tailieunhanh - Mechanical Engineer's Reference Book 2011 Part 3

Tham khảo tài liệu 'mechanical engineer's reference book 2011 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Instrumentation 3 25 Figure Super-linear variable capacitor SLVC courtesy ASL transducers. Audiofrequency AF power supplies are used for resistance and inductance measurements though . is of course also effective for resistance. Circuits used for capacitance measurement must take account of the stray capacitance that occurs between nearby conductors unless they are specifically screened from each other. Thus in Figure capacitance variations between its lead and earth either within or outside the screened cable are indistinguishable to the measuring circuit from transducer capacitance changes. Various arrangements can be adopted to overcome this problem. The relatively small capactiance in most transducers corresponds to a very high impedance at lower frequencies and this is an argument for working at higher frequencies but in fact AF bridge systems with very high sensitivities are available and give good performance when spurious effects are eliminated. Many considerations come into the choice of transducer technique. As a very simple summary it may be suggested that resistance devices are simple and inexpensive inductance devices while tending to be larger and more complicated have a long history of development and mass production. Capacitance devices simple and sensitive in principle need more elaborate circuitry but may well give the best approach for particualrly onerous requirements. Sometimes the force needed to move a transducer element is important. In general the force is less for capacitors than for inductors while with variable resistors it may be less repeatable. Optical methods of position measurement Some classical experiments in physics depend on optical interference. If two coherent light beams are superposed they reinforce or cancel each other according to whether they are in or out of phase and this phase difference depends on the different lengths of the paths they have travelled. If they have travelled the same distance or .