Today, dimensions in the micrometer range are common in all industries, says Ruediger Keller, product manager for integrated measuring systems at Bosch Rexroth, Schweinfurt, Germany (boschrexroth-us.com).
This level of accuracy is driving companies to install direct-measuring systems on motion axes. "And the demand is, of course, for systems that work without any additional costly and bulky components," says Keller.
One solution is a noncontact, inductive position-sensing system built into Rexroth ball and roller-rail guides. It takes up no extra space and, thanks to rugged construction, withstands harsh environments, he says.
No single displacement-measuring system is best for every application, notes Keller. For simple tasks, indirect systems such as rotary encoders on motors calculate positioning data with sufficient accuracy. Although this method offers clear advantages in velocity-control loops, mechanical errors affect results. Consequently, positioning accuracy is only around 0.1 mm — not accurate enough for machinetool applications.
Direct-measurement, linear-displacement systems are a step up. In these devices, a sensor reads position from a scale along the entire length of the linear axis. Combined with numericaldisplacement interpolation, the units deliver positioning accuracy in the micron range.
Several versions are available. Glass scales with optical sensors, for example, are precise but highly sensitive to contamination. Magnetic varieties are relatively robust, though external magnetic disturbances can falsify measurements. The scale's permanent magnetic field can also attract dirt. The most rugged solution for highly accurate position sensing, says Keller, is an inductive linear-measuring device integrated in ball and roller-rail systems. The linear-motion guides have a graduated scale with a precise pitch of 1,000 m. Pitch is etched into the scale using a photolithographic process much like that used in semiconductor fabrication. The steel scale is embedded in a groove running the length of the guide rail and protected by a tightly welded stainlesssteel band. It withstands extreme vibrations and impacts to 10 g. Oil, metalworking fluids, water, and metal shavings have no effect on measurements, and the system is impervious to magnetic and electrical interference.
A probe housing a sensor and signal-conditioning electronics bolts to the runner block and measures position. Relative, noncontact motion between scale and probe generates sinusoidal measurement signals, delivered as incremental analog signals (1 Vpp) with a periodic length of 1,000 m.
Interpolating the analog voltage produces square-wave signals. Depending on the interpolation factor, resolution can be as high as 0.25 m. In industrial applications, positioning accuracy is ±3 m over 2,000 mm.
Scales are available in one-piece lengths to 4,000 mm. An advantage, especially over long travel distances, is that several runner blocks can read the scale independently of one another, allowing simultaneous positioning of several movements at once. Real-time evaluation electronics guarantees reliable positioning data even for motions to 5 m/sec. Direct measurement on the rail also eliminates the need for complex arrangements to compensate for parallelism offsets common with external sensors.