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| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Measuring Methods | | Measuring Methods | | | | | | | | | | Spherosyn Technology - Incremental cont'd The phase changes by 360° for each pitch of movement. This output signal is at the fundamental frequency of 10 kHz and has a peak-to-peak amplitude of approximately 5V around a DC level of 5V. Thus the position measured is absolute over a single element, i.e. for every 12.7mm increment. Figure 3 , shows a phase shift of 90° that equates directly to a position of 3.175mm relative to the zero phase position. Phase change of 90° relating to '/4 of a pitch 3.175mm for Spherosyn technology. To achieve linear measurement, the total position is constructed by the addition of the absolute measurement value and the sum of the number of elements traversed since the encoder was referenced. Encoders or position sensors can be broadly categorised into two families, DC operation or AC operation. In the former class lie optical and magnetic encoders both rotary and linear. Devices that use AC excitation are either inductive or capacitive. Examples of rotary inductive devices are resolvers and syncros whilst linear devices include LVDTs, Inductosyn and Newall In AC systems, the signals containing the positional data are modulated AC signals at the fundamental operating frequency of the device. In DC systems, the signals are modulated DC, i.e. slowly varying DC levels. DC signals are particularly subject to offset errors, drift and low frequency noise. | | | | | | Spherosyn Technology Absolute | | | | | | Offset errors can be countered by the use of techniques such as chopper stabilisation which, effectively, converts the signal to AC to eliminate the offset and then converts back. In AC systems the nulling of offset errors is inherent in the AC coupling used and no complex techniques need be applied. Drift is a problem in DC systems, particularly optical where the lamps, LEDs or solar cells are subject to long-term ageing. Inductive systems are inherently stable being based on fixed physical properties such as turns ratios and permeability of the encoder parts. These do not change with time. Low frequency noise, particularly mains power frequencies, can interfere with DC signals and cannot be blocked without severely degrading the system's response time.AC systems, working at a precise, fixed frequency, will employ low and high frequency filters without impacting upon response speed. A criticism often aimed at inductive encoders is that their relatively long pitch length requires a much larger interpolation level for a given resolution than for an optical grating. This is true, but it is not mentioned that accurate interpolation is much more easily achieved, for the reasons given above, on AC systems than DC. The accuracies and resolutions that can be obtained from resolvers match those of their optical rotary counterparts. The same is true for Newall's linear encoders versus its linear optical or magnetic competitors. | | | | | | | | | | The Newall Absolute SHG-A* Encoder is a breakthrough in linear measurement technology. Uniquely coded inserts are placed between the precision nickel chrome elements in the scale. The inserts are locked in position as part of the manufacturing process and contain a small magnetic target that can be detected by a series of hall sensors contained within the readerhead. The density of the inserts and the detectors within the readerhead allows the system to determine fully absolute position at any point in time. Once the encoder has internally determined the true absolute position it is then a matter for the DSP processing to handle communications of the positional data to the outside world through the use of communications protocols such as SSI (Synchronous Serial Interface), Fanuc, RS232, RS485 etc. Furthermore, the internal positional information can be used to accurately emulate other forms of Pseudo-Absolute interfaces such | | | | | | as Distance-Coded. Being a DSP based absolute system capable of a high level of processing, the encoders are error mapped during manufacturing against a laser interferometer. This error map is stored in FLASH memory allowing it to be applied in real-time thus resulting in a highly accurate system. Distance-Coded references (Pseudo Absolute) Distance-Coded reference markers allow the controller to acquire absolute position by moving the encoder system across 2 uniquely spaced reference marks. By using its internal absolute position count, a variant of the Absolute can mimic the Distance-Coded index marks that are generated by glass scales. | | | | | | | | | | | | | | | | | | 01100212100012200101 | | | | | | | | | | ii | | | | | | Figure 3 | | | | | | | | | | One Fitch | | | | | | | | | | 200101 = Element 14 | | | | | | | | | | A 3gna | | | | | | | | | | Absolute Position (mm) = Element No. x 12.7 + Position on current Element | | | | | | | | | | Amplitude | | | | | | | | | | Measured 9gna | | | | | | | | | | NEWALL MEASUREMENT SYSTEMS 3 El TECHNOLOGIES, INC. 7 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
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