Search Results (16)

To minimize mechanical damage caused by an apple picking robot end-effector during the apple grasping process, and on the basis of optimizing the minimum stable grasping force of apple, a variable impedance control strategy based on a reinforcement learning deep deterministic policy gradient (DDPG) algorithm is proposed to achieve compliant grasping control for apples. Firstly, according to the apple contact force model, the gradient flow algorithm is adopted to optimize grasping force in terms of the friction cone, force balancing condition, and stability assessment index and to obtain a minimum stable grasping force for apples. Secondly, based on the analysis of the influence of impedance parameters on the control system, a variable impedance control based on the DDPG algorithm is designed, with the reward function adopted so as to improve the control performance. Then, the improved control strategy is used to train the optimized impedance control. Finally, simulation and experimental results indicate that the proposed variable impedance control outperforms the traditional impedance control by reducing the peak grasping force from 4.49 N to 4.18 N while achieving a 0.6 s faster adjustment time and a 0.24 N narrower grasping force fluctuation range. The improved impedance control successfully tracks desired grasping forces for apples of varying sizes and significantly reduces mechanical damage during apple harvesting. Full article

This paper is an attempt to investigate the rock indentation behaviors of a conical pick under different loading rates (1, 2, 3, and 4 mm/min), indenter types (sharp and blunt indenters), and types of rock (concrete, limestone, granite). Serial indentation tests by indenters were first performed by an automatic universal testing machine and monitored by an i-SPEED high-speed camera to record the peak pick force, indentation depth, rock fracture area, and rock failure process. Accordingly, the effect of loading rates, rock brittleness, and pick type on rock indentation behaviors was subsequently analyzed for a sound understanding of rock fragmentation mechanisms with indenters. It was found that higher loading rates necessitate a higher pick force and indentation depth to achieve rock fragmentation, resulting in a larger fractured area. Notably, a positive linear relationship exists between loading rates, rock-breaking forces, and fracture areas. A sharp indenter induces multiple cycles of repeated crushing and chipping phases, resulting in an arcuate-shaped fracture pattern with a smaller fractured area. Conversely, the rounded blunt indenter leads to a single stage of compression, with cracks propagating directly through the rock specimen, producing a larger fractured area. In addition, rock brittleness is another key factor to control rock failure efficiency, with tensile strength serving as a significant component. Full article

Cables and tendons are crucial elements in bridge engineering but also are vulnerable structural elements because they are usually subjected to fatigue and corrosion problems. Thus, vibration-based non-destructive techniques have been used for external post-tensioning tendon assessment. Regarding continuous monitoring systems, tendon assessment is carried out through the continuous tracking of its natural frequencies and the subsequent estimation of the tension force, as this parameter is essential for the bridge’s overall structural performance, thus providing useful information about bridge safety. However, for long-term monitoring assessment, two main challenges have to be addressed regarding practical applications: (i) double-peak spectra and other spurious factors that affect the frequency estimation, and (ii) temperature dependency, which needs to be carefully treated since frequency/tension variation may be explained by temperature variation, thus masking potential structural anomalies. On this subject, this paper presents the experimental long-term monitoring of several post-tensioning external tendons in a high-speed railway bridge in which a sectorized weighted peak-picking frequency identification procedure is proposed for frequency estimation, alongside a cascade clustering process, which allows meaningful frequency estimates to be selected. Finally, the selected frequency estimates, which show variations from 1 to 2% for all analyzed frequencies, are used for the long-term assessment of the tension force. Full article

This study introduces a simplified method for predicting the optimal cutting conditions to maximize excavation efficiency based on tool forces. A laboratory-scale linear rock-cutting test was conducted using a conical pick on Finike limestone. The tool forces and their ratios were analyzed in relation to cutting parameters such as penetration depth and spacing. While the cutting force (FC) and normal force (FN) increased with the penetration depth and spacing, this relationship could not predict the optimal cutting conditions. The ratio of the mean normal force to the mean cutting force (FN_m/FC_m) increased with the penetration depth and the ratio of spacing to penetration depth (s/d). However, even while including this relationship, predicting optimal cutting conditions remained challenging. The ratio of the peak cutting force to the mean cutting force (FC_p/FC_m) reached a maximum value at a specific s/d, which is similar to the relationship between the specific energy (SE) and s/d. The optimal s/d obtained through the SE methodology was found to be between 3 and 5, and FC_p/FC_m reached a maximum at s/d. The error between the optimal s/d and the s/d in which FC_p/FC_m was maximized was less than 5%. Therefore, it was confirmed that the optimal cutting conditions could be predicted through the relationship between FC_p/FC_m and s/d. Additionally, by using the results from previous studies, the optimal cutting conditions obtained from the SE methodology and the proposed methodology were found to agree within a margin of error of 20%. The proposed methodology can be beneficial for the design of cutter heads and the operation of excavation machines. Full article

Manipulation involves both fine tactile feedback, with dynamic transients perceived by fingerpad mechanoreceptors, and kinaesthetic force feedback, involving the whole hand musculoskeletal structure. In teleoperation experiments, these fundamental aspects are usually divided between different setups at the operator side: those making use of lightweight gloves and optical tracking systems, oriented toward tactile-only feedback, and those implementing exoskeletons or grounded manipulators as haptic devices delivering kinaesthetic force feedback. At the level of hand interfaces, exoskeletons providing kinaesthetic force feedback undergo a trade-off between maximum rendered forces and bandpass of the embedded actuators, making these systems unable to properly render tactile feedback. To overcome these limitations, here, we investigate a full upper limb exoskeleton, covering all the upper limb body segments from shoulder to finger phalanxes, coupled with linear voice coil actuators at the fingertips. These are developed to render wide-bandwidth tactile feedback together with the kinaesthetic force feedback provided by the hand exoskeleton. We investigate the system in a pick-and-place teleoperation task, under two different feedback conditions (visual-only and visuo-haptic). The performance based on measured interaction forces and the number of correct trials are evaluated and compared. The study demonstrates the overall feasibility and effectiveness of a complex full upper limb exoskeleton (seven limb-actuated DoFs plus five hand DoFs) capable of combined kinaesthetic and tactile haptic feedback. Quantitative results show significant performance improvements when haptic feedback is provided, in particular for the mean and peak exerted forces, and for the correct rate of the pick-and-place task. Full article

To address the issues of significant soil blockage and high potato damage rates in current potato picking machines, this study developed a toggle lever-type potato picker designed to minimize potato damage and improve operational efficiency. Design calculations were performed for the picker components, and kinematic analyses were conducted for the toggle lever. Single-factor experiments were carried out to determine the variation in performance parameters of the potato picker under different experimental conditions. Discrete element simulations were performed to measure the peak soil height before the pick-up shovel and the peak force on potatoes during the pick-up process. A Box–Behnken response surface experiment was conducted using toggle lever speed, machine forward speed, and shovel angle as experiments factors. Subsequently, an analysis of variance was performed, and a mathematical regression model was established based on the experiments results. The findings revealed that at a toggle lever speed of 50 r/min, machine forward speed of 0.9 m/s, and shovel angle of 19°; the potato leakage rate was 2.32%, and the potato damage rate was 2.72%, thereby meeting the requirements stipulated by potato mechanized picking technology regulations. Full article

During the picking process of the apple harvesting robot, the attitude of the end effector holding the apple and the movement method of separating the apple directly affect the success rate of picking. In order to improve the stability of the picking process, reduce the gripping force, and avoid apple dislodgement and damage, this work studies the new apple-picking pattern of the flexible three-fingered end-effector based on the analysis of the existing apple-picking pattern. First, two new three-finger grasping postures for wrapping the apple horizontally and vertically on the inside of the fingers are proposed, and a new method of separating the stem with a circular-pull-down motion of the end-effector picking the apple is designed. Then, the pressure on the apple under different picking patterns was analyzed, and a branch–stem–apple simulation model was established. Combining the constraint conditions such as the angle between the apple stem and the vertical direction, the movement speed, the root impulse, and so on, the optimal angle of apple circular movement and the force required to realize the movement are obtained through dynamic simulation experiments. Finally, the experiments of apple picking patterns were carried out with the flexible three-fingered end-effector. The experiment shows that the best angle for apple picking is 15°~20° using the circular-pull-down movement separation method. In terms of average grasping force peaks and pressures, the combination of the vertical holding posture of the inner finger and the circular-pull-down movement separation method is the best picking pattern. In this pattern, the average peak exerts force on the inner side of a single finger is about 8.52 N, and the pressure is about 20.9 KPa. Full article

The task of gripping has been identified as the rate-limiting step in the development of tree-fruit harvesting systems. There is, however, no set of universally adopted ‘specifications’ with standardized measurement procedures for the characterization of gripper performance in the harvest of soft tree fruit. A set of metrics were defined for evaluation of the performance of end effectors used in soft tree-fruit harvesting based on (i) laboratory-based trials using metrics termed ‘picking area’, which was the cross-sectional area in a plane normal to the direction of approach of the gripper to the fruit in which a fruit was successfully harvested by the gripper; ‘picking volume’, which was the volume of space in which fruit was successfully harvested by the gripper; and ‘grasp force’, which was the peak force involved in removing a fruit from the grasp of a gripper; (ii) orchard-based trials using metrics termed ‘detachment success’ and ‘harvest success’, i.e., the % of harvest attempts of fruit on tree (of a given canopy architecture) that resulted in stalk breakage and return of fruit to a receiving area, respectively; and (iii) postharvest damage in terms of a score based on the percentage of fruit and severity of the damage. Evaluations were made of external (skin) damage visible 1 h after gripping and of internal (flesh) damage after ripening of the fruit. The use of the metrics was illustrated in an empirical evaluation of nine gripper designs in the harvest of mango fruit in the context of fruit weight and orientation to the gripper. A design using six flexible fingers achieved a picking area of ~150 cm² and a picking volume of 467 cm³ in laboratory trials involving a 636 g phantom fruit as well as detachment and harvest efficiency rates of 74 and 65%, respectively, in orchard trials with no postharvest damage associated with the harvest of unripe fruit. Additional metrics are also proposed. Use of these metrics in future studies of fruit harvesting is recommended for literature–performance comparisons. Full article

To improve the seedling-picking efficiency of the vegetable transplanter and reduce the damage rate of the seedling pot, a reciprocating seedling-picking device driven by full air pressure was designed. In this paper, the structure and working principle of the pneumatic seedling-picking device are introduced. Through the mechanical analysis between the seedling-picking claw and the seedling pot, working parameters such as the stroke and driving force of the pneumatic seedling-picking claw clamping cylinder were determined. According to the action sequence of the seedling-picking mechanism, which is horizontally dispersed and longitudinally conveyed, the pneumatic control scheme of the seedling-picking and -dropping system was formulated. The simulation model for the control loop of the longitudinal cylinder was created with AMESim simulation software, and the simulation analysis was carried out. The Box–Behnken response surface design optimization method was used to determine the best operating parameters of the cylinder. The optimized peak value of shock vibration at the end of the cylinder was optimized from −65.64 mm·s⁻² to 35.41 mm·s⁻², proving that the optimization of pneumatic working parameters has a positive effect on the success rate of seedling picking. The bench test of the seedling-picking mechanism was conducted on 72-hole plug seedlings with two picking frequencies of 120 plants·min⁻¹ and 144 plants·min⁻¹, respectively, and the average seedling leakage rate, seedling damage rate, and seedling pot damage rate at different picking frequencies were counted. The experimental results show that under the two seedling-picking frequencies, the average success rate of seedling picking and throwing after optimization is increased from 96.4% and 92.4% to 97.9% and 95.3%, respectively. This is in line with the requirements of high-speed seedling picking and confirms the rationality of the seedling-picking mechanism design. Full article

Conical picks are applied to rock breaking, and the cutting parameters greatly influence cutting performance and rock damage. A conical pick rock-cutting numerical simulation model and software to provide a quantitative analysis of rock damage are established to research the influence of cutting parameters on cutting force and rock damage. The research results indicated that the average peak cutting force decreases with an increased cutting angle, and the average peak cutting force increases with a decreased cutting speed. The average peak cutting force of the second conical pick increases as the distance between double conical picks increases in the process of rock-cutting with double conical picks. There is a positive correlation between the volume of rock damage, the volume of rock breaking, and the cutting force of the conical pick. The research results provide a reference for optimizing the cutting parameters of conical picks. Full article

Prediction of rock fracturing capacity demands particular requirements for the exploitation of mineral resources, especially for the parameter design of conical pick performance for hard rock fragmentation, which must take into account differences in rock mechanical properties. Among these parameters, the peak cutting force (PCF) is important in designing, selecting, and optimizing the cutting head of mining equipment and a cutability index of rocks. Taking high lithological tolerance as demand traction, this study proposes a theoretical model for estimating the peak cutting force of conical picks based on the improved projection profile method for which the influence of alloy head, pick body structure, and installation parameters are taken into consideration. Besides, experimental results corresponding to different numbers of rock samples are used to verify the accuracy and stability of the theoretical model. Meanwhile, the comparison of performance in cutting force estimation between this model and four other existing theoretical models is conducted. The results found that the new method has the highest correlation coefficient with the experimental results and the lowest root mean square error comparing with other models, i.e., the estimation performance of this method has high lithological tolerance when the rock type increases and the lithology changes. Consequently, the proposed peak cutting force estimation of improved projection profile method will provide a more valid and accurate prediction for rock fracturing capacity with large differences in rock mechanical properties. Full article

In recent decades, many researchers have focused on the design and development of exoskeletons. Several strategies have been proposed to develop increasingly more efficient and biomimetic mechanisms. However, existing exoskeletons tend to be expensive and only available for a few people. This paper introduces a new gravity-balanced upper-limb exoskeleton suited for rehabilitation applications and designed with the main objective of reducing the cost of the components and materials. Regarding mechanics, the proposed design significantly reduces the motor torque requirements, because a high cost is usually associated with high-torque actuation. Regarding the electronics, we aim to exploit the microprocessor peripherals to obtain parallel and real-time execution of communication and control tasks without relying on expensive RTOSs. Regarding sensing, we avoid the use of expensive force sensors. Advanced control and rehabilitation features are implemented, and an intuitive user interface is developed. To experimentally validate the functionality of the proposed exoskeleton, a rehabilitation exercise in the form of a pick-and-place task is considered. Experimentally, peak torques are reduced by 89% for the shoulder and by 84% for the elbow. Full article

Accurate estimation of cable tension is crucial for the structural health monitoring of cable-supported structures. Identifying the cable’s force from its vibration data is probably the most widely adopted method of cable tension estimation. According to string theory, the accuracy of estimated cable tension is highly related to identified modal parameters including natural frequencies and frequency order. To alleviate the factors that impact the accuracy of modal parameters when using the peak-picking method in wireless sensor networks, a fully automated and robust identifying method is proposed in this paper. This novel method was implemented on the Xnode wireless sensor system and validated with the data obtained from Jindo Bridge. The experiment results indicate that, through this method, the wireless sensor is able to distinguish the cognizable power spectrum, extract the peaks, eliminate false frequencies and determine frequency orders automatically to estimate cable tension force without any manual intervention or preprocessing. Meanwhile, the results of natural frequencies, corresponding orders and cable tension force obtained from the Xnode system show excellent agreement with the results obtained using the Matlab program method. This demonstrates the effectiveness and reliability of the Xnode estimation system. Furthermore, this method is also appropriate for other high-performance wireless sensor network systems to realize self-identification of cable in long-term monitoring. Full article

Cutter suction dredgers are important pieces of rock excavation equipment in port and waterway construction. It is valuable but difficult to properly estimate the cutting force on the chisel pick of the cutter suction dredger. In this paper, an analytical model, called the crushed zone expansion induced tensile failure model (CEIT model), is proposed for rock cutting with a chisel pick in order to predict the peak cutting force (Fc) more accurately. First, a review of the existing models for rock cutting with a chisel pick is presented. Next, based on the tensile breakage theory, cavity expansion theory and some hypotheses, the mathematical formula of the CEIT model is obtained. Different from that in the previous models, the effect of the rock on both sides of the chisel pick on Fc, defined as the sidewall effect is considered in the CEIT model. Then, the predicted Fc by the CEIT model is compared with the predicted Fc by existing theoretical models and experimental results to check the validity of the CEIT model. The results show that the CEIT model can well capture the relationships of Fc to the cutting parameters, including cutting width, cutting depth, and rake angle, and can predict the experimental results much better than the existing models. Finally, the sidewall effect and its influence factors according to the CEIT model are discussed. Full article

The impact of hot-melt extrusion (HME) on the solid-state properties of four methacrylic (Eudragit^® L100-55, Eudragit^® EPO, Eudragit^® RSPO, Eudragit^® RLPO) and four polyvinyl (Kollidon^® VA64, Kollicoat^® IR, Kollidon^® SR, and Soluplus^®) polymers was studied. Overall, HME decreased Tg but increased electrostatic charge and surface free energy. Packing density decreased with electrostatic charge, whereas Carr’s and Hausner indices showed a peak curve dependency. Overall, HME reduced work of compaction (Wc), deformability (expressed as Heckel P_Y and Kawakita 1/b model parameters and as slope S′ of derivative force/displacement curve), and tablet strength (TS) but increased elastic recovery (ER). TS showed a better correlation with S′ than P_Y and 1/b. Principal component analysis (PCA) organized the data of neat and extruded polymers into three principal components explaining 72.45% of the variance. The first included Wc, S′ and TS with positive loadings expressing compaction, and ER with negative loading opposing compaction; the second included P_Y, 1/b, and surface free energy expressing interactivity with positive loadings opposing tap density or close packing. Hierarchical cluster analysis (HCA) assembled polymers of similar solid-state properties regardless of HME treatment into a major cluster with rescaled distance Cluster Combine Index (CCI) < 5 and several other weaker clusters. Polymers in the major cluster were: neat and extruded Eudragit^® RSPO, Kollicoat^® IR, Kollidon^® SR, Soluplus^®, and extruded Eudragit^® L100-55. It is suggested that PCA may be used to distinguish variables having similar or dissimilar activity, whereas HCA can be used to cluster polymers based on solid-state properties and pick exchangeable ones (e.g., for sustain release or dissolution improvement) when the need arises. Full article