Technical Field

The present invention relates to a method of gripping a top stack of flexible substrates arranged in a substrate pile by means of a pair of grippers, said grippers being arranged on one arm each of a two-arm robot and each having a lower finger and a corresponding upper finger, wherein said lower fingers are arranged to be inserted into said substrate pile below a bottom substrate of said top stack and said corresponding upper fingers are arranged to abut on a top substrate of said substrate pile in order for all fingers to jointly grip said top stack.

In this context, flexible substrates is meant to comprise all sorts of flexible substrates which can be provided in piles, that is for instance paper sheets, whether printed or not, plastic substrates, sheets of metal foil, or even sheets of pastry or pasta.

Prior Art

A method according to the preamble is known from the published document WO 2012/069056 A1 . According to the known method a gripping action is performed in order to aerate a gripped stack of flexible substrates, which is an important task for instance in a printing factory, because insufficiently aerated substrates tend to stick together and cause

misalignment of individual or a plurality of substrates in a following production stage and hence lead to production disturbances. Object of the Invention

Although a robot thanks to its endurance is very useful in this case in point, it has been found that a gripping action by means of grippers of a two- arm robot from time to time can cause damage to single or multiple flexible substrates. Damages noticed are wrinkling or even tearing of substrates in contact with the grippers or scratches caused by the grippers on prints on the substrates.

Further, a stack being incorrectly gripped by said pair of grippers, can lead to quality problems downstream an aeration step as well, such as a cutting step. In such a step the correct number or proper alignment of substrates in a stack to be cut is vital for quality, which an incorrectly assembled or gripped stack is prone to impede.

In the light of the above an object of the invention is to create a method eliminating or at least mitigating the above quality related problems.

Brief Summary of the Invention

According to the invention in a method according to the preamble this is achieved by means of the steps of scanning the topography of said substrate pile by means of a sensor arranged on one of said grippers in order to gather outline data and top contour data of said substrate pile; of inserting of a first one of said lower fingers below said bottom substrate at a pile location determined by means of said topography; inserting of a second one of said lower fingers alongside the inserted first one; of moving of at least one of the inserted lower fingers along said substrate pile, on a path determined by means of said topography, to a final gripping position; and of eventually having both grippers grip the top stack thus determined.

By scanning the topography according to the method of the invention, knowledge of both the outline and the top contour of a top stack to be gripped is acquired and later on used to handle the grippers of the two-arm robot in an optimal way, wherein the coordination between the scanning action and the later gripping action is simplified by involving one of the grippers in both.

Preferably, the method according to the invention comprises the additional step of having a first sensor on a first one of said grippers scan the topography of a first part of said substrate pile, while a second sensor on a second one of said grippers simultaneously is scanning the topography of a second part of said substrate pile. By this the time for scanning can be reduced considerably compared to scanning by means of just one sensor and gripper. When scanning by means of two sensors and grippers, it is preferred to let said grippers work in parallel and to have said first sensor scan the topography of a first half of said substrate pile and to have said second sensor scan the topography of a second half of said substrate pile. In that way the time for the scanning operation can be reduced to half the time compared to scanning by means of just one gripper and sensor.

In order to facilitate the whole process, according to an embodiment of the invention there is the additional step of having said pair of grippers emit patterns of visible light in order to assist an operator to place a substrate pile correctly in front of said two-arm robot. By this moves of the robot can be shortened and hence time be save.

Preferably, according to a preferred embodiment of the invention said sensors are used to emit patterns of visible light in order to assist an operator to place a substrate pile correctly in front of said two-arm robot, which reduces costs and facilitates assembly.

In its simplest but still useful form, the visible light is emitted vertically downwards, wherein each of said patterns comprises a single dot of visible light. To project such a simple pattern an ordinary light emitting diode does fine.

Brief Description of the Drawings

In the drawings a preferred embodiment of the method according to the invention is illustrated schematically, wherein:

Fig. 1 is a perspective view showing a two-arm robot behind a first pile of flexible substrates;

Fig. 2 is a perspective view showing a two-arm robot behind a second pile of flexible substrates;

Fig. 3 is a perspective view showing a two-arm robot behind a third pile of flexible substrates; and

Fig. 4 is a perspective view showing the two-arm robot from behind when gripping a stack of flexible substrates.

Detailed Description of a Preferred Embodiment Below the method according to the preferred embodiment of the invention will be described in detail with reference being had to the

accompanying drawings. In these similar reference numbers are used to denote corresponding items.

In figs. 1 -4 a robot 1 is shown schematically. It comprises a base 2, which can be mounted on a stand (not shown) and bears a top 3 in a rotatable way. The top 3 has two arms 4, 5 hinged thereto, said arms being multiply pivoted and holding grippers 6, 7 at their distal ends.

Each gripper 6, 7 comprises an upper finger 8, 9 and a lower finger 10, 1 1 , respectively, the upper 8, 9 and lower 10, 1 1 fingers being movable towards or away from each other in order to grip or release items.

With its pair of grippers 6, 7 the two-arm robot 1 is according to a method of the present invention used to handle items in the form of flexible substrates 12, which are supplied in a pile 13 on a support 14, such as a pallet. The flexible substrates 12 can be of many different kinds, such as paper, plastic or metal sheets, and may be blank, coated or bear a print.

If a plurality of flexible substrates 12 are laid out in a pile 13, their flexibility can cause a top 13' of the pile 13 to deviate considerably from an all flat shape, especially if each substrate 12 bears a print not covering the entire substrate 12, or if the support 14 does not support the pile 13 evenly. The result of this is illustrated in figs. 1 -3 by means of a first, a second and a third pile 13, respectively, all three deviating from a strictly parallelepipedic shape. Thus, for instance the first pile 13 of fig. 1 has a convex top 13', the second pile 13 of fig. 2 a concave top 13' and the third pile 13 of fig. 3 an undulated top 13'.

Now, if a two-arm robot 1 is to grip just a stack 15 of flexible substrates 12 from the top 13' of a pile 13, in order to size the stack 15 correctly the robot 1 has to have knowledge of the exact position of its grippers 6, 7 in relation to the pile 13 as such and to the top 13' of the pile 13 in particular. To this the lower fingers 10, 1 1 of the grippers 6, 7 are provided with sensors (c.f. arrows 16, 17 indicating their placement in figs. 1 -3), which by emitting light vertically downwards (c.f. light beams 18, 19 in figs. 1 -3) and by measuring light reflected from a surface hit by the light can help provide the robot 1 with the position information needed.

To this the grippers 6, 7 are while measuring moved above the pile 13 in order scan a topography of the pile 13 by gathering outline data and top contour data of the pile 13. This is done most effectively by moving the grippers 6, 7 in parallel and by having their sensors 16, 17 scan the

topography of a first half of the pile 13 and of a second half of the pile 13, respectively.

Once the scanning is done, the topography information is used to place the grippers 6, 7 in front of and close to a side 13" of the pile 13. Then a first lower finger, such as the lower finger 10 of the gripper 6, is inserted below a bottom substrate 12 of a future stack 15 at a pile height determined by means of said topography and the desired stack size or thickness, while the corresponding upper finger, such as the upper finger 8 of the gripper 6, is placed above a top substrate 12 of the pile 13.

When said first lower finger 10 is inserted, the second lower finger, that is in this case the lower finger 1 1 of the gripper 7, is inserted along the finger 10 into a void created by that finger 10, such that said second lower finger 1 1 is placed immediately under the same bottom substrate as said first lower finger 10 and the corresponding upper finger, such as the upper finger 9 of the gripper 7, is placed above the top substrate 12 of the pile 13.

The next step according to the method of the invention is again to use the topography information to move at least one of the inserted lower fingers 10, 1 1 along the side 13" of the pile 13, such that spaced apart positions according to fig. 4 are achieved for the grippers 6, 7. While moving a lower finger 10, 1 1 the gripper 6, 7 involved does not necessarily have to follow a straight horizontal line, but can deviate upwards or downwards in

correspondence with the pile topography. Thus, the risk of damaging the flexible substrates 12 during the gripper moving action, e.g. by wrinkling or even tearing them or by scratching prints on them, can be avoided or at least mitigated. And further, the risk of incorrectly gripping or sizing a stack 15 of substrates 12 is minimized too. I one embodiment of the method according to the invention the light emitted by the sensors of the grippers 6, 7 can be light visible by humans. Thus, a clearly visible pattern, such as simple dots 20, 21 , can be projected downwards in order to assist an operator to place a substrate pile 13 correctly in front of said two-arm robot 1 .