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Configuration Dependent Indexing Solutions

~ Motion Control Expert

This use case explains how to an external motion controller may be used as indexer in assembly systems with static or dynamic indexing configurations. 

Application Challenge

Many assembly systems utilize a conveyor belt that will “index” a subassembly to a designated work area. There are various ways to implement indexing and the overall complexity of the assembly process will dictate the most efficient implementation. The challenge often is to understand the implications of different indexing approaches and select the most appropriate one. This note will demonstrate three discrete configurations for indexing, each addressing different levels of manufacturing sophistication, within the context of an ultrasonic welder: Static Simple Indexing, Static Complex Indexing and Dynamic Indexing. In each case, the motion controller will perform indexing functs/digital_drives benefits of reduced hardware, simpler code structure and greater machine reliability are easily achieved by off-loading the motion process control to an external controller.

Ultrasonic Welding Machine
Figure 1: Ultrasonic Welding Machine


Design Considerations

Discrete Indexing Configurations

The configuration terms employed above are for reasons of elucidation only. The Static Simple configuration is best suited to simple operations that have fixed positions with few I/O points. The Static Complex configuration suits semi-automated processes that contain a set of destination positions, in a possibly complex sequence defined by several I/O points. The Dynamic configuration applies to a system where the set of sequential positions is frequently changed to accommodate new parts.

In each indexing approach, an ION®/CME Digital Drives will be used to control the motion of the conveyor belt because the ION/CME (C-Motion® Engine) Digital Drive extensive I/O options and real-time response to input or output commands, as well as its ability to store user code, offer great flexibility to the machine designer. In this application, the operator loads a main assembly with subassemblies on the conveyor belt. A two-button, finger-safe start is an input to the ION/CME Digital Drive, which triggers the process of indexing the next part in place under the bowl feeder or welder. The ION/CME Digital Drive also drives outputs that start the ultrasonic welder. After the welding operation is completed, the ION/CME Digital Drive indexes the conveyor belt such that the part advances to the next work area and the next part is moved in place. The state of the inputs to the ION/CME Digital Drive determines the distance and timing of the move.

In the case of Static Simple Indexing, the operator of the ultrasonic welder supervises the index process. This human supervision is adequate because this application is simple and the process has very few variables. Because of the simplicity of this process, there is no need to employ the use of a PLC; user code loaded directly on the ION/CME Digital Drive controls the I/O signals. This reduces hardware count and results in the use of a single programming language with a simple code structure, which lowers overall cost.

When designing a machine for Static Complex Indexing, the ION/CME Digital Drive is connected to a PLC and becomes a sub-process of the larger automation process. Because of the variability in the distance and timing of the move, the input signals to the ION/CME Digital Drive are significantly more complex. This necessitates the use of a greater number of inputs, control of which is offloaded to the PLC, which places the complex part handling process under the supervision of the PLC. User code on the ION/CME indexes the conveyor belt, as and when requested to do so by the PLC. To ensure that the PLC properly controls the ION/CME Digial Drive, a simple 3-wire handshake is used. Other input lines are used to select one of several different index distances, thereby giving the system greater manufacturing flexibility.

If the index position requirements are greater than the I/O line configuration allows, then the Dynamic Indexing method should be used. In this case, the ION/CME Digital Drive is used for basic user interfacing, such as the start button, while the configuration and automation control comes from a PC. Since indexing positions are expected to change on the fly, and because intensive error checking needs to be performed, PLC code no longer suffices. Instead, the code is written in a language better suited to this function (such as C or C++ code that resides on a PC host or the ION/CME itself). This alone significantly reduces development time and increases the benefits of lowered cost.

Going Further

Ladder Logic, used in PLC programming, is one such language whose code structure is complex and ill-suited to motion control applications. When programming basic motion functions such as position, velocity acceleration and deceleration using Ladder Logic, the PLC programmer needs to calculate and monitor these variables to ensure the move executes as intended. When using an external controller like the ION/CME Digital Drive, this capability is built into the controller itself. Because of the limiting structure of Ladder Logic, just the simple function of calculating a trapezoidal move on a PLC will require significantly more lines of code than a motion controller or PC. This simplified language leads to redundant processors and hardware. By choosing the hardware, programming language and code structure that optimizes each sub-system, the entire system is optimized and has a lower design cost while maintaining a high degree of machine reliability and robustness.

ION Digital DrivesION Digital Drives provide high-performance motion control supporting multiple motor types including DC brush, Brushless DC, step, and microstepping motors, and are based on PMD Corp.'s Magellan® positioning ICs, which provides user-selectable profile modes including S-curve, trapezoidal, velocity contouring, and electronic gearing.


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