Method for controlling a print head

Abstract

A method of printing an image onto a surface using a print head carried by a robot arm, comprising: obtaining an image, the image exceeding a predetermined print width (w) of the print head; splitting the image into at least two image strips and generating associated print head paths, each image strip being contained within a reduced print width (w0), the reduced print width being smaller than the predetermined print width; detecting a deviation from the print head paths when the robot arm is fed a first control signal in a dry run; modifying the image strips by applying a local lateral offset to compensate for the detected deviation; and printing the modified image strips onto the surface while feeding the robot arm with a control signal identical to the first control signal.

Description

Technical Field

[0001] This disclosure relates to the field of robot control for automated painting, and more particularly to a method for printing an image onto a surface using a robot-carried printhead. Background Technology

[0002] Pixel printing involves applying one or more paint colors to selected areas of a flat or curved surface to produce an image. Images wider than the printhead are produced by printing multiple parallel image strips, which are generated assuming the printhead follows a corresponding reference path on the surface. Ideally, the actual path of the printhead follows the reference path. However, due to wear, mechanical inaccuracies, suboptimal position sensors, or imperfect controllability of printhead movement, path deviations from the reference path must be anticipated. Unfortunately, these path deviations can introduce alignment errors in the printed image, such as visible gaps or overlaps.

[0003] DE102010004496 discloses a robot with a printhead and a method for controlling the printhead matrix and correcting trajectory deviations. During production operations, a three-dimensional path deviation between the desired path and the actual path of the printhead is detected using an integrated path detection sensor. If a deviation is detected, the printhead matrix is ​​controlled based on that deviation. Summary of the Invention

[0004] One object of this disclosure is to provide a method and apparatus for compensating for deviations from the intended printhead path. Providing such compensation is particularly interesting when the printhead is carried by a robotic arm. Another object is to compensate for path deviations without replacing existing robotic equipment or permanently modifying the equipment.

[0005] At least some of these objectives are achieved by means of the invention as defined by the independent claims. The dependent claims relate to advantageous embodiments of the invention.

[0006] In a first aspect of the invention, a method is provided for printing an image onto a surface using a printhead carried by a robotic arm. The image may be a monochrome image or include multiple colors. The method relates to cases where an image exceeding a predetermined print width w of the printhead is to be printed. If the image can be printed via a single pass of the printhead, a deviation from a reference printhead path will cause displacement of the printed image, but generally will not produce visible defects of the type discussed above. In a next step, the image is split into at least two image strips, and associated printhead paths are generated, each image strip contained within a reduced print width w0 less than the predetermined print width. In response to a deviation from the printhead path detected when the robotic arm is fed a first control signal, the image strips are modified to compensate for the detected deviation by applying a local lateral offset. Preferably, when a deviation is detected during dry operation, the first control signal is equivalent to a future control signal to be used during production operation (i.e., print run).

[0007] Conceptually, an image stripe can be understood as a pattern of pixels in a local reference frame centered on a point on the printhead, where for each vertical position y, a pixel is valid only in a region with a decreasing print width w0. Inactive pixels occupying a total width w-w0 also form part of the image stripe. Before any lateral offset has been applied, inactive pixels can be represented as having an equal width w. L =w R = (w-w0) / 2 for the two edges. Lateral offsets do not change the values ​​of the active pixels, but rather move them to the left (or right) as needed. To make a d-unit lateral offset, increase the width w of the right (left) edge. R And make the width w of the left (right) edge L Shrink this amount:

[0008]

[0009]

[0010] As long as 2|d|≤w-w0, the region with active pixels will fit within the print width w. If the widths of the edges with inactive pixels are not equal, the region will be within the print width w. L ≠w R This allows for a larger offset in one direction, up to w-w0 in extreme cases.

[0011] Compared to the aforementioned background technology (where path deviations are detected during production operations), the first aspect of the present invention enables more efficient compensation for systematic (or non-random) errors related to wear, mechanical inaccuracies, suboptimal position sensors, or imperfect controllability of printhead movement. Systematic errors tend to repeat consistently in each run, although they may be superimposed with process noise and random errors. Therefore, it is advantageous to accurately detect the systematic error components during the initial run and derive compensation (local lateral offset) suitable for application to all production runs. In fact, the operator has a reasonable opportunity to detect whether the printhead position sensing is accurate during the initial run (and take appropriate action), but monitoring accuracy and checking for faults and incidents throughout the entire production operation can be a tedious task. The initial run can be a dry run or a printing run.

[0012] Another advantage associated with this invention is that accurate printhead positioning, useful during initial operation, is only optional in production. Thus, even robotic arms lacking highly accurate position sensors can be used with this compensation method, i.e., if a detachable, highly accurate position sensor is used to sense path deviations. Furthermore, a single detachable position sensor can be used with multiple robotic arms, limiting capital investment. As used herein, a position sensor is “detachable” if the sensor components (which may include active or passive components) that must be positioned on the printhead during sensing can be non-destructively separated from the printhead after sensing has ended. Alternatively, non-contact position sensing, such as position sensing via a camera system, can be used.

[0013] In a second aspect of the invention, a controller is provided adapted to control a printhead carried by a robotic arm to print an image onto a surface. The controller includes a first interface, a second interface, and a processing circuitry system. The first interface is configured to receive image data and position sensor signals, the second interface is configured to output printhead control signals, and the processing circuitry system is configured to perform the described method.

[0014] The present invention also relates to a computer program comprising instructions for causing a computer (specifically, a controller) to perform the methods described above. The computer program may be stored or distributed on a data carrier. As used herein, a "data carrier" may be a transient data carrier (such as modulated electromagnetic or optical waves) or a non-transient data carrier. Non-transient data carriers include volatile and non-volatile memories, such as permanent and non-permanent storage media of magnetic, optical, or solid-state types. Still within the scope of "data carrier," such memory may be fixed-mount or portable.

[0015] Generally, unless otherwise expressly defined herein, all terms used in the claims shall be interpreted according to their ordinary meaning in the technical field. Unless otherwise expressly stated, all references to “element, device, component, element, step, etc.” shall be interpreted as referring to at least one instance of an element, device, component, element, step, etc. Unless expressly stated otherwise, the steps of any method disclosed herein need not be performed in the exact order described. Attached Figure Description

[0016] Now, aspects and embodiments will be described by way of example with reference to the accompanying drawings, in which...

[0017] Figure 1 This is a flowchart of a method for printing an image onto a surface using a robot-carried printhead, according to an embodiment of the present invention.

[0018] Figure 2 A printing system according to an embodiment of the present invention is shown, comprising a robotic arm equipped with a printhead and an associated controller;

[0019] Figure 3 shows the active side of two example printheads;

[0020] Figure 4A and Figure 4B The illustration shows the splitting of an image into flat image strips with associated printhead paths;

[0021] Figure 4C The diagram illustrates the detected deviation from the printhead path;

[0022] Figure 4D It shows that it is adapted for compensation Figure 4C The modified image stripe contours showing deviations in the image; and

[0023] Figure 5 illustrates the case where an image is split into non-linear segments with associated printhead paths. Detailed Implementation

[0024] The various aspects of this disclosure will now be described more fully with reference to the accompanying drawings, which illustrate certain embodiments of the invention. However, these aspects may be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example, and thus this disclosure will be thorough and complete, fully conveying the scope of all aspects of the invention to those skilled in the art. Throughout the description, the same reference numerals refer to the same elements.

[0025] Figure 1 This is a flowchart of method 100, which uses a robot-carried printhead to print an image onto a surface. Example image A, representing the character "1", is shown below. Figure 4AAs shown. Example surface 290 and printhead 230 are as follows. Figure 2 As shown.

[0026] Figure 2 A robotic arm 220 is also shown, comprising multiple components and linear / rotary joints, and equipped with an end effector in the form of a printhead 230. The movement accuracy of the robotic arm 220 is limited; that is, when control signals command the robotic arm 220 to take a specific position or posture, this is achieved only within tolerances. Position guidance tolerance can be expressed as the maximum permissible error in the position of the tool center point (TCP). A typical value for position guidance tolerance in an industrial painting robot can be 0.5 mm. Posture guidance tolerance can be expressed as the maximum permissible linear / angular error of the 220 joints of the robotic arm. Additionally or alternatively, tolerances can express the maximum permissible error in posture or position self-reported by the robotic arm 220. Tolerances according to any of these options can be specified by the commercial provider of the robotic arm 220, or can be calculated at the user side based on position measurements with an accuracy superior to the robot's tolerances.

[0027] A controller 210 is provided for controlling the printhead 230. Figure 2 In the illustrated embodiment, the controller 210 is multifunctional, and in a sense, it also acts as a robot controller for controlling the actuators (not shown) in the robot arm 220. In other embodiments, the controller 210 may be implemented as a dedicated printhead controller, i.e., adapted to operate as a separate entity in parallel with the robot controller.

[0028] Figure 2 The example controller 210 has a first interface 212 configured to accept image data and a position sensor signal generated by a position sensor 232 associated with the printhead. The design, type, and measurement principle of the position sensor 232 are not essential to this invention. For example, the position sensor 232 may be an optical position sensor, a position sensor equipped with a laser, or a camera system including one or more cameras. It should be understood that, although Figure 2A position sensor 232 attached to the printhead 230 is shown, but non-contact position sensing, depending on the options mentioned, may not require any active sensing components to be arranged at the printhead 230; it may even be unnecessary to provide visual markings, references, etc., for the printhead 230. Where positioning techniques require the arrangement (installation) of active or passive components of the position sensor 232 at the printhead 230, the use of removable components is advantageous. If the removable component has been secured with temporary adhesive or releasable mechanical fasteners during the detection phase, it can be removed without damage once deviation detection is complete. This protects the component from contamination or mechanical damage during the operation of the printhead 230. Furthermore, it ensures that the surface 290 is not obstructed, facilitating monitoring and inspection.

[0029] The tolerance of the position sensor 232 is preferably smaller than that of the robotic arm 220. In some embodiments, the tolerance of the position sensor 232 is 0.1 mm or less. These embodiments can be applied to printing images on surfaces with a total range of about 0.1 meters, 1 meter, or 10 meters. Moreover, a position sensor 232 with a tolerance of 0.1 mm or less is also meaningful for use with a robotic arm 220 with a tolerance of 0.5 mm or greater.

[0030] The controller 210 also includes a second interface 214 configured to output printhead control signals. Figure 2 In the illustrated multi-functional embodiment, the second interface 214 is also configured to provide control signals to the robotic arm 220. The processing circuitry 216 in the controller 210 is configured to execute the method 100, which will be described next. Alternatively, the controller 210 may accept image data via an operator input / output interface (not shown) or a network interface (not shown).

[0031] Figure 3A and Figure 3B An example printhead 230 suitable for this method 100 is shown. Connection toward the robotic arm 220 is suggested on the right side of each figure. The illustrated printheads 230 (each printhead may include an inkjet head or a spray gun) are adapted to move substantially along the longitudinal direction indicated by y (vertical in the figures) during printing. The longitudinal direction is generally aligned with the printhead path as the printhead 230 moves.

[0032] exist Figure 3A and Figure 3BIn this context, hollow circles represent individually controllable pixels, which may structurally correspond to nozzles or other components used for depositing ink, paint, dye, etc., on surface 290. It should be understood that while the example pixel arrangements discussed herein may involve a single color, more advanced printheads 230 may include multiple such pixel arrangements corresponding to a given color. Linear pixel arrangements, such as... Figure 3A As shown, and Figure 3B The diagram illustrates a matrix-shaped pixel arrangement. The horizontal range w of the pixels corresponds to the printing width of the print head 230. As used in this disclosure, the pitch of the print head 230 is the pixel resolution in the direction transverse to the longitudinal direction (horizontal in the diagram). Therefore, Figure 3A The pitch e of the linear pixel arrangement corresponds to the spacing between adjacent pixels. Figure 3B The pitch e of the matrix-like pixel arrangement corresponds to the lateral spacing. In some embodiments, to ensure efficient compensation for detected path deviations, the pitch e of the printhead 230 is smaller than the (position guidance) tolerance of the robotic arm 220.

[0033] Return to Figure 1 The first step 110 of method 100 is to obtain image A, for example, in a compressed or uncompressed image format and / or bitmap or vector graphics. In this specification, it is assumed that image A exceeds the predetermined print width w of printhead 230. Image A exceeds this width w unless it can be printed in a continuous run, wherein no segment of the printed pattern is adjacent (or continuous or tangent) to another segment of the printed pattern; such adjacent segments may suffer from alignment errors if the robotic arm has a non-negligible positional tolerance. For example, even though it is theoretically possible to print a solid circle with a diameter greater than w by moving printhead 230 in a helical path, aligning consecutive turns with each other would be a challenging task, and the option of splitting the circle into multiple image strips might be preferred; therefore, a solid circle wider than w can be considered an image exceeding the print width w. In contrast, an image of any length that is not wider than w anywhere does not exceed the print width w.

[0034] In the second step 112, in order for the print head 230 to print image A, image A is split into at least two image strips, and associated print head paths are generated, each image strip being contained within a reduced print width w0. The reduced print width w0 is smaller than the print width w of the print head 230. It should be noted that the criterion for whether splitting is necessary refers to the print width w, but the width of the image strip is the reduced print width w0 or smaller.

[0035] To illustrate the effect of step 112, Figure 4A Image A, which exceeds the print width w, is shown. Figure 4BThe same image A is shown divided into three image strips, A1, A2, and A3 (defined by solid vertical lines). Each image strip in A1, A2, and A3 is less than w0 units wide and is associated with printhead paths C1, C2, and C3 (as shown by vertical dashed lines). Printhead path C k Corresponding to the movement of printhead 230. More precisely, printhead path C k This can be understood as a trajectory traced by the projection of a reference point onto the surface 290 of the printhead 230. The reference point could be, for example, the center of the pixel arrangement of the printhead 230. Regarding image strip A... k Printhead path C k It can be an approximate centerline.

[0036] Figure 5 illustrates the... Figure 5A In the case where image A is not a straight line (curved) and is split into two image strips A1 and A2, and the associated printhead paths C1 and C2, as shown below. Figure 5B As shown, image strips A1 and A2 have non-straight boundaries, but on the other hand, they have relatively regular geometry relative to the longitudinal components, thus allowing the printhead 230 to be fully utilized along the entire length of each image strip A1 and A2. In the case shown in Figure 5, non-linear splitting is advantageous due to the elongated and curved shape of the resulting image A; conceivably, linear splitting of image A would produce smaller and more numerous straight image strips, where abrupt, non-orthogonal or slanted ends would be produced when only a portion of the printhead 230 is utilized.

[0037] In the third step 114 of method 100, when the robot arm is fed a first control signal, a deviation from the printhead paths C1, C2, C3 is detected. The first control signal can command the robot arm 220 to move the printhead 230 along paths C1, C2, C3. The third step 114 can be performed during dry operation, but it can also be performed during printing operations, including production operations. Figure 4C The illustration shows a possible result of the third step 114, where the first printhead path C1 and the third printhead path C3 have only negligible deviations. However, for the second printhead path C2, a deviation is detected. This deviation is not constant over the length of path C2. It can be represented as a sign difference function d2(y) between the printhead path C2 and the actual trajectory of the printhead 230, where for each longitudinal coordinate y, the difference d2(y) is measured in the lateral direction x.

[0038] For a planar surface 290, deviation detection is very simple. If surface 290 is curved, the deviation at that point on surface 290 can be detected in the tangential plane at that point. In the tangential plane, deviation detection is also constrained to the lateral direction, that is, the deviation should be orthogonal to the printhead path.

[0039] In step 116, the image stripes are modified to compensate for the detected deviations by applying a local lateral offset. In the running example, since no deviations from the first printhead path C1 and the third printhead path C3 are detected, the first image stripe A1 and the third image stripe A3 do not need modification. However, for the second image stripe A2, it is suitable to apply a variable local lateral offset over the length of path C2, thereby eliminating the deviation. The purpose of the compensation is to terminate the pixel pattern of the second image stripe A2 at or near its intended position on surface 290. The local lateral offset can be, for example, the negative value of the difference function -d2(y). Figure 4D The left and right contours of the region with active pixels in the second image strip A2 are shown after this offset is applied.

[0040] In further developments of step 116, this is achieved by modifying not only the associated image strip A in 116.1. k Furthermore, modify one or two adjacent image stripes A in 116.2. k±1 This allows for more efficient and / or less disruptive compensation with a printhead path C. k The deviation. This can, for example, make it possible to compensate for relatively large deviations. Figure 4D As illustrated, the lateral offset -d2(y) causes the area with active pixels to almost touch the right boundary of image strip A2, which is within the feasible limit of printhead 230. In the notation described above, the width of the right edge...

[0041]

[0042] The local values ​​are brought close to zero. Therefore, in the development mentioned in step 116 of the fourth step, the deviation is compensated across several image strip distributions by the following: shifting the first image strip A1 by a constant D, shifting the second image strip A2 by D-d2(y) units, and shifting the third image strip A3 by D units. By selecting D, the deviation is made to... This joint offset of the image stripes ensures that for all y, w R >0, although the cost is to shift the printed image A on surface 290 by D units.

[0043] In other embodiments of step 116, deviations of all detected printhead paths C1, C2, and C3 are considered together. For example, compensation can be found by solving a system of equations. Separate systems of equations can be formulated and solved for different longitudinal segments of printhead paths C1, C2, and C3, where smoothness conditions (repair conditions) between consecutive segments can be applied. Alternatively or additionally, modified image stripes can be obtained by solving the optimization problem using an objective function that takes into account the detected deviations and penalizes large local lateral offsets and / or poor alignment of the image stripes.

[0044] In an optional step 118 of method 100, the modified image stripe is printed onto surface 290 while a control signal equivalent to the first control signal is fed to robot arm 220. Step 118 can be performed during production operations. Since it can be reasonably assumed that the systematic (non-accidental) components of the deviation of printhead 230 from printhead paths C1, C2, C3 will repeat in almost the same manner, effective near-complete compensation can be expected.

[0045] The foregoing has primarily described various aspects of this disclosure with reference to some embodiments. However, as will be readily apparent to those skilled in the art, other embodiments besides those disclosed above are also possible within the scope of this invention, as defined in the appended claims.

Claims

1. A method (100) for printing an image (A) onto a surface (290) using a print head (230) carried by a robotic arm (220), the method comprising: Obtain an image (110) that exceeds the predetermined print width (w) of the print head. The image is split (112) into at least two image strips (A1, A2, ...) and associated printhead paths (C1, C2, ...) are generated, each image strip being contained within a reduced print width (w0) that is smaller than the predetermined print width; When the robot arm is fed a first control signal, the deviation between the detection (114) and the print head path is detected, and the first control signal commands the robot arm to move the print head along the print head path; as well as The image stripes (116) are modified by applying a local lateral offset to compensate for the detected deviation.

2. The method according to claim 1, further comprising: When a control signal equivalent to the first control signal is fed to the robot arm (220), a modified image stripe is printed (118) onto the surface (290).

3. The method according to claim 1 or 2, wherein the deviation is detected during dry operation (114).

4. The method according to claim 1 or 2, wherein the deviation is detected during the printing run (114).

5. The method according to claim 1 or 2, wherein the deviation is detected (114) using a detachable position sensor (232).

6. The method according to claim 1 or 2, wherein the deviation is detected (114) using a position sensor (232) with a tolerance smaller than that of the robot arm.

7. The method according to claim 1 or 2, wherein the deviation is detected (114) using a position sensor (232) with a tolerance of 0.1 mm or less.

8. The method according to claim 1 or 2, wherein the deviation is detected using an optical or laser-equipped position sensor or camera system (114).

9. The method according to claim 1 or 2, wherein the printhead (230) has a plurality of individually controllable pixels, the pitch (e) defined by the plurality of individually controllable pixels being smaller than the tolerance of the robotic arm (220).

10. The method according to claim 1 or 2, wherein the modification (116) comprises: Modify (116.1) and the detected deviation in the printhead path (C) k At least the corresponding image strip (A) k );as well as Modify (116.2) at least one adjacent image stripe (A k±1 ).

11. The method according to claim 1 or 2, wherein the modification (116) comprises: Take into account the detected biases together.

12. The method according to claim 1 or 2, wherein the modification (116) comprises: A lateral offset (d) is applied, which is at most equal to the difference between the print width (w) and the reduced print width (w0).

13. The method according to claim 1 or 2, wherein the printhead (230) comprises an inkjet head and / or a spray gun.

14. A controller (210) adapted to control a printhead (230) carried by a robotic arm (220) to print an image (A) onto a surface (290), said controller (210) comprising: The first interface (212) is configured to receive image data and position sensor signals; The second interface (214) is configured to output printhead control signals; and The processing circuit (216) is configured to perform the method according to any one of claims 1 to 13.

15. A computer program comprising instructions for causing a controller (210) according to claim 14 to perform the method (100) according to any one of claims 1 to 13.

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