A recent study in Energies reviews the key characteristics of an optical encoder's accuracy, resolution, and repeatability. The experimental reading head for a Moiré effect-based linear optical encoder is utilized to investigate the impact of various reading head designs on an optical encoder's performance under varied mounting errors and dynamic circumstances.
An optical encoder's resolution, precision, and repeatability are the metrics that describe and assess its performance. Similar to other sensors, the optical encoder for a given application is typically chosen based on these factors. However, many adverse effects directly impact how well an encoder performs. A deeper understanding of an encoder's performance and a strategy for figuring out these aspects is required to comprehend how to reduce these negative impacts.
Linear Optical Encoders as Measurement Devices
Optical encoders are the measurement tools employed in industrial applications owing to their high precision, strong repeatability, and high resolution. They transform linear or angular motion into electrical signals that show the position and direction of motion. This information can be utilized to calculate the speed and control of moving components of various technical machines and other equipment after additional signal processing by a numerical control system.
Applications of Linear Optical Encoders
A variety of applications use linear optical encoders. The advantage of using incremental encoders is that they can accurately measure the reading head's movement and absolute position.
The mechanical design of linear optical encoders can be enclosed or open. The measuring scale and reading head of enclosed-type linear optical encoders based on the Moiré fringe scanning technique are shielded from debris, chips, and coolant by aluminum extrusion.
The measuring scale for open-type linear encoders is fastened to the application support and the scanning head. Such a configuration system runs on the non-contact reflected scanning concept, which operates without mechanical touch. Coordinate measurement machines (CMMs), high-accuracy machine tools, and other precision measuring equipment, positioning stages, etc. are only a few examples of the machines that employ them. New open-type laser encoders can identify and assess multi-degrees-of-freedom linear motion errors and measure linear displacement with an accuracy of less than 1 mm owing to an interferodiffractive grating scanning technology.
Different linear optical encoders with particular accuracy and resolution are needed for various applications. An optical encoder only functions in specific environmental circumstances created by various technical operations carried out in machines.
A typical set of factors that invariably affect an encoder's performance include temperature changes carried out by heat generated during the metal cutting process and using coolants, deformations, mechanical vibrations, various mounting errors, and translations.
Limitations of Linear Optical Encoders
Numerous variables affect the measurement precision and the production of high-quality electrical signals produced by optical encoders. These variables include unfavorable changes in the gap and tilt between the optical components, fluctuations in the optical noise caused by flaws in the measuring scale, changes in the amount of light received and the sensitivity of the photodetectors in use. These factors result in a poor contrast in the Moiré fringe and electrical signal distortion.
Strategies to Enhance Linear Optical Encoders
With adequately engineered reading heads, the majority of errors associated with optical encoders can be reduced. Engineering solutions and mechanical constructions are crucial. The space between the scanning reticle and the measuring scale must be maintained, and a flexible spring-based suspension must connect the scanning carriage and the reading head housing.
In addition to being flexible and absorbing any application irregularities, the optical encoder must also be rigid enough to prevent any extra scanning carriage. This can be done using solid components that maintain contact while the reading head moves in both directions, including stainless steel pins and toughened plates.
Correct linear encoder mounting is another issue that frequently arises in practice. The reading head is typically mounted to the moving unit, while an aluminum extrusion of the enclosed encoder is attached to the rigid support of an application. A second steel or aluminum plate is frequently used to support the reading head. The assembly's final alignment can alter the reading head's initial location in several ways.
Several variables influence the performance of a linear optical encoder. Since the encoder's accuracy, repeatability, and resolution are the most significant performance indicators, maintaining their stability is essential. One of the critical elements in achieving this goal is a reading head encoder that is appropriately designed.
This study examined the effects of mounting deviations on displacement, measuring accuracy, and repeatability while testing an experimental reading head with three distinct mechanical designs to determine which construction was least subjected to mounting errors. The dynamic response of experimental linear optical encode to an electrodynamic shaker excitation was captured, and the inaccuracies caused by mechanical vibrations were identified using a well-established methodology documented in the literature.
All measuring systems' dynamic behavior was affected by mechanical structure variations. The resonance frequencies of each design varied. When designing, great care must be taken to minimize any possibly hazardous impacts of these frequencies. Errors can be produced immediately by any additional motion unrelated to the displacement measurement. The experimentally produced inaccuracies varied between each reading head design and exceeded 1 mm in size.
The higher frequency harmonics excite the electrical encoder signals due to optical component oscillations. Diffusion of the analog signal could result from this. The high resolution of the encoder cannot be achieved due to distorted signals, which can result in sub-divisional errors (cyclic errors caused by analog signal flaws) and interfere with the interpolation process.
Gurauskis, D., Przystupa, K., Kilikevičius, A., Skowron, M., Jonas, M., Michałowska, J., & Kilikevičienė, K. (2022). Performance Analysis of an Experimental Linear Encoder’s Reading Head under Different Mounting and Dynamic Conditions. Energies, 15(16), 6088. https://www.mdpi.com/1996-1073/15/16/6088/htm
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