Development and Application of Fully Digital Motor Actuator-EEWORLD

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introduction

The all-digital electric actuator designed in this paper is a further improvement on the motor drive circuit of the 9610R series all-electronic electric actuator of Xiangyi Electronic and Electrical Equipment Factory. We replace the original analog control with digital control based on 80C196 single-chip microcomputer to improve the control accuracy and operation reliability. At the same time, for the convenience of debugging, the infrared remote control function and the communication function based on CAN bus are added to meet the needs of modern industrial control.

1. Features of the original fully electronic electric actuator

Development and Application of Fully Digital Motor Actuator

The original 9610R series of fully electronic electric actuators uses a 220V AC unidirectional power supply as the driving power supply, a unidirectional AC motor as the driving motor, and a high-performance conductive plastic potentiometer as the position feedback.

The principle of the servo amplifier is shown in Figure 1.

①When UY=0,

K_=Uo/Ux=-[(R4+R5)/R5]×(R6/R1)

②When Ux=0,

K+=Uo/UY=[R3/(R2+R3)]×[(R4+R5)/R5]×(1+R6/R1)

According to the principle of linear superposition, Uo=K+UY+K_UX.

As can be seen from the above, since it is difficult to achieve perfect matching of resistors, the original 9610R electric actuator has the problem of asymmetric forward and reverse rotation of the motor. The motor drive circuit is shown in Figure 2.

Development and Application of Fully Digital Motor Actuator

In Figure 2, Uo is the voltage signal from the servo amplifier. When Uo>0.7V, the motor rotates forward; when Uo<-0.7V, the motor rotates reversely. C1 is the capacitor that controls the motor braking.

The redesigned all-digital electric actuator has improved the motor drive circuit, using the duration of the ±12V switch signal to control the forward and reverse rotation of the motor, and realizing the brake and reverse cutoff functions of the electric actuator. The new motor drive circuit is shown in Figure 3.

In Figure 3, Ukp and Ukn are two high-speed output pins of 80C196, respectively, and T2-1/T2-2, T3-1/T3-2, T4-1/T4-2, T5-1/T5-2, T6-1/T6-2, and T7-1/T7-2 are six opto-isolators. When Uk is at a high level of +5V, T2-1/T2-2 is turned on, and T*-1/T6-2 is turned on to make the motor rotate forward; when Uk changes from a high level to a low level, T4-1/T4-2 is turned on instantly, making T7-1/T7-2 turn on instantly, and the motor reverses instantly, and the motor stops after the capacitor discharges; similarly, when Uk is at a low level of 0V, the motor reverses. In this way, the forward and reverse control of the motor is realized.

Development and Application of Fully Digital Motor Actuator

The system output and the driving circuit are completely photoelectrically isolated, which can improve the system's anti-interference ability and reliability.

Development and Application of Fully Digital Motor Actuator

2 Control system structure

The control system structure of the fully digital electric actuator with 80C196KC single chip microcomputer as the core is shown in Figure 4. In Figure 4, in addition to the 80C196KC single chip microcomputer, X25043 is also selected to realize the power-off protection function, and MAX7219 is used to drive the LED digital tube to display the given value and feedback value of the valve position as well as the status and control mode of the valve position; at the same time, the threshold feedback signal is directly converted into a 4-20mA signal by an improved 4-20mA constant current circuit and sent to the indoor analog secondary meter for display to ensure the compatibility of its analog and digital control. The valve position feedback and valve position analog given signal are converted into a 10-bit digital signal by using the A/D conversion port inside the 80C196KC, and the valve position fault (blocking, over-limit) is judged by software, and the fault processing (alarm or shutdown) is performed. The single chip microcomputer system is isolated from the motor drive circuit by MOC3061 photoelectric isolation at the control output end and the fault processing end to achieve the purpose of anti-interference.

Development and Application of Fully Digital Motor Actuator

The 1838 infrared remote control receiver and decoder integrated chip is used to receive infrared remote control signals from the remote control. The CAN controller uses Philips' SJA1000 integrated chip, and the CAN bus driver uses the 82C250 integrated chip. The 6N137 fast optical isolation is used between the SJA1000 and the CAN bus driver 82C250 for photoelectric isolation, and the single-chip microcomputer interface realizes the communication function between the single-chip microcomputer and the host computer.

The main hardware circuits of each part are introduced as follows.

(1) Improved 4-20mA constant current circuit

The entire constant current circuit consists of an integrated 4-channel operational amplifier LM324, 6 precision resistors, 1 adjustable resistor, 1 ceramic capacitor and 1 diode. The circuit structure is very simple, as shown in Figure 5. In Figure 5, R1=R2=R3=R4=R5=100kΩ, R6=200Ω, and R7 is an adjustable resistor of 0~100Ω.

From the circuit in Figure 5, we can see that when the four resistors R2, R3, R4, and R5 are well matched, U1-U2=U1, and by adjusting R7, R6+R7=250Ω, so that Io=U1/250Ω, the purpose of converting 1-5V voltage into 4-20mA is achieved, and no matter how the load at the output end changes, this relationship will not change, achieving the purpose of constant current. In order to make the load that the constant current circuit can carry as large as possible, the power supply of the integrated operational amplifier LM324 is best to use a +18V power supply.

(2) Infrared remote control receiving circuit

As an electric actuator, when used in industrial process control, it is often difficult to debug at the installation location. Using infrared remote control can be a good solution, which can save some work required for conventional debugging, such as opening the control box cover to debug circuit changes, etc. The infrared remote control receiving chip uses the infrared remote control receiving and decoding integrated chip 1838. The circuit is shown in Figure 6.

In Figure 6, the resistor and capacitor form a decoupling circuit to suppress power supply interference; no other external components are required, and the center frequency is 38kHz. However, since the gain of the 1838 integrated chip is high and cannot be adjusted, and there is no shielding, it is particularly susceptible to external interference, so shielding measures must be taken. The best way is to use metal materials to make a shielding box, put the 1838 in it, and leave only the infrared interface outside.

We choose a universal infrared remote controller as the debugging device for the electric actuator. The 80C196KC single chip microcomputer first detects the command code of each button of the remote controller, and then assigns the required debugging commands to them respectively, which can greatly shorten the development cycle.

Development and Application of Fully Digital Motor Actuator

(3) Communication between upper and lower computers

CAN (Cantrol Area Network) is the abbreviation of control area network. It was first introduced by BOSCH in Germany and is used for data communication between measurement and execution components inside automobiles. Its bus specification has been formulated as an international standard by the ISO International Standards Organization and is widely used in the field of discrete control. Its signal transmission medium is twisted pair. The communication rate is as high as 1Mbps/40m, the direct transmission distance can reach up to 10km/5kbps, and the maximum number of connected devices can reach 110.

CAN signal transmission adopts short frame structure, and the number of effective bytes in each frame is 8, so the transmission time is short and the probability of interference is low. When a node has a serious error, it has the function of automatic shutdown to cut off the connection between the node and the bus, so that other nodes on the bus and their communications are not affected, and it has strong anti-interference ability. The CAN bus communication interface circuit is shown in Figure 7.

The AD15 port of 80C196KC is used as the chip select signal of SJA1000, so the address occupied by CAN controller SJA1000 is: 8000H ~ 80FFH. The purpose of using CAN bus transceiver PCA82C250 is to further improve the driving ability of CAN bus. Its working mode is provided by RS control pin, which depends on the slope resistor (resistance value of 200kΩ adjustable resistor).

Development and Application of Fully Digital Motor Actuator

The host computer can monitor the instruments and control equipment with CAN communication interface in the industrial field through a HK-CAN30B PCI bus non-intelligent isolation communication board produced by Huakong Company.

(4) Power-off protection and anti-interference measures

The system uses Maxim's X25043 as the component for current protection. X25043 has three common functions: watchdog timer, voltage monitoring and E2PROM, combined in a single package. X25043 is very suitable for this system that requires the board space to be as small as possible. The circuit is shown in Figure 8.

The watchdog timer of X25043 provides an independent protection system for the microcontroller 80C196. The selectable timeout periods are: 1.4s, 600ms, 200ms, and can also be disabled. When the system fails, after exceeding the selected timeout period, the X25043 watchdog will respond with a RESET signal to reset the system. The X25043 low VCC detection circuit can be used to protect the system from low voltage conditions. When VCC drops to the minimum VCC detection level, RESET becomes low and resets the system until VCC rises to the minimum VCC detection level for 200ms. In addition, X25043 also has a 512×8-bit serial E2PROM, which makes this system unnecessary to expand the data storage RAM.

The anti-interference measures of the system include hardware measures and software measures. In terms of hardware: ① Optical isolation is used in the input and output channels for signal transmission, the optoelectronic isolator MOC3061 is used in the motor drive circuit, and the fast optical isolation 6N137 is used in the upper and lower computer communication circuits; ② A 0.01μF ceramic capacitor is placed in each integrated circuit chip to eliminate most high-frequency interference; ③ The analog ground is separated from the digital ground; ④ In terms of CPU anti-interference measures, in addition to the power-off protection circuit, manual reset and automatic power-on reset circuits are also configured. In terms of software: ① Instruction redundancy, two NOP instructions are inserted after some double-byte and three-byte instructions to ensure that the runaway program is quickly included in the correct control track; ② Use software traps to forcibly lead the captured program to the program that handles program errors; ③ Enable the internal watchdog timer of 80C196KC; ④ Use software digital filtering for A/D input signals.

Development and Application of Fully Digital Motor Actuator

3 System software design

The system program flowchart is shown in Figure 9. First, the program is initialized, including the initialization of hardware and variables. Then, the program determines the global variable RUN. If RUN=0, it means that the program has terminated, then it jumps to the end of the program to reset the watchdog, and then jumps to the front of the program to determine the RUN flag and execute the loop; if RUN≠0, the program executes the main loop and resets the watchdog. In this way, the execution of the program is controlled by setting the RUN variable.

Only basic operations, such as data input and output, are processed in the interrupt program; some complex data processing, such as software filtering of input channels, are processed in the main loop. In the main program, a global variable is assigned to each interrupt as an interrupt flag. When an interrupt occurs, this flag is set to 1. In the main loop, the program judges each flag bit in turn to decide whether to execute the corresponding subroutine, that is, a procedure or function. After processing the corresponding interrupt service in the main program, the corresponding interrupt flag must be cleared.

The functions of the main program include: determining the valve position and valve position status, LED display of the valve position and valve position status, valve position control output, judging whether the valve is blocked to protect the motor from overheating, infrared remote control command decoding and remote control command control output. In the valve position control output, the valve position is controlled by controlling the time of the motor's forward and reverse rotation, and the A/D sampling cycle is controlled to be very short, such as 10ms, or even shorter. The motor's action is controlled by this cycle, and the motor is kept inactive within the required valve position accuracy range of 0.5%, so as to ensure the accuracy of the valve position control and avoid the valve position from vibrating back and forth.

4 Conclusion

By improving the motor drive circuit and 4-20mA constant current circuit of the 9610R series all-electronic electric actuator, the operating accuracy and reliability of the actuator during analog control have been further improved. The control accuracy has been improved. Its intelligence makes remote maintenance possible; the development of its remote control system makes the debugging of the electric actuator more convenient.

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