What Are Capacitive Field Effect Sensors And How Do They Work?
Fences protecting key infrastructure are ideal for capacitive field effect solutions.
Capacitive field effect sensor perimeter solutions operate on the basis of tuned circuits which create resonant frequencies that are broken by intruders to trigger alarm events. Such sensors can be reliable, low draw, economical to install and highly accurate with well designed zoning.
CAPACITIVE field effect electronic perimeter intrusion systems function on the basis of a property called electrical resonance. The way capacitive field effect sensors work is based on the fact that if a capacitor and an inductor wired in parallel have a voltage applied then current will flow through the circuit.
The current will be the sum of the current flowing through both capacitor and inductor and if the current’s frequency in the circuit is raised the current in the inductor will fall until, at a particular frequency, they will be identical. This is the resonant frequency and when it’s attained you get waves of current surging between inductor and capacitor in a process of storing and discharging with very little additional current coming from the supply.
Such an oscillating current in such a circuit can be larger than the current such a circuit would ordinarily draw from its power supply and this means that it’s possible to sense a change in inductance or capacitance in the circuit which causes a loss of this resonance balance and leads to a immediate draw of current from the circuit’s supply. A capacitive field effect sensor will detect this current draw change when an intruder moves in a capacitive field or touches part of the circuit’s wiring.
Typically, in a capacitive perimeter security solution the capacitor will be the conductive structure of what is being protected – like a chain link fence – or it might be a layer of air between a pair of electrodes. Meanwhile, the inductor forms part of the analyser circuit wired to the protected structure. The 3 types of capacitive sensors include capacitive relays, capacitive effect cables and capacitive field effect sensors.
The first of these and the least commonly used are capacitive relays. These depend on the property of an object being protected to conduct electricity and become part of the circuit. Typically, these highly sensitive capacitive relays are applied to safes, cabinets and metal doors. Such circuits face challenges, including the fact they are prone to all manner of interference from their surrounds, including static, EMI and changing internal resistance. All these issues can make poorly designed capacitive relays highly unstable.
Meanwhile, capacitive effect cables are very basic capacitive static electricity creating cables that are generally installed in strands on top of chain link fences or masonry walls. Each wire has a low voltage AC signal with a frequency of around 3kHz. This signal is not strong enough to generate an air-field capacitor, but it does propagate a measurable capacitance into the air between wires and grounded struts on which the wires are strung. Total capacitance of this circuit is the combination of a pair of capacitive cables of the same length and the air capacitance with the cable being dominant. Any intruder touching the cable alters capacitance and trips an alarm circuit.
Some capacitive field effect sensors employ the ability of air to become a capacitor by storing a charge. In this case the field is a pattern of static electricity distribution created when a capacitor is formed using air as the dielectric between 2 electrodes.
The way such a sensor is set up, a fence is fitted with horizontal wires (there will be 3 wires in higher quality systems) on struts that form continuous electrodes if supplied with small currents. In simple systems single wires can be used to form a positive electrode and the grounded fence structure can be used to create a potential difference across the air gap that separates them. When 3 wires are used, one strand is positive, one ground and one is the sensing wire. It’s also possible for 4 wires to be used in a system to offer a higher level of protection.
When tuned properly, there’s virtually no mains current draw and when the air capacitor field is disturbed by a moving intruder, it stops resonating and a larger than normal current will be drawn. Depending on value and time thresholds of this current draw, the controller will decide what sort of an intrusion the system is experiencing.
A good system that’s well tuned for a site can establish whether or not a intrusion-based change took place all in the one direction, it can work out the speed of the change and the size of the intruder – a bird, a bilby or a human being.
Good systems use 3 or more band bass filters – a lower frequency filter to ignore blown leaves or rubbish, a higher frequency filter to take out wind vibration in wires and a third filter to take care of lightning strikes. Typically, a system will alarm if frequencies detected at the controller have an amplitude typical of a human being-sized object, shows a frequency consistent with an object loitering or moving around or crawling in the detection zone, or a signal change that continues over a long period of time.
Quality capacitive field solutions are not only reliable but they have excellent detection rates between 98-100 per cent and can be installed in zones – they are among the best sensors available for higher security fence applications and from an economy point of view on larger sites.
Integrators considered such applications should ensure fences supporting them are of sufficient height and that these fences cannot be easily dug or crawled under. That means anchoring, burying or embedding footings and fixings in concrete. Important, too, is that the fence be taut, and not flex due to wind or the opening of gates. You’ll also need a clear area on both sides of the fence of about half a metre, which is more than achievable on most sites. This clear ground should be devoid of plant growth as it may cause false alarms.