In PLC programming it’s typical to think in terms of discrete outputs. Since motion controllers have evolved from their origins in CNC milling and industrial robots, the sequential nature of their programming (in the form of G-code or proprietary robot programming languages) has never fit well with ladder logic programming, and gives rise to what I call the Ladder Logic/Motion Controller Impedance Mismatch. In fact TwinCAT 3 does support kinematics packages for industrial robots, and you can even purchase a Codian Delta Robot through Beckhoff and use TwinCAT NC I with a delta robot kinematics package to control the robot directly from TwinCAT 3. It’s no co-incidence that Fanuc is both a leading supplier of industrial robots and CNC mill controllers. Industrial robots, such as those from ABB, Fanuc, Motoman, etc., are actually controlled by interpolated motion controllers internally. G-code is the language that CNC mills (and FDM-type 3D printers) use to instruct the motion controller on the path to follow to create a finished part. This add-on actually includes a built-in G-code interpreter. TwinCAT 3 offers this as an add-on to TwinCAT NC, called “TwinCAT NC I”. If you need your axes to work in a co-ordinated way, such as in a 3-axis CNC mill where you need to follow a complex path in 3 dimensions, then you need “interpolated motion”. That is, each axis is controlled independently. The TwinCAT NC module is just a basic motion controller that gives you “point to point” motion. In fact Beckhoff originally got its start in motion control systems, so TwinCAT 3 includes one of the better motion controller systems available today. It’s the same NC as “CNC machines”, which can mill (or cut) away material from a block of steel with accuracies better than a thousandth of an inch. The “NC” in TwinCAT NC means “numerical control”. The base package is 10 axes, and you can add more axes for more money. The latter is licensed depending on how many “axes” (i.e. The PLC system is licensed as TwinCAT PLC, and the motion control system is called TwinCAT NC. In TwinCAT 3 they are separately licensed modules. Since around 2000 we’ve seen more and more convergence of Motion Control and PLC systems, but they are still separately packaged modules within the system. Motion Control systems are not new, but they have a parallel lineage to PLCs. A servo requires a motor and a position feedback device such as a resolver or an encoder, and it controls the position of the motor using a feedback control system. Motion Control refers to the use of servo (and stepper) motors in your system. This chapter is part of the TwinCAT 3 Tutorial.
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