Waterjet Drive Systems: Traction Drive
I’m really excited about the new Traction Drive system now being used in most of OMAX’s high-precision bridge-style cutting tables. We have found it to be so cost-effective to manufacture and so robust in operation that we are now featuring it in a simplified version in our low-cost MAXIEM products. The Traction Drive system is the first linear motion system designed specifically for abrasive waterjets. It has three elements working together to provide a precision motion control system that is ideally suited for our applications.
The first element is the actual traction drive mechanism. This mechanism uses the time-tested steel-on-steel traction capability found in the railroad locomotive. A hardened steel drive wheel mounted to a movable carriage assembly is pressed against a hardened steel rail by the action of a spring flexure mechanism. Rotation of the drive wheel by a brushless synchronous motor causes the carriage to move along the rail. This drive system is extremely robust and operates well in the worst imaginable wet and abrasive-laden environment, as demonstrated by the millions of miles traveled by railroad locomotives in conditions ranging from heavy rain to drifting sand. It provides smooth movement over a wide range of speeds and accelerations and can be designed to provide only the forces actually needed to smoothly move the cutting nozzle, resulting in a high level of operator safety. No lubricants are required for the drive surfaces, which further enhances reliability and minimizes wear in a contaminated environment. The drive is low in cost compared to traditional linear drive systems and can be easily expanded in size at very little cost by merely lengthening the steel guide rail. In addition, off-the-shelf extruded rail guides can be used to minimize manufacturing costs.
The potential drawback of the traction drive mechanism is that it is what we engineers refer to as a “loosely coupled” drive system. There is no continuous positive mechanical linkage between the moving carriage and the rail, as found between a ballscrew and ballnut or a rack and pinion. Thus the need for the second element—a precision magnetic linear encoder system. Not long ago, long-length linear encoders were both exotic and expensive. This is no longer the case. Now these precision magnetic devices are readily available in virtually limitless lengths at very reasonable cost. They are reliable and quite insensitive to contaminants and dirty conditions. Moreover, they are highly accurate and have a resolution down to 1 micron (0.00004”), more than adequate for the most precise abrasive waterjet applications. Indeed the inherent accuracy of this system is on par with the accuracy of a much more expensive linear motor drive, which also relies on a linear encoder for determining position.
The final element of the traction drive system is the electronic Drive Control. The traction drive mechanism provides the motive power to smoothly move the carriage along the rail. The linear encoder provides the data to determine precisely where the carriage is. The Drive Control links these, along with input from the machine control computer in a unique feedback/feedforward control loop. Conceptually, the system functions as follows: The Drive Control first takes input from the machine control computer to determine the desired direction, acceleration, speed and distance of motion based on the tool path and pre-computed velocity profile associated with the part being made. The Drive Control then instructs the traction drive motor to provide the desired carriage acceleration, speed and distance based on an assumed coupling between the drive wheel and the rail. The Drive Control monitors the actual carriage motion based on the input from the linear encoder sensor and adjusts its instructions to the traction drive motor based on what is actually happening. This is the “feedback” part of the control. In addition the Drive Control modifies its intended future commands to the traction drive based on the currently measured drive wheel/rail coupling. This is the “feedforward” part of the control. The end result of this feedback/feedforward control approach is that the mechanically “loosely coupled” drive system becomes a “close coupled” drive that automatically adjusts for changes in conditions, such as possible contaminants on the rails and long-term wear.
Both extensive laboratory tests and real operations have shown the drive to be robust and reliable. It is also accurate: capable of producing parts with tolerances in the range of +/-0.001” or better. Finally, actual experience in manufacturing the Traction Drive shows that it can indeed be very cost-effective compared to more traditional machine tool drive systems. It should help point the way to lower overall costs for precision abrasive waterjet cutting systems. That is indeed good news because it will broaden the market for this technology, thus putting the flexibility and efficiency of the abrasive waterjet cutting process into the hands of a greater number of manufacturers, large and small. As readers of this blog already know, my personal goal is an abrasive waterjet cutting system in every machine shop. The Traction Drive is another step toward that goal.
Dr. John Olsen