Intelligent model car chassis technology

Intelligent model car chassis technology

　 Abstract: Aiming at the chassis of the model car used in the intelligent car competition, this paper introduces the steering wheel alignment parameters, the selection of the vehicle's center of gravity, the principles of sideslip, etc. from the perspective of automobile theory. By testing the steering wheel alignment parameters, the performance of the steering gear, and the steering stability of the model car, the influence rules between these adjustment parameters are obtained. This can provide a certain reference for the relevant participating teams in algorithm formulation, simulation parameter setting, and adjustment and optimization of hardware structures such as chassis and steering gear.

　　Keywords: front wheel alignment; steering gear; steering; turning radius

　　introduction

　　This article introduces the principles of steering wheel alignment, vehicle center of gravity selection, and sideslip from the perspective of automobile theory, and conducts a series of tests on the chassis of the competition model car, including the selection of steering wheel alignment parameters, steering gear performance test, and model car steering steady-state test. The influence rules between these adjustment parameters are obtained, hoping to provide some reference for the relevant participating teams in algorithm formulation, simulation parameter setting, and adjustment and optimization of hardware structures such as chassis and steering gear.

　　Automobile chassis related performance

　　Steering wheel alignment parameters

　　For a car, in order to maintain the stability of the vehicle's straight-line driving, make it automatically return to the center when turning, and make steering easy, the wheel alignment parameters must be determined, including kingpin inclination, kingpin inclination, front wheel camber and front wheel toe.

　　Caster Angle

　　The caster angle forms a self-aligning torque after the wheel deflects, which hinders the wheel from deflecting. The larger the caster angle, the higher the vehicle speed, and the stronger the automatic self-aligning force after the wheel deflects. However, if the self-aligning torque is too large, it will cause the front wheel to self-align too violently, accelerate the front wheel shimmy, and make the steering heavy. The caster angle is usually 1° to 3°.

　　Kingpin inclination angle

　　In the front and rear direction of the car, the kingpin is tilted inward at an angle. The angle between the kingpin axis and the vertical line is called the kingpin inclination angle. When the steering wheel of the car deflects under the action of external force, the wheel and the entire front of the car will be lifted to a certain height due to the kingpin inclination. After the external force disappears, the wheel will try to return to the original middle position under the action of gravity. Usually the kingpin inclination angle is not more than 8°.

　　Front wheel camber

　　In the horizontal plane of the car, the center plane of the front wheel tilts outward at an angle, which is called the front wheel camber. On the one hand, the front wheel camber can make the wheel roll close to the vertical road surface and slide to reduce steering resistance, making the car steering easier; on the other hand, it reduces the load on the bearing and its locking nut, increases service life and improves safety. Generally, the front wheel camber is about 1°, but for vehicles with high speed and sharp steering requirements, the front wheel camber can be reduced or even negative.

　　Front wheel toe

　　Looking down at the wheels, the rotation planes of the two front wheels of the car are not completely parallel, but slightly angled. This phenomenon is called front wheel toe-in. The function of wheel toe-in is to reduce or eliminate the adverse consequences caused by the camber of the front wheels. The two coordinate with each other to ensure that the front wheels roll without sliding during the driving of the car. The front wheel toe-in is generally 0 to 12 mm. The camber of the front wheels of modern cars tends to decrease or even become negative, so the front wheel toe-in should also decrease accordingly or even become negative.

　　The influence of center of gravity on vehicle performance

　　The position of the center of gravity of a car is usually expressed by the horizontal distance from the center of gravity to the center line of the front axle and the height from the center of gravity to the horizontal road surface. The position of the center of gravity can be measured by experimental methods and estimation methods.

　　Impact on power performance

　　The normal driving of the car must meet the driving-adhesion conditions:

　　That is, the driving force of the car must be greater than or equal to the sum of the slope resistance, rolling resistance, and air resistance, and equal to the adhesion of the car's driving wheels. Adhesion is related to the road adhesion coefficient and the axle load of the driving shaft, and the axle load of the driving shaft depends on the horizontal position of the center of gravity. Therefore, the position of the center of gravity must ensure that the driving wheels can provide sufficient adhesion. Considering this aspect alone, the closer the center of gravity is to the driving shaft, the better.

　　Impact on braking performance

　　The braking performance of a car requires a large braking deceleration, a short braking distance, and good braking direction stability, that is, it is not easy for the front wheel to lose steering, the rear wheel to slip, and run off. The stability of the braking direction is related to the locking order of the front and rear wheels, and the locking order is related to the center of gravity position. If the center of gravity position ensures that the synchronous adhesion coefficient of the car (β is the proportion of the front braking force to the braking force of the whole vehicle, and b is the horizontal distance from the center of gravity to the rear axle) is equal to the adhesion coefficient of the common road surface of the car, then the braking stability is good; if the center of gravity moves forward, b increases, and the rear axle is prone to slip, which is very dangerous for high-speed cars; if the center of gravity moves backward, b decreases, and the front wheels are prone to lose steering ability.

　　Impact on passability

　　When a car is driving on a steep slope or making a sharp turn at high speed, it may tip over. To avoid this danger, the center of gravity should be lowered as much as possible while ensuring the minimum ground clearance.

　　Based on the above analysis, after installing many circuit boards, the vertical position of the center of gravity of the model car should be as low as possible, and the horizontal position should be on the center line of the car close to the rear axle.

　　Car side sliding

　　In order to ensure that the steering wheels of the car roll in a straight line without lateral slip, the wheel camber angle and wheel toe must be properly matched. When the wheel toe value and wheel camber angle are not matched properly, the wheel may not roll purely during straight driving, resulting in lateral slip. If this slip phenomenon is too serious, it will destroy the adhesion conditions of the wheel and make the car lose its directional driving ability. Side slip is divided into the following situations.

　　Directional slide

　　Random slide

　　Turning and sliding

　　Braking skid

　　If the front wheels lock and slip first during braking, the car may skid.

　　Some compensation measures can be taken to reduce sideslip. For directional sideslip, the basic means is to use the Q-type sideslip generated by the toe-in of the front wheels to compensate for the W-type sideslip generated by the camber. The properties of the Q-type sideslip are: the size of the sideslip is equal to the size of the toe angle; the direction of the sideslip is opposite to the direction of the toe angle, and is related to the direction of the vehicle's travel; it has nothing to do with the quality of the road surface. For random sideslip, the main approach is to change the independent suspension structure. For example, the random sideslip of the wheels of the double wishbone independent suspension axle of this car model can be solved by changing the length of the upper and lower wishbones using the comprehensive theory of the four-bar mechanism, so that the wheelbase does not change much during the model's driving process, thereby reducing random sideslip. For steering sideslip, it mainly depends on selecting a suitable kingpin angle, reasonably matching the kingpin inclination and caster angles, and making the steering inner wheel produce camber or increase camber as much as possible, and making the steering outer wheel produce inclination or reduce camber.

　　Model car chassis performance

　　The chassis of the model car adopts an equal-length double wishbone independent suspension (as shown in Figure 1). When the wheel bounces up and down, the wheel plane does not tilt, but the wheelbase will change greatly, so the possibility of lateral slippage of the wheel is greater. There are 6 adjustable parameters in this car, among which the kingpin inclination angle has little effect on the performance of the model car and can be set to.

　　Figure 1 Front wheel toe adjustment

　　Caster Angle

　　The kingpin caster angle can be increased by increasing the number of shims. There are 4 shims in total, 2 front and 2 rear, the caster angle is 0; 1 front and 3 rear, the caster angle is; 0 front and 4 rear, the caster angle is.

　　For this model car, if you want to make it steer more flexibly, the caster angle can be selected; if you want to increase the self-centering torque, the caster angle can be selected.

　　Front wheel camber

　　It is closely related to the side slip of the model car and needs to be matched with the front wheel toe, which can be set to .

　　Front wheel toe

　　The front wheel is steered by the steering gear driving the left and right tie rods. After the vertical position of the kingpin is determined, the toe-in of the front wheel can be changed by changing the length of the left and right tie rods. The left rod is short, and the adjustable range is 10.8mm to 18.1mm; the right rod is long, and the adjustable range is 29.2mm to 37.6mm (as shown in the red circle in Figure 1).

　　Chassis ground clearance

　　The ground clearance of the front half of the chassis can be adjusted by adding or removing shims between the lower arm and the bottom plate of the independent suspension. The shims are available in two sizes: 1mm and 2mm. Without a shim, the ground clearance of the front of the car is 9mm, so the adjustment range of the ground clearance is 9mm to 12mm. From existing experience, after the sensor is installed, if this distance is too small, the model car will have a lower passability when climbing a slope; if it is too large, it will affect the sensitivity of the sensor.

　　Rear suspension longitudinal shock absorber spring preload

　　Adding a gasket at the red circle in Figure 2 can increase the preload force of the spring.

　　Figure 2 Suspension preload adjustment

　　Servo performance test

　　A variable resistor is connected to the shaft of the servo. The variable resistor has three connectors. One end of the connector on both sides is connected to a 5V power supply, the other end is grounded, and the middle connector is connected to an oscilloscope, which measures voltage. When the servo drives the front wheel to rotate, the resistance of the variable resistor changes accordingly, and the voltage value of the oscilloscope also changes, that is, the voltage is matched with the steering angle of the servo. In this way, the rate of change of the steering angle of the servo can be known by measuring the change of voltage over time. From the experiment, it can be seen that the servo rotates from the maximum angle on one side to the maximum angle on the other side at an approximately uniform speed. Combined with the measurement of the maximum angle of the front wheel, it can be estimated that the speed of the servo is about 2.42rad/s-2.52rad/s. According to the relevant knowledge of automobile theory, the performance of the servo is relatively soft, which can be adjusted by increasing the toe of the front wheel.

　　The test of the servo performance is mainly used to set the simulation parameters. At the same time, the estimated servo speed also has a certain reference significance for the program response speed and the vehicle speed limit during steering.

　　Steady-state steering test for model cars

　　This section discusses the relationship between the servo PWM duty cycle and the vehicle speed and turning radius. In the test, the servo PWM duty cycle is set to 6 gears, represented by 1, 2, 3, 4, 5, and 6 respectively. The larger the number, the larger the turning angle. As shown in Figure 1, the vehicle speed-turning radius correspondence diagram when the servo angle is gear 1. From the test, it can be seen that at the same turning angle, the turning radius and vehicle speed are roughly linearly related.

　　According to the relevant data of the model car, the theoretical turning radius can be calculated as 275mm by the following formula. This value is close to the turning radius at 0.31m/s in the model car test; when the speed of the model car is >1.4m/s, side slipping begins to occur.

　　Conclusion

　　Through theoretical analysis and experimental testing, this paper analyzes the adjustment of steering wheel alignment parameters, key selection, side slip control, chassis height adjustment, steering performance and steering stability of the model car for intelligent car competition, and gives adjustment suggestions for relevant parameters of the model car. Since the steering parameters of the above model car affect each other, this paper only gives the adjustment trend of each parameter, and the best matching value still needs to be obtained according to track debugging.