Dynamic Modelling and Open loop Control of a 2 DOF Twin Rotor Multi-Input Multi-Output System

A dynamic model for the characterisation of a two-degree-of-freedom (DOF) twin rotor multi-input multi-output system (TRMS) in hover is extracted using a black-box system identification technique. Its behaviour in certain aspects resembles that of a helicopter, with a significant cross-coupling between longitudinal and lateral directional motions. Hence, it is an interesting identification and control problem. The extracted model is employed for designing and implementing a feedforward/open-loop control. Open-loop control is often the preliminary step for development of more complex feedback control laws. Hence, this paper also investigates open-loop control strategies using shaped command inputs for resonance suppression in the TRMS. Digital low-pass and band-stop shaped inputs are used on the TRMS test-bed, based on the identified vibrational modes. A comparative performance study is carried out and the results presented. The low-pass filter is shown to exhibit better vibration reduction. When modal coupling exists, decoupled feedback controllers are incapable of eliminating vibration. In such cases, generating motion by shaped reference inputs is clearly advantageous.