plane, and FIG. It is a figure which shows the state after moving centering on each of A.
The robot arm moves around the point B from the lower right side of the point B, passes between the points B and A, moves around the point A from the upper left side of the point B, and moves above the point A. Pierce through. In the initial state of the robot arm attached to the carriage, the vertical rotary joints are extended horizontally and all are braked. The starting position of the carriage is at the lower left side of point B, and the most advanced of the robot arm is positioned at the lower left side of point B. The environment detection sensor 9 feeds back the position information of the point B to the arm control system, and the carriage carrying the robot arm starts to move to the right in the horizontal direction. When the first longitudinal rotary joint 201 has passed point B (elapsed immediately below point B), the carriage stops, the rotary joint 201 stops braking, and the wire controls the joint in the counterclockwise direction. Rotate 45 degrees before braking. The carriage continues to move forward in the horizontal direction, and when the second longitudinal rotary joint 202 passes point B, the carriage stops, the rotary joint 202 stops braking, and the wire controls the joint. Then, the brake is applied after rotating 45 degrees counterclockwise. The carriage continues to move forward in the horizontal direction. In this way, the above-described movement is sequentially repeated until the rotary joint 204 rotates 45 degrees counterclockwise, and then the carriage is stopped. In this case, the tip of the robot arm is located between point B and point A, and the tip is directed leftward in the horizontal direction. In the course of moving around the point B, one vertical rotation joint is rotated each time the carriage stops.

  Subsequently, the rear end of the robot arm continues to move around the point B, and the tip that passes between the points B and A moves around the point A.

  The carriage continues to move to the right in the horizontal direction, and when the fifth longitudinal rotary joint 205 passes point B, the carriage stops, the rotary joint 202 stops braking, and the rotary joint 205 is controlled. Then it rotates 45 degrees counterclockwise and brakes. Subsequently, the rotary joint 201 stops braking, and the wire is braked after controlling the rotary joint 201 and rotating 90 degrees clockwise. The carriage continues to move to the right in the horizontal direction, and analogizes sequentially until the tip of the robot arm is extended to the right in the horizontal direction.

  The movement that the rear end of the arm is centered on the point B and the movement that the end of the arm is centered on the point A are sequential movements, that is, each time the carriage stops, the two joints are rotated.

  In order to specifically explain the control process of the joint, the motion of the carriage adopts intermittent motion. In the actual operation of the multi-jointed arm, if the carriage moves linearly and the rotation coordination of the non-braking joints is satisfied, the joint will rotate at the same time and move the obstacles when the carriage travels at a uniform speed. You can get over.

The above is a process in which the robot arm performs a passing motion around each of two points on the vertical plane. From the above, it can be seen that the coordinated movement between the immediately following joint and the carriage repeats the coordinated movement between the immediately preceding joint and the carriage. Therefore, simple and reliable control can be realized using the memory function of the joint program. It can be seen from the Fourier series that simple cosine or sine waveforms are superimposed on each other, no matter how complex the waveform trajectory is. The articulated arm of FIG. 5 can be converted to an S-shape (similar to a cosine waveform when viewed from the side), that is, it can satisfy most trajectories or posture generation, and multiple types of work can pass through a narrow labyrinth passage. The request has been completed.
The present invention does not limit the sequential connection of the vertical rotary joint and the horizontal rotary joint. For example, one or more vertical rotary joints are connected in sequence before the horizontal rotary joint is connected. You can connect. Further, the rotary joints may be arbitrarily combined according to space limitations and motion requirements. For example, when only a movement in a plane is required, only a plurality of longitudinal rotational joints or lateral rotational joints may be connected.

  Meanwhile, according to the present invention, a brake for a robot arm is provided. The upper joint mount 41 and the upper connection component 411 are connected by bolts, and the upper connection component 411 is attached to the pivot 100 by a bearing 108 to form a rotating pair, and the bearing 108 mainly receives a radial force. A stop ring 103 is installed at the center of the pivot 100 in the axial direction, and the case 107 of the drive unit 1 is integrally connected to the stop ring 103 and the upper connecting part 411 by a rivet 105. Since the inner diameter of the retaining ring 103 is larger than the outer diameter of the pivot 100, the retaining ring 103 and the pivot 100 have no positioning relationship by connection.

  The lower joint mount 42 and the lower connection component 421 are connected by bolts. The lower connection part 421 is fixedly connected to the pivot 100 by the connection part 102 and the conical pin 101. Therefore, the pivot 100 does not rotate relative to the lower joint mount and is fixedly connected. A movable ring 104 is installed on each of the pivots on both sides of the stop ring 103, and the movable ring 104 and the pivot 100 are connected by adopting a spline shift. That is, the movable ring and the pivot are positioned circumferentially and there is no relative rotation between them.

  Teeth 1031 are provided on both sides of the retaining ring 103. The movable ring 104 has teeth 1041 blended with the retaining ring 103, and the movable ring and the retaining ring control braking by the meshing of the tooth surfaces. A plurality of stepped holes are arranged at equal intervals along the circumferential direction of the case 107, and a carrier rod 1073, a spring 1072, and a biasing bolt 1071 are attached to the inside of the hole. . Coils 106 connected to external circuits are installed on both sides of the movable ring 104 in the axial direction.

  When braking, the coil 106 is cut off, and the teeth 1041 of the movable ring 104 and the teeth 1031 of the stop ring 103 are engaged with each other due to the pressure of the spring 1072, and the pivot 100 cannot rotate. As a result, the rotary joint cannot rotate and enters a braking state.

  When the brake is released, the coil 106 is energized, and the coil sucks the movable ring 104 and slides it to the both sides in the axial direction, so that the teeth 1031 and the teeth 1041 do not mesh with each other, and the upper joint mount is performed by the wire penetrating the wire hole 412 When 41 is pulled up, the upper connecting part 411 and the stop ring 103 that are integrally connected to the upper joint mount 41 rotate simultaneously.

  The articulated robot arm of the present invention has a unity design unity as compared with a conventional industrial robot, has a compact configuration and good compatibility, and reduces manufacturing and maintenance costs. In addition, it has a high degree of freedom and agility, can improve the application range and work space, and can adapt to different work duties and changes in the surrounding environment with an optimal posture by changing its joint angle. Moreover, rigidity can be improved by adopting a wire-type multi-joint arm. The articulated flexible robot arm cancels the joint motor and employs an electric push rod to drive the wire to move and rotate the rotating joint. Therefore, by using a wire and a brake in combination instead of the joint motor, the complexity of the joint structure and the consumed electric power are reduced, and the control design is simple and easy to implement.

DESCRIPTION OF SYMBOLS 1 Drive part 2 Rotating joint 3 Rotating joint 5 Upper wire 6 Lower wire 7 Left wire 8 Right wire 11 Push rod 111 Upper push rod 112 Lower push rod 113 Left push rod 114 Right push rod

Claims (8)

A drive unit that drives a plurality of push rods to perform a reciprocating linear motion;
A vertical rotary joint and a horizontal rotary joint connected alternately and orthogonally, wherein the vertical rotary joint pivot axis is perpendicular to the horizontal rotary joint pivot axis, and the vertical rotary joint joint And a rotary joint part in which the lateral rotary joint can rotate and stop around the respective pivot axis,
A plurality of wires each having one end connected to each push rod and sequentially penetrating each rotary joint of the rotary joint, and then the other end penetrating through the most advanced rotary joint;
A brake attached to each rotary joint,
A multi-joint flexible robot arm that controls the rotation of the rotary joint by tightening or loosening a wire using a reciprocating linear motion of a push rod.   The articulated flexible robot arm according to claim 1, wherein the brake is a non-energized electromagnetic brake.   2. The multi-joint flexible robot arm according to claim 1, wherein only one of the vertical rotary joint and the horizontal rotary joint rotates at the same control timing.   The multi-joint flexible robot arm according to claim 1, wherein an environmental sensor is attached to the most advanced rotary joint.   The articulated flexible robot arm according to claim 1, wherein a position sensor is installed on the push rod.   The articulated flexible robot arm according to claim 1, wherein an encoder for detecting a rotation speed of the push rod motor is installed on the push rod. A stop ring that is fixedly connected to the rotary joint coaxially and whose both end surfaces are tooth surfaces,
A spline and a pivot shaft are slidably connected to both ends of the retaining ring, the end surface adjacent to the retaining ring is a tooth surface, and a plurality of springs are provided on the outer side in the axial direction to push out the movable ring and the retaining ring. Two movable rings that engage the surface;
A brake comprising: an electromagnetic coil installed outside the movable ring in the axial direction;
When the electromagnetic coil is energized, the movable ring is sucked and slid until it is separated from the stop ring, and the rotary joint is in a non-braking state. When the electromagnetic coil is not energized, the movable ring is stopped by the elastic force of the spring. The brake applied to the multi-joint flexible robot arm according to any one of claims 1 to 6, wherein the rotary joint is brought into a braking state by sliding to a state in which the tooth joint engages with the tooth surface. .   The brake according to claim 7, wherein the degree of biasing of the movable ring and the stop ring is adjusted by adjusting the amount of compression of the spring through the biasing screw.