Siemens PLC Two-Axis Synchronized Motion Control Guide
Question: In Siemens PLC with TIA Portal, I have configured two positioning axes. How can I achieve synchronized tracking between them?
Synchronizing two axes in a Siemens PLC environment is a common requirement in applications like conveyor tracking, flying saws, printing presses, and packaging machines. With the integrated motion control functions in TIA Portal and S7-1200/1500 PLCs, you can implement precise synchronization using technology objects such as TO_PositioningAxis and TO_SynchronousAxis. This article explains the key methods, configuration steps, and practical considerations for achieving two-axis synchronization.
Understanding Synchronization Methods
Siemens offers several motion control instructions to synchronize axes. The choice depends on the relationship you need between the master and slave axes:
| Method | Instruction | Typical Use Case |
|---|---|---|
| Gearing | MC_GearIn | Constant speed ratio between axes (e.g., conveyor belts) |
| Camming | MC_CamIn | Non-linear, position-dependent relationship (e.g., rotary knife) |
| Electronic Line Shaft | MC_GearIn with virtual master | Multiple axes following a common virtual axis |
Step-by-Step Configuration in TIA Portal
To set up synchronized motion, you need to configure technology objects and write the PLC program. Here’s a typical workflow using MC_GearIn for gearing:
1. Hardware Configuration
Ensure your PLC supports motion control (e.g., S7-1200 with firmware V4.2 or higher, S7-1500). Add the drives and motors to the project. For example, using SINAMICS V90 PN with PROFINET connection.
2. Create Technology Objects
In the TIA Portal project tree, under “Technology Objects”, add two TO_PositioningAxis objects. Configure the drive interface, mechanics (lead screw, gearbox), and dynamics (velocity, acceleration limits).
3. Define the Master Axis
One axis will act as the master. It can be a real axis (actual motor) or a virtual axis (TO_VirtualAxis). Virtual axes are useful for line shaft applications where no physical motor exists.
4. Program the Synchronization
Use the MC_GearIn instruction in your program. This block requires the master and slave axis inputs, a gear ratio (numerator/denominator), and an acceleration/deceleration for engaging. The slave will follow the master’s position and velocity according to the ratio.
5. Monitoring and Diagnostics
Use the TIA Portal trace function or watch tables to monitor actual positions, velocities, and following errors. The technology object provides status bits like “InSync” to confirm synchronization.
Example: Gearing Two Axes with a 2:1 Ratio
Suppose you have a master axis (Axis1) and a slave axis (Axis2). You want Axis2 to move at twice the speed of Axis1. The gear ratio is 2/1. In the MC_GearIn block, set RatioNumerator = 2, RatioDenominator = 1. When you execute the instruction, Axis2 will accelerate to match the scaled velocity of Axis1 and maintain the position relationship.
Note: Always ensure the slave axis can physically achieve the required dynamics. Check the maximum velocity and acceleration settings in the technology object to avoid errors.
Advanced Synchronization: Camming
For non-linear relationships, use MC_CamIn. You need to define a cam profile (a table of master/slave positions) using the TO_Cam technology object. The cam can be created in the TIA Portal cam editor or imported from a file. Typical applications include:
- Rotary knife: slave axis follows a specific profile to cut material on the fly.
- Pick and place: slave axis moves in a complex path synchronized with a conveyor.
- Winding machines: controlling tension with variable speed ratios.
Troubleshooting Common Issues
| Symptom | Possible Cause | Solution |
|---|---|---|
| Slave axis does not move | MC_GearIn not executed or master not moving | Check Execute pin and master axis status |
| Following error too large | Dynamics limits too low or mechanical binding | Increase acceleration/jerk or check mechanics |
| Synchronization lost after stop | Master axis stopped or MC_GearIn aborted | Use MC_GearOut to disengage properly |
Best Practices for Reliable Synchronization
- Use a virtual master for multiple slaves to simplify coordination and allow easy speed changes.
- Set appropriate dynamics in the technology object to match the mechanical system’s capabilities.
- Monitor the “InSync” status before starting dependent processes.
- Implement error handling for communication losses or drive faults.
- Test with trace to verify the actual motion profile matches expectations.
Siemens PLCs provide powerful and flexible motion control capabilities. By understanding the synchronization instructions and properly configuring technology objects, you can achieve precise multi-axis coordination for a wide range of industrial automation applications. Always refer to the official Siemens documentation for your specific PLC and TIA Portal version for detailed parameter descriptions and updates.
Further Reading: Explore topics like electronic camming with variable profiles, using MC_Phasing for offset adjustments, and integrating safety functions with synchronized axes.
