[AutoChips AC7801x motor demo board review] Unboxing + hardware circuit introduction

我的名字真的真的很长

[AutoChips AC7801x motor demo board review] Unboxing + hardware circuit introduction [Copy link]

 

First of all, I would like to thank EEWORLD and AutoChips for giving me the opportunity to evaluate this board. I am very happy to get a mature motor development board from a large company to do an evaluation before I prepare to make my own brushless motor driver board. This is also a good learning opportunity for me.

My major in college is motors, electrical appliances and power electronics. I am also very interested in motor control, especially brushless motor control. Before this, I had been looking for a suitable motor development board, but after searching the market, I found that there was no suitable consumer-grade motor development board. Maybe this market is indeed a bit small. After all, most of the product developers in the company make their own boards, but this makes the learning cost very high. Beginners often cannot design hardware circuits well. If there is a problem after blindly starting, it is difficult to confirm whether it is a hardware problem or a software problem. Fortunately, AutoChips provides such a development board, and the price is not expensive. The only regret is that it seems that it is not available on Taobao at present? Fortunately, I got this evaluation opportunity and was able to learn the design of this development board so as to lay a foundation for my own development board design in the future.

Let's get back to the topic. Let's take a look at the appearance of this board.

This is the front of the board. You can see that the left half of the board is the main control board, which is responsible for the communication interface and control part, and the right half is the power driver board, which is responsible for inverting the input DC voltage and inputting it into the motor and sampling the motor phase current, etc. There is a separation gap between the main control board and the driver board in the form of a splicing board. The two boards can be separated, but I was reluctant to break it when I saw such a new board. A flat cable is used to connect the main control board and the driver board to transmit signals.

This is the back of the board. You can see that there are no components on the back. A window is opened at the position of the power MOS tube to facilitate heat dissipation.

Before starting software development and debugging, we first use the information on the official website to analyze the hardware design of this development board. After all, if the hardware structure is not clear, it is difficult to start with the software.

There are four schematic diagrams provided by the official website. We analyze them one by one. This is the first one, which mainly focuses on the power supply part.

From the schematic diagram, we can see that the input 12V voltage is reduced to 5V and 3.3V by two switching buck chips after input, but there is a part of the circuit that confuses me:

Here you can see that the 3.3V and 5V power supplies are connected together. Isn't this a short circuit? After seeing those 0R resistors, I seem to understand that this may be an optional configuration, but after careful analysis, it is still wrong. Even if the 0R resistors are disconnected, there is no way to separate the 3.3V and 5V. Is it wrong in the schematic diagram? No, it must not be. This is the information posted on the official website. How could such a low-level mistake be made? Puzzled, I picked up the development board and wanted to see the real thing. As a result, I suddenly realized that there was no 3.3V buck chip soldered on the development board. It turned out that this is indeed a design with optional voltage, but the voltage is not selected by 0R resistors, but by directly not soldering in hardware.

Let's look at the next circuit diagram:

This circuit diagram is relatively complicated. Let's look at it part by part.

Here is the minimum system circuit of the main control microcontroller. There is nothing much to say. It is worth noting that the six channels of PWM1 are pulled down here to prevent the upper and lower arms of the H-bridge from being turned on at the same time and burning the MOS tube due to uncertain electrical levels when powered on.

This part is the schematic diagram of some onboard peripherals, including four user buttons, a reset button, and a potentiometer. Another part is a reserved interface for external PWM input, so that the motor speed can be controlled by external PWM input. It should be noted that an ADC is used to read the values of the four buttons in the four user buttons. This method cleverly saves IO port resources, which is worth learning when we design the board.

This part is the USB to serial port circuit, which uses the CH340G chip.

Next, let's look at the third schematic diagram

This part is mainly the communication interface circuit, which mainly includes CAN interface, LIN interface, SPI interface, and encoder and Hall sensor interface used in closed-loop control. The encoder interface and Hall sensor interface have pull-up resistors to facilitate the use of open-drain output devices, and each interface is equipped with a 10nf filter capacitor to filter out high-frequency interference.

Next is the last and most complicated circuit diagram.

This part mainly includes the pre-drive circuit of the power MOS tube, the power MOS tube circuit, the phase current sampling circuit, the back electromotive force sampling circuit during sensorless driving, the bus voltage sampling circuit, and the overcurrent protection circuit.

Next, let's take a closer look at the functions of each part. I would like to state here that I am still in the learning stage about the drive circuit of the brushless motor. I can only analyze the function of each part of the circuit to the best of my ability. There will inevitably be errors and omissions. You are welcome to point out the mistakes.

This is the driving circuit of the power MOS tube. The power tube used is the domestically produced NCE6990 from Wuxi Xinjie Energy. The parameters of this MOS tube are as follows:

It can be seen that the rated parameters are still very good. Because six NMOS are used, the high-end drive MOS tube needs a bootstrap boost circuit to work, so IR2101S is used to drive the MOS tube. At the same time, this part also includes the phase current sampling resistor and the bus current sampling resistor.

This part of the circuit is used to amplify the phase current sampling value. The MCP6022 rail-to-rail operational amplifier is used to form a differential amplifier circuit. The output voltage after amplification by the amplifier circuit is input into the sampling end of the ADC.

This part of the circuit is the back-electromotive force sampling circuit for sensorless driving. The phase voltage of each phase is input into the sampling end of the ADC after voltage division. At the same time, in order to detect the zero-crossing point of the back-electromotive force, a voltage V_REF at the center point of the motor's three-phase winding Y connection is also required. This voltage is also input into the sampling end of the ADC.

This part is the bus current acquisition and overcurrent protection part. The bus current acquisition uses the MCP6021 high-speed, broadband operational amplifier, and its specific parameters are as follows:

The differential amplifier circuit composed of MCP6021 amplifies the differential voltage obtained by the bus current sampling resistor, and the amplification factor is Av=24/(1+1)=12, that is, it is amplified by 12 times. The output voltage is sent to the ADC for sampling, and the other is input to the LMV7235 voltage comparator of the subsequent stage to compare with the reference voltage. When overcurrent occurs, the comparator outputs a low level and is sent to the brake terminal of the PWM timer to forcibly stop the PWM output.

The last part is the bus voltage acquisition circuit, which uses a resistor voltage divider with a voltage division coefficient of 0.048.

So far, the analysis of the hardware circuit of this development board has been completed. Due to limited capabilities, there are inevitably errors and omissions. You are welcome to correct me.

Finally, let's summarize this development board. Overall, this board is very comprehensive. It has everything it should have. It has a lot of reserved interfaces and supports multiple communication methods. It also supports encoder drive, Hall sensor drive, and sensorless drive for brushless motors. At the same time, the protection circuit is also well done, integrating overcurrent protection and voltage detection functions (undervoltage and overvoltage protection are realized through software, which is more flexible).

At the same time, this board is also very cost-effective in terms of materials. A rough calculation shows that the BOM cost of the operational amplifier alone is close to 20 yuan. Of course, this is the retail price. The actual batch production will be lower, but it should be more than 10 yuan. From this, it can be seen that the materials used in this development board are still very good. And one thing that needs to be praised is that this development board has reserved many test points when it was designed, which greatly facilitates subsequent development and debugging. When testing, you only need to find the test points corresponding to the signal that needs to be observed, and you can easily use an oscilloscope to observe the waveform. From this point, it can be seen that this board is truly designed for users.

This review ends here. In the next article, I will complete the development environment setup and try to use this board to drive the two motors I have on hand. Stay tuned.