Analysis and Countermeasures of Automotive EPB Wire Harness Wire Breakage

Abstract: In view of the fracture problem of automobile EPB wire harness in the real vehicle simulated bending test, the factors related to wire fracture were comprehensively analyzed, and the reliability of the automobile EPB wire harness wire against bending fatigue fracture was analyzed and evaluated using experimental analysis methods. It was proposed that Several solutions were proposed and their implementation effects studied. The results show that optimizing the copper conductor grain size and wire structure, increasing the number of core wires, and reducing the volume ratio of the wire's outer protective sheath are relatively practical, and can effectively reduce the probability of automotive EPB wiring harness wire breakage.

0 Preface

With the rapid development of the automobile industry, the safety of automobiles is the first indicator of automobile design and manufacturing. Through the use of modern technology and advanced means, various possible ways and solutions to further improve safety can make automobiles as transportation safer. As a braking system, the PEB system is a major safety system and an important factor in measuring automobile safety standards. The extensive application of EPB  electronic parking systems in automobiles plays an important role in improving the safety and ride comfort of automobiles. The EPB wiring harness is an integral part of the system, and its reliability directly affects the reliability of the entire automotive EPB system. This article mainly studies the reliability of automotive EPB wire harnesses against bending and fracture.

As part of the automobile wiring harness, the automotive EPB wire harness integrates EPB wires and ABS wires and is installed in the suspension area of ​​the body chassis. This area is subject to both impact and corrosion from the outside of the body and a large amount of mechanical movement of the longitudinal swing arm, so the PEB's resistance to the wire harness is Bending properties put forward very high requirements. During the driving process of the car, due to uneven road conditions, the swing of the longitudinal swing arm in the body chassis suspension system pulls the EPB wire harness to continuously bend at high frequencies, causing the wires in this section to bend and break. This failure mode is the focus of this article.

Figure 1 Automotive EPB wiring harness working environment and EPB system structure

1 Automotive EPB system

Automotive EPB (Electrical Park Brake) system is the abbreviation of electronic parking system. It replaces the traditional lever handbrake but is safer. It will not change the braking effect due to the driver's strength, turning the traditional lever handbrake into a within reach. button. It is a technology that realizes parking brake by electronic control.

The system includes EPB push button switch, electronic control unit ECU, automobile EPB wiring harness and ABS wiring harness, ABS speed sensor , brake motor , reduction gear mechanism and brake caliper and other components. The ABS speed sensor converts the vehicle speed signal detected while the car is driving into an electrical signal, and transmits the electrical signal to the ECU through the EPB wiring harness. The ECU then issues instructions to control the brake caliper to brake the wheels. The EPB wire harness plays the role of a bridge for transmitting electrical signals.

2 EPB wire bending and fracture phenomena and related factors

2.1 Wire breakage phenomenon

The EPB wire harness is fixed to the support arms and longitudinal swing arms of the body chassis through wire harness rubber sheaths, plastic buckles, brackets and other parts. When the car is driving, the wheels bump up and down due to uneven road surfaces, which causes the longitudinal swing arms on the chassis. It makes a reciprocating motion similar to a pendulum around the fixed point of the body support beam.

The EPB wire harness bending test simulates the movement of the wire harness in the real vehicle environment, bending and swinging at a frequency of 2.5Hz in the range of -30°C to normal temperature. The test requires that the number of wire harness bends must be within the required lifespan to ensure that the appearance of the wire harness is not damaged. No signal interruption. The bending test found that the breaking position of the wire harness was near the fixed point of the moving section.

Photos of the fracture location and cross-section of the conductor during the test show that the core wire fracture surface has both a flat cross-section and a rounded cross-section similar to that of a bullet. It shows that the copper conductor is subject to both shear force and tensile force when subjected to external forces.

2.2 Conductor force analysis

When the vehicle is driving, the EPB wire harness is pulled by the longitudinal swing arm of the body chassis to form high-frequency symmetrical bends, causing local bending, deformation and damage to the wires. The accumulation of local cyclic plastic deformation is the fundamental cause of metal fatigue damage. Its bending form and stress analysis are as follows.

Figure 2 Wire harness bending principle and force diagram

The copper wires are severely stretched and deformed in the outer area of ​​the arc, and mutual extrusion deformation is formed in the inner area of ​​the arc.

① Axial tensile force F1: The bumps of the vehicle cause the longitudinal swing arm to swing up and down, and the wire harness is pulled in a certain direction and bent. During this process, an angle ∠a is formed, and the traction force F generates an axial pulling force F1 in the direction ∠a; F1=F *Cos a, a decrease in the angle will cause an increase in the axial tension F1.

② Radial shear force F2: The wire harness is pulled in a certain direction and bends, forming an angle ∠a. The traction force F produces a component force in the direction ∠a, which is the shear force F2; F2=F*sin a, the increase in the angle of a will Causes the shear force F2 to increase. The repeated alternation of tension and shear forces causes micro deformation of the core wire. The superposition of hundreds of thousands of micro deformations eventually leads to the occurrence of fatigue fracture, a failure mode.

2.3 EPB wire harness conductor material

2.3.1 EPB wiring harness belongs to automobile wiring harness, and its conductors are made of multi-core copper wires. The physical properties of the copper material in the core wires determine the basic mechanical properties of the EPB wiring harness conductors. Copper is a metal crystal, and the relationship between the size of its grains and the strength of the metal shows that the smaller the grains, the better the mechanical properties of the metal, such as strength, toughness, and plasticity. Grain refinement is one of the important means to improve the mechanical properties of metals. From the Hall-Petch relationship:

σy represents the yield limit of the material;

σ0 represents the lattice friction resistance produced when moving a single dislocation;

Ky is a constant related to the type and nature of the material and the grain size;

d average grain diameter.

The effect of grain refinement on metal strength is described by the HP relationship. The metallographic structure test shows that the crystal grain size in the metallographic structure of the broken copper wire is relatively large, the average grain size, and the uniformity of the grains affect the toughness and strength of the copper core wire, see Figure 4.

Figure 3 Metallographic analysis diagram of broken conductor core wire

2.3.2 The ratio of copper material in the EPB wire harness core wire affects the elongation at break of the core wire. The conductors in this case use tin-plated core wires, which reduces the proportion of copper in the core wires.

Figure 5 shows that the elongation at break of tinned copper core wire is smaller than that of bare copper core wire. The galvanized copper core wire with reduced elongation at break reduces the bending resistance of the EPB wire harness and increases the risk of breakage.

Figure 4 Elongation at break of wires with different coatings

2.3.3 The wire structure affects the toughness of the wire, thereby affecting the bending resistance of the EPB wire harness. The greater the number of core wires, the higher the overall toughness of the wire, which is more conducive to EPB wire harness resistance to fatigue fracture caused by bending. The wires of the EPB wiring harness in this case include 2 2.5mm2 copper wires and 2 0.5mm2 copper wires.

Among them, the core wire diameter of the 0.5mm2 conductor is 0.15mm, and the number is 28. The number of core wires is too small and the diameter is too large, which affects the mechanical properties of the conductor as a whole.

2.4 Analysis of EPB wire harness outer protective layer

The integrated cable of the broken EPB wire harness contains four wires and an outer protective layer of PVC material. The protective layer and the four wires are tightly fitted together, almost like a rigid body. Analysis shows that the four wires inside the outer protective layer lack buffer space during the bending process and are difficult to expand or contract, causing severe stress concentration and breakage.

2.5 EPB wiring harness installation point analysis

The EPB wire harness is fixed on the body chassis through mechanical interference fit. The mounting point is made of polyurethane elastomer PUR. The material hardness reaches 95A. The edges lack rounded corners. The lack of elasticity and buffering effect when subjected to force is another factor that causes the EPB wire harness to break. factors. In this case, the lack of elastic buffering and the energy absorption effect on bending stress lead to fatigue fracture due to shear stress concentration at the fixed point of the wire harness.

2.6 EPB harness length analysis

After test analysis, the length of the EPB wire harness wire has a limited size margin in the dynamic area, and the wire harness is obviously tightened between the two fixed points. During the bending process, the moving area of ​​the wire harness is subject to tension, and the bending angle increases due to the strengthening of the wire harness. The shear stress concentration caused the wire harness to break.