Although MicroE optical encoders, by Celera Motion, have generous alignment tolerances for most application, special care must be taken while aligning to small rotary scales with a diameter of 20 mm or less.
The geometries and layout of rotary and linear grating patterns differ significantly, resulting in tighter alignment tolerances in some cases (Figures 1a and 1b).
Figure 1a. Linear grating pattern layout
Figure 1b. Rotary grating pattern layout
The grating features are wedge shaped with rotary scales. They are thicker at the outer diameter and thinner at the inside diameter. This means that the period of spaces and lines change with the increase of the radius. The period is exactly 20 µm at the nominal optical diameter, and this is where the sensor must be positioned for optimum performance.
The lines in linear scales are parallel and straight, so there is no change in modulation or period. For large diameter rotary scales, the encoder system will have the same alignment tolerances as a linear system and the period change is small. The wedge shape of the main track lines becomes more distinct as diameters become smaller. This will place more restrictions on alignment in the radial direction.
General terminology for encoder alignment is defined as follows: X being the direction of travel along the scale, Y being the lateral direction to the left and right, and Z being the gap or height between the scale and the sensor. Theta X, Y or Z is the rotations about those axes (Figure 2).
Figure 2. Sensor Orientation Axes
The Y direction is the axis that corresponds to the radial direction on a rotary scale. To show the differences in acceptable Y misalignment, alignment tolerance studies have been conducted with rotary and linear scales (Figure 3). The graph in Figure 3 shows the signal strength of the encoder sensor as a function of misalignment.
In the horizontal axis, zero represents perfect, nominal alignment in accordance with the interface drawings. The vertical axis is normalized sin/cos amplitude. The specified alignment tolerance for a linear or large rotary grating is +/-200 µm, shown as vertical red dotted lines in the graph. As can be seen in the green trace, this provides ample margin for the effects of thermal changes or for additional misalignments in other axes.
Figure 3. Y alignment tolerance study
The blue trace in Figure 3 distinctly shows how the sin/cos amplitude drops off for more suddenly for the standard 12 mm disk; model R1206. The signal amplitude has dropped to only 20% of the ideal at +/-200 µm. The acceptable misalignment must be limited to +/-100 µm or less to maintain appropriate signal strength for accurate interpolation.
It is important to provide sufficient adjustability to the encoder sensor head mount when designing a mounting scheme for a MicroE encoder system, especially one with a small diameter rotary scale. This will enable fine adjustment to the sensor/scale orientation to offset inevitable tolerance stack-up problems. It is not recommended to depend on benching surfaces for small diameter rotary scales.
This information has been sourced, reviewed and adapted from materials provided by Celera Motion.
For more information on this source, please visit Celera Motion.