Strain Wave Gearing Research Articles

Series Clutched Actuation for Collision-Tolerant High-Speed Robots

Abstract Collisions at high-speed can severely damage robots with non-backdrivable drivetrains. Adding an overload clutch in series can improve the robot’s collision tolerance without compromising its high dynamic performance. This paper aims at determining the speed above which overload clutches are required in a two-link manipulator arm. Furthermore, the optimal clutch topology as function of the impact velocity is investigated. Third, it is evaluated if adding clutches can lower the impact force on the arm. Finally, the maximum speed is identified below which impact-aware robot control is possible. The latter requires that none of the clutches decouple during an intentional collision with the environment. These answers are obtained through collision simulations and experiments with a custom build two-link arm. It was found that adding a clutch reduces the torque experienced by the drivetrain by an order of magnitude and below the limit momentary peak torque of the strain wave gears that are used. Adding a clutch to the elbow joint of the two-link arm was effective in protecting the shoulder as well if the impact occurred at the tool center point. With respect to a rigid elbow joint, the clutched elbow joint reduced the collision force at the tool by only 8%. To demonstrate that the arm is impact-aware, a box of 8 kg is approached, impacted, and pushed at 1 m/s without decoupling a clutch, nor damaging the robot’s hardware.

An experimental study of the sensorized strain wave gear RT1-T and its capabilities for torque control in robotic joints.

The idea of sensorizing a strain wave gear to measure the transmitted torque has been reported since the 1980s. The strain in the elastic flex spline is typically measured by strain gages attached to it. The resulting voltages relate to the transmitted torque in the gear. However, periodic inaccuracies in the measured torque signal (sensing ripple), resulting from positioning inaccuracies of strain gages on the flex spline, prevented this technology from being used outside a lab environment. Regardless of these difficulties, measuring the torque directly in the strain wave gear would bring many advantages, especially in robotic applications, where design space is highly limited. Traditionally, robotic joints are equipped with link-sided torque sensors, which reduce the available design volume, lower the joint stiffness, and require complex cable routing. This paper presents an experimental study of a novel sensorized strain wave gear named RT1-T, which was developed by Schaeffler Technologies. The study was implemented on a joint testbed, including a high-resolution reference torque sensor at the link side. In addition to the measurement accuracy and linearity, a torque ripple analysis is performed. The joint torque control capabilities are determined along dynamic trajectories and compared to the performance achieved with a link-sided reference sensor. The sensor employed in the testbed has a static torque error of 0.42 Nm and an average closed-loop torque control error of 0.65 Nm above the reference sensor.

Dynamics Modeling of Robot Joints with Asymmetric Load-Dependent Friction

This paper presents a strain wave gear model intended for industrial robots that are operated in both forward drive and backward drive under different load conditions. We show that a simple joint model captures the joint dynamics well at one load, but fails to capture the joint dynamics when the load is changed. Similarly, the simple model cannot capture the efficiency of the joint in both forward drive and backward drive. We propose a tooth model with sliding friction to resolve these issues. This model is simple to simulate and can thus be used for real-time control; the proposed model captures the load dependent friction and difference in efficiency in backward drive and forward drive. We validate the simulation performance of the tooth models on data from a Universal Robots UR10e robot with different payloads which shows an improvement compared to the standard rigid joint model.

Elucidation on step-out phenomenon of magnetic strain wave gear by multi flexible body dynamics and displacement measurement

Elucidation on step-out phenomenon of magnetic strain wave gear by multi flexible body dynamics and displacement measurement

Intelligent Fault Diagnosis of Robotic Strain Wave Gear Reducer Using Area-Metric-Based Sampling

The main challenge with rotating machine fault diagnosis is the condition monitoring of machines undergoing nonstationary operations. One possible way of efficiently handling this situation is to use the deep learning (DL) method. However, most DL methods have difficulties when the issue of imbalanced datasets occurs. This paper proposes a novel framework to mitigate this issue by developing an area-metric-based sampling method. In the proposed process, the new sampling scheme can identify which locations of the datasets can potentially have a high degree of surprise. The basic idea of the proposed method is whenever significant deviations from the area metrics are observed to populate more sample points. In addition, to improve the training accuracy of the DL method, the obtained sampled datasets are transformed into a continuous wavelet transform (CWT)-based scalogram representing the time–frequency component. The dilated convolutional neural network (CNN) is also introduced as a classification process with the altered images. The efficacy of the proposed method is demonstrated with fault diagnosis problems for welding robots. The obtained results are also compared with existing methods.

A Novel Strain Wave Gear Reducer with Double Flexsplines

Strain wave gear reducers, also known as harmonic drives, are widely used in industrial robots and collaborative robots. The zero-backlash feature is very important for these applications. However, this places extremely high demands on the machining accuracy of the strain wave gear reducer. Excessive manufacturing errors will lead to excessive backlash, affecting transmission accuracy or making installation difficult, and the flexible spline (or flexspline) is prone to wear, resulting in reduced accuracy with use. This study proposes a novel strain wave gear reducer with a double flexspline structure. The original “circular spline” which was rigid will be redesigned to be slightly flexible and deformable with an additional deformation adjustment structure, which reduces the requirements for machining accuracy, and realizes the same zero-backlash characteristic of traditional structure. The experimental results show that the new strain wave gear reducer has extremely low lost motion, hysteresis loss, and high torsional rigidity. The new strain wave gear reducer provides a more economical way to realize the zero-backlash reducer and helps to solve the problem of the accuracy decline of the strain wave gear reducer due to the wear of the flexspline.

Proposal of strain wave gear actuated by magnetic force

Lower limb exoskeletons with actuators aids disabled individuals in walking. However, the exoskeleton weight is greater than that of conventional medical devices such as orthoses and walking frames. In this study, we propose a novel strain wave gear for light exoskeletons, which is 50% lighter than conventional commercial strain wave gears. This gear uses magnets and yokes, rather than mechanical elements. The thin-walled elastic cylinder is deformed by the magnetic force in the gear, and the torque is transmitted by tooth engagements. The maximum torque was calculated to be 10.5 Nm using flexible multibody dynamics analyses with a magnetic force database. The outcome was efficient in development of lightweight machines.

Prehľad doterajšieho výskumu výroby harmonického prevodu

Harmonic Drive, also called Harmonic Drive gear, harmonic gear, or strain wave gearing, mechanical speed-changing device, invented in the 1950s, that reduces the gear ratio of a rotary machine to increase torque. It operates on a principle different from that of conventional speed changers. Harmonic drive products are unique precision speed reducers playing important roles in robots, semiconductor manufacturing systems, factory automation equipment and furthermore, in aerospace equipment that may convey our human dreams.

Circumferential spatial tooth profile modification of flexspline for strain wave gear (A novel method and a case study)

Circumferential spatial tooth profile modification of flexspline for strain wave gear (A novel method and a case study)

Кинематика волновых зубчатых передач с генератором волн внешнего деформирования

The development of automatic control systems, tracking systems and industrial robotics requires the creation of new generation drive systems with high efficiency, kinematic accuracy, reliability, durability with small axial and radial dimensions and weight. To implement tracking systems with power differentials, transmission mechanisms with two degrees of freedom are required to provide the transformation and summation of two movements of drive motors into the required movement of the drive output shaft. These requirements in single-channel tracking systems are well met by strain wave gears (SWG) with low-inertia disk and ring wave generators. They allow for wide layout possibilities, the ability to route cables through the hollow central shaft of the drive, high torsional rigidity and low moments of inertia, which are in the range of 250 to 400 times less than those of cam wave generators, thus ensuring high drive speed. There are practically no publications, except domestic ones, devoted to the study of SWG with ring wave generators. This scientific direction of research is carried out at the Bauman Moscow State Technical University. Structural schemes of SWG with ring generators of external deformation waves are considered, the features of the kinematics of these gears are studied, and recommendations for improving their quality indicatorsare proposed

A Preliminary Experimental Study on the Effects of Wear on the Torsional Stiffness of Strain Wave Gears

Strain wave gears, also known as harmonic drives, are employed in a wide range of fields such as robotics and aerospace, where light weights, precision, and reliability are essential to the correct execution of the tasks. For this reason, their understanding and optimization are of high interest for both academia and industry. Previous studies have been mainly focused on investigating and modeling the working principle of strain wave gears in nominal operating conditions. On the contrary, the present paper describes the results of an experimental campaign aimed to introduce wear in gears of two different suppliers and its impact on the gear torsional stiffness. Results show how the change in the gear performance strongly depends both on the gear manufacturer and the location of wear. For the analyzed components, a damaged wave generator–flexspline interface reduces the gear stiffness up to one-fourth of its nominal value, while the non-nominal shape of the teeth jeopardizes the gearbox performance, leading up to just 4% of the nominal stiffness values, and resulting in backlash. Such data can be used to properly model the presence of wear in strain wave gears and to train data-driven diagnostics and prognostics routines to effectively detect such a fault.

Comparison of conventional and split-cam strain wave gear through initial load and stress distribution

ABSTRACT This paper presents an analysis of stress developed in flex-gear (FG) cup with conventional strain wave generating (SWG) cam and split-cam SWG in strain wave gear/harmonic drive (HD), using initial load distribution by tooth pairs as a boundary condition in finite element method (FEM). The initial load distribution by tooth pairs is achieved with the assembly of ring gear (RG) and deformed FG before the application of any external load. A closed-form solution to the analytical method describes the exact initial load distribution by tooth pairs in the presence of secondary contacts. However, with spit-cam SWG the phenomenon of secondary contact is completely eradicated. Therefore, the load is distributed by contacting tooth pairs that lie only in the line of action and the tooth pair that lies along the major axis of split-cam SWG experiences maximum load compared to conventional SWG. The strain developed in FG cup based on the load distribution with both SWG cams obtained from FEM is verified with the experiment. The comparison shows a rise of 44 microns in circumferential strain around the major axis of deformed FG, when RG is assembled. This testifies the presence of initial load, which is distributed among all interfering tooth pairs before applying external load.

Prognostic health management of the robotic strain wave gear reducer based on variable speed of operation: a data-driven via deep learning approach

ABSTRACT The robotic reducer is prone to failure because of its unique characteristics. Data from vibration and acoustic emission sensors have been used for the prognostics of the reducer. However, various issues are associated with such traditional techniques. Hence, our research group proposes a novel approach to utilize the embedded setup of the electrical current to detect the mechanical fault of the robotic reducer in the actual industrial robot. Previously, a comprehensive approach of feature engineering was proposed to classify the mechanical fault for the robotic reducer. However, handcraft-based feature extraction is quite a tedious task, and computationally expensive. These features require a well-designed feature extractor, and the features need to be manually optimized before feeding into classifiers. In addition, the handcrafted features are problem-specific, and are complicated to generalize. To resolve these challenges, deep features are extracted to classify the fault and generalize for two different motion profiles under different working conditions. In the proposed research work, the fault characteristic is generalized for variable speed of operations considering various kinds of scenarios. In this research work, the generalization capability of the proposed approach is comprehensively evaluated. For that purpose, the data under different working conditions such as of lower speeds, higher speeds, and speed sequestration are used as unseen data to validate the model. The authenticity of the presented approach can be supported by the performance evaluation for fault classification of the different motion profiles and speed of operations.

A general mathematical modeling and finite element analysis of a strain wave gear possessing a double-circular-arc-with-a-common-tangent profile

In this study, the characteristics of a strain wave gear (SWG) with a double-circular-arc with a common tangent (DCACT) profile were simulated using two-dimensional (2D) and three-dimensional (3D) finite element analysis (FEA). First, theoretically generated tooth profiles for the flexible spline (FS) and circular spline (CS) of an SWG were developed according to the theory of gearing. An automated mesh-generating computer program was developed in Visual C++ to establish 2D and 3D finite element models of the SWG based on its derived geometry. An FEA was then performed to explore the meshing characteristics, including the stress distribution, torsional angle, torsional stiffness, and number of engaged teeth, of the SWG. Continuous meshing of multiple tooth pairs between the FS and CS from the 2D FEA verified the conjugation of tooth profiles. The coning effect on the FS during assembly of the SWG was also predicted and discussed from the 3D FEA results. The 3D FEA produced more accurate and practical results than the 2D FEA did. Additionally, 3D FEAs of a modified (with longitudinal crowning) and unmodified (without longitudinal crowning) FS were performed and compared. Finally, a 3D FEA of a modified FS was successfully conducted to predict cyclic contact stress, root stress, and transmission error.

Design, analysis, and optimization of a strain wave gear with a novel tooth profile

In this study, an optimization process was designed for a strain wave gear (SWG) through finite element analysis (FEA) to improve its performance. Two optimization steps are executed in this process. In the first optimization step, a novel two-dimensional (2D) tooth profile is obtained using a double Bezier curve with a common tangent line profile (DBCCTLP), and the mathematical model of this profile is derived. Subsequently, the obtained tooth profile is optimized through 2D FEA based on the torsional stiffness and number of engaged teeth. A three-dimensional (3D) model of an SWG is subsequently established to explore its 3D meshing performance through FEA and optimize the parameters of its flexible spline (FS). In the second optimization step, 3D FEA is conducted to predict the torsional stiffness, hysteresis loss, and stress of the SWG using the DBCCTLP-based optimum tooth profile. The optimum structural design parameters for a 3D FS cup were determined through 3D FEA and optimization. Finally, the transmission error and cyclic stress for the initial and optimum SWG design were successfully simulated and compared.

A novel cylindrical skiving tool with error-free flank faces for internal circular splines

Strain wave gearing is widely applied in robotics and aerospace because of its zero-backlash and high gear ratio. Its circular spline (CS) is a rigid internal gear engaged with a flex spline (FS), driven by the wave generator's rotation. Obtaining the CS's tooth profile is difficult because the FS has a deformed ellipse shape caused by the wave generator. Furthermore, most CSs are manufactured by gear shaping and thus are lower precision and productivity comparing to external gears produced by gear hobbing. Power skiving is recently applied to overcome this problem. Mathematical models of CS and its skiving tool are established here. Moreover, a cylindrical tool is submitted to make tool manufacturing easier than a conical tool. A novel design for the error-free cylindrical tool and the skived gear is eventually derived. By modifying machine settings of the skiving process, it provides a relief angle to avoid the cylindrical tool rubbing with the internal gear. This manufacturing process is conducted on a six-axis CNC bevel gear cutting machine. An error analysis of the VERICUT cutting simulation result confirms the correctness of the proposed models.

Performance analysis of asymmetric toothed strain wave gear

Strain Wave Gear (SWG) or Harmonic Drive (HD) shows moderate performance under symmetrical involute teeth. Teeth interference, excessive teeth meshing, vibration due to improper teeth engagement are the major drawbacks of strain wave gear under symmetrical involute teeth. In the present paper, an attempt has been made to improve the performance of strain wave gear by replacing the symmetrical involute teeth with asymmetrical involute teeth. Present investigation finds that under ‘asymmetric toothed strain wave gear’, aspects such as teeth interference, teeth engagement and teeth contact stress of strain wave gear are improving clearly and as a consequence some other aspects such as individual teeth strength, load carrying capacity, tooth endurance are also improving inherently. In the present analysis for symmetrical teeth 20°–20° pressure angle and for asymmetrical teeth, three different combinations of pressure angles such as 25°–20°, 30°–20° and 35°–20° are taken.

Properties of Elasto-Hydrodynamic Oil Film in Meshing of Harmonic Drive Gears.

Among the essential issues facing designers of strain wave gears, the provision for correct lubrication should be of paramount importance. The present paper presents the results of research on elasto-hydrodynamic oil film in meshing of a harmonic drive with an involute tooth profile. The research was carried out based on theoretical models developed by Dowson and Higginson. For the discussed structural problem, results of the study are presented graphically in the form of static characteristics of the oil film. Correct operation regimes were determined for two different oils. The paper also provides a review of information concerning the design and principle of operation of strain wave transmission.

Development of a precision drive unit at K.K.K. 99 Ltd. to be universally used as robot joints with high powerdensity and modular construction

A highly innovative gear unit set has been developed and manufactured to be widely used in medical industry, manufacturing robots or high precision manipulators. The main elements of the drive unit are the gear ratio reducing strain wave gear, the torque-motor and the force-torque measuring sensor, which can be combined arbitrarily. In one unit the motor series size is given but its power output (longer motor) and the gear reduction ratio can be varied. Assembling these parts, the result of this RnD project is a high precision drive with minimal dimensions, which, on itself, is able to position rotating movement precisely and while at halt it can also keep that position with high torsional stiffness. What makes the modular precision drive unit whole, is the controlling system. The above-mentioned machine elements can be assembled into different configurations so it can become part of a SCARA, humanoid or Delta robot. As a result of this project the created precision drive prototypes could provide tests and results that may influence the mechatronic and mechanical engineering profession.

Development and manufacturing of an experimental flat wheel strain wave gear unit at K.K.K. 99 Ltd.

The functions of the main components of a flat-wheel harmonic drive are similar to the functions of a traditional harmonic drive, but the features of the flexible and the solid wheels are different. This paper reports the development and investigation of the experimental flat wheel harmonic drives. The development project was sponsored by Ministry for National Economy.