Introduction

Computer-controlled water-jet or abrasive-jet cutters are a special type of machine tool capable of cutting many types of materials, including 6-inch steel billet, foam, plastics, and stone, to name a few. One major manufacturer of these water-jet machine tools uses the Galil motion controller to handle all of their machine motion control and I/O functions.

In a water-jet cutting application, the work piece is placed on a stable surface, typically underwater. The high pressure stream is then used to abrade away excess material, leaving the finished product behind. The desired shape is typically drawn in CAD and then coverted to X-Y motions of the water-jet nozzle. There are a total of three axes of motion for this particular water-jet machine: two axes are slaved in a gantry configuration to perform the X motion, and a Y axis is coordinated with the X axis to trace out the appropriate pattern. The actual machine layout is shown in Figure 1.

Problem

The key to a water-jet cutting machine is precise coordination of X-Y position along with tight control of speed and acceleration. Accuracies must be maintained up to ±0.003 inch in some cases, with speeds in excess of 500 inches/min. This water-jet manufacturer was having problems following their desired profile with their previous controller, and contacted Galil to help with the motion controller.

Solution

The customer selected the Ethernet-based DMC-2133 to run their system (Figure 2). In order to solve their accuracy problems, three key features of the Galil controller were used. One, backlash was eliminated electronically on each of the positioning axes to increase the tooling accuracy. Two, a special gantry mode was used to tightly couple the two gantry axes. And three, the Galil vector mode (VM) was used to allow tightly coordinated motion over circles, arcs, and linear segments. This section details how the three features mentioned were used to provide the optimum performance for their tool.

Eliminating Backlash

The Galil controller offers two methods for eliminating backlash from a system. One is to use the standard dual loop feature (DV1). In this mode, the controller closes a single positioning loop around two feedback devices. One encoder located on the motor is responsible for the stability (derivative gain KD), while another encoder located on the load is responsible for the positioning (proportional/integral gain KP/KI). This gives the benefits of both a stable and accurate system.

The other option, which this customer-selected, is a standard controller upgrade for backlash compensation. This is an offset applied to positional moves based on the direction of the backlash, and is handled within the controller firmware. This was the better option for this customer since their large table size precluded the use of linear encoders.

Gearing (GA, GR, GM)

This application required that one of the axes be geared to the X axis for the gantry configuration. To meet this need, a gearing relationship was set between the X and Z axes. Furthermore, gantry mode (GM) was used to more tightly couple the two axes (ST, AB, and the abort input do not break the gearing relationship. Only GR0 stops the gearing).

GAZ=CX gear z axis to x axis commanded position for gantry operation

GRZ=1 set gear ratio to 1:1

GMZ=1 use gantry mode

Vector (VM, VP, CR)

The Galil controller's vector mode was used to follow the motion paths determined by their software. Vector mode allows two axes to be linked together to perform linear and circular interpolation such that complex X-Y patterns can be executed. Vector speeds anywhere along the path can be specified to tailor the motion profile based on geometry.

Conclusion

Through the use of three key features of the Galil controller, this water-jet cutting application was a success. The features above were implemented into the system, and when used in conjunction with the manufacturer's software and graphical interface, provided the end customer with a high performance, easy-to-use machine tool.