Vibetek cable

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Vibetek Cable

All Vibetek cables utilise the piezoelectric effect, discovered in 1880 by the famous French physicist Pierre Curie and his brother Jacques. Piezoelectric materials have the interesting property of converting mechanical energy into electrical energy and vice versa. Most piezoelectric materials are hard and brittle; examples are crystal quartz and certain ceramics such as lead zirconate titanate (PZT). However, in the 1970s it was discovered that certain thermoplastics could be processed to make them piezoelectric. This meant that flexible piezoelectric sheet, film and cables could be made, so paving the way for the development of the Vibetek range of microphonic sensor cables in the 1980s. Vibetek cables are coaxial in configuration and use polyvinylidene fluoride (PVDF) as the dielectric between the inner core and the outer braiding. The cable effectively behaves as an extended microphone, converting stress, strain, vibration, impact, sound or pressure change into minute electrical signals.

As with all piezoelectric sensors, Vibetek displays a linearly increasing electrical output as the level of mechanical disturbance increases, and will monitor signals ranging in frequency from a few mHz to hundreds of kHz or more.

Picture of Vibetek cable cross section

Vibetek Sensor Cable Specification

Picture of Vibetek cable specification

Interface Circuits

A Vibetek sensor cable can be modeled as a voltage source in series with a capacitance, where the capacitance represents the total capacitance of the Vibetek cable. This means that Vibetek cables have a very high source impedance, and care needs to be taken when designing interface amplifier/buffer circuits. For example, a 1 metre length of Vibetek 10, with a capacitance of 700pF, has an effective output impedance of 22M at a frequency of 10Hz. This clearly requires the input impedance of the interface circuit to be high also. A low noise, non inverting JFET voltage amplifier is the simplest interface option, using, for example, a TL071 op-amp.

Picture of Vibetek equivalent circuit

When using non inverting op-amps with capacitive sources, R3 is necessary to provide a DC bleed path to earth. This should be a high resistance, ideally >10M. It is recommended, particularly when using Vibetek 10, that transient protection be provided for interface circuits, using, eg, back to back diodes. This will prevent damage by the high voltage spikes that can be generated when the cable experiences an impact. A charge amplifier can also provide a suitable interface for use with Vibetek, but care needs to be taken with the design of this type of circuit if it is to be used at very low frequencies.

Picture of Vibetek op-amp circuit

Vibetek Applications

Vibetek has many practical applications, including:

Vibetek in Security

Vibetek cable has been in continuous use in the perimeter security industry for many years - mainly as a microphonic sensor on perimeter fences. The Vibetek cable is simply attached to the fence, using cable ties, and effectively converts the entire fence into a microphone that “listens” for any sounds that may be related to intrusion events. An electronic signal analyser continuously monitors the the sensor cable and automatically discriminates between environmentally generated noises, caused by wind and rain etc, and signals caused by attempts to breach the fence by, for example, someone climbing over or cutting the fence fabric. When the latter is detected, the analyser signals the event to the central alarm system. It is usually possible to confirm the alarm event by “listening in” to the audio signals

There are many reasons why Vibetek sensor cables have been so successful in perimeter security:

Picture of security fence Picture of security fence

Vibetek as an Acoustic Transducer

Vibetek is an excellent acoustic transducer; this is why it has been so successful in perimeter security applications. However, it has also found applications as a high fidelity pickup for musical instruments, including pianos and acoustic guitars.

Picture of guitar pickup Picture of piano pickup

Vibetek Linearity

The output of Vibetek, like that of all piezoelectric sensors, is directly proportional to the applied stress or strain, over a wide range of pressures and deformations. Vibetek is unaffected by being permanently subjected to high pressure, ie it does not exhibit pressure ageing. This means that it will continue to monitor changes in stress or strain even whilst it has a heavy object resting on it, or it is being subjected to large hydrostatic pressures by immersion in a liquid.

The graph below shows the output voltage of Vibetek 3 when subjected to increasing hydrostatic pressure, demonstrating the excellent linearity of the cables. (Vibetek 5 and Vibetek 10 show similar linearity)

Vibetek 3 Variation in Output with Applied Hydrostatic Pressure

Picture of Vibetek linerarity graph axis Picture of Vibetek linerarity graph
      Hydrostatic Pressure in MPa

Vibetek Jacket Material

With many years of deployment in a number of different applications and locations, the environmental performance of Vibetek is proven. The key to its longevity is its outer jacket, a special black polyolefin material with outstanding weathering resistance. It is the same material that covers outdoor power and data cables and many outdoor fibre optic cables. The Vibetek jacket is waterproof and immune to degradation by UV light; it is specifically designed for outdoor use, either exposed to the elements or buried in the ground. Standard weatherproof cable ties can be used to attach Vibetek to a fence; in this case a tie wrap tool will considerably speed up installation. (If the cable is directly buried in the ground, care should be taken to avoid stones etc causing mechanical damage to the jacket. This can be avoided by surrounding the cable with a layer of sand.)

In a perimeter security system, where the cable is attached to a fence and there is a high risk of the cable being deliberately cut, the use of conduit should be considered. The conduit can either be the standard rigid electrical type or made of flexible stainless steel. Either will give the cable considerable protection against malicious damage. Ormal Electronics supplies all standard grades of Vibetek cable pre-installed in 7mm OD flexible stainless steel conduit, ready for installation. This is delivered on reels ready for use. Click on the picture below for more information about this product.

Picture of stainless steel conduit

Variation in Sensitivity of Vibetek Cable

The polarisation process that is used to give Vibetek cable its piezoelectric activity produces a cable that has minimal variation in sensitivity over cable length. This is clearly important where the cable is to be used as a linear microphone in such applications as perimeter security. The graph below shows the typical variation in hydrostatic sensitivity (d3h) along a 300 metre length of Vibetek 3. The data were obtained by cutting up a continuous 300 metre length of cable into 50 metre pieces and measuring the d3h of the outer end of each piece. The variation in sensitivity over such lengths is typically better than + 5%.

Picture of Vibetek polarisation graph axis Picture of Vibetek polarisation graph
      Distance Along Cable in Metres

Temperature Effects

The sensitivity of Vibetek shows very little temperature dependence over the range -40°C to +65°C - see the graph below. The cables have been in use Worldwide for many years, in locations ranging from the Tropics to the Arctic. However, like all piezoelectric materials, the piezoelectric properties can be destroyed by excessive temperature. With Vibetek, care should be taken to avoid heating the cable for long periods above about 70°C, as there will be some permanent loss of sensitivity at these temperatures. For example, there is typically a loss of about 15% in the hydrostatic sensitivity of Vibetek 3 after a cable has been heated at 80°C for 1 hour, after which the sensitivity stabilises at its new value. Further temperature increase will produce more sensitivity loss, until eventually all of the piezoelectric output is lost.

At low temperatures (< 0°C) a slight temporary increase in sensitivity is observed. No permanent damage will be caused by temperature excursions below -40°C, although the jacket will have stiffened considerably and may be susceptible to mechanical damage.

Picture of Vibetek temperature graph