Abstract: This paper introduces the design principle of the automatic shifting mechanism of an automotive transmission, the working principle of an automatic shifting robot powered by a servo motor, and the software design method of using PLC to control automatic shifting. The automatic shifting mechanism is characterized by safety, reliability, and high success rate. Keywords: PLC, shifting robot, gear selection, shifting, transmission 1. Introduction Automatic shifting refers to the ability to complete a series of gear changes according to operating instructions to reach a predetermined gear without stopping the vehicle during testing. In automotive transmission system and gearbox performance testing, it is often necessary to continuously change the gears of the transmission to meet the test requirements. Currently, most tests use a static shifting mode, meaning the vehicle must stop before each shift. This frequent starting and stopping is not only energy-intensive but also time-consuming. Moreover, in some tests, such as synchronizer tests, dynamic shifting is necessary to effectively test the synchronizer's performance. Therefore, it is necessary to study the implementation of automatic shifting in transmissions. 2. System Composition 2.1 The gear shifting robot in an automotive transmission uses a combination of two levers for selecting and shifting gears. Mimicking the manual gear shifting mechanism in a moving car, a flexible wire moves back and forth within a sleeve to push and pull the levers to complete the shifting operation. The selection and shifting wires have several fixed points on their straight sections; different combinations correspond to different gears. The clutch wire also has two fixed points on its straight section, corresponding to the clutch's engaged and disengaged states. The gear shifting robot is a specialized actuator for changing gears in automotive transmissions. There are various implementation methods for gear shifting robots, such as those using hydraulic or pneumatic power sources. However, these methods suffer from limitations such as unadjustable tension and incomplete movements, affecting the success rate of gear shifts. This gear shifting robot uses three DC servo motors as its power source, converting the motor's rotational motion into linear motion via high-precision ball screws to push and pull the levers for selecting, shifting, and clutching. The advantages of this method are its simple structure and sensitive operation. Since the operating mechanism is connected to the transmission via a flexible shaft, its installation location is not limited by environmental conditions (as shown in Figure 1). The motor driver employs a specially designed servo control system with adjustable speed and torque, and excellent stall characteristics. This allows for precise adjustment of the shifting operation in terms of stroke, speed, and force, thereby improving the shifting success rate. Position signal acquisition uses an encoder disk and high-speed counter, which offers advantages such as high accuracy, ease of adjustment, and long service life. A programmable logic controller (PLC) is used to control the shifting robot, enabling long-term, safe, and reliable operation. Another advantage is its suitability for different types of gearboxes; only the counter settings and gear assignments need to be readjusted. 2.2 Control Method 2.2.1 Based on the control requirements of this gear-shifting mechanism, the PLC selection requires 23 inputs and 23 outputs, as well as 3 high-speed counter channels. Since the entire gear-shifting process takes approximately 1 second, a fast response speed is also required. The Mitsubishi FX2N series is a small, mid-range unit-type PLC with a 16-bit CPU and a dedicated logic processor, featuring direct output functionality. This ensures the output is unaffected by the scan cycle, resulting in fast execution. It also includes an internal high-speed counter with a maximum frequency of 3kHz. Its powerful functionality and flexible combination make it well-suited. Specifically, the FX2N-64MR-ES model is selected, with 32 inputs and 32 outputs, providing sufficient margin. M represents the basic expansion unit, R represents relay output, E represents expansion unit, and S represents thyristor output. The FX-20P-E portable programmer is chosen, featuring menu-driven function selection and enabling online or offline programming for convenience. 2.2.2 Control Scheme: The gear-shifting robot can be controlled independently or via a host computer. Independent operation is performed using buttons on the control panel. The control panel is simple and easy to use. It features eight indicator lights showing the gear position (1-6 gears, neutral, and reverse), two clutch engagement/disengagement indicator lights, a shift failure warning light, a PLC running indicator, one automatic mode indicator light, and one power indicator light, making the shift status readily apparent. A manual/automatic switch allows for switching between manual and automatic control modes. In automatic mode, there are two buttons for selection: upshift and downshift. When a button is pressed, the system automatically identifies the gear position at that moment. If the operation logic is not followed, the system will not respond. If the operation logic is followed (gear shifting can only be done sequentially, skipping gears is not allowed), the system will perform a series of operations until the requirement is met. If the shift fails (the shifting time exceeds the specified range and the lever has not yet reached the predetermined position), the system will automatically downshift (the shift lever returns to the zero position) and engage the clutch. At this time, the shift lever remains in its original position and a shift failure signal is issued, asking whether to continue. There are two other selection buttons on the panel. If you select to continue, the system will disengage the clutch and continue shifting, repeating the above operations. If you select to stop, the system will automatically return to neutral (the shift lever also returns to the zero position) and wait for the next instruction. To meet different operational requirements in the test and to simulate the actual situation of frequently shifting from any gear to neutral during normal driving, this system has this function, which can be completed by simply selecting the stop operation. This system is designed with a communication interface with a host computer, allowing direct reading of the system status and completion of all operations in automatic mode. In manual mode, six buttons are available for selection, corresponding to the forward and reverse rotation of the three servo motors. Operation is done inching, primarily for positioning and debugging during system maintenance or installation. The panel also features six adjustable potentiometers for adjusting the speed and torque of various actions. Furthermore, the system incorporates necessary filtering, overvoltage protection, and self-calibration to address origin drift caused by mechanical backlash. 2.2.3 Counter Parameter Setting: Tighten the connecting bolts on the gearbox side of the three flexible shafts, disengage the bolts on the lead screw side from the connecting plate, place the gear selection and shift levers in the middle position, and place the clutch lever in the engaged position. Move the selection, shift, and clutch levers to the other position. The measured extension length of the flexible shaft is then *l* (actual stroke, in millimeters). Due to mechanical clearance, the actual value of *l* can be taken as an intermediate value. The ball screw pitch is 4 mm. The encoder outputs 100 pulses per revolution, so the counter setting value s is: s = 100 * l/4. Moving the connecting plate to the right is the positive direction, and the counter is positive; moving the connecting plate to the left is the negative direction, and the counter is negative. The programmable controller can determine the direction and travel of the connecting plate based on the number of pulses read. The corresponding numbers for each point are as follows: a0, a1, a2, a3 are the 4 points for gear selection (some gearboxes do not have a3 point), b0, b1, b2 are the 3 points for gear shifting, and c0, c1 are the 2 points for clutch operation. The correspondence between each point and register number is shown in the table below. The points using two registers employ an interval comparison function. 2.2.4 The timer parameters are set as follows: There are 7 timers (T2-T8) with shift timeouts, corresponding to register numbers D32-D38, typically set to 1.2-1.5 seconds. There are 14 timers for the time interval between gear selection and shift operations, corresponding to register numbers D40-D53, typically set to 0.3 seconds. Negative counting is implemented in software. Because the motor continues to rotate due to inertia after the motor reverse command disappears, the negative count switch must have a certain delay to prevent the loss of negative count pulses. Timers T10-T12 are used to complete this delay, typically set to about 0.5 seconds. Timers T0-T1 are for issuing continue and stop commands from the computer. Their purpose is to extend the short pulse signals issued by the computer to ensure the system can accurately read the commands; typically set to 1 second. 3. Software Design The software uses ladder logic programming. The main program sets the counters at each gear position of the transmission (using functions FNC11D and FNC10D). Then, it performs gear selection and shifting operations according to the instructions. Its flowchart is as follows: 4. Conclusion The problem of automatic gear shifting in automobile transmission has been difficult to solve in the tests of automobile transmission system, transmission and other related tests. The main problem is that the force and speed during the shifting process are difficult to match with the running state, resulting in a low success rate of shifting and affecting the application of automatic shifting. The shifting robot used in this test uses three servo motors as the power source. Its driver adopts a specially designed servo control system with adjustable speed and torque. PLC control is used, which has high reliability. The position signal acquisition uses a code disk plus a high-speed counter, which has high accuracy. In the design, the upshifting and downshifting operations and the forward and reverse rotation of the three servo motors are all interlocked from the input end, output end and intermediate relay. There will be no operation disorder or output short circuit. The test results show that this design is simple and practical. The success rate of shifting once is over 90%, and the second shift is successful. It basically solves the problem of automatic shifting of transmission. References: [1] Yin Zhengqi, Power output capability and power matching of servo system, Electrical Transmission, 1998(2), 38-44. [2] Tang Yunmiao, Electrical Machines - Electromechanical Energy Conversion, Machinery Industry Press, 1986. [3] MITSUBISHI Micro Programmable Controller Programming Manual. 2000.9.