Design of six-speed control eddy current retarder based on PIC microcontroller

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Design of six-speed control eddy current retarder based on PIC microcontroller [Copy link]

Aiming at the manufacturing process of domestic heavy trucks, a new type of eddy current retarder based on PIC microcontroller is designed. The retarder is divided into six gears for regulation, which can well realize the functions of constant speed, on-site programming and sound and light prompt gear. The

eddy current retarder is a high-efficiency automobile braking auxiliary device. It can not only make the car convenient to slow down and drive at a constant speed when driving on a slope, but also can slow down in time and easily on a highway or in poor road conditions, so it can greatly improve the safety and comfort of the car when driving.

At present, there are two forms of retarders used abroad. One is to use the pressure of liquid, which is called "hydraulic retarder". The hydraulic retarder has many disadvantages such as complex structure and difficult maintenance. The other is "eddy current retarder". Compared with the hydraulic retarder, it has a simple structure and is easy to maintain. However, most foreign eddy current retarders use relays as control elements (such as TELMA in France). Due to the frequent closure of the relay, the contact life is low, and the braking force cannot be adjusted continuously and evenly. Some products use high-power contactless switches as control elements (such as Germany's KLOFT, etc.). Although the life of the components is greatly improved, they often fail when used on domestic vehicles. After a long period of investigation, it was found that the cause of the failure was that foreign manufacturers did not take into account the manufacturing process of domestic vehicles and the operating habits of drivers, making the product structure and control method unsuitable for domestic conditions. The eddy current retarder introduced in this article is a kind of auxiliary braking device suitable for domestic cars specially developed to address the above problems.

System structure of eddy current retarder

The eddy current retarder system is mainly composed of an electronic control part, a power module, an excitation coil and a rotor. The electronic control part calculates the appropriate braking force according to the driver's brake gear signal and speed feedback signal, and controls the conduction of the power module. The power module provides a suitable current to the excitation coil to generate a magnetic field and form an eddy current in the rotor. The size of this eddy current is proportional to the rotation speed of the rotor. The magnetic field formed by eddy currents produces a torque in the same direction as the rotor's rotation. Due to the relationship between action and reaction, the rotor produces a torque in the opposite direction of its own rotation. This torque is a function of the rotor speed and the stator magnetic field current. The magnitude of the magnetic flux is related to the number of turns of the excitation coil and the magnitude of the excitation current passing through. The rotor is installed at both ends of the drive shaft, which is equivalent to applying a braking resistance torque to the rotation of the drive shaft to achieve a deceleration effect. The current required for the retarder is directly supplied by the car battery. The schematic diagram is shown in Figure 1.

The eddy current retarder adopts a discrete structure, which separates the electronic control unit and the power module. The electronic control part is installed in the cab, and the power module is installed on the rear axle under the car. Since the working environment

on the car is relatively harsh, this structure can greatly reduce the impact of electromagnetic interference and the environment on the control unit. Only two signal lines are used to transmit the control signal in the design. The control core of the system uses the CMOS microcontroller PIC16C63A developed by Microchip of the United States. The system hardware circuit is shown in Figure 2.

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The control signal from the gear control handle must be input shaped and de-interference circuit before it can be transmitted to the single-chip microcomputer. Then the single-chip microcomputer calculates the appropriate braking force in real time and outputs the corresponding PWM control signal to achieve deceleration.

The voltage detection circuit forms a "line and" logic relationship between the two comparator output ends of LM393, forming an over-voltage and under-voltage protection. When the voltage range exceeds 18-30V, the output signal is sent to the single-chip microcomputer to realize the voltage protection function.

In order to facilitate the driver's operation, the electronic control part is equipped with 6 red LEDs and a green LED and a buzzer to provide sound and light display of the gear position.

In addition, considering that the braking force required for different vehicles and road conditions is inconsistent, 8 sets of different braking force parameters are compiled in the design. Since the program memory of PIC16C63A is 4K×14, the 1K E2PROM AT24C01A of Atmel Company is expanded to store these parameters. The communication interface circuit uses transistors to enhance the driving capability, while improving the transmission distance and anti-interference.

Power module hardware design

Traditional eddy current retarders mostly use relays to directly drive the excitation coil. In order to avoid the drawbacks of frequent relay closure, low contact life and arcing during shutdown, this system uses INFINEON's high-power MOS tube to replace the relay as the actuator. Considering the high current working requirements of the automobile retarder (the normal operating current is about 40A), BTS550PE3146 is selected. The power switch tube has overvoltage, overtemperature and short-circuit protection, the average working current is 97A, and the short-circuit current can reach 180A. The intelligent power switch is equipped with a freewheeling diode to effectively ensure the reliable operation of the system. There are a total of 4 high-power tubes, each of which controls a set of stator excitation coils; the excitation current is determined by the conduction time of the switch tube to achieve the adjustment of the braking force.

It should be noted that since the eddy current retarder achieves deceleration in a divergent energy consumption manner, the temperature will be very high during the working process, so the power module is equipped with a heat sink for cooling; considering the complex working environment of the car, a capacitor network and a varistor are also designed to absorb overvoltage.

System control design

● Design implementation method

In order to meet the different braking force requirements of the car under different conditions, 6 gears and 1 constant speed gear are designed in the operating system of this eddy current retarder: in the first gear, the two sets of coils are half open; in the second gear, the two sets of coils are fully open; in the third gear, the two sets of coils are fully open, and the two sets of coils are one-quarter open; in the fourth gear, the two sets of coils are fully open, and the two sets of coils are half open; in the fifth gear, the two sets of coils are fully open, and the two sets of coils are three-quarters open; in the sixth gear, the four sets of coils are fully open. When the constant speed button is pressed, the microcontroller records the current running speed in time, and the internal program can realize automatic tracking and adjustment of the braking force to meet the requirements of uniform speed operation.

The braking torque change curve of the existing eddy current retarder is generally hard, and long-term use will cause damage to the rear axle of the car. In this design, the PIC microcontroller outputs a control signal with adjustable pulse width to control the excitation current, thereby realizing the soft characteristics of the braking force, so that the braking force gradually increases. At the same time, the excitation current size is changed by changing the output frequency of PWM to achieve the purpose of changing the size of the car's braking force.

The control of the stator coil adopts the driving wheel co-control mode, that is, the 4 groups of excitation coils (a total of 8) are evenly divided into two groups, and the control signal transmitted by the electronic control unit adopts a diagonally symmetrical control method to energize the coil, and each signal controls a pair of stator coils. This control method overcomes the problem of inconsistent braking force on the left and right sides of traditional brakes and avoids the phenomenon of vehicle deviation.

When the vehicle speed is too low, the retarder does not need to work, so a threshold value is set in the system program. When the vehicle speed is lower than 5km/h, the controller has no current output. The green light on the controller is the ready/constant speed indicator light. The light is set to stay on to indicate that the retarder is working; if the retarder works in a constant speed state, the light and buzzer are set to flash and beep at a frequency of 0.5Hz.

● The programming switch

system calls various braking force parameters stored in the E2PROM by toggling the programming switch to adapt to the needs of different vehicles and road conditions. There are 8 programming switches in total. 6, 7, and 8 are used to set the size of the braking force of each gear. When 6, 7, and 8 are ON/ON/ON, the braking force is 100%; ON/ON/OFF: 95%; ON/OFF/ON: 90%; ON/OFF/OFF: 85%; OFF/ON/ON;: 80%; OFF/ON/OFF: 75%; OFF/OFF/ON: 70%; OFF/OFF/OFF: 65%; Programming switch 5 is used to set the change of braking force between each gear. When it is ON, the braking force of each gear increases evenly. When it is OFF, the 1st and 2nd gears increase quickly, and the 3rd, 4th, 5th, and 6th gears increase slowly, as shown in Figure 3. Programming switch 4 is used to set the soft characteristics of the controller. When it is ON, the braking force of the controller gradually increases, and the delay time in the sixth gear is about 0.4s. When it is OFF, the controller cancels the soft characteristics. Switches 1, 2, and 3 are used as reserved functions.

Test results

The eddy current retarder has completed the design of the electronic control part prototype and has entered the field experiment stage. Two eddy current retarder prototypes were installed on two 9-ton Jiefang trucks of Ruili Group Company in Ruian City, Zhejiang Province, and operated without failure for more than 1,000 hours. The table above shows the current data measured during the test.