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Hardware Design and Test of a Gear-Shifting Control System of a Multi-gear Transmission for Electric Vehicles

  • Feng Tian
  • Liqi SuiEmail author
  • Yuanfan Zeng
  • Bo Li
  • Xingyue Zhou
  • Lijun Wang
  • Hongxu Chen
Article
  • 48 Downloads

Abstract

The performance of electric vehicles is affected by the shift quality of multi-gear transmission. The realization of dual-target tracking control requires the transmission control unit (TCU) to accurately measure and process the input signals of the gear-shifting control system and precisely control the drive motor torque and the position of shift motors. An electric-vehicle-dedicated TCU was designed to meet the above design requirements. Its function modules included a single-chip control circuit, shift position signal sampling circuit, signal conditioning circuit of the rotational speed and angle, controller area network communication circuit, and shift motor drive circuit. A hardware-in-the-loop simulation test system showed that the TCU design scheme met measurement accuracy requirements and coordinated the actions of the shift actuator and motor control unit to achieve fast and smooth shifting before the road test. The power interruption time of the shifting process was within 350 ms. The reliability of the TCU design was further verified in a 150,000-km vehicle road test.

Keywords

Electric vehicles Transmission control unit Multi-gear transmission Dual-target tracking control Hardware design Gear-shifting control 

1 Introduction

The multi-gear transmission of electric vehicles can widen the torque output range of the drive motor and improve vehicle dynamics. Meanwhile, the technology reduces the maximum torque requirement of the drive motor, so that the electric vehicle can fully utilize the high-efficiency area of the motor and improve the economy of the vehicle [1, 2]. These advantages have greatly promoted the development of multi-speed electric vehicles. At the same time, the automatic multi-gear transmission system can automatically shift gears according to road conditions and the driver’s intentions, improving driver comfort, and ease of operation [3]. Automatic shifting is the developmental trend of electric-vehicle power systems.

The control core of the gear-shifting control system of multi-gear transmission is the transmission control unit (TCU). TCU needs to communicate with vehicle control unit (VCU) and motor control unit (MCU), accurately collects shift signals, and controls the torque of driving motor, so as to achieve high-quality shift. TCU should complete the “zero” fault of shifting action throughout its life cycle. Otherwise, drivers and passengers may face personal safety risks [4].

The present paper designed an electric-vehicle-dedicated four-speed TCU according to the dual-target tracking control requirements of multi-gear transmission, which has an automatic shifting function and is compatible with two-speed transmission. Section 2 introduces the hardware design requirements and presents a detailed discussion on the TCU input and output system, hardware architecture, and circuit design. Section 3 analyzes the vehicle shifting process by building a hardware-in-the-loop simulation system. Simulation test results verify that the TCU design meets measurement accuracy requirements. The TCU coordinates the actions of the shift actuator and the motor controller to achieve fast and smooth shifting. Section 4 estimates the controller area network (CAN) communication delay and verifies the reliability of the TCU hardware design through real-vehicle testing. Finally, the TCU shift quality and performance are summarized in Sect. 5.

2 TCU Hardware Design

2.1 TCU Hardware Design Requirements

The hardware design requirements for the reliable shifting of the TCU are as follows:
  1. (1)

    Requirements for the measurements of the rotational speed and angle

     
In the gear-shifting control of multi-gear transmission, it is necessary to separately measure changes in the rotational speed and angle of the input shaft and output shaft of transmission and thereby control the drive motor to adjust the differences in the rotational speed and angle [5, 6]. The TCU requires a speed measurement accuracy of ± 1 rpm and an angle measurement accuracy of ± 1° for the output shaft. CAN communication is adopted to obtain the angle information of the transmission input shaft, and the accuracy of the angle calculation is thus readily affected by the CAN communication delay and its small fluctuations when the motor rotates at high speed. It is therefore necessary to estimate and compensate for the CAN communication delay in real time. In addition, to ensure the accuracy of the angle measurement of the input shaft, the resolution of the electrical angle of the motor rotor provided by the MCU is no less than 4096 pulses per revolution.
  1. (2)

    Requirements for the measurement of the shift fork position

     
Accurately collecting the position information of the shift fork provides information on the gear position and determines whether the shift fork reaches the target position. The position information is also used as a feedback link of the shift motor control, so that the shift stroke is controlled within a range of 11 mm ± 1 mm. Once the actual position of the shift fork deviates from the allowable error range, the TCU should be able to correct the position or take appropriate protective measures and report the error.
  1. (3)

    Accurate control of the torque of the drive motor

     
The drive motor torque is controlled by CAN commands transmitted from TCU to MCU. To accurately control the drive motor rotor and shorten the time required for speed adjustment, the transmission cycle of the torque control command is required to be less than 5 ms and the communication delay time is estimated and compensated for in real time. In the process of actively adjusting the angle difference, the drive motor also needs to be able to switch between driving and braking frequently. In addition, the maximum torque rise rate of the drive motor should be as high as possible to shorten the speed regulation time, and the steady-state accuracy is within ± 5%.
  1. (4)

    Requirements for the position control of shift motors.

     

The shift motor rotor is connected to a shift fork and sleeve using a speed reducer, so that accurately controlling the position of the shift motor can accurately control the position of the shift fork. The transmission adopts a four-gear configuration, in which one shift motor controls the first and second gears and the other shift motor controls the third and fourth gears. Forward rotation represents upshifting while reverse rotation represents downshifting, and the two shift motors thus need to be able to operate alternately in four quadrants. The stroke of the shift fork is 11 mm, and the required gear-shifting control accuracy is ± 1 mm. Otherwise, there may be a risk of disengagement or failure to reach the target gear position. In addition, the output power of the shift motor needs to overcome the inertial force of the shift fork and actuator itself.

2.2 Design Principle Analysis

2.2.1 TCU Input and Output System

A signal flow graph of the TCU input and output system is shown in Fig. 1. The input signals are a rotational speed signal \(\omega_{\text{m}}\) and a rotational angle signal \(\theta_{\text{m}}\) fed back by the motor rotary encoder of the input shaft, a rotational speed signal \(\omega_{\text{slv}}\) and a rotational angle signal \(\theta_{\text{slv}}\) fed back by the photoelectric encoder of the output shaft, two sleeve position signals \(x_{1}\) and \(x_{2}\) fed back by two shift actuator position sensors, and an acceleration signal, brake signal, and gear signals \(P, R, N, D\) transmitted by a CAN0 bus. The output signals are a torque signal \(T_{\text{m}}\) that actively controls the drive motor and two torque signals \(T_{{{\text{m}}1}}\) and \(T_{{{\text{m}}2}}\) that control two shift motors. To ensure the normal operation of the driving motor, motor temperature and phase current signals are fed back to the MCU.
Fig. 1

Signal flow graph of the TCU input and output system

During the shifting process, the TCU outputs the drive motor torque \(T_{\text{m}}\), which actively controls the rotational speed and angle, so that the tooth tip of the sleeve in the transmission faces the slot of the gear ring [1, 5]. The TCU outputs the shift motor torque signals \(T_{{{\text{m}}1}}\) and \(T_{{{\text{m}}2}}\), which drive the sleeve to move \(\Delta x_{1}\) and \(\Delta x_{2}\), respectively, so that the shift fork pushes the sleeve to the target gear position.

2.2.2 Dual-Target Tracking Control

Figure 2 shows a multi-gear transmission system for electric vehicles. Its structural feature is the removal of the synchronizer.
Fig. 2

A multi-gear transmission system without synchronizer for electric vehicles

In an electric-drive mechanical transmission system without a clutch or synchronizer, a type of “zero speed difference” and “zero angle difference” dual tracking control of the relative speed and angle is realized between the gear ring and the sleeve through active control of the torque of the driving motor [5]. In the circumferential direction, the equations of motion for the sleeve and the gear ring are [1, 7]
$$\left\{ {\begin{array}{*{20}l} {\dot{\omega }_{\text{slv}} = - \frac{{T_{\text{fv}} }}{{J_{\text{out}} }}} \hfill \\ {\dot{\omega }_{\text{gr}} = \frac{{i_{{{\text{g}}0}} \cdot i_{{{\text{g}}1}} \cdot (T_{\text{m}} - T_{\text{fg}} )}}{{J_{\text{in}} }}} \hfill \\ {\dot{\theta }_{\text{slv}} = \omega_{\text{slv}} } \hfill \\ {\dot{\theta }_{\text{gr}} = \omega_{\text{gr}} } \hfill \\ \end{array} } \right.$$
(1)
where \(\omega_{\text{slv}}\) is the speed of the sleeve; \(\omega_{\text{gr}}\) is the speed of the gear ring; \(\theta_{\text{slv}}\) is the angle of the sleeve; \(\theta_{\text{gr}}\) is the angle of the gear ring; \(T_{\text{fv}}\) is the resistance moment of the vehicle; \(J_{\text{out}}\) is the equivalent moment of inertia at the output of the transmission; \(i_{{{\text{g}}0}}\) is the transmission ratio between the input shaft and the intermediate; \(i_{{{\text{g}}1}}\) is the transmission ratio between the intermediate shaft and the first gear; \(T_{\text{m}}\) is the torque output from the motor; \(T_{\text{fg}}\) is the oil resistance in the transmission; and \(J_{\text{in}}\) is the equivalent moment of inertia (including input shaft, intermediate shaft, etc.) at the gear ring.
In dual-target tracking control, the variables of concern are the relative rotational speeds and angles of the sleeve and gear ring. The system is characterized with ∆ωslv-gr and ∆θslv-gr, and the equation of state is
$$\left\{ {\begin{array}{*{20}l} { \Delta \dot{\omega }_{\text{slv-gr}} = \frac{{ - i_{{{\text{g}}0}} \cdot i_{{{\text{g}}1}} }}{{J_{\text{in}} }} \cdot T_{\text{m}} + \Delta f} \hfill \\ {\Delta \dot{\theta }_{\text{slv-gr}} = \Delta \omega_{\text{slv-gr}} } \hfill \\ \end{array} } \right.$$
(2)
where the system disturbance is \(\Delta f = (i_{{{\text{g}}0}} \cdot i_{{{\text{g}}1}} \cdot T_{\text{fg}} )/J_{\text{in}} - T_{\text{fv}} /J_{\text{out}}\). \(T_{\text{fg}}\) is relatively small and \(J_{\text{out}}\) is relatively large and the effect of the disturbance on the relative rotational speed ∆ωslv-gr is therefore much weaker than the motor torque \(T_{\text{m}}\). Assuming that \(\Delta f\) is a constant, the control variable is the output torque \(T_{\text{m}} \in [ - T_{ \hbox{max} } , T_{ \hbox{max} } ]\).
Before performing “zero speed difference” and “zero angle difference” dual-target tracking control, the initial state \(\Delta \omega_{\text{slv-gr}} (t_{0} ) = \Delta \omega_{{{\text{slv-gr}}0}}\). Here \(\Delta \omega_{{{\text{slv-gr}}0}} > 0, \Delta \theta_{\text{slv-gr}} (t_{0} ) = \Delta \theta_{{{\text{slv-gr}}0}} , \;{\text{and}}\; \Delta \theta_{{{\text{slv-gr}}0}} \in \left[ { - \pi /N, \pi /N} \right]\), where N is the number of teeth of the sleeve and the gear ring. After the adjustment is complete, the end condition of the system is \(\Delta \omega_{\text{slv-gr}} \left( {t_{\text{f}} } \right) = 0,\; \Delta \theta_{\text{slv-gr}} \left( {t_{f} } \right) = k \cdot \left( {2\pi /N} \right),\) where k = 0, 1, 2, …, and the angular acceleration difference equals to 0. The control objective is that the duration of the dual-target tracking control process is sufficiently short, and the performance functional is therefore
$$J = \int\limits_{{t_{0} }}^{{t_{\text{f}} }} {1\,{\text{d}}t}$$
(3)
The optimization goal is
$$T_{\text{m}}^{*} = \mathop {\arg \;\hbox{min} }\limits_{{T_{\text{m}} \in [ - T_{{max} } ,T_{{max} } ]}} J$$
(4)

The dual-target tracking control method is used in the gear-shifting control system. Precise control of the motor torque and shifts is required by the TCU through the rotational speed and angle difference.

2.3 TCU Hardware Architecture

In realizing dual-target tracking control, the TCU hardware architecture is analyzed to help optimize and improve hardware design. The main function of the TCU is to coordinate the actions of the shift actuator, VCU, and MCU for fast and smooth shifting and to shorten the power interruption time of the shifting process to within 350 ms.

The present paper processed the TCU input and output signals using a modular design; that is, different signals are collected and processed by different modules to obtain accurate input and output signals. The overall block diagram of the TCU hardware is shown in Fig. 3. The hardware mainly comprises a single-chip minimum system module, DC–DC power module, shift motor control module, speed and angle pulse shaping module, shift actuator position signal acquisition module, shift motor current sampling module, CAN communication module, and hardware protection circuit.
Fig. 3

Overall block diagram of the TCU hardware design

2.4 Design of TCU Hardware Circuits

Important circuits of the TCU are analyzed and designed according to the input and output signals and hardware architecture requirements.

2.4.1 Design of the Single-Chip Control Circuit

The single-chip control circuit is the key to ensuring the successful shift of the transmission. Figure 4 shows a diagram of the single-chip minimum system circuit, including a crystal oscillator circuit, power supply circuit, reset circuit, reference voltage circuit, and debug interface circuit. An Infineon 16-bit microcontroller was adopted. The central processing unit had a maximum frequency of 80 MHz and an instruction cycle of 12.5 ns. The external clock input signal was provided by an 8-MHz crystal oscillator, and the required operating frequency could be configured by an on-chip phase-locked loop. The external input power supply was 9–24 V DC, and 5 V DC was obtained through a power reverse polarity protection diode and a TLE6365G step-down DC–DC voltage converter chip. An LC filter circuit was used to suppress the voltage ripple interference to a small amplitude and provide a reliable power supply for the microcontroller. The reset circuit could be re-run the microcontroller program by pressing a button. The 4.1-V reference voltage provided a reference for the AD conversion circuit. The debug interface performed program download and online simulation debugging through the on-chip debug system (OCDS) interface.
Fig. 4

Single-chip minimum circuit

2.4.2 Design of the Conditioning Circuit for Speed and Angle Signals

The measurement accuracy of the rotational speed and angle signals of the input and output shafts of the multi-gear transmission affects the shifting time and quality. Accurate measurement of the speed and angle signals assists speed and angle synchronization of the sleeve and gear ring, which is the key to achieving fast and smooth shifting [7, 8, 9]. In the vehicle test, the speed and angle sensor of the transmission output shaft uses an incremental photoelectric encoder, as shown in Fig. 5a. The photoelectric encoder has the advantages of a simple principle, compact structure, convenient installation, and stable performance, and the resolution of the quadruple frequency output is 4000 p/r. The incremental encoder directly outputs two channels (e.g., A and B) to indicate the rotational speed and angle of the transmission output shaft, plus an optional index marker pulse channel (e.g., Z) according to the principle of photoelectric conversion. The pulse phase difference between phases A and B is 90°. The output axis has forward rotation if the pulse of phase A is ahead of the pulse of phase B and has reverse rotation otherwise. The Z line is a zero pulse line, and the photoelectric encoder generates one Z signal every revolution. The Z signal is used to clear the counter and accumulated error.
Fig. 5

a Incremental photoelectric encoder and b conditioning circuit for speed and angle signals

To accurately measure the rotational speed and angle signals, an RC filter and Schmitt inverter circuit were designed as shown in Fig. 5b. The output signal of the encoder was filtered by an RC low-pass filter circuit, and the three-phase signal waveform of phases A, B, and Z was shaped by a Schmitt trigger. Additionally, a pull-up resistor was added to allow a high low-level output voltage. A BAT54S Schottky barrier double diode was used to limit the input pulse voltage to between 0 and 5 V and protect the microcontroller. The TCU accurately calculated the rotational speed and angle by receiving the frequency of phases A, B, and Z.

2.4.3 Design of the Signal Acquisition Circuit for the Shift Fork Position

A resistive displacement sensor was used to measure the position signal of the shift fork. The position sensor was connected to the shift actuator. The shift motor rotated the sensor to produce an opening for the output of the position signal of the shift fork.

The TCU used an AD conversion module to acquire the position signal. A voltage follower was designed at the front end of the AD sampling, as shown in Fig. 6, to improve the sampling accuracy and driving capability of the AD conversion. A dual operational amplifier, which had the advantages of rail-to-rail input/output, low offset (100 μV), low noise \((4.5\,{\text{nV/}}\sqrt {\text{Hz}} )\), and high-speed operation (50-MHz gain bandwidth), was adopted.
Fig. 6

Acquisition circuit of the position signal of the shift fork

The voltage follower had the function of increasing the input impedance and reducing the output impedance, which effectively ensured the accuracy of sampling. The integration circuit could attenuate the sudden change in the gear position signal due to abnormal fluctuation of the sampling voltage. The RC low-pass filter circuit at the AD input could be used to filter charge injection effect.

2.4.4 Design of the High-Speed CAN Communication Circuit

CAN communication has advantages of a short transmission time, low probability of interference, and good error detection capability. CAN communication has up to 110 nodes connected to each other through a two-wire bus. The nodes are independent of the master and slave and have different priorities. CAN communication can send and receive data in a point-to-point and point-to-multipoint manner.

High-speed CAN communication can shorten the message reception and transmission delay and improve communication among the TCU, VCU, and MCU. The transmitted signals include the accelerator pedal signal, brake pedal signal, gear position signal, drive motor torque, input shaft speed, and angle signals.

Figure 7 shows a high-speed CAN communication circuit. TJA1050 transceiver and ZJYS81 common mode filter chips were selected to improve electromagnetic compatibility. A 3.3-V transient voltage suppression diode was connected between CANH and CANL to protect the driver chip from abnormal voltage damage. The hardware requirements for CAN communication are as follows. First, the wiring harness of the CAN bus adopted a double-layer shielded twisted pair with a diameter of about 0.5 mm2. Second, a 124-Ω terminating resistor and differential matching were adopted to solve signal reflection problems in signal integrity and match the CAN communication impedance. Third, the CAN communication line should be as far as possible from the high-voltage and high-current power lines of the motor and battery.
Fig. 7

High-speed CAN communication circuit

2.4.5 Shift Motor Drive Circuit Design

A brushless direct-current motor (BLDC) has the advantages of good speed regulation, high power density, high efficiency, and stable performance [10]. The BLDC overcomes the congenital defects of the brushed DC motor and replaces the mechanical commutator with an electronic commutator. Two BLDCs are therefore used as shift motors, and their three-phase bridge driving circuits are designed. One BLDC controls the first and second gears, while the other controls the third and fourth gears.

Two special chips are used for three-phase bridge drivers integrated by short-circuit detection, current detection, shoot-through protection, overcurrent protection, two-bit error diagnosis, and high output performance, meeting the typical requirements of automotive applications. The duty cycle is adjustable within the range of 0–100%, and the three high-side and three low-side output stages are sufficient to drive the metal–oxide–semiconductor field-effect transistor (MOSFET) with rise and fall times of approximately 150 ns. The basic structure of the three-phase full-bridge drive circuit is shown in Fig. 8. Six negative–positive–negative (NPN) MOSFETs and one current sampling resistor \(R_{\text{s}}\) are used for each driver.
Fig. 8

Basic structure of three-phase full-bridge drive circuit

The CCU6 function module of the single chip first outputs six complementary pulse width modulation (PWM) pulse signals with a dead time. PWM signals are sent to two driver chips via signal conditioning and a multiplexer. Only one driver chip can be controlled at a time, and the error of two BLDC simultaneous shifts is thus successfully solved. The TCU controls the turn-on and turn-off of the MOSFET by outputting the PWM of different duty cycles and changes the current of the shift motor. Through the current sampling resistor \(R_{\text{s}}\) and AD conversion, the current value is converted into a digital quantity and handed over to the single-chip program to realize the current closed-loop control of the shift motor. By collecting the Hall signals through the counter module, the closed-loop control of the speed of the shift motor is realized. Whether the shift fork has reached the target position can be determined from the position feedback signal of the sleeve. The current, speed and position closed-loop control of the shifting motor allows precise control of the four-quadrant operation and the driving of the shift fork to reach the target gear position.

2.5 Physical Map of the TCU

The TCU is fabricated according to the hardware architecture and functions as shown in Fig. 9. The basic functions of each module were tested, and it was confirmed that the module can be used in the transmission shift test.
Fig. 9

Physical map of the TCU

3 Hardware-in-the-Loop Simulation Test

3.1 Simulation Test Environment

After completing the hardware circuit design of the TCU, a hardware-in-the-loop shifting simulation and test were carried out to verify the TCU functions. The test platform is shown in Fig. 10. The platform allows transmission shift testing with a driver and virtual environment. The main components are the load motor, planetary reduction mechanism, transmission without synchronizer, drive motor, shift actuator and shift motor, MCU, TCU, National Instruments (NI) USB data acquisition card, host computer display, driving operation equipment, data processing server, driving scene displays, and monitoring displays [11]. The interface for monitoring the shifting process displays the road conditions, shifting process, gear position, shift time, vehicle speed, and driver operation information in real time such that testers can intuitively monitor the shifting process.
Fig. 10

Hardware-in-loop shifting simulation and test platform

3.2 Shift Process Analysis and Functional Verification

The main functions of the TCU are to coordinate the actions of the shift actuator and MCU and thus achieve fast and smooth shifting and to shorten the power interruption time of the shifting process. During the hardware-in-the-loop simulation test, the rotational speed and angle signals of the output shaft of the drive motor and load motor were provided by the photoelectric encoder inside the motors. The TCU filtered the high-frequency wave interference of phases A, B, and Z and achieved accurate acquisition.

The entire process is divided into three steps: drive motor unloading, the gear-shifting control process, and drive motor loading. The unloading and loading of the motor are beyond the scope of the present study, and only the shift control process is considered. The upshift from first gear to second gear involves the shift fork moving from the first gear to neutral, start of speed synchronization, start of angle synchronization, driving motor torque switching, end of speed and angle synchronization, shifting the shift fork from neutral to second gear, and reaching the target position.

At the beginning of the upshifting process, the gear ring speed was higher than the sleeve speed as shown in Fig. 11, so brake speed regulation of the drive motor was thus needed. The TCU sent a negative torque signal to the MCU to decrease the speed of the gear ring. Then, by switching the torque of the motor, the difference in the rotational speed between the sleeve and gear ring was rapidly reduced and speed synchronization was achieved. Since the gearbox was opened (see Fig. 10), the lubricating oil inside was completely released, so the friction between the gears became much larger and uneven, which led to speed oscillation before the speed synchronization. In order to achieve speed synchronization quickly, the maximum negative torque of the motor was used for speed regulation. As the speed difference decreased, the motor torque decreased synchronously, but it still caused the negative overshoot in the speed synchronization phase. After two or three overshoots, the speed is synchronized. Similarly, angle synchronization was also achieved through the precise control of motor torque.
Fig. 11

Speed and angle synchronization of the sleeve and gear ring

During the gear-shifting control process, the speed and angle synchronization times were, respectively, about 120 and 75 ms. The speed measurement accuracy was ± 1 rpm. The speed control accuracy was within ± 5 rpm, and the rotational angle measurement accuracy was ± 0.5°. These test results meet the design requirements of the TCU. The gear angle difference was maintained within a small range. The gear angle difference deviated from the midpoint position because there was a gear side clearance between the gear pairs and the spline connection pairs in the transmission when the drive motor was reloaded.

Figure 12 shows the gear-shifting control process of the shift fork position of the TCU. When the speeds and angles of the sleeve and gear ring synchronize, the TCU accurately controls the position of the shifting motor by controlling the PWM signals and achieves a direct engagement of the sleeve and the gear ring [12, 13]. There was little shift shock in this process, and the displacement accuracy was within the control range of 11 mm ± 1 mm. The shift fork took about 45 ms shifting from the first gear to the neutral position and about 45 ms shifting from the neutral position to the second gear. The shift fork has gone through the start, acceleration, and arrival at the detection position, and it was not obstructed from entering the target gear position. After the shift fork reached the target gear position, it maintained a stable gear position and waited for the next shift.
Fig. 12

Gear-shifting control process of the shift fork position

After 6 months of the hardware-in-the-loop simulation test, the TCU realized precise measurement and control of input and output signals. The average shift time for 200,000 shifts was about 300 ms, and the shifting process was smooth with little shift shock. The simulation test results show that the TCU meets the shifting requirements of the multi-gear transmission and provides a good foundation for the subsequent vehicle test.

4 Vehicle Test of the TCU

4.1 Estimation of the CAN Communication Delay

The CAN communication delay time cannot be ignored because dual-target tracking control has high requirements of the real-time performance of the system. Shift shock will otherwise occur. Therefore, the CAN communication delay needs to be estimated and compensated for in real time.

The CAN communication delay is divided into four parts: the generation delay, queue delay, transmission delay, and reception delay. As the level of the microprocessor increases, the generation and reception delays become negligible and the delay time \(R_{\text{m}}\) can thus be expressed as the sum of the queue delay \(t_{\text{m}}\) and the transmission delay \(C_{\text{m}}\):
$$R_{\text{m}} = t_{\text{m}} + C_{\text{m}}$$
(5)
The message queue delay in the CAN communication network comprises the arbitration delay \(t_{\text{arbi}}\) and non-arbitration delay \(t_{\text{nonarbi}}\). According to prioritization theory and the probability density of each frame message sent on the bus, the mathematical expectation of the queue delay \(t_{\text{m}}\) is expressed as [14, 15]:
$$\begin{aligned} t_{\text{m}} & = t_{\text{arbi}} + t_{\text{nonarbi}} = \mathop \sum \limits_{\forall j \ne m} \left[ {\frac{{C_{j}^{2} }}{{2T_{j} }}} \right] \\ & \quad + \mathop \sum \limits_{\forall j \in h\left( m \right)} \left[ {\frac{{t_{\text{m}} + J_{j} + f_{\text{bit}} }}{{T_{j} }}} \right]C_{j} \\ \end{aligned}$$
(6)
where \(C_{j}\) is the transmission delay of message frame \(j\); \(T_{j}\) is the transmission period of periodic message frame \(j\); \(J_{j}\) is the maximum period error generated by message frame \(j\); and \(f_{\text{bit}}\) is the time required to transmit one data bit on the transmission medium.
The transmission delay refers to the time from when the message frame occupies the bus to when it leaves the bus. The transmission delay of the standard data frame is only related to the message frame itself and the bus parameters. The mathematical model is [16, 17]
$$C_{\text{m}} = \left( {44 + 8S_{\text{m}} + \frac{{34 + 8S_{\text{m}} }}{5}} \right)f_{\text{bit}} + P_{\text{cons}}$$
(7)
where \(C_{\text{m}}\) is the transmission delay time; \(S_{\text{m}}\) is the number of bytes in the data frame; and \(P_{\text{cons}}\) is a constant relating to the electrical characteristics of the physical transmission medium.

Using the mathematical model, the CAN communication delay time can be compensated for in real time using the calculated delay time. The priority of the key parameter message is improved in a targeted manner; for example, using the speed and angle signals at the output of the transmission, the target torque sent to the MCU, and the actual torque fed back from the MCU. Communication between the TCU and MCU is realized using a proprietary CAN, and the transmission period is set to 5 ms. By adopting the above measures, the CAN communication delay error can reach the control requirement of less than 50 μs, meeting the requirements for dual-target tracking control of the TCU.

4.2 Test Vehicle Modification

Compared with a hardware-in-the-loop simulation test, an actual-vehicle test often encounters unpredictable interference generated by the entire vehicle power system and the severe external test environment. The present study carried out a real-vehicle road test to verify the shift quality and reliability of the TCU. The test road was located in a mountainous area and had a total length of about 4 km and a maximum slope of about 20%. Figure 13 shows a logistics vehicle on the road of the transmission shift test.
Fig. 13

Logistics vehicle on the transmission shift test road

The test vehicle was a 4.5-tonne logistics vehicle. The vehicle was modified before the test. The basic parameters of the vehicle after modification are given in Table 1.
Table 1

Basic parameters of the logistics vehicle

Parameter

Value

Unit

Motor rated power

50

kW

Motor peak power

90

kW

Motor rated torque

160

Nm

Motor peak torque

350

Nm

Motor rated speed

3000

r/min

Motor maximum speed

6000

r/min

First gear ratio

3.81

/

Second gear ratio

2.54

/

Third gear ratio

1.54

/

Fourth gear ratio

1

/

Main reducer speed ratio

5.83

/

Wheel rolling radius

0.37

m

Figure 14 shows the layout of the drive system after vehicle modification. The MCU and a power distribution system were arranged under the front of the vehicle. The drive motor and transmission were connected in a straight line and longitudinally arranged. The shift actuator was located on the side of the four-gear transmission. There were two shifting motors, one controlling the first and second gears and the other controlling the third and fourth gears. The TCU was located on the side of the frame. Metal enclosures, aviation plugs, and high-temperature shielded wires were adopted in the TCU to protect against water, dust, and electromagnetic interference. The transmission output shaft was connected to the final drive, and the differential was mounted on the rear axle via a drive shaft.
Fig. 14

Driveline of the test vehicle

The test vehicle completed a 150,000-km test to verify the reliability of the TCU. Test results show that the TCU had a good gear-shifting control effect, increasing the driving performance and economy of the whole vehicle. The power interruption time was short during the shifting process, and the drive comfort was improved.

5 Conclusions

The present paper carried out the hardware design and testing of a TCU for the gear-shifting control system of a multi-gear transmission for electric vehicles. According to the TCU input and output system and hardware architecture, the circuit design of the key functional modules was completed, realizing dual-target tracking control. Substantial simulation and vehicle test results show that the TCU accurately measures and controls the input and output signals, resulting in a short shift time and minimum shift shock. The power interruption time of the shifting process was within 350 ms. The results of the study thus verify the accuracy, reliability, and stability of the TCU hardware design. The gear-shifting control system will be further tested on other models. This study will promote the development of transmission technology for electric vehicles.

Notes

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51775291), Provincial-College Cooperation Project (2019YFSY0008), and Sichuan Science and Technology Project (Grant No. 2019JDRC0002).

Compliance with Ethical Standards

Conflict of interest

On behalf of all authors, the corresponding authors state that there is no conflict of interest.

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Copyright information

© China Society of Automotive Engineers (China SAE) 2019

Authors and Affiliations

  1. 1.State Key Laboratory of Automotive Safety and Energy, School of Vehicle and MobilityTsinghua UniversityBeijingChina
  2. 2.Beijing SpacecraftsChina Academy of Space TechnologyBeijingChina
  3. 3.Yibin Fengchuan Power Technology Co., Ltd.YibinChina

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