Systems and methods for providing contact detection in an articulated arm

A sensing manipulator of an articulated arm is disclosed. The sensing manipulator includes a compliant section and a movement detection system provided along a first direction of the compliant section such that movement of the compliant section along both the first direction and at least one direction transverse to said first direction, are detectable by the movement detection system.

BACKGROUND

The invention generally relates to robotic and other sortation systems, and relates in particular to articulated arm systems for use in sortation systems.

Systems and methods using mechanical compliance to improve robot performance during grasping and manipulation are known. Purpose-built compliant elements exist commercially that function as safety guards, such as, for example, position sensors sold by ABB Automation Technology Products AB of Sweden. These devices may include magnetic breakaway or spring elements that deflect when contact between the robot and the environment is made. Additionally, these designs can include rudimentary on/off sensing of a breakaway state, which is often used as a stop signal to the robot controller.

More modern robotic systems in industry and academia have incorporated flexible elements and deformation sensors in the joints of a robot arm (see for example, the Baxter Robot sold by Rethink Robotics, Inc. of Boston, Mass. and the DLR Lightweight Robot III developed by the Institute of Robotics and Mechanics at German Aerospace Center in Germany). Through the combined sensing of deformation at each joint, an approximation of the force at the end-effector may be deduced. Such an implementation is undesirable in certain applications however (for example, due to unnecessary added compliance that may degrade the positional accuracy of the end-effector, added mechanical complexity and cost, and decreased payload capabilities of the robotic system), with the added complication that any highly flexible end-effector on the robot arm causes the loads transmitted through to the joints to be fairly small and difficult to reliably measure.

Force sensors are also known to be used in robotic manipulation systems. A typical force sensor consists of a rigid plate instrumented with several micro-scale deformation sensors such as strain gauges. This plate is commonly placed between the robot end-effector and the robot arm, and used to sense forces and torques acting on the end-effector. These sensors tend to be expensive and difficult to calibrate accurately since they measure deflections or strain on very small scales. Furthermore, a force sensor mounted between the end-effector and robot arm suffers from the issue mentioned above for joint-sensors, namely that highly flexible elements on the end-effector will not create significant forces for detection at the force sensor.

There remains a need therefore for an improved sensing system for robotic and other sortation systems.

SUMMARY

In accordance with an embodiment, the invention provides a sensing manipulator of an articulated arm. The sensing manipulator includes a compliant section and a movement detection system provided along a first direction of the compliant section such that movement of the compliant section along both the first direction and at least one direction transverse to said first direction, are detectable by the movement detection system.

In accordance with another embodiment, the sensing manipulator includes a compliant section providing movement of the compliant section in at least two degrees of freedom, and a movement detection system providing output data regarding movement of the compliant section in the at least two degrees of freedom.

In accordance with a further embodiment, the invention provides a method of sensing the position an orientation of an object held by a manipulator at an end effector of a robotic system. The method includes the steps of engaging the object in a working environment of the robotic system, perceiving an initial position of a movement detection system, lifting the object against gravity, and perceiving at least two of load, pitch, roll and yaw of the object with respect to the initial position of the movement detection system.

The drawings are shown for illustrative purposed only.

DETAILED DESCRIPTION

The invention provides in accordance with an embodiment, a novel sensing manipulator that tracks the physical deformation of a robot end-effector as it makes contact with an environment, including an object within the environment. Many robot end-effector designs rely on flexible passively-compliant elements that deform to accommodate the environment. This compliance is used to improve the quality and reliability of contact during grasping and manipulation, and to reduce the impact loads applied to both the robot and objects during contact.

The novel sensing manipulator discussed herein in accordance with certain embodiments tracks these various modes of deformation, and provides this information for use in higher-level automation software to determine significant details about the state of end-effector contact with the environment. This mode of sensing eliminates the need for an additional complex mechanical element traditionally used to sense forces or add compliance to a robot system, while minimally altering the stiffness and inertia of the pre-existing hardware. Placing the sensor as close as possible to the contact site, in accordance with an embodiment, ensures it is able to obtain signals relevant to the manipulation task unaltered by the dynamics of transmission through the robot structure.

In accordance with certain embodiments, sensing manipulators of the present invention may have several primary features with many ancillary benefits, summarized here and discussed in more detail below.

The position deformation sensor design methodology provides A) a sensing strategy that can sense the deformation of a compliant element along multiple axes simultaneously, B) a sensing system that can be applied to a variety of pre-existing compliant elements and eliminates the need for new mechanical complexity along the serial chain of a robot arm, C) a sensor solution that minimally affects the stiffness or inertia of existing compliant elements, and D) a sensor that is placed near the end-effector contact surface to obtain data that is both highly sensitive and is unaltered by the dynamics of force transmission through the robot.

The novel software and algorithms of certain embodiments of the invention further provide A) software strategies that use the sensor information to detect the presence or absence of contact with the world, and B) software strategies that detect the amount of force and torque imparted on the end-effector due to the external load of the object and grasping configuration.

This general approach of deflection sensing and algorithms applied to process the resultant data, is illustrated via several examples as follows. The design and methodology may be understood initially by considering a simplified illustration of the deflection sensor design as shown inFIG. 1.FIG. 1shows a deformation sensor application diagram in accordance with an embodiment of the present invention, where the deformation sensor is positioned adjacent the environment such that the sensing of the deflection sensor ofFIG. 1occurs at the point of contact with the environment.

In particular, the robotic system10includes a movement detection system12such as a deflection sensor that is provided with a compliant interface14such as a vacuum cup, for engaging an environment16. The movement detection system12and the compliant interface14are coupled to an end effector18attached to a robotic mass20of the robotic system. The compliant interface may be formed in a shape of a tubular or conical bellows using a flexible material as shown at14and14ainFIGS. 2A and 2Brespectively. Note that the compliant interface may move in not only a direction as shown at A, but may also move in second directions shown at B (as shown) and D (into and out of the page) that are transverse to the first direction, as well as directions as shown at C that are partially transverse to the first direction. Also note the compliant interface is not necessarily a part of the deflection sensor itself, but may, in certain embodiments, be a natural part of the manipulation system.

The deformation sensor may be applied to systems where the deformation is not tightly constrained but rather provides multi-axis sensing, meaning that deformation may occur linearly, rotationally, or along complex paths. The ability to allow for and sense this complex deformation is a key differentiator from prior art systems. Several technologies can be applied to provide sensors to the compliant interface. It is important that this sensing not restrict or impede the compliant motion, or add significant inertia or mass. Several sensors could be applied to measure the deformation including but not limited to; flex sensors (such as flex-sensitive resistors or capacitive sensors), magnetic field sensors (such as a compass or hall-effect sensors), or potentiometers.

FIG. 3shows a sensing manipulator30in accordance with another embodiment of the invention wherein the sensing manipulator includes a movement detection system32. The movement detection system32includes a static 3-axis magnetic field sensor34that is aligned against a magnet36attached to the central part of the compliant cup38by a ring50. A vacuum is provided at an open end46of the complaint cup38. As the compliant cup38moves, so too does the ring40. As the ring40around the cup moves, so too does a bracket42as well as a magnet46, which movement is detected with respect to the magnet sensor44attached to the articulated arm54for sensing the axial flexure of the vacuum cup from which translations/roll/pitch/of the cup. When the magnetic field sensor is employed, the system may determine not only movements in the elongated direction (x) of the deflection sensor with respect to the articulated arm, but also movements in directions (y and z) that are transverse to the elongated direction of the deflection sensor as well as directions that are partially transverse to the elongated direction of the deflection sensor.

With reference toFIG. 4, in accordance with a further embodiment, the system may include an articulated arm80to which is attached an end effector82, again, which may be a tubular or conical shaped bellows. The end effector82also includes a sensor84that includes an attachment band86on the bellows, as well as a bracket88attached to magnetic field sensor90, and a magnet92is mounted on the articulated arm80. As the bellows moves in any of three directions (e.g., toward and away from the articulated arm as shown diagrammatically at A, in directions transverse to the direction A as shown at B, and directions partially transverse to the direction A as shown at C. The magnetic field sensor90may communicate (e.g., wirelessly) with a controller90, which may also communicate with a flow monitor94to determine whether a high flow grasp of an object is sufficient for continued grasp and transport as discussed further below. In certain embodiment, for example, the system may return the object if the air flow is insufficient to carry the load, or may increase the air flow to safely maintain the load.

FIGS. 5A and 5Bshow an object160being lifted from a surface162by the end effector82that includes the load detection device ofFIG. 5. Upon engaging the object160, the system notes the position of the detection device. Once the object160is lifted (FIG. 5B), the system notes the change in the sensor output. In this example, the load provided by the object160is relatively light.FIGS. 6A and 6B, however, show the end effector lifting a heavy object.

FIGS. 6A and 6Bshow an object170being lifted from a surface172by the end effector82that includes the load detection device ofFIG. 5. Upon engaging the object170, the system notes the position of the detection device. Once the object170is lifted (FIG. 6B), the system notes the change in the position of the detection device. As noted above, in this example, the object170is heavy, presenting a higher load.

The system may also detect whether a load is not sufficiently balanced.FIGS. 7A and 7Bshow an object180being lifted from a surface182by the end effector82that includes the load detection device ofFIG. 4. Upon engaging the object180, the system notes the position of the detection device. Once the object180is lifted (FIG. 7B), the system notes the change in the position of the detection device. In this example, the object180presents a non-balanced load. The compliant element may therefore, undergo substantial translational and angular deformation.

Various further platform applications include the following. The deformation sensor concept is designed to integrate with existing passive and active compliant components of a robot end-effector. In the above embodiments, suction cups are used as examples of compliant members. Many different compliant elements however, could be used based on the end-effector selected. In accordance with a further embodiment, the invention provides a movement detection system that includes force-sensitive resistors.FIGS. 8 and 9, for example, show a sensing manipulator200together with a vacuum cup202wherein the movement detection system includes an array (e.g., three) of detectors204for sensing the axial flexure of the vacuum cup from which translations/roll/pitch/of the cup can be deduced. In particular, the force-sensitive resistors may include a conductive polymer that is printed on a surface, wherein the conductive polymer changes it resistance in a predictable manner when a force is applied to the surface. The sensing manipulator200may be attached to a robotic arm via a mounting element208(which couples to a robotic arm mount that passes between two of the detectors204). A vacuum may be provided at an open end206of the vacuum cup202for engaging an object210(as shown inFIG. 9).

Another such alternative compliant element example is the use of a two-fingered robot gripper either at the wrist (as shown inFIG. 10) or on the finger tips (as shown inFIGS. 11A and 11B). Normally compliance is built in at the fingertips or directly behind the wrist of the gripper. A deflection sensor could easily be adapted to accommodate similar alternative designs. In particular,FIG. 10shows a sensing manipulator220in accordance with a further embodiment of the present invention that is attached to a robotic arm222. The sensing manipulator220includes a compliant section224and a sensing section226that includes a two finger gripper end effector228. As shown at D and E, the sensing section226may provide sensing of the position and orientation of the end effector228with respect to the robotic arm222, e.g., by magnetic or capacitive sensing.

FIG. 11shows a sensing manipulator230that is attached to a robotic arm232. The sensing manipulator230includes a gripper234that includes two jaws236. On or both jaws is provided a compliant element238, and on the compliant element238is provided a magnet242. With further reference toFIG. 12(which shows an enlarged view of a portion of one jaw236) a corresponding magnetic sensor240is provided on the jaw. When the compliant element238is under a load (as shown by a force as shown at F), the sensor242will move respect to the sensor240, providing position and orientation sensing data.

The stiffness and sensitivity of the compliant material are also important considerations. Note fromFIG. 1that the location of sensing is along the preexisting compliant structure of the robot system. This allows a system using the deformation sensor to maintain it's original stiffness and compliance properties, unlike prior art solutions. Also important to note is the target location for the deformation sensor in the system. The more distal the sensor is the closer it is to the interaction point, where non-linear complicating effects from the robot are less significant.

The software may involve high-level automation software that uses the data output from the deformation to make a series of important decisions as follows.

Contact State

The most straightforward application of the sensor is thresholding the deformation values from the sensor to detect when contact with the world has occurred. If any axis of deformation moves outside nominal levels, then robot motion can be stopped and appropriate gripping strategy motions may be executed (such as pushing more or less on the environment as needed).

When approaching an object for grasping, a robot arm will often first make contact with the object by pushing into it (either intentionally or unintentionally). Compliance is often used in robotic systems by allowing the end-effector to passively re-adjust to the environment by bending against the contact point. By using the deformation sensor to sense this angle of deflection, and then actively controlling the robot to re-adjust and compensate for the deflection by re-positioning itself, grasps can be made more reliable and centered on the object.

Force Sensing

Given a model of how the compliant element deflects under load, the deformation changes may be mapped to forces and torques on the end-effector. This may allow for a number of force-sensing strategies, such as force-guided insertions and grasps, and force-guided placement of objects on surfaces.

Post-Grasp Centerpoint Sensing and Adjustment

Similar to the above two points, after an object is grasped and lifted, gravitational effects will cause the robot end-effector to deflect under the load. Depending on the location of the grasp point with respect to the center-of-mass of the object, this may cause various deformations in the compliant element of the end-effector. Also, a poorly chosen grasp location on a heavy object can induce oscillations between the compliant components and object. The deformation sensor would be capable of sensing both these effects, and may be used to guide the robot to accept or reject grasps and give important information about the direction of the misalignment.

Human and Robot Safety

Due to centripetal effects the end-effector is often the most dangerous point on a moving robot arm. During motions where no environmental interaction is expected the deformation sensor can be monitored for changes and the robot stopped when unexpected events occur. The deformation has advantages over the more traditional joint-level or wrist-level safety guards on a robot, since it is designed into the low-inertia low-mass endpoint of the robot, and has the potential to respond before any damage has been done to the robot, environment, or human obstacles.

The deformation sensing strategy presented here provides a framework that allows sensitive high-resolution sensing of contact between a robot and it's environment, while minimally altering the physical attributes of the robot's compliance. Given a model or properly tuned heuristics the sensor may be used to resolve important information for robot decision making to improve manipulation task performance.