With machines and robots becoming increasingly sophisticated and called upon to carry out more difficult tasks, the motion control that defines the movements of these machines must be as precise as possible to prevent mechanical failures, avoid accidents and increase throughput.
Many complex challenges and constraints are faced by system designers. To meet system design requirements, it is often essential to minimize the weight and size of motion control components. Similarly, adjacent components have to be combined into an optimized assembly in order to meet cost and performance targets.
As humans and machines interact on an increasing scale, reliability has become more important than ever before. For more than 20 years, Celera Motion has been developing MicroE miniature encoder products and collaborating closely with OEMs to design highly optimized motion control solutions in the most challenging applications. In addition, Celera Motion’s depth of application expertise, wide range of miniature optical encoders and proven reliability have allowed leading OEMs to realize the design goals of their state-of-the-art systems. In addition, Celera Motion’s depth of application expertise, wide range of miniature optical encoders and proven reliability have allowed leading OEMs to realize the design goals of their state-of-the-art systems.
The Solution: PurePrecision™ Technology
Celera Motion’s patented and innovative PurePrecision™ technology has been the core optical technology platform for more than two decades. The company leverages this technology to provide precision optical encoders to the market and these products are considerably easier and smaller to align and install than other competitive products. Using Celera Motion’s PurePrecision technology, designers of precision motion control systems in semiconductor, medical, advanced scientific and industrial applications are able to design lighter, smaller and more energy-efficient instruments and machines. PurePrecision technology is especially suitable for those applications where the encoder is deployed in controlled environments requiring small overall footprint, compact sensor size and high accuracy associated with glass gratings.
PurePrecision™ optical encoder technology is a patented, laser-based, reflective diffraction technique unique to Celera Motion. It employs a basic principal of optical diffraction known as Talbot Imaging. PurePrecision™ optics does not need any intermediate lenses, reticles or filters, allowing Celera Motion to design the industry’s smallest high-resolution optical encoders with generous alignment tolerances.
How PurePrecision Works
Encoders operate by sensing a sensor’s relative position to that of a grating or scale. Celera Motion develops non-contact optical “kit encoders”, where the grating and sensor are separately installed into a single machine element with a pre-existing bearing structure. The sensor head produces a light source, which reflects off the grating and is subsequently collected by the detector that is also integrated in the sensor head. The outcome is a series of pulses which can be used to find out speed and position.
The following information provides a better understanding about the principles behind the Celera Motion innovations that result in the smallest encoder products when compared to its competitors.
A coherent light source is needed. Celera Motion uses a vertical cavity surface emitting laser (VCSEL) for its low power consumption, extremely small size and stable operation. The laser produces a diverging beam which lights up a diffraction grating pattern printed on the scale, and the grating pattern is subsequently produced using either laser written lines on a metal tape scale or chrome deposition on a glass scale.
A 20 µm pitch diffraction pattern (10 µm space, 10 µm line, etc.) is generally employed, but there are some products that use 40 µm pitch diffraction pattern (20 µm space, 20 µm line etc.). This is called an amplitude grating, which diffracts the light and creates a high contrast interference pattern of dark and bright directly back onto a highly specialized detector array (Figure 2). The widths of the detector cell are optimized to reject undesirable orders of diffraction.
The diffracted light produces discrete Talbot planes of constructive interference on which the detector can be placed. As the relative position of the detector and scale changes, this diffraction grating pattern translates across the detector array leading to a sinusoidal change in all the detector cells. The 3rd Talbot plane is utilized in the example shown in Figure 1. For every 20 µm on the detector, there are four quads or cells. Each detector cell produces one of four sinusoidal signals with an electrical period of 20 µm; Sin+, Cos+, Sin- or Cos-, as shown in Figure 3. Identical cells are tied together electrically and then fed into the processing circuitry of the sensor.
Figure 2 shows the grating’s index mark, which acts as a cylindrical lens and this lens, in turn, creates a single band of light directed at the index detector cells. Once the index mark is crossed, this band of light passes over the three detector cells, i.e., A, B and C. Each cell produces an analog signal that is represented by the black traces in the diagram shown in Figure 4. These analog signals are mathematically combined as B minus A minus C, as illustrated in the blue trace.
It is the zero-crossings of this composite signal that define the falling and rising edges of the digital Index Window signal indicated in red. The spacing of the three detector cells are selected to produce a stable 20 µm wide Index Window, in spite of the small differences in optical power.
With PurePrecision technology, Celera Motion has delivered precision optical encoders that are considerably easier and smaller to align and install in comparison to other competitive products. The products based on this technology are suitable for relatively clean environments and demand high accuracy associated with glass gratings.
For the wider range of applications where the encoder is subjected to a degree of contamination, products based on Celera Motion’s VeraPath™ technology is designed with next-generation signal processing and electronics and sophisticated optical filtering to ensure reliable performance.
Key Products Lines Using PurePrecision Technology Include:
- Mercury II™
This information has been sourced, reviewed and adapted from materials provided by Celera Motion.
For more information on this source, please visit Celera Motion.