Application of MC68HC908GP32 in gear shift protection of tractor transmission

0 Introduction
    The travel system of a certain aircraft tractor produced by our company adopts a hydrostatic drive mode: the engine directly drives the hydraulic piston pump, and the high-pressure oil output by the piston pump drives two hydraulic piston motors in parallel. The motor is connected to the front and rear axles through the gearbox and universal joint to achieve four-wheel drive. The gearbox used in this scheme has strict requirements on the gear speed during gear shifting. If the driver does not strictly follow the instructions, gear shifting often occurs, which may cause the gearbox to be scrapped. Therefore, in this design, we use a single-chip microcomputer to design the gearbox shift protection module, which successfully solves the problem of gear colliding due to misoperation.

1 Introduction to MC68HC908GP32
    The MC68HC08 series microcontroller is an upgraded product launched by Motorola to completely replace the MC68HC05 series microcontroller. The MC68HC08 has made many optimizations and enhancements in terms of functions and features. The three most important optimizations are:
    (1) The phase-locked loop technology is used to reduce the external crystal frequency to 32KHz, while the internal clock can reach 32MHz, greatly reducing the system noise;
    (2) The Flash technology (MC68HC908 series) is applied to make online programming possible.
    (3) 78 instructions are expanded on the basis of MC68HC05, and the instruction set is greatly optimized. The stack pointer is changed from a fixed 8-bit to a floating 16-bit, making it possible to use C language.
    (4) The MC68HC908GP32 is a general-purpose product in the MC68HC08 family, with 32KB internal Flash memory, which can encrypt programs.
    The new MC68HC908GP32 in Motorola's 68HC08 microcontroller family is very suitable for controlling the tractor hydraulic system because of its numerous internal functional modules, rich instruction set, powerful control functions and high reliability.

2 System Design
2.1 Working Principle of DANA 360 2-speed Transmission The
    360 ​​2-speed transmission is designed by DANA Corporation of the United States for construction machinery vehicles. Its speed and torque meet high performance requirements. Its speed change principle is that the hydraulic cylinder drives the gear meshing position to change the transmission ratio, thereby achieving the purpose of speed change. Its action is performed by connecting the external hydraulic source and the shift solenoid valve, as shown in Figure 1. The gearbox has an integrated shift in place travel switch and can be equipped with a speed sensor. The gearbox has a simple structure and high reliability, but it has a relatively high rotation speed requirement during gear shifting. When the gear speed is higher than a certain threshold, the gear shifting will cause the gear to clash.

2.2 Selection of speed sensor
    Instantaneous speed measurement is achieved through a magnetoelectric sensor, which is installed at the output gear end of the gearbox. The speed range is 0 to 3500 rpm, and the final detection frequency can be converted to 0 to 6.3KHz according to the number of teeth.
    Here we selected the s12 speed sensor of Beijing Spectrum Company. Its specific technical indicators are as follows:
    Detection distance: test frequency 0 to 10KHz, measurement distance GAP ≥ 2.0mm
    Power supply voltage: Vs = 24V
    Output waveform: rectangular square wave, high level is about 10V, low level is less than 0.7V
    Power-on output: high level
    Output mode: NPN output
    Since the sensor outputs a high level of about 10V, in order to avoid damage to the internal circuit of the sensor caused by abnormal external circuits, an optical coupler is used for isolation before the single-chip microcomputer collects the signal. The pulse frequency of the gearbox output is (0-6.3)kHz, so the selected isolation optical coupler should be able to pass pulse signals above 10kHz. The optocoupler selected here is Toshiba's T12521-4. This type of optocoupler can adjust the pulse passing capacity by adjusting the pull-up resistor of the receiving part C pole.
2.3 Circuit design
2.3.1 Power supply and single-chip reset and clock module The
    tractor power supply is 24V, and the single-chip operating voltage is 5V. Here, two three-terminal voltage regulators L7812 and L7805 are used for two-stage voltage regulation to obtain the system power supply, and two light-emitting diodes are added to the 24V and 15V power supplies as power supply indicators. The reset and clock modules use typical circuits, and the external crystal oscillator uses a 32.768KHz clock. Through the internal PLL phase-locked loop module of the single-chip microcomputer, the bus clock can be multiplied to a maximum of 8MHz. The use of low-frequency crystal oscillators greatly reduces the electromagnetic interference of the crystal oscillator circuit of the microcontroller system and improves the reliability of the system. At the same time, since the output frequency of the phase-locked loop frequency synthesizer can be modified by software setting, it provides greater flexibility for user program design.

2.3.2 Input and output module
    The specific 10 allocation of input and output design is shown in Table 1:

In the table, the signal form DI represents the switch input, PI represents the frequency input, and D0 represents the switch output.
    The input is isolated by the photocoupler TLP521-4, and an LED is added as an indicator at the input end, as shown in Figure 3. The output IO drives the photocoupler TL2250, and the TLP250 photocoupler can directly drive the low-power N-channel MOSFET IRF740, and then drive the relay and buzzer. The relay contact is directly output to the high and low-speed electromagnets as a control signal, as shown in Figure 4. Figure 3 Input part Figure 4 Output part [page]

2.4 Software Design
    The working process of this single-chip control system is as follows: When the high-speed or low-speed switch is turned, the current gearbox output speed is detected to determine whether the speed exceeds the set threshold. If the speed is not 0 and does not exceed the set threshold, the high-speed or low-speed electromagnet is energized, and the shift cylinder moves until the shift position. At this time, the travel switch is activated, the electromagnet power is turned off, and the shift is completed.
    When the speed is 0 or exceeds the set threshold, the buzzer alarms, and the system cyclically scans the speed input until the speed is not 0 and does not exceed the set threshold, the alarm is canceled, the high-speed or low-speed electromagnet is energized, the shift cylinder moves until the shift position, and the travel switch is activated, the electromagnet power is turned off, and the shift is completed. The principle of the main program is shown in Figure 5.

The speed measurement is realized by the counting function of the timer/counter. In MC68HC908GP32, T0 and T1 are timer/counters with the function of adding 1. Input an external signal on the I/0 pin of the channel, and the input capture function can be used to monitor the jump of the input signal. Initialize Tl to allow input capture interrupts, set the input capture interrupt to the rising and falling edge trigger mode, and every time a sensor pulse reaches the T1 pin of the microcontroller, immediately apply for an interrupt to add 1 to the corresponding internal RAM count value of the microcontroller. Initialize T0 as a timer, and generate an interrupt once the timing time is 500ms, read the microcontroller count RAM and clear it. The program principle block diagram is shown in Figure 6:

3 Conclusion
    Through the design of hardware and software, the online detection of gearbox speed and the intelligent protection of gearbox shifting in real time are realized. The design makes full use of the rich on-chip resources and superior programming and control performance of MC68HC908GP32, integrating mechatronics technology and intelligent instrument technology. The control scheme given in this article has been debugged and can run smoothly, and I hope it can be of benefit to readers.