Brief analysis of Mitsubishi PLC high-speed counter and encoder application

1. Ordinary counter

Let's learn about ordinary counters first, taking C0 as an example. See the figure below. When X0 has a rising edge, C0 will count once. When C0 counts to 1000, it will stop counting and the C0 counter switch will act. When programming, it is recommended to use the RST instruction to reset C0, otherwise C0 will be in an overflow state after the count exceeds the limit.

2. High-speed counter

The difference between high-number counter and ordinary counter is:

1. High-frequency counting can identify pulses with higher frequencies

2. The high-speed counter can be called. The input terminal X will not be reflected in the program. For example, in the following table, after calling the counter C235, there will be no X0 input terminal in the program. Just connect the wires at the input terminal.

As shown in the following table, this is our single-phase high-speed counter

If I connect the photoelectric sensor to X0, then C235 is its dedicated counter. Every signal sensed by X0 will be counted by C235. We can use the following program to store the number of pulses sensed by X0 into D235. (Similarly, C236 records the number of pulses of X1; C237 records the number of pulses of X2...),

Before starting the counter, there are generally two programs to write:

To start the special register corresponding to the counter (for example, C235 corresponds to M8235, C236 corresponds to M8236, etc.), you need to define the direction first, whether it is count up or count down.

It is recommended to reset C235 before starting. (Someone asked if C235 can still count after overflow. The answer is that it can still count, but it is 32 bits, so it can only count to 32 bits)

Of course, the counting frequency of the counter has a limit. The receiving speed of the ordinary FX series X point is 50KHz, which means that it can receive and conduct 50,000 times in 1 second.

3. Use of encoder (taking incremental encoder as an example)

The incremental encoder can use the principle of photoelectric conversion to output three groups of square wave pulses A, B and Z; the phase difference between the two groups of pulses A and B is 90 degrees, which can determine the rotation direction of the motor, and the Z phase outputs one pulse per revolution for reference point positioning. The principle of this encoder is simple, mechanically averaged, and has a service life of tens of thousands of hours. It has strong anti-interference ability and high reliability. However, it cannot output the absolute position information of the shaft rotation.

According to the encoder principle, we connect the encoder to the PLC, as shown in the following figure (the figure below takes C251 as an example)

Two-phase high-speed counter (C251~C255) The A-phase and B-phase signals determine whether the counter counts up or down. When the A-phase is ON and the B-phase changes from OFF to ON, it counts up; when the A-phase is ON and the B-phase changes from ON to OFF, it counts down.

The following figure shows the basic instructions for counting using C251.

The first instruction: Before starting C251, perform a reset operation

The second instruction: X12 is used to start the C251 counter. X12 is not used for data collection (X1 and X2 are used for data collection and cannot appear in the program).

The third instruction: After C251 counts overflow, start Y2 operation

The fourth instruction: M8251 is the forward and reverse flag. For example, when the encoder runs in the forward direction, M8251 is connected, and vice versa.

Note: After the C251 counter overflows, it can continue counting.