these interventions and typically a byproduct. Whereas in closed-loop, it is possible to realize vehicle motion control request via the haptic feedback control using the angular position signal overlay. There are two advantages: (i) the two functions directly interact with each other and thus provides a better steering torque response, when driver is in the loop and (ii) the haptic feedback impedance and admittance control both behave as a position controller for vehicle motion control function with driver out of the loop.
It should also be noted that the results from coupled stability analysis (in Section 3.2.2) are valid for similar closed-loop interconnected systems such as hardware-in-the-loop test rigs, force-feedback in driving simulator, etc. Based on the discussion above and Chapter 3 and 4, some general conclusions can be derived as shown below, connected to the research questions.
(a) Lower level control:
− Better inner-loop (or closed-loop) performance in impedance control than admittance control. The admittance control, in general, is affected by higher and/or variable mechanical (or hardware) impedance.
− The linear time invariant admittance control is relatively less robust than impedance control for variable mechanical impedance. Hence, the perfor-mance and robustness are consequently affected.
− The interdependent objectives: (i) reference tracking, (ii) hardware impedance compensation and (iii) robustness could inherently conflict with each other in admittance control. However in impedance control, these objectives can be simultaneously satisfied.
(b) Higher level control:
− It is possible to realize faster reference dynamics (on the outer-loop) in admit-tance control than impedance control; assuming the admitadmit-tance closed-loop control bandwidth (on the inner-loop) is sufficiently high.
− For non-linear Coulomb friction dynamics, the impedance reference requires a feedforward implementation as compared to the feedback connection in the admittance reference.
(c) Comparatively easier to realize the angular position overlay in a closed-loop set-ting, for merging haptic feedback control and vehicle motion control functionali-ties, than open loop control.
5.2 Future work
The reference steering feedback response in the higher level control (regardless of the steering system) needs significant development. Some of the important elements, which should be taken care of as next steps, are as follows:
50 Chapter 5. Summary
• To append the driver excitation reference generator for higher vehicle lateral ac-celeration range (by including tire and vehicle non-linearity).
• Development of a robust steering rack force observer for realization of external road excitation in the reference generator.
• To improve the steering feedback response for lower vehicle speed range (or park-ing maneuvers).
Similarly, the lower level control will be investigated further on the following aspects.
• Improving the performance and robustness of admittance control in electric power assisted steering system. This is to compensate the equivalent steering rack me-chanical impedance with multivariable feedback control approach (by including torsion bar torque signal).
• Improving the performance and robustness of admittance control in force-feedback system. This is to overcome the drivers’ arm inertia uncertainty with multivariable feedback control approach (by including torsion bar torque signal).
The vision is to develop the closed-loop haptic controller, for a personalized steering feedback response. Basically, it means that the driver could choose a weighting on different excitation sources. For instance, a virtual steering feedback is provided in case the driver wants to completely isolate the external road feedback. On the contrary, a more realistic road feedback is provided to the driver for limit handling maneuvers.
This thesis has presented the shortcoming of the existing open loop control (for electric power assisted steering and steer-by-wire force-feedback systems) and enabled a step towards understanding of closed-loop steering feedback control function. The improvement points, as mentioned in this chapter, will be considered in the next steps.
The experiments will also be performed to evaluate the closed-loop steering feedback control function on a prototype vehicle, with real-time test equipment, for both the steering systems.
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