Techniques for Nonlinear Real-time Hybrid Testing

Real-time hybrid testing is a powerful experimental technique but it cannot presently be used for a majority of practical engineering applications because of problems with compensating for delays in the control loop. This work seeks to address this challenge using a novel combination of forward prediction and adaptive tuning of the mechanical properties of the system. Exploitation of the method in commercial settings will lead to improved quality and reduced time-to-market, and in an academic context it will offer a valuable experimental tool across a breadth of fields.

Real-time hybrid testing (RTHT) is a technique for performing experiments on components of large or complex structures in a laboratory. The component is physically present in the laboratory, but the rest of the system exists only in a computer simulation. Motors, actuators, and sensors create a virtual link between the two so they behave as one system. The technique can offer radical time and cost savings in industrial development programmes and provides a uniquely powerful research tool, but its potential is currently limited by delays in the motors and actuators linking the two parts of the system. State-of-the-art delay compensation techniques remain ineffective in the presence of strong nonlinearities, as found in many practical engineering systems such as automotive shock absorbers, aircraft control mechanisms, and materials used to design safe buildings and bridges. This project addresses the difficulties in performing real-time hybrid tests of common nonlinear components, to allow the realisation of the currently untapped potential of RTHT.

A novel approach is taken, drawing inspiration from the mechanisms used in the human body to compensate delays in nerve signal transmissions. A combination of two strategies is employed: forward prediction of the forces and positions required, and tuning of the mechanical properties of the system to compensate for errors in those force and position predictions. Two key research questions are to be addressed, concerning firstly the ability to tune the system to match the desired mechanical properties, and secondly the extent of the improvements this makes to the RTHT capabilities. Three objectives are identified, centring on the experimental investigation of this configuration and the evaluation of its performance: analytical studies of the system configuration will be conducted to determine appropriate component sizing; the system will be built and the effectiveness of tuning of the mechanical properties will be evaluated; the final objective is to conduct full RTHT experiments to determine the extent of improvements to fidelity and stability of the results.

There are a variety of reasons for wanting to conduct hybrid tests. One field where they have established marked success is earthquake testing of large civil structures. The motivation here is threefold: firstly, large structures such as buildings or bridges cannot be fitted into a laboratory for testing in their entirety. Secondly, the conditions experienced in an earthquake are not readily reproduced for an entire building. Thirdly, experimental testing may deliberately or inadvertently lead to failure, which in this case would be dangerous and expensive. Other examples include simulations of satellite docking in zero gravity, testing of aeronautical equipment without the danger or expense of flight testing, and testing of automotive components before the full vehicle has been manufactured. Businesses and researchers in these fields are poised to take advantage of new techniques, which will greatly expand the classes of system which can benefit from real-time hybrid testing.

The benefits to businesses in terms of increased productivity and reduced costs, and academics in terms of better research tools, will translate to end users and the general public in terms of better quality of products, higher standards of living and improved safety.

The impact of this real-time hybrid testing (RTHT) research will be to increase the capacity of engineers to design, build and test components of complex systems in a laboratory. The ability to test components in a realistic environment before the complete system has been built will permit much shorter project timescales, as has been proven by hardware-in-the-loop (HiL) testing for embedded electronic controllers. HiL testing is estimated to reduce project lead times by 15-50% with the associated cost savings. The adoption of RTHT in UK industry has the potential to offer similar efficiency benefits. The automotive and aerospace industries will be targeted in particular, with the project partner involvement of Cobham and Anthony Best Dynamics ensuring the work successfully targets the needs of these sectors.

The aerospace sector will benefit not only from reducing the high cost associated with flight testing of new equipment but also from a reduced risk to flight test personnel. More comprehensive test programmes will lead to improved designs which will in turn make vehicles more efficient, less expensive, and safer in operation - to the benefit of the environment, the economy and the general public respectively.

The automotive sector relies heavily on optimised development programmes and as such has been one of the major driving forces behind the development of HiL testing. It is likely that this sector will also be at the forefront of the exploitation of RTHT technology.

Another community that is always quick to assimilate improvements to RTHT is the earthquake engineering community. This is because of their unique requirements for the techniques, due to the scale of the structures and the magnitudes of the earthquake forces. In this area the proposed work stands to contribute to the design of safety features for large civil structures in areas of high seismic activity and further afield. These improvements will save lives.

The potential of the control techniques to influence research into man-machine interaction in robot control opens up further interesting possibilities. Man-machine interaction is important to new manufacturing processes, particularly for composite materials, where closer cooperation between people and machines will improve productivity and reduce costs in production lines. Better technology will also lead to a safer environment for workers.

As well as the benefits to UK industry and the safety of the public and workers established above, the outcomes from the research will be extended to the general public through educational outreach events. Working with schools and educational charities such as the Royal Institution of Great Britain, the engineering principles and the applications will be disseminated to children and adults alike to foster interest and appreciation. In particular, a video will be produced for the Royal Institution's Ri Channel, and the work will feed into Dr. du Bois' continued involvement with the Science and Engineering Masterclass series. This will help bolster future generations of engineers to the benefit of the UK economy.