Developing a Hybrid Simulation of Autonomous Air-to-Air Refuelling Procedures Using NI VeriStand and PXI Express

Dr. Thomas Richardson, University of Bristol

"The highly technical challenges involved in operating a full-scale robotics test bed in real time, integrated with a full nonlinear flight dynamics environment, could only be addressed using the real-time NI hardware and software."

- Dr. Thomas Richardson, University of Bristol

The Challenge:

Accelerating the development of autonomous air-to-air refuelling technology by creating a low-cost, full-scale, reliable, and safe sensor-in-the-loop robotic test environment that can interface with external controllers, sensors, and software models.

The Solution:

Using NI VeriStand software with modular NI PXI Express hardware to combine the execution of flight dynamic models with the ability to communicate with commercial robot controllers and bespoke sensors, all of which were monitored and controlled via an intuitive user interface.


Dr. Thomas Richardson - University of Bristol
Dr. Jon du. Bois - University of Bath
Ujjar Bhandari - University of Bristol
Dr. Peter Thomas - University of Bristol
Steve Bullock - University of Bristol



The Importance of Automation

Air-to-air refuelling (AAR) is the process of transferring fuel from one aircraft to another during flight. Probe-and-drogue, one of the main systems for AAR, uses a flexible hose that trails from the tanker aircraft. At the end of the hose are a reception coupling, a drogue, and an aerodynamic canopy used to stabilise the coupling.


A rigid arm, called the probe, typically extends from the nose or fuselage of the receiver aircraft. Currently, the pilot of the receiver aircraft must manually adjust the aircraft position to engage the probe with the reception coupling so fuel can be transferred between the two aircraft. AAR can be a demanding procedure to perform because it requires advanced training and fast response times.


As part of the Autonomous Systems Technology Related Airborne Evaluation & Assessment (ASTRAEA) programme, researchers at the Bristol University Department of Aerospace Engineering collaborated with Cobham Mission Equipment to develop a safe and reliable hybrid test environment to accelerate the development of autonomous AAR technology for unmanned systems. This laboratory-based robotic test environment would provide the initial system development at a significantly lower cost than actual flight tests and help to minimize risk in hardware and software demonstration and certification.


System Overview

The Relative Motion Robotics (RMR) facility at Bristol University offered a safe, reliable, and repeatable environment for the development of autonomous AAR technology. It consisted of two six degrees-of-freedom (6DoF) robotic manipulators, one of which was mounted on a linear track. We also mounted full-scale probe-and-drogue refuelling hardware on the robots, which accommodated a large range of relative motion for simulating the final 10 m approach and drogue capture during the aerial refuelling procedure.


Traditionally, these types of robots followed predetermined motion paths and were not configured for the level of real-time control required in this application. In order to overcome this limitation, we facilitated low-level control of the industrial controllers through the Open Robot Control Architecture (ORCA) developed by researchers at the University of Lund and a bespoke interface developed at Bristol with NI hardware.


An NI PXI real-time system served as the central hub for sensor-in-the-loop operation of the RMR facility. We used a real-time PXI controller running NI VeriStand software to seamlessly integrate full flight dynamic simulation models that were previously written with The Mathworks, Inc. Simulink® software. These models executed deterministically on the controller, acting on the information provided by the sensors mounted on the robotic arms. The simulation models also accounted for the tanker and receiver aircraft trajectory, navigation, and flight control systems; the dynamics of the aircraft with hose/drogue assemblies; and atmospheric disturbances.



We extended the NI VeriStand configuration-based software on the PXI Express controller using a supervisory custom device developed with NI LabVIEW system design software. The supervisory custom device executed parallel to the simulation models and orchestrated test procedures whilst preprocessing the position commands sent to the ORCA and robot controllers. The supervisory process also acted as the hub for all signals associated with the full system.


We also configured the modular PXI Express system with the necessary controller area network (CAN) and Ethernet interfaces for communication with the RMR sensor suite. The sensor suite helped the probe manipulator target and then track the movement of the drogue robot. The necessary sensor positions were subsequently passed to the PXI Express controller via a PXI CAN interface, before being fed into the aircraft simulation models executing in NI VeriStand.



Advantages of the NI Solution

When presented with the original requirements, NI’s dedicated academic engineers offered a variety of potential solutions at the right cost and proactively helped us identify challenges and ways to overcome them.


We decided on the PXI modular hardware platform alongside NI VeriStand and LabVIEW software, because we realized early on in the project that NI tools provided significant advantages over the less flexible alternatives we considered. NI VeriStand provided an out-of-the-box configuration-based environment that helped us easily import models compiled with The Mathworks, Inc. Simulink® software, before configuring the model input and output parameters with physical PXI hardware channels.


The NI VeriStand host running on a Windows 7 PC also provided an intuitive workspace, that interfaced seamlessly with the models executing deterministically on the NI PXI real-time system. The workspace not only provided quick access to all the parameters associated with the simulation, but also the ability for editing the user interface while in operation, should more specific model parameters require monitoring during a test.


Although NI VeriStand is configuration based, it provided us with the flexibility to create our own custom device. We developed the custom device with the user-friendly LabVIEW system design software, easing integration of the necessary supervisory processes whilst also ensuring they executed deterministically on the NI PXI Express real-time controller.



The integration between the NI hardware and software used in our solution was seamless and helped us interact with the external robot controllers, simulation models, and sensor hardware efficiently. We were able to take advantage of powerful configuration-based, yet expandable, software tools.


The NI software tools were easy to use. However, NI UK technical support was always available to answer questions and assisted us in accelerating our research and reducing overall development times.


The RMR facility project was an overriding success. NI tools acted as the vitally important hub so we could convert manufacturing robotic manipulators and off–the-shelf sensors into a real-time controlled, safe, and extendable autonomous AAR testing environment.


With the NI system at the centre, the RMR control system has been used to further develop high–fidelity, multientity flight dynamics models. We have presented sample results from a simulated AAR exercise on the facility to industry and at conferences worldwide. These results demonstrate the suitability of the RMR for conducting advanced tests of aerial refuelling hardware and sensors for the purpose of developing automated aerial refuelling capabilities.


The University of Bristol would like to thank NI for its support throughout the project and for helping us develop a world-leading facility. The highly technical challenges involved in operating a full-scale robotics test bed in real time, integrated with a full nonlinear flight dynamics environment, could only be addressed using the real-time NI hardware and software. We look forward to continuing to develop the facility and working with NI on this and future projects.


Author Information:

Dr. Thomas Richardson
University of Bristol
University of Bristol, Queen's Building, University Walk, Clifton
Bristol BS8 1TR
United Kingdom

Figure 1. The Relative Motion Robotics (RMR) Facility at the University of Bristol, Where Robot 1 in the Foreground has an Aircraft Refuelling Probe Mounted to it and Robot 2 in the Background has a Refuelling Drogue Attached to it.
Figure 2. Layout of the Sensor-in-the-Loop Robotic Test Environment for Air to Air Refuelling
Figure 3. Close up of the Sensing Hardware Mounted on Robot 1 Above the Aircraft Refuelling Probe
Figure 4. PXIe Hardware Running the VeriStand Real-Time Engine Alongside the VeriStand Workspace User-Interface Displaying the Parameters of the Flight Dynamics Models and Sensor Readings