Automatic control system of ship power station based on CAN bus technology

0. Introduction
Ship power stations are generally composed of fuel engines, generators, and main distribution panels, as shown in Figure 1. Each component has its own input and output signals. The traditional control method is to connect the respective inputs or outputs to the corresponding controllers, and the corresponding controllers realize the control of a single device. For example, when the power grid detects that the load is large, it automatically generates a standby generator start signal. After receiving the signal, the standby engine controller automatically starts. After a delay and voltage establishment, the automatic paralleling device controls the generator to be connected to the power grid for operation. During the operation, the load distribution device automatically adjusts the load. If the load of the power grid is small, after load balancing distribution, the load of each generator is too small, the system will automatically unload an original standby generator, and it will automatically shut down and return to the standby state after a delay from the power grid, realizing the automatic decoupling of the power station.

1. CAN bus and ship power station
With the development of shipping and the improvement of requirements for ship power stations, bus technology is gradually used in ship control technology, and distributed systems are gradually becoming the protagonist in new design systems. Among them, the controller area network (CAN) module is a serial interface that can be used to communicate with other peripherals or single-chip microcomputers. This interface/protocol is designed to allow communication in a noisy environment. Based on the CAN bus and combined with single-chip microcomputer (MCU) technology, this paper
realizes unmanned automatic control and remote monitoring of the power supply station system consisting of three engines, three generators, and three main distribution panels in the ship power station.
CAN has several important characteristics: First, the bus protocol is completely open, and the relevant control words and registers can be directly obtained from the relevant CAN chip or MCU. As long as the relevant registers are effectively set, the CAN bus module can automatically communicate, and the MCU can directly process the CAN communication information by reading or writing; second, CAN is a bottom-level protocol, and users can completely customize the high-level protocol on this basis; third, the bus has a mature market use and reliable anti-interference characteristics. Therefore, CAN bus is increasingly used in ship control systems.
The ship power station can be divided into the following parts according to the control function:
1) Engine start and stop control
2) Generator voltage control and reactive power distribution control
3) Generator signal detection and protection control
4) Generator automatic parallel control
5) Generator power management control

Each of the above controls has a corresponding sensor, signal transmitter and execution controller to match it. This system implements each link or component with a single-chip microcomputer with a CAN bus. The system structure is shown in Figure 2. The system is divided into three layers. The highest layer is a power management controller (PMU) of a power grid, which detects the power consumption of the power grid and sends a start or stop signal or a load increase or decrease signal to the corresponding controller in the middle layer according to the situation. The middle layer is the controller required by each generator, which adjusts and controls their own power signals, such as voltage or current, as needed. The lowest layer is the sensor and actuator layer, which is composed of one or several sensors or actuators to form a CAN bus unit. All components are hung on a CAN bus network. To ensure the reliability of the system, each unit is physically equipped with dual CAN interfaces, and the entire network constitutes two CAN networks, that is, the CAN bus realizes redundant control. In theory, any controller can control any sensor or actuator, which can realize redundant control of the controller. In fact, the controllers with corresponding functions of the three generators are made redundant with each other, and controllers of different natures are not redundant. However, the highest-level controller (PMU) has the functions of the middle and all controllers, which can realize downward redundant control.

The switch input transmitter 1 detects some basic signals of the fuel engine and converts them into CAN bus interface signals. These basic signals include: cooling water pressure, lubricating oil temperature, oil level in the oil pan, engine standby status, engine automatic control position, fuel pressure, starting air pressure, etc.; CAN bus 2 is the network bus for the entire system communication. The figure shows a bus, but in order to ensure the reliability of the system, two buses are used. Each unit has two bus interfaces to achieve dual bus redundancy; the start-stop output actuator 3 is a relay output with a CAN interface to control the engine start, stop and emergency stop solenoid valve; the engine start-stop and protection controller 4 is the control core that controls the engine to run or stop. On the one hand, it receives signals such as control buttons On the one hand, it receives the signal of CAN bus and controls the engine according to these command signals; the plus and minus output actuator 5 is a relay output controller with CAN bus and local manual output, which is used to control the input of the speed governor in the engine to adjust the speed or load; the fuel engine 6 is controlled by the start or stop solenoid valve and adjusts the running speed or output power according to the speed governor; the speed and other analog input transmitters 7 are sensors that detect some important parameters such as engine speed, cooling water temperature, lubricating oil pressure, exhaust temperature, etc. and convert the signals into signals of CAN bus interface; the power station power management controller 8 is the regulator of the entire control system. The control unit detects the status of the power grid and each engine, realizes automatic control of frequency and load regulation, or realizes automatic starting, or controls automatic unloading and decoupling; the voltage regulating actuator 9 is a phase compound excitation automatic voltage regulating controller with CAN bus control, which adjusts the trigger angle of its bypass thyristor according to the command from the CAN bus or the signal of the regulating knob provided by the actuator, thereby realizing the voltage regulation control of the engine; the generator 10 receives the excitation regulation of 9, and is driven by the prime mover 6 to output electric energy to the power distribution device; the automatic voltage regulating and reactive power controller 11 adjusts its output to the unit 9 according to the voltage and current signals of the generator, and at the same time needs to judge its reactive power and power factor value, so as to realize constant voltage and Reactive power distribution balance; the voltage and current signal input transmitter 12 detects the voltage and current signals output by the generator and the phase difference between the two, calculates the power value, reactive power value, power factor value and other electrical quantities required, and converts them into digital signals and provides them to other required links through the CAN interface; the main switch protection controller 13 mainly realizes the overcurrent, undervoltage and reverse power protection of the generator. Its input signal is provided by link 12, and the main switch and distribution panel status signal is input as an auxiliary signal. It controls the disconnection control of the main switch; the input transmitter 14 of the buttons on the screen converts all operation signals such as the buttons on the distribution panel into standard CAN The interface signal is provided to the relevant links on the CAN network for use; the main distribution panel 15 is a distribution device containing the main switch, relevant relay circuits, and relevant equipment installed inside it; the main switch parallel controller 16 will detect the power difference between the generator and the power grid, and achieve synchronization and automatic parallel connection between the two by adjusting the generator; the main switch closing/opening actuator 17 is a relay output link with a CAN interface, which is matched with the main switch to realize the energy storage, closing or opening control of the main switch; the power supply bus 18 is the power grid powered by the three generators of the ship power station, and all external electrical equipment are powered by this power grid.

2. MCU unit with CAN bus interface
As can be seen from Figure 2, all relevant links of the control system need to be equipped with a CAN bus interface, including sensor signal input and control output. Some of the signal transmission links also need to be calculated and analyzed. Basically, all units with CAN bus need to be equipped with a single-chip microcomputer MCU. The MCU collects the required signals or outputs control signals, and the bus interface is realized by the MCU and CAN. To facilitate the implementation of this function, an MCU with a CAN bus interface is selected. This system uses the PIC30 series control chip of MICROCHIP to implement it. The main features of its built-in CAN module are as follows:
• Implement CAN protocol: CAN 1.2, CAN 2.0A and CAN 2.0B
• Standard and extended data frames
• Data length is 0 to 8 bytes
• Programmable bit rate reaches 1 Mb/s
• Support remote data frames
• Double-buffered receiver with two priority-based received message storage buffers

According to the above description of the characteristics of the MCU with CAN communication interface, combined with the application occasion, with the relevant circuit, a variety of transmitters and actuators with dual CAN interfaces can be realized. The MCU adopts dsPIC30f5011, which has two built-in CAN interfaces. The MCU and CAN interface circuit are shown in Figure 3. C1Tx is the sending signal of CAN bus No. 1, C1Rx is the receiving signal of CAN bus No. 1, C2Tx is the sending signal of CAN bus No. 2, and C2Tx is the receiving signal of CAN bus No. 2. The peripheral switch quantity can realize a maximum of 48 inputs or outputs, and the analog quantity can realize a maximum of 16 inputs. The MCU and the periphery are optically isolated by the high-speed chip 6N137. The CAN bus transceiver adopts the standard PCA82C250, and its output is a differential signal, defined as a pair of CANH and CANL, and is hung on the entire system CAN bus network by twisted pair. A small capacitor connected in parallel between CANH and CANL can filter out high-frequency interference on the bus and provide a certain degree of electromagnetic radiation protection. In addition, a 120-ohm resistor is connected in parallel between the two wires CANH and CANL at the terminal of the CAN bus to eliminate signal reflection.
The MCU has powerful functions, 16-bit CPU, 66K program memory, 4K RAM, 1K EEPROM, 16 × 16-bit working registers, and the highest clock can use 10M crystal oscillator multiplied by 16 times, so it can adapt to general applications. Due to its built-in DSP core, it can also handle data processing that requires fast response, and generally does not require expansion to meet the needs. The specific signal input and output that needs to be realized can be realized with a suitable peripheral interface circuit; the control functions that need to be realized can also be realized programmably. In this system, except for the power station power management controller, which has relatively complex requirements and needs to further enhance the system configuration, other controllers and signal interfaces or transmitters are all implemented using the above circuit. The transmitter or actuator with CAN interface implements the CAN bus interface at one end as shown in Figure 3, and the other end is equipped with the corresponding peripheral circuit of MCU, which can realize the input and output of different functions with CAN bus. The details are as follows:
2.1 Common signal transmitters with CAN bus
Common signals are divided into switch input, 0-5V, 4-20mA, thermal resistor, thermocouple, etc. Among them, the switch signal input adopts optocoupler isolation input, and the analog input adopts high-performance instrumentation operational amplifier. Thermal resistors, thermocouples, etc. all use standard signal conditioning circuits, so for MCU, the input is a standard 0-5V signal, corresponding to the maximum range of the sensor. The 16 analog inputs after signal conditioning are connected to RB0-15 of MCU, so that the MCU can sample the 16 analog inputs. The switch input signal transmitter isolates the input signal and sends it to the PORTB-G port of the MCU. A maximum of 48 inputs can be achieved. Except for the use of optocoupler isolation circuit, no other circuits are required in the middle.

2.2 Orthogonal pulse coding input of the coded signal transmitter with CAN bus has two-phase input of A and B, that is, the phase difference is 90 degrees, and the maximum frequency can reach 20KHz. Due to the high frequency, the photoelectric isolation should use high-speed optocoupler for sampling. The signal capture interface RD8-11 of the MCU can be used to realize 2 groups of 4 orthogonal pulse coding inputs. The corresponding registers of the MCU can be configured to realize the counting and positive and negative judgment of the orthogonal pulse coding.
2.3 Electric quantity signal transmitter with CAN bus

The sampling of the electric quantity signal needs to collect the voltage and current signals and convert them into effective values, and condition them into the 0-5V signal required by the MCU. At the same time, it needs to convert the sine-square wave and send it to the interrupt interface of the MCU for phase calculation. The voltage signal conditioning circuit is shown in Figure 4. The current-type voltage transformer converts the sampled voltage signal into a mA current signal, which is amplified into a voltage signal by the operational amplifier U2. The detection signal of the AC voltage zero-crossing square wave is obtained by the U1A comparison circuit for frequency conversion and phase calculation. The operational amplifiers U1B and U1C circuits constitute a rectifier circuit, and the U1D circuit is a filter circuit, and its output is the 0-5V voltage signal required by the MCU. The voltage signal sampling takes into account the limited variation range, and selects 1.5 times the rated voltage corresponding to the maximum MCU input 5V. The current signal varies greatly, especially when the large motor starts, the current can reach 6-8 times its rated current. The current protection control also needs to be able to effectively achieve 8-10 times the protection control. Therefore, the sampling of the same current is divided into three levels to achieve, one is 2 times the rated current corresponding to the MCU input 5V, one is 4 times the rated current corresponding to the MCU 5V input, and the other is 10 times the rated current corresponding to the 5V input. The circuit principle is similar to that shown in Figure 4. In this way, the sampling of the three-phase voltage and current output of a generator requires 12 analog inputs, 3 voltage zero-crossing interrupt inputs, and 3 current zero-crossing interrupt inputs. According to the above signals, MCU will not only obtain the corresponding voltage and current values, but also calculate the phase difference, power factor, active power, reactive power, apparent power, active energy statistics, etc. At the same time, it is necessary to determine whether there are overvoltage, undervoltage, long-delay overcurrent, short-delay overcurrent, instantaneous overcurrent, reverse power and other fault signals according to the rated value, so this electrical quantity transmitter has multiple functions.

2.4 Actuator with CAN bus
The I/O port of MCU can be configured as output. After configuring the corresponding I/O as output according to the needs, it is connected to the photoelectric coupling unit, and its output is then driven by the transistor to realize the output of the relay. The control of the actuator is that its control power is sent to the actuator through the contacts of the relay to control its forward and reverse operation to achieve corresponding regulation, or to control the on and off of the solenoid valve circuit, etc. In some special occasions, the output of the MCU can be photoelectrically coupled and then driven by the transistor to drive the MOSFET to achieve PWM regulation control or the action regulation of related actuators.

3. Various controllers with CAN bus interface
The CAN interface of the controller with CAN bus is the same as that of the above MCU. In the controller, input and output are not the main functions. The main functions are the computing power, storage capacity, control capacity, display driver, etc. of the MCU. Therefore, the MCU used is a relatively high-end MCU in the PIC series, and its hardware circuit is also similar. In addition to the CAN interface circuit used in Figure 3, some expansion circuits with I2C are used for coordination, such as EEPROM, clock circuit, etc., as shown in Figure 5, where SCL and SDA are the I2C interfaces of the MCU, defined as clock line and data line, A0, A1, A2 are selection signals when the same device is used at the same time, controlled by the MCU, U3 is the clock chip DS1307, and is the clock source crystal oscillator, and 24C08 is the EEPROM of the I2C interface. If other functions are required, they can be expanded on the original I2C bus interface circuit. On this hardware basis, the information on the bus is received through CAN, and each controller compiles the corresponding software according to its required functions, and sends the corresponding output signal to the corresponding output CAN interface module through CAN. Controllers are divided into engine controllers, automatic voltage regulation and reactive power regulation controllers, power distribution protection controllers, synchronous paralleling controllers, power management controllers, etc. according to their specific locations and functions.
4. Redundant control technology

In addition to the main power management controller PMU, the hardware circuits of the other controllers are similar, and the functions they implement are different. However, the control redundancy between them can be achieved through software. Therefore, in the actual design,
two sets of control programs are designed in each controller. Under normal circumstances, one set of the main program is working, and the other set is used as a backup for other controllers to read the data on the CAN bus, but the backup program does not perform output actions. When a controller in the system fails, after there is no heartbeat signal of the normal operation of the controller on the CAN bus network, the backup controller wakes up its backup program and outputs it to replace the failed controller, and the corresponding display appears on the working controller. The relationship between the mutual redundancy and backup of each control in the system design is shown in Table 1, in which the power management controller can be used as a backup for other controllers.

In addition to the redundancy of the controller, the aforementioned CAN buses all use dual CAN interfaces, and the actual lines are also corresponding dual CAN networks. When one of the CAN buses fails, the system can automatically enable the backup CAN network, thereby realizing redundant control of the CAN bus.
5. Conclusion
The ship power station control system adopts a distributed structure, standardized hardware design, and modular software design, making the entire system design combination more flexible. This design method also has a certain reference value for the development of other projects. The actual operation effect of the system is good and the work is reliable, which shows that the use of CAN bus technology in ship power stations is successful and can be promoted.
Innovation of this article: The CAN protocol developed by Yunchu is used in the field of automobile manufacturing. Now the CAN technology is transplanted to the control of ship power stations, realizing unmanned operation, process automatic control and remote monitoring of ship power stations, improving the degree of ship automation and improving system performance.