WO2006117025A1

WO2006117025A1 - An industrial robot with a releasable safety coupling mechanism

Info

Publication number: WO2006117025A1
Application number: PCT/EP2005/056823
Authority: WO
Inventors: Torgny BROGÅRDH
Original assignee: Abb Ab
Priority date: 2005-05-02
Filing date: 2005-12-15
Publication date: 2006-11-09

Abstract

An industrial robot comprising an actuator (10) and a mechanical transmission arrangement (11, 4, 2, 3, 5) transmitting energy from the actuator to robot movement. The transmission arrangement comprises a first part (3) connected to the motor and a second part (2) connected to the first part via a releasable mechanical coupling (7, 8, 9) arranged such that energy from the actuator is transferred from the first part to the second part via the coupling, and the coupling is arranged such that it will be released if a force larger than a defined release force is applied between the first and the second part.

Description

AN INDUSTRIAL ROBOT WITH A RELEASABLE SAFETY COUPLING MECHANISM

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to the field of industrial robots.

An industrial robot includes a plurality of stiff link elements, which are movable relative each other. The link elements are either rotatable relative to each other around a rotational axis or linearly displaceable relative to each other. The link elements are often denoted robot arms. The number of axes of the robot varies due to the type of robot. However, usually the number of axes is between 3 and 6. There are different types of robots, such as serial kinematic manipulators and parallel kinematic manipulators.

A serial kinematic robot can be viewed as a chain of stiff link elements. Two adjacent link elements are joined with each other so that they are rotatable relative to each other. The first link element in the chain is the base of the robot and the last link element usually constitutes a tool attachment. The joint connected to the tool attachment is denoted the wrist of the robot.

A parallel kinematic robot comprises at least one stationary element, a movable element, denoted a platform, and at least three arms. Each arm comprises a link arrangement connected to the movable platform. These link arrangements transfer forces to the movable platform.

It is a desire today to allow direct human-robot interaction, for example during robot programming. A lot of R&D work has been made the latest ten years to make industrial robots intrinsically safe, to be able to admit a direct human-robot interaction. The reason for this is that there is a considerable need to make the use of robots much easier. Thus, if people can collaborate directly with the robots, new possibilities will open up for installa- tion, calibration, programming, program editing, exception handling, reconfiguration and adaption to changing environment. However, the safety level of robots manufactured today is not high enough to allow interactivity.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide an industrial robot that improves safety of direct human-robot interactions.

This object is achieved by an industrial robot as defined in claim 1. Such a robot comprises an actuator and a mechanical transmission arrangement for transmitting energy from the actuator to robot movement, and is characterized in that the transmission arrangement comprises a first part connected to the actuator and a second part connected to the first part via a releasable mechanical coupling arranged such that energy from the actuator is transferred from the first part to the second part via the coupling, and the coupling is arranged such that it will be released if a force larger than a defined release force is applied between the first and the second part. The first part is, for example, an actuated arm fixed to a rotary shaft of the actuator, or a first part of a gear train between the actuator and the robot arm. The second part is, for example, one of the arms of the robot, or a second part of the gear train between the actuator and the robot arm. The actuator is, for example, a motor.

If there is a collision between the robot and a human, and the force on the coupling, due to the collision, exceeds the defined release force, the coupling will release, and thereby transmis- sion energy from the actuator will no longer be transferred from the first part to the second part, and the second part is allowed to move freely relative the first part of the transmission arrangement.

The invention makes use of a loaded coupling, for example spring loaded, that will be released at a collision. The robot is rigid as long as the force on the coupling is below a defined release force and becomes compliant when the force is larger than the defined release force. Thereby, the safety for the robot operator is increased.

The present invention provides a mechanism for limiting the forces obtained at a collision between the robot and a human, which is simple, low cost, and does not require a large space. The invention guarantees that a defined maximum force will not be exceeded at a collision between the transmission arrangement and a human. The invention can be used for all the axis of a serial robot. The invention makes low arm inertia possible, meaning low actuation power and actuation forces/torques and therefore low force levels at an impact between robot and hu- man.

According to an embodiment of the invention, the coupling comprises a protrusion and a grove adapted for receiving at least a part of the protrusion, and a force applying member applying a force on the coupling such that the protrusion and the grove are pressed against each other, and the level of the applied force sets the level of the release force. If the force on the coupling, due to a collision, exceeds the force applied to the coupling by the force applying member, the coupling will be released. The force applying member is, for example, a spring. This coupling is simple and cheap to produce.

According to an embodiment of the invention, the protrusion has an outer diameter, which is larger than the inner diameter of the grove. When the force on the coupling exceeds the defined release force, the protrusion should be moved away from the grove in order to release the coupling. The diameter of the protrusion should be somewhat larger than the diameter of the grove to prevent the protrusion from penetrating to far into the grove and thereby make it impossible to release the coupling.

According to an embodiment of the invention, the force applying member is arranged so that the force applied to the coupling is adjustable. Thereby, it is possible to adjust the defined release force and thus the maximum force permitted at a collision.

According to an embodiment of the invention, the robot comprises a detector for detecting when the coupling is released and the robot is adapted to turn off the actuator upon detecting that the coupling is released. This embodiment further improves the safety at a collision.

According to an embodiment of the invention, the robot comprises a plurality of actuators for providing robot motions and the robot is adapted to turn off the plurality of actuators upon detecting that the coupling is released. This embodiment further improves the safety at a collision.

According to an embodiment of the invention, the robot comprising a second coupling adapted to rigidly connect the first part to the second part. As an alternative, the robot comprises a mechanism to lock the safety coupling in order to change to a rigid coupling. In some applications it is desirable to have a rigid connection between the first and second part of the robot. This embodiment makes it possible to select whether the robot should be intrinsically safe or rigid by selecting between a re- leasable and a rigid coupling. For example, during programming it is preferable that the robot is intrinsically safe, and when the robot is in production a rigid connection is to prefer. The solution to this problem is to use the releasable coupling during pro- gramming of the robot and the rigid coupling when the robot is in production. According to an embodiment of the invention, the robot comprises at least two, or more preferably at least three, releasable mechanical couplings arranged such that they will be released if a force larger than a defined release force is applied between the first and the second part, in order to avoid too big movements of the second part of the robot after a release of the safety coupling.

According to an embodiment of the invention, the transmission arrangement comprises a shaft and the second part is mounted on the shaft such that the second part is allowed to rotate freely relative the shaft when the coupling is released. The first part of the transmission arrangement is fixedly mounted relative to the shaft. The coupling is such that it will open at a certain torque on the shaft, which is defined by the release force. Thus, at a collision between the robot and a human, and the force on the coupling exceeds the defined release force, the coupling will release, and the second part moves freely relative to the shaft and thereby the forces on the human during and after the collision are limited.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Fig. 1 shows a graph of a desired relation between position er- ror and force.

Figs. 2a-c show an industrial robot including a releasable coupling according to a first embodiments of the invention. Figure 2a shows a side view of an arm structure of the robot, and figure 2b shows a cross section A-A through the arm structure in figure 2a. Fig. 3 shows a graph of the relation between position/angle error and force for the industrial robot shown in figures 2a-c.

Fig. 4 shows an industrial robot according to a second embodiment of the invention.

Figs. 5-7 show examples of releasable mechanical couplings.

Figs. 8-10 show an industrial robot according to a third embodiment of the invention.

Fig. 11 shows an example of the invention implemented on the first and second axes of an industrial robot.

Fig. 12 shows an example of the invention implemented on the third axes of an industrial robot.

Figs. 13a-b show an industrial robot according to a fourth embodiment of the invention.

Fig. 14 shows a robot having a releasable as well as a non- releasable coupling.

Figs. 15-17 show examples of the invention implemented on a parallel kinematic robot.

Figs. 18-19 show further examples of the releasable mechanical coupling.

Figs. 20-22 show further examples of the invention implemented on a parallel kinematic robot.

Figs. 23-24 show further examples of the releasable mechanical coupling. Fig. 25 shows an example of a releasable mechanical coupling suitable for a parallel kinematic robot.

Fig. 26 shows an example of a parallel kinematic robot.

Fig. 27 shows another example of the invention implemented on the parallel kinematic robot shown in figure 29.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

An intrinsically safe and industrially useful robot should be compliant if it hits a human accidentally, and simultaneously be rigid and accurate under normal movements. This means that when the forces, that the robot structure can exert on a human, inside the working range of the robot are not large enough to cause serious injuries, the robot is allowed to be stiff, but when the forces exceed the limit for being dangerous, the robot must be compliant in order to not build up any forces causing injuries. The force F to position error dx shown in Fig 1 can represent this behavior.

Figs 2a-c show an industrial robot according to a first embodi- ment of the invention. The arm structure comprises an actuator in the form of a motor 10, a transmission arrangement including a shaft 4 connected to the motor via a gear box 11 , bearings 5 on the shaft 4, a mounting structure 1 for the robot joint, a safe robot arm 2, an actuated arm 3 fixed to the shaft 4, and a safe releasable mechanical coupling 7 arranged between the robot arm 2 and the actuated arm 3. The robot arm 2 is rotatable about an axis through the shaft 4. The robot arm 2 is mounted on the shaft 4 with one of the bearings 5, and accordingly can rotate freely relative the shaft 4. The releasable mechanical coupling 7 comprises a grove 8a in the safe arm 2, a protruding part 8b arranged in the actuated arm 3, a spring member 9 giv- ing a defined force on the protrusion 8b, and a spring holder 6, which preferably is adjustable. The spring member 9 is arranged such that the force F_spring on the coupling 7 is directed essentially perpendicular to the longitudinal extension of the arm 2.

The grove is adapted to receive at least a part of the protruding part δb. In this example, the grove δa is a cylindrical hole and the protruding part δb is a cylinder with a spherical head. The diameter of the sphere is somewhat larger than the diameter of the hole in order to prevent the sphere from penetrate into the hole. The actuated arm 3 is provided with a through hole and the protruding part δb and the spring member 9 is arranged inside the hole. The spring member 9 is arranged such that it applies a defined force on the protruding part δb in the direction of the grove, such that the protruding part is pressed against the periphery of the grove. The spring holder 6 adjusts the size of the force. By using a releasable mechanical coupling 7 between the robot arm 2 and an actuated arm 3 and by designing the coupling is such a way that it will open at a certain torque on the shaft 4 a not reversible relationship between torque and angle deviation can be achieved as shown in Fig 3. This behavior is even safer than the behavior in Fig 1 since a human cannot be jammed between the robot and for example a wall as is the case for a robot having the characteristics of Fig 1.

Fig 2c shows the releasable coupling in Fig 2b in a view from above cut along the line B-B, magnified to show the force relationships. In this case a sphere δc replaces the cylinder δb in Fig 2. The spring holder 6 is possible to screw into the hole in the actuated arm 3 to adjust the spring force. Roughly the arm force F_arm and spring force F_spring depicted in the Fig 2c will be achieved, and when F_margin goes to zero the releasable coupling between the actuated arm 3 and the robot arm 2 will open and the robot arm 2 can swing freely about the shaft 4. Thus, when an error occurs, for example a collision between the robot arm and a human, causing a too high arm force F_arm, the coupling is released and transmission of energy from the motor to the robot arm is no longer possible and the robot arm 2 swings freely about the shaft 4. When the error has been corrected, it is easy to manually move the robot arm 2 in position relative the actu- ated arm 3 again to close the coupling.

In order to increase the rigidity of the coupling 7 between the actuated arm and the robot arm, two actuated arms 3a-b, one on each side of the robot arm 2, can be used according to Fig 4. Two releasable couplings 7a-b are arranged between the actuated arms 3a-b and the robot arm 2.

Figs 5 and 6 exemplify other design possibilities for the coupling between the actuated arm 3 and the robot arm 2. As shown in Fig 5, a protruding part 8d having a convex coupling surface is arranged on the robot arm 2 side, and a spring loaded hollow cylinder defining a hollow space 12a is arranged on the actuated arm 3 side. In Fig 6 a socket type of spring-loaded element 12b is arranged on the actuated arm 3 side, giving a ball and socket joint between the actuated arm and the robot arm. The spring- loaded element 12b is provided with a grove 12c. Of course, the spring loaded coupling component 12d can also be mounted on the robot arm 2 as shown in fig 7. In this figure the concave surface is a conical grove or a V-grove 12f in the actuated arm 3. Instead of a spring, it is possible to have a controlled force on the coupling by means of a hydraulic, pneumatic or magnetic device.

The arrangement in Fig 1 can be slightly changed to get the possibility to catch the robot arm 2 with the actuated arm 3 when the coupling is released by, for example, a collision. This is shown in Fig 8, where the actuated arm is a disc 3c with at least one coupling 6 on each side of the coupling used in the normal case. It is in this design an advantage to be sure that the motor driving the actuated arm is electrically disconnected and that a mechanical brake is activated as soon as the coupling is re- leased. To guarantee that this will happen an electric switch can be integrated into the coupling element 6, as exemplified in Figs 9 and 10. In Fig 9 an electric switch 23 will disconnect the motor and brake voltage when the element 12b no longer has any con- tact with the convex part 8d of the robot arm 2. In Fig 10 the element 18 contains an electric contact 24, which in the normal case connects a first wire 25 with a second wire 26, but as soon the coupling is released, the wires 25 and 26 are disconnected and the motor is disconnected and the mechanical brake is en- gaged. The contact fuction can also be implemented by different types of sensors measuring the position of the moving coupling element. Examples of such sensors are capacitive, inductive, strain and optical sensors.

Fig 1 1 shows the two axes on a serial robot with releasable couplings according to an embodiment of the invention. The second axis unit 1 ,2,3,4,6,7,8,9,10,11 of the robot is the same as shown in Fig 2. What is added is the first axis unit of the robot, with a vertical shaft 31 driven by a motor 32 via a gear box 33 and con- nected to a disc 28 containing two releasable couplings 29 and 30 for the transmission of the motor torque to a part 27, which supports the second axis unit. The part 27 can rotate freely relative the shaft 31 since it is mounted on the axis 31 with a bearing 5. This means that when the couplings 29,30 are released because the transmitted torque is too high, then the second axis unit can rotate free around the shaft 31. The drive unit 32,33 is mounted on a floor platform 34. The safety couplings 29 and 30 can be of any of the types as shown in Figs 5, 6, 7, 9 and 10.

Fig 12 exemplifies how a safety coupling can be used for the third axis of the robot. A motor and gearbox of the third axis actuates a lever 39 via a safety coupling 41 between an actuated arm 40 and the lever 39. The lever 39 is connected to an upper arm 35 via a parallelogram bar 37, which is connected to the lever 39 with a bearing/shaft 42 and to the upper arm 35 with a bearing/shaft 43. The safety coupling 41 can be of any of the types as shown in Figs 5, 6, 7, 9 and 10.

Instead of having the safety coupling between an actuated arm and a robot arm, which are both mounted on a common actuated shaft, it is possible to use a not activated extra joint48 on the actuated arm 35, around which the safe robot arm 44 can swing according to Figs 13a-b. Fig 13b is a cross section through along the line C-C in Fig 13a. As shown in the figures, the actu- ated arm 35 is actuated via the bar 37 and the lever 39, which is connected to the motor and gear box at the joint 4,5. The actuated arm 35 can rotate around a bearing 46 on the shaft 45 mounted on the lower robot arm 2. On the actuated arm 35, the safe upper robot arm 44 is mounted to swing around the shaft 47 with the bearing 48. The shaft 47 is mounted on the actuated arm 35 and at back end of the upper robot arm 44 there is a re- leasable coupling 49 arranged between the safe upper robot arm 44 and the actuated arm 35.

If the robot in production must move heavy objects or tools or make very fast movements with high acceleration, the arm forces and torques needed will be higher than can be tolerated from a safety point of view. However, in many applications it is most important to have an intrinsically safe robot during calibra- tion and programming. Therefore one solution is to use the re- leasable couplings only during programming and to activate a rigid connection between the actuated arm and the robot arm in production. This is exemplified in Fig. 14, where a rigid coupling 50 is arranged between the safe arm 2 and the actuated arm 3. The rigid coupling 50 includes a piston 51a, which is pushed into a cylindric hole in the robot arm 2 by an actuation device 51 b in order to shortcut the safety coupling 7. Of course different locking devices can be used to shortcut the safety coupling. It is also possible to have a locking mechanism integrated into the safety coupling to lock the safety coupling itself. Fig. 15 shows an example of a parallel kinematic manipulator having three linear actuator modules 93, 94, 95 each with an actuated carriage, 96, 97 respectively 98. On each carriage an arm 99, 100, 101 is mounted and these arms hold a six link parallel kinematic structure connected to an actuated platform 102. Each link 104 has a joint in each end. The advantage with a parallel structure is low moving mass also for robots with a large workspace, which means low inertia and low risk for injuries at a collision. However, the arms 99, 100, 101 and the carriages 96, 97, 98 will have a larger mass and there is a risk, at least when the robot must handle big loads or run with high acceleration or speed, that the robot will anyhow be dangerous. To handle this problem the arms can be equipped with safe releasable couplings, as exemplified in Fig. 16.

Fig 16 shows a first design of a safe arm 99, seen from above in Fig. 15. The arm 99 is mounted on a bearing on a vertical shaft 105, which is fixed to the carriage 96. The arm 107 is also fixed to the carriage 96, and the arm 99 is connected to the arm 107 by means of a releasable coupling 106. This design is actually the same as shown in Fig. 13, where 35 corresponds to 107 and 44 corresponds to 99.

Fig 17 shows an alternative safety coupling 108,109,110 be- tween the arm 99 and an arm 111 fixed to the carriage 96. As in Fig 16 the arm 99 is mounted on the carriage 96 with a bearing and shaft 105 in such a way that it can swing freely in the horizontal plane if the safety coupling is released. The safety coupling includes a link 108, which in at least one end has a cou- pling 109,110 that will be released at a certain force. Figs 18,19 show some design possibilities for the couplings 109,110.

Fig 18 shows an implementation of the safety coupling 109,110 in Fig 17 using a ball 1 12 and socket 114 coupling. The ball 112 is made from a magnetic material and the socket 1 14 is attached to the ball by means of the magnetic forces from magnets 113 mounted in the socket. When a too high force is exerted on the arm 99 relative to the arm 111 , a ball and socket coupling will be detached and the arm 99 can swing freely around the shaft 105.

Fig 19 shows another implementation of the safety coupling 109,1 10 in Fig 17 with a double link 108a-b, each link having a ball 115a-b, 118a-b at its ends. The arms 99,11 1 are provided with groves. The balls are forced into the groves in the arms 99,111 with springs 1 16 and 117. Of course, the groves can in- stead be mounted on the links 108a-b and the balls be fixed to the arms 99 and 1 11.

Fig 20 shows an alternative to the embodiment shown in Fig 17. In Fig 20 the safety arrangement includes a linear track 120, on which a carriage 121 is mounted. The carriage 121 carries the safe arm 99 and at collision the safety coupling 109 and/or 110 will be disconnected and the carriage 121 can move along the track 120 and thus give away for collision forces. Of course, one of the couplings 109 and 110 can be replaces by a fixed joint and at least one of the couplings should have an electric switch that directly disengages the motors. This is also the case for the safety arrangement in Fig 17.

Instead of using a special link 108 as a safety coupling, a re- leasable coupling as in any of the figures 2, 5, 6 and 7 can be used between the linear track 120 and the carriage 121 as shown in Fig 21. Of course this safety coupling can be combined with a locking mechanism between 120 and 121 in the same way as a locking mechanism 51 is used in Fig 14 between 2 and 3. Another possibility is to integrate a locking mechanism into the safety coupling as exemplified in Fig 22. Here the shaft 51 can be pushed into the part 8 of the releasable safety coupling and thereby the safety coupling will no longer have the possibility to release on the high forces between the carriage 121 and the safety track 120. If considerable safety force levels are needed, then the spherical elements can be replaced by cylindrical elements to get a larger contact surface between the two elements that need to be separated at a certain force level. Thus, Fig 23 illustrates one arrangement using a cylinder 122, which is forced into a groove 123 in a coupling element by a force, illustrated by an arrow 124. A force, illustrated by an arrow 125, corresponds to the force between the cylinder 122 and the coupling element and is the force that must have the safety characteristics of Fig 1.

Fig 24 shows a possible arrangement for obtaining the force 124 in Fig 23. Here the force is obtained by a combination of a spring 126 and a magnetic circuit 126 - 130 - 129 - 122. Since the cylinder 122 and the groove 123 can be made long, several springs and magnetic circuits can be used in parallel along the cylinder.

Instead of having two safety couplings in both ends, or in one end of a link, as in the Figs 17 - 20, it is possible to use a cou- pling somewhere on the safety link 108. For example, the safety link 108 in Fig 20 can be replaced with not releasable joints in its ends and a safety coupling can be mounted in the link as shown in Fig 25. The link 108 is divided into two parts 108a-b. The safety coupling in Fig 25 is very simple; it includes a ball 134 on the link part 108b and a fork 135,131 ,132 on the link part 108a. The parts 131 and 132 of the fork work as blade springs, and the ball 134 is forced between these spring and is given a stable position by simple holes 133 in the spring elements 132,131. This type of safety link can of course also be used for the parallel bar 37 in Fig 13a, as can the safety links in Figs 19 and 20 and variants of these. In Fig 25 also a locking possibility is indicated by a rod 136, which can lock the link 108b to the link 108a by insertion of the rod into the hole 137 in order to achieve a rigid coupling. Also indicated in the figure is an elec- trie contact 138, which works as a safety switch for the motors. The safety couplings are also very useful to make arm type of parallel kinematic robots safe. In 26 actuators 142, 143, 144, which are fixed to the central column 145, rotate the arms 139, 140 and 141 around a vertical axis. A structure of six links con- nects the three arms with a manipulated platform 146. Each link (as the link 147) has a joint of at least 2 DOF in each end, one joint mounted on the arm (as the joint 147a) and one joint mounted on the manipulated platform (as joint 147b). At a dangerous collision it is important that the arms will give away and Fig 27 shows one possibility to obtain this. In Fig 27 the arm 140 in Fig 26 is seen from above. The arm 140 is here mounted on the actuator 143 using a 1 DOF joint 148 with a vertical axis. The arm 140 is kept in place by a beam 149, which is mounted on a part 151 using a 1 DOF joint 150 (also with a vertical axis). The part 151 is fixedly mounted on the actuator 143. In the other end of the beam 149 there is a safety coupling 152 of any of the types described before. When the safety coupling is released the safety coupling will move in a track 153 and as soon as the safety coupling starts moving the electric switch 154 will discon- nect the actuators from the power.

Claims

1. An industrial robot comprising an actuator (10) and a me- chanical transmission arrangement (2,3,4,5,1 1 ;

93,96,99,111 ,108) for transmitting energy from the actuator to robot movement, characterized in that the transmission arrangement comprises a first part (3;35;1 11 ,108) operatively connected to the motor and a second part (2;44;99) connected to the first part via a releasable mechanical coupling (7;49;109,1 10) arranged such that energy from the actuator is transferred from the first part to the second part via the coupling, and the coupling is arranged such that it will be released if a force larger than a defined release force is applied between the first and the second part.

2. An industrial robot according to claim 1 , wherein said coupling comprises a protrusion (8b,8c,8d,12d;122) and a grove (8a,12a,12c,12f;123) adapted for receiving at least a part of said protrusion, and a force applying member (6,9;126) applying a force on the coupling such that the protrusion and the grove are pressed against each other, and the level of the force sets the level of the release force.

3. An industrial robot according to claim 2, wherein the protrusion has an outer diameter, which is larger than the diameter of the contact area between the protrusion and the grove.

4. An industrial robot according to claim 2 or 3, wherein said force applying member (6,9) is arranged so that the force applied is adjustable.

5. An industrial robot according to any of the previous claims, wherein the robot comprises a detector (23;24,26,25;138;154) for detecting when the coupling is released, and the robot is adapted to turn off the actuators upon detecting that the coupling is released.

6. An industrial robot according to claim 6, wherein the robot comprises a plurality of actuators (10,32,79) for providing robot motions and the robot is adapted to turn off said plurality of actuators upon detecting that the coupling is released.

7. An industrial robot according to any of the previous claims, wherein the robot comprising a second coupling (50;136) adapted to rigidly connect the first part to the second part.

8. An industrial robot according to any of the previous claims, wherein the robot comprises a mechanism (51 ;136) to lock the safety coupling.

9. An industrial robot according to any of the previous claims, wherein the robot comprising at least two releasable mechanical couplings (6) arranged such that they will be released if a force larger than a defined release force is applied between the first and the second part.