Digital Transducers

Digital transducers are devices that produce a pulse-form electrical signal (digital signal) corresponding to a train of mechanical events. The simplest type is the magnetic pickup, consisting of a coil wound round a permanent magnet, one pole of the magnet being located close to a ferrous object (ill. 11-1). Movement of the ferrous object will induce a current in the coil. If a number of ferrous objects move past the coil, then a corresponding train of current pulses will be generated in the coil. Thus, if the ferrous object is a toothed wheel mounted on a rotating shaft, each revolution of the shaft will generate N pulses of electrical signal, where N is the number of teeth in the wheel (ill. 11-2). If these pulses are counted and divided by N, this give an exact figure for the rate of revolutions of the shaft (the magnetic pickup in this case being used as a revolution counter). If the wheel has 60 teeth, the pulse frequency generated in the pickup will be the same as the rotational speed of the shaft in rpm. But, of course, the wheel does not need to have exactly 60 teeth. In practice, larger wheels with a smaller number of teeth (the preferred choice would be 30 teeth or 15 teeth) generally give better performance.

Good results can also be obtained by using pickoffs with shafts having a single indentation or projection to provide a single output pulse per revolution. Shaped cams may also be detected. Where the member to be monitored is nonferrous, the introduction of mild steel studs to provide the appropriate configuration is usually satisfactory. Moving permanent magnets in place of ferrous masses will provide a substantially higher output amplitude from the pickoff, and this is often of great value at low speeds, or where large air gaps are essential.

Apart from having no physical contact with the component being monitored (in this case the rotating shaft), the electrical information derived has the advantage of being in digital form suitable for working a digital display or readout with no need for analog-to-digital conversion.

ill. 11-1. A magnetic pickup for detecting motion of a ferrous object.

ill. 11-2. A magnetic pickup can be used in conjunction with a toothed wheel for measuring the angular speed of a rotating shaft.

SIGNAL OUTPUT

The strength of the signal generated, or pulse amplitude, depends on a number of factors, some dependent on the geometry of the installation and others on the design of the pickup. The most significant factors are:

- The speed at which the magnetic flux is interrupted or modulated. To a first-order approximation, the output amplitude is directly proportional to the speed or passage of ferrous objects past the pick off pole piece but is attenuated by coil and iron losses that predominate at high frequencies. The output of magnetic pickoffs continues to rise with speed to frequencies in excess of 10 kHz; in some cases, to beyond 15 kHz.

- The distance separating the transducer pole-piece and the moving ferrous object. To a first-order approximation, the output amplitude is inversely proportional to this distance. For very low- speed applications a separation of only 0.127 mm (0.005 in.) may be necessary, whereas for high-speed applications a separation of up to 2.540 mm (0.100 in.) or more may be satisfactory.

- The mass of metal modulating the flux. The relationship between the mass of iron passing the transducer pole piece and the output amplitude is complex and is, to some extent, related to the fourth factor. In general terms, and within the constraints of con figuration, the output is greater for a greater mass of moving iron.

- The configuration of the iron circuit. The design of mod em magnetic pickoffs is a compromise to achieve a high output volt age over a wide speed range in a small physical size. it's necessary that the active pole of the pickoff be small so that the sensing area is reduced to a level where the transducer may be used effectively with small gear wheels.

If the pole pieces are of circular section, then to generate maximum output it's desirable that one moving object should clear the active area before the next object enters it. When used with gear wheels, the best effects will be obtained if the tooth form is rectangular, the tooth width being equal to, or greater than, the pole- piece diameter, the gear width being at least twice the pole-piece diameter. The gap between teeth should also exceed the pole-piece diameter. However, the provision of these optimum conditions is seldom necessary in practice, and the use of standard dp or similar gear wheels usually gives entirely acceptable results. It should be borne in mind, however, that in installations where the gap between teeth is substantially less than the pole-piece diameter, the output of pickoffs will be well below optimum.

Diametrically split gear wheels should be avoided because they will almost certainly produce magnetic discontinuities that will affect performance.

LIMITATIONS OF MAGNETIC PICKUPS

The main limitation of a magnetic pickup is that there is a lower limit of speed monitoring where the output falls to a level where the signal-to-noise ratio is too poor to decode accurately. To a large extent this is also dependent on the sensitivity of the input circuits of the associated electronics. For low-speed measurement, there fore, pre-amplification and /or circuits with a sensitivity of not less than 1 V are usually required.

Another significant limitation is that it's difficult to provide a very high number of pulses per revolution without the use of large- diameter wheels having a large number of teeth, or step-up gearing to drive the wheel being monitored from the machine under investigation.

ROTARY DIGITIZERS

Rotary digitizers are another form of transducer generating electrical signal pulses when monitoring shaft rotation. They are much more sensitive than magnetic pickups and , in addition to measuring shaft rotational speed (working as pulse tachometers), can also accurately signal incremental shaft movements. In the latter application they are normally known as incremental shaft encoders, shaft angle encoders, or incremental pulse transducers.

In fact, there are two distinct forms of shaft encoders: the incremental encoder, and the absolute encoder. The incremental en coder or rotary pulse generator produces a train of electrical pulses resulting from angular rotation of the input shaft and interfaces with a digital counter to provide information giving the magnitude of angular rotation. The absolute encoder produces a number of electrical logic level outputs in parallel in binary form. it's a considerably more complex and expensive device.

An incremental or rotary pulse generator that produces a single train of pulses can be used for both shaft position and angular velocity measurements. It does not distinguish between different directions of angular movement. For that reason it's used only as a revolution counter. However, the addition of a second pulse train that changes its phase relation to the first pulse chain when the direction of rotation is reversed enables the transducer to detect direction of rotation via a suitable logic circuit. Such circuits can present this information in various ways, depending on actual requirements, and rotary digitizers of this type are known as dual- pulse rotary pulse generators (RPGs).

The following is a description of how they work, based on the industrial hardware developed and manufactured by Trump-Ross.

Early forms of the RPG included electrical commutator type devices that caused interruptions of electrical power. Magnetic principles were also utilized in a number of ways, some of which included variable inductance techniques and reed switches operated by permanent magnets.

When light-sensitive devices, such as photoelectric cells, became available, it became a simple matter to drill a number of holes in a circle and use these as shutters to interrupt the electric power. In recent years light-sensitive devices have become extremely efficient and compact, resulting in the miniaturization of the once ponderous RPG. Advances in photographic reproduction techniques have enabled the disks that are attached to the RPG shaft to carry extremely fine-line patterns.

Typical low-cost RPGs have line densities up to 2540 on a 2-in. diameter circle. More sophisticated and expensive encoders sometimes pack in as many as 30,000 lines on a 2- or 3-in, circle.

The obvious benefit of using the optoelectronic approach is the elimination of any form of electrical contact, thus enabling the RPG to provide high reliability and long product life. In addition, the lack of contact eliminates any possibility of radio-frequency noise generation and contact surface wear. The use of very high frequency response circuitry and the obviously very high speed of light itself enable the RPG to produce electrical pulses at frequencies approaching 1 million pulses per second.

THE DUAL-PULSE RPG

The more standard dual-channel RPG, which is designed to pro duce bidirectional-type pulse trains, must switch the incoming dc power in a form that is most acceptable to the logic circuit receiving the electrical signals.

A common electrical configuration is that which uses +5 Vdc as the power supply (this voltage level is normal to most logic systems), and the RPG switches the voltage level alternately from + 5 V to a level approaching 0 V. The duration of the switched-on power is normally one-half of an angular pulse increment. If we call the +5 V output level logic 1 and the voltage level approaching zero as logic 0, we can associate the output signal more readily in terms understandable to the logic designer.

The electrical pulse train has a conventional configuration where the logic 1 level has the same angular duration as the logic 0. The 1:1 ratio of these states is known as the symmetry of the signal and is a closely controlled parameter of an RPG. The number of switched cycles per revolution of the input shaft of the RPG is known as the line count and is interchangeable with the term resolution. The degree to which the occurrence of a discrete pulse coincides with the incremental angular displacement of the shaft is known as pulse accuracy. The term stability is the extent of repeatability, or consistency, of the signal configuration under actual operating conditions. For a dual-channel RPG the relation of the second pulse train to the first is of considerable importance. In order to provide the maximum discrimination by the logic system, which is detecting the direction of rotation, it's necessary to place the switching point (the leading or trailing edge of the square wave) at a position that equally divides the logic 1 or logic o states of the other channel, or one-quarter of a full square-wave cycle. This is known as 90 electrical degrees of phase shift, or Quadrature.

THE DISK and THE SIGNAL

The effective use of the rotary pulse generator does not depend on an intimate knowledge of the function of the device. For all practical purposes the system designer is only concerned with his two means of communication with the unit. The first, and quite obvious, requirement is to provide some form of mechanical connection to the input shaft in order to cause the shaft to rotate in proportion to the mechanical motion of the machine. The second, and equally obvious requirement, is to connect the electrical out put of the unit to the control black box.

Experience, however, indicates that a more precise knowledge of the internal functioning of the RPG will assist the systems designer, and the ultimate user, in a more satisfactory selection and a greater understanding of the application and handling of the device.

All of the Trump-Ross rotary pulse generators operate with the use of some form of light source that shines through a transparent disc. A series of radial lines is printed on one side of the transparent disc, producing a complete circle of alternately transparent and opaque sectors (ill. 11-3).

The light shining through the disk is optically corrected to pro duce a parallel beam. The technical term for light correction is collimation.

ill. 11-3. A pair of rotating disks can be used to measure angular speed.

Because most RPGs have a relatively high number of lines on the disc, it's generally quite difficult to shine the light through one slit only and get a satisfactory electrical signal from the light- sensitive device receiving the light. it's normal, therefore, to utilize some form of grating that, when placed close to the encoder disc, will cause the light to be shuttered through many lines. This larger amount of light can then be utilized to produce a substantial and reliable signal (ill. 11-4).

ill. 11-4. At A, a narrow-beam light source must be used with a single rotating disk. At B, the addition of a grating allows a wider beam to be used.

The distance between the grating and the disc should be as small as is physically possible in order to reduce the effects of poor collimation and stray light.

Conventional rotary pulse generators normally use tungsten- filament lamps as the light source. The filament is generally very small, essentially appearing as a point source of light. The tungsten lamp, when operated at a voltage level that is significantly less than the manufacturer’s rated voltage, has a useful life far in excess of the rated life of most RPGs. All Trump-Ross products have a built- in voltage reduction that assures a normal lamp life of 20 years or more.

For excessive shock and vibration, solid-state light-emitting diodes (LEDs) are used. The LED appears to have an almost in finite life when properly used. However, the lower energy output, coupled with sensitivity due to temperature variations and difficulties in collimating the light output, tends to present problems in matching the electrical performance of the tungsten-filament lamp.

The most commonly used light-sensitive detector is the silicon photodiode, or solar cell. The large area of the solar cell is uniquely suitable to the large area of light that can be transmitted through the disc and grating.

A further refinement of the RPG involves the use of push-pull- type signal generation. The principle of push-pull involves two sets of gratings positioned in such a way that light is passed through one grating while blocked in the other. This situation reverses as the disc is rotated through half a count cycle. The push-pull technique ensures that the geometrical form of the electrical signal does not change if there is a variation in the electrical power that energizes the signal source.

ELECTRONIC INTERFACE

High integrity is required from the electronic interface, the performance of which must be independent of variations of power- supply voltage, line resistance and capacitance, temperature changes, and other external factors that may tend to affect the electric signal generated by the RPG. Typically this is met by state-of- the-art solid-state circuitry.

SELECTING THE RIGHT RPG

The rotary pulse generator is an integral part of a widely varied range of digital control systems. Each application has its own peculiar set of conditions, and it's wise to match the physical and electrical configuration of the RPG to meet its particular environment. The instrument-type application requires an instrument-like RPG. The unit on a rolling mill, e.g., must be heavy in construction in order to conform to the general atmosphere of the application.

The physical interface of the RPG to the machine or the element on the machine to which the shaft is connected is extremely important in order to protect the device.

Proper alignment of the fixing holes and shafts and the use of very flexible, but angularly stiff, couplings are mandatory. Electrical connections should be made, by the proper use of rugged disconnects and armored conduit, insensitive to the roughest treatment.

The RPG user must be prepared to receive equipment into his plant that may be unfamiliar to receiving personnel. Many machine- tool and heavy-equipment manufacturers experience “failures” of RPGs that may be traced to gross mishandling by personnel who are more familiar with large machine components. The incoming inspection department of these companies can systematically ruin RPGs by improper testing procedures.

Rotary pulse generators, like any other device, have a limited life. The life expectancy may be determined by the life of the power source, such as the lamp filament; or, on the other hand, the mechanical life may be the limiting factor, particularly if the shaft of the RPG is too heavily loaded.

The user must satisfy himself that he is buying a product that has a normal life expectancy in excess of five times the required life.

There is no substitute for experience. Life expectancy can best be judged by the history and experience that the RPG supplier can provide, plus the experience of the other users of the product.

Rotary pulse generators are frequently quite sophisticated and represents a complex electromechanical package. With the advent of solid-state devices, the resulting miniaturization allows the manufacturer to package much of the amplification and switching circuitry within the unit. The net result is a highly desirable package, which, unfortunately, has a higher potential for failure.

The occasional failure of an RPG necessitates, in most cases, some repair or replacement of the unit. Repair work is rarely possible in the field, and the unit must then be returned to the manufacturer. The speed with which the manufacturer can turn the unit around will materially affect the machine’s down time.

Most manufacturers will warrant their product for one year and will be able to present plenty of evidence to show that their product should have a long life expectancy. The user would be foolish, however, to rely blindly on the reliability of the unit, particularly if the device is used in a critical application.

Installation of RPGs without a reasonable backup quantity of spare units is very shortsighted. The economics of such a policy can look very bleak when a highly complex, large machine is down for the want of a transistor.

(We gratefully acknowledge Trump-Ross for extensive quotations from their literature on the subject of RPGs.)

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Fundamentals of Transducers (all articles)