Motion System Design

Plotting a good design

Most products require protective packaging for shipment and handling, and paper remains the number one choice of material because cardboard or corrugated cartons offer many design options. It is important to test prototype designs to determine their functionality. It is too expensive and imprecise to cut them by hand, so multifunctional cutting plotters from Lasercomb, Notzingen, Germany, are used to convert CAD data directly into finished, cut samples.

Lasercomb required a small plotter with all the features of its big brother, but a straightforward transfer into a smaller version was not possible; cabling costs alone would have taken it out of the price range. So it was decided to use a compact CNC controller in the top-hat module. Incorporating two CANopen nodes also minimized cabling costs. One node is responsible for inputs/outputs as a control bus and the other — cycled at a higher frequency — operates as a control bus for the drives. This ensures rapid and precise guidance of the tool in the tool head. At the same time, it was possible to reduce the number of trailing cables to the head from 17 to just one, including the power supply.

The ProDigi plotter provides maximum performance despite its compact construction. Depending on the model, the tool head travels work surfaces from 800 ×1,300 to 3,100 × 1,700 mm at speeds to 100 m/min and acceleration of 5 m/s. In full cutting operation, it achieves up to 30 m/min, and as a drawing plotter, it reaches 50 m/min. A change of tools on the head is all that is required for reconfiguration; electronics adjust cutting or drawing parameters via sensitive controllable motors.

The plotter and the tools are powered by compact dc motors from MicroMo Electronics Inc., a member of the FAULHABER Group, Clearwater, Fla. Four motors must be mounted in a limited space for controlling the Z-axis (i.e. knife stroke movement) and the C-axis (i.e. horizontal movement of the head). Each pair of motors is responsible for a tool fixture. All motors are pre-configured with connection cables and incremental encoders. The connectors are then simply attached to a “switching panel” that provides the connections for the motor and the add-on modules as well as the special plug configuration for the control side. In an emergency, this allows the end-user to replace the motor using plug-and-play, which means resoldering of connections required in the past is not necessary. A seal against paper dust also improves the lifetime of the motors — without disruptive foreign bodies, the frictional pairing of brushes/collectors is subject to much less wear.

To guide the cutting knife to the precise height, a 28 W motor with 180 mNm torque at a diameter of 35 mm and length of 57 mm is installed in the head. With a lifting spindle as a self-locking gear reduction, the cutting knife can be precisely positioned with zero backlash. Controller and miniature drives call for different solutions: optimally implemented, only a single trailing cable is now required for the controller and power supply. The drive is hidden under the cover to save space; complete motor gear units with an 80 W motor offering 530 mNm and flanged metal planet gears are used for the C-axis. The dc motor's high dynamics permit digital table height compensation (vertical tool tracking) over the entire work surface, enabling a high degree of precision. This ensures a constant distance from the tool to the vacuum table and also to the material to be processed.

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