drive current control
The torque output by the motor is one of the important key parameters of the motor performance. The torque of the motor is related to the current of the motor. Based on the synchronous rotating coordinate system, Te is the torque of the motor, and id and iq are the currents of the d and q axes respectively. Therefore, the precise control of the torque is the precise control of the current.
When performing current control on the permanent magnet synchronous motor, the set currents Iq_Ref and Id_Ref of the d and q axes are input into the current PI loop, and converted into the set voltages Vq and Vd of the d and q axes, which are transformed into Vα, Vβ is converted into a, b, and c three-phase voltages and input to the motor through space vector modulation technology (SVPWM). Through the sampling resistor on the motor controller, the three-phase currents ia, ib, and ic obtained are transformed into Iα, Iβ by Clark, together with the motor position parameter θ obtained by the encoder on the motor, transformed into two axes of d and q by Park The actual current is fed back as the input quantity of the current loop control, thus completing the control of the primary current loop.
The method is a method for controlling the output of the electromagnetic torque of the motor by controlling the current of the winding of the permanent magnet synchronous motor. Reasonable design of the parameters of the current loop can weaken the fluctuation of the dq axis voltage, make the response speed of the system faster and the operation more stable.
Reasonable design of the parameters of the current loop can weaken the fluctuation of the dq axis voltage, make the response speed of the system faster and the operation more stable. Among them, the current closed loop is composed of four links: PI regulator, delay, PWM, and motor conversion. The transfer function is shown in the figure below.
The above control method is usually easy to implement and has good control performance. When the target torque required by the motor is different, it only needs to give different target currents, and the motor can be controlled to output the corresponding torque through the current loop to realize the power steering control of commercial vehicles. This method meets the basic needs of commercial vehicles, and driving current control is often used when commercial vehicles are running at low speeds.
Running speed control
Under different working conditions and usage environments, it is often not enough to only control the output torque of the motor. It is necessary to adjust the speed of the motor so that the motor can respond quickly. The steady-state accuracy and fast response without overshoot of the motor speed control are hot issues in current research, and the speed control is based on the current control method.
When controlling the running speed of the permanent magnet synchronous motor, the external acceleration control forms a speed loop on the basis of the current loop control. Input the set speed Speed_Ref of the motor into the speed PI loop to obtain the set currents Iq_Ref and Id_Ref of the d and q axes, and input them to the subsequent current loop for motor control, and compare the calculated motor speed parameter ω with the speed set value Speed_Ref The error value feedback calculation, as the input quantity of the speed loop control, completes a complete speed loop control, and its control block diagram is shown in the figure.
In the above speed loop control method, the set current input of the d-axis is set to 0, because the d-axis current does not generate an output force for the rotation of the drive motor, and it is usually set to 0 for input for the convenience of speed control. This method can control the motor speed well, but in practical application, it will be limited by the power of the system voltage inverter. The maximum output power of the motor and the maximum torque output at different speeds of the motor depend on the capability of the inverter, so the performance of the inverter has a great influence on the control method.
Rotation Angle Control
For the development of the active steering function of the commercial vehicle steering system, it is necessary to control the rotation angle of the motor. Active steering functions include centering control and corner servo control. The centering control of the vehicle is that under different steering conditions, the steering wheel and wheels can quickly return to the middle position when the driver does not have a rotational torque on the steering wheel. The angle servo control is to make the steering wheel angle follow the target angle control, and for the motor, it is a closed-loop control in which the actual angle of the motor tracks the desired angle.
When controlling the rotation angle of the permanent magnet synchronous motor, the position loop is formed by adding an angle control on the basis of the speed and current double-loop control. Input the set rotation angle Angle_Ref of the motor into the angle P loop to obtain the target speed Speed_Ref, which is input to the subsequent speed and current double loops for motor control, and the obtained motor current position parameter θ and the angle set value Angle_Ref are used for error value feedback calculation, as the position The input of the loop control controls the rotation angle of the motor.
In practical applications, because the counts returned by the motor encoder need to be calculated to obtain the motor speed ω, when the motor speed is slow, there is a large error in the obtained speed. If you want to reduce this error, you can only increase the unit time to obtain multiple sets of motor speed calculation values, but this will increase the delay of the entire system. In order to avoid errors caused by intermediate speed calculation links and system delays as much as possible, the speed loop can be removed, and only the position and current double loops are used to control the motor. At the same time, in order to make the motor achieve a more accurate position, the motor will inevitably accelerate and stop when it rotates. It cannot be controlled by pure P, and I and D should be added to realize PID loop control.
The control effect of this method is better than that of single-loop control, and it can respond to the motor rotation angle control command in time, so it is widely used in practice. Under the control of the position and current double loops, the target steering angle command is sent to the EHCS system through other control systems, and the motor rotation angle can be controlled to drive the wheels to steer, and finally realize the active steering function of commercial vehicles.