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The formation of sub-surface defects during friction stir welding has limited the adoption of the process in high volume production. A potential exists to eliminate/reduce the need for costly post weld inspection of such defects through the development of an in-process defect detection method based on a measured process output. The current state of in-process defect detection consists primarily of applying “black box” methods of correlating process outputs to defect occurrence without a fundamental physical understanding of what is producing the change in the output. This approach constrains the application of such methods when altering any aspect of the friction stir welding process. The current study seeks to provide a fundamental physical explanation as to what is driving the oscillation of friction stir welding process forces at the tool rotational frequency, as well as what is occurring when the tool interacts with defects and the oscillatory process forces are altered. A novel understanding was enabled through the synchronization of force measurements with angular position measurements of tool features. The results suggest that the eccentric motion of the tool and/or the rotation of a slanted shoulder surface are the primary drivers of process force oscillations. A fundamental explanation of the interaction between features on the tool probe and defective volumes has been proposed. The physical understanding helps to explain how altering the process will alter the force transients on which the detection method is based, which will enable a more robust and transferable method.

The goal of this research was to examine how altering the amount of friction stir tool eccentricity while controlling the amount of slant in the tool shoulder (drivers of oscillatory process forces) effects the generation of process force transients during sub-surface void interaction. The knowledge gained will help improve the accuracy of force-based void monitoring methods that have the potential to reduce the need for post-weld inspection. Process force transients during sub-surface void formation were examined for multiple tools with varying magnitudes of kinematic runout. The eccentric motion of the tool produced oscillations in the process forces at the tools rotational frequency that became distorted when features (flats) on the tool probe interacted with voided volumes, generating an amplitude in the force signals at three times the tool rotational frequency (for three-flat tools). A larger tool eccentricity generates a larger amplitude in the force signals at the tool’s rotational frequency that holds a larger potential to create a distortion during void interaction. It was determined that once void becomes large enough to produce an interaction that generates an amplitude at the third harmonic larger than 30% of the amplitude at the rotational frequency in a weld with no interaction (amplitude solely at rotational frequency), the trailing edge of the tool shoulder cannot fully consolidate the void, i.e., it will remain in the final weld. Additionally, once the void exceeds a certain size, the amplitudes of the third harmonics saturate at 70% of the amplitude at the rotational frequency during full consolidation. The interaction between the eccentric probe and sub-surface void was isolated by ensuring any geometric imperfection in the shoulder (slant) with respect to the rotational axis was removed. The results suggest that geometric imperfections (eccentricity and slant) with respect to the tool’s rotational axis must be known when developing a void monitoring method from force transients of this nature.

Worker posture, task time and performance are often affected when one-handed manual dexterous tasks are performed in small overhead spaces under an obscured view. A common method used for supplementing visual feedback in these cases is a hand-held telescopic mirror, but that involves working with both arms extended overhead, and is often accompanied by awkward neck and shoulder postures. A video camera was considered as an alternative to using a mirror for visual feedback and reducing overhead reach. A mirror, a borescope and an omnidirectional camera were evaluated while laboratory participants performed three one-handed simulated manufacturing tasks in a small overhead enclosure. Videos were recorded for quantifying the time that postures were assumed while performing the tasks. The average time that both arms were above mid-shoulder height for the omnidirectional camera was more than 2.5 times less than for the mirror and borescope. The average proportion of neck strain time was 0.01% (or less) for both the omnidirectional camera and the borescope, compared to 83.68% for the mirror. No significant differences were observed in task completion times between the three modalities. Hence, an omnidirectional camera can provide visibility while reducing straining postures for manufacturing operations involving overhead work.

The objective of this research is to develop a fundamental understanding of the interaction between features on the friction stir tool probe and volumetric sub-surface defects formed during welding. This will guide the development of real-time defect monitoring methods that will promote process adoption in high volume and high-reliability applications. A single-head laser Doppler vibrometer system was used to produce a non-contact measurement of the eccentric motion of a friction stir tool during welding. When features on the tool probe interacted with voided volumes, the tool was momentarily deflected into the voided volume. The distortion signals in the tool position, measured with the laser vibrometer, are correlated with distortions in measured process forces and defect size. The results add understanding to the changes in forces signals that hold potential for defect monitoring and suggest that monitoring may be possible through a motion-based measurement (accelerometer) from the tool side.

Manipulations of a constrained object often use a non-rigid grasp that allows the object to rotate relative to the end effector. This orientation slip strategy is often present in natural human demonstrations, yet it is generally overlooked in methods to identify constraints from such demonstrations. In this paper, we present a method to model and recognize prehensile orientation slip in human demonstrations of constrained interactions. Using only observations of an end effector, we can detect the type of constraint, parameters of the constraint, and orientation slip properties. Our method uses a novel hierarchical model selection method that is informed by multiple origins of physics-based evidence. A study with eight participants shows that orientation slip occurs in natural demonstrations and confirms that it can be detected by our method., Comment: 2021 International Conference on Robotics and Automation (ICRA 2021)

The objectives of this work are to investigate power and energy consumption of friction stir welding over a wide range of welding parameters and to provide a welding parameter map synthesized with their relations to welding quality. Friction stir welding has been known as an energy efficient welding technology because the whole process takes place in the solid-state, i.e., no excessive heat is needed to melt the workpiece. However, the study of the energy consumption and resulting welding quality across a wide parameter window has not been established inclusively. In this work, a series of 6061-T6 aluminum butt welds were created by friction stir welding across various welding parameters (spindle speeds and weld speeds). In order to understand the overall power and energy consumption, both mechanical process power and electrical wall plug power were measured on the 3-axis CNC mill used to perform the welds. Additionally, temperatures near the workpiece surface and weld roots were measured. The resulting weld quality was assessed by conducting tensile tests of weld cross-sections to determine the location of failures. The results of the power/energy, temperatures, and tensile tests were incorporated, producing an inclusive welding parameter map. The welding parameter map helps identify minimum power and minimum energy consumption points within the process constraints needed for maintaining good welding quality. The general approach presented in this paper can be applied to different welding setups and conditions.

The objective of this work is to develop a method for detecting the creation of discontinuities ( i.e. , voids, volume defects) during friction stir welding. Friction stir welding is inherently cost effective, however, the need for significant weld inspection can make the process cost prohibitive. A new approach to weld inspection is required in which an in-situ characterization of weld quality can be obtained, reducing the need for post-process inspection. To this end, friction stir welds with subsurface voids and without voids were created. The subsurface voids were generated by reducing the friction stir tool rotation frequency and increasing the tool traverse speed to create “colder” welds. Process forces were measured during welding, and the void sizes were measured post-process by computerized tomography ( i.e. , 3D X-ray imaging). Two parameters, based on frequency domain content and time-domain average of the force signals, were found to be correlated with void size. Criteria for subsurface void detection and size prediction were developed and shown to be in good agreement with experimental observations. With the proper choice of data acquisition system and frequency analyzer the occurrence of subsurface voids can be detected in real time.

The objective of this work is to investigate the effects of welding parameter variation and backing plate diffusivity on energy consumption in friction stir welding (FSW). FSW is a solid-state welding process, where joints are created below the solidus temperature of the workpiece. As a result, FSW is known for being energy efficient and environmentally friendly. However, a method of quantifying the energy consumption in FSW, for a variety of operating conditions, has not been well established. In order to understand the energy consumption of FSW, both process power and wall plug power were measured on a 3-axis CNC mill at various feed rates, spindle speeds, and with different backing plates. The results of monitoring the process power suggest that using a low thermal diffusivity backing plate has a beneficial effect on the instantaneous power consumption and on the total energy consumption during a weld. Conversely, tensile tests also showed that welds over a high thermal diffusivity backing plate consistently failed inside the weld zone, corresponding to a lower welding temperature and lack of penetration in the weld zone. These results suggest that using a low thermal diffusivity backing plate can both reduce energy consumption during FSW while also helping to achieve full penetration welds, while high feed rate is good for both welding productivity and reducing energy consumption per length of weld.

The objective of the work is the reduction and eventual avoidance of post welding inspections that are currently needed to ensure defect-free welds. An analytical thermal model of the FSW process along with an analytical disturbance model is developed. This disturbance model relates defect formation to variations in the measured temperature and is based on experimental process identification. A dynamic disturbance observer computes an estimate of the disturbance signal, which is further processed in order to provide information about the presence of defects along the weld. Experiments for one kind of disturbance verify that the observer shows good tracking behavior.

The goal of this work is to develop a method for detecting the creation of discontinuities during friction stir welding. This in situ weld monitoring method could significantly reduce the need for post-process inspection. A process force model and a discontinuity force model were created based on the state-of-the-art understanding of flow around an friction stir welding (FSW) tool. These models are used to predict the FSW forces and size of discontinuities formed in the weld. Friction stir welds with discontinuities and welds without discontinuities were created, and the differences in force dynamics were observed. In this paper, discontinuities were generated by reducing the tool rotation frequency and increasing the tool traverse speed in order to create “cold” welds. Experimental force data for welds with discontinuities and welds without discontinuities compared favorably with the predicted forces. The model currently overpredicts the discontinuity size.

Hybrid welding/joining of lightweight metals to carbon fiber reinforced polymers (CFRPs) typically relies on the adhesive bond created when the molten polymer matrix hardens in contact with the metallic surface. It is hypothesized that these bonds can be improved upon by fully displacing the polymer and infiltrating the carbon fibers with the metallic constituent to create load-bearing fibers that bridge the two materials. Friction stir welding (FSW) holds potential to melt and displace the polymer matrix, plasticize the metal constituent, and force the plasticized metal to flow around the fibers. Preliminary investigations were performed by FSW in AA 6061-T6 plates sandwiched against dry carbon fiber bundles. The FSW process plasticizes the aluminum while applying pressure, forcing the material to flow around the fibers. Cross-sectional images of the samples were used to measure the distance of infiltration of the aluminum into the carbon fiber bed. A fiber infiltration model previously developed to describe the infiltration of carbon fibers with epoxy resins during resin transfer molding was applied to this solid-state infiltration situation, thus modeling the plasticized aluminum as a fluid with an effective viscosity. Promising agreement was seen between the measured distances of infiltration and the predicted distances of infiltration when using effective viscosity values predicted by computational fluid dynamics (CFD) simulations of FSW found in literature. This work indicates that the well-established epoxy infiltration model can form the basis of a model to describe solid-state infiltration of carbon fibers with a plasticized metal.

The objective of this work is to establish nominal friction stir [butt] welding process parameters for joining 4.76-mm-thick aluminum alloys 6061-T6 and 7075-T6 and to improve the joint quality via programmed tool offsets. In addition, dynamic tool–workpiece interface temperatures are measured during welding and used to explain the effects of alloy placement and weld tool offset from the joint. Weld tool offsets into the retreating side AA7075 increase the measured tensile strength of the dissimilar joint. The increased joint strength is facilitated by lower average weld temperatures with increasing amount of AA7075 stirred into the nugget.

A real-time wireless temperature measurement system has been developed and successfully implemented for closed-loop control of tool shoulder–workpiece interface temperature. The system employs two thermocouples in through holes and measures the shoulder and pin interface temperatures with an angular resolution as small as 10°. Both temperatures correlate with weld quality (mechanical testing and weld cross sections), e.g., all welds in 4.76-mm-thick 6061-T6 with an average shoulder interface temperature below 520 °C and an average pin interface temperature below 460 °C fail in the weld zone instead of the heat-affected zone, have unacceptable tensile strengths and in some cases voids. Similarly, welds with shoulder temperatures above the solidus temperature result in a degradation of the weld quality. It was found that a shoulder interface temperature of 533 °C results in the highest weld quality; hence, this temperature should be used as the setpoint temperature in the control system with a constant travel speed of 400 mm/min. The temperature measurement strategy was shown to be able to indicate welds with insufficient shoulder–workpiece contact, thus potentially identifying and preventing welds with detrimental weld quality due to lack of penetration. It was shown that backing plates of different thermal diffusivity change the heat flow out of the weld zone, hence weld temperature, and caused a measurable impact on the weld strength. By changing other process parameters, e.g., through a temperature control system, weld quality can be maintained in the presence of such changing thermal boundary conditions.

In this paper a shared control strategy is presented that allows a skilled operator to identify irregularities that occur during robotic friction stir welding (FSW) and assist the robotic system in producing an appropriate response. Human operators are adept at identifying disturbances; however, the complexity of the friction stir welding process makes it difficult for the operator to respond. While examining the capabilities of shared control in friction stir welding, this paper focuses on responding to defects that are caused by a lack of workpiece material during butt welding, such as gaps. A compensation strategy is presented that combines the human operator's perceptual strengths with an automated procedure for adjustment of the process parameters (i.e. travel angle and plunge depth). Experiments comparing four control strategies are performed while welding 5083-H116 aluminum. Through our experiments we demonstrate that if the FSW control task is appropriately shared between the human operator and the computer control system, the weld quality (strength) can be improved (from 9 ksi to 31 ksi for a gap size of 2.5 mm) as compared with the nominal case in which no corrections are made.

The current methods for welding aluminum alloys to carbon fiber composites rely heavily on the adhesive bond created when a melted polymer solidifies in contact with the aluminum surface. This bond may be improved by achieving mechanical interlocking of the carbon fibers and the aluminum alloy, so that load bearing fibers contribute to the bond strength. Friction Stir Welding (FSW) holds potential as a process to achieve such interlocking by locally heating and plasticizing the metal under significant load to cause infiltration of the metal into the carbon fiber weave. During preliminary investigations, AA6061-T6 was friction stir welded in contact with dry carbon fiber bundles, and the infiltration of aluminum into the fibers was measured. The results were compared to an accepted polymer infiltration model that was adapted for aluminum infiltration, giving promising initial agreement. This suggests that the plasticized aluminum in FSW can be treated as a fluid with an effective viscosity, and that well established polymer infiltration models can form the base of a metal composite FSW model.

This paper explores common process variations encountered in friction stir welding (FSW) and the limits to which acceptable joint strength is maintained while welding with a robotic FSW system. Part fit-up and mating variations are common in manufacturing, yet the limits to which a friction stir welding process can weld without major process adjustment are unclear. The effects on joint strength and mechanical properties of several of the most common mating variations (i.e., faying surface gap, misalignment, mismatch, etc.) are experimentally determined as individual effects as well as among common welding parameters. Experimental results on 5-mm-thick aluminum alloy 5083-H111 show that ultimate tensile strength, yield strength, and elongation begin to decrease from nominal weld conditions when either the tool offset distance from weld centerline or gap in abutted plates exceeds 25% of the average pin diameter (6 mm). In addition, vertical plate mismatch and lack of penetration can be tolerated up to 2.5% and 10%, respectively, before adverse effects on mechanical properties are observed. The work also indicates that of all the mating variations tested in this study, tool misalignment, followed by travel angle, has the most significant effect on the measured joint strength. Process stability testing has shown that the FSW process is able to endure part fit-up and mating variations within a defined tolerance, giving the practitioner an awareness of how well stock workpiece tolerances must be controlled before joint strength is adversely effected.

Several surgical techniques and devices have been developed to help patients born with limb deformities, limb length inequalities, and extreme short stature. People with such ailments often experience pain, dysfunction, and joint degeneration. The primary method of treating such deformities is an osteotomy followed by callus distraction [1]. Commonplace lengthening devices are external fixators and intramedullary devices, but each has its drawbacks. Traditional external fixators, such as the Ilizarov device and Taylor spatial frame, are cumbersome, painful, and produce large residual scars [2]. Due to pin tract infection rates of 10–20%, lengthening with these methods requires careful surveillance [3]. Intramedullary lengthening devices can cause severe complications such as intramedullary infection [4]. Surgeons have recently experienced success with a motorized, intramedullary nail (Fitbone), but pediatric use of this device can be limited due to interference with open growth plates [5]. The investigators have designed an extramedullary device that retains the attractive qualities of an intramedullary nail, without the risk of deep infection or damage to growth plates. Additionally, the device can be equipped with a six-axis force-torque sensor capable of measuring forces and moments in real time.

The objective of this work is to develop an improved temperature measurement system for friction stir welding (FSW). FSW is a solid-state joining process enabling welds with excellent metallurgical and mechanical properties, as well as significant energy consumption and cost savings compared to traditional fusion welding processes. The measurement of temperatures during FSW is needed for process monitoring, heat transfer model verification and process control, but current methods have limitations due to their restricted spatial and temporal resolution. Previous work showed that temperatures at the tool shoulder-workpiece interface can be measured and utilized for closed-loop control of temperature. Adding an additional thermocouple at the tool pin-workpiece interface and performing a calibration of the measurement to gain better insight into the temperature distribution in the weld zone improved the method. Both thermocouples were placed in through holes right at the interface of tool so that the sheaths are in direct contact with the workpiece material. This measurement strategy reveals dynamic temperature variations at the shoulder and the pin within a single rotation of the tool in real-time. It was found that the highest temperatures are at the shoulder interface between the advancing side and the trailing edge of the tool, closer to the advancing side. The temperature distribution was mostly affected by travel speed and the temperature difference within one tool rotation was found to be between 10 °C and 50 °C, depending on the process parameters. The dynamic temperature measurements obtained with the current system are of unmatched resolution, fast, and reliable and are likely to be of interest for both fundamental studies and process control of FSW.

Use of robotic friction stir welding (FSW) has gained in popularity as robotic systems can accommodate more complex part geometries while providing high applied tool forces required for proper weld formation. However, even the largest robotic FSW systems suffer from high compliance as compared to most custom engineered FSW machines or modified computer numerical control (CNC) mills. The increased compliance of robotic FSW systems can significantly alter the process dynamics such that control of traditional weld parameters, including plunge depth, is more difficult. To address this, closed-loop control of plunge force has been proposed and implemented on a number of systems. However, due to process parameter and condition variations commonly found in a production environment, force control can lead to oscillatory or unstable conditions and can, in extreme cases, cause the tool to plunge through the workpiece. To address the issues associated with robotic force control, the use of simultaneous tool interface temperature control has been proposed. In this paper, we describe the development and evaluation of a closed-loop control system for robotic friction stir welding that simultaneously controls plunge force and tool interface temperature by varying spindle speed and commanded vertical tool position. The controller was implemented on an industrial robotic FSW system. The system is equipped with a custom real-time wireless temperature measurement system and a force dynamometer. In support of controller development, a linear process model has been developed that captures the dynamic relations between the process inputs and outputs. Process validation identification experiments were performed and it was found that the interface temperature is affected by both spindle speed and commanded vertical tool position while axial force is affected primarily by commanded vertical tool position. The combined control system was shown to possess good command tracking and disturbance rejection characteristics. Axial force and interface temperature was successfully maintained during both thermal and geometric disturbances, and thus weld quality can be maintained for a variety of conditions in which each control strategy applied independently could fail. Finally, it was shown through the use of the control process model, that the attainable closed-loop bandwidth is primarily limited by the inherent compliance of the robotic system, as compared to most custom engineered FSW machines, where instrumentation delay is the primary limiting factor. These limitations did not prevent the implementation of the control system, but are merely observations that we were able to work around.

The objectives of this work are to develop an improved temperature measurement system for Friction Stir Welding (FSW). FSW is a novel joining technology enabling welds with excellent metallurgical and mechanical properties, as well as significant energy consumption and cost savings compared to traditional fusion welding processes. The measurement of temperatures during FSW is employed for process monitoring, heat transfer model verification and process control, but current methods have limitations due to their restricted spatial and temporal resolution and have found only few industrial applications so far. Thermocouples, which are most commonly used, are either placed too far away from the weld zone or are destructively embedded into the weld path, and therefore fail to provide suitable data about the dynamic thermal phenomena at the tool-workpiece interface. Previous work showed that temperatures at the tool shoulder-workpiece interface can be measured and utilized for closed-loop control of temperature. The method is improved by adding an additional thermocouple at the tool pin-workpiece interface to gain better insight into the temperature distribution in the weld zone. Both thermocouples were placed in through holes right at the interface of tool and workpiece so that the sheaths are in contact with the workpiece material. This measurement strategy reveals dynamic temperature variations at the shoulder and the pin within a single rotation of the tool in real-time. Due to the thermocouple’s limited response time and inherent delays due to physical heat conduction, the temperature response is experiencing attenuation in magnitude and a phase lag. Heat transfer models were constructed to correct for this issue. It was found that the highest temperatures are between the advancing side and the trailing edge of the tool. Further work is needed to increase the accuracy of the correction. Experimental results show that the weld quality is sensitive to the measured interface temperatures, but that temperature is not the only factor influencing the weld quality. The dynamic temperature measurements obtained with the current system are of unmatched resolution, fast and reliable and are likely to be of interest for both fundamental studies and process control of FSW.

The objectives of this work are to determine an accurate temperature feedback strategy and to develop a closed-loop feedback control system for temperature in friction stir welding (FSW). FSW is a novel joining technology enabling welds with excellent metallurgical and mechanical properties, as well as significant energy consumption and cost savings. However, numerous parameter and condition variations are present in the FSW production environment that can adversely affect weld quality, which has made extensive automation of this process impossible to date. To enable large scale automation while maintaining weld quality, techniques to control the FSW process in the presence of unknown disturbances must be developed. One process variable that must be controlled to maintain uniform weld quality under the inherent workpiece variability (thermal constraints, material properties, geometry, etc.) is the weld zone temperature. Our hypothesis is that the weld zone temperature can be controlled, which can help in controlling the weld quality. A wireless data acquisition system was built to measure temperatures at the tool-workpiece interface. A thermocouple was placed in a through hole right at the interface of tool and workpiece so that the tip is in contact with the workpiece material. This measurement strategy reveals temperature variations within a single rotation of the tool in real time. In order to automate the system, a first order process model with transport delay was experimentally developed that captures the physics between spindle speed and measured interface temperature. The model has a time constant of 110 ms and a delay time of 85 ms. Using this temperature measurement technique, a closed-loop temperature control system with a bandwidth of 0.3 Hz was developed. Interface temperatures in the range from 555 °C to 575 °C were commanded to an integral controller, which regulated the spindle speed between 850 rpm and 1250 rpm to adjust the heat generation and achieve the desired interface temperatures in 6061-T6 aluminum. To simulate changes in thermal boundary conditions, backing plates of different thermal diffusivities were found to effectively alter the heat flow, hence, weld zone temperature. The integral controller that manipulates spindle speed is applied when welding during these intentionally introduced weld disturbances. The measured temperature stayed within ±5 °C after introducing the disturbance, compared to a 50 °C change in temperature when no control was applied.

In recent years, flexible robotic catheter systems have been developed for minimally invasive cardiac surgery. These systems commonly include a passive flexible catheter, which is biocompatible, and an active servo system which provides the actuation for the flexible catheter. To relate control actuation motion of the servo system to catheter motion, models of continuous catheters have been developed. A typical approach is to assume a kinematic model in which the catheter maintains constant curvature along its length when articulated. However, this approach cannot explain the nonlinear behavior of the catheter when the effect of internal friction is considered. In this paper we present a lumped-parameter modeling approach that allows for the inclusion of nonlinear effects, including friction. The proposed approach has been used to model a prototype catheter where internal friction is modeled using a modified Dahl friction model. To evaluate the efficacy of the modeling approach, the simulation results were compared to an experimental catheter prototype. Results show that the proposed approach significantly improves simulation accuracy as compared to the case where friction is not considered. In addition, the effects of friction on catheter performance were investigated using the developed modeling approach.

Robotic catheters are gaining widespread use in the medical community for cardiac, neurological and other surgical interventions. However, many of the catheters used in these procedures exhibit non-linear behavior, and thus present many difficulties in implementing effective open-loop control. Systems such as this have been shown to benefit from closed-loop control, however very little investigation has been done into 3D closed loop control of this class of manipulators. Initial investigations by the authors have shown greatly improved positioning accuracy and response with closed-loop control based on feedback from an electromagnetic localization sensor. This paper describes the control approach and experimental results, and provides a robotic catheter system overview.

This paper presents an investigation of the effects of friction stir weld tool travel angle and machine compliance on joint efficiency of butt welded 5083-H111 aluminum alloy in the presence of joint gaps. Friction stir welds are produced with a CNC mill and an industrial robot at travel angles of 1 deg, 3 deg, and 5 deg with gaps from 0 mm to 2 mm, in 0.5 mm increments. Results indicate that the more rigid mill resulted in higher joint efficiencies than the relatively compliant robot when welding gaps greater than 1 mm with a 3 deg travel angle using our test setup. The results also show that when gaps exceed 1 mm welds made with a travel (tilt) angle of 5 deg are able to generate higher joint efficiencies than welds made with a travel angle of 1 deg and 3 deg. Based on tool geometry and workpiece dimensions, a simple model is presented that is able to estimate the joint efficiency of friction stir welds as a function of gap width, travel angle, and plunge depth. This model can be used as an assistive tool in optimizing weld process parameters and tool design when welding over gaps. Experimental results show that the model is able to estimate the joint efficiency for the test cases presented in this paper.