Abstract: This paper focuses on the steering wheel positioning parameters, vehicle center of gravity selection, side slip and other principles with reference to the theory of automobile theory, and focuses on the model vehicle chassis for smart car racing. The chassis of the vehicle obtained the influence law of these adjustment parameters by testing the steering wheel positioning parameters, the steering gear performance and the steady state of the model vehicle. This technology can provide some reference for the relevant teams in algorithm formulation, simulation parameter setting, hardware structure adjustment and optimization of chassis and steering gear.
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introduction
This paper introduces the principles of steering wheel positioning, vehicle center of gravity selection, side slip, etc. from the perspective of automobile theory, and carries out a series of tests for the model vehicle chassis, including the selection of steering wheel positioning parameters, servo performance test and model. The car steering steady state test, the influence law of these adjustment parameters is obtained, and it is hoped that the relevant participating teams can provide certain reference in algorithm formulation, simulation parameter setting, hardware structure adjustment and optimization of chassis and steering gear.
Model car chassis related performance
Model car chassis steering wheel positioning parameters
For the car, in order to maintain the stability of the vehicle in straight-line driving, so that the turning automatically returns to the right and the steering is light, the wheel positioning parameters must be determined, including the kingpin backward tilt, the kingpin tilt, the front wheel camber and the front wheel toe.
Model car chassis kingpin inclination
In the front-rear direction of the car, the kingpin is inclined at an angle inward, and the angle between the axis of the kingpin and the perpendicular is called the kingpin inclination angle. When the steering wheel of the car deflects under the action of external force, the wheel will be lifted to a certain height along with the front part of the whole car due to the inward tilt of the kingpin. After the external force disappears, the wheel will try to return to the original middle under the action of gravity. position. Usually the kingpin has a camber angle of no more than 8°.
Model car chassis front wheel camber
In the transverse plane of the car, the center plane of the front wheel is inclined outward by an angle called the front wheel camber. On the one hand, the camber angle of the front wheel can make the wheel roll close to the vertical road surface and slide to reduce the steering resistance, so that the car can be turned lightly; on the other hand, the load of the bearing and its lock nut is reduced, the service life is increased, and the safety is improved. Generally, the front wheel camber angle is about 1°, but for vehicles with high speed and sharp steering requirements, the front wheel camber angle can be reduced or even negative.
Model car chassis kingpin caster
The caster caster angle creates a positive moment after the wheel is deflected, preventing the wheel from deflecting. The larger the caster angle is, the higher the vehicle speed is. The stronger the back force is after the wheel is deflected, but the positive moment is too large, which will cause the front wheel to be too strong, accelerate the front wheel and make the steering heavy. Usually the caster angle is 1° to 3°.
Model car chassis front wheel toe
Looking down on the wheel, the plane of rotation of the two front wheels of the car is not completely parallel, but is slightly angled. This phenomenon is called the front wheel toe. The role of the toe of the wheel is to reduce or eliminate the adverse consequences caused by the camber of the front wheel. The two are coordinated to ensure that the front wheel rolls without slipping during the driving of the car. The front wheel toe is generally 0 to 12 mm. However, the front wheel camber angle of modern cars tends to decrease or even negative, and the front wheel toe should be reduced or even negative.
The influence of the position of the center of gravity on the performance of the car
The position of the center of gravity of the car is usually expressed by the horizontal distance of the center of gravity from the centerline of the front axle and the height of the center of gravity from the horizontal road. The position of the center of gravity can be measured by experimental methods and estimation methods.
Influence of model car chassis on braking performance
Automobile braking performance requires large braking deceleration, short braking distance, and good braking direction stability, that is, it is not easy to cause front wheel loss steering, rear wheel side slip and deviation. The stability of the braking direction is related to the order of the front and rear wheels, and the order of the lock is related to the position of the center of gravity. If the position of the center of gravity ensures the synchronous adhesion coefficient of the car (β is the ratio of the front braking force to the braking force of the vehicle brake, b is The horizontal distance from the center of gravity to the rear axle is equal to the common road surface adhesion coefficient of the automobile, and the braking stability is better; if the center of gravity moves forward, b increases, and the rear axle slips easily, which is dangerous to high-speed cars; , b decreases, the front wheel is easy to lose steering ability.
Influence of model car chassis on dynamic performance
The car must meet the drive-attach condition for normal driving:
That is, the driving force of the automobile must be greater than or equal to the sum of the slope resistance, the rolling resistance, and the air resistance and equal to the adhesion of the driving wheel of the automobile. The adhesion is related to the road surface adhesion coefficient and the axle load of the drive shaft. The axle load of the drive shaft depends on the horizontal position of the center of gravity, so the position of the center of gravity must ensure that the drive wheel can provide sufficient adhesion. Only in this respect, the closer the center of gravity is to the drive shaft, the better.
Impact on passability
When the car is driving on a steep side slope or a high-speed sharp turn, lateral overturning will occur. To avoid this danger, the center of gravity should be reduced as much as possible while ensuring the minimum ground clearance.
Based on the above analysis, after installing many circuit boards, it should be as far as possible to ensure that the vertical position of the center of gravity of the model car is as low as possible, and the horizontal position should be close to the rear axle on the center line of the car.
Car skid
In order to ensure the linear rolling of the car steering wheel without lateral slip, the wheel camber angle and the wheel toe are required to be properly matched. When the wheel toe value is not properly matched with the wheel camber angle, the wheel may not be purely rolling during straight running. A lateral slip phenomenon occurs. When this slip phenomenon is too serious, the adhesion condition of the wheel will be destroyed, and the car will lose its directional driving ability. Skidding is divided into the following situations.
Directional skid
Random skid
Steering slip
Brake side slip
If the front wheel first locks and drags during the braking process, side slip may occur.
Some compensation measures can be taken to reduce side slip. For oriented side slip, the class B side slip generated by the front wheel toe is used to compensate for the W-type side slip generated by the camber. The nature of the Q-type skid is: the size of the side slip is equal to the size of the toe angle; the direction of the side slip is opposite to the direction of the toe angle, which is related to the direction of travel of the vehicle; it has nothing to do with the quality of the road. For the random side slip, the main purpose is to change the independent suspension structure. For example, the random side slip of the double wishbone independent suspension axle wheel of the model can change the length of the upper and lower cross arms by the four-bar linkage theory to make the model run. During the process, the track change is not large, thereby reducing the random side slip. For steering side slip, mainly choose the appropriate kingpin angle, reasonably match the kingpin tilting and backward tilting angle, and make the steering inner wheel camber or increase the camber as much as possible, so that the steering outer wheel produces inward tilt or reduces camber.
Model car chassis performance
The model vehicle chassis adopts an isometric double wishbone type independent suspension (Fig. 1). When the wheel is bouncing up and down, the wheel plane is not inclined, but the wheel track will change greatly, so the wheel may be laterally slipped. More sexual. The vehicle has a total of 6 parameters adjustable, in which the main pin inclination angle has little effect on the performance of the model car, and can be set.
Figure 1 Front wheel toe adjustment
King pin back angle
The caster angle can be increased by increasing the number of spacers. There are 4 pads, the first 2 and the 2nd, and the back tilt angle is 0; the front 1 and the back 3 are the back tilt angle; the front 0 and the rear 4 are the back tilt angle.
For this model car, if you want to make it flexible, the caster angle can be selected. If you want to increase the return torque, the back rake angle can be selected.
Front wheel camber
It has a large relationship with the side slip of the model car and needs to be matched with the front wheel toe.
Chassis ground clearance
Between the swing arm and the bottom plate of the independent suspension, the ground clearance of the front half of the chassis can be adjusted by adding or removing spacers. The gaskets are available in 1mm and 2mm sizes. A piece of gasket is not added, and the gap between the front and the ground is 9mm, so the adjustment range of the ground clearance is 9mm~12mm. From the existing experience, after the sensor is installed, the distance is too small, which will reduce the passage of the model car when climbing; if it is too large, it will affect the sensitivity of the sensor.
Model car chassis front wheel toe
The front wheel is driven by the steering gear to drive the left and right tie rods. After the position of the kingpin in the vertical direction is determined, the length of the front wheel toe can be changed by changing the length of the left and right tie rods. The left lever is short, the adjustable range is 10.8mm to 18.1mm; the right lever is long, and the adjustable range is 29.2mm to 37.6mm (as shown in the red circle in Figure 1).
Rear suspension longitudinal damping spring preload
Adding a spacer at the red circle of Figure 2 increases the preload of the spring.
Figure 2 suspension preload adjustment
Steering gear performance test
A varistor is connected to the shaft of the steering gear. The varistor has three connectors. One end of the connector on both sides is connected to the 5V power supply, and the other end is grounded. The middle connector is connected to the oscilloscope, and the oscilloscope measures the voltage. When the steering gear drives the front wheel to rotate, the resistance of the varistor changes, and the voltage value of the oscilloscope also changes, that is, the voltage corresponds to the rotation angle of the steering gear. Thus, by measuring the change of voltage with time, the steering angle of the steering gear can be known. Rate of change. It can be seen from the test that the steering gear is rotated from the maximum angle of one side to the maximum angle of the other side at a uniform speed. Combined with the measurement of the maximum rotation angle of the front wheel, the speed of the steering gear can be estimated to be about 2.42 rad/s to 2.52 rad/s. According to the relevant knowledge of automobile theory, the performance of the steering gear is soft and can be adjusted by increasing the front wheel toe.
The test of the servo performance is mainly used to set the simulation parameters. At the same time, the estimated servo speed also has certain reference significance for the corresponding speed of the program and the speed limit during steering.
Model car steady state steering test
This section discusses the relationship between the duty cycle of the servo PWM and the vehicle speed and turning radius. In the test, the PWM duty ratio of the steering gear is set to 6 gears, which are represented by 1, 2, 3, 4, 5, and 6, respectively, and the larger the number, the larger the angle of rotation. Figure 1 shows the vehicle speed-turning radius corresponding map when the steering angle is the gear 1. It can be seen from the test that the turning radius is approximately linear with the vehicle speed at the same corner.
According to the relevant data of the model car, the theoretical turning radius can be calculated as 275mm. This value is similar to the turning radius at 0.31m/s in the model vehicle test; the side slip phenomenon begins to occur when the model car speed is >1.4m/s.
Conclusion
In this paper, the steering wheel positioning parameter adjustment, chassis height adjustment, key selection, side slip control, steering performance and steering stability of the model car for smart car racing are analyzed, and the relevant parameters of the model car are given. Adjustment suggestions. In addition, since the parameters of the above model car about the steering are mutually influential, this paper only gives the adjustment trend of each parameter, and the best matching value needs to be obtained according to the track debugging.
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