Solutions for long-range linear motion

Short-range linear motion of machine tools is often achieved using ballscrews and nuts, but this design can become problematic as travel length increases. In a typical ballscrew-and-nut drive, the nut is fixed to the moving table. As the screw rotates, the nut translates along the axis of motion. In this configuration, the servomotor directly sees the rotary inertia of the screw. Rotary inertia is the resistance of the servomotor rotor and attached screw to rotational acceleration. 

As the desired motion elongates, so does the screw. As the screw length increases, so does its rotary inertia. As its rotary inertia increases, the acceleration achievable by the limited motor torque decreases. In the traditional ballscrew-and-nut design, long motions have low accelerations. In addition, as screw length increases, the screw becomes thin and flexible, and, without support, the screw can begin to whirl like a jump rope. However, it is difficult to support the screw and still allow the nut to pass. 

Courtesy of Atlanta Drive Systems

Figure 1. A rack-and-pinion drive for a linear axis. 

For these reasons and others, fixed-nut, rotating-screw designs are practically limited to about 4 ' of travel. If the desired motion is longer, another solution is needed. In machine tools with long travels, two basic designs are common: rack-and-pinion and linear drives. 

In a rack-and-pinion drive, the servomotor is attached to the small pinion gear, and the teeth mesh with the teeth on a long-toothed track—the rack (Figure 1). The rack is often fixed to the machine base, and the servomotor and pinion are mounted to the moving component. As in a gear, motion quality is directly related to teeth quality. 

A common problem in these types of drives is backlash, a pause in motion caused by the clearance between teeth when the direction of motion is reversed. In addition to backlash being reduced by improving the precision of the teeth, it can be reduced by using a spring-loaded split pinion. In this design, the pinion is split into two parts: one side in contact with the rack in the direction of forward motion and one side in contact with the rack for motion in the opposite direction. The preload between the two parts is set with a spring, and the spring’s force is greater than the operating force, so contact is never lost in either direction. 

Backlash can also be removed by using two servomotors, loading two pinions against each other. The advantage of this design is the preload is programmable. In most cases, the positional feedback for a rack-and-pinion drive is a rotary encoder on the servomotor. Such systems can have intermediate gears to increase the transmission ratio, making the servomotor insensitive to the inertia of the moving axes or cutting forces.

Courtesy of IntelLiDrives

Figure 2. A direct-drive linear motor.

The second common design for long-range linear motion is the linear drive (Figure 2), which is like an unrolled servomotor. In the linear drive, what was the stator of a servomotor is now configured in a long straight line, similar to the rack in a rack-and-pinion drive. It is typically fixed to the nonmoving machine bed. As a result, the rotor is configured as a slider that travels along the stator.

In the same way that the magnetic field is manipulated in a servomotor to cause rotation, the magnetic field is manipulated in a direct-drive linear motor to cause linear motion. 

Linear motors can have high accelerations, and the length of a linear motor drive is almost unlimited. Under high acceleration, linear motors can exhibit a kind of backlash that results from the flexibility of the structural components, loaded first in one direction and then another. 

The feedback for a linear motor drive cannot come from a rotary encoder, as it does on a rack-and-pinion drive. Instead, it is common to use a linear encoder, which looks like an unrolled rotary encoder. Whereas a rotary encoder is arranged in a circle, a linear encoder is arranged in a long, straight line, and it lies alongside the stator of the linear motor.

Alternatively, a laser interferometer could provide the feedback. Linear encoders and laser interferometers are suitable for rack-and-pinion systems as well, but they are generally more expensive than a rotary encoder mounted to the servomotor on the pinion—the common solution.

While direct-drive linear motors can have a high acceleration, they directly see the inertia of the moving object. It is not easy to imagine a way to change the transmission ratio for a linear motor. If the moving mass does not change much, this is a minor problem, but when workpiece mass is significant compared to the machine structure, and the mass significantly changes during machining, the acceleration of the linear drive will change throughout the cutting operation. 

In machine tools with parallel kinematics, such as hexapods, the mass of the moving spindle does not change, but the “apparent mass” of the spindle does change, depending on the machine configuration and the desired acceleration direction. Control strategies to account for the changing workpiece mass or the apparently changing workpiece mass represent an active and challenging research area for machine tool designers. CTE

About the Author: Dr. Scott Smith is a professor and chair of the Department of Mechanical Engineering at the William States Lee College of Engineering, University of North Carolina at Charlotte, specializing in machine tool structural dynamics. Contact him via e-mail at kssmith@uncc.edu.