The motor is the power source of the electric power steering system, and its function is to output appropriate auxiliary torque according to the command of the ECU. The motor has a great impact on the performance of the EPS system, so the EPS system has high requirements for the motor. It requires not only large torque, small torque fluctuation, small moment of inertia, high power density, but also high reliability and long life. Permanent magnet synchronous motor PMSM and DC brushless motor BLDC not only have the advantages of AC motor such as simple structure, reliable operation and convenient maintenance, but also have many characteristics of DC motor such as high operation efficiency, no excitation loss and good speed regulation performance. Therefore, these two three-phase motors are widely used in various EPS systems.

2. EPS system classification

According to the installation position and mechanical structure of the motor, the electric power steering system can be divided into: column mounted, pinion mounted and rack mounted.

As shown in Figure 2, in the column assisted EPS system, the power assisted motor is installed on the column and connected to the steering shaft through the deceleration and torque increasing mechanism to directly drive the steering shaft for power assisted steering. Such a system has a simple and compact structure and is easy to install. However, because the booster motor is installed in the cockpit and is affected by the spatial layout and noise, the motor has a small volume and small output torque, and is generally only used on small and compact vehicles.

In the pinion assisted EPS system, the power assisted motor and the reduction and torque increasing mechanism are connected to the pinion, and the gear is directly driven to realize the power assisted steering. Since the power assisted motor is not installed in the passenger compartment, a larger motor can be used to obtain a higher power assisted torque without worrying about the noise caused by too large motor moment of inertia. See Figure 3 for details.

In the rack assisted EPS system, the power assist motor and the deceleration torque increasing mechanism directly drive the rack to provide power, so it can provide greater power assistance. However, the whole system is complex in structure and high in cost, so it is suitable for luxury cars and commercial vehicles.

3. Advantages and disadvantages of power steering system

• The parking and steering force is obviously reduced, so that the driver can easily stop and steer. The steering force during driving is also reduced, which is conducive to reducing driver fatigue.

  
  

• It reduces the number of extreme turns of the steering wheel from one end to the other, usually 2.5 to 3 turns.

  
  

• It reduces the influence of road surface interference on the steering wheel.

4. Main performance requirements of EPS

• Safety: failure safety; Power assistance works in parallel with conventional mechanical steering mechanisms.

  
  

• Sensitivity: In addition to providing enough power assistance under various driving conditions, it should also allow the steering wheel to maintain a high enough road feel. When making a sharp turn, the steering force is relatively large at first, and the driver feels that the steering wheel is relatively heavy. Then, with the increase of lateral acceleration, the steering wheel becomes lighter. The feeling of lightening is a warning signal to remind the driver that the wheel is about to slip. The steering force generally increases with the angle of the steering wheel leaving the straight driving position, which is actually because the steering resistance moment of the front wheel to the kingpin axis increases with its angle.

5. Main requirements for steering system

• Ensure that the car has a minimum turning radius that is small enough to enable it to turn within a limited area of the site; (C-factor) extension: the minimum turning radius refers to the radius of the track circle that the center of the outer steering wheel rolls over on the support plane when the steering wheel is turned to the limit position and the car turns and runs at the lowest stable speed. To a large extent, it represents the ability of vehicles to pass through narrow and curved areas or bypass obstacles that cannot be crossed. The smaller the turning radius, the better the mobility of the car. The minimum turning radius is determined by the wheelbase of the vehicle in order to ensure the pure rolling of the wheels without sliding during steering. Long wheelbase means large turning radius, but vehicles with long wheelbase have better ride comfort.

• Ensure that all wheels rotate around a common instantaneous steering center when the car turns, and the sideslip angle of all wheels tends to be consistent and as small as possible, so as to extend tire life and prevent tire noise.

  
  

• It is easy to operate. When the car turns, the tangential force (steering force) exerted by the driver on the steering wheel shall not exceed 150-200N for cars and 500N for trucks. In the case of power steering, parking and steering can generally be guaranteed, and the maximum steering force is generally significantly less than the above limit value.

  
  

• After turning, the steering wheel can automatically return to the right position and keep the car in a stable straight driving state.

  
  

• The requirements of handling stability at high speed and fast steering at low speed (the number of turns required for the driver to turn the steering wheel is relatively small) and portability are considered. For mechanical steering system, it is necessary to adopt appropriate steering transmission ratio. For the power steering system, it is necessary to provide proper power assistance as well as proper steering transmission ratio.

  
  

• The recoil transmitted to the steering wheel due to the impact of the ground on the steering wheel should be as small as possible to reduce the fatigue of the driver. This performance requirement is of great significance to vehicles running on rough roads.

Extension: this is the rear steering gear, which has a dust cover; The front steering gear has a small cover.  

6. Relevant national standards for EPS

When the power assistance of the steering gear fails, the steering is completely conducted by the hand power of the driver.

GB-17675-1999 Basic requirements for automobile steering system: when turning forward at a speed of 10km/h and a turning diameter of 24m, the steering force shall be less than 245N without power steering, and when the power steering fails with power steering, the steering force shall be less than 588N.

GB15740-2006 Anti theft Performance Requirements

GB11557-2011 Provisions on Prevention of Driver Injury Caused by Vehicle Steering Mechanism

7. Tuning and steering performance evaluation

General three wheel training: mule cart, DV, PV

The chassis calibration can be divided into three stages: first, the first round of calibration. At this time, there is no sample vehicle. The second stage of calibration is the calibration that has entered the sample vehicle stage. The third stage is already very close to the mass production stage. At this time, we need to make fine adjustments.

• In the first round of adjustment, there is no sample vehicle, no reference vehicle, and no spare parts at this stage. It is just a concept, only basic design parameters. Usually, at this stage, it is necessary to find a vehicle with relatively similar wheelbase, track width, vehicle weight, suspension form and size, and use it as a reference for restructuring to ensure that the hard points designed at present can be consistent and achieve the target load.

• The second round of adjustment is the adjustment for entering the sample vehicle stage. From a real point of view, in order to start targeted adjustment for the next step of mass production, at this stage, it is necessary to confirm the status of all parts, then confirm the parameters of each part, determine that they are the most close to the standard, and adjust them to the most close to mass production status.

• Next, enter the third phase of calibration, which is basically in the mass production phase. There is little time left for us before mass production, so fine tuning is carried out at this stage. For example, it is mainly about the damping force to confirm whether the suspension state meets the standard. Acceptance and calibration is also important to ensure that the performance of parts can meet the requirements and challenges of various environments. EPS shall be adjusted after chassis adjustment.