The DZRN homing instruction is used to return a servo motor to its reference point. The command "DZRN K20000 K3000 X12 Y0" indicates that the motor starts moving at a pulse frequency of 20,000 Hz. When the rising edge of X12 is detected, the frequency drops to 3,000 Hz. Once the falling edge of X12 is sensed, the pulse output stops. Y0 is the output pin responsible for sending the pulses.
The DDRVI instruction is used for relative positioning. For example, "DDRVI K10000 K1000 Y0 Y2" means that when X2 is activated, the motor moves 10,000 pulses from the current position at a frequency of 1 kHz. The direction of movement is controlled by Y2, which is set to a positive state.
Servo motors are commonly operated in position control mode. The wiring diagram provided in the manual might seem complex, but it's manageable once you understand the key ports. CN1 is the control port, and CN2 is the encoder feedback port. Typically, only five lines on CN1 are used: 32, 33, 34, 35, and 4. Lines 32 and 33 are for pulse input, while 34 and 35 are for direction. For simplicity, you can connect 32 to +24V, 33 to 0V, 34 to +24V, and 35 to 0V, creating a basic working system.
In some cases, you can combine 32 and 34 to share the same power supply, making the setup even simpler. However, the motor still requires continuous pulse signals to move. Each pulse corresponds to a small rotation—like 360 degrees divided by 10,000 pulses, which results in very small steps. To achieve continuous motion, the PLC must provide a steady stream of pulses at a higher frequency.
Mitsubishi PLCs like the FX1S and FX1N offer built-in 24V DC power supplies. The output points Y0, Y1, and others are connected to COM terminals. For instance, Y0 connects to COM0, and Y2/Y3 connect to COM2. This allows you to control both pulse and direction signals using these outputs.
Generating a pulse signal can be done with a timer or an ALT instruction. The ALT instruction toggles the output, allowing you to create a square wave for pulse generation. While this method is simple, it’s not commonly used for precise servo control due to limitations in timing accuracy.
For more advanced control, Mitsubishi provides instructions like DRVA and DRVI. DDRVA is for absolute positioning, where S1 represents the number of pulses from the origin (between -999,999 and +999,999), and S2 sets the speed. Y0 and Y2 are used for pulse and direction outputs, respectively. DDRVI works similarly but uses the current position as the starting point.
One major advantage of using DDRVA and DDRVI is that the PLC automatically handles the direction signal. You just need to specify whether the movement is positive or negative. The direction output (Y2) will adjust accordingly without manual control. Additionally, the PLC keeps track of the pulses sent via special registers like D8140 and D8141, which store the total pulse count.
It’s important to note that the high-speed outputs on FX1S models are limited to Y0 and Y1. To control multiple servos, you can pair Y0 with Y2 for one axis and Y1 with Y3 for another. This efficient use of resources helps maximize the capabilities of the PLC in motion control applications.
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