While manipulating objects is relatively easy for humans, reliably grasping arbitrary objects remains an open challenge for robots. Resolving it would advance the application of robotics to industrial use cases, such as part assembly, binning, and sorting. Likewise, it would advance the area of assistive robotics, where the robot interacts with its surroundings in support of human needs. Robotic grasping involves perception, planning, and control. As a starting point, knowing which object to grab and how to do so are essential. Consequently, accurate and diverse detection of robotic grasp candidates for target objects should lead to a better grasp path planning and improve the overall performance of grasp-based manipulation tasks.
The proposed solution utilizes a deep learning strategy for identifying suitable grasp configurations from an input image. In the past decade, deep learning has achieved major success on detection, classification, and regression tasks [1, 2, 3]. Its key strength is the ability to leverage large quantities of labelled and unlabelled data to learn powerful representations without hand-engineering the feature space. Deep neural networks have been shown to outperform hand-designed features and reach state-of-the-art performance.
In this research problem, we are interested in tackling the problem of identifying viable candidate robotic grasps of objects in a RGB-D image. The envisioned gripper is a parallel plate gripper (or similar in functionality). The principal difficulty comes from the variable shapes and poses of objects as imaged by a camera. The structural features defining successful grasps for each object may be different; all possible features should be identified through the learning process. The proposed architecture associated to the grasp configuration estimation problem relies on the strengths of deep convolutional neural networks (CNNs) at detection and classification. Within this architecture, the identification of grasp configuration for objects is broken down into a grasp detection processes followed by a more refined grasp orientation classification process, both embedded within two coupled networks.
The proposed architecture includes a grasp region proposal network for identification of potential grasp regions. The network then partitions the grasp configuration estimation problem into regression over the bounding box parameters, and classification of the orientation angles, from RGB-D data. Importantly, the orientation classifier also includes aNo Orientation competing class to reject spuriously identified regions for which no single orientation classification performs well, and to act as a competing no-grasp class. The proposed approach predicts grasp candidates in more realistic situations where no, single, and multiple objects may be visible; it also predicts multiple grasps with confidence scores (see Fig. rectangle, scores not shown for clarity). A new multi-objects grasp dataset is collected for evaluation with the traditional performance metric of false-positives-per-image. We show how the multi-grasp output and associated confidence scores could inform a subsequent planning process that takes advantage of the multi-grasp output for improved grasping success.
The main contributions of this paper are threefold:
(1) A deep network architecture that predicts multiple grasp candidates in situations when none, single or multiple objects are in the view. Compared to baseline methods, the classification-based approach demonstrates improved outcomes on the Cornell dataset benchmark, achieving state-of-the-art performance on image-wise and object-wise splits.
(2) A multi-object, multi-grasp dataset is collected and manually annotated with grasp configuration ground-truth as the Cornell dataset. We demonstrate the generalization capabilities of the architecture and its prediction performance on the multi-grasp dataset with respect to false grasp candidates per image versus grasp miss rate. The dataset is available at github.com/ivalab/grasp_multiObject.
Experiments with a 7 degree of freedom manipulator and a time-of-flight RGB-D sensor quantify the system’s ability to grasp a variety of household objects placed at random locations and orientations. Comparison to published works shows that the approach is effective, achieving a sensible balance for real-time object pick-up with an 88% success rate and less than 0.25 s from image to prediction to plan.
Ii Related Work
Research on grasping has evolved significantly over the last two decades. Altogether, the review papers [5, 6, 7, 8] provide a good context for the overall field. This section reviews grasping with an emphasis on learning-based approaches and on representation learning.
Early work on perception-based learning approaches to grasping goes back to 
, which identified a low dimensional feature space for identifying and ranking grasps. Importantly it showed that learning-based methods could generalize to novel objects. Since then, machine learning methods have evolved alongside grasping strategies. Exploiting the input/output learning properties of machine learning (ML) systems, proposed to learn the image to grasp mapping through the manual design of convolutional networks. As an end-to-end system, reconstruction of the object’s 3D geometry is not needed to arrive at a grasp hypothesis. The system was trained using synthetic imagery, then demonstrated successful grasping on real objects. Likewise, 
employed a CNN-like feature space with random forests for grasp identification. In addition to where to grasp, several efforts learn the grasp approach or pre-grasp strategy[12, 13], while some focus on whether the hypothesized grasp is likely to succeed . Many of these approaches exploited contemporary machine learning algorithms with manually defined feature spaces.
At the turn of this decade, two advances led to new methods for improving grasp identification: the introduction of low-cost depth cameras, and the advent of computational frameworks to facilitate the construction and training of CNNs. The advent of consumer depth cameras enabled models of grasping mapping to encode richer features. In particular 
represented grasps as a 2D oriented rectangle in the image space with the local surface normal as the approaching vector; this grasp configuration vector has been adopted as the well-accepted formulation. Generally, the early methods using depth cameras sought to recover the 3D geometry from point clouds for grasp planning, with manually derived feature space used in the learning process. Deep learning approaches arrived later 
and were quickly adopted by the computer vision community.
Deep learning avoids the need for engineering feature spaces, with the trade-off that larger datasets are needed. The trade-off is usually mitigated through the use of pre-training on pre-existing computer vision datasets followed by fine-tuning on a smaller, problem-specific dataset. Following a sliding window approach,  trained a two stage multi-modal network, with the first stage generating hypothesis for a more accurate second stage. Similarly,  first performed an image-wide pre-processing step to identify candidate object regions, followed by application of a CNN classifier for each region. To avoid sliding windows or image-wide search, end-to-end approaches are trained to output a single grasp configuration from the input data [20, 21, 22]. A regression-based approach is used in . It required compensation through image partitioning since the grasp configuration space is non-convex. Following the architecture in ,  experimented on real objects with physical grasping experiments. The two-stage network in  first output a learnt feature, which was then used to provide a single grasp output. These approaches may suffer from averaging effects associated to the single-output nature of the mapping. Guo et al.  employed a two-stage process with a feature learning CNN, followed by specific deep network branches (graspable, bounding box, and orientation). Alternatively, [24, 25] predicted grasp scores over the set of all possible grasps (which may be discretized). Such networks admit the inclusion of end-effector uncertainty. Most deep network approaches mentioned above start with the strong prior that every image contains a single object with a single grasp target (except ). This assumption does not generically hold in practice: many objects have multiple grasp options and a scene may contain more than one object.
Another line of research is to learn the mapping from vision input to robot motion to achieve grasping. To directly plan grasps, Lu et al. 
proposed to predict and maximize grasp success by inferring grasp configurations from vision input for grasp planning. Research on empirical grasp planning with reinforcement learning (RL) acquired samples from robots in real experiments. The training time involved several weeks and led to limitation of its scalability. The work  collected over 800k data points with up to 14 robotic arms running in parallel for learning visual servoing. The training time involved over 2 months. Generalization performance of RL solution to environmental changes remains unknown.
Inspired by , we propose to incorporate a grasp region proposal network
to generate candidate regions for feature extraction. Furthermore, we propose to transform grasp configuration from a regression problem formulated in previous works[20, 21] into a combination of region detection and orientation classification problems (with null hypothesis competition). We utilize ResNet , the current state-of-the-art deep convolutional neural network, for feature extraction and grasp prediction. Compared to previous approaches, our method considers more realistic scenarios with multiple objects in a scene. The proposed architecture predicts multiple grasps with corresponding confidence scores, which aids the subsequent planning process and actual grasping.
Iii Problem Statement
Given corresponding RGB and depth images of a novel object, the objective is to identify the grasp configurations for potential grasp candidates of an object for the purpose of manipulation. The 5-dimensional grasp rectangle is the grasp representation employed . It is a simplification of the 7-dimensional representation  and describes the location, orientation, and opening distance of a parallel plate gripper prior to closing on an object. The 2D orientated rectangle, shown in Fig. 2 depicts the gripper’s location , orientation , and opening distance . An additional parameter describing the length completes the bounding box grasp configuration,
Thinking of the center of the bounding box with its local axes aligned to the and variables, respectively, the first three parameters represent the frame of the bounding box in the image, while the last two describe the dimensions of the box.
Much like [20, 23], the proposed approach should avoid sliding window such as in  for real-time implementation purposes. We avoid the time-consuming sliding-window approach by harnessing the capacity of neural networks to perform bounding box regression, and thereby to predict candidate regions on the full image directly. Furthermore, we preserve all possible grasps and output all ranked candidates, instead of regressing a single outcome. To induce a richer feature representation and learn more of the structural cues, we propose to use a deeper network model compared to previous works [20, 18, 27], with the aim of improving feature extraction for robotic grasp detection. We adopt the ResNet-50  with 50 layers, which has more capacity and should learn better than the AlexNet  used in previous works (8 layers). ResNet is known for its residual learning concept to overcome the challenge of learning mapping functions. A residual block is designed as an incorporation of a skip connection with standard convolutional neural network. This design allows the block to bypass the input, and encourage convolutional layers to predict the residual for the final mapping function of a residual block.
The next three subsections describe the overall architecture of the system. It includes integration of the proposal network with a candidate grasp region generator; a description of our choice to define grasp parameter estimation as a combination of regression and classification problems; and an explanation of the multi-grasp detection architecture.
Iv-a Grasp Proposals
The first stage of the deep network aims to generate grasp proposals across the whole image, avoiding the need for a separate object segmentation pipeline. Inspired by Region Proposal Network (RPN) 
, we reset each orientated ground truth bounding box to have vertical height and horizontal width, as shown in Fig. regionprop. RPN shares a common set of convolutional layers of ResNet-50 in our architecture, and outputs a 512-d feature which is then fed into two sibling fully connected layers. The two outputs specify both probability of grasp proposal and proposal bounding box for each anchor.
At each anchor, by default 3 scales and 3 aspect ratios are used for grasp reset bounding box shape variations. RPN works as sliding a mini-network over the final convolutional feature map of ResNet-50. Hence predictions would be generated in total. Let denote the 4-dimensional vector specifying the reset of the -th grasp configuration, and denote the probability of the -th grasp proposal, we define the loss of grasp proposal net to be:
where for no grasp and when a grasp is specified. The variable is the ground truth grasp coordinate corresponding to .
Iv-B Grasp Orientation as Classification
Many prior approaches [21, 20] regress to a single 5-dimensional grasp representation for a RGB-D input image. Yet to predict either on (planar pose) or on (orientation) involves predicting coordinates that lies in a non-Euclidean (non-convex) space where regression and its standard L2 loss may not perform well. Rather than performing regression, our multi-grasp localiztion pipeline multi-grasp quantizes the orientations component of the grasp representation into angles, and formulates the input/ouput mapping a classification task for grasp orientation. It is similar to , except that we also add a non-grasp collecting orientation class for explicit competition with a null hypotheis. If none of the orientation classifiers outputs a score higher than the non-grasp class, then the grasp proposal is considered incorrect and rejected. In contrast,  separates this step using an additional grasp confidence score, which may not capture well the orientation-dependent properties of grasps. The value of the non-grasp class is that it is necessary for the downstream multi-object, multi-grasp component of the final algorithm. The total number of classes is . Denote by where the -th grasp configuration with classification label is associated with the angle . For the case of no possible orientation (i.e., the region is not graspable), the output label is and there is no associated orientation. Here, as illustrated in Fig. 5.
Iv-C Multi-Grasp Detection
After the region proposal stage of the deep network, the last stage aims to identify grasp configurations for candidates. This last stage classifies the predicted region proposals from previous stage into regions for grasp configuration parameter . At the same time the last stage also refines the proposal bounding box to a non-oriented grasp bounding box .
To process the region proposals efficiently, we integrate an ROI pooling layer  into ResNet-50 so that it may share ResNet’s convolutional layers. Sharing the feature map with previous layers avoids re-computation of features within the region of interest. An ROI pooling layer stacks all of the features of the identified grasp proposals, which then get fed to two sibling fully connected layers for orientation parameter classification and bounding box regression . The ROI pooling layer receives its input from the last convolutional layer of ResNet-50 (layer 41).
Let denote the probability of class
after a softmax layer, and
denote the corresponding predicted grasp bounding box. Define the loss function of the grasp configuration prediction to be:
where is the ground truth grasp bounding box.
With the modified architecture of ResNet-50 model, end-to-end training for grasp detection and grasp parameter estimation employs the total loss:
The streamlined system generates grasp proposals at the ROI layer, stacks all ROIs using the shared feature, and the additional neurons of the two sibling layers output grasp bounding boxes and orientations, or reject the proposed region.
V Experiments and Evaluation
Evaluation of the grasp identification algorithm utilizes the Cornell Dataset for benchmarking against other state-of-the-art algorithms. To demonstrate the multi-object, multi-grasp capabilities, a new dataset is carefully collected and manually annotated. Both datasets consist of color and depth images for multiple modalities. In practical not all possible grasps are covered by the labelled ground truth, yet the grasp rectangles are comprehensive and representative for diverse examples of good candidates. The scoring criteria takes into account the potential sparsity of the grasp configuration by including an acceptable proximity radius to the ground truth grasp configuration.
The Cornell Dataset  consists of 885 images of 244 different objects, with several images taken of each object in various orientations or poses. Each distinct image is labelled with multiple ground truth grasps corresponding to possible ways to grab the object.
Since the Cornell Dataset scenarios consist of one object in one image, we collect a Multi-Object Dataset for the evaluation of the multi-object/multi-grasp case. Our dataset is meant for evaluation and consists of 96 images with 3-5 different objects in a single image. We follow the same protocol as the Cornell Dataset by taking several images of each set of objects in various orientations or poses. Multiple ground truth grasps for each object in each image are annotated using the same configuration definition.
V-a Cornell Data Preprocessing
To reuse the pre-trained weights of ResNet-50 on COCO-2014 dataset , the Cornell dataset is preprocessed to fit the input format of the ResNet-50 network. For comparison purposes, we follow the same procedure in  and substitute the blue channel with the depth channel. Since RGB data lies between 0 to 255, the depth information is normalized to the same range. The mean image is chosen to be 144, while the pixels on the depth image with no information were replaced with zeros. For the data preparation, we perform extensive data augmentation. First, the images are center cropped to obtain a 351x351 region. Then the cropped image is randomly rotated between 0 to 360 degree and center cropped to 321x321 in size. The rotated image is randomly translated in x and y direction by up to 50 pixels. The preprocessing generates 1000 augmented data for each image. Finally the image is resized to 227x227 to fit the input of ResNet-50 architecture.
To avoid over-fitting and precondition the learning process, we start with the pretrained ResNet-50. As shown in Fig 3, we implement grasp proposal layer after the third residual block by sharing the feature map. The proposals are then sent to the ROI layer and fed into the fourth residual layer. The average pool outputs are then fed to two fully connected layers for final classification and regression. All new layers beyond ResNet-50 are trained from scratch.
Because the orientation is specified as a class label and assigned a specific orientation, the Cornell dataset needs to be converted into the expected output format of the proposed network. We equally divide 180 degrees into regions (due to symmetry of the parallel gripper) and assign the continuous ground truth orientation to the nearest discrete orientation.
For training, we train the whole network end-to-end for 5 epochs on a single nVidia Titan-X (Maxwell architecture). The initial learning rate is set to 0.0001. And we divide the learning rate by 10 every 10000 iterations. Tensorflow is the implementation framework with cudnn-5.1.10 and cuda-8.0 packages. The code will be publicly released.
V-D Evaluation Metric
Accuracy evaluation of the grasp parameters involves checking for proximity to the ground truth according to established criteria . A candidate grasp configuration is reported correct if both:
the difference of angle between predicted grasp and ground truth is within 30 , and
the Jaccard index of the predicted graspand the ground truth is greater than 0.25, e.g.,
The Jaccard index is similar to the Intersection over Union (IoU) threshold for object detection.
Vi-a Single-object Single-grasp
Testing of the proposed architecture on the Cornell Dataset, and comparison with prior works lead to Table sosg. For this single-object/single-grasp test, the highest output score of all grasp candidates output is chosen as the final output. The proposed architecture outperforms all competitive methods. On image-wise split, our architecture reaches 96.0% accuracy; on object-wise split for unseen objects, 96.1% accuracy is achieved. We also tested our proposed architecture by replacing ResNet-50 with VGG-16 architecture, a smaller deep net with 16 layers. With VGG-16, our model still outperforms competitive approaches. Yet the deeper ResNet-50 achieve 4.4% more on unseen objects. Furthermore, we experiment on RGB images without depth information with ResNet-50 version and both image-wise and object-wise split perform slightly worse than our proposed approach, indicating the effectiveness of depth. The third column contains the run-time of methods that have reported it, as well as the runtime of the proposed method. Computationally, our architecture detects and localize multiple grasps in 0.120s, which is around 8 fps and is close to usable in real time applications. The VGG-16 architecture doubles the speed with some prediction accuracy loss.
Table Jthresh contains the outcomes of stricter Jaccard indexes for the ResNet-50 model. Performance decreases with stricter conditions but maintains competitiveness even at 0.40 IoU condition. Typical output of the system is given in Fig. 6a, where four grasps are identified. Limiting the output to a single grasp leads to the outputs depicted in Fig. 6b. In the multi-grasp case, our system not only predicts universal grasps learned from ground truth, but also contains candidate grasps not contained in the ground truth, Fig. 6c.
|Prediction Accuracy (%)||fps|
|Jiang et al. ||60.5||58.3||0.02|
|Lenz et al. ||73.9||75.6||0.07|
|Redmon et al. ||88.0||87.1||3.31|
|Wang et al. ||81.8||N/A||7.10|
|Asif et al. ||88.2||87.5||–|
|Kumra et al. ||89.2||88.9||16.03|
|Mahler et al. ||93.0||N/A||1.25|
|Guo et al. ||93.2||89.1||–|
|Ours: VGG-16 (RGB-D)||95.5||91.7||17.24|
|Ours: Res-50 (RGB)||94.4||95.5||8.33|
|Ours: Res-50 (RGB-D)||96.0||96.1||8.33|
Vi-B Single-object Multi-grasp
For realistic robotic application, a viable grasp usually depends both on the object and its surroundings. Given that one grasp candidate may be impossible to achieve, there is benefit to provide a rank ordered list of grasp candidates. Our system provides a list of high quality grasp candidates for a subsequent planner to select from. Fig. 7c shows samples of the predicted grasps and corresponding ground truths. To evaluate the performance of the multi-grasp detector, we employ the same scoring system as with the single grasp, then generate the miss rate as a function of the number of false positives per image (FPPI) by varying the detection threshold, see Fig. 7a for the single-object multi-grasp case. The model achieves and miss rate at 1 FPPI for object-wise split and image-wise split, respectively. A false positive means an incorrect grasp candidate for the object. Thus, accepting that there may be 1 incorrect candidate grasp per image, the system successfully detects 72% (75%) of possible grasps for object-wise (image-wise) split. The model performs slightly better in image-wise split than object-wise split due to unseen objects in the latter.
Vi-C Multi-object Multi-grasp
Here, we apply the proposed architecture to a multi-object multi-grasp task using our Multi-Object dataset. The trained network is the same trained network we’ve been reporting the results for (trained only on the Cornell dataset with both image-split and object-split variants). Testing involves evaluating against the multi-object dataset, and represents a cross domain application with unseen objects.
Fig. 7b depicts the plot of miss rate versus FPPI. At 1FPPI, the system achieves and prediction accuracy with image-split model and object-split networks, respectively. Visualizations of predicted grasp candidates are depicted in Fig. 8. The model successfully locates multiple grasp candidates on multiple new objects in the scene with very few false positives, and hence is practical for robotic manipulations.
|object||Top-1||Nearest to center|
Vi-D Physical Grasping
, performance is given for both the vision sub-system and the subsequently executed grasp movement. The dual scores aid in understanding sources of error for the overall experiment. To evaluate the vision sub-system, each RGB-D input of vision sub-system is saved to disk and annotated with the same protocol as the Cornell and Multi-Object datasets. The evaluation metric uses Jaccard indexand angle difference 30 thresholds. A set of 10 commonly seen objects was collected for the experiment. For each experiment, an object is randomly placed on a reachable surface at different locations and orientations. Each object is tested times. The outcome of physical grasping is marked as pass or fail.
Table pge shows the performance of both the vision sub-system and the physical grasping sub-system for two different policies. For the first (Top-1), we used the the grasp candidate with the highest confidence score. For the second, the planner chose the grasp candidate closest to the center of the object from the top- candidates ( in this experiment). In a physical grasping scenario in the real world, a grasp candidate close to the center of mass should be potentially helpful by creating a more balanced grasp for most objects. The lowest performing objects are those with a less even distribution of mass or shape (screwdriver and pliers), meaning that object-specific grasp priors coupled to the multi-grasp output might improve grasping performance. We leave this for future work as our system does not perform object recognition. Also, the tested objects are unseen.
|approach||Time (s)||Settings||Success (%)|
|||13.50||–||30||100||84 / 89*|
* Outcomes are for Baxter / PR2 robots, respectively, with the diffence arising from the different gripper spans.
Table pgc compares our experimental outcomes with state-of-the-art published works. The testing sets for reported experiments may include different object class/instance. Even though the object classes may be the same, the actual objects used could differ. Nevertheless, the comparison should provide some context for grasping performance and computational costs relative to other published approaches. The experiments in  had 7 objects in common with ours. On the common subset,  reports a success rate with a 60 execution time, while ours achieves with a 15 execution time (mostly a consequence of joint rate limits). The approach described in  reported success rate on 10 household objects and when using a cross entropy method  to sample and re-fit grasp candidates (at the cost of greater grasp detection time). The RL approach taking several weeks achieved on seen and unseen objects . Not included in the table are the reported results of , due to different experimental conditions. They reported 90% success on grasping objects from a bin with replacement, and 80% without replacement (100 trials using unseen objects). Our approach achieves in real-time, with subsequent planning of the redundant manipulator taking 0.1 secs. Overall it exhibits a good balance between accuracy and speed for real world object grasping tasks.
We presented a novel grasping detection system to predict grasp candidates for novel objects in RGB-D images. Compared to previous works, our architecture is able to predict multiple candidate grasps instead of single outcome, which shows promise to aid a subsequent grasp planning process. Our regression as classification approach transforms orientation regression to a classification task, which takes advantage of the high classification performance of CNNs for improved grasp detection outcomes. We evaluated our system on the Cornell grasping dataset for comparison with state-of-the-art system using a common performance metric and methodology to show the effectiveness of our design. We also performed experiments on self-collected multi-object dataset for multi-object multi-grasp scenario. Acceptable grasp detection rates are achieved for the case of 1 false grasp per image. Physical grasping experiments show a small performance loss (8.3%) when physically grasping the object based on correct candidate grasps found. The outcomes might be improved by fusing the multi-grasp output with object-specific grasp priors, which we leave to future work. All code and data will be publicly released.
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