One of the difficulties of a hybrid electric vehicle (HEV) powertrain with two electrical driving axles is the ability to distribute the electrical current of one high-voltage battery appropriately to the two independent electrical motors. Depending on vehicle driving conditions or the maximization of the entire efficiency chain of the system, a suitable control strategy is necessary.
At the 2012 SAE World Congress, researchers from IAV will present their proposal for an input-output feedback linearization strategy to cope with a nonlinear system subject to input constraints. (See http://papers.sae.org/2012-01-1007.) This approach needs an external, state-dependent saturation element, which translates the state-dependent control input saturation to the new feedback linearizing input, while preserving the properties of the differential geometric framework.
Hybrid structures can be divided into serial, parallel, and combined HEV architectures. A combined HEV concept requires that some nodes are physically linked together. For example, the Toyota Prius structure, which constitutes a power-split architecture, includes a planetary gear set.
In the proposed IAV HEV architecture, the powertrain comprises of a parallel HEV structure with two independently controllable electrical axles. With this physical implementation of a HEV, the structure has two full degrees of freedom to satisfy the power demands from the driver.
In a hybrid structure with two or more motors, one generally suffers from the current distribution limits of just one battery. An easy solution is to force the motors to be in different operating quadrants.
This fairly easy solution will require that one of the electrical elements is in generator operation and the other in motor operation. This, however, may fail for certain vehicle operating conditions of the HEV.
For example, many car maneuvers with two electrical axles will require that both electrical axles provide positive torque to avoid critical vehicle driving conditions. To access the full potential of the proposed hybrid structure, a control scheme is necessary for proper distribution of the current limits, especially in the case of battery or machine ratings.
The hybrid operating strategy must consider different operating constellations of electrical motors that are technically possible for one and the same vehicle operating point. For instance, external interventions from the vehicle stability program will determine the torque limits for the primary electrical axle to satisfy certain vehicle driving conditions for safety, or the hybrid operating strategy chooses torque limits to improve system efficiencies. In general, one false component of the control strategy could lead to dangerous vehicle driving conditions and system inefficiencies. These issues lead to a layered control scheme, with the current limits distribution control strategy located at the bottom layer.
Input-output feedback linearization can easily and in a systematic way be applied to nonlinear processes with affine control inputs. The methodology has the advantage that the design procedure can be divided into two independent development steps.
The first step is the transformation of the nonlinear process into a linear process using a nonlinear coordinate transformation. In the second step, the classical control theory can be applied to obtain a linear controller for a stable, fast, or well-damped controlled process. Therefore, adjustments for the process response will not lead to a time-consuming nonlinear redesign of the controller.
The input-output feedback linearization technique has been addressed by others. However, this control design method is significantly diminished in the face of input constraints. To be applicable to the current limits distribution problem, a technique that uses state-dependent constraints for the transformed input variable is necessary.