While it is tempting to think of wire as an ideal conductor that simply gets a signal from point A to point B without alteration, this is often not the case – particularly for long cable runs. In the real world, appreciable length of field wiring will introduce its own effects, meaning that the signal produced at the sensor can be quite different from the signal received at the monitoring system. To assist you in this important topic, we have recently produced a completely revised Application Note that provides comprehensive guidance on the effects of field wiring and how to mitigate them. In this article, we give you a sneak preview of what you will find in the full, Application Note which contains extensive examples with values from real cable. In addition to the Application Note, the download includes a cable length “calculator” – an Excel® spreadsheet that allows you to enter relevant cable parameters and solve for length, cut-off frequency, and more. Combined, the Application Note arms you with the background you need to understand the calculations and principles involved, while the calculator streamlines the calculations and allows you to focus on the results rather than the tedium of manually solving for the unknown values.
In an ideal world, the cable connecting a sensor to its associated monitoring system would look as depicted in the below figure – a perfectly lossless conductor that would not alter the signal whatsoever.
Ideal field wiring (top) has no resistance, inductance, or capacitance. Real field wiring (bottom) exhibits distributed resistance, inductance, and capacitance as shown.
In the real world, however, the cable exhibits, resistance, inductance, and capacitance and must therefore be modeled as depicted in the above figure. These values for R, L, and C are typically expressed on a per-meter or per-kilometer basis when SI units are used.
These distributed parameters act to alter and distort the sensor signal in four undesirable ways:
The distributed cable parameters form a low-pass filter that allows low frequencies to pass with little or no attenuation but effectively blocks high-frequency components of the signal from passing. This results in an altered waveform that may be unsuitable for conducting proper diagnostics. Indeed, the signal may be so degraded that it is unsuitable not just for diagnostics, but protection as well.
For speed measurements, the signal is typically a rectangular pulse train but distorts due to the impact of wiring capacitance on the so-called slew-rate. The original pulse train shape has abrupt vertical changes because the speed sensor is often observing a toothed surface, a key, a keyway, or some other physical discontinuity in the surface observed by the sensor. If there were no limitations on slew rate, such vertical discontinuities could be reproduced. In real wiring, however, the capacitance limits the ability for instantaneous changes in signal voltage and the crisp, rectangular pulse train begins to instead resemble a series of smoothed pulses where the abrupt edges are rounded. While slew-rate rarely affects vibration and dynamic pressure signals, it can frequently affect speed signals.
Cable resistance forms a voltage divider that drops some of the original signal voltage produced by the sensor across the wiring instead of entirely across the input impedance of the monitor. This has the undesirable effect of decreasing the signal-to-noise ratio.
IEPE (Integrated Electronic Piezo-Electric) devices are vulnerable to an effect that creates asymmetry in the waveform when there is insufficient margin between the source of constant current (usually the monitoring system) and the minimum constant current required by the device (typically around 4mA). This asymmetry is essentially a slew-rate effect that affects only the positive portion of the signal. It arises due to cable capacitance and the phase difference it creates between current and voltage, allowing sufficient current during the negative part of the waveform but not the positive.
Depending on how the signal is transmitted from the sensor to the monitor, the effects just mentioned can be mitigated to some degree.
When the signal is transmitted as a varying voltage, we refer to this as voltage-mode signal transmission. Although voltage-mode is considered the conventional method for transmitting vibration signals, and is commonplace, it is unfortunately the most vulnerable to the four effects discussed above. As such, voltage-mode transmission tends to constrain the length of field wiring more than alternative modes. Many manufacturers, however, do not support anything other than voltage-mode transmission with their sensors.
When the signal is instead transmitted as a varying current (instead of a varying voltage), we refer to this as current-mode signal transmission. Vibro-meter pioneered this mode of signal transmission because it can mitigate many of the effects discussed above and allow much longer field wiring lengths than with conventional voltage-mode transmission. Most of our measurement chains, whether using integral signal conditioning (such as many of our CE-series accelerometers) or external signal conditioning (such as via our IPC 707 and IQS900 devices) support the option for either voltage-mode or current-mode transmission, ensuring that lengthy cable runs can be addressed more flexibly than if constrained to only voltage-mode operation.
There are selected situations in which neither voltage-mode nor current-mode transmission are adequate over long distances. One notable example is in applications where the cable is literally underwater – such as submerged vertical pumps. By using frequency modulation to transmit the signal and then demodulating at the monitor, longer field wiring distances can be supported than by other means. This can be particularly useful for transmission of speed (tachometer) and phase signals. It can also be useful for quasi-static signals where a conventional 4-20mA scheme proves inadequate.
While many readers are aware of galvanic isolation as an alternative to Zener barriers for achieving intrinsic safety in hazardous area installations, few may be aware that vibro-meter’s GSI (Galvanic Separation Instrument) technology offers other benefits and can be used even when not addressing a hazardous area. Devices such as our GSI 127 (top) and its forerunners not only help eliminate ground loops, they provide lower impedances that translate to longer wiring distances even when using voltage-mode transmission.
Vibro-meter has developed, perfected, and incorporated special algorithms into many of its monitoring platforms that automatically adjust threshold and hysteresis for speed and phase measurements, allowing even heavily degraded pulse train signals to provide accurate speed and phase measurements. These capabilities mean that the necessity to preserve waveform shape is less critical if the fundamental pulse train frequency itself can be preserved – even if smoothed rather than crisp and rectangular. This allows speed signals to be provided over much longer field wiring distances than would otherwise be practical.
While signal degradation is always a constraint due to field wiring length, there is another constraint that occurs when the installation will be in a hazardous area and intrinsically safe (I.S.) methods will be used to achieve compliance: stored energy. Both cable inductance and cable capacitance have the effect of storing energy. When a fault in the wiring or device occurs, I.S. constrains a resulting spark from releasing sufficient energy to ignite the surrounding flammable atmosphere. It thus does not prevent a spark – it simply limits the energy in the spark. Consequently, approvals for intrinsic safety provide strict limits on the total amount of inductance and capacitance that can exist in the hazardous area, and thus the amount of energy that can be stored and released as a spark. When one thinks of the capacitance and inductance values as a “budget”, a certain amount of this budget is consumed by the measurement chain components (sensor, interconnect cable, signal conditioner) leaving the remainder of this budget for the field wiring. Although these effects are not discussed in the subject Application Note, they are discussed in other vibro-meter publications. Users should thus be aware that signal degradation is not the only constraint that exists on field wiring length when the installation involves a hazardous area and I.S. wiring. Indeed, in such installations, the user must compute the wiring length constraint under both signal degradation and stored energy criteria and then use the more restrictive of the two separate constraints.
The considerations introduced in this article are discussed in substantial depth in our newly revised and expanded Application Note. Equations are provided that model the effects summarized here and examples are provided using typical vibro-meter devices and field wiring types. As a result, users will be equipped to compute maximum field wiring length given a particular cut-off frequency, to compute the maximum frequency transmitted without attenuation given a particular field wiring length, to understand and quantify the advantages of current-mode transmission versus voltage-mode transmission, to understand and quantify the advantages of our GSI technology, and to understand numerous other aspects of field wiring length constraints and potential options available to mitigate some of these constraints. In addition to this Application Note, we are pleased to provide a highly useful Field Wiring Calculator – an Excel® spreadsheet with a simple and intuitive user interface that allows the user to easily make calculations related to field wiring length.