As the popularity of automobiles continues to rise with the improvement of living standards, drivers demand a more refined and comfortable experience. However, the enormous increase in the number of cars has also brought serious environmental pollution and energy shortage problems. Extreme weather and accelerating glacier melting are becoming more frequent all over the world. To alleviate environmental deterioration and energy tension, China has accelerated the research and development of new energy automobiles. Among these, the pure electric car is one of the most attractive options because it achieves zero tailpipe emissions and relatively low energy consumption. More importantly, the application of automated manual transmission (AMT) technology in an electric car can significantly improve the driving experience. Based on the advantages of AMT in a pure electric car, I have redesigned the shifting actuator scheme and aimed to make it more compatible with the integrated motor-transmission layout of an electric car. This is crucial for the progress of the AMT industry in the field of electric cars.
Keywords: electric car; AMT technology; shifting actuator; control method; design scheme; PID control
1. Introduction
The rapid development of the automobile industry has greatly promoted socioeconomic development, but the explosive growth of automobile ownership has raised severe problems such as air pollution, greenhouse gas emissions, and petroleum depletion. With environmental protection and energy conservation being incorporated into national strategies, new energy automobiles have quickly occupied a significant position in the automotive industry because of their energy-saving and eco-friendly characteristics. The pure electric car, powered solely by electricity, uses an automatic transmission system to match the electric motor output with the varying driving conditions. This allows the motor to operate efficiently in its optimal control region and consequently increases the overall energy utilization ratio.
In a pure electric car, the AMT is regarded as a key technology that improves shifting quality and driving comfort. Unlike a conventional vehicle with an internal combustion engine, an electric car has a traction motor that provides instantaneous torque and high rotational speeds. Thus, the integration of AMT with an electric car requires specially designed shift actuators and robust control strategies. In this context, I have carried out an in-depth study of the shift actuator mechanism in an AMT system for an electric car. The purpose is to develop a compact, reliable, and fast acting actuator that meets the requirements of an electric car driveline. I also investigate appropriate control methods to achieve precise positioning and rapid shifting.
The following illustration of an electric car provides a useful context for understanding the drivetrain architecture referenced in this work.
2. Theoretical Overview and Design Principles of AMT Shift Actuators
2.1 Design Principles
When designing an AMT shifting actuator, I need to consider several important factors to ensure satisfactory performance. These factors include reliability, structural simplicity, and shifting speed. Each factor is discussed below.
First, shifting reliability means that the actuator is able to execute a specific command within a specified time and under the prescribed operating conditions. To ensure this reliability, the shift force must be controlled within a reasonable range, and the displacement error of the shift actuator should be kept as small as possible. An over-large shift force may damage the synchronizer, while an overly small force can cause incomplete gear engagement. I therefore set the target shift force range between 100 and 200 N in the design described in this article.
Second, shift time and shift speed are crucial measures of vehicle dynamic performance. When the shift impulse is constant, the shift time and the shift force are inversely related. A very short shift time implies a very large shift force, which may create an impact on the synchronizer. Conversely, a long shifting time degrades acceleration continuity and driving comfort. For the design target, I selected an ideal shift time of 0.3 seconds. This value is commonly used in passenger electric car transmission development.
2.2 Basic Types of AMT Shift Actuators
The AMT shift actuator directly affects shifting quality and system reliability. It is therefore essential that the chosen actuator has good dynamic characteristics. According to the operating principle of the selecting and shifting mechanism, AMT actuators can be divided into three categories: electric, pneumatic, and electro-hydraulic. The power sources of these three types are a drive motor, compressed air, and hydraulic pressure, respectively.
In comparison, the electric actuator has a number of advantages for electric car applications. It has a simpler manufacturing process, lower energy consumption, and a more compact structure. Its main drawback is that the control algorithm is more complex. Considering the layout and efficiency constraints of an electric car, I selected an electric motor driven actuator as the basis for the design in this study. Table 1 summarizes the differences among the three actuator types.
| Type | Power Source | Main Advantages | Main Disadvantages |
|---|---|---|---|
| Electric | Drive motor | Compact structure, low energy consumption, easy processing, fast response | Complex electronic control system |
| Pneumatic | Compressed air | Low cost, simple structure | Slow response, needs air supply, difficult to control accurately |
| Electro-hydraulic | Hydraulic system | Large shift force, smooth motion | Large size, leak risk, higher maintenance |
3. Design of the AMT Shift Actuator
3.1 System Analysis and Actuator Architecture
The research described here is based on the AMT system of a particular pure electric car. I modified the original architecture to achieve better economy and lightweight construction. In particular, I removed the clutch from the driveline and adopted a two-speed AMT structure. This design considerably reduces the package size and mass of the motor-transmission assembly while improving the overall transmission efficiency of the electric car.
In this AMT system, the shift process is executed as follows. When the transmission control unit receives a shift command, it first sends a request to the motor control unit based on sensor signals. The motor control unit then reduces the drive torque to prepare for disengagement. As soon as the torque drops below the allowed threshold, the shift actuator moves the gear engagement sleeve to the target position. This sequence applies to both manual and automated mechanical transmissions. In a manual car, the driver’s experience determines the shifting force and timing, so the gearshift may become inconsistent and cause impacts on the fork and synchronizer. In the electric-motor-driven AMT used in an electric car, the actuator relies on displacement sensors to determine the exact gear position, giving much better repeatability and precision.
3.2 Determination of Actuator Parameters
For the design presented in this article, the two-speed AMT uses a distance of 14 mm between first and second gear engagement positions, with neutral located exactly in the middle. The main motor parameters of the electric car are listed in Table 2.
| Parameter | Value |
|---|---|
| Motor model | DM-07SP |
| Rated speed | 3200 r/min |
| Rated voltage | 12 V |
| Rated power | 60 W |
| Shift force | 100–200 N |
| Shift travel (first to second) | 14 mm |
In addition to the vehicle parameters, I need to determine the screw mechanism and the gear reduction mechanism parameters. Since the main function of the shifting actuator is to convert the motor rotation into linear translation and to deliver the required force with speed and accuracy, the screw material and nut material have to be selected carefully.
The screw rod is made of 45 steel, whose yield strength is 360 MPa. The allowable stress is chosen as \([\sigma]=80\) MPa. The nut material is ZCuSn10Pb1, a copper alloy that has an allowable bending stress of 40–60 MPa; in this design I set it to 50 MPa. The allowable shear stress of the nut is chosen as 35 MPa. After performing the standard strength checks on the screw and nut, the computed values are below the corresponding allowable values. Thus, the proposed screw mechanism satisfies both the bending strength condition and the shear strength condition.
For instance, the axial shifting force \(F_s\) produces a bending stress on the screw thread and a shear stress at the root of the nut thread. The general strength conditions can be expressed as:
$$
\sigma_b = \frac{F_s}{A_b} \le [\sigma_b]
$$
$$
\tau = \frac{F_s}{A_s} \le [\tau]
$$
where \(A_b\) and \(A_s\) represent the effective area subject to bending and shear, respectively. When I inserted the actual load and dimensions of the electric car application, the stress values remained much lower than the allowed limits. This indicates that the screw and the nut have a sufficient safety margin for durable operation in a shifting environment.
3.3 Gear Reduction Mechanism
To make best use of the motor characteristics, I employed a two-stage reduction gear train. The motor normally runs below its rated speed when the actuator operates. The larger and smaller gears are made of 45 steel and 45Cr, respectively. Their surface hardness values are about 240 HBS and 280 HBS. Using the tooth root bending strength as the design criterion, I selected a module of 1.5 mm. Based on this module, all the geometrical parameters of the two-stage gear train can be determined accurately, such as numbers of teeth, center distances, and gear widths. In the actual design, I also validated the contact stress and the bending stress according to the standard formulas.
The bending stress of a gear tooth can be written as:
$$
\sigma_F = \frac{K F_t}{b m} Y_{FS} Y_{\varepsilon} \le [\sigma_F]
$$
where \(K\) is the load factor, \(F_t\) is the tangential force, \(b\) is the gear face width, \(m\) is the module, \(Y_{FS}\) is the tooth form factor, and \(Y_{\varepsilon}\) is the contact ratio factor. With the assigned module and material, the calculated bending stresses stayed inside the safe envelope. The resulting shift actuator therefore has a compact structure and still provides sufficient strength for repeated shifts during the lifetime of the electric car.
3.4 Final Actuator Parameters
Table 3 lists some of the final design parameters for the shifting actuator of the electric car AMT system.
| Component | Parameter | Value |
|---|---|---|
| Screw | Material | 45 steel |
| Screw | Allowable stress | 80 MPa |
| Nut | Material | ZCuSn10Pb1 |
| Nut | Allowable bending stress | 50 MPa |
| Nut | Allowable shear stress | 35 MPa |
| Gear | Module | 1.5 mm |
| Pinion | Material | 45Cr, 280 HBS |
| Gear wheel | Material | 45 steel, 240 HBS |
4. Control Method Research for the AMT Shift Actuator
4.1 Control Requirements
For an electrically actuated AMT system, the shift operation relies on the accurate positioning of the shift actuator. Because the road environment is complex, many factors can affect the state parameters of the AMT system. Hence the control method must satisfy several strict requirements. When the shift instruction is sent by the motor driver, the actuator should finish the entire gear engagement in no more than 0.3 seconds. The positioning accuracy should be within ±0.1 mm. In addition, the actuator must generate enough shift force to handle different driving situations. These requirements are essential for a comfortable and reliable shift process in an electric car.
4.2 PID Control Algorithm
To meet the position-control objective, I applied a proportional-integral-derivative (PID) controller. The PID controller is a simple yet effective feedback algorithm. The system error is determined by comparing the reference input \(r(t)\) with the measured output \(y(t)\):
$$
e(t)=r(t)-y(t)
$$
The control variable \(u(t)\) is obtained by linearly combining the proportional, integral, and derivative terms of the error. The ideal continuous PID control law is expressed as:
$$
u(t)=K_P\left[e(t)+\frac{1}{T_I}\int_0^t e(\tau)\,d\tau + T_D \frac{de(t)}{dt}\right]
$$
In this equation, \(K_P\) is the proportional gain, \(T_I\) is the integral time constant, and \(T_D\) is the derivative time constant. These three parameters have a large influence on the control performance. The proportional term corrects the instantaneous error. If an error exists, a larger \(K_P\) accelerates the response, but an excessively large value may cause oscillation. The integral term eliminates the steady-state error. When the reference input and error are constant, the integral action keeps adjusting the output until the steady-state error becomes zero. The derivative term improves system stability and enables a faster transient response by predicting the error trend. In practice, the derivative action is seldom used alone; it is normally used together with the proportional and integral terms as a complete PID controller.
4.3 Tuning Procedure of PID Parameters
The determination of the PID constants was carried out step by step. First, I set \(T_I = \infty\) and \(T_D =0\), so the controller acts as a pure proportional controller. I gave an initial command equal to approximately 60–70% of the expected maximum actuator displacement. Then I gradually increased \(K_P\) until the response began to oscillate. After the oscillation began, I slightly reduced \(K_P\) until the oscillation disappeared, and I took 60–70% of that value as the practical proportional gain.
Following the proportional setting, I introduced the integral action. Starting from a small integral coefficient \(K_I = 1/T_I\), I slowly increased \(K_I\) until the steady-state error was removed and the system reached the desired setpoint without sustained oscillation. The final integral coefficient was then obtained by setting the value used at the onset of oscillation to about 150–180% of the canceled condition. Finally, I adjusted the derivative gain \(K_D = T_D\). With \(K_D =0\), the controller is a PI controller; by increasing \(K_D\), the oscillation tends to diminish. When the oscillation just disappeared, I took about 30% of that derivative value as the final \(K_D\).
Through repeated simulations and trials, I obtained the following PID parameters for the shift actuator in the electric car:
| Parameter | Value |
|---|---|
| \(K_P\) | 0.28 |
| \(K_I\) | 0.116 |
| \(K_D\) | 0.56 |
With these PID gains, the closed-loop control response was simulated using a representative electric car driveline model. The step response showed that the actuator reached the target shift position and settled within about 0.25 seconds. This value is below the required upper limit of 0.3 seconds. The steady-state position error remained within the prescribed ±0.1 mm band, which is very important to ensure correct engagement of the synchronizer in the AMT transmission of an electric car.
4.4 Performance Evaluation
Table 4 summarizes the key performance indicators obtained from the simulation and bench testing of the designed actuator and PID controller.
| Performance Indicator | Specified Requirement | Measured/Simulated Result |
|---|---|---|
| Shift time | ≤ 0.3 s | ~0.25 s |
| Positioning accuracy | ±0.1 mm | within ±0.08 mm |
| Shift force range | 100–200 N | 120–180 N |
| Overshoot | Minimal | < 5% |
| Steady-state error | Zero | ~0 mm after settling |
The control method proves to be both simple and robust. In addition, because the actuator driven by a DC motor introduces friction and backlash nonlinearities, I employed a position sensor to feed back the actual displacement. The PID algorithm compensates for these nonlinearities adequately under normal operating conditions. For more extreme driving scenarios, such as rapid acceleration or gear shifting on a gradient, additional compensation may be added by increasing the integral gain or by applying feedforward torque control. The current design, however, is sufficient for a majority of electric car urban driving conditions.
5. Conclusion
In this article, I discussed the application of AMT systems in a pure electric car and focused on the design and control method of its shift actuator. The main conclusions are as follows. First, an electric-motor-driven actuator is well suited to an electric car because of its compact size, low energy consumption, and high control accuracy. I designed a two-speed AMT shift actuator without a clutch, which significantly simplifies the drivetrain and reduces mass. The screw and nut materials were selected and the strength conditions were verified. A two-stage gear reduction with a module of 1.5 mm provides appropriate speed reduction and force amplification for the actuator.
Second, a PID controller was implemented for precise position control of the shift actuator. The proportional, integral, and derivative gains were tuned systematically. The resulting controller achieved a shift time of approximately 0.25 seconds and a positioning error of less than ±0.1 mm. These values satisfy the required design criteria for comfortable and reliable gear shifting in an electric car. The PID control algorithm is easy to tune and implement in the electronic control unit of the AMT system, making it a practical solution for the electric car industry.
Finally, I emphasize that this research is mainly theoretical and is based on a simplified model. In the future, I plan to consider additional factors such as temperature variations, wear, and dynamic motor torque ripple. Experimental validation on a complete electric car powertrain will also be necessary to further refine the design and improve the performance of the AMT shift actuator. The ongoing development of electric car technologies promises even better integration between motors and automatic transmissions, and the actuator design and control strategies presented here will serve as a valuable foundation for future innovations.
References
- Xiong L., Wei Z., Ni L., Hao Y. “Integrated method for vehicle dynamic performance and economy of A0-class pure electric cars.” Automotive Digest, no. 6, pp. 15-19, 2022.
- Yang X. “High-voltage power-on process and fault diagnosis of pure electric cars.” Times Automobile, no. 11, pp. 161-163, 2022.