[求助]大侠帮忙呀！帮小弟翻译下数控方面的（毕业设计用）

hero631 · 发表于 2007-3-14 17:05:00

Typically, a CNC machine's signal reference is a common wire of its computer logic power supply connected to the machine's ground plate.In these machines, you should consider two forms of ground-related noise:• The noise appearing on the input power conductors relative to the equipment ground (typically the enclosure or chassis ground), and• The noise or difference in electrical potential that appears between the grounds of interconnected equipment. When trying to avoid electrical noise, distancing from the source of noise is usually a great help, but watch out for misperceptions about ground noise problems (see sidebar "Myths About Ground Loops," on page 30.)Potential pitfalls.      Some CNC machine manufacturers recommend, or even require, the addition of a supplemental ground rod at the CNC machine. (See sidebar "Manufacturers Speak Out About Machine Grounding and Ground Rods,"on page 30.) In most cases, this is an 8-ftcopper rod driven next to the machine, often through the concrete floor, and bonded to the machine's ground plate. The NEC does permit such an installation, as long as the ground rod is bonded to the building electrode grounding system (Fig.3 on page 28). However, this type of installation may still invite stray ground currents.         Different manufacturers state different benefits from the supplemental rod. However, Held experience at sites with the supplemental ground rods has shown that the rod may actually increase the risk of CNC electronics damage. These sites were found to be prone to damage of internal electronics after thunderstorms or utility power system faults.  Basically, the supplemental ground invites stray currents into the machine by effectively connecting the electronic controller between the service grounding electrode system and the supplemental ground rod. A rise in earth ground potential near an electrode causes a large amount of current to flow on the grounding conductor between the electrodes and pass through the CNC machine. Such large currents are possible when lightning strikes nearby or when utility power line faults occur outside the facility. Current-induced electromagnetic stress is the suspected failure mode. The NEC, being strictly an electrical fire safety code, does not consider this condition. Drawing a comparison with other electronic equipment (computers, electronic motor drives, and PLCs), none subscribe to such a local grounding practice. The analogy would he to drive a separate ground rod in the office, and connect it to the logic ground inside your personal computer. IEEE 1100-2005 (The Emerald Book), which focuses on powering and grounding electronic equipment, does not recommend such a practice. Instead, it recommends that a single-point ground from individual electronic cabinets be individually bonded to a local ground gridBeware of isolated ground rod Code violation.     When a CNC machine manufacturer recommends or absolutely requires an additional earth ground rod located at the CNC machine, the end-user is ultimately responsible for the installation to meet NEC and local code requirements.         Although permitted by the NEC, the additional rod is intended as a supplemental grounding electrode, which means you must bond it to the rest of the building grounding system, Too often, this ground rod, which incorrectly might be thought of as "isolated and dedicated" to the CNC machine and from the otherwise "noisy" building ground, is not bonded to the building electrical system ground (Fig. 3). This installation creates an electric shock hazard and is a violation of the NEC.     In effect, this effort to further distance the CNC machine from the "noisy" building ground using an isolated ground rod creates new hazards to both people and machine during lightning storms or power system ground faults. When these occur, dangerous potential differences can exist between the incorrectly isolated ground rod and the rest of the building's grounding systems.Temporary solutions.     Because existing codes or standards don't provide specific grounding practices for CNC machines, an initiative is currently under way to bring end-users, CNC machine manufacturers, consultants, and utility personnel together to work out and publish a best powering and grounding practice for these very important tools.Until then, what should you do to ground CNC equipment?For safety, you must strictly follow the NEC's provisions. This starts with proper grounding of the power source (service entrance or separately derived system). Other requirements include bonding to the grounding electrode system, which connects all available electrodes (building steel, metal water pipes, ground rings, ground rods, etc.) and using properly sized and installed equipment grounding conductors routed with the associated power conductors. Also, you should avoid any connections of the grounded circuit conductor (typically the neutral) to ground, except at the power source.         Because the CNC machine manufacturer is most knowledgeable of the CNC's circuit design and its susceptibilities, it's in the best position to recommend additional grounding, shielding, or installation practices. You should follow these recommendations, as long as they do not violate local or national electrical codes. Also, make sure they aren't based on myths about ground loops or misconceptions about the need for an isolated ground.     Keep in mind that not all operational problems are the result of inadequate grounding, and that proper grounding can't solve all problems. Good system design, which includes facility lightning protection (where appropriate), surge protection, the use of dedicated circuits for the CNC machine, and the segregation of large cyclical or other "disturbing" loads on power feeders separate from the CNC machine, is important to avoid operational problems. I-or more severe power supply variations, such as voltage distortion, sags, swells, and interruptions, you may need some form of power conditioning.

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[求助]大侠帮忙呀！帮小弟翻译下数控方面的（毕业设计用）

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