Patent Application
20160082501
This invention relates generally to automatic riveting machines used for large-scale manufacturing operations using fasteners, such as for commercial aircraft, and more specifically concerns a system for preventing damage to an aircraft workpiece or the like in the event of a fastener misalignment.
Automatic riveting machines can have various configurations, including a C-frame arrangement, such as shown in
These machines use a CMC for control of rivet upset and logic. A CMC is both a logic controller and a motion controller, controlled by one processor or multiple processors connected together. The CMC controls upper and lower rams (or front and back rams) of a riveting machine and applies logic and timing to the motion control of the machine, as well as recognizing input and output information. Examples of CMCs include Delta Tau PMAC controller, Fanuc controllers and Siemens controllers.
A large ram force produced by an actuator is necessary to upset a rivet after it has been positioned in the workpiece opening or to drive a bolt into an interference fit in an opening in the workpiece. A rivet or bolt may in some cases not successfully initially enter the opening, because it is jammed between the ram die and the workpiece or turned sideways (laid down). In either case, when the riveting or interference force is applied to the misaligned fastener, the resulting damage to the workpiece can result in the entire workpiece being ruined, with a substantial monetary loss.
Accordingly, it is desirable that the manufacturing apparatus be able to automatically detect when a fastener (rivet or bolt) is not positioned properly in the workpiece prior to the application of the large ram force. One previous approach in solving this problem uses a camera to ensure proper insertion of the fastener. While this has been generally successful, it has limitations with respect to certain types of fastener misalignment and is insensitive to the case where the rivet is misaligned perpendicular to the view of the camera. Such a vision system also is expensive and has the further disadvantage of slowing down the fastening process, because the machine must actually stop during every cycle to perform a vision check.
In another previous approach, the push-away of the lower clamp portion of a riveting system is sensed. A lower clamp is held against the workpiece pneumatically in riveting operations. In a normal rivet cycle, the lower clamp is not pushed away but in the case of a rivet jam or a sideways, laid-down situation, the lower clamp is pushed away from the workpiece. While this technique is effective in reducing damage, it does not prevent it, since the clamp motion which is sensed has already resulted in at least some workpiece damage before the riveting force is interrupted. The lower clamp push-away technique cannot be used for bolts, however, because the lower clamp is not held pneumatically against the workpiece for bolt insertion. The full unexpected force can be detected by a load cell arrangement but catastrophic damage is done to the workpiece before the motion of the ram is stopped.
Accordingly, existing systems for preventing damage due to misaligned rivets and bolts are not completely satisfactory. It is important that all or virtually all instances of misalignment be quickly recognized and the fastener process interrupted prior to the application of fastening force and resulting damage to the workpiece.
Accordingly, the system for accomplishing riveting or bolt insertion into an opening in a workpiece without damage to the workpiece includes: a ram assembly, having fingers at a forward end thereof for grasping a fastener; an actuator for moving the ram assembly under control of a cycle motion controller for initially inserting the fastener into an opening in the workpiece and thereafter accomplishing an insertion cycle for the fastener to complete insertion of the fastener in the opening; a protective air gap assembly responsive to movement of the ram assembly toward the workpiece, including an air gap which is maintained by a selected amount of force; and a sensor assembly mounted and operable to determine closing of the air gap due to movement of the ram assembly toward the workpiece, wherein the sensor has a signal state which is monitored by the cycle motion controller, the insertion cycle being interrupted prior to damage being done to the workpiece in the event that the air gap begins to close too early or too late relative to closure of the air gap when the fastener is properly initially inserted in the workpiece opening.
In the present invention, there is an air gap 36 of approximately 10 mm located between an upper end of anvil socket 22 and a top end 35 of housing 26. A spring 38 is positioned within a slot 37 in the anvil socket, the spring extending into air gap 36. Spring 38 holds the air gap open with about 60-75 pounds of force. The air gap 36 in effect creates a lost motion of the ram-to-die arrangement during insertion of the rivet into the opening 39 of the workpiece. The spring force passes through the ram-to-die connection when the rivet is fully inserted into the opening. The system further includes a sensor assembly 40 attached to the housing, the sensor assembly including a lower sensing element 41, while a flag member 42 is attached to anvil socket 22. In operation, as the flag member 42 moves with the anvil socket, the air gap decreases and sensing element 41 is uncovered, changing the signal state of the sensor. This occurs whether the rivet is properly inserted or not. However, if the change of state is early, i.e. prior to the normal expected time for a properly inserted rivet, as explained in more detail below, the CMC recognizes an error and interrupts (halts) the riveting cycle, i.e. the application of riveting force to the rivet. The action of the sensor assembly, the flag and the cycle motion controller (CMC) is also explained in more detail below.
Referring now to
With a properly inserted rivet,
In more detail, referring to
The present system can also be used to identify when a bolt has been jammed in an opening in the workpiece or is positioned sideways, although the vast majority of use of the present air gap system is for rivets. The shank diameter of a bolt fastener is typically at least 0.001 inches larger than the diameter of the opening, while the threads at the forward end (tip) of the bolt have an outside diameter which is smaller than the diameter of the opening. Accordingly, during successful bolt insertion, the threaded portion first slips into the opening without resistance, and then as the shank begins to enter the opening, a significant amount of force, typically thousands of pounds, is required to drive it the rest of the way in. The position where the threads are fully inserted and the shank is just beginning to enter the opening is referred to as the “stake” position. Using the present system, detection of staking can be achieved, because the force required to press the bolt in further is much higher than the force that the air gap spring exerts, so that the air gap will begin to close immediately upon the occurrence of staking.
For a given bolt length, the distance between the bolt head and the point where the bolt shank begins to transition to the threaded portion is well known. This distance determines how far the bolt will protrude from the panel when it is staked. If a bolt which has been staked is too long or too short, the bolt will protrude from the panel by the wrong distance. When the air gap sensor is triggered, the known position of the bolt-inserting ram can be used to measure the protrusion distance. The machine is programmed with the nominal acceptable tolerance protrusion distance for each length of bolt that the machine installs, so that an acceptance tolerance band can be specified.
If the measured protrusion deviates from the nominal acceptable protrusion distance by more than the specified tolerance, the normal cycle of insertion is interrupted. Typically, when the air gap begins to close too early, i.e. before tolerance range 63 in
On the other hand, if the position of the inserter ram moves past the tolerance range 69, to the right on position axis 86, without the sensor changing state, the motion controller will recognize this error as well and interrupt the cycle, preventing any damage to the workpiece. An error message is also generated. This is typically indicative of a sideways laid down bolt or an oversize hole or an undersize bolt. The force profile for a normal bolt cycle and the successful insertion is referenced at 98.
Accordingly, the present invention is capable of identifying jammed rivets and bolts as well as sideways rivets and bolts and to interrupt the normal high force cycle action to prevent damage to the workpiece, as well as generating an error message. In operation, the insertion ram is driven toward the workpiece by a servo-motor, which is controlled by a cycle motion controller. The protective air gap is monitored by a combination of a sensor assembly and the cycle motion controller. If the sensor changes state early or changes state late (only for an interference fit bolt), the cycle motion controller will recognize the error and will interrupt the rivet/bolt insertion force cycle and generate an error message.
There is some system reaction time, referred to as a delay, between a failed/incomplete insertion and the interruption of the insertion cycle. Most of the delay is due to the time it takes for the ram to actually decelerate to a halt after it received the interrupting signal from the cycle motion controller.
The fastener insertion system, in terms of maximum possible deceleration, is limited. If the insertion system is driven toward the workpiece at its maximum speed, a long distance may be required for it to decelerate and come to a complete stop, preventing damage to the workpiece. The time it takes to decelerate and therefore the distance traveled during deceleration can be reduced by driving the system at a reduced speed.
Accordingly, a system has been described and shown which identifies a misaligned rivet or bolt, specifically, those not properly entering the opening in the workpiece. After identification of such a circumstance, an error signal is sent which is then transmitted to the cycle motion controller, which interrupts the insertion process, saving the workpiece from damage.
Although a preferred embodiment of the invention has been disclosed for purposes of illustration, it should be understood that various changes, modifications and substitutions may be incorporated in the embodiment without departing from the spirit of the invention, which is defined by the claims which follow.
