4. Accurate Positioning
When particularly accurate positioning is required, reservations on positioning speed have to be taken into account. Higher accuracy can be obtained by using a smaller settling window; i.e., by reducing the window enter 0 (0x406) and window exit 0 (0x407)parameters.
The achievable positioning accuracy is limited by the grating period of the scale, the accuracy of the sensor, and the interpolation factor of the sensor electronics. For higher accuracy consider acquiring:
- stages with finer grating periods
- controllers with internal interpolation
As mentioned in chapter 2, a sensor is used (see Fig. 1; also referred to as encoder) to determine the position and velocity of the stage. In PILine® positioning systems, optical and magnetic incremental and absolute sensors can be used depending on the required accuracy, energy consumption and cost efficiency. In most cases, optical incremental sensors are used. These sensors determine the distance to a given reference point by recording a periodic pattern (referred to as grating) on a scale.
The grating periods utilized range from a few to several tens of micrometers. Using two photodiodes with a 90° phase shift, two sinusoidal signals are generated allowing for detection of the direction of motion. These signals are then processed by an interpolation circuit which splits each period into several equally-spaced pulses. The final resolution corresponds to that of the grating period divided by the interpolation factor.
Using a PILine® stage with a state-of-the-art PIOne (PI Optical Nano Encoder) sensor with a sensor signal period of 0.5 μm and an interpolation factor of >1,000, sub-nm resolution can be achieved.
Typically, interpolators with interpolation factors of 256 to 8192 are integrated directly into the stage electronics. Some PILine® stages have a switch for bypassing this inbuilt interpolator. In this case, the output changes from internally interpolated A/B counts to raw sine/cosine signals, which can be processed further by an external interpolator.
The new PILine® C-867.1U controller features an integrated interpolator with an interpolation factor of up to 20,000, enabling manifold increasing the resolution of your existing stages. Applications that require precise positioning as well as very slow-motion benefit from this gain in resolution (refer to Fig. 10).