Free Trial

Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.

chAPter | 37 Design and Construction of Instruments 653 angular transducer in its housing, and a rear-mounted plate (Figure 37.17(b)) for pre-amplifier fixing and cable clamping. The motor produces a torque which rotates the shaft in the bearings. Position information is provided by the transducer, which produces an output via the pre-amplifier. The associ- ated electronic servocontrol unit provides the power output stage, position feedback, and loop- stabilization components to control the shaft angle to an arc second. This accuracy is attainable by the use of precise ball bearings which maintain radial and axial movement to within 10 nm. The shaft, motor casing, and transducer components are manufactured by precise turning of a nonmagnetic stainless steel bar and finished by fine bead blasting. The motor and transducer electrodes (not shown) are glued in place with a thin layer of epoxy resin and in the latter case finished by turning to final size. The two parts of the transducer stator are jigged concen- tric and held together by three screws in threaded holes A, leaving a precisely determined gap in which the transducer rotor (Figure 37.17(c)) rotates. The transducer rotor is also turned, but the screens are precision ground to size, as this determines the transducer sensitivity and range. The screens are earthed, via the shaft and a hardened gold rotating point contact held against vibration by a spring (Figure 37.17(d)). The spring is chemically milled from thin epoxy fiberglass, gold, and aluminum, and machining toler- ances are typically 25 nm for turning, 3 nm for grinding and 0.1 mm for bending. The only external feature is the clamping ring at the shaft end for axial fixing (standard servo type size 20). This is provided because radial force could distort the thin wall sec- tion and cause transducer errors. references Birbeck, G., "Mechanical design," in A Guide to Instrument Design, SiMA and BSiRA, Taylor and Francis, London (1963). Clayton, G. B., Operational Amplifiers, Butterworths, London (1979). Furse, J. E., "Kinematic design of fine mechanisms in instruments," in Instrument Science and Technology, Volume 2, ed. E. B. Jones, Adam Hilger, Bristol, U.K. (1983). Horowitz, P., and Hill, W., The Art of Electronics, Cambridge University Press, Cambridge (1989). Kibble, B. P., and Rayner, G. H., Co-Axial AC Bridges, Adam Hilger, Bris- tol, U.K. (1984). Morrell, R., Handbook of Properties of Technical and Engineering Ceram- ics Part 1, An introduction for the engineer and designer, HMSo, London (1985). oberg, E., and Jones, F. d., Machinery Handbook, The Machinery Publish- ing Company (1979).