The control of traffic lights must be safe and reliable, and the control procedures should be easy to modify according to road traffic flow. The traffic light control program designed with the custom clock pulse segment formed by the PLC timer as the basic unit of the program has a simple structure and clear ideas. Without changing the program structure, the new traffic light can be quickly obtained through the adjustment of the timer setting Control laws. The following uses Mitsubishi FX2N PLC as the core control element to clarify the control design ideas and control scheme of traffic lights.

1 Work tasks and timing analysis

1.1 Work tasks

As shown in Figure 1, two traffic lights are installed in the east-west direction and north-south direction respectively. The control requirements are as follows: when the start button SB1 is pressed, the signal light starts to work, and the green light in the east-west direction and the red light in the north-south direction are on at the same time. The regularity of the lights on and off between the lights (see Figure 2): Between 0 and 30 s, the north and south red lights are always on, the east and west green lights are on for 25 s, and the flashing is 3 s (1 time per second), and the east and west yellow lights are on for 2 s; 30 ~ Between 55 s, the east and west red lights are always on, the north and south green lights are on for 20 s, flashing for 3 s (1 time per second), and the north and south yellow lights are on for 2 s; thereafter, they return to the law of extinction between 0 and 55 s, and have been Continue until the stop button SB2 is pressed to end the operation.

1.2 Timing analysis

Analysis of the timing charts of the various signal lights shown in Figure 2 can be summarized as follows:

(1) The east-west green light starts at time 0 s, turns on for 25 s, flashes for 3 s, and turns off for 27 s; then turns on for 25 s, flashes for 3 s, and turns off for 27 s; and so on.

(2) The east-west yellow light starts at the 28th s. Turns on for 2 s and turns off for 53 s; turns on for 2 s and turns off for 53 s again;

(3) The east-west red light starts at time 0 s, extinguishes for 30 s and turns on for 25 s; then turns off for 30 s and turns on for 25 s; and so on.

(4) From the 30th s, the green light of the north and south starts at 20 s, flashes for 3 s, and turns off for 32 s; then lights up for 20 s, flashes for 3 s, and turns off for 32 s; and so on.

(5) From the 0th s, the yellow lights in the north and south begin to turn off for 53 s and turn on for 2 s; then turn off for 53 s and turn on for 2 s; and so on.

(6) The red light from north to south starts at the 0 s moment, turns on for 30 s and turns off for 25 s; then turns on for 30 s and turns off for 25 s; and so on.

2 PLC custom clock pulse

2.1 PLC clock pulse

The PLC clock pulse refers to a square wave signal that exhibits periodic changes through the output relay. Periodicity is the basic characteristic of the clock pulse signal. The PLC pulse signal has two basic parameters, period D and duty cycle D, and two secondary parameters, ON period t1 and OFF period t2. The sum of the ON period t1 and the OFF period t2 is the period of the clock pulse, that is, T = t1 + t2. The duty ratio refers to the time ratio of the ON period t1 in a period T, that is, the duty ratio D = t1 / T = t1 / (t1 + t2).

The special auxiliary relays M8011, M8012, M8013, M8014 of Mitsubishi FX2N PLC provide four kinds of clock pulses with a period of 0.01 s, 0.1 s, 1.0 s, and 60 s. The duty ratio is 50%. The output is all ON and then OFF . The inherent clock pulses of these PLCs obviously cannot meet the timing control problems of the traffic traffic light timing control in this article. The timing control problems that can be arbitrarily set in the ON period t1 and OFF period t2 are required for the output ON / OFF sequence. This requires development and design of this The custom clock pulse program required by the timing control.

2.2 The concept and basic characteristics of PLC self-defined clock pulse

As shown in FIG. 3, a custom clock pulse refers to a clock pulse that can define a period T, a duty ratio D (that is, defines an ON period t1, an OFF period t2), and controls the sequence of outputting ON / OFF signals.

The custom clock pulse program segment should have two basic characteristics: First, the fixed structure of the program, which can be flexibly referenced as a subprogram. The second is the flexibility of program parameter modification. According to the control object without changing the program structure, the cycle T, duty cycle D, and ON / OFF output can be set successively by changing the set value.

2.3 PLC custom clock pulse program design

The custom clock program design is shown in Figure 4. Figure 4 (a) shows that the program outputs the ON signal first, and Figure 4 (b) shows that the program outputs the OFF signal first. Comparing Figure 4 (a) and Figure 4 (b), we can find that the state of the auxiliary contact of the timer T0 connected in series with the output relay Y000 will determine:

(1) Is the output ON first or OFF first?

(2) Timers T0 and T1 control the division of labor between the ON period t1 and the OFF period t2. As shown in FIG. 4 (a), when the T0 movable contact is connected in series with the output relay Y000, the ON signal is output first and then the OFF signal is output, T0 controls the ON period t1, and T1 controls the OFF period t2. In the diagram shown in Fig. 4 (b), when the T0 moving contact is connected in series with the output relay Y000, the OFF signal is output first, and then the ON signal is output. T0 controls the OFF period t2, and T1 controls the ON period t1.

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