Design of deep sea remote motor control system based on STM32F103

Abstract: The power system of the survey equipment used in deep-sea scientific expeditions is often powered by lithium batteries and brushed DC motors. However, lithium battery power supply has many disadvantages, and the mechanical commutation part of the brushed DC motor is prone to failure due to the harsh working environment. In view of this situation, a remote permanent magnet synchronous motor control system based on the STM 32F103 microcontroller is designed. The coaxial cable is used for power supply, making the system more efficient and reliable, and effectively extending the operation time.

Deep-sea motor control system is a key technology in deep-sea operations such as deep-sea scientific investigation, geological exploration, biological resource collection, and deep-sea salvage. It is of great significance to make the motor run efficiently and reliably in the complex environment of the deep sea. At present, in China's deep-sea scientific investigation, brushed DC motors are usually used as power sources, and underwater lithium batteries are used to power them. Since lithium batteries are expensive and need to be charged from time to time, which seriously affects the effective operation time, the use of water power supply, that is, remote control, has great practical significance. On the other hand, brushed DC motors are immersed in high-pressure oil for a long time, and the brushes and commutators are easily damaged due to the harsh deep-sea operation environment. The permanent magnet synchronous motor uses electronic commutation instead of mechanical commutation, which not only has the speed regulation performance of DC motors, but also has a small size and high efficiency. The rotor of the permanent magnet synchronous motor uses permanent magnets, so the excitation circuit is omitted , and thus has a higher power factor. In recent years, due to the extensive development and utilization of new rare earth permanent magnet materials, the performance of permanent magnet synchronous motors has been greatly improved. my country has abundant rare earth resources, so it is believed that permanent magnet synchronous motors will be more widely used.

1 System control principle

Figure 1 is a block diagram of the deep-sea remote motor control system. The energy and data hybrid transmission coaxial cable not only supplies power to the entire underwater system but also provides a line for the host computer and the control system to communicate. The power supply voltage is 1 kV. The data coupling communication module is responsible for separating or superimposing the modulated signal on the coaxial cable, while the DC/DC power supply is responsible for reducing the 1 kV high voltage on the coaxial cable to the 300 V working voltage of the motor and generating a 15 V voltage for the control system. The STM32F103 microcontroller exchanges data with the data coupling communication module through the RS232 isolated by the optocoupler , that is, receives instructions or feedbacks the working status of the motor. Since the motor will generate large harmonics when running, it will interfere with the data signal on the coaxial cable. In severe cases, it will cause remote control errors and cause malfunctions of the underwater system. Therefore, the designed control system is required to respond well to the control instructions issued by the host computer.

Figure 1. Block diagram of deep-sea remote motor control system

2 System Design

2.1 Coaxial cable for hybrid transmission of energy and data

The coaxial cable for mixed transmission of energy and data is a key part in realizing remote control. The transmission voltage waveform on the cable is shown in Figure 2.

The principle of hybrid transmission is to superimpose the DC power supply and data signal at the transmitting end, and then transmit them through the coaxial cable after coupling. At the receiving end, a filter is used to separate the power supply and data. In this way, only one coaxial cable is used to realize the power supply and control of the control system.

Figure 2 Schematic diagram of coaxial cable transmission signal

2.2 Data coupling communication module

The data coupling communication module is mainly composed of two parts: data coupler and modulation and demodulation circuit. The modulation and demodulation circuit modulates and demodulates the data signal to achieve long-distance transmission of the signal. The data coupler is essentially a filter, which is the main part of realizing the hybrid transmission function. Its role in the whole system is shown in Figure 3.

Since both high-voltage power supply and data signals need to pass through this, the filter network is required to withstand high voltage, and the transmission power signal loss should be small and efficient. There is one set of this filter network at both ends of the coaxial cable above water and underwater, and their structures are exactly the same.

Figure 3 The role of the data coupler

2.3 PMSM Motor

PMSM can be divided into surface-mounted, plug-in and embedded types according to the different ways of installing permanent magnets on the rotor. Since the magnetic permeability of permanent magnets is very close to that of air, the inductance of the surface-mounted permanent magnet rotor is basically equal to the direct-axis inductance, that is, Ld = Lq, which belongs to the hidden pole motor. Since its inductance is relatively small, the induced current can be obtained quickly, and no magnetic resistance torque is generated, so the torque linearity is relatively good. The motor used in this design adopts this structure.

The stator of a PMSM is similar to the stator of a common three-phase synchronous motor with electric excitation. If the induced electromotive force (back electromotive force) generated by the permanent magnet is the same as the induced electromotive force generated by the excitation coil and is also sinusoidal, then the mathematical model of the PMSM is basically the same as that of the electric excitation synchronous motor. The structure of a two-pole PMSM is shown in Figure 4.

Figure 4 Structure diagram of two-pole PM SM

The directions of the a, b, and c axes are the directions of the axes of the three-phase windings. The voltage equation and flux equation of the PMSM in the abc three-axis coordinate system are:

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Where rs is the phase resistance of the stator winding , Lms and L Is are the excitation inductance and leakage inductance of the stator winding respectively, r is the rotor electrical angle, and Φm is the flux generated by the permanent magnet. From formula (2), it can be seen that the flux of the three-phase stator is mutually coupled, and they are all functions of the rotor position, which brings difficulty to control.

In the 1970s, Siemens engineer F. Blaschke first proposed the vector control theory to solve the torque control problem of AC motors, which greatly improved the characteristics of AC motors. Vector control uses the vector transformation method to decouple the magnetic flux and torque control of AC motors, making the control of AC motors equivalent to that of DC motors. In Figure 2, the three-phase static abc coordinate system is transformed into a rotating dq coordinate system using the coordinate transformation theory, where the d axis is the direction of the fundamental magnetic field of the permanent magnet rotor, and the q axis leads the d axis by 90 electrical degrees along the direction of rotation. The rotation speed of the rotor reference coordinate is the rotor speed. The stator voltage equation and flux equation on the dq coordinate axis are simplified to:

The electromagnetic torque of the surface mounted PMSM can be calculated as follows, where P is the number of pole pairs of the motor:

Substituting equation (4) into equation (5), and knowing that Ld = Lq of the surface mounted PMSM, the electromagnetic torque expression can be finally obtained as:

From formula (6), it can be seen that controlling the q-axis current of the stator can control the electromagnetic torque of the motor.

2.4 IPM power drive and current sampling module

The power part of this design uses the FSBB20CH60 IPM module of FAIRCH ILD . The maximum working voltage of the MOS tube integrated in this power intelligent module is 600 V, the maximum working current is 20 A, it has a strong self-protection circuit, and has one fault output. The use of the power module not only reduces the size of the system, but also has stronger reliability than the solution of using the power tube plus the driver chip. The three-phase voltage outputs U, V, and W of FSBB20CH60 are connected to the ABC phases of PMSM respectively. Nu, Nv, and Nw are the lower half-bridge outputs of the three half-bridges, and are connected to the current sampling resistors respectively, with a resistance of 15m#. Now take the U phase as an example to illustrate the current sampling method. As shown in Figure 5, Nu and N terminals are connected to the positive and negative terminals of the operational amplifier respectively. Since the phase current may flow into the winding or out of the winding, the voltage signal is positive or negative. The ADC input voltage range of STM32F103 is 0 V to 3.3 V, so an offset voltage VOFFSET is required. The current calculation method is shown in formula (7).

Where R158 = R159 = 3.9 kΩ, R152 = R153 = 1 kΩ, simplifying to get equation (9), and then substituting equation (10) to get the value of current IU.

Figure 5 Current sampling schematic diagram

2.5 Optocoupler Isolation RS232

In order to prevent the motor control system from damaging the data coupling communication module due to high voltage breakdown, optocoupler isolation measures are taken between the STM32F103 microcontroller and the data coupling communication module. The schematic diagram of optocoupler isolation RS232 is shown in Figure 6. The isolation chip uses 4N35 . Due to the speed limitation of the optocoupler device and the small amount of data to be transmitted, the communication rate of RS232 is set to 9600 baud.

Figure 6 Optocoupler isolation RS232

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2.6 STM32F103 microcontroller

ST's STM32F103 controller uses ARM's latest Cortex M3 core, which implements 1-25DM IPS/MH z on Harvard structure, 3-stage pipeline with branch instruction prediction, Thumb 2 instruction set, and the highest operating frequency can reach 72MH z. STM32F103 integrates an advanced timer TIM1 on the chip, which can output six complementary PWM waves with dead zone and has input interrupt function. When overcurrent occurs on the power device, the interrupt function is used to turn off the PWM output to protect the power device. The motor control software flow chart is shown in Figure 7.

Figure 7 Motor control software flow chart

2.7 DC/DC and power management module

The DC/DC power supply reduces the 1 kV power voltage on the coaxial cable to the 300 V operating voltage of the motor and generates a 15 V voltage for the FSBB20CH60 power module, which is the gate drive level of its integrated MOS tube. The power management module uses LM7805 and LM1117 to further reduce the 15 V voltage to generate a 3.3 V voltage for the STM32F103 microcontroller and other devices in the control system.

3 System Testing

The power supply of the water part adopts a high-performance regulated DC power supply to reduce ripple interference, and the output voltage is 1 kV. The motor load gradually increases, and the experimental data is shown in Table 1. Among them, Us, Is and Ps are the output voltage, current and power of the regulated DC power supply, respectively, and U1, I1 and P1 are the output voltage, current and power of the DC/DC module, respectively. The experimental results show that from no-load to close to the rated power range, STM32F103 can normally receive the start-stop, acceleration and deceleration instructions sent by the host computer through the RS232 isolated by optical coupling, and the motor runs well. It can be seen that the normal communication on the coaxial cable is not affected when the motor is running, which meets the design requirements.

Table 1 Motor control when the load gradually increases

4 Conclusion

This paper uses STMicroelectronics' latest ARM CortexM 3 microcontroller STM32F103 to control the PMSM motor. The STM32F103 has the necessary circuits for motor control, such as high-speed dual AD and advanced timer, and has a high operating frequency. At the same time, the hybrid data and energy transmission technology is used to realize both power supply and remote control of deep-sea power equipment, overcoming many drawbacks of using lithium batteries for power supply.

This equipment has been successfully used on my country's scientific research vessel "Ocean No. 1". Practice has proved that it is more flexible and efficient than traditional methods, greatly increases scientific research operation time, reduces equipment maintenance times, and has good application prospects.