## 3 Control System Design  
Figure 4 shows the block diagram of the proposed boost converter-fed BLDC machine control system, which is composed of a power circuit, a control circuit and a remote monitoring circuit. The power circuit mainly deals with the driving and commutating of the boost converter-fed BLDC machine according to the control signals. The control circuit, the key to the system, is mainly responsible for power management, conditioning and sampling, isolation and protection, and digital signal processing (DSP). The power management circuits convert 270 V DC bus-voltage to different required levels according to the distribution scheme of the system. The conditioning and sampling circuits realize the conversion from analog signals, such as current, voltage, and temperature, to digital signals through sensor detections, amplifiers, filters, and analog to digital converter (ADC) chips. The isolation and protection circuits are designed to realize electrical isolation between power and control parts. The hardware protection circuits consist of some comparators and a complex programmable logic device (CPLD). DSP is used to calculate the sampling output of ADC and realize the closed-loop control. The remote monitoring circuits realize the anti-interference data transmission between the personal computer (PC) and DSP by utilizing a series communication interface (SCI) and a shielded RS-485 transceiver. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Control Factors**} & \multicolumn{2}{c}{**Results**} \\ \hline S & \(K_{1}\) & \(K_{2}\) & \(K_{3}\) & \(u_{1}\) & \(u_{2}\) \\ \hline
1 & 1 & 1 & 1 & 0 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The boost converter operating logic. To attain a small static tracing error, anti-disturbance ability, and rapid dynamic performance, the system adopts a control strategy that combines normal mode regulation with boost mode adjustment, as the block diagram shows in Figure 5.  
It can be seen from Figure 5 that the boost mode adjustment process consists of signal detection, control synthesis of \(S\), \(K_{1}\), \(K_{2}\) and \(K_{3}\), and a DC-link voltage closed-loop calculation. It ensures a reliable power supply under over-load or under-voltage fault situations according to the Equation (11). Normal mode regulation deals with rated states by adopting a conventional dual closed-loop proportional integral derivative (PID) control strategy, which is still the workhorse in industries despite the great evolution and rapid development of hardware [24]. The outer loop is the speed loop, which adopts proportional integral (PI) action, aimed at stabilizing speed and resisting load-disturbance. In contrast, the inner current loop adopts P action to deal with current stabilization and voltage-fluctuation resistance. Voltage, current and load torque are detected by corresponding sensors and converted into digital signals using A/D sampling circuits. Angular speed is calculated by hall signals utilizing event capture (eCAP) module of the DSP chip. The manual control signal is the backup control signal of the boost converter for debugging. There is almost no need for the BLDC machine here to adopt the flux-weakening control method. On one hand, this system is for an aerospace oil pump and the flux-weakening region does not suit the requirements of such pumps. On the other hand, the drive system may suffer from a flux-weakening failure, which may decrease the reliability of the whole system. Figure 4: Control block diagram. Figure 5: Control strategy block diagram. When a PID controller is utilized, it provides a pole and two negative real zeros for the system. The pole helps to improve the system order-type, eliminate or reduce the steady-state error, and improve the steady-state performance. The negative real zeros can increase the phase margin, adjust the relative stability, and improve the dynamic performance.