JPH01121194A - Aligner - Google Patents

Abstract

PURPOSE: To securely insert an inserting member into an engaging part by disposing a compliance mechanism between a mounting main body and a holding device mounting body. CONSTITUTION: A compliance mechanism 34 having a compliance for a deviation in a surface perpendicular to the center shaft of an inserting member (pin) 22, a compliance for a deviation along the center shaft of the inserting member 22, a compliance for a deviation in a rotating direction about the center shaft of the inserting member 22, and a compliance for a deviation by an inclination relative to the center shaft of the inserting member 22 is disposed between the mounting main body 40 at the top end of a transfer body (robot arm) 10 and a holding device mounting body (hand body) 16. As a result, even when the inserting member 22 and an engaging part (engaging hole) 28 of a member to be inserted (substrate) 30 are deviated aslant from each other, the inserting member 22 is aligned correctly with the engaging part 28 by the action of the compliance mechanism 34 to allow insertion.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is directed to, for example, an automatic assembly device that automatically performs alignment with respect to a fitting hole when a member is fitted into a fitting hole. Regarding the centering device that can be used.

[Prior Art] Generally, in an automatic assembly device, in order to insert a member into a predetermined fitting hole, the member and the fitting hole must be aligned with each other, in other words, their central axes must be aligned with each other. It is necessary to perform centering so that they are located on the same straight line.

In order to align the member and the fitting hole with each other in this manner, a compliance mechanism is conventionally employed as shown in FIGS. 7A to 7C. In this compliance mechanism, the pin a can be reliably fitted into the fitting hole C formed in the member to be attached, even if the center axes of the pins are slightly shifted from each other.

Here, in the conventional compliance mechanism, as shown in the figure, a pin a is inserted into an insertion hole e formed in a holder d, and is sucked by a suction pipe f connected to a vacuum pump (not shown). and is held by the holder d.

This holder d is fixed to a movable plate g. Two elastic leaf springs h are fixed to the movable plate g via a presser plate j and a screw j so as to be parallel to each other.

Further, the other end of the above-mentioned leaf spring h is fixed to the intermediate plate k by a presser plate °C and a screw j. Further, on this intermediate plate k, two other mutually parallel leaf springs m extend along the direction orthogonal to the two leaf springs h mentioned above, and hold the presser plate n and the screw j. It is fixed through.

At the other ends of these other two leaf springs m, there is a mounting base 0
is fixed by a presser plate n and a screw j, and furthermore, this mounting base 0 is fixed to an arm q by a screw p. Here, arm q is attached to a movable arm (not shown) of a robot or the like in an automatic assembly device.

In the conventional compliance mechanism configured in this way, the movable plate g is in a state where it is easy to move eight times along the left and right direction (X-axis direction) in FIG. 7A due to the deflection of the leaf spring h, and Due to the deflection of the plate spring m, the plate becomes easily movable along the direction perpendicular to the plane of the paper (X-axis direction). In this way, the holder d fixed to the movable plate g is placed on a horizontal plane (x-y plane).
You will be able to move in each direction within the space.

Here, such a compliance mechanism is attached to a robot hand of an automatic assembly device, and a holder d
The robot hand moves the pin a, which has been suctioned and held, to a predetermined position, and operates to fit it into the fitting hole C formed in the member to be attached. Here, if the center axis of the pin a to be fitted and the center axis of the fitting hole C to be fitted are slightly misaligned, as shown in Fig. 7B, the opening of the fitting hole C The lower edge of the pin a comes into contact with the chamfered portion r, which is a chamfered edge.

As the robot arm 9 further descends from this contact state, the downward force on the robot arm q is
A component force is applied at the contact portion between the lower edge of the bottle a and the chamfered portion r, and a component force in the lateral direction in the figure acts on the bottle a.

As a result, the movable plate g is deflected laterally while remaining parallel to the intermediate plate, and in this way, the bin a is displaced from the 7th C while remaining parallel to the initial position. As shown in the figure, it will be fitted into the fitting hole C.

[Problems to be Solved by the Invention] However, although conventional compliance mechanisms can perform alignment operations with a simple configuration, they have the following problems. It is.

In other words, in this conventional alignment device, if the center axis of pin a and the center axis of fitting hole C are parallel to each other but are simply shifted in the horizontal direction, compliance is reliably achieved. After absorbing this deviation, it is possible to allow the pin a to fit into the fitting hole C. However, if the center axis of the pin a and the center axis of the fitting hole C are diagonally misaligned, this conventional alignment device cannot handle the situation, and the holder d may be destroyed or the fitting operation may be difficult. A problem arises in that the process stops midway.

This invention has been made in view of the above-mentioned problems, and an object of the present invention is to ensure that even when the fitting member and the fitting portion are deviated in an oblique direction, the fitting member and the fitting portion can be reliably connected to each other. It is an object of the present invention to provide an alignment device having compliance that allows insertion.

[Means for Solving the Problems] In order to solve the above-mentioned problems and achieve the purpose, the alignment device according to the present invention holds the fitting member at the tip of the transfer body via a holder, By moving the transport body,
In the alignment device for aligning the center axis of the fitting member and the center axis of the fitting portion so that they are aligned with each other when fitting the fitting member into the fitting portion of the fitting member, the transfer body A mounting body that is attached to the tip of the holder, a holder mount to which the holder is attached, and a holder mount that is disposed between these mount bodies and the holder mount, and is arranged to prevent misalignment in a plane perpendicular to the central axis of the fitting member. a compliance mechanism that has compliance with respect to deviation of the fitting member along the central axis, compliance with deviation in the rotational direction of the fitting member around the central axis, and compliance with deviation in inclination of the fitting member with respect to the central axis. It is characterized by the following:

[Function] In the alignment device configured as described above, the compliance mechanism provided therein has compliance with respect to misalignment in a plane perpendicular to the central axis of the fitting member,
It has compliance against displacement along the central axis of the fitted member, compliance against rotational deviation around the central axis of the fitted member, and compliance against deviation in inclination of the fitted member with respect to the central axis. Even if a misalignment occurs between the fitting member and the fitting portion, the fitting member and the fitting portion can be reliably compliant in an aligned state.

[Embodiment] Below, the configuration of an embodiment of the alignment device according to the present invention will be described in detail with reference to FIGS. 1 to 4B of the accompanying drawings.

FIG. 1 shows a state in which a hand portion 14 is attached to the tip of a robot arm 1o, which constitutes a part of an automatic assembly device, via an alignment device 12 of this embodiment. This robot arm 10 is freely driven by a drive mechanism (not shown) in eight directions in the left-right direction (X-axis direction), the direction perpendicular to the page (X-axis direction), and the vertical direction (X-axis direction) in the figure. It is something that will be done.

On the other hand, the hunt portion 14 includes a disc-shaped hunt body 16 and a holder 2o fixed to the lower surface of the hand body 16 via a bolt 18.

The lower part of this holder 20 extends along the vertical direction,
An insertion hole 24 is formed that is open at the lower surface and into which a pin 22 as a fitting member is inserted.

Here, a suction pipe 26 connected to a suction pump (not shown) is connected to the upper part of this insertion hole 24 so as to communicate therewith. By driving this suction pump, the inside of the insertion hole 24 is maintained in a negative pressure state, and the pin 22 is suctioned and held within the insertion hole 24 by this negative pressure.

Further, a fitting hole 28 into which the pin 22 is fitted is formed in a substrate 30 as a member to be fitted. This board 30
is mounted horizontally on a base (not shown),
In this state, the fitting hole 28 is set to extend in the vertical direction. Incidentally, a tapered surface 32 having a predetermined inclination with respect to a horizontal plane is formed on the upper end edge of the fitting hole 28 in order to facilitate the insertion of the pin 22.

Here, the alignment device 12 of the above-described embodiment includes a compliance mechanism 34 disposed so as to have a central axis coaxial with the central axis of the robot arm 1o. This alignment device 12 includes a disk-shaped mounting body 4o fixed to the lower end of the robot arm 10 via a bolt 38.

Here, in the above-described compliance mechanism 34, in a normal state, when no external force is acting on the holder 2o, the central axis of the robot arm 1o and the central axis of the holder 20 are aligned perpendicularly to each other. When an external force in a horizontal plane is applied to the holder 20, it can be elastically maintained in a state aligned along the holder 20, and can be allowed to deviate softly within a predetermined range in the horizontal plane in response to this external force. It is set as follows.

The configuration of the compliance mechanism 34 that characterizes this embodiment will be explained below.

That is, this compliance mechanism 34 has a first part integrally formed on the lower surface of the mounting body 4° fixed to the lower end of the robot arm 10 so as to be coaxial with the central axis of the robot arm 1o and protrude downward. The shaft member 34a is provided. On the other hand, the upper surface of the hand body 16 is provided with a holder 2o.
A second shaft member 34b is fixed via a bolt 38 so as to be set coaxially with the central axis of and protrude upward. The mutually opposing ends of these first and second shaft members 34a and 34b are opposed to each other so as to be spaced apart by a predetermined distance.

Here, outer flange portions 34c and 34 are provided at the lower end of the first shaft member 34a and the upper end of the second shaft member 34b, respectively.
d is integrally formed.

Further, this compliance mechanism 34 includes first and second
A plurality of support pins 34e are arranged so as to simultaneously surround the opposite ends of the shaft members 34a and 34b.
It is equipped with

Each support bin 34d is integrally provided with outer flanges 34f and 34g at its upper and lower ends, respectively. In detail, each outer flange portion 3 of these support bins 34d
4f and 34g are the first and second shaft members 34a mentioned above.
, 34b, and the number of cylinders is set to six.

Further, as shown in FIG. 2, on the outer periphery of the outer flange portions 34c and 34d at the ends of the first and second shaft members 34a and 34b, there are provided support pins 34e corresponding to the number of support pins 34e. ,
A plurality of V grooves 34m are formed at equal intervals so as to extend along the vertical direction. A corresponding support pin 34e is fitted and supported in each groove 34m. In this way, the relative position of the second shaft member 34b with respect to the first shaft member 34a is elastically defined. In addition, an outer flange 34f of each support pin 34d,
An annular cut groove 34h is formed on the outer circumferential surface of the portion other than 34g.

These support bins 34e are connected to both shaft members 34a, 34.
In the state surrounding b, an outer flange 34f integrally formed on the upper end of the support pin 34e engages with the outer flange portion 34c of the first shaft member 34a from above, and the lower end of the support pin 34e The outer flange portion 34 of the second shaft member 34b is integrally formed with the outer flange 34g.
d engages from above. In this way, the hand body 1
6 is suspended from the mounting body 40 of the compliance mechanism 34 so as to be swingable around the central axis.

Further, as shown in FIG. 2, in a state in which these support pins 34e surround both shaft members 34a and 34b, similarly to the above-mentioned embodiment, these support pins 34a are collectively surrounded, and each A ring-shaped urging member 34i is provided in the cut groove 34h to urge the support pins 34e to elastically press against the circumferential surfaces of the opposing ends of the first and second shaft members 34a and 34b. are stored respectively. In this embodiment, this biasing member 34i
is formed from a finely wound ring-shaped coil spring.

On the other hand, as shown in FIG. 1, recesses 34j and 34k are formed in the lower surface of the first shaft member 34a and the upper surface of the second shaft member 34b, respectively, so as to face each other. These recesses 34j.

A coil spring 34° C. is disposed with its upper end and lower end housed in the coil spring 34k. This coil spring 341 is provided to bias the first and second shaft members 34a, 34b in a direction away from each other.

In addition, as shown in FIG. 1, this alignment device 12 includes:
When the robot arm 10 moves laterally at high speed, its inertia causes the second shaft member 34b to move against the first shaft member 34a.
A locking mechanism 38 is provided to prevent lateral deflection. This locking mechanism 38 includes a pair of cylinder chambers 38a that are formed at symmetrical positions within the first shaft member 34a with a recess 34j therebetween.

A piston member 38b is housed in each cylinder chamber 38a so as to be able to protrude downward. Here, each piston member 38b is urged upward by a coil spring 38c, and due to the urging force of this coil spring 38c, the piston member 38b is integrally brought into contact with the upper surface of 38a so that the upper end of the piston member 38b projects upward. At the position where they stopped in contact with each other,
A retracted position of the piston member 38b is defined.

Further, a lock hole 38e into which the tip of the corresponding piston member 38b is fitted is formed on the upper surface of the second shaft member 34b at a position opposite to the tip of each piston member 38b. Here, a connecting pipe 38f connected to a pressurizing pump mechanism (not shown) is connected to a portion of each cylinder chamber 38a described above above the upper end of the piston member 38b.

Then, compressed air (pressurized air) is generated from this pressurization pump mechanism.
is introduced into the cylinder chamber 38a via the connecting pipe 38f, each piston member 38b is pushed downward from the retracted position against the urging force of the corresponding coil spring 38c, and biased to the lock position. be done. In addition, in this lock position, the lower end of each piston member 38b will fit into the corresponding lock hole 38e.

In this way, the locking mechanism 38 is activated, thereby locking the first and second shaft members 34a.

34b are laterally locked to each other and will move laterally together.

When the hand portion 14 is attached to the tip of the robot arm 10 via the alignment device 12 having the compliance mechanism 34 configured as described above, the substrate 3
The fitting operation of fitting the one-wooden bottle 22 into the one-wooden fitting hole 28 formed in the hole 28 will be described below.

First, with the bottle 22 held by the holder 20 under suction, the robot arm 10 is controlled to move by a control mechanism (not shown) so as to fit the bottle 22 into the fitting hole 28 formed in the substrate 30.

That is, in this control mechanism, x of the fitting hole 28 is set in advance.
- Position information on the y plane and the robot arm 10
The three-dimensional position of the bottle 22, that is, the position information of the bottle 22 to be inserted, is input, and this robot arm 10
Movement is controlled by control operations of the control mechanism based on the position information.

Here, this position information is accurate, the robot arm 10 is moved and driven according to the control contents of the control mechanism, and
When the fitting hole 28 is positioned according to the set value, the bottle 22 is moved 8 times directly above the fitting hole 28 and then vertically lowered, so that the bottle 22 is properly positioned. It will fit into the fitting hole 28.

However, the positioning of the fitting hole 28 is not accurate, and
- If there is a slight deviation from the set value in the y plane, or if the position of the robot arm 10 is slightly deviation from the posture specified by the control mechanism due to an error in the drive system, such as backlash in the gear, , so-called deviation occurs in the diagonal direction.

Here, on the robot arm 10 side, if the fitting hole 28 is formed with an oblique deviation even though it is set accurately, the bottle 22 that has been vertically lowered due to the lowering of the robot arm 10 As shown in FIGS. 1 and 3, the lower edge thereof comes into contact with the tapered surface 32 of the fitting hole 28. Then, as the robot arm 10 further descends, the lower end edge of the pin 22 moves to the tapered surface 3.
2 will receive a component force F directed in the horizontal direction.

Here, as the pin 22 receives such a component force F in the horizontal direction, this component force F is applied to each second second portion of the compliance mechanism 34 via the holder 20 and the hand body 16.
This will act on the shaft member 34b. Therefore, when this component force F is not applied, the pair of upper and lower biasing members 34i elastically move the first and second shaft members 34a and 34b perpendicular to each other, as shown in FIG. 4A. From the axially aligned state, as shown in FIG. 4B,
The support pin 34 resists the urging force of these urging members 34i.
By tilting diagonally, the second shaft member 34b moves horizontally.

Further, in this compliance mechanism 34, the first
The second shaft members 34a and 34b are set to be relatively swingable around their central axes, and as a result, from the fitting hole 28 and the central axis (Z axis) of the bottle 22 With respect to the inclination (α), the second shaft member 34b is once soft with compliance and can be tilted with respect to the first shaft member 34a. In this way, in this compliance mechanism 34, the inclination (displacement) of the fitting hole 28 and the bottle 22 from the central axis is well absorbed, and the bottle 22 is reliably fitted into the fitting hole 28. become.

In this way, the diagonal misalignment between the pin 22 and the fitting hole 28 is elastically absorbed by the misalignment and inclination of the first and second shaft members 34a and 34b in the compliance mechanism 34, and the pin 22 is The pins 22 are brought into alignment with each other in the holes 28, and as the robot arm 10 is lowered, the pins 22 are properly fitted into the fitting holes 28.

After the operation of fitting the pin 22 into the fitting hole 28 is completed, the drive of the suction pump (not shown) is stopped, and when the robot arm 10 is driven upward, the holding state of the pin 22 in the holder 20 is released. , the hand part 14 is the bottle 22
When released, it will rise independently. Then, when the bottle 22 is completely separated from the holder 20, the above-mentioned component force F no longer acts on the holder 20. As a result, the component force acting on the second shaft member 34b in the compliance mechanism 34 is eliminated, and the second shaft member 34b is 4B
The biased state shown in the figure is successfully returned to the aligned state shown in FIG. 4A.

In this way, the alignment operation in the alignment device 12, in other words, the elastic deflection/return operation in the compliance mechanism 34 is completed.

On the other hand, since the above-mentioned coil spring 34° C. is provided, this compliance mechanism 34 can be operated in the vertical direction (
2 axes)).

Therefore, when fitting the pin 22 to the deepest part of the fitting hole 28, even if the depth of the fitting hole 28 is shorter than the set value or the length of the pin 22 is long, the coil Due to the elastic contraction of the spring 34Il, deviations from these set values are reliably absorbed, and due to the so-called compliance regarding the Z-axis, the robot arm 10 is properly fitted without being subjected to impact.

Further, as described above, the first and second shaft members 34a,
Since the support pin 34e is supported so as to be fitted into the V groove 34m formed in each of the shaft members 34b, the relative angular positions of the first and second shaft members 34a and 34b are elastically defined. It turns out. As a result, the horizontal plane (x-y
Compliance occurs when the angle in the plane) is deflected. In this way, even if a deviation (θ) occurs around the Z-axis, the compliance mechanism 34 can exhibit compliance corresponding to this deviation (θ).

As described in detail above, the alignment device 12 in one embodiment
In addition to the deviations in the y-axis, y-axis, and X-axis directions, Z
Displacement in the rotational direction around the axis (θ), tilt with respect to the y-axis (α
), it is possible to respond with compliance and deflect, and when the component forces based on these diagonal deviations are released, it is possible to reliably return to the original consistent state. be.

It goes without saying that this invention is not limited to the configuration of the one embodiment described above, and can be modified in various ways without departing from the gist of the invention.

For example, in the above-described embodiment, the pin 22 is held in the insertion hole 24 formed in the holder 22 under vacuum suction, but the pin 22 is not limited to this, and a magnet may be used. It may be a magnetic attraction mechanism, or it may be a mechanical locking mechanism using mechanical fingers.

Further, in the compliance mechanism 34 of the above-described embodiment, a ring-shaped coil spring is used as the biasing member 34i, but the structure is not limited to this, and FIG. As shown as a modification example, a ring-shaped elastic rubber band 34n may be used. In this case, compared to the case where a ring-shaped coil spring is used, each support pin 34c has a cut groove 34.
There is no need to form d.

Furthermore, as shown in a second modification example in FIG. 5B, by widening the width of this ring-shaped elastic rubber band 34n, it is possible to use only one elastic rubber band 340 instead of a pair of upper and lower elastic bands. Become.

Here, in the above-mentioned embodiment, the substrate 30 includes:
The description has been made of a single shaft case in which a single wooden fitting hole 28 is formed, one pin 22 is inserted and held in the holder 20, and this pin 22 is inserted into the fitting hole 28. The alignment device 12 of this embodiment is not only applicable in this manner, but also includes two pieces of wood inserted and held in the holder 20 into the fitting holes 28 of the two pieces formed on the substrate 30. It can also be applied to a case of two shafts into which the pin 22 is inserted.

That is, in such a two-axis case, as shown in FIG. 6A, the line segment C1 connecting the centers of the two pins 22 and the line segment J12 connecting the centers of the two fitting holes 28 are , if they are deviated while maintaining parallel states, the amount of deviation ΔX along the X-axis direction and the amount of deviation Δy along the y-axis direction are the same as in the prior art. 28 and the tapered surface 32 have similar values, and can be reliably handled without rotation of the hand main body 16 relative to the robot arm 10.

On the other hand, as shown in FIG. 6B, a line segment C1 connecting the centers of the two wooden pins 22 and a line segment 1 connecting the centers of the two fitting holes 28
t2 are deviated such that they intersect at a predetermined angle Δθ, the deviation amount ΔX along the X-axis direction and the deviation amount Δy along the X-axis direction are 28
At the contact part with the tapered surface 32, the value becomes different,
This involves rotation of the hand body 16 relative to the robot arm 10. Even in such a case, the compliance mechanism 3 in the alignment device 12 of this embodiment
4.36 allows rotation of the hand main body 16 relative to the robot arm 10 within a predetermined range, so it can reliably deal with such angular deviation in the rotation direction.

Furthermore, the alignment device 12 of this embodiment is applicable not only to the two-axis fitting operation described above, but also to multi-axis fitting operations.

As described in detail above, in this embodiment, the alignment device 12 is capable of detecting deviations along the X-axis, y-axis, and Z-axis, deviations (θ) in the rotational direction about the Z-axis, and ,
It has compliance with the inclination deviation (α) with respect to the Z axis, and it deviates softly in response to the deviation in the diagonal direction, and when the shear force based on this diagonal deviation is released, it reliably It is possible to return to the original, consistent state.

[Effects of the Invention] As detailed above, the alignment device according to the present invention has the following effects:
By holding the fitting member at the tip of the transfer body via a holder and moving the transfer body, when fitting the fitting member into the fitting part of the member to be fitted, the center axis of the fitting member and the fitting are aligned. In an alignment device for aligning the center axes of the parts, the attachment body is attached to the tip of the transfer body, the holder attachment body is attached to the holder, and the attachment body and Arranged between the retainer mounting body,
Compliance against misalignment in a plane perpendicular to the central axis of the fitted member, compliance against misalignment along the central axis of the fitted member, compliance against deviation in the rotational direction around the central axis of the fitted member, and the center of the fitted member It is characterized by having a compliance mechanism that has compliance with respect to deviations in inclination with respect to the axis.

Therefore, according to the present invention, even if the fitting member and the fitting part are not deviated diagonally, the alignment device has compliance that allows the fitting member to reliably fit into the fitting part. will be provided.

[Brief explanation of the drawing]

FIG. 1 is a partially sectional front view showing the structure of an embodiment of the alignment device according to the present invention; FIG. 2 is a sectional view taken along line II-II in FIG. 1; Fig. 3 is a plan view showing the misaligned state in the x-y axis plane; Fig. 4A is a front view partially showing the state in which the first and second shaft members are aligned with each other; Fig. 4B is a plan view showing the state in which the first and second shaft members are aligned with each other; In order to absorb the misalignment with the hole,
FIG. 5A is a front view partially showing the configuration of a first modified example of one embodiment; FIG. 5B is a front view partially showing a state in which the second shaft members are shifted from each other; FIG. 5B is an embodiment A front view partially showing the configuration of the second modification example: Fig. 6A is a top view showing the case where the misalignment is in a parallel state in the case of two axes; Fig. 6B is a top view showing the case where the misalignment is in a rotating state in the case of two axes. FIGS. 7A to 7C are front views showing the configuration and operation of a conventional alignment device. In the figure, 10... Robot arm, 12... Alignment device, 14... Hand part, 16... Hand body, 18
...Bolt, 20...Holder, 22...Pin, 2
4... Insertion hole, 26... Suction tube, 28... Fitting hole, 30... Board, 32... Tapered surface, 34... Compliance mechanism, 34a... First shaft member, 34
b...Second shaft member, 34c; 34d...Outer flange portion, 34e...Support pin, 34f, 34g...
・Outer flange part, 34h...cut groove, 34i...
Biasing member, 34j; 34k... recess, 34jl!・・・
・Coil spring, 34m...V groove, 34n; 34
o... Elastic rubber band, 36... Bolt, 38... Lock mechanism, 38a... Cylinder chamber, 38b... Piston member, 38c... Coil spring, 38d...
・Stopper member, 38e...Lock hole, 38f...
It is a connecting pipe. ↓ Figure 2 Figure 4A Figure 5A

Claims (1)

[Claims] By holding the fitting member at the tip of the transfer body via a holder and moving the transfer body, when the fitting member is fitted into the fitting portion of the member to be fitted, the fitting member is An alignment device for aligning a central axis and a central axis of a fitting part so that they are aligned with each other, comprising: a mounting body attached to the tip of the transfer body; and a holder mounting body to which the holder is attached. is arranged between the mounting body and the holder mounting body, and has compliance with displacement in a plane orthogonal to the central axis of the fitting member, compliance with displacement along the central axis of the fitting member, and compliance of the fitting member with What is claimed is: 1. An alignment device comprising a compliance mechanism that has compliance with respect to deviations in the rotational direction about a central axis and compliance with deviations in inclination of a fitting member with respect to the central axis.