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Control of power steering motor based on XC164CS

because permanent magnet synchronous motor (PMSM) has the characteristics of simple structure, small volume, light weight, low loss and high efficiency, compared with DC motor, it has no mechanical commutator and brush, and compared with asynchronous motor, it does not need reactive excitation current, so the power factor is high, which will be the first choice of automotive electronic control products. But its control algorithm mainly has two kinds:

first, according to the principle of space voltage vector PWM, eight possible switching states form six possible magnetic field directions (plus two zero vectors). In one PWM cycle, the three-phase current is collected in real time, and the stator magnetic field orientation theory is used to calculate the respective action time of the six space voltage vectors, so as to realize the servo control of permanent magnet synchronous motor. The advantages are high precision, good dynamic performance and small size. Therefore, the main control chip is required to have high computing speed and data processing ability in order to achieve real-time control

second, the microcomputer calculates the torque and corresponding current of the motor, and then calculates the instantaneous value of the three-phase current according to the current rotor position angle. Through the current closed-loop control, the actual stator current of the motor is close to the command current by using the voltage source PWM inverter with fast tracking characteristics, so as to realize the servo control of the permanent magnet synchronous motor. The current closed loop is generally composed of analog circuits such as digital to analog converter and current comparator

due to the high product cost and complex control system, the above two control methods hinder a large number of promotion and application in automotive electronic products

as an important part of automobile, the steering system has developed from mechanical steering system and hydraulic power steering system to today's electric power steering system [1]. Electric power steering system is a kind of steering system that directly depends on the motor to provide auxiliary torque. Compared with traditional steering technology, automotive electric power steering system (EPS) has many advantages, such as energy saving, convenient operation and driving safety, and can reduce the labor intensity of drivers. The traditional drive motor is mainly DC brush motor, and there are also products using brushless DC motor, but its drive mode is 60 ° commutation three-phase six state working mode. Due to 60 ° commutation, the control accuracy is ± 30 °, so there is a large torque fluctuation

based on the working principle of permanent magnet synchronous motor, this paper proposes a field oriented control algorithm of permanent magnet synchronous motor based on XC164CS single chip microcomputer to realize the control of automotive electric power steering motor

working principle of permanent magnet synchronous motor

because DC motor has very excellent linear mechanical characteristics, wide speed regulation range, large starting torque, simple control circuit and other advantages, it has been widely used in various driving devices and servo systems for a long time. However, the mechanical commutator and brush of DC motor make its structure complex and reliability poor. In particular, the changing contact resistance, sparks, noise, etc. are particularly prominent in low-voltage and high current applications. For example, for this system, the output power of DC motor is 340W, and when the rated voltage is 12V, the input current is as high as 50A. The current impact and noise generated by mechanical commutator and brush will seriously affect the performance of the system [2]

the three-phase sine wave permanent magnet synchronous motor with field oriented control not only retains the advantages of DC motor, but also has the advantages of simple structure, reliable operation and convenient maintenance of asynchronous motor. At present, it is widely used in high-end electrical products

in the permanent magnet synchronous motor system with field oriented control, the position angle of the rotor is detected in real time ε, Make the current space vector always remain on the q-axis, that is δ= 90 °, so that the permanent magnet synchronous motor has the same excellent linear torque characteristics as the DC motor. The magnitude of the electromagnetic torque will be directly proportional to the magnitude I of the current space vector. Select rotor position angle ε The zero point of is coincident with the a-axis, as shown in Figure 1 [3]

Figure 1 vector diagram of permanent magnet synchronous motor

realize the field oriented control algorithm and realize the control algorithm

according to the EPS system control principle block diagram in Figure 2 and the vector diagram of permanent magnet synchronous motor in Figure 1, when the saturation of motor core, eddy current and hysteresis loss of motor are ignored, and the rotor has no damping winding, the electromagnetic torque and stator flux linkage equation is [4]:

tem = NP( ψ diq- ψ qid) (1)

ψ d=Ldid+ ψ f (2)

ψ Q = LQ IQ

the stator voltage equation is: (3)

in formula (3), NP is the number of poles of the motor, RS is the stator resistance, LD and LQ are the equivalent inductance of the stator inductance on the D and Q axes (for the PSMS selected in this system, LD = LQ), ψ F is the constant determined by the motor structure of the flux linkage of the rotor magnetic field passing through the stator winding. UD and UQ are DQ axis voltage, ω M is the electrical angular velocity of the rotor, u is the space voltage vector synthesized by UD and UQ, and ω M rotation

when the stator current vector is controlled to fall on the q-axis, that is, id=0, IQ = I

from equations (1), (2) and (3), the DQ axis voltage equation and electromagnetic torque can be simplified as: (4)

through park inverse transformation and Clarke inverse transformation, the following is simplified: (5)

where Ke is the motor potential constant, β Is the offset angle, which is related to the current and can be calculated by UD and UQ

The EPS system controls the DQ axis voltages UD and UQ of the permanent magnet synchronous motor, so that the current space vector is always maintained on the q axis, so that the permanent magnet synchronous motor has the same excellent linear torque characteristics as the DC motor. Through the above method, for the upper control strategy of EPS system, its control object is equivalent to a DC motor. Therefore, a large number of EPS control strategies based on DC motor can be easily applied to this system

the control principle block diagram of EPS system based on Infineon XC164CS MCU is shown in Figure 2. The actuator is a permanent magnet synchronous motor. Since it essentially supplies power to the current controlled voltage source inverter (SPWM), the functions in the dotted box in Figure 2 are realized by the single chip microcomputer software. The power assisted motor control principle of EPS is as follows, which is shown in Figure 2: the required power assisted torque or return torque calculated according to the control strategy of the upper layer (the rotating torque of releasing both hands and restoring the steering wheel to the middle position) is the input command torque tm* of the motor control, from which the command current iqef is generated. At the same time, the system collects the current IA, IC, and calculates ID, IQ through Clarke transform and park transform. Where IQ is compared with the command current iqef, and UQ is generated through the PI regulator; The system sets idef=0, and compares it with the ID of feedback. Through PI regulator, UD can be generated. Calculated by UD and UQ β, The three-phase stator voltages UA, Ub and UC are calculated by park inverse transformation and Clarke inverse transformation. By adjusting the three-phase stator voltage, id=0, iq=iqef, the motor output electromagnetic torque follows the command torque tem*

Figure 2 EPS power motor control schematic diagram

pwm output mode

for the generation of six PWM control signals, regular sampling SPWM method is adopted. In order to reduce the working noise of permanent magnet synchronous motor, the six PWM carrier frequency is 20kHz; The sampling period of the system is 1ms (that is, the stator voltage amplitude is changed only once every 1ms), so that MCU has sufficient time to complete a large number of control strategy operations. The drives of v1v4, v3v6 and v5v2 are complementary and symmetrical, controlling the phase voltages of three-phase windings a, B and C respectively, and the sampling centers of output waveforms are symmetrical, as shown in Figure 3. If the duty cycle of any phase dmx=50%, the fundamental voltage of that phase in a PWM cycle is zero. The output control equation of three-phase PWM is as follows:

dma = 49 × Umsin( ε+β)+ 50

Dmb=49 × The fabric products produced by UMS based on SABIC pp514m12 polymer show better moisture absorption and barrier properties in hydrostatic test( ε+β- 120°)+50 (6)

Dmc=49 × Umsin( ε+β- 240 °) + 50

, where: um is the ratio of the amplitude of space voltage vector u to the maximum output phase voltage

formula (6) using coefficient 49 can avoid the state of duty cycle dmx=100% or dmx=0% of any phase, which makes the bootstrap circuit of IR2110 work abnormally, resulting in the failure of the upper bridge arm to make and break normally. The offset of 50 makes the PWM waveform meet the centrosymmetry, and makes DMX positive, simplifying the control calculation

in the EPS system, by adjusting the three-phase duty cycle, the three-phase stator voltage of the permanent magnet synchronous motor is controlled to realize the field oriented control of the permanent magnet synchronous motor for steering assistance

Figure 3 PWM output waveform

ECU development based on Infineon XC164CS single chip microcomputer

according to the characteristics of the controller, the ECU design is divided into three modules: MCU module, signal conditioning module and motor drive output module

mcu module

in ECU, MCU is the core of control. Different from industrial electronics, automotive entertainment electronics and other products, it should be based on the MCU that is safe, reliable and successfully applied in the automotive control system. Infineon's automotive MCU is widely used in the middle and high-end automotive control system in Europe. Infineon's XC164CS chip is a 16 bit embedded microcontroller, and the main frequency of the system is 40MHz; Access to internal ROM and flash memory is 64 bit wide; Seven PWM outputs, six of which can be configured as complementary symmetrical outputs with adjustable dead time; 14 channel high-speed ADC, with single channel conversion time less than 2 μ s; Built in dual can drive function module and OCDs (on chip debug system) interface [5][6]. In EPS system, steering wheel angle and torque, phase A and C current of motor, total current, temperature of motor and power MOSFET are 10 analog quantities, which are input to P5 port of MCU through respective conditioning circuits; The rotation angle of the motor is detected by the resolver, converted into 10 digit digital quantity through RDC, and input by P0 port; Six channel PWM is output by P1l; Control input and protection output are realized by P9 port; Fault records are saved by EEPROM

signal processing module

signal processing module is mainly composed of 10 channel analog signal amplification and filtering circuit, switching input and output level conversion circuit, RDC corner detection circuit, etc. Rotor position detection, angle digital conversion, and steering wheel torque angle signal acquisition and processing are the key parameters of EPS system

rotor position detection

the position sensor of permanent magnet synchronous motor detects the position angle of the rotor in real time, calculates the three-phase duty cycle according to the position of the rotor, and controls the conduction and cut-off of MOSFET. Therefore, the detection accuracy of rotor position angle is related to the accuracy of current space vector positioning on the q-axis, and it is the key to improve the performance of the system

magnetoresistive resolver is a kind of signal element whose output voltage changes with the rotor angle. Its structure is solid and durable. It is not afraid of vibration and shock. It can work at high temperature. It has a strong ability to adapt to the environment and is more suitable for installation in the strong vibration and shock environment such as the automobile chassis. Its stator has a group of excitation windings and two groups of output windings with a spatial difference of 90 °. The rotor is a permanent magnet, and its magnetic pole direction is consistent with that of the permanent magnet synchronous motor. Its unique shape can form a special air gap magnetic field between the stator and rotor. When the excitation winding is excited by a certain frequency of AC voltage UE, the voltage amplitude of output windings U1 and U2 and the rotor angle ε Into sine and cosine function. Namely:

ue = ESIN ω t

U1= α Esin ω t Cos ε (7)

U2= α Esin ω t sin ε

in formula (7): e is the excitation potential, α Is the transformation ratio. ω Is the excitation frequency

therefore, the motor rotor position angle can be calculated by detecting UE, U1, U2 ε。

corner digital conversion circuit

the corner conversion circuit adopts rdc19222 chip, as shown in Figure 4, which consists of 7V ~ 10kHz excitation circuit and corner conversion circuit

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