The passive nature of fiber optic technology provides a fundamental solution to interference plagued transmission problems
DENNIS HORWITZ, MICRONOR INC.
IN MANY INDUSTRIAL automation or motion control applications, equipment can be exposed to EMI generated by nearby equipment. These noisy electrical environments can induce errors in the communications link between the (rotary or linear) encoder and the system controller. Missing encoder counts can cause the equipment to move to the wrong position or can simply have an adverse effect on the ongoing process. In a manufacturing application, this exposure can lead to out-of-tolerance or defective parts or products. In a test application, EMI can lead to incorrect equipment settings, bad readings, and false results. These encoder communication errors are costly in terms of time lost and money and materials wasted.By definition, a benign environment is neutral or unthreatening. Examples might be a manufacturing plant, machine shop, or engineering lab operating within a controlled air-conditioned environment or, alternatively, working at a comfortable ambient temperature. In these benign environments, the electronics of conventional encoders are neither thermally stressed (typically operating within the “industrial” electronics range of 0–70°C) nor directly exposed to other adverse elements or conditions (washdown, corrosive chemicals, shock, vibration, etc.). In the majority of these relatively benign applications and conditions, noise problems can be reduced or eliminated by re-routing cables or by upgrading to more expensive, higher performance shielded cabling. A harsh environment pushes at least one environmental parameter to some extreme. Elements in a harsh environment can include temperature (extreme heat, cold, or cycling), humidity, pressure or vacuum, altitude, shock, or vibration. Other adverse factors might be electrical noise (EMI or RFI); radiation; or wet environments such as wash-down, immersion, or wind-driven rain. Contaminants like sand, dirt, or oil and corrosive agents such as detergents, chemicals, jet fuel, and salt air will compromise a work environment. Needless to say, extremely hazardous or explosive environments pose formidable problems. Conventional rotary encoders incorporate electronics in which performance diminishes under any or all of these extremes. On the other hand, an all-optical approach can outperform conventional electrical technology. An all-optical encoder features total passivity and electrical isolation—i.e., it cannot generate or be affected by electromagnetic interference and receives input only via fiber optic links. These encoders are also chemically passive and immune to lightning and atmospheric static. Moreover, such encoders operate over a wider range of temperature (-60°C to +150°C and beyond) and are far easier to install. Lightweight cabling replaces heavy and bulky multi-wire harnesses. Another significant installation factor is that remote placement of the encoder is feasible; and with glass fiber, transmissions over very long distances—over 1000 meters or more—are possible.Click below to read a pdf of the article.