Basic knowledge of electric power steering

1.EPS composition
For EPS to work normally, EPS ECU must first know the direction of rotation of the motor and the current of the motor, so the driver’s steering intention and steering torque must be known. In addition, for stability, as the vehicle speed increases, the boosting effect should be reduced, and the degree of reduction should be sensed by the vehicle speed sensor.

In the past, EPS mainly used dual-coil torque sensors, which could only detect the torque of the steering wheel, but could not detect the angle and angular velocity of the steering wheel rotation, which made it difficult to achieve precise control. Therefore, its development trend tends to be the torque sensor and the rotation angle sensor. It is integrated and adopts non-contact structure, such as magnetic ring-Hall type, magnetic ring-magnetoresistive type, photoelectric type, microwave type and other sensors to adapt to the development of automobile intelligence and integration.

(1) Sensor
①Torque sensor. The function of the torque sensor is to collect the torque applied by the driver on the steering wheel, and input it to the ECU after processing. This signal is one of the main control signals of EPS.

As shown in Figure 1, the integrated circuit composed of magnetoresistive elements is used as the sensor head, and the signal wheel is a multi-pole magnetic rotor that rotates synchronously with the steering column. The torsion bar) is driven by the steering column to drag the pinion to rotate, thereby pushing the rack to move. Due to the twisting of the torsion rod during turning, the magnetic rotor and the sensor head using the magnetoresistive effect are staggered, so that the magnetic flux passing through the magnetoresistive element changes, and this change is amplified and then input to the ECU.

Basic knowledge of electric power steering
Figure 1 Magnetic ring-reluctance torque sensor

②Rotation angle sensor. The function of the rotation angle sensor is to collect the steering angle and angular velocity applied by the driver on the steering wheel, and input them to the ECU after processing. This signal is one of the main control signals of EPS. When the signal fails, the emergency operation mode is activated, replaced by the substitute value, and the electronic power steering still works, but the fault indicator light is always on.

The rotation angle sensor is usually photoelectric, which uses the grating principle to measure the angle. The composition of the photoelectric angle sensor is shown in Figure 2. The encoder disc is divided into three layers. The innermost ring layer has nothing to do with the principle. The middle layer is a signal disc with uniform openings, and the outermost ring layer is a signal disc with uneven openings. The light source is between the two outer signal plates.

Basic knowledge of electric power steering
Figure 2 The composition of the photoelectric angle sensor

The light beam hits the sensor through the aperture, generating a voltage signal. If the light is blocked, the voltage disappears. Since the opening of the signal plate in the middle layer is uniform, and two different voltage sequences of high and low are generated when rotating, the voltage signal generated by the signal plate is also a regular signal. The outer signal plate generates irregular high and low voltage signals due to irregular openings. Comparing the two signals of regular and irregular, the system can calculate the angle of the encoder disk. The outer signal disc determines the starting point of the movement.

In addition to basic signals such as steering wheel angle, steering wheel torque and vehicle speed, the input signal of ECU also has various auxiliary signals such as vehicle yaw rate, lateral acceleration, front axle load, etc., which are mainly used to judge the ground adhesion. Change to correct the steering motor current.

③Current sensor.
current sensor. Located on the EPS ECU, it is used to detect the current of the motor loop, which can be inductive or Hall type.

(2) ECU
The structure of EPS ECU is shown in Figure 3. ECU is the control core of EPS. It performs calculation and analysis based on the input signals of each sensor to obtain the optimal value of control parameters, and then sends control commands to the motor and clutch to control their actions. The control system and control algorithm of ECU are also one of the key technologies of EPS. The control system is required to have good anti-interference ability, real-time control, and safety protection and fault self-diagnosis functions. The ECU adopts a hardware system with 8-bit or 16-bit single-chip microcomputer as the core, and writes appropriate control programs to realize the control of steering wheel torque, steering speed, steering return characteristics, steering road feel, and motor current.

Basic knowledge of electric power steering
Figure 3 EPS ECU structure

F/S relay is also called fail-safe relay or main relay. EPS ECU monitors the input and output of the system and the drive current of the motor to realize the fault self-diagnosis function. If a problem occurs, the system is shut down through the action of the fail-safe relay in the control unit, and the commutation FET (FET is a field effect transistor) , Mainly used for motor commutation) the drive signal is cancelled. In this way, the steering assist is cancelled, the system returns to manual steering, and the FPS warning light flashes to alert the driver.

Some control units have a temperature sensor inside. When the temperature exceeds 100°C, it starts to reduce the current of the motor to prevent the electronic components from failing due to overheating. When the current decays to less than 60% of the limit, the fault warning light is on. The motor relay is the relay on the motor circuit. The current sensor monitors the motor loop current for feedback.

EPS ECU working conditions: when the ignition switch is turned on (or the vehicle power is in the “OK” gear), power on the ECU and the electric power steering device is in a standby state. When the car is driving and turning, the EPS ECU sends out control commands (electrical signals) to the motor and electromagnetic clutch after comparing and calculating according to the signals of the vehicle speed sensor and the torque sensor, so that the motor generates torque in the corresponding direction, which is used after the transmission mechanism decelerates and increases the torque. For steering assist. According to different vehicle speeds, EPS ECU controls the current of the motor. For example, when the vehicle speed reaches a certain level (for example, when the vehicle speed is above 30km/h), the ECU cuts off the signals of the motor and electromagnetic clutch, and the system changes to the normal steering mode; when the vehicle speed drops to a certain level (For example, when it is below 27km/h), the ECU turns on the electric motor and the electromagnetic clutch to control the current, and the system becomes the electric power steering assist state.

(3) Motor
The function of the motor is to output a suitable assist torque according to the control command of the ECU, and it is the power source of the EPS. The road feel of a car when steering is closely related to the performance of the motor. It is required that the power assist increases slowly at small turning angles and fast at large turning angles. The power assist is large at low speeds and small at high speeds, and the steering wheel has good followability to the steering wheel. The motor is the key technology of EPS, which requires good control performance, low speed, high torque, fast response, small fluctuation, small size and high reliability. Commonly used motors include permanent magnet synchronous motors and brushless DC motors. They not only retain the excellent mechanical characteristics and adjustment characteristics of ordinary DC motors, but also have simple structure and reliable operation. The permanent magnet synchronous motor has small torque ripple, fast response and compact structure; and if it can ensure the generation of a constant magnetic field, the permanent magnet synchronous motor can obtain the required boost torque by adjusting the armature current by the simplest PWM method, thereby simplifying The software and hardware design of ECU is becoming the mainstream of application.

2. working principle of EPS
Figure 4 shows a structural block diagram of a steering shaft assisted EPS system, which relies on the motor to assist the steering shaft. The electric power steering system is composed of steering wheel rotation direction and torque sensor, vehicle speed sensor, rotation angle sensor, power assist mechanism, power steering motor and ECU (MCU).

Basic knowledge of electric power steering
Figure 4 The structure block diagram of the steering shaft assisted EPS system

When the car is not turning, the motor does not work.

When the driver operates the steering wheel, the torsion bar connected to the steering wheel is deformed, and its deformation angle is proportional to the torque applied to the steering wheel. The torque sensor converts the deformation angle of the torsion bar into a linear voltage output signal T. The signal, the vehicle speed signal V, and the rotation angle sensor signal W are sent to the controller ECU. Based on these signals, the ECU performs arithmetic processing in combination with the detected current feedback signal of the booster motor, and determines the magnitude and direction of the motor boost current from the target motor current curve. This current is the required assist torque. After the electromagnetic clutch decelerates and increases the torque through the deceleration mechanism, it is added to the steering shaft to obtain a steering force compatible with the driving conditions of the car. When the ECU detects an abnormal signal, it immediately disconnects the electromagnetic clutch, exits the boost mode, and lights up the fault warning light at the same time.

At different vehicle speeds, the steering assist current is different, so the steering wheel rotation torque is different. Generally, the ECU stores eight target motor current curves in the left and right directions. When the steering wheel rotates to the maximum angle position and remains at this position, the steering assist is also When the maximum is reached, the control unit reduces the current supplied to the motor to prevent the motor from overloading and damaging the motor.

The driver has the best driving feeling under various working conditions, mainly because of the following three points.
① Good straight-line driving ability (the steering wheel is returned to the center position by the electronic mechanical steering system)
②Easy and gentle steering force input.
③There is no uncomfortable reaction force even when driving on uneven roads.

EPS ECU usually has the following three working modes.
①Normal control mode: In response to the signals from the torque sensor and the rotation angle sensor, it provides steering assistance in the left and right directions.
②Return to control mode: After the steering is completed, use this mode to help the steering return to a straight line.
③Damping control mode: Improve the road feel and dampen the adverse reaction of recoil with the change of vehicle speed, so as to maintain straight driving.

How to power steering is controlled by a constant characteristic map program stored in the control unit. Up to 16 different characteristic maps can be stored in the control unit. The storage quantity of the characteristic map is set in the production plant according to different vehicle equipment (such as the weight of the vehicle).

Some EPS ECUs store eight kinds of characteristic graphs, and they are divided into light and heavy characteristic curves according to different vehicle loads.

The characteristic curve shows that the total amount of power-assisted steering torque given by the motor is determined by the input steering torque and vehicle speed.

The EPS ECU calculates the necessary assist torque and controls the motor to start working according to the steering force, vehicle speed, steering wheel angle, steering wheel speed, and characteristic curves stored in the control unit.

When the summer tires are replaced with winter tires, the four-wheel alignment is incorrect, the side wind, etc., the center line of the car will deviate from the center line of the car. Cause driver fatigue.

The straight-line driving function is an extension of the active return function. When there is no force provided, the system generates a boost to make the wheels return to the center position.

If the control unit or steering system is changed, the steering ECU can be matched through the adaptive function of the detector. In addition, the general EPS ECU is directly connected to the motor, and the whole is replaced when it is damaged.