Proportional Electromagnet Control Technology Based on 51 Single Chip Microcomputer

introduction

As an actuator, the proportional solenoid is one of the key products of mechatronics and is widely used in various automatic control systems. The proportional solenoid has a large thrust, simple structure, convenient maintenance, and low cost. It is a widely used electro-mechanical converter [1]. The characteristics and working reliability of the proportional solenoid have a very important impact on the entire control system and is one of the key components that determine the quality of the control system. As an electro-mechanical conversion element, the proportional solenoid converts the current signal input by the proportional control amplifier into a displacement or force signal output.

Proportional electromagnets are suitable for control circuits of proportional control amplifiers with a DC voltage of 24 V, as power elements that continuously and proportionally control the motion, speed and direction of the system's actuators. The thrust of a proportional electromagnet within its rated travel range is proportional to the current flowing into its coil, and it can be used as a linear power element in other devices that require automatic force control, such as automatic throttle control. When the electromagnet and the single-chip microcomputer form an automatic control system together, the design of the interface circuit between the single-chip microcomputer and the proportional electromagnet is a key because the working voltage and current of the electromagnet are relatively high.

With the development of microelectronics and computer technology, the demand for proportional solenoids is increasing day by day, and they are used in various control fields. The following introduces the control technology of proportional solenoids.

1 Basic principles and characteristics of PWM drive

PWM (Pulse Width Modulation) technology is a control technology that uses the on and off of semiconductor switching devices to convert DC voltage into a voltage pulse train, and achieves voltage and frequency conversion by controlling the voltage pulse width and the pulse train period [2]. That is, a series of equal-amplitude rectangular pulses with different pulse widths are used to approximate a required current or voltage signal.

PWM drive circuit is a drive form widely used in high-precision control systems. This circuit can achieve a wide range of speed and position control, and has incomparable advantages over conventional drive methods. The advantages of PWM drive circuit are simple, fast, linear, and efficient, making it widely used in many fields of measurement, communication, power control, and conversion. This design uses the characteristics of PWM drive circuit, which requires few high-power controllable devices, wide speed regulation range, good speed, high efficiency, and low power consumption. The PWM signal directly output by the C8051F005 microcontroller passes through the drive circuit, and then cooperates with a suitable control algorithm (PID algorithm or fuzzy control algorithm, etc.) to control the proportional electromagnet, which can achieve precise control of the clutch, and has a good reference value for the study of electronic clutch control systems.

2 Proportional solenoid and single chip microcomputer interface circuit

2.1 MCU Overview

The single-chip microcomputer used in this control system is the C8051F005 single-chip microcomputer launched by Silabs in the United States [3]. It is a fully integrated mixed-signal system-level MCU chip with a true 12-bit multi-channel ADC, a programmable gain amplifier, two 12-bit DACs, two voltage comparators, a voltage reference, a microcontroller core with 32 KB Flash memory and compatible with 8051, as well as hardware-implemented (not simulated by bit operations in user software) I2C/SMBus, UART, SPI serial interfaces and a programmable counter/timer array (PCA) with 5 capture/compare modules, as well as four general-purpose 16-bit timers and 4-byte wide general-purpose digital I/O ports. The C8051F005 has 2,304 bytes of RAM and an execution speed of up to 25 MIPS; it has an on-chip VDD monitor, WDT and clock oscillator, and is a truly independent system-on-chip that can effectively manage analog and digital peripherals. The Flash memory also has in-system reprogramming capability for non-volatile data storage and allows field updates of the 8051 firmware. The MCU can shut down any or all peripherals to reduce power consumption.

The C8051F005 microcontroller can operate with a voltage of 2.7 to 3.6 V in the industrial temperature range (-45 to +85 °C). The port I/O, RST and JTAG pins all allow a 5 V input signal voltage.

2.2 PWM signal output and proportional solenoid drive circuit

The C8051F005 MCU has an on-chip programmable counter/timer array PCA. The PCA includes a dedicated 16-bit counter/timer time base and five programmable capture/compare modules. The clock of the time base can be one of the following four clock sources: system clock/12, system clock/4, timer 0 overflow or external clock input (ECI).

Each capture/compare module has its own I/O line (CEXn line). When it is allowed to work, the CEXn line is connected to a pin of the port through the function selection switch. Each capture/compare module has 4 working modes: edge-triggered capture, software timer, high-speed output, and pulse width modulation (PWM). The I/O and external clock input of the PCA capture/compare module can be connected to the port I/O pin of the MCU through the digital cross switch.

The procedure for outputting an 8-bit PWM signal (with variable duty cycle) from PCA is as follows:

$include (c8051F005.inc)
ORG 0000H
LJMP MAIN
ORG 0073H; Timer 3 interrupt entry
LJMP INTERT33
MAIN:
MOV WDTCN, #0DEH; Disable watchdog timer
MOV WDTCN, #0ADH
MOV OSCICN, #84H; Select internal oscillator as 12 MHz
MOV XBR0, #08H; Select CEX0 pin to connect to P0.0
MOV XBR2, #40H; Enable function selection switch
ORL PRT0CF, #00000001B; Select P0.0 as push-pull mode
MOV TMR3RLL, #0B0H; Assign initial value to low byte of timer 3
MOV TMR3RLH, #0A0H; Assign initial value to high byte of timer 3
MOV PCA0CPH0, #0FFH; Assign initial value to high byte of PCA capture module
MOV PCA0CPL0, #0FFH; assign initial value to low byte of PCA capture module
MOV PCA0MD, #08H; select system clock as clock source of PCA, disable CF interrupt
MOV PCA0CPM0, #42H; select 8-bit pulse width modulation output mode, and start
MOV PCA0CN, #40H; enable PCA to work
MOV IE, #080H; CPU opens interrupt
MOV EIE2, #1; T3 opens interrupt
MOV TMR3CN, #00000110B; start T3, T3 uses system clock source
SJMP ＄
INTERT33:
MOV A, TMR3CN; clear T3 flag TF3
ANL A, #7FH
MOV TMR3CN, A
DEC PCA0CPH0; duty cycle change
RETI

According to the system design requirements, PWM signals with different duty cycles can be obtained by modifying PCA0CPH0.

This control system uses the PCA of the C8051F005 microcontroller to achieve 8-bit resolution PWM output by software. The PWM signal is connected to the port I/O pin output of the MCU through the CEXn line through the function selection switch. The PWM output signal can drive the proportional electromagnet through the drive circuit. The module's capture/compare registers PCA0CPLn and PCA0CPHn store the high-level time value of the PWM output signal duty cycle. If the duty cycle needs to be changed, the value of PCA0CPHn can be changed during operation. The displacement of the proportional electromagnet push rod is proportional to the value of PCA0CPHn.

In the drive circuit, the voltage signal of the PWM output needs to be converted into the control current signal of the proportional electromagnet, and a good proportional characteristic relationship must be ensured. Using the transfer characteristic of the field effect transistor [4], when the voltage VDS between the drain and source of the field effect transistor remains unchanged, the relationship between the drain current ID and the voltage VGS between the gate and source is called the "transfer characteristic".

This control circuit uses a high-power field effect transistor IRL3803, whose current output is sufficient to drive the action of the proportional electromagnet. The drain current ID of IRL3803 and the gate-source voltage VGS have a good linear relationship. The gate is connected to the P0.0 port of the C8051F005 microcontroller (the PWM signal output pin selected by software programming PCA), and the drain of IRL3803 is connected to the proportional electromagnet. In this circuit, the proportional electromagnet is GP80, with a rated suction force of 120 N, a stroke of 8 mm, and a rated voltage of 24 V.

When controlling, the magnitude of the control current is achieved by changing the "duty cycle" of the electrical signal input to the proportional solenoid switch. The larger the duty cycle, the greater the control current passing through the solenoid coil, and the greater the displacement of the control output.

The proportional solenoid drive circuit is shown in Figure 1. In the drive circuit, R1 is a current limiting resistor that turns on the IRL3803 tube; D1 is a guide diode that provides the correct voltage polarity to the IRL3803 tube; and diode D2 plays a protective role to prevent damage to the proportional solenoid when overvoltage occurs. The proportional solenoid is directly powered by a 24 V voltage.

Conclusion

The C8051F005 single-chip microcomputer has high integration and few peripheral circuits. Its high-speed execution of instructions can accurately control the proportional electromagnet. The C8051F005 core is compatible with the ordinary 51 series, and the instructions are simple and easy to learn, which can shorten the system development cycle. Proportional electromagnets have been widely used as electro-mechanical conversion devices. The proportional electromagnet control system based on the C8051F005 single-chip microcomputer can meet the requirements of high precision and good stability. The hardware circuit is simple and the operation is reliable. In the application system, the fixed duty cycle or variable duty cycle PWM signal is directly output by the I/O port of the single-chip microcomputer according to the needs. With a certain control algorithm, the software programming is clear and easy to implement, and it has great promotion value.

References

[1] Lu Yongxiang. Hydraulic and Pneumatic Handbook [M]. Beijing: Machinery Industry Press, 2002.
[2] Jin Ying, Pan Zaiping. Research on Pulse Width Modulation Control Method [J]. Bulletin of Science and Technology, 2005(1):3033.
[3] Zhang Yingxin, et al. Principle and Application of C8051F Series SOC Microcontroller [M]. Beijing: Beijing Institute of Technology Press, 2001.
[4] Yang Suxing. A Brief Tutorial on the Basics of Analog Electronic Technology [M]. 2nd Edition. Beijing: Higher Education Press, 1999.
[5] Wang Shangyong, Yang Qing. Electronic Control Technology of Diesel Engine [M]. Beijing: Machinery Industry Press, 2005.

Ren Guizhou (graduate student), whose main research direction is automotive electronic technology;
Qu Jinyu (associate professor), whose main research direction is automotive electrical and electronic technology.