# Improvement on Flux Weakening Control Strategy for Electric Vehicle Applications
## 5 Optimized Control Strategy for Flux Weakening Operation  
This paper proposes a FF control loop based on the estimation of the \(i_{sd}\) current setpoint, according to Equation (8). The objective of the proposed FF loop is improving  
Figure 11: Experimental results of outer voltage control [17]. Current angle (orange solid line) and angular mechanical speed (blue solid line).  
Figure 12: Experimental results of Jahns’ control [16]. Current angle (orange solid line) and angular mechanical speed (blue solid line).  
the robustness of control strategy during abrupt torque transition which could lead to unwanted brake operation mode of the electrical machine.  
Experimental results show that the proposed approach leads to a good estimation of the d-axis current during braking operation in FW region. Especially, thanks to the FF control loop, the issue related to current angle calculation of Figures 11 and 12 is overcome.  
The introduced FF loop works as follows: if an abrupt torque transition occurs in FW region and the \(i_{sd}^{*}\) setpoint computed with the integral approach is lower than the \(i_{sd-safe}\) value of Equation (8), then the \(i_{sd-safe}\) is set as d-axis current setpoint. In so doing, the performance of the electrical machine as a motor is preserved and it won't move towards brake operation at high speed as a consequence of an unexpected torque transition. According to its action, the new \(i_{sd}\) current contribution is named \(i_{sd-safe}\). The corresponding improved control scheme proposed in [17] is shown in Figure 13.  
Figure 14 shows the experimental results of the improved outer voltage method of [17] with the \(i_{sd-safe}\) FF contribution. Figure 15 shows the current angle in acceleration and deceleration from standstill to 6000 rpm and back. During deceleration, the value of the angle will be overridden by the \(i_{sd-safe}\) of Equation (8) and the unwanted effect on the speed slope at the beginning phase of braking operation disappears providing a higher braking quality..  
Figure 14: Experimental results of outer voltage method of [17] with proposed FW control. Speed profile and \(i_{sd},i_{sq}\) currents.  
Figure 13: Control scheme of outer voltage method of [17] with the proposed \(i_{sd-safe}\) strategy.  
The presented FF solution was implemented also in [16] as well. In Figure 16 the additional \(i_{sd-safe}\) block is shown, while Figure 17 shows the behaviour in the experimental setup in acceleration and deceleration from standstill to 6000 rpm and back. The shape of the current angle in [16] is very close to that shown in Figure 15 for the outer voltage scheme. The experimental results with improved Jahns' control scheme confirm the improved behavior and repetitiveness due to FF, \(i_{sd-safe}\) loop in case of braking action.  
In addition to the \(i_{sd-safe}\) block, a particular voltage-based _anti-wind up_ method has been introduced in both enhanced control schemes with the aim to further improve their performance in FW, with the aim to keep the desired voltage request around the the saturation edge.  
The _anti-wind up_ block multiplies by an appropriate gain, denoted as \(\alpha\), the voltage error between the requested voltage vector amplitude and the available one. The output of the _anti-wind up_ block is then used to limit the integral terms of current regulators. In this way, the deep saturation of current regulators, due to battery voltage limit, is avoided. Consequently the dynamic performance improves, since current regulators will ask from the system the voltage that it can be realized. This solution is not particularly suitable for outer voltage strategy, since, it derives the current angle from voltage saturation.  
Figure 16: Proposed improved Jahns’ control scheme.  
Figure 15: Experimental results of outer voltage method of [17] with proposed FW control: current angle (orange solid line) and angular mechanical speed (blue solid line).