Application skills/Development of microcomputer digital trigger

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Application skills/Development of microcomputer digital trigger [Copy link]

	Abstract : The trigger circuit is an important part of the thyristor rectifier. Using microcomputer technology and MCS-8096 single-chip microcomputer as the control core, a new type of microcomputer digital trigger is designed, and its hardware and software detailed implementation methods are given.
	Keywords : thyristor digital trigger single chip microcomputer microcomputer technology
	Thyristor rectifier devices have been widely used in industry. The trigger control of thyristors is the key link in the application. The control accuracy and precision of the trigger circuit will directly determine the working performance of the main circuit. The trigger circuit of thyristors can be roughly divided into two categories: analog and digital. The analog circuit is composed of discrete components, which is large in size and has low control accuracy. It is difficult to meet the standard and is rarely used now; the thyristor trigger signal is essentially a discrete quantity that can be realized by a digital signal. At present, a large number of digital trigger products have been launched. These digital triggers are generally composed of zero-crossing detectors, counters, pulse distributors and other parts. With the development of microelectronics technology, the widespread application of microcomputers is characterized by the design of digital triggers with single-chip microcomputers as the control core, which can greatly simplify the composition of hardware circuits and improve the control accuracy of triggers. The resolution of the trigger angle α can reach 0.1°~0.01°, or even higher. In addition, due to the programmability of the software, the adjustment range of the microcomputer digital trigger is quite flexible and can meet many needs.
	The MCS-96 series microcontrollers have built-in A/D conversion channels, high-speed input ports HIS and high-speed output ports HSO, which are extremely convenient for pulse detection and generation. The six parallel high-speed outputs HSO can trigger an event at the time specified by the program. After the triggering moment of HSO is determined in the content addressable storage area CAM, a trigger pulse can be generated on the HSO port when the specified time arrives, and the rising and falling edges of the trigger pulse can be set at the same time. Using this feature of HSO, it is very convenient to form a trigger circuit for a thyristor rectifier.
	This article focuses on the three-phase fully controlled bridge rectifier circuit and designs the hardware and software of the trigger.
	1 Trigger Hardware Design
	The hardware circuit of the microcomputer digital trigger is mainly based on MCS-8096 as the control core, including input signal preprocessing circuit, synchronous pulse generation circuit, pulse formation and output circuit, memory expansion and auxiliary circuit, etc. The hardware block diagram is shown in Figure 1.
	1.1 Synchronous pulse generation circuit
	In various thyristor rectifier circuits, the trigger pulse of each thyristor must have a relatively fixed phase relationship with the AC main power supply voltage applied to the thyristor (that is, the trigger moment of each tube differs from a certain fixed phase point of the main power supply voltage by a control angle α). The pulse corresponding to this trigger moment is called a synchronization pulse, and the circuit that completes this task is a synchronization pulse generation circuit. Digital triggers are divided into absolute triggering and relative triggering according to different triggering methods of synchronization pulses. The so-called absolute triggering method means that the formation moment of each trigger pulse is determined by the synchronization reference, which requires six synchronization reference AC voltages in a three-phase bridge circuit; while the relative triggering method only requires one synchronization reference. After the first pulse is generated by the synchronization reference, the first trigger pulse is used as the reference for the next trigger pulse. In a three-phase bridge circuit, the difference between two adjacent trigger pulses is 60° electrical angle, but because the grid frequency fluctuates around 50Hz, it is necessary to track and measure the grid cycle.
	The synchronous pulse voltage can be phase voltage Ua or line voltage Uac. When the line voltage Uac is used as the synchronous voltage, the synchronous reference in the three-phase bridge circuit is exactly the reference of α=0°; and when the phase voltage Ua is used as the synchronous voltage, there is a phase difference of -30°. In the synchronous pulse circuit of this device, the line voltage Uac is used as the synchronous voltage. The circuit is shown in Figure 2. After the line voltage Uac is stepped down, it is added to the voltage comparator composed of LM339, and the high and low level square waves are output. After the differential circuit composed of the optoelectronic isolator TIL117 and the capacitor and resistor, a differential signal is formed. This differential signal is the synchronous phase pulse, and its period is the period of the power grid.
	1.2 Trigger pulse isolation, drive and output circuit
	In order to prevent interference and meet the gate of the thyristor's requirements for trigger pulse power, the trigger pulse sent by the microcontroller must be isolated and driven before it can be added to the gate of the thyristor. This circuit consists of a buffer, a photoelectric isolator, a transformer and other devices, as shown in Figure 3.
	When there is no pulse signal at the high-speed output port HSO of the single-chip microcomputer 8096, the photoelectric isolator TIL117 is cut off, the transistor BG is cut off, and the transformer has no pulse output; when there is a pulse signal at HSO, the photoelectric isolator TIL117 is turned on, thereby turning on the corresponding transistor BG, so that the trigger pulse is output through the pulse transformer T, prompting the thyristor to be triggered and turned on.
	1.3 Input signal preprocessing circuit
	The main function of the input signal preprocessing circuit is to generate a pulse phase shift control signal. Since the 8096 has four 10-bit A/D conversion channels, there is no need to connect an external A/D conversion circuit. However, the 8096 microcontroller A/D converter has certain requirements for the external control voltage, and it only allows the standard voltage of 0 to +5V to be converted. The actual input not only has different amplitudes but also different polarities, so an input signal preprocessing circuit is set. Its main task is to determine the polarity of the input signal, extract the amplitude of the input signal, and convert the external voltage signal into a standard voltage signal of 0 to 5V.
	In addition, in the microcomputer digital trigger circuit, since the 8096 microcontroller has a 64kB addressing space, except for 256 internal special memories, the rest of the space needs to be expanded, so the hardware circuit also includes a memory expansion circuit for storing system control programs, real-time sampling data, various intermediate results, etc., as well as a reset circuit, an analog reference high-precision 5V power supply, a 12MHz crystal oscillator and a microcontroller accessory circuit for display.
	2 Trigger Software Design
	The software design of the trigger is mainly divided into several parts: main program, pulse synchronization and phase shift, and pulse formation and output, which are explained as follows.
	2.1 Main Program
	The main program is the system program, which mainly completes functions such as system initialization, display of α angle and waiting for interruption. The main program block diagram is shown in Figure 4.
	2.2 Pulse synchronization and phase shift
	In this device, when the rising edge of the synchronization pulse signal occurs, the HIS.0 interrupt of 8096 responds immediately, obtains and calculates the α value to achieve pulse synchronization and phase shift.
	The power grid cycle is calculated using the time difference between the rising edges of adjacent synchronization signals. Assume that the count value of timer T1 is t1 when the previous synchronization pulse reference arrives, and the count value of timer T1 is t2 when the current synchronization reference arrives, then the power wind cycle T=t2-t1. The time corresponding to a unit electrical angle is T/360°, and the time corresponding to the electrical angle is T1U=αT/360°. T1U is the time when the first pulse is released after the rising edge of the synchronization pulse occurs. The change in the time when the first pulse is generated means that the pulse is phase shifted. The subroutine flowchart of pulse synchronization and phase shift is shown in Figure 5.
	2.3 Pulse Formation and Output
	By utilizing the functions of 8096 hardware and software timers, high-speed output channel HSO and high-speed input channel HIS, and using software timing interrupts, the output of six trigger pulses is realized at the six HSO ports.
	When the positive edge of the synchronization signal occurs, it immediately causes an external interrupt of HIS.0. The pulse synchronization and phase shift subroutine calculates the timing value T1U corresponding to the rising edge of the first pulse in each cycle. The timing value T1D of the falling edge of the pulse is determined by its pulse width. If the electrical angle corresponding to the pulse width is 15°, then T1D = (α+15°) T/360. The values of T1U and T1D are placed in the content addressable storage area CAM of HSO. HSO compares with timer T1 and outputs a high level at T1U and a low level at T1D, thus forming trigger pulse No. 1.
	When the rising edge of pulse 1 arrives, HSO generates an interrupt. According to the current value, plus the phase difference Δα between two adjacent pulses (Δα=60° in the three-phase bridge circuit), the timing value of pulse 2 is: rising edge setting value T2U=（α+60°）T/360°, falling edge timing value T2D=（α+75°）T/75°）/360°. Similarly, when the rising edges of pulses 2 to 5 are generated, HSO interrupts are also caused, generating trigger pulses 3 to 6.
	The microcomputer digital trigger designed with HIS, HSO and 10-bit A/D conversion channel for pulse formation and output can realize all the functions of the digital trigger circuit without adding many other peripheral circuits, making the hardware circuit design of the trigger simple and easy to implement. In the design of this device, changing the size of the analog input Ux can easily realize the α angle phase shift, and there is a large phase shift range. The time interval between the two interruptions of HIS.0 can dynamically track the changes in the power grid cycle. In addition, only a slight modification of the software program is needed to make this microcomputer digital trigger not only for rectification, but also for inversion. Therefore, the design of this device is reasonable and has high control accuracy. In addition, this system can be expanded, the single-chip microcomputer is connected to the host computer, and the host computer issues the adjustment command of the α angle, which can make the entire control system more perfect.