In scenarios such as urban distribution, emergency material delivery, and warehousing and picking, people expect UAVs to independently complete the entire "pickup-transport-drop" process. However, mainstream multi-rotor usually adopts Bottom-mounted gripper or robotic arm, the package hangs under the body, causing problems such as a downward shift in the center of gravity and an increase in the moment of inertia. In order to break through this structural bottleneck, the team of Associate Professor Zhao Huichan of the Department of Mechanical Engineering of Tsinghua University (Soft Robotics Lab) published a paper in the top issue of Robotics in May this year.The International Journal of Robotics Research Publish a paper "A Center-less Quadrotor Design with a Soft Enveloping Grasper for Aerial Grasping and Delivery Tasks". Thesis proposed"Hollow quadcopter + flexible covered gripper” architecture, by hollowing out the center of the fuselage as a cargo compartment and embedding inflatable silicone grippers on the inner wall, the center of mass is kept on the plane of the propeller, taking into account stable flight and efficient grabbing, and provides a compact and robust new solution for terminal logistics.

01 Research background

At present, mainstream aerial grabbing systems mostly adopt the following two ideas:1. Suspended robotic arm/gripper: Commonly used in aerial operation research, the gripper is installed at the bottom of the UAV. The grabbing action will cause the center of gravity to shift, reduce the stability of the flight controller, and make the control complex.2. Special grabbing mechanism Solutions such as magnetic suction, suction cups, and flexible grippers have high success rates for specific targets, but they are highly dependent on the packaging material and size, and their versatility and load capacity are limited.

At the same time, the central cabin of traditional multi-rotors is occupied by flight controllers, batteries and other equipment, and cargo can only be hung, which further amplifies changes in the center of mass and relies on manual or fixed base station loading and unloading.

face challenges

• The center of mass and inertia change drastically, weakening posture stability

• The coupling between the grabbing mechanism and the flight controller is complex and difficult to control.

• Highly dependent on manual work/base stations, it is difficult to truly achieve end-to-end autonomous delivery

02 Technical Highlights

This research focuses on the center of mass during UAV aerial grabbing and delivery.Control, grab adaptability and landing tolerance Addressing the three core difficulties, a high-performance flight platform is proposed that integrates a central load layout, flexible covering jaws and passive load guidance, which significantly improves the multi-rotor's comprehensive capabilities in load control, mission adaptability and operating efficiency.

Hollow quadcopter design

The Center-less Quadrotor proposed by the research team breaks the traditional "center-axis arm" layout and uses a quad-rotor frame designed around a hollow load bay to place the load directly in the center of the UAV.

Image source: Yu Herng Tan et al., "The International Journal of Robotics Research" (2025), paper "A Center-less Quadrotor Design with a Soft Enveloping Grasper for Aerial Grasping and Delivery Tasks".

Advantages:
1. Ensure that the load is always located near the center of gravity of the UAV;
2. Reduce attitude interference during flight and improve control consistency;
3. The load can be freely entered and exited through the upper and lower channels, supporting a variety of operating modes; 4. The power battery, flight controller and ESC are respectively fixed in the four corner beams, avoiding heat dissipation, wiring and structural conflicts caused by the stacking of traditional center cabins.

Flexible covered gripper

In order to be compatible with packages of different sizes and materials, the research team laid a single-cavity silicone soft sleeve on the inner wall of the 90mm hollow warehouse, and used an onboard air pump to inflate and drive to achieve full-coverage grabbing. Tests have shown that the clamping jaw can hold objects up to 500g and reliably carry loads up to 60% of its own weight. It has the following characteristics: 1. Ring-shaped silicone clamping layer with built-in air cavity. After ventilation, it expands inward to cover the target, filling it in an average of 9.8 seconds and resetting after deflation in 5.8 seconds; 2. High adaptability: can adapt to objects of various sizes and shapes; 3. High safety: soft materials reduce the squeezing force on objects, suitable for grabbing fragile objects.

Image source: Yu Herng Tan et al., "The International Journal of Robotics Research" (2025), paper "A Center-less Quadrotor Design with a Soft Enveloping Grasper for Aerial Grasping and Delivery Tasks".

Passive load guidance system (PPP)

Targeting crawl-time UAVlocalization error To solve the problem, the team innovatively designed a passive load guidance system and integrated a PTFE telescopic ring under the body:

• During the flight phase, the diameter of the ring remains about 210mm, forming a capture window that is much larger than the gripper; at the moment of landing, the guide ring is pressed and retracts along the slide rail, with a diameter of about 80mm, pushing the package into the flexible gripper located in the center of the fuselage;
• It does not require motors or sensors and has a lightweight structure, which expands the effective capture area by about 6.9 times, significantly relaxes UAV landing localization accuracy requirements and improves the success rate of autonomous capture.

Image source: Yu Herng Tan et al., "The International Journal of Robotics Research" (2025), paper "A Center-less Quadrotor Design with a Soft Enveloping Grasper for Aerial Grasping and Delivery Tasks".

03 Experimental testing

Multi-object "grab-move-throw" demonstration

The UAV is equipped with a flexible gripper and has completed flight tests for grabbing a variety of typical objects, including beverage cans, glasses, soft packaging boxes, and special-shaped toys. In addition, the system also supports extended functions such as mid-air inverted grabbing and vertical column docking, which has good practicality and expansion potential.

• Grab-takeoff success rate 100\%

• During transportation, the mean square error of roll/pitch attitude remains ≤ 0.6 °

• Release point horizontal error\<5 cm

Image source: Yu Herng Tan et al., "The International Journal of Robotics Research" (2025), paper "A Center-less Quadrotor Design with a Soft Enveloping Grasper for Aerial Grasping and Delivery Tasks".

Passive load guidance testing

The team tested the guidance effect of the Passive Payload Guidance System (PPP) during autonomous landing.test scenario

• The UAV lands above the target object, and the PPP deploys to guide the object into the gripper area.

• Through the ground reaction force, the ring guide automatically shrinks and accurately sends the object into the grasping range.

Test results

• The guide ring expands the capture area by approximately 6.9 times, significantly relaxing the landing localization error.

• No motors or sensors are required, which can reduce reliance on high-precision environmental perception and control to a certain extent.

Image source: Yu Herng Tan et al., "The International Journal of Robotics Research" (2025), paper "A Center-less Quadrotor Design with a Soft Enveloping Grasper for Aerial Grasping and Delivery Tasks".

Resource Express
Paper link:
https://journals.sagepub.com/doi/10.1177/02783649251339772
The content of the article is only for academic exchange and technology sharing. The copyright of the graphic and text materials belongs to the original author and the journal. If there is any infringement, please contact us to delete it.