Controller Area Network (CAN) is a type of fieldbus, a serial communication network that effectively supports distributed or real-time control, and is widely recognized as one of the most promising fieldbuses. In industrial control systems, electric actuators are crucial execution units in electric unit combination instruments. They consist of two completely independent parts: a control circuit and an actuator. They receive electrical control signals from the regulator and linearly convert them into mechanical angles or linear displacements to manipulate dampers, baffles, valves, and other regulating mechanisms for automatic control. In recent years, with the rapid development of microelectronics and control technology, electric actuators have also experienced rapid growth. In particular, some foreign manufacturers have successively launched conventional intelligent electric actuators with fieldbus communication protocols, and the CAN intelligent electric actuator is one of the most promising. This paper introduces a CAN intelligent electric actuator that uses a brushless DC motor for control, achieving high control precision and realizing digital valve position detection. This improves the accuracy and reliability of valve position measurement and can replace the currently widely used potentiometer and differential transformer analog measurement methods. 0. Hardware Design of the Control Circuit The principle structure of the CAN intelligent electric actuator is shown in Figure 1. Figure 1 shows the schematic diagram of the CAN intelligent electric actuator. The entire circuit mainly consists of five parts: the host part, mainly composed of the P89C58 microcontroller; the CAN bus control and interface part, composed of SJA1000, 82C250, opto-isolation circuit, etc.; the motor control part, composed of the MC33035 brushless DC motor dedicated control chip; the valve position detection part; and the LCD display part. 1 CAN Bus and Interface Part The CAN bus control and interface, as shown in Figure 2, mainly includes the independent CAN communication controller SJA1000, the CAN bus transceiver 82C250, and the high-speed optocoupler 6N137. The P89C58 microcontroller first initializes the SJA1000 and controls the SJA1000 to perform data communication tasks. The AD0~AD7 pins of the SJA1000 are connected to the P0 port of the 89C52, the CS pin is connected to the P2.0 port of the 89C52, and the remaining pins are connected accordingly. [align=center]Figure 2 CAN Bus and Interface Section[/align] To enhance the anti-interference capability of the CAN bus nodes, the TX0 and RX0 pins of the SJA1000 are not directly connected to the TXD and RXD pins of the 82C250. Instead, they are connected to the 82C250 via a high-speed optocoupler 6N137. This effectively achieves electrical isolation between the various CAN nodes on the bus. The interface between the 82C250 and the CAN bus also employs certain safety and anti-interference measures. A 5Ω resistor is connected in series between its CANH and CANL pins and the CAN bus, which can limit current and protect the 82C250 from overcurrent. Small capacitors can also be connected in parallel to the CANH and CANL pins to filter out high-frequency interference on the bus and prevent electromagnetic radiation. A slope resistor is connected to the Rs pin of the 82C250. The resistance value can be adjusted appropriately according to the bus communication speed, generally between 16 and 140kΩ. 2. Motor Control Circuit The motor control chip in this electric actuator uses the high-performance second-generation single-unit brushless DC motor controller MC33035 developed by ON Semiconductor. This controller includes a rotor position decoder for correct rectification timing, and a reference level for temperature compensation from sensors; it also features a programmable sawtooth wave oscillator, an error signal amplifier, a pulse modulator comparator, three open-collector top-drive outputs, and three high-current totem-pole bottom-drive outputs ideal for driving power MOSFETs. The principle module of the motor control circuit is shown in Figure 3. [align=center] Figure 3 Principle module diagram of the motor control circuit[/align] The functions of this motor control circuit include PWM open-loop speed control, enable control, and forward/reverse control. Motor speed control utilizes the function of the 4-10 bit decoder/driver 74LS145, with a signal provided by the microcontroller P89C58, which is then supplied to the pin speed voltage via a voltage divider circuit. The motor's enable and forward/reverse control signals are also controlled by a microcontroller, which sends level signals to the 89C2051, performs level conversion, and outputs the signals to the corresponding pins of the MC33035 to control the motor's operating state. 3. Valve Position Detection Circuit and LCD Display To achieve digital detection of the valve position, an Analog Devices (ADI) 16-bit Σ-Δ (charge-balanced) A/D converter, the AD7705, is used in the A/D conversion circuit. The AD7705 includes a front-end analog adjustment circuit consisting of a buffer and a programmable gain amplifier (PGA), a Σ-Δ modulator, and a programmable digital filter. It can directly amplify signals from different swing ranges of the sensor to near the full-scale voltage of the A/D converter before A/D conversion, achieving a non-linear 16-bit error-free data output with 0.003% accuracy. Its gain and data output update rate are programmable, and it also allows selection of an input analog buffer, self-calibration, and system calibration modes. By connecting an external high-precision conductive plastic potentiometer sensor, the voltage value obtained after voltage division from the reference voltage value reflects the valve position. The system uses a 128×64 dot matrix LCD manufactured by TRULY. All its controllers, scanning circuits, and display RAM are integrated on the back of the LCD screen, and it features LED backlighting. This module uses a single power supply and consists of four parts: a large-scale dot matrix display controller KS0107, an LCD screen array driver circuit KS0108, display memory, and the LCD screen itself. Intelligent Actuator Software Design The software design mainly includes three aspects: the CAN bus communication module, the electric actuator control, and the LCD display. The CAN bus communication section initializes the SJA1000 and returns it to normal operation after initialization, transmitting the actuator's operating status to the host computer via the bus. The operator can also issue control commands from the host computer to operate the actuator. The electric actuator control section mainly implements the valve position control function and can also control the valve closing speed by controlling the motor speed. The LCD display section mainly displays the valve position and actuator operating status, and can also display the control signals sent to the actuator by the host computer. The system program flowchart is shown in Figure 4. [align=center] Figure 4 System Program Flowchart[/align]