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Detailed Implementation Techniques for PLC and Servo System Coordination Control in Precision Positioning Applications

In the field of automation control, precision positioning is akin to a “needlework” technique, where the perfect combination of PLC and servo systems serves as the needle and thread of this “needlework” technique. Currently, various high-precision devices in the market, such as CNC machine tools, packaging machinery, and printing equipment, largely rely on the collaboration between PLC and servo systems for their control principles. Today, let us unveil the technical secrets of this “golden duo” in precision positioning applications.

1.

Basic Concept Analysis

A PLC (Programmable Logic Controller) can be understood as the “brain” in an industrial environment, responsible for processing various logic and calculations. If we compare a PLC to a computer we use daily, then the servo system is akin to a precision robotic arm controlled by the computer, capable of moving accurately to specified positions according to instructions.

The servo system mainly consists of a servo driver and a servo motor. The servo driver receives commands from the PLC and then controls the servo motor to rotate at specific angles or speeds. This is similar to a remote control (PLC) sending commands, with the receiver (servo driver) receiving the signal and then controlling the toy car (servo motor) to move as required.

2.

Hardware Connection Methods

The connection between PLC and servo systems mainly has three methods:

1. Pulse + Direction Control Mode: The most common connection method, where the PLC sends pulse and direction signals, and the servo driver receives them to control the motor’s rotation. Each pulse corresponds to a fixed angle of rotation for the motor, and the direction signal determines whether it rotates forward or backward. This method iscost-effective but has limited precision.

2. Analog Control Mode: The PLC outputs a ±10V analog voltage, which the servo driver converts into speed or torque commands. This method isfast-responding but is susceptible to interference.

1. Bus Control Mode: Communication via industrial buses (such as PROFINET, EtherCAT, etc.) is currently themost advanced method, featuring high precision, high speed, and rich functionality.

! PLC and servo system connection schematic

3.

Key Parameter Settings

Setting parameters for the servo system is crucial for precision positioning, mainly including the following aspects:

1. Electronic Gear Ratio: Determines the actual movement distance corresponding to each pulse, calculated as:

   Electronic Gear Ratio = Number of pulses required for the load mechanism to move 1mm / Encoder resolution

2. Position Loop Gain (Kp): Affects the system’s response speed and stability; too high can cause oscillation, while too low results in slow response.

3. Speed Loop Gain (Ki): Affects the precision of speed control and needs to match the position loop gain.

Deceleration Time

1. : Properly setting acceleration and deceleration times canavoid mechanical shocks and extend equipment lifespan.

4.

Practical Application Case

Taking a precision positioning system for a pick-and-place machine as an example:

The pick-and-place machine needs to place electronic components accurately on a PCB, with a precision requirement of ±0.02mm. The system uses Mitsubishi FX5U PLC in conjunction with MR-J4 series servo drivers, communicating via SSCNET III bus.

PLC program snippet (ladder diagram):

LD X0           // Start button
OUT M0          // Start flag
LD M0           // Start flag
DMOV K1000 D100 // Set target position (unit: pulses)
DMOV K5000 D102 // Set speed (unit: pulses/second)
DMOV K1000 D104 // Set acceleration (unit: pulses/second²)
DMOV K1000 D106 // Set deceleration (unit: pulses/second²)
MOV K1 D108     // Set operation code (1 indicates absolute positioning)
SET M100        // Trigger positioning operation

The execution logic of this program is: when the start button is pressed, the PLC sets the position, speed, and acceleration/deceleration parameters, then triggers an absolute positioning operation, and the servo motor moves to the specified position according to the set parameters.

5.

Common Issues and Solutions

1. Inaccurate Positioning:

• Check if the electronic gear ratio setting is correct
• Check if there are any gaps in the mechanical transmission parts
• Consider increasing the encoder resolution

System Oscillation:

• Appropriately reduce the position loop gain
• Increase acceleration and deceleration time
• Check if the mechanical structure is loose

Position Overshoot:

• Adjust feedforward gain
• Increase deceleration time
• Enable S-curve acceleration and deceleration function

6.

Practical Recommendations

1. When selecting a servo system, do not solely pursue high performance; instead, choose a suitable model based on actual precision requirements to control costs.

2. When designing the mechanical structure, try to minimize transmission links; using direct drive methods can improve precision.

3. Regularly check the brakes, bearings, and other mechanical components of the servo system to prevent wear from causing precision degradation.

4. For applications requiring high precision, consider adding measurement feedback systems, such as grating rulers or magnetic scales.

Practical exercises can include building a simple single-axis positioning system using a small PLC and servo motor to achieve reciprocating motion at different positions, observing changes in system response by adjusting parameters.

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