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4. Construction of Electronic
Instruments
Electronic instruments can be categorized by the way they are intended to be used physically, resulting in certain types of construction:
1. Site mounting.
2. Panel mounting.
3. Bench mounting.
4. Rack mounting.
5. Portable instruments.
FIG. 7 instruments for site mounting. Courtesy of Solartron instruments.
4.1 Site mounting
The overriding consideration here is usually to get close to the physical process which is being measured or controlled.
This usually results in the need to tolerate harsh environmental conditions such as extreme temperature, physical shock, muck, and water. Signal conditioners and data-acquisition subsystems, which are connected to transducers and actuators, produce signals suitable for transmission over long distances, possibly to a central instrumentation and control system some miles away. Whole computerized systems are also available with ruggedized construction for use in less hostile environments.
The internal construction is usually very simple, since there are few, if any, controls or displays. FIG. 7 shows an interesting example which tackles the common problem of wire terminations. The molded plastic enclosure is sealed at the front with a rubber "o" ring and is designed to pass the iPC 65 "hosepipe" test (see BS 5490 or the NEMA 4 and 4X standard). The main electronic circuit is on one printed circuit board mounted on pillars, connected to one of a variety of optional interface cards. The unit is easily bolted to a wall or the side of a machine.
4.2 Panel mounting
A convenient way for an instrument engineer to construct a system is to mount the various instruments which require control or readout on a panel with the wiring and other system components protected inside a cabinet. Instruments designed for this purpose generally fit into one of a number of DIN standard cut-outs (see DIN 43 700). FIG. 8 is an example illustrating the following features:
1. The enclosure is an extruded aluminum tube.
2. Internal construction is based around five printed circuit boards, onto which the electronic displays are soldered.
The PCBs plug together and can be replaced easily for servicing.
3. All user connections are at the rear, for permanent or semi-permanent installation.
FIG. 8 Panel-mounting instrument. Courtesy of Systemteknik AB.
FIG. 9 (a) Bench-mounting instrument. Courtesy of Automatic Systems Laboratories
Ltd. (b) General assembly drawing of instrument shown in FIG. 9(a).
4.3 Bench-mounting Instruments
Instruments which require an external power source but a degree of portability are usually for benchtop operation. Size is important, since bench space is always in short supply.
Instruments in this category often have a wide range of controls and a display requiring careful attention to ergonomics. FIG. 9(a) shows a typical instrument, where the following points are worth noting:
1. The user inputs are at the front for easy access.
2. There is a large clear display for comfortable viewing.
3. The carrying handle doubles up as a tilt bar.
4. It has modular internal construction with connectors for quick servicing.
The general assembly drawing for this instrument is included as FIG. 9(b), to show how the parts fit together.
4.4 rack-mounting Instruments
Most large electronic instrumentation systems are constructed in 19-inch wide metal cabinets of variable height (in units of 1.75 inch 1U). These can be for bench mounting, free standing, or wall mounting. Large instruments are normally designed for bench operation or rack mounting for which optional brackets are supplied. Smaller modules plug into sub-racks which can then be bolted into a 19-inch cabinet.
FIG. 10 shows some of the elements of a modular instrumentation system with the following points:
1. The modules are standard Eurocard sizes and widths (DIN 41914 or IEC 297).
2. The connectors are to DIN standard (DIN 41612).
3. The subrack uses standard mechanical components and can form part of a much larger instrumentation system.
The degree of modularity and standardization enables the user to mix a wide range of instruments and components from a large number of different suppliers worldwide.
FIG. 10 Rack-based modular instruments. Courtesy of Schroff ( U.K.) Ltd.
and Automatic Systems Laboratories Ltd.
FIG. 11 Portable instruments. Courtesy of Solomat SA.
4.5 Portable Instruments
Truly portable instruments are now common, due to the reduction in size and power consumption of electronic circuitry. FIG. 11 shows good examples which incorporate the following features:
1. Lightweight, low-cost molded plastic case.
2. Low-power CMoS circuitry and liquid crystal display (LCd).
3. Battery power source gives long operating life.
Size reduction is mainly from circuit integration onto silicon and the use of small outline components.
4.6 Encapsulation
For particularly severe conditions, especially with regard to vibration, groups of electronic components are some times encapsulated (familiarly referred to as "potting"). This involves casting them in a suitable material, commonly epoxy resin. This holds the components very securely in position, and they are also protected from the atmosphere to which the instrument is exposed. To give further protection against stress (for instance, from differential thermal expansion), a complex procedure is occasionally used, with com pliant silicone rubber introduced as well as the harder resin.
Some epoxies are strong up to 300ºC. At higher temperatures (450ºC) they are destroyed, allowing encapsulated components to be recovered if they are themselves heat resistant. Normally an encapsulated group would be thrown away if any fault developed inside it.
5. Mechanical Instruments
Mechanical instruments are mainly used to interface between the physical world and electronic instrumentation.
Examples are:
1. Displacement transducers (linear and rotary).
2. Force transducers (load cells).
3. Accelerometers.
Such transducers often have to endure a wide temperature range, shock, and vibration, requiring careful selection of materials and construction.
Many matters contribute to good mechanical design and construction, some of which are brought out in the devices described in other sections of this guide. We add to that here by showing details of one or two instruments where particular principles of design can be seen. Before that, however, we give a more general outline of kinematic design, a way of proceeding that can be of great value for designing instruments.
5.1 Kinematic design
A particular approach sometimes used for high-precision mechanical items is called kinematic design. When the relative location of two bodies must be constrained, so that there is either no movement or a closely controlled movement between them, it represents a way of overcoming the uncertainties that arise from the impossibility of achieving geometrical perfection in construction. A simple illustration is two flat surfaces in contact. If they can be regarded as ideal geometrical planes, then the relative movement of the two bodies is accurately defined. However, it is expensive to approach geometrical perfection, and the imperfections of the surfaces mean that the relative position of the two parts will depend upon contact between high spots, and will vary slightly if the points of application of the forces holding them together are varied. The points of contact can be reduced, for instance, to four with a conventional chair, but it is notorious that a four-legged chair can rock unless the bottoms of the legs match the floor perfectly. Kinematic design calls for a three-legged chair, to avoid the redundancy of having its position decided by four points of contact. More generally, a rigid solid body has 6 degrees of freedom which can be used to fix its position in space. These are often thought of as three Cartesian coordinates to give the position of one point of the body, and when that has been settled, rotation about three mutually perpendicular axes describes the body's attitude in space. The essence of kinematic design is that each degree of freedom should be constrained in an identifiable localized way. Consider again the three-legged stool on a flat surface.
The Z-coordinate of the tip of the leg has been constrained, as has rotation about two axes in the flat surface. There is still freedom of X- and- Y-coordinates and for rotation about an axis perpendicular to the surface: 3 degrees of freedom removed by the three constraints between the leg-tips and the surface.
A classical way of introducing six constraints and so locating one body relative to another is to have three V-grooves in one body and three hemispheres attached to the other body, as shown in FIG. 12. When the hemi spheres enter the grooves (which should be deep enough for contact to be made with their sides and not their edges), each has two constraints from touching two sides, making a total of six.
FIG. 12 Kinematic design: three-legged laboratory stands, to illustrate
that six contacts fully constrain a body. (Kelvin clamp, as also used for theodolite
mounts.)
If one degree of freedom, say, linear displacement, is required, five spheres can be used in a precise groove as in FIG. 13. Each corresponds to a restricted movement.
For the required mating, it is important that contact should approximate to point contact and that the construction materials should be hard enough to allow very little deformation perpendicular to the surface under the loads normally encountered. The sphere-on-plane configuration described is one possible arrangement: crossed cylinders are similar in their behavior and may be easier to construct.
Elastic hinges may be thought of as an extension of kinematic design. A conventional type of door hinge is expensive to produce if friction and play are to be greatly reduced, particularly for small devices. An alternative approach may be adopted when small, repeatable rotations must be catered for. Under this approach, some part is markedly weakened, as in FIG. 14, so that the bending caused by a turning moment is concentrated in a small region. There is elastic resistance to deformation but very little friction and high repeatability.
The advantages of kinematic design may be listed as:
1. Commonly, only simple machining operations are needed at critical points.
2. Wide tolerances on these operations should not affect repeatability, though they may downgrade absolute performance.
3. Only small forces are needed. Often gravity is sufficient or a light spring if the direction relative to the vertical may change from time to time.
4. Analysis and prediction of behavior is simplified.
The main disadvantage arises if large forces have to be accommodated. Kinematically designed constraints normally work with small forces holding parts together, and if these forces are overcome-even momentarily under the inertia forces of vibration-there can be serious malfunction.
Indeed, the lack of symmetry in behavior under kinematic design can prove a more general disadvantage (for instance, when considering the effects of wear).
Of course, the small additional complexity often means that it is not worth changing to kinematic design. Sometimes a compromise approach is adopted, such as localizing the points of contact between large structures without making them literal spheres on planes. In any case, when considering movements and locations in instruments it is helpful to bear the ideas of kinematic design in mind as a background to analysis.
FIG. 13 Kinematic design: five constraints allow linear movement.
FIG. 14 Principle of elastic hinge.
FIG. 15 Rugged proximity transducer.
5.2 Proximity transducer
This is a simple device which is used to detect the presence of an earthed surface which affects the capacitance between the two electrodes E1 and E2 in FIG. 15. In a special application it is required to operate at a temperature cycling between 200ºC and 400ºC in a corrosive atmosphere and survive shocks of 1,000 g. design points to note are:
1. The device is machined out of the solid to avoid a weak weld at position A.
2. The temperature cycling causes thermal stresses which are taken up by the spring washer B (special mnemonic spring material for high temperatures).
3. The ceramic insulator blocks are under compression for increased strength.
5.3 Load cell
As discussed in section 7, a load cell converts force into movement against the reaction of a spring. The movement is then measured by a displacement transducer and converted into electrical form.
The load cell in FIG. 16 consists of four stiff members and four flexures, machined out of a solid block of high quality spring material in the shape of a parallelogram. The members M1, M2 and M3, M4 remain parallel as the force F bends the flexures at the thin sections (called hinges).
Any torque, caused by the load being offset from the vertical, will result in a small twisting of the load cell, but this is kept within the required limit by arranging the rotational stiffness to be much greater than the vertical stiffness. This is determined by the width.
The trapezoidal construction is far better in this respect than a normal cantilever, which would result in a nonlinear response.
5.4 Combined Actuator transducer
FIG. 17 illustrates a more complex example, requiring a number of processing techniques to fabricate the complete item. The combined actuator transducer (CAT) is a low-volume product with applications in automatic optical instruments for mirror positioning. The major bought-in components are a torque motor and a miniature pre-amplifier produced by specialist suppliers. The motor incorporates the most advanced rare-earth magnetic materials for compactness and stability, and the pre-amplifier uses small outline electronic components, surface mounted on a copper/fiber glass printed circuit board.
FIG. 16 Load cell spring mechanism.
FIG. 17 Combined actuator/ transducer. (a) Whole assembly; (b) fixing
plate; (c) transducer rotor; (d) spring contact.
The assembled CAT is shown in FIG. 17(a). It consists of three sections: the motor in its housing, a capacitive angular transducer in its housing, and a rear-mounted plate (FIG. 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 associated electronic servo-control 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 concentric and held together by three screws in threaded holes A, leaving a precisely determined gap in which the transducer rotor (FIG. 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 (FIG. 17(d)). The spring is chemically milled from thin beryllium copper sheet.
The shaft with motor and transducer rotor glued in place, motor stator and casing, transducer stator and bearings are then assembled and held together with three screws as B. The fixing plate, assembled with cable, clamp, and pre amplifier separately, is added, mounted on standard stand offs, the wires made off, and then finally the cover put on.
In addition to the processes mentioned, the manufacture of this unit requires drilling, reaming, bending, screen printing, soldering, heat treatment, and anodizing. Materials include copper, PTFE, stainless steel, samarium cobalt, epoxy fiberglass, gold, and aluminum, and machining tolerances 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 section 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, Bristol, U.K. (1984).
Morrell, R., Handbook of Properties of Technical and Engineering Ceramics Part 1, An introduction for the engineer and designer, HMSo, London (1985).
Oberg, E., and Jones, F. d., Machinery Handbook, The Machinery Publishing Company (1979).
Shields, J., Adhesives Handbook, Butterworths, London (revised 3rd ed., 1985).
Smith, S. T., and Chetwynd, J., Foundations of Ultraprecision Mechanism Design, Gordon & Breach, London (1992).
The standards referred to in the text are:
BS 5252 (1976) and 4800 (1981): Framework for color coordination for building purposes BS 5490 (1977 and 1985): Environmental protection provided by enclosures.
DIN 43 700 (1982): Cutout dimensions for panel mounting instruments.
DIN 41612: Standards for Eurocard connectors.
DIN 41914 and IEC 297: Standards for Eurocards.
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