Software and hardware design of automatic optical inspection system based on TMS320VC5509A DSP chip

TCF792A and CF792B are single-phase and three-phase general digital phase control trigger circuits. This series of devices has a single-phase synchronous input signal and a digital frequency division phase shift of 120°, which can be adapted to single-phase and three-phase trigger circuits. TCF792A is mainly suitable for frequency adjustment in a wide range of 10～500 Hz (an external 20 MHz crystal oscillator is required, and special order is required for more than 500 Hz); while TCF792B is mainly suitable for frequency adjustment in the 50 Hz power frequency range (no external crystal oscillator is required) . Both can choose rectangular wave or modulated wave output, and the pulse width is adjustable. In terms of Circuit function, this series of trigger circuits are fully compatible with TC787, TC788, TC790A, TC790B, TCA785 and KJ004, KJ04l, KJ04 and other single-phase and three-phase phase-shifted trigger circuits, and the price is low. Since the voltage is used to control the pulse width, no phase-shifting capacitor is needed, so it is convenient to construct an inverter power trigger circuit with variable amplitude and pulse width modulation (PWM).

In addition, because it adds the synchronization signal lag compensation function, its compensation range is 0°~60°, so strong interference signals can be removed through deep filtering, and the compensation angle can also be adjusted to accurately determine the zero point of the synchronization signal. The addition of this function not only enables it to be used as a precise zero-crossing switch, but also simplifies the wiring group of the rectifier transformer, which can be used for any phase rectifier circuit, etc. In addition, the trigger angle of this series of devices can be controlled by selecting the traditional sawtooth linear output or cosine function output. When the cosine function is used for output, the rectified output voltage has a linear relationship with the control voltage. This series of devices can be used for single-phase, three-phase half-control and full-control bridge thyristor rectification triggering, single and three-phase AC voltage regulation anti-parallel and bidirectional thyristor triggering, and can also be used for control circuits such as transistor-type variable-frequency and variable-voltage inverters. Since the angle used is the control unit, it can effectively prevent the out-of-control and subversion phenomena caused by frequency changes.

2. TCF792 principle

Figure 1 shows the schematic structure of TCF792. The device’s power supply voltage is 5 V, and its input and output ports are compatible with TTL levels, so it can be easily connected to other digital circuits. The synchronization signal of TCF792 adopts a square wave, which is input by pin 7. Its falling edge should be the zero-crossing synchronization point where the phase A voltage changes from negative to positive.

After the periodic signal is frequency multiplied by 180, a pulse signal with a period width of 2° is formed, and then it enters the digital operation control unit to form a control signal of 2°, 4°, 60°, 120°, 180°, etc. This produces modulated wave pulses, phase distribution and other signals. The phase shift angle control voltage, pulse width control voltage, and lag phase compensation voltage are switched by multiple switches and enter the 10-bit A/D conversion circuit. Its resolution is up to 0.05%, which can meet the needs of industrial control. The converted value is sent to the arithmetic control unit, and its arithmetic beat is generated by the oscillator. The oscillator is divided into an internal oscillator and an external oscillator circuit. TCF792A uses an external crystal oscillator circuit; TCF792B uses an internal oscillator circuit, which is fixed by the manufacturer during manufacturing. Full control double pulse or half control single pulse, rectangular wave or modulating wave, sawtooth waveform or cosine function waveform (referring to the relationship between phase shift angle and control voltage), positive phase sequence output or reverse phase sequence output by pins 15, 17 respectively , 18, 19 control. If pin 16 is grounded, all outputs will be blocked, which is generally used for overload or short circuit protection. The device contains both an automatic power-on reset circuit and a hardware watchdog circuit. When the circuit interference disrupts the running beat of the digital circuit, it plays a corrective role, so that the output circuit quickly resumes normal operation.

TCF792 adopts standard DIP20 and SOP20 packages. Its function and usage are as follows: Pin 1 is RST, used for reset, and grounded through a 1 kΩ resistor. Pin 2 is +A, pin 3 is one A, respectively used for pulse output (low level is effective, the maximum sink current is 20 mA, and the weak pull-up resistor is included). Pin 4 is XTL2, in which TCF792B is not connected to a crystal oscillator; TCF792A is connected to a 20 MHz crystal oscillator. Pin 5 is XTLl, used as the input end of the crystal oscillator. Pin 6 is +B, pulse output (low level is effective, the maximum sink current is 20 mA, and the pull-up resistor is included). Pin 7 is Tb, used for synchronization signal input (square wave input, the falling edge is valid, and the starting point of the positive half wave of Va is the reference for the falling edge). Pin 8 is a B, pin 9 is +C, and pin 11 is a C, respectively used for pulse output (low level is effective, the maximum sink current is 20 mA, and a weak pull-up resistor is included). Pin 10 is GND. Pin 12 is Vk, used as a control voltage potential input terminal, its input range is 0 ~ Vcc, the linear corresponding control phase shift angle is 2 ° ~ 178 °. Pin 13 is Mk, the pulse width voltage potential input terminal. When rectangular wave pulse is selected, the input range is 0~Vcc, and the linear corresponding pulse width phase angle is 2°～178°; when the modulation pulse is selected, the input range is 0~Vcc, the pulse width phase angle corresponding to linear is 0°~60°. Pin 14 is Xb, which is used as the phase compensation potential input terminal. The input range is 0～Vcc. The linear corresponding forward control pulse angle is 0°~60°. There is no compensation when this port is grounded. Pin 15 is Bk. When the port is floating or connected to a 10 kΩ pull-up resistor, it is a three-phase fully-controlled dual-pulse output, that is, when the phase is triggered, a pulse can be sent to the previous trigger port at the same time; when the port is grounded, it is a three-phase Half-controlled single pulse output. Pin 16 is Jz. When this port is floating or 10 kΩ is connected with a pull-up resistor, it is normal output; when this port is grounded, output is prohibited, all output pulse ports are high level, and the response time is less than 60°. Pin 17 is Tz. When this port is floating or connected to a 10 kΩ pull-up resistor, it is a rectangular wave output; when this port is grounded, it is a modulated pulse output. The modulating wave period is 4° and the duty cycle is 500%. Pin 18 is Cos. When the port is floating or connected to a 10 kΩ pull-up resistor, the output and input are in a sawtooth wave relationship; when the port is grounded, the cosine function is selected, and the relationship between the output phase shift angle α and the control voltage Vk is α=argcos[ (Vcc－2Vk)/Vcc]the rectified output voltage has a linear relationship with the control voltage. Pin 19 is Fx. When the port is left floating or connected to a 10 kΩ pull-up resistor, it is a positive sequence output, that is, +A, one C, +B, one A, +C, and one B; when the port is grounded, it is a reverse sequence output , Namely +A, one B, +C, one A, +B, one C. Pin 20 is Vcc, Vcc is 5.5 ~ 3. 8 V; The power consumption is 4 ~ 7 mA; The limit voltage is 5.5 V. It is worth noting that: ①The limit range of any port voltage is 0.3V~Vcc, compatible with TTL level; the industrial temperature range is -40℃～+85℃; high antistatic (ESD) protection; resistance to 4 kV fast pulse Interference test; strong anti-interference ability; three-phase asymmetry is less than 0.5°. ②The digital circuit should pay attention to the anti-interference design, and adopt the optocoupler isolation design as much as possible.

3. Three-phase full-wave half-controlled rectifier circuit

Figure 2 shows the principle wiring diagram of the power frequency three-phase thyristor full-wave half-controlled rectifier circuit. Among them, the grid voltage Uac is clipped by R1, R2, R3 and VD1, VD2, and then enters the negative input of the R4 input voltage comparator. When Uac is a positive half wave, the comparator outputs a zero level; when Uac is a negative half wave When the wave is high, the comparator outputs a high level to convert the sine wave output into a square wave output. The input range of Uac is 5 to 400 V. The photoelectric coupler U2 is used for circuit isolation and has an anti-interference effect. The capacitor C1 acts to filter out the burr near the zero-crossing point in the Uac signal, and its value can be determined according to the size of the burr in the actual waveform. Due to the phase lag caused by filtering, the output of RW2 can be adjusted to pin 14 for potential compensation. Pin 2 of TCF7928 is +A output signal, low level is effective. The photocoupler U3 is used for circuit isolation, and its meaning is the same as that of U2. The right side of U2 is the pulse amplifier circuit, whose parameters can be adjusted according to the thyristor model. The pulse width can be adjusted by RW1, the adjustment range is 0°～180°, and the actual value is about 200Us. The potential on the firing angle control voltage input terminal Vk (pin 12) is obtained by the voltage drop on R20, and its value depends on the output current of the photocoupler U4. The photoelectric coupler adopts the linear PC817 one A, and its output current is proportional to the input current. Transistor VT2 and R21 constitute a constant current source circuit, the size of which is controlled by the base potential. Uac in Figure 2 is rectified and filtered by resistor-capacitor voltage divider and added to both ends of RW3 to adjust RW3, which can change the base potential input to VT2, thereby changing the phase shift trigger angle and adjusting the thyristor rectified output voltage. The use of 5 V and 18 V power supplies to isolate the circuit is conducive to the anti-interference ability of the device. Its pin 15 is grounded, and half-controlled single pulse is selected; pin 18 is grounded, and the cosine function output is selected. The relationship between the output phase shift angle α and the control voltage Vk is:

formula

It can be seen from formula (3) that Ud and Vk have a linear function relationship. It is worth noting that: in the example, the voltage at both ends of RW3 does not use a regulated power supply, but the voltage after Uac is rectified and stepped down. It can be seen from the above formula that when Uac drops, Ud drops, Vk also drops proportionally at this time, and Vk drops will reduce the control angle, thereby compensating for the drop in rectified voltage caused by Uac drops. Assuming that the trigger angle is 90° during normal operation, when the grid voltage drops by 10%, it is calculated that Ud drops only by 1%. It can be seen that it has a certain constant voltage rectification function, and the circuit principle is simple.

Power frequency three-phase thyristor full-wave half-controlled rectifier circuit principle wiring diagram

4. Conclusion

It can be seen from the above example that the TCF792 digital phase control circuit has the advantages of high precision, ease of use, reliability, no need for debugging, few external components and excellent performance. At the same time, due to the single-phase synchronization signal, its application is as convenient as the single-phase trigger circuit, which facilitates the design of the trigger board. So it will become a new generation of digital phase shifting circuit.