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Shock Sensors: A New Approach to Accuracy?

Shock sensors represent a valuable tool in intrusion detection systems, as well as in a host of value-added smart solutions. Most alarm-based systems can be split into two parts: protection of the interior of sites and protection of the perimeter. While many consider the perimeter as the very border of a protected space, such as a fence or gate, the perimeter of buildings is a vital consideration when dealing with early detection of intruders. With new standards and more efficient technologies, is the shock sensor about to become more valued by system integrators and end users?

Detecting intrusion in the interior of a building has traditionally been the preferred method of detection for security alarm systems, predominantly because of standards and regulations relating to police first response. The interior of a secured and closed building is usually a controlled environment: it is effectively a sterile zone when the alarm system is set. No motion is expected, and doors and windows should not be opened.

Traditionally with intruder detection systems, the interior of a building or structure is protected by PIRs, dual tech space detectors and internal beam-based sensors. While predominantly used for building perimeter protection, contacts on doors and windows sit between the perimeter and interior, for one simple reason.

It should be remembered that while contacts are commonly used to detect unauthorised opening of external doors and windows in order to trigger an alarm, the intruder must actually manage to open the door or window. While this does deliver an early warning of intrusion, often before the criminal has gained entry to the premises, it can often result in damage being caused, which equates to a cost for the end user.

Perimeter based detection devices include external PIRs and dual technology units, beam-based devices and shock sensors. The latter can also be used for internal detection, such as on safes, secure storage areas, high risk spaces (such as doors of cash-handling areas, etc.), but when used on external doors and windows can deliver the earliest warning of an attempted intrusion, often earlier than a contact device and usually before significant damage is caused by an intruder.

As such, in terms of cost savings, shock sensors represent a better return on investment than simple contacts. Shock sensors can detect the initial attempt to break through or force open a door or window, and are ideal for locations likely to be targeted for attack, such as ground-level doors, windows on flat roofs, skylights, etc.. Most shock sensors can also include a contact detector, so it’s always best to use combined shock and contact units on vulnerable doors and windows.

The reality is that many integrators don’t use them often enough, because the risk of nuisance activations from innocuous vibrations or non-threatening impacts is too high. This isn’t to say that shock sensors are (or were) unreliable, but the technology used in the past did have certain limitations.

As a result, many sites miss out on an important part of perimeter protection because system designers consider shock sensors to be best suited to the protection of secure assets inside the building.

However, following developments in other technology sectors, the security sector can now leverage technologies which advance the accuracy and performance of shock sensors. These advances, combined with new standards, should allow a wider use of shock sensors, adding greater protection to many intruder detection systems.

Technology issues

Historically, shock detectors made use of piezoelectric sensors. The word piezo is taken from the Greek for pressure, which gives a clue as to how the technology works. In the 19th century, it was discovered that pressure generated electrical charges in some crystals including quartz and tourmaline. This is referred to as the piezoelectric effect. It is also the case that electrical fields can cause deformation in piezoelectric material, which is the inverse piezoelectric effect.

The creation of piezoelectric ceramics allowed economies of scale to be introduced, which paved the way for wider uses of the technology across a range of industries. One of the more prominent uses of the technology has been in the manufacturing of speakers, where electrical signals are converted into vibrations to generate sound. Piezoelectric sensors are also commonly used in process control, manufacturing and processing, heavy industry, etc..

The security industry introduced shock sensors using piezo technology to replace older displacement technologies, which were unreliable and costly to implement. Piezo technology has been in use for a number of years, and for some time it represented the best option for the detection of shocks, impacts and vibrations.

However, recently accelerometer-based sensing technology has become more widely available, allowing the design and manufacture of shock sensors which are more reliable and efficient, and which also add a number of benefits when it comes to detector location and use.

Accelerometers are capable of delivering higher degrees of accuracy and efficiency when compared with piezoelectric technology. An accelerometer is an electromechanical device used to sense movement or vibrations. The use of accelerometers crosses multiple disciplines in the industrial, commercial and consumer markets.

For example, accelerometers are widely used to protect hard drives in a range of IT-based devices and applications. If a PC or server is subjected to impact, excessive vibrations or a fall, accelerometers detect the threat and immediately switch off the hard drive to avoid damaging the reading heads or the hard drive platter.

Accelerometers are also used in cars to deploy airbags when an impact occurs, and in smartphones to rotate the display depending on how the phone is held.

Advances in the use of accelerometers have created significant economies of scale, and this enables security manufacturers to increasingly include the devices in advanced shock and vibration sensors.

Accelerometers can be configured to be highly sensitive to sub-surface shock transmissions, which are created when an intruder attempts a forced entry. Despite having a high degree of sensitivity to impacts and vibrations, accelerometer-based devices are not affected by airborne high frequency noise, which has proved problematic with piezo technology in the past.

High frequency noise has meant some applications have been unsuitable for the deployment of shock sensors due to environmental conditions, but the problem is eliminated with shock sensors which make use of accelerometers.

When deploying traditional shock sensors, there are often limitations as to where they can be installed, with an important emphasis on device orientation. Again, this can mean that some uses are either limited or not possible.

Accelerometers can detect in three dimensions, which allows them to be mounted in different ways. It also expands the possibilities for use, making them suitable for a range of added-value implementations which might be outside of traditional security and safety.

New standards

In order to ensure shock sensors deployed as part of security systems are accurate, reliable and effective, a new standard has been created which dictates the performance of detectors in graded alarm systems. It is vital that integrators offering systems which require a police first response or adherence to the standards use sensors which meet the new regulations.

The EN50131-2-8 European Standard for vibration detectors mandates stringent performance criteria across a wide range of surface materials. As building practices evolve, so the importance for sensors to work across a wide range of substrates becomes increasingly important.

Because shock sensors will be used on a wide range of materials such as glazed windows, wood, metal and concrete, it is essential the right level of sensitivity can be selected. The new standard covers this and ensures that compliant shock sensors are capable of delivering protection in a wide range of installation scenarios. It is vital appropriate shock sensors are used, as non-compliant devices may lead to a system no longer being recognised as a graded solution.

In summary

Shock sensors offer a credible level of protection. New technology, coupled with the new standards, ensure the right level of protection can be achieved.

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