Patent Description:
In high precision robotic drilling applications, for example in the aerospace industry, numerous holes may need to be drilled at various precisely defined locations in a component. The holes may need to be oriented at different angles as well as different positions, so a drilling tool having multiple axes of movement may be required. A robotic arm equipped with a drilling tool may be used for this purpose. While such robotic arms can achieve reasonable degrees of accuracy due to feedback from internal encoders on each joint, positional errors may occur in three main different ways. <FIG> illustrates the origin of three different types of possible errors in a robotic arm. Kinematic errors (<FIG>)) result in positional or rotational errors at each joint. Compliance and process forces (<FIG>)) result in errors arising from forces on the robotic arm that create displacement without necessarily resulting in any movement being measured. Backlash, or lost motion, errors (<FIG>)) result from slack being taken up in various gearing throughout the robotic arm. Each of these types of error may combine to result in a reduced positional and orientation accuracy when using a robotic arm for a drilling operation.

In robotic drilling operations, a clamp may be used to ensure that a drilling tool is held in position against a component to be machined. The clamp may be pneumatically actuated, applying a high pressure against the workpiece. The pressure can force the drill out of its intended position, leading to inaccurate positioning. In addition, such pressures may cause damage to the workpiece, which may not be evident if for example the workpiece is formed of a composite material.

<CIT> discloses a machining device including the features of the preamble of present claim <NUM>, for machining large-area, freely formed components on an aircraft, comprising a guide frame for guiding a tool spindle, a holding device for accommodating the tool spindle, wherein the guide frame is slidably mounted on the holding device via a linear bearing, a support element which can be connected or is connected to the guide frame for supporting the processing device on a surface of the component to be processed, an element for adjusting the contact pressure with which the processing device is pressed against the surface of the component, and a servo unit with which the guide frame relative to the holding device is displaceable. A method for machining large-area, free-form components is disclosed, comprising: positioning the machining device relative to the large-area, free-form component to be machined and pressing the machining device against the component with a contact pressure.

<CIT> discloses a drilling apparatus including: a drill rotatable about a center axis and capable of advancing and retracting along the axis; and a pressing unit for pressing a work in an advancing direction of the drill. The drilling apparatus advances the drill while rotating the drill about the center axis to form a hole in the work, in a state in which the work is pressed by the pressing unit. A pressing force applied on the work by the pressing unit is set to a predetermined pressing force based on machining reaction applied to the drill from the work during drilling and the pressing force causing deformation of the work in the advancing direction of the drill. The predetermined pressing force can suppress deformation of the work and displacement of the drill due to the machining reaction. The machining reaction and the pressing force are calculated beforehand in a drilling test.

In accordance with a first aspect of the invention, which is defined by the appended claims, there is provided a clamp configured for attachment to a drilling tool of a robotic drill, the clamp comprising:.

An advantage is that the combination of a servo motor driven actuation mechanism with a force sensor enables a controlled clamping force to be applied to the workpiece to be machined, reducing or controlling positioning errors that may otherwise result.

The workpiece contacting portion may have a non-slip surface for contacting the workpiece surface. The non-slip surface may for example comprise a rubber layer or coating. The rubber may for example be neoprene. Such a non-slip surface further reduces the possibility of the drilling tool moving out of position during a clamping operation.

The arms and end piece may be of unitary construction. The frame having a C-shape prevents deflection, distortion and/or skidding of the frame when the surface of the workpiece contacting portion contacts the workpiece and a clamping force is applied.

Rotation of the rods may be synchronised by a pulley or timing belt extending between the pair of arms. Synchronising rotation of the rods enables the frame to be actuated so that the force applied at the workpiece contacting portion is concentric with the drilling direction.

In accordance with a second aspect there is provided a robotic drilling system, comprising:.

The robotic drilling system may comprise a metrology system for measuring a position of the drilling tool relative to the workpiece to be machined. The metrology system may comprise a first plurality of datums located on the robotic drill and a second plurality of datums located on the workpiece holder. The controller may be configured to receive signals from the first and/or second plurality of datums to determine a position of the drilling tool relative to the workpiece.

The metrology system may be an optical metrology system, and the first and second plurality of datums may comprise optical emitters. The metrology system may comprise a plurality of optical sensors arranged to determine a position of the tool relative to the workpiece holder based on signals received by the optical sensors from the first and second plurality of optical emitters.

In a first mode, the controller may be configured to:.

The controller may be configured to determine and store a bias for a plurality of predetermined locations on the workpiece.

In a second mode, the controller may be configured to:.

The controller may be configured to measure a position of the drilling tool after actuating the servo motor and to update the stored bias for the predetermined location. The system may thereby be continually updated during drilling operations to maintain positional accuracy.

The use of the metrology system to determine a bias for each location on the workpiece to be machined allows an offset correction or bias to be made that is specific to each location, since different locations on a complex workpiece will result in different compliances coming into play on the robotic drill. For a robotic arm, for example, the compliance will differ depending on the orientation of the arm relative to the workpiece holder. By operating the robotic drilling system in the first mode, which may be run without any drilling operations taking place, a set of biases for each predetermined location where drilling is to take place can be determined for a given workpiece, which can be applied for subsequent workpieces of nominally identical structure, thereby improving overall accuracy and repeatability.

According to a third aspect there is provided a computer-implemented method for operating a robotic drill, the method comprising, in a first mode:.

The first mode of the method may be repeated for a plurality of predetermined locations on the workpiece.

The method may further comprise, in a second mode:.

The second mode of the method may be repeated for the predetermined locations on the workpiece.

According to a fourth aspect there is provided a method for operating a robotic drill, the method comprising:.

The method may be repeated for a plurality of predetermined locations on the workpiece.

According to a fifth aspect there is provided a computer program comprising instructions for causing a computerised controller to perform the method according to the third or fourth aspects. The computer program may be recorded on a non-transitory storage medium.

The invention is described in further detail below by way of example and with reference to the accompanying drawings, in which:.

<FIG>) show a robotic arm <NUM>, illustrating different sources of positional inaccuracy, as described in the background section above.

<FIG> is a schematic drawing of an example clamp <NUM> configured for attachment to a drilling tool <NUM> of a robotic drill, which may for example comprise a robotic arm of the type shown in <FIG>. The clamp <NUM> comprises an attachment portion <NUM> for attachment to the drilling tool <NUM>, and a frame <NUM> that is linearly moveable relative to the attachment portion <NUM> along a central axis <NUM> of the drilling tool <NUM>, i.e. along a rotational axis of a drill bit <NUM> attached to the drilling tool <NUM>. The drilling tool <NUM> is configured to drill a workpiece by actuating the drill bit <NUM> along the central axis <NUM> in the direction indicated by arrow <NUM>.

The clamp <NUM> comprises an actuation mechanism comprising a servo motor <NUM> configured to drive linear movement of the frame <NUM> relative to the attachment portion <NUM>, i.e. relative to the drilling tool <NUM>. A workpiece contacting portion <NUM> at a distal end <NUM> of the frame <NUM> comprises a surface <NUM> for contacting a surface of a workpiece to be drilled and an aperture <NUM> allowing for passage of the drill bit <NUM> through to the workpiece surface.

A force sensor <NUM> is arranged to measure a force acting on the workpiece contacting portion <NUM> in the drilling direction. The force sensor <NUM> may for example form part of the workpiece contacting portion <NUM> or may be provided elsewhere in the frame <NUM> or attachment portion <NUM> to measure a force acting along the central axis <NUM> between the workpiece and the drilling tool <NUM>.

The workpiece contacting portion <NUM> may have a non-slip surface <NUM> for contacting the workpiece.

The frame <NUM> in the example of <FIG> generally has a C-shape, with a pair of arms 211a, 211b extending in the drilling direction on either side of the central axis <NUM> and a distal end piece <NUM> extending between the pair of arms 211a, 211b. The arms 211a, 211b and the distal end piece <NUM> may be of unitary construction, for example formed of a single piece of metal, to aid stiffness and prevent distortion of the frame <NUM> during use. Other shapes may also be possible, and the frame <NUM> may have more than two arms 211a, 211b.

The servo motor <NUM> drives each of the arms 211a, 211b of the frame <NUM> by driving respective rods 213a, 213b extending along each arm 211a, 211b. A pulley or timing belt <NUM> extends between the pair of arms 211a, 211b across the distal end section <NUM>, allowing for the rotation of the rods 213a, 213b to be synchronised so that the frame <NUM> moves in a uniform linear direction along the central axis <NUM>, thereby applying a force on the workpiece parallel with the drilling direction.

A plurality of datums <NUM> may be provided, which may be attached to the part of the clamp that is secured to the drilling tool <NUM>, i.e. the attachment portion <NUM>, to allow for a metrology system to determine a location of the drilling tool <NUM>, described in further detail below. Typically at least three datums <NUM> will be required to enable a precise location and orientation in three-dimensional space to be determined.

<FIG> illustrates schematically an example robotic drilling system <NUM>. The system <NUM> comprises a robotic drill <NUM> comprising a drilling tool <NUM>, on to which a clamp <NUM> of the type described above is attached. A workpiece holder <NUM> is provided for holding a workpiece (not shown) to be machined. A controller <NUM> is connected to the robotic drill <NUM> for control of the robotic drill <NUM> and clamp <NUM>. The controller <NUM> actuates the robotic drill <NUM> to contact the surface of the workpiece contacting portion <NUM> (<FIG>) to the surface of a workpiece to be machined and actuates the servo motor <NUM> in the clamp <NUM> to drive the clamp <NUM> on to the workpiece while measuring a force from the force sensor <NUM> until a predetermined force has been reached. The controller <NUM> may then operate the drilling tool <NUM> to drill a hole into the workpiece.

The system <NUM> may comprise a metrology system for measuring a position of the drilling tool <NUM> relative to the workpiece to be machined. The metrology system comprises a first plurality of datums <NUM> on the robotic drill <NUM>, specifically on the part of the clamp <NUM> that is attached to the drilling tool <NUM>, i.e. the attachment portion <NUM>, and a second plurality of datums <NUM> on the workpiece holder <NUM>. Each plurality of datums <NUM>, <NUM> comprises at least three datums to enable accurate positioning and orientation of the workpiece holder <NUM> and drilling tool <NUM> in three-dimensional space relative to each other. The datums <NUM>, <NUM> may for example be light emitting elements, for example light emitting diodes. A plurality of light detecting elements <NUM> may be arranged to receive light from each of the light emitting elements <NUM>, <NUM>, signals from which are received by a metrology control unit <NUM>. The metrology control unit <NUM> also controls operation of the light emitting elements <NUM>, <NUM>. The metrology control unit <NUM> may provide position information to a computer <NUM>, which also communicates with the controller <NUM>.

Although the controller <NUM>, computer <NUM> and metrology control unit <NUM> are illustrated as separate components in <FIG>, these components may be contained in, or considered as being, a single controller, or a controller may be considered to be distributed between and among the different functional components <NUM>, <NUM>, <NUM>.

The metrology system allows the robotic drilling system <NUM> to measure the relative locations of the drilling tool <NUM> and workpiece holder <NUM>, thereby allowing the controller <NUM> to compensate for any difference in location of the drilling tool <NUM> after actuation of the clamp <NUM> before a drilling operation. This may for example be carried out during each drilling operation or may be carried out prior to performing any drilling operations on a workpiece.

<FIG> is a schematic flow diagram illustrating a method of operation of the system <NUM> in a first mode, in which a bias is measured and determined for each of a plurality of predetermined drilling locations on a workpiece. In a first step <NUM>, the robotic drill is actuated to contact the surface of the workpiece contacting portion of the clamp to the workpiece to be machined. In a second step <NUM>, a first position of the drilling tool relative to the workpiece is measured using the metrology system. In a third step <NUM>, the servo motor is actuated to drive the clamp on to the workpiece to be machined, while measuring a force from the force sensor until a predetermined force has been reached. In a fourth step <NUM>, a second position of the drilling tool relative to the workpiece is measured using the metrology system. In a fifth step <NUM>, a bias is determined from a difference between the first and second measured positions. In a sixth step <NUM>, the determined bias is stored for the predetermined location. The method may then be repeated for further predetermined locations on the workpiece until all locations for drilling operations have been covered.

In a second mode of operation, the controller may perform the method as outlined in <FIG>. In a first step <NUM>, a stored bias for a predetermined location of a workpiece to be machined is retrieved. In a second step <NUM>, the robotic drill is actuated to contact the surface of the workpiece contacting portion of the clamp at the predetermined location offset by the retrieved bias. In a third step <NUM>, the servo motor is actuated to drive the clamp on to the workpiece while measuring a force from the force sensor until a predetermined force has been reached. In a fourth step <NUM>, the drilling tool is operated to drill a hole at the predetermined location of the workpiece. The method may then be repeated until all locations for drilling have been covered.

The position of the drilling tool may continue to be measured during the second mode of operation, which can be used to update a stored bias for the predetermined location.

The second mode of operation may be carried out separately from the first mode, i.e. with the stored bias for each predetermined location having been previously determined.

The optical metrology system may operate by locating the position of multiple LEDs on the drilling tool <NUM> and workpiece holder <NUM> so that when the robotic drill <NUM> performs a drilling operation the optical metrology system enables the drilling tool to drill a hole in the workpiece to a greater degree of accuracy than may be possible using positional encoders on the robotic drill alone. The optical metrology system may for example have a positional accuracy within around <NUM> of a nominal target position, The bias between an unclamped and clamped position can be used by the controller to predict what bias or offset to apply to the robotic drill for future drilling operations. Multiple bias measurements may be incorporated into a machine learning algorithm to predict a bias to be used for a robotic drilling system.

<FIG> is a drawing indicating various component parts of an example robotic drilling system <NUM>, including a robotic drill <NUM> with a tool frame <NUM> comprising a drilling tool and clamp of the type described above. The robotic drill <NUM> is mounted on a robot base frame <NUM> that is associated with a robot system origin <NUM>. A workpiece <NUM> to be machined is mounted on a workpiece holder <NUM>, which may be attached to the robot base frame <NUM>. In operation, the robot tool frame <NUM> is moved from the system origin <NUM> to a target <NUM>, which may be determined by a 3D CAD model relative to an origin <NUM> of the workpiece <NUM> or workpiece holder <NUM>. Operation of the robotic drill <NUM> may then be as described above.

<FIG> illustrates a more detailed example of a clamp attached to a drilling tool <NUM> of a robotic drill <NUM>, the clamp having the general form as shown in <FIG> and described above. The clamp comprises an electro-mechanical servo-driven mechanism <NUM> for high precision clamp-up, and has a non-skid surface <NUM> at the clamp "nose", or workpiece contacting portion. An integrated force feedback system <NUM> enables the servo to be driven until a desired force is met. Linear rails <NUM> with a pulley or timing belt ensure that force is applied to the structure, i.e. the workpiece to be machined, that is central to the clamp nose and not offset, which can amplify any skid. A rigid C-shaped frame <NUM> is designed to prevent distortion, deflection and skid of the clamp nose.

Claim 1:
A clamp (<NUM>) configured for attachment to a drilling tool (<NUM>) of a robotic drill, the clamp (<NUM>) comprising:
an attachment portion (<NUM>) configured for attachment to the drilling tool (<NUM>);
a frame (<NUM>) linearly moveable relative to the attachment portion (<NUM>) along a central axis (<NUM>) of the clamp (<NUM>) concentric with a drilling direction (<NUM>) of the drilling tool (<NUM>);
an actuation mechanism comprising a servo motor (<NUM>) configured to drive linear movement of the frame (<NUM>) relative to the drilling tool (<NUM>);
a workpiece contacting portion (<NUM>) at a distal end (<NUM>) of the frame (<NUM>), comprising a surface (<NUM>) for contacting a surface of a workpiece to be drilled and an aperture (<NUM>) allowing for passage of a drill bit (<NUM>) of the drilling tool (<NUM>) through to the workpiece surface; and
a force sensor (<NUM>) arranged to measure a force acting on the workpiece contacting portion (<NUM>) in the drilling direction (<NUM>),
wherein the frame (<NUM>) has a pair of arms (211a, 211b) extending in the drilling direction (<NUM>) on either side of the central axis (<NUM>) and a distal end piece (<NUM>) extending between the pair of arms (211a, 211b),
characterised in that the servo motor (<NUM>) is configured to drive each of the arms (211a, 211b) of the frame (<NUM>) by rotation of respective rods (213a, 213b) extending along each arm (211a, 211b).