Description

The invention is related to a robot system, comprising a robot arm with a moveable tip and at least two degrees of freedom in movement and a robot controller which is foreseen to control the movement of the tip of the robot arm along a predetermined movement path in between a start- and an end- position.

It is known that industrial robots are widely used in industrial production such as welding, press automation or handling. A typical robot comprises a robot arm with for example 5, 6 or 7 members which are connected by respective joints in a slewable or rotatable manner at their respective ends so that a respective number of degrees of freedom in movement is gained. Each joint of the robot arm is typically driven by a dedicated servo motor wherein the position of the tip of the robot arm depends on the joint angles. A related robot controller normally comprises a computing device with a robot program running thereon containing a desired trajectory respectively movement path and a related movement speed of the tip of the robot arm. In many cases a tool such as a gripper is foreseen at the tip of the robot arm, so that the robot program basically describes the related movement path and movement velocity of a reference point of the tool, the so far called tool center point (TCP).

The robot controller is foreseen to generate control signals for the servo motors of each joint so that the desired movement of the tip of the robot arm respectively the tool center point (TCP) is executed according to the data within the robot program. Typically motion instructions of a robot program are specified in a programming language that is being interpreted during its execution by the robot controller. In this way, the motion trajectory is calculated on-line, taking the motion targets respectively coordinates of the movement path that are specified in the robot program as input. However, in many applications, industrial robots are intended to repeat pre-planned motion trajectories, which are optimized (off-line) before running it in the desired application. With the on-line trajectory calculation capability of today's industrial robot controllers, results of off-line optimization are converted to motion targets, and the actual trajectory is calculated on-line during execution of the robot program.

In many cases a robot does not execute an independent standalone application, moreover the movement of a robot arm has to be synchronized with for example the movement of another collaborating production device such as a press or another robot which can be seen as master device. In the case that the movement of the master device is deviating from its expected movement speed for example, the movement of the robot has also to become adapted accordingly, otherwise the movement of the master device and the robot would no longer be synchronized and a collision might be the consequence.

Disadvantageously within the state of the art is, that robot movements must be synchronized with the movements of the master device and corrections are made on the online calculated trajectory based on the difference between desired position of the tip of the robot arm respectively the TCP and its measured actual position.

Making corrections on the online calculated trajectory based on the difference between desired and measured position has following drawbacks:

• typically there is a need for dedicated hardware or software components that maintain the relationship between master axis position of the master device and the desired robot tool position and that determine the difference and provide the result to the interface of the robot controller,

• there is a time delay between robot position tracking and trajectory correction, which causes a delay of the robot movement and has negative impact on the results, • in case that the velocity of the external axis differs too much to the planned velocity, the difference between actual and desired robot tool position becomes too large, which can cause problem (e.g. collision, unintended stop).

The objective of the invention is to provide a robot system which enables a synchronized robot movement and avoids the drawbacks mentioned above. This is solved by a robot system of the aforementioned kind. This is characterized in that the

• the robot controller comprises input means for a control signal which is foreseen to represent values in between a first and a second value limit,

• wherein the robot controller is foreseen to assign each value of the control signal to a related position of the tip of the robot arm along the predetermined movement path and

• wherein the robot controller is foreseen to control the movement of the tip of the robot arm along the predetermined movement path dependent on the respective values of the control signal.

The basic idea of the invention consists in using an external axis of a master device as master axis of robot motion control. Hereby an offline optimized trajectory respectively movement path with explicitly defined master-slave relationship to the control signal is used to generate the desired robot joint positions and the desired robot movement therewith online.

According to the invention the movement of the robot arm along the predetermined trajectory respectively movement path is directly dependent on the actual value of the control signal at the input means, which is foreseen to represent the current position of the external master axis. Thus, in case of a slowing down or speeding up the devolution of the control signal representing the position of the master axis, the speed of the robot movement will be in a direct relationship thereto so that external master axis and movement of the robot are fully synchronized.

A predetermined trajectory respectively the movement path can be generated with any software tools, but can also be generated offline with a virtual robot controller. A predetermined trajectory is preferably optimized for the normal operation and consid- ers also conditions like operation under maximum speed of master axis or speed variations.

The control signal itself could be as well represented by a digital signal with respective data values as also by an analogous voltage signal or the like. The control signal represents values in between a first and a second value limit.

The predetermined trajectory as a function of master axis position is used in online operation on the robot controller to generate desired robot positions (TCP and/or joint values, velocity and acceleration) for robot servo motor control. Desired robot positions are created based on the master axis position, which might become fed from an external sensor (e.g. encoder), motion control (e.g. machine control PLC) or a source for generating a control signal according to a mathematical function such as a sinusoidal course.

In case that the course of the control signal is as dynamic, that the specified maximum robot speed is reached or exceeded, a signal might be raised up, indicating that the synchronization will be lost. Thus it is possible to configure the action to be taken when the specified maximum robot speed is reached or a change of master axis speed is larger than a specified limit, for example trying to follow the master axis as fast as possible, stopping movement or invoking a defined robot function or the like.

According to another embodiment of the robot system according to the invention the robot controller is foreseen to control the position of the tip of the robot arm along the predetermined movement path respectively trajectory in that way, that the tip distance from the path start position is increasing when the value of the control signal is increasing. Thus the dependency in between control signal and movement of the tip of the robot arm is defined in that way, that the required dynamic of the robot movement is as lower as lower the given dynamic of the course of the control signal is. So it is ensured that the movement speed of the robot does not exceed a certain limit if the dynamic of the course of the control signal is limited.

According to another embodiment of the invention the robot controller is foreseen to control the position of the tip of the robot arm along the predetermined movement path in that way, that the tip is at the path start position when the control signal value corresponds to its first limit and that the tip is at the path end position when the control signal value corresponds to its second limit. In a further preferred embodiment the relationship in between the control signal and the robot movement along the predetermined movement path is linear so that a fully linear synchronization is gained.

According to another embodiment of the invention source means are provided for generating a master signal with values in between the first and the second value limit, which is foreseen to be supplied to the input means as control signal. The source means might generate an analogous signal such as a voltage signal but also a digital data signal.

According to another embodiment of the invention the source means for the master signal are foreseen to generate the master signal based on a mathematical function. This might be for example a sinusoidal function. Background for this embodiment is, that the master signal of the source means is not only provided to one, but also to at least a further robot or production device, that's movement is dependent on the (same) control signal. So it is possible for example, that a sinusoidal signal controlling the up- and down movement of a press for example is additionally supplied to the input means of the robot. Thus a synchronized movement of the two or more robots or devices depending on this control signal is gained in an advantageous way without the need for a sensor determining the actual position of the master axis of the master device.

According to an alternative embodiment of the invention the robot system further comprises a device with an external movement axis which is moveable in between a first and a second axis limit, wherein sensor means for determining the position of the external movement axis are foreseen and wherein the source means for the master signal are foreseen to generate the master signal in that way, that it describes consecutive positions of the external movement axis with consecutive values in between the first and second value limit, so that the movement of the tip of the robot arm along the predetermined movement path is interconnected with the actual position of the external movement axis. In this example a sensor means is used for the generation of the master signal. This might be required, if the movement of the master de- vice is controlled in a purely mechanical way, for example by a rotating excentric causing an up- and down- movement of a press, so that no respective control signal is available.

According to another embodiment of the invention the position of the external movement axis at its first limit corresponds to the position of the tip of the robot arm at the start position of the predetermined movement path and/or the position of the external movement axis at its second axis limit corresponds to the position of the tip of the robot arm at the end position of the predetermined movement path. Thus the movement of the external master axis is preferably linearly assigned to the movement of the tip of the robot arm along the predetermined movement path.

According to another embodiment of the invention the start- and end- position of the predetermined movement path are identical and source means are foreseen to generate a continuously increasing master signal and to set its value abrupt to the first value limit when reaching the second value limit. Background for this embodiment is that also periodical movements within a loop or as hysteresis are enabled therewith, wherein there is no backwards movement of the tip of the robot arm back along the same path. In other words a kind of hysteresis behavior is possible, so that for example the trajectory of a robot arm to a target has not necessarily to correspond to the trajectory back from the target.

Suitable source means for the generation of such a master signal could generate the master signal either based on a mathematical function or for example an angle sensor of a rotating disk or excentric. Not a suitable source means is for example a linear distance meter determining the actual gap in between the upper and lower part of a press, since no continuously increasing master signal is generated.

According to another embodiment of the invention the robot controller is foreseen to check, whether the robot arm is capable to perform the desired movement speed of its tip along the predetermined movement path. Even each point of the predetermined movement path can be assumed to be reachable by the tip of the robot arm respectively by the TCP of a tool mounted thereon, in case of a too high dynamic of the course of the control- respectively master signal the desired movement speed of the robot arm resulting therefrom might be higher than the robot arm is able to perform. In case of exceeding the admitted limits respective counteraction can be initiated, in the easiest case a stop of the robot system. Further actions could be:

• robot arm moves as fast as it can,

• robot arm stops on path,

• robot arm stops as fast as it can.

According to another embodiment of the invention a tool with a tool center point is mounted on the tip of the robot arm, wherein the robot controller is foreseen to control the movement of the tip of the robot arm in that way, that the tool center point (TCP) of the tool is moving along the predetermined movement path instead of the tip itself. The definition of a tool center point as reference for moving along a trajectory facilitates programming in a good way.

According to a further embodiment of the invention the robot system comprises at least a further robot respectively robot arm that's tip is moveable along a further predetermined movement path dependent on the respective values of the same control- respectively master- signal. Thus a synchronization of the movements of two or more robots is enabled in an easy way by using a common master signal for synchronization.

Further advantageous embodiments of the invention are mentioned in the dependent claims.

The invention will now be further explained by means of an exemplary embodiment and with reference to the accompanying drawings, in which:

Figure 1 shows an exemplary control scheme of a robot system according to prior art,

Figure 2 shows an exemplary control scheme of a robot system according to the invention,

Figure 3 shows an exemplary robot system,

Figure 4 shows an a graph a control signal interconnected with predetermined

movement path and Figure 5 shows an external movement axis interconnected with predetermined movement path.

Figure 1 shows an exemplary control scheme of a robot system according to prior art in a sketch 10. A robot controller comprises a computing device with a robot program running thereon, which comprises motion instructions describing a predetermined movement path respectively trajectory. Based thereon the following steps are performed by the robot controller:

• Geometrical path interpolation. Typically the movement path is determined by a sequence of coordinates wherefrom the trajectory is derived of. Also a dynamic behavior of the robot is considered herby, for example a sharp edge in the trajectory which would arise when linear connecting the coordinates is rounded automatically.

• Creating joint trajectory. A robot respectively a robot arm comprises typically several members which are linked to a kinematic chain by respective joints, wherein each joint is driven by a dedicated servo motor. The end of the kinematic chain - the tip of the robot arm - is foreseen to perform a movement along the predetermined movement path. Thus each joint has to be operated according to a respective related joint trajectory, so that the end of the kinematic chain is moving as desired.

• Creating desired servo positions. Each servo motor has to be operated in that way that the joint related thereto is performing the respective desired joint trajectory.

While synchronizing the movement of the robot arm with an external axis, a sensor or control device is permanently providing the actual position of the axis to be synchronized with the robot controller, which determines, whether there is a deviation from the desired synchronized state or not.

In case of a deviation, the positions become adjusted, e.g. the joint velocity becomes slowed down respectively speeded up so that the synchronized state is achieved again. Those temporarily adjusted positions become provided to the robot axis control which drives the servo motors accordingly, so that the synchronized state to the master axis is ensured.

Figure 2 shows an exemplary control scheme of a robot system according to the invention in a sketch 20. In contrast to prior art there are no longer any motion instructions which have to become geometrically interpolated to define a movement path, wherefrom the joint trajectory is derived of.

Moreover, the actual position of the external axis, which might be provided by a sensor or control device, is directly provided is input for a function assigning each value of the control signal directly to a related position of the tip of the robot arm along the predetermined movement path. Such a function might be realized for example based on a look-up table, which is an extreme fast solution.

Having determined the desired position of the robot arm along the predetermined movement path, the desired respectively desired positions of the servo motors are derived therefrom and the robot joints are controlled accordingly.

Figure 3 shows an exemplary robot system 30 in a schematic overview. A robot with a robot arm 32 and a tip 34 at its distal end is controlled by a robot controller 36. The robot arm 32 might have six degrees of freedom in movement and a total length of 2,5m. The robot controller 36 comprises as well a computing device as several power electronic based amplifiers for generating the driver signals for the respective servo motors of the joints of the robot arm 32.

The robot controller 36 is foreseen to control the movement of the tip 34 of the robot arm 32 along a predetermined movement path 38 in between a start- and an end- position dependent on the respective values of a control signal 52, which is provided to the robot controller 36 at input means 50. The respective positions along the predetermined movement path 38 include three coordinates x, y, z for the positioning as such as three further values Θχ, Θγ, Θζ, for the orientation of the tip 34 of the robot arm 32. A not depicted function module is foreseen to assign each value of the con- trol signal 52 in between a first and a second limit directly to a related position of the tip 34 of the robot arm 32 along the predetermined movement path 38.

The control signal 52 is provided by source means 48, in this case a signal generator for a sinusoidal signal which is basically foreseen to control a press drive 44 of a press with an upper 40 and lower 42 part, which are moving up- and down against each other along an external movement axis 46.

Figure 4 shows a schematic graph 60 with a control signal 62 interconnected with a predetermined movement path 68. The control signal 62 might represent values in between a first 64 and a second 66 value limit. Dependent thereon a not depicted tip of a robot arm moves in between a start- position 70 and an end- position 72 of the predetermined movement path 68.

Figure 5 shows in a sketch 80 the course 84 of an external movement axis 90 which is indirectly interconnected with a predetermined movement path 86 since both are controlled by a common master signal 82. The external movement axis 90 describes a sinusoidal course 84 in between a first axis limit 92 and a second axis limit 94 dependent on the master signal 82 which rises from 0% to 100%. Also dependent on the master signal 82 the position of a not depicted tip of a robot arm moves along the hysteresis-like predetermined movement path 86 with common start- and end- position 88. Thus it is possible to move repeatedly along the same hysteresis like predefined movement path several by continuously rising the master signal 82 which will be set automatically to 0% if it exceeds 100%.

List of reference signs

10 exemplary control scheme of a robot system according to prior art

20 exemplary control scheme of a robot system according to the invention

30 exemplary robot system

32 robot arm

34 tip of robot arm

36 robot controller

38 predetermined movement path

40 upper part of press

42 lower part of press

44 press drive

46 external movement axis (of press)

48 source means

50 input means

52 control signal

54 connection to robot

60 control signal interconnected with predetermined movement path

62 control signal

64 first value limit

66 second value limit

68 predetermined movement path

70 start- position of predetermined movement path

72 end- position of predetermined movement path

80 external movement axis interconnected with predetermined movement path

82 master signal

84 course of external movement axis

86 predetermined movement path

88 common start- and end- position of predetermined movement path

90 external movement axis

92 first axis limit

94 second axis limit