Linear motor aligning multiple magnet tracks
Background
Linear motors are designed to be "ironless" or "air-core" and do not generate cogging forces when the drive moves through the magnetic track. However, force ripple is another form of disturbance that can originate from many sources, one of the main causes being misalignment of the magnets relative to the electromagnetic field. This technical how-to explains how unwanted force ripple due to misalignment of the magnetic tracks can be avoided.
Why align magnetic railways?
Allient (Airex) offers various magnetic tracks for the C Series or P Series with lengths of 24, 36, or 48 inches. However, many applications require longer travel distances than the options listed above. Each magnetic segment is manufactured 0.010 inches shorter than its model number indicates (a 24-inch segment is 23.99 inches long) to compensate for the thermal expansion of each segment. Although long travel distances can be achieved by simply stringing the magnetic segments together, avoiding the aforementioned force ripple requires precise placement of the individual segments relative to each other. Force fluctuations are reduced by maintaining an accurate magnetic field spacing across the gap between the rail segments. To achieve the required alignment of the magnetic rail, dowel pins can be precisely inserted into the fitting surface of the rail so that they engage with the precision bores in the magnetic rail and achieve the desired alignment.
Aligning magnetic tracks correctly
Proper installation of the magnetic tracks is done in two steps. In the first step, the correct magnetic polarity between the magnetic tracks must be ensured. Allient uses markings to indicate the correct alignment. The correct polarity is achieved when each magnetic track is positioned so that the track markings point in the same direction (see illustration).
The second step is to maintain the correct spacing between the magnetic tracks. Allient recommends using the dowel pin holes in each segment. These holes are marked as the first hole on each segment. Place the dowel pins at a distance "X" (center to center) from the mounting surface. The locating pins should not be higher than 0.23 inches for P20 rails and 0.10 inches for P12 and P15 rails. Please refer to Figure 1 and Table 2 for pin diameters and "X" distances.
| Series | Pin Ø | Spacing X |
|---|---|---|
| Unit | mm | mm |
| P12 | 4.95 | 60.96 |
| P15 | 4.95 | 60.96 |
| P20 | 9.04 | 60.96 |
Summary
The process described here deals with how to maintain the correct field spacing when using more than one magnetic track. This is the only way to avoid unwanted force fluctuations at the joints. Applications that require multiple track lengths therefore need an additional step to ensure uniform force transmission.