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Overcoming Synchronization Problems When Coordinating Robots with PLC

I recently made a sales call with Ed Novak, North American Sales Manager at Trio Motion, to an integrator that frequently works with SCARA and 6-axis robots.

In our preliminary discussion, I mentioned that there are frequently synchronization problems when coordinating robots with PLC based motion control. The customer said they had experienced problems on their last machine. They spent days adjusting their program in order to get the timing of the robot and several integrated step motors just right.

It was a perfect time for Ed to explain the benefits of EtherCAT versus Ethernet/IP and the benefits of a Trio RPS (Robot Programming System) over a PLC.

Ethercat controller versus Ethernet/IP PLC

Ethernet/IP is implemented over TCP/IP protocol. Packets are read at every node. Repeatability is heavily affected by bandwidth and the amount of traffic. TCP/IP packets arrive in any order at various time periods. It cannot guarantee execution in a consistent time (IEEE 1588 synchronization) period without additional hardware.

EtherCAT, on the other hand, was specifically designed for deterministic real-time motion control and does not use TCP/IP. EtherCAT masters send a “telegram” of data that passes through each node device. Each EtherCAT node contains an ASIC chip. The nodes read their own input data and adds output data to the telegram before it goes to the next node, “processing on-the-fly”.

EtherCAT features a distributed clock. The master sends a broadcast to all nodes to a certain address in the ASIC chip. Upon reception of this message, all nodes will latch the value of their internal clock twice – once when the message is received and once when it returns. The master can then read all latched values and calculate the delay for each node. Consistent sub-millisecond update rates are achievable.

The Trio RPS.

Trio EtherCAT Motion Coordinators can control a variety of robot mechanisms, such as SCARA or 6-axis robots, plus other machine control axes of up to 128 axes of servo or step motors (depending on the Trio model). The update time period with all nodes and I/O is conservatively 1 millisecond.

The advantage: controlling your special mechanism (using built-in kinematics) along with the entire machine process, all in the same environment – robot and machine working in harmony!

TRIO’s RPS includes:

  • A high-level robot programming language building on the fast-effective multi-tasking TrioBASIC.

  • Kinematics transformations package configurable for the majority of robot mechanism types.

  • Robot visualization tool in Motion Perfect v4 (MPv4) – See your robot move in real-time for faster development and testing.

  • Pendant and Teach Programming System – This allows programs, robot tools and points to be built, edited and proven in an easy to use way.

  • A special robot tool within MPv4 for robot configuration, editing points, frames and tools. Complex programs may also be edited inside MPv4.

The Trio RPS is vendor neutral. The following link (13-axis dual SCARA robots) shows a 13-axis system consisting of 2 SCARA robots, 3 rotary axes (2 are camming), 1 linear axis and one indexing conveyor.

The most demanding industrial robot applications require precise and timely coordination with automated production equipment. Using the familiar PLC to tie your system together could be a big mistake. Contact Axis NJ, who working with Trio, offers a superior platform for robotic cells integrated with automation systems.

J.R. Cover


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