Design of electric windows of door control module

The door system that was previously controlled by mechanical methods is now gradually being changed to electronic control. More and more low-end cars are also beginning to use electronically controlled door control systems, using CAN or LIN bus communication technology to achieve communication between the four doors. The window anti-pinch function is one of the difficulties of the door control system. The door control system has a variety of fault diagnosis capabilities and can promptly identify faults such as short circuits, open circuits, overheating, and overloads.

This article focuses on the hardware and software design of the electric window part in combination with the project practice of the design of the automobile door control module. The startup characteristics and fault detection characteristics of the intelligent power chip BTS7960 during normal operation are studied and analyzed, and the test results are given.

The overall design of the door control module

Figure 1 is a schematic diagram of the door control module, in which the microcontroller XC164CS is used to control the switching actions of all power devices, monitor the system status at a regular time, receive appropriate fault feedback signals, and exchange fault information and key control information with the central body controller and other door controllers through the vehicle network (such as CAN bus), so as to display the fault content on the user interface in a timely manner and control the door in real time, ensuring driving safety.

Figure 1 Overall principle block diagram of the gate control module

The 16-bit microcontroller XC164CS is based on the enhanced C166S V2 structure, combining the advantages of RISC and CISC processors, and realizing powerful computing and control capabilities through the DSP function of the MAC unit. The XC164CS integrates a powerful CPU core and a set of powerful peripheral units on a single chip, making the connection very effective and convenient. The

electric windows use two half-bridge intelligent power driver chips BTS7960B combined into an H-bridge driver, the driver chips for the central door lock, rearview mirror and heater use TLE6208-3G, BTS7741G and BSP752R respectively, and the driver chip for the headlights uses BTS724. These devices have provided complete fault detection and protection functions, thus avoiding the use of too many discrete components, greatly reducing the module size, and improving the module's EMC (electromagnetic compatibility) characteristics.

The circuit of the door control module is mainly composed of the following parts: power supply circuit, electric window drive circuit, rearview mirror drive circuit, heater drive circuit, central door lock drive circuit, headlight drive circuit, CAN bus interface circuit and key interface circuit, etc.

Hardware design of electric window

1 Electric window drive circuit and start-up characteristics

This window control system drives the DC motor to rotate through the intelligent power chip BTS7960. The interface circuit of BTS7960 is shown in Figure 2. The pins 7960INH1, 7960IN1, 7960IS1, 7960INH2, 7960IN2 and 7960IS2 in the figure are connected to the I/O ports P9.4, P1L.4, P5.6, P9.5, P1L.5 and P5.7 of XC164CS respectively.

Figure 2 BTS7960 interface connection diagram [page]

BTS7960 is a high-current half-bridge high-integration chip for motor drive. It has a P-channel high-side MOSFET, an N-channel low-side MOSFET and a driver IC. The P-channel high-side switch eliminates the need for a charge pump, thereby reducing EMI. The integrated driver IC has the functions of logic level input, current diagnosis, slope adjustment, dead time generation, and over-temperature, over-voltage, under-voltage, over-current and short-circuit protection. The typical value of the on-state resistance of BTS7960 is 16mΩ, and the drive current can reach 43A. Therefore, even in the cold winter in the north, the safe start of the window can still be guaranteed.

As shown in Figure 3, two BTS7960s form a full-bridge to drive the window up or down. When T1 and T4 are turned on, the window rises; when T2 and T3 are turned on, the window falls. The system has no active braking process. After the window is moved, the upper tube triggers the signal to stop, and the anti-parallel diode of the lower tube of the bridge arm is used for continuous flow until the current is 0A. The continuous flow process lasts for 250ms, which is enough to meet the high power requirements of the window motor. In order to avoid the current spike when the window motor starts, the soft start function is realized by controlling the PWM signal with a frequency of 20kHz for the lower bridge arm switch tube.

2 BTS7960 fault detection characteristics

As shown in Figure 3, the chip of BTS7960 is a half bridge. The INH pin is high level to enable BTS7960. The IN pin is used to determine which MOSFET is turned on. When IN=1 and INH=1, the high-side MOSFET is turned on and the OUT pin outputs a high level; when IN=0 and INH=1, the low-side MOSFET is turned on and the OUT pin outputs a low level. The size of the external resistor on the SR pin can adjust the time when the MOS tube is turned on and off, and has the function of preventing electromagnetic interference. The IS pin is the current detection output pin.

Figure 3 Schematic diagram of full-bridge drive circuit

The IS pin of BTS7960 has a current detection function. In normal mode, the current flowing out of the IS pin is proportional to the current flowing through the high-side MOS tube. If RIS=1kΩ, then V IS=I load/8.5; under fault conditions, the current flowing out of the IS pin is equal to I IS(lim) (about 4.5mA), and the final effect is that IS is high. As shown in Figure 4, Figure (a) shows the current output of the IS pin in normal mode, and Figure (b) shows the current output on the IS pin under fault conditions.

The experimental conditions of the BTS7960 short-circuit fault experiment are as follows: +12.45V battery voltage, +5V power supply, 2.0m short-circuit wire (R=0.2Ω), cross-sectional area of ​​0.75 mm, connected to a 1kΩ resistor and a light-emitting diode. The wire between VS and the positive electrode of the battery is 1.5m long (R=0.15Ω). As shown in Figure 5, V IS is the voltage of the IS pin to ground, VL is the voltage of the OUT pin to ground, and IL is the short-circuit current flowing through the BTS7960 when a short-circuit fault occurs to ground.

Figure 4. Working principle diagram of the current sensing pin IS of BTS7960

Whether it is power-on first and then short-circuit or short-circuit first and then power-on, BTS7960 presents the same protection characteristics, so only one of them will be described below.

Figure 5 BTS7960 short-circuit test circuit diagram

Figures 6 and 7 are the first and second half of the waveform of the short-circuit test of BTS7960, which is short-circuited first and then powered on. At the moment of short circuit, the output current rises rapidly. Within 80μs, the current rises to a peak value, which can reach about 62A. At this time, BTS7960 detects a short circuit fault, turns off the MOS tube, and the output current drops to 0A. The part indicated by the purple arrow is obviously turned off. The part sandwiched by the dotted line in the figure is the process of turning off and maintaining the MOS tube. The whole process lasts for about 80μs. At the moment of short circuit conduction, the output voltage of the OUT pin is about 5V, which is the voltage division value of the total resistance between the short-circuit wire and the battery and the ground; during the MOS tube shutdown, the output voltage of the OUT pin is 0V. At the moment when the current drops sharply, a weak reverse electromotive force is induced on the short-circuit wire, so the output voltage of the OUT pin will show a short-term negative voltage. The state detection pin IS fluctuates around 5V, which has the characteristic of fluctuating up and down with the short-circuit current. During the entire short-circuit process, BTS7960 periodically shuts down the MOS tube to prevent the short-circuit current from causing the chip to continue to heat up, causing the chip to overheat and burn, thereby effectively protecting the chip. Finally, BTS7960 completely shuts down the MOS tube, and the short-circuit current slowly drops to 0A. The IS pin recovers to the normal level by its own cooling about 500μs after the MOS tube is completely shut down.

Figure 2 The first half of the BTS7960 short-circuit test waveform [page]

Figure 7 The second half of the BTS7960 short-circuit test waveform

Software design of electric window

1 Software design of driver chip BTS7960 In the

electric window part, BTS7960 drives the DC motor to rotate in hardware to make the window rise or fall. Two BTS7960Bs are used to form a full bridge.

The interface signals between BTS7960 and the microcontroller include IN1, IN2, INH1 and INH2; IS1 and IS2 are current detection signals.

Window rise: IN1=1, IN2=0, INH1/2=1; Window fall: IN1=0, IN2=1, INH1/2=1.
The entire driving process can be divided into four stages: soft start, full PWM output, continuous flow and stop. The soft start function is realized by PWM control of the lower bridge arm switch tube during the window lifting process. The PWM frequency is 20kHz and the soft start lasts for 200ms. During this process, the duty cycle gradually increases from 0% to 100%, divided into 10 segments, and each segment lasts for 20ms. The PWM signal is applied to the INH pin of the bridge arm where the lower tube is located. When the bridge arm is turned off (INH=0), the current continues to flow through the anti-parallel diode of the upper tube. After the PWM signal realizes soft start, the current waveform when the electric window is started is shown in Figure 8. It can be seen from the figure that the current spike is effectively suppressed.

This system does not have an active braking process. After the window is moved, the switch tube will continue to work for about 250ms. This is the continuous flow process. During this period, the trigger signal of the upper tube stops, and the current continues through the anti-parallel diode of the lower tube of the bridge arm (at this time, the trigger signal of the lower tube of the original other bridge arm needs to continue to be given, such as when the positive continuous flow: IN1=1, INH1=0, IN2=0, INH2=1), until the current is 0. However, if overheating occurs, this continuous flow process is not required.

Motor stalling is not allowed because it will cause overcurrent. BTS7960 can detect the current of the switch tube by itself, and perform current/voltage conversion through the current of the 2.2kΩ sampling resistor. The sampling voltage passes through a simple RC filter network and a protection resistor (not added) to AN0/AN1 for analog-to-digital conversion. When the current is detected to be greater than 15A, it can be determined that the motor is in a stalled state. At this time, the microcontroller stops triggering the motor (current flow is still required), and the user can restart the window.

The faults to be detected in the window part include overheating of the two switch tubes on the upper bridge arm and open circuit of the load. The first detection method is to detect the overheating of the upper tube through the built-in temperature detection function of BTS7960. When overheating occurs, the device automatically shuts down all output circuits, and the output level of the IS pin is high; the second is to require an auxiliary transistor to detect the open circuit, which can be achieved by detecting the current value of the IS pin, and the microcontroller needs to provide a CTRLWIN signal.

Figure 8 Electric window soft start current waveform

2 Software Design of the Main Program of Electric Window

The software control of this electric window control system is based on state conversion. By comparing the system state with the control command, a judgment is made to determine the action that the current system should perform. The program divides the operating states of the electric window into the following: WINDOW_OFF, WINDOW_UP_PWM, WINDOW_UP, WINDOW_UP_FREE, WINDOW_UP_STOP, WINDOW_DOWN_PWM, WINDOW_DOWN, WINDOW_DOWN_FREE and WINDOW_DOWN_STOP. When the electric window is in the OFF state and receives an increase or decrease command, the program will make the window enter the PWM increasing state first to achieve soft start. When the PWM full duty cycle is reached, the window will enter the UP or DOWN state. If a command to stop or reverse the electric window is received during the PWM increasing state or the PWM full duty cycle operation, the program will make the window enter the continuous flow state. After the continuous flow is completed, the window enters the STOP state. If an open circuit or overvoltage fault is detected in any state, the window will enter the OFF state. (end)