Inertia Detectors

Inertia detectors sense disturbances and vibrations, but are particularly sensitive to low frequencies. Thus a gentle prising or levering of a structural member, or the motion generated by climbing a perimeter fence which may be ignored by a vibration or impact sensor, will trigger an inertia detector. As it's name implies, the mobile component has a high mass to give it inertia, so what when the environment is disturbed, the housing moves but the component does not. Contacts are thereby broken, thus activating the inertia detector alarm.

In one inertia detector model, the component is a gold-plated ball seated on a pair of contacts, so forming a normally closed switch. Displacement of the ball from either contact breaks the circuit. A high-security version inertia detector has in addition a closely spaced ring around the ball. This serves as a normally open switch, so that any disturbance causes the ball to touch the ring as well as break contact with its supporting electrodes.

The maximum current that can be switched by this inertia detector is low because the area of the ball actually in contact with it supports is minute. A large current passed through such a small area could cause pitting and eve welding to occur. The specified current is 0.2 mA, with an applied voltage not exceeding 2 volts. This is much lower than the 500 mA or so that the vibration sensor with its much larger contact area will permit. It is also lower than the current applied by many control units to the loop circuit. It thus needs its own control unit, which must not exceed the maximum current, and must process the result to distinguish between false alarms and intrusion.

With another inertia detector model, a lever having a weight at its free end is supported by a gold-plated rod which rests on a contact piece. An angled extension of the lever is terminated by another gold-plated rod that rests against the under-surface of a second contact piece above it. The lever and rods are held against the terminal pieces by the downward pull of the weight, but any relative movement between weight and housing causes loss of contact.

This inertia detector requires an analyzer to control the current, set the sensitivity and sort out the true from the false alarms. It reacts if there is a series of small shocks, several medium shocks, or one large one.

With other inertia detectors the response can be pre-set to a given number of impulses in a specified time. Below this, the alarm is not triggered, so random shocks against a perimeter fence by children playing, for example, have no effect.