11 Display, recording and presentation of measurement data

 11.2.2 Ultra-violet recorders

The earlier discussion about galvanometric recorders concluded that restrictions on how far the system moment of inertia and spring constants can be reduced limited the maximum bandwidth to about 100 Hz. Ultra-violet recorders work on very similar principles to standard galvanometric chart recorders, but achieve a very significant reduction in system inertia and spring constants by mounting a narrow mirror rather than a pen system on the moving coil. This mirror reflects a beam of ultra-violet light onto ultra-violet sensitive paper. It is usual to find several of these mirror-galvanometer systems mounted in parallel within one instrument to provide a multi-channel recording capability, as illustrated in Figure 11.10. This arrangement enables signals at frequencies up to 13 kHz to be recorded with a typical inaccuracy of ±2% f.s. Whilst it is possible to obtain satisfactory permanent signal recordings by this method, special precautions are necessary to protect the ultra-violet-sensitive paper from light before use and to spray a fixing lacquer on it after recording. Such instruments must also be handled with extreme care, because the mirror galvanometers and their delicate mounting systems are easily damaged by relatively small shocks. In addition, ultraviolet recorders are significantly more expensive than standard chart recorders.

11.2.3 Fibre-optic recorders (recording oscilloscopes)

The fibre optic recorder uses a fibre-optic system to direct light onto light-sensitive paper. Fibre-optic recorders are similar to oscilloscopes in construction, insofar as they have an electron gun and focusing system that directs a stream of electrons onto one point on a fluorescent screen, and for this reason they are alternatively known as recording oscilloscopes. The screen is usually a long thin one instead of the square type found in an oscilloscope and only one set of deflection plates is provided. The signal to be recorded is applied to the deflection plates and the movement of the focused spot of electrons on the screen is proportional to the signal amplitude. A narrow strip of fibre optics in contact with the fluorescent screen transmits the motion of the spot to photosensitive paper held in close proximity to the other end of the fibre-optic strip. By driving the photosensitive paper at a constant speed past the fibre-optic strip, a time history of the measured signal is obtained. Such recorders are much more expensive than ultra-violet recorders but have an even higher bandwidth up to 1 MHz.

Whilst the construction above is the more common in fibre-optic recorders, a second type also exists that uses a conventional square screen instead of a long thin one. This has a square faceplate attached to the screen housing a square array of fibre-optics. The other side of the fibre-optic system is in contact with chart paper. The effect of this is to provide a hard copy of the typical form of display obtainable on a cathode ray oscilloscope.

 

11.2.4 Hybrid chart recorders

Hybrid chart recorders represent the latest generation of chart recorder and basically consist of a potentiometric chart recorder with an added microprocessor. The microprocessor provides for selection of range and chart speed, and also allows specification of alarm modes and levels to detect when measured variables go outside acceptable limits. Additional information can also be printed on charts, such as names, times and dates of variables recorded. Microprocessor-based, hybrid versions of circular chart recorders also now exist.

 

11.2.5 Magnetic tape recorders

Magnetic tape recorders can record analogue signals up to 80 kHz in frequency. As the speed of the tape transport can be switched between several values, signals can be recorded at high speed and replayed at a lower speed. Such time scaling of the recorded information allows a hard copy of the signal behaviour to be obtained from instruments such as ultra-violet and galvanometric recorders whose bandwidth is insufficient to allow direct signal recording. A 200 Hz signal cannot be recorded directly on a chart recorder, but if it is recorded on a magnetic tape recorder running at high speed and then replayed at a speed ten times lower, its frequency will be time scaled to 20 Hz which can be recorded on a chart recorder. Instrumentation tape recorders typically have between four and ten channels, allowing many signals to be recorded simultaneously.

The two basic types of analogue tape recording technique are direct recording and frequency-modulated recording. Direct recording offers the best data bandwidth but the accuracy of signal amplitude recording is quite poor, and this seriously limits the usefulness of this technique in most applications. The frequency-modulated technique offers better amplitude-recording accuracy, with a typical inaccuracy of ±5% at signal frequencies of 80 kHz. In consequence, this technique is very much more common than direct recording.

 

11.2.6 Digital recorders

For some time, the only technique available for recording signals at frequencies higher than 80 kHz has been to use a digital processor. As the signals to be recorded are usually in analogue form, a prerequisite for this is an analogue-to-digital (A–D) converter board to sample the analogue signals and convert them to digital form. The relevant aspects of computer hardware necessary to achieve this were covered in Chapter 9. Correct choice of the sampling interval is always necessary to ensure that an accurate digital record of the signal is obtained and problems of aliasing etc. are not encountered, as explained in Chapter 5. Some prior analogue signal conditioning may also be required in some circumstances, again as mentioned in Chapter 5.

Until a few years ago, the process of recording data digitally was carried out by standard computer equipment equipped with the necessary analogue interface boards etc., and the process was known as data-logging. More recently, purpose-designed digital recorders have become available for this purpose. These are usually multi-channel, and are available from many suppliers. Typically, a 10-bit A–D converter is used, which gives a 0.1% measurement resolution. Alternatively, a 12-bit converter gives 0.025% resolution. Specifications typically quoted for digital recorders are frequency response of 25 kHz, maximum sampling frequency of 200 MHz and data storage up to 4000 data points per channel.

When there is a requirement to view recorded data, for instance to look at the behaviour of parameters in a production process immediately before a fault has occurred in the process, it is usually necessary to use the digital recorder in conjunction with a chart recorder*, applying speed scaling as appropriate to allow for the difference in frequency-response capability between a digital recorder and a chart recorder. However, in these circumstances, it is only necessary to use the chart recorder to display the process parameters for the time period of interest. This saves a large amount of paper compared with the alternative of running the chart recorder continuously if a digital recorder is not used as the main data-capture mechanism.

As an alternative to chart recorders when hard-copy records are required, numerical data can be readily output from digital recorders onto alphanumeric digital printers in the form of dot-matrix, inkjet or laser printing devices. However, when there are trends in data, the graphical display of the time history of a variable provided by a chart recorder shows up the trends much more readily.

As an alternative to hard-copy displays of measured variables when there is a need to view their behaviour over a particular time period, there is an increasing trend to use a computer monitor to display the variables graphically. Digital recorders with this kind of display facility are frequently known as paperless recorders.

 

11.2.7 Storage oscilloscopes

Storage oscilloscopes exist in both analogue and digital forms, although the latter is now much more common. An analogue storage oscilloscope is a conventional oscilloscope that has a special phosphorescent coating on its screen that allows it to ‘hold’ a trace for up to one hour. This can be photographed if a permanent record of the measured signal is required.

The digital storage oscilloscope, commonly referred to simply as a digital oscilloscope, is merely a conventional analogue oscilloscope that has digital storage capabilities. The components of a digital oscilloscope are illustrated in Figure 11.11. The input analogue measurement signal is sampled at a high frequency, converted to digital form by an analogue-to-digital converter, and stored in memory as a record of the amplitude/time history of the measured signal. Subsequently, the digital signal is converted back into an analogue signal that has the same amplitude/time characteristics as the original signal, and this is applied to the xy deflector plates of the analogue part of the oscilloscope at a frequency that is sufficient to ensure that the display on the screen is refreshed without inducing ‘flicker’. One difference compared with a normal analogue oscilloscope is that the output display consists of a sequence of dots rather than a continuous trace. The density of the dots depends partly on the sampling frequency of the input signal and partly on the rate at which the digitized signal is converted back into analogue form. However, when used to measure signals in the medium-frequency range, the dot density is high enough to give the display a pseudo-continuous appearance.

Digital oscilloscopes generally offer a higher level of performance than analogue versions, as well as having the ability to either display a measurement signal in real time or else store it for future display. However, there are also additional advantages. The digitization of the measured signal means that it is possible for the instrument to compute many waveform parameters such as minimum and maximum values, r.m.s. values, mean values, rise time and signal frequency. Such parameter values can be presented on the oscilloscope screen on demand. Also, digital oscilloscopes can record transient signals when used in single-sweep mode. This task is very difficult when using analogue oscilloscopes because of the difficulties in achieving the necessary synchronization. If permanent, hard-copy records of signals are required, digital oscilloscopes usually have analogue output terminals that permit stored signals to be transferred into a chart recorder.