Robust Control of Diesel Drivelines

The original contribution of this thesis is to provide insight into research of non-traditional control techniques for automotive power train applications, culminating in experimental evidence of much improved performance and reduced commissioning costs. This includes much work on the technique of Observer Based Robust Control (OBRC) which, before the research documented in this thesis commenced, was only in its infancy with some promise being shown through the simulation of electric drive applications (Dodds, 2007). The thesis, therefore, contributes to the process of bringing this new control technique nearer maturity. OBRC is based on an observer designed to provide information enabling effective control of an automotive power train application and its performance assessment. Comparison with traditional and other robust control techniques is included. The observer in OBRC is is designed to estimate the equivalent disturbance input, referred to the control input to a plant. This represents plant modelling errors as well as external disturbances. The equivalent disturbance estimate is applied to the real plant input to cancel its effect, thereby reducing the control problem to that of controlling the known real-time model of the plant employed in the observer. One of the disadvantages of conventional robust control methods, such as those based on sliding mode control, is that relatively high gain control loops are closed around the uncertain plant. This increases the risk of instability due to the dynamic elements, such as sensor lags, that are not included in the assumed plant model. The initial reason for investigating OBRC is that the high gain loops are applied to the known plant model in the observer and that the stability of these loops, taken in isolation, can therefore be guaranteed. It was found that the observer gains are limited only by the finite sampling frequency of the digital processor. In theory, infinite observer gains would yield ideal robustness. However, in practice only finite gains are possible. The aforementioned application of the equivalent disturbance estimate to the real plant input effectively transfers the high gain loops from the plant model in the observer to the real plant. This meant that closed loop stability could not be guaranteed under all circumstances. In view of this, it was decided that sliding mode control should not be excluded from the set of controllers for comparison. Since the control chatter associated with basic sliding mode control has to be eliminated for the vehicle application, polynomial control (a continuous version of the discrete RST controller) with robust pole assignment is included. This polynomial control can be regarded as equivalent to the sliding mode control with a boundary layer, but without the uncertainty associated with the choice of the boundary layer width. Two more controllers, based on the Internal Model Control (IMC) and H-infinity, are included for comparison on the basis that their design methodologies do not demand high gains. The various control techniques are demonstrated and compared via their application to Diesel Drivelines for commercial road vehicles.
One of the operational problems with conventional PI engine speed controllers is the need for time consuming initial controller tuning. This requires different sets of gains for each gear selection, including idle (i.e., neutral) and later retuning to compensate for changes in the driveline characteristics with component aging. A major advantage of OBRC in this application is the elimination of the tuning procedure. Of particular interest is the fact that the order of the system is increased by two when a gear is engaged due to a vibration mode created by the finite torsional compliance of the propeller shaft and other driveline components. Since this driven mechanical load can be represented by its inverse dynamic model in a feedback path whose output acts at the same point as the control variable, the OBRC compensates for this automatically without the need for any parametric changes. The simulations and experimental work were carried out on the DAF 12 litre diesel engine. The comparative study was carried out, not only with respect to the main application of Diesel Drivelines, but also using academic examples that are even more demanding of the controllers’ capabilities.
A key parameter in an engine configuration is the saturation limit on the injected fuel rate, which is highly dependent on the engine capacity from 10 litres to 16 litres. One example was introduced with a saturation block and the abilities of the various controllers under this constraint were assessed. The H-infinity controller could not handle such a saturation constraint, which is common practice in automotive applications and therefore this had to deemed unsuitable. The remaining controllers were able to operate with fuel rate saturation.
The overall conclusion is that the controllers based on OBRC, polynomial control and IMC are capable of a similar performance with appropriate controller parameter settings. However all are subject to the trade-off between the conflicting requirements of short response times and robustness.