WO2020041221A1

WO2020041221A1 - Customized robotic grippers with feedback sensors

Info

Publication number: WO2020041221A1
Application number: PCT/US2019/047124
Authority: WO
Inventors: Daniel Rau; Mark R. BURHOP; Reed Williams; Elena Arvanitis; Livio Dalloro
Original assignee: Siemens Aktiengesellschaft; Siemens Corporation
Priority date: 2018-08-20
Filing date: 2019-08-20
Publication date: 2020-02-27

Abstract

A robotic gripper includes a pair of gripper fingers constructed by an additive manufacturing process using a dual extruder to print a conductive material and a flexible nonconductive material. Each of the gripper fingers is composed of the nonconductive material, and a gripping surface of at least one of the gripper fingers includes an embedded sensor comprising traces of a conductive material forming a ground electrode and a sensor electrode. The embedded sensor integrates a capacitive sensor that provides a capacitance measurement and a strain sensor that provides a strain measurement of gripping surface deformation.

Description

CUSTOMIZED ROBOTIC GRIPPERS WITH FEEDBACK SENSORS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 62/719,748, filed on August 20, 2018, U.S. Provisional Application No. 62/719,738, filed on August 20, 2018, and U.S. Provisional Application No. 62/719,741 , filed on August 20, 2018, the content of which is incorporated herein in its entirety.

TECHNICAL FIELD

[0001] This application relates to additive manufacturing. More particularly, this application relates to construction of flexible robotic grippers of a customized shape with embedded feedback sensors.

BACKGROUND

[0002] Commercially available robotic grippers can be costly. Consequently, such grippers are designed with a basic shape intended to accommodate a wide variety shaped objects, without any specialized capability. Such grippers struggle with objects having odd shapes, including round or curved surfaces. Due to the high cost of the grippers, it is prohibitive to manufacture customized grippers using traditional manufacturing methods (i.e. molding, machining). Also, embedding features within grippers using traditional manufacturing methods requires additional assembly.

[0003] Recent proposed approaches using 3D printed grippers includes systems with dual extruders to enable the deposition of both conductive and non-conductive materials to create a single feedback sensor (e.g., a capacitive sensor to sense liquid level, a strain sensor to sense contact with an object). However, such sensors are highly sensitive to factors that reduce their ability to accurately sense. A 3D printed strain sensor is affected by noise, change in resistance over time due to electrical heating, and repeatability issues. A 3D printed capacitive sensor is also affected by noise and interference and their functionality is greatly affected by the material they are sensing (i.e. plastic is much harder to sense than a human due to its substantially lower permittivity). These factors can reduce the sensor's ability to provide accurate and reliable contact measurements.

SUMMARY

[0004] Aspects according to embodiments of the present disclosure include using additive manufacturing to construct grippers with customized shapes based on what object form is to be gripped and 3D printed with embedded feedback sensors. The feedback sensors are used to determine whether an object has been grasped by the gripper. Use of additive manufacturing allows the gripper to be manufactured completely autonomously and in a wide variety of shapes and sizes while minimizing the human labor needed to produce the gripper. In addition, a modular design of the actuation system for the gripper allows differently shaped grippers to be quickly installed and removed. An integration of a capacitive and strain sensor into at least one sensor allows the redundant sensing of contact with other objects. By continuously sampling both strain and capacitive data from the sensor, the reliability of the sensor to detect contact with a wide variety of objects in a wide variety of conditions is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Non-limiting and non-exhaustive embodiments of the present embodiments are described with reference to the following FIGURES, wherein like reference numerals refer to like elements throughout the drawings unless otherwise specified.

[0006] FIG. 1 shows a first example of a modular gripper with symmetrical grips and an actuation assembly produced by an additive manufacturing process in accordance with embodiments of the disclosure.

[0007] FIG. 2 shows a second example of a modular gripper with one curved grip and an actuation assembly produced by an additive manufacturing process in accordance with embodiments of the disclosure.

[0008] FIG. 3 shows an example of a printed sensor for embedding in the gripper in accordance with embodiments of the disclosure.

[0009] FIG. 4 shows an example section of a gripper finger having an embedded sensor in accordance with embodiments of the disclosure.

[0010] FIG. 5 is graphical diagram of an example for sensor response during gripper operation in accordance with embodiments of the disclosure.

[0011] FIG. 6 shows an example for a mounting configuration for an alternative actuation in accordance with embodiments of the disclosure.

[0012] FIG. 7 shows an example of an enhanced strain sensor configuration in accordance with embodiments of the disclosure. DETAILED DESCRIPTION

[0013] Methods and systems are disclosed for using additive manufacturing to produce customized shapes for grippers that allow more reliable grasping of objects. Printing of a customized gripper can be achieved rapidly (e.g., in a few hours) with no human labor required. Differently shaped grippers can quickly be changed out using a modular design. Embedded feedback sensors provide a signal indicating that the gripper is holding an object while not diminishing the object grasping functionality of the gripper, or without necessitating enlargement of the gripper to accommodate the sensors.

[0014] FIG. 1 shows a first example of a modular gripper with symmetrical grips and an actuation assembly produced by an additive manufacturing process in accordance with embodiments of the disclosure. An additive manufacturing process, such as 3D printing via fused deposition modeling may be employed to produce the gripper 101. In an embodiment, a 3D printer with micron thickness of 50, 100, or 200 microns may used to fabricate the gripper 101.

[0015] The gripper 101 includes an actuation assembly 1 15, which may include one or more hinged parts, each part separately printed and shaped with interlocking structures such as gear sprockets and hinges which can be assembled by metal fasteners. The actuation assembly 1 15 may include a coupling point 1 16 for engagement with a motor shaft. The gripper 101 , includes two gripper fingers 1 1 1a, 1 1 1 b, which may be symmetrical as shown in FIG. 1. In an embodiment, the gripper fingers 1 1 1a, 1 1 1 b are composed of a flexible material that can conform to the object being grasped. As an example, a polylactic acid material may be used to print the gripper fingers 1 1 1 a, 1 1 1 b. The flexible material may be modified to increase stiffness for supporting a grasp of heavier objects. In this example, gripper fingers 111a, 111b are modeled in the Fin Ray design, which is useful as a general gripper for a robot to pick up a wide range of objects. One improvement provided by the gripper 101 is the modular design which allows for quick exchange of one or both of gripper fingers 111a, 111b with a different custom printed finger, such as shown in FIG.2. To replace fingers 111a, 111b, fasteners (e.g., such as metal pins, dowels, screws) at coupling points 112, 113 may be disengaged for quick disassembly and then replaced for reassembly. In an embodiment, actuator assembly 115 may be omitted, and gripper fingers 111a, 111b may be attached, via fasteners at coupling points 112, 113, to an existing gripper actuator as replacements for rigid robotic gripper fingers. Coupling points 112, 113 may be holes fabricated in the material during the 3D print process, or may be drilled after the gripper fingers 111a, 111b are printed. Accordingly, retooling for the robotic gripper is improved.

[0016] FIG.2 shows a second example of a modular gripper with one curved grip finger and an actuation assembly produced by an additive manufacturing process in accordance with embodiments of the disclosure. The gripper 201 is produced by an additive manufacturing process as described above for gripper 101. The gripper 201 shown in FIG.2, however, has an improved tandem of gripper fingers 211a, 211b in that finger 211a is 3D printed to a custom shape conducive to picking up curved objects. In an embodiment, the curve of finger 211a may have a diameter that is approximately identical to one or more objects intended to be grasped by the gripper 201, for a customized fit. Placement of coupling points 213 is in a location and pattern for matching coupling points 1 13 of gripper finger 1 1 1 a, allowing replacement gripper finger 21 1 a to be easily swapped.

[0017] The gripper finger topology as shown in FIGs. 1 and 2 are presented as non- limiting examples, which may be substituted with the gripper surface having one or more different geometric profiles and tandem combinations customized as needed to conform to objects that are to be grasped, are within the scope of the described embodiments. For example, a tandem of two curved gripper fingers similar to finger 21 1 a may be constructed and deployed.

[0018] FIG. 3 shows an example of a printed sensor for embedding in the gripper 101 , 201 in accordance with embodiments of the disclosure. Printed sensor 300 includes traces of a ground electrode structure 301 and a sensor electrode 302 and terminals 303 for connection to an external control circuit for the robotic gripper (e.g., screw terminals for connection to external wiring). During the additive printing process, a filament deposition modeling system with dual extruders enables the deposition of both conductive material for sensor 300, and a non-conductive material (e.g., to create a dielectric layer) to construct the embedded sensor 320. The additive manufacturing process may be performed in separate stages for each of the conductive material structure 31 1 and nonconductive material structure 312, and then physically assembled as a separate step to produce an embedded sensor assembly 320. Alternatively, dual extruders may print both layers as a single print process to construct the embedded sensor 320. In an embodiment, the nonconductive structure 312 is constructed with a material that is substantially thermoplastic polyurethane (e.g. , a flexible thermoplastic polyurethane). In other embodiments, the nonconductive structure 312 may be constructed with another material that is flexible, such as for example a silicone compound or a rubberized material. The traces for conductive material structure 31 1 may be created by printing a material having infused conductive carbon particles. In an embodiment, the binder material for the carbon may be substantially polylactic acid. Alternative substitutes for the polylactic acid include other thermoplastic polymers such as acrylonitrile butadiene styrene (ABS) and nylon. In an embodiment, the electrical resistivity of the printed sensor elements is approximately 0.15 ohm-meter or within a range of 0.15 and 0.30 ohm-meter.

[0019] FIG. 4 shows an example section of a gripper finger 400 having an embedded sensor 420 in accordance with embodiments of the disclosure. The gripper finger structure 41 1 is constructed by printing additional layers of the nonconductive material 312 of FIG. 3 in an additive manufacturing process. Printed in a manner as described above with embedded sensor 320, the embedded sensor 420 shown in FIG. 4 is arranged on the gripping surface of a gripper finger structure 41 1 . Other gripper finger structures, such as curved gripper finger 21 1a, may be constructed in a similar manner to integrate an embedded sensor, such as sensor 220 shown in FIG. 2.

[0020] Embedded sensor 420 may integrate two types of sensors for a robust feedback control of the gripper to determine whether an object has been grasped. In an embodiment, the sensor 420 is capable of continuously sampling both strain and capacitance as redundant sensing to allow for a more accurate detection of a wide range of objects. In an embodiment, a capacitive sensor is formed using the combination of sensor electrode structures 401 and 402 by grounding sensor electrode 401 and energizing electrode 402, and measuring the detected change in capacitive field as the sensor approaches an object to be grasped. The capacitive measurement may be enhanced in the sensor circuit using external circuit components which may include a Wheatstone bridge to enhance the low voltage signal for more accurate readings. Terminals 403 may be used to electrically couple the sensor electrodes 401 , 402 to the remainder of the sensor circuit either directly or via wire jumpers to robotic electrical terminals typically arranged in the proximity of the gripper assembly. The strain sensor of the dual embedded sensor 420 is formed by applying electrode 401 as a resistor within a strain sensor circuit, where strain direction 410 is measured by detecting a change in resistance as electrode 401 is stretched and deflected along the strain direction 410. The strain occurs when the flexible gripper finger 41 1 flexes in response to gripping surface 414 wrapping around a grasped object 425. The capacitive sensor provides a capacitance measurement used to determine if an object is in close proximity to the sensor while the strain sensor provides a strain measurement of the surface deformation of the gripper finger used to determine if the gripper is deforming to hold the object 425. The footprint for the embedded sensor 420, with integrated dual sensor types, may be scaled to occupy the same amount of area as would a single sensor using conventional techniques. Accordingly, there is no detriment in implementing the additional sensing capability of the integrated sensor.

[0021] FIG. 5 is graphical diagram of an example for sensor response during gripper operation in accordance with embodiments of the disclosure. In an embodiment, once a gripper 101 , 201 is deployed on the robot and the embedded sensor 320 is electrically coupled to a monitor and control circuit, a strain sensor measurement 501 and capacitive measurement 502 may be monitored and displayed as shown in FIG. 5. The benefit of the integrated sensors is demonstrated by the dual feedback response that can be analyzed by a controller using the robust feedback to provide improved gripper control compared to conventional gripper control techniques that rely on a less reliable, single sensor feedback. Sensing of grounded objects and ungrounded objects can be distinguishable by the relative difference between capacitive sensor measurements at 51 1 , 512 and 514. The first five pulses at 511 represent five gripper actuations sensor feedback during five gripper actuations as the gripper sensor senses a grounded, conductive object resulting in an increase in capacitance when the sensor neared the object signaling the motor to stop and reverse direction. At 512, a nonconductive object is substituted for grasping, demonstrated by the lower capacitive sensor measurements at 512 and 514 when detecting the capacitive object. At 513, the strain sensor measurements take over for control of the gripper, rapidly increasing until reaching a threshold signaling the motor to stop and reverse direction for a set distance to avoid excessive compression of the object and to protect the motor from overloading.

[0022] FIG. 6 shows an example for a mounting configuration for an alternative actuation in accordance with embodiments of the disclosure. In an embodiment, the actuation for gripper fingers 1 1 1 a, 11 1 b may be in an arrangement controlled by rigid fingers of the robot instead of a single small motor mount, in which case the actuator assembly 115 may be omitted. With this example, gripper fingers 11 1 a, 1 1 1 b are fabricated to be appended onto rigid robotic fingers 151 already integrated with an actuator, coupled as accessory extensions to the existing robotic fingers. Couplers 152 may be constructed by an additive manufacturing process (e.g., the same process used to print the gripper fingers 1 1 1a, 1 1 1 b) for mating with rigid fingers 151 of a robot hand 155. For example, couplers 152 may have a void region shaped to tightly wrap around rigid robotic fingers. Gripper fingers 1 1 1 a, 1 11 b may then be attached to the couplers 152 using a fastener at the coupling points 113. As an alternative embodiment, gripper fingers 1 11 a, 1 1 1 b may be constructed to mate directly to a rigid robotic finger 151 by a similar void region as coupler 152 (e.g., integrating a coupler 152 during the additive manufacturing process, eliminating the need for fasteners). Accordingly, the modular configuration of gripper fingers 1 1 1 a, 1 1 1 b is advantageous for flexibility in deployment, in this case converting a rigid gripper to a "soft" gripper with the sensor enhancements embedded onto one or both gripping surfaces 1 14 as described above.

[0023] FIG. 7 shows an example of an enhanced strain sensor configuration of the dual sensor electrodes according to embodiments of this disclosure. Electrodes 701 , 702 are printed and arranged in a manner similar to that described above for electrodes 401 , 402. In an embodiment, electrodes may be arranged to form layers, such as electrode 703 as shown in FIG. 7, which can be printed in a layer above or below the layer containing electrodes 701 , 702. The combination of electrodes 701 and 703 may form an enhanced strain sensor by which electrode 703 can sense a change in resistance when the gripper surface is deflected in a direction transverse to the electrode 701. For example, electrode 701 may sense strain in a lengthwise direction along gripper finger (i.e. , to sense strain along the length of gripper finger) while electrode 703 may sense strain across the height of the gripper finger (or object's top/bottom) direction. Such diverse strain detection may be advantageous, for example, when an object to grasp is shaped narrowly along the gripper finger, such as a rod-

-I Q- shaped object 71 1 , and enhances the grasping detection for the gripping surface deflection in a vertical flex 712 around such object.

[0024] The system and processes of the figures are not exclusive. Other systems, processes and menus may be derived in accordance with the principles of the invention to accomplish the same objectives. Although this invention has been described with reference to particular embodiments, it is to be understood that the embodiments and variations shown and described herein are for illustration purposes only. Modifications to the current design may be implemented by those skilled in the art, without departing from the scope of the invention. As described herein, the various systems, subsystems, agents, managers and processes can be implemented using hardware components, software components, and/or combinations thereof. No claim element herein is to be construed under the provisions of 35 U.S.C. 1 12(f), unless the element is expressly recited using the phrase“means for.”

Claims

CLAIMS What is claimed is:

1. A product-by-process for constructing a robotic gripper by additive

manufacturing, comprising:

a pair of gripper fingers constructed by an additive manufacturing process using a dual extruder to print a conductive material and a flexible nonconductive material, wherein each of the gripper fingers is composed of the nonconductive material, and a gripping surface of at least one of the gripper fingers includes an embedded sensor comprising traces of a conductive material forming a ground electrode and a sensor electrode;

wherein the embedded sensor integrates a capacitive sensor that provides a capacitance measurement and a strain sensor that provides a strain

measurement of gripping surface deformation.

2. The product-by-process of claim 1 , wherein the dual extruder uses a filament deposition modeling system to print.

3. The product-by-process of claim 1 , wherein the nonconductive material is substantially thermoplastic polyurethane.

4. The product-by-process of claim 1 , wherein the conductive material is

substantially polylactic acid containing conductive carbon particles.

5. The product-by-process of claim 1 , wherein at least one of the gripper fingers is constructed to produce the gripping surface having a curvature with a diameter approximately identical to one or more objects to be grasped by the robotic gripper.

6. The product-by-process of claim 1 , wherein at least one of the gripper fingers is constructed to produce the gripping surface having a geometric profile customized to conform to one or more objects to be grasped by the robotic gripper.

7. The product-by-process of claim 1 , wherein the embedded sensor comprises a first electrode and a second electrode arranged on the gripping surface to form a capacitive sensor to perform mutual capacitive sensing of non-grounded objects, wherein the strain sensor uses the first electrode to detect deflection of the gripping surface.

8. A robotic gripper, comprising:

a pair of modular gripper fingers constructed of a flexible nonconductive material, wherein a gripping surface of at least one of the gripper fingers includes an embedded sensor comprising traces of a conductive material forming a ground electrode and a sensor electrode;

measurement of gripping surface deformation.

9. The robotic gripper of claim 8, wherein the strain sensor comprises a first electrode arranged to sense strain along the length of the gripper finger and a second electrode to sense strain across the height of the gripper finger.

10. The robotic gripper of claim 9, wherein the coupling point is configured for coupling as an accessory extension to a rigid finger of a robot.

1 1 . The robotic gripper of claim 9, wherein the coupling point is configured for fastening to an actuator as a replacement for a rigid finger of a robot.

12. The robotic gripper of claim 8, wherein, wherein the nonconductive material is substantially thermoplastic polyurethane.

13. The robotic gripper of claim 8, wherein, wherein the conductive material is substantially polylactic acid containing conductive carbon particles.

14. The robotic gripper of claim 8, wherein at least one of the gripper fingers comprises a gripping surface having a curvature with a diameter approximately identical to one or more objects to be grasped by the robotic gripper.

15. The robotic gripper of claim 8, wherein the embedded sensor comprises a first electrode and a second electrode arranged on the gripping surface to form a capacitive sensor to perform mutual capacitive sensing of non-grounded objects, wherein the strain sensor uses the first electrode to detect deflection of the gripping surface.