Vibration sensors (sometimes called shock sensors) have to be chosen for each application and type of vibration. Rotational vibration is very different to axial vibration (along a single plane) and with the latter, the sensor will need to be correctly aligned.
It's worth looking at the different types of vibration sensors available before we look at choosing one for each application:
Accelerometers measure the vibration, or acceleration of motion of a structure. A transducer converts mechanical force caused by vibration or a change in motion, into an electrical current using the piezoelectric effect. There are two types, high impedance and low impedance.
High impedance accelerometers produce an electrical charge which is connected directly to the measurement instruments.
Low impedance accelerometers have a charge accelerometer and a built-in circuit that converts the charge into a low impedance voltage. This type is more easily interfaced with standard instrumentation and is more commonly used in the industry.
A strain gauge measures the strain on a machine component. The sensor resistance varies with applied force, so it converts force, pressure, tension or weight into a change in electrical resistance which can then be measured. When there is a strain applied to a metallic wire, the length of that wire increases and the diameter decreases. This will change the resistance of the wire which will allow measurement of strain on our machine component.
Eddy-Current sensors are non-contact devices that measure the position and/or change of position of a conductive component. These sensors operate with magnetic fields. The sensor has a probe which creates an alternating current at the tip of the probe. The alternating current creates small currents in the component we are monitoring called eddy currents and the sensor monitors the interaction of these two magnetic fields. As the field interaction changes the sensor will produce a voltage proportional to the change in the interaction of the two fields.
Mechanical vibration sensors like the SW-18020P use a sprint and small weight within a sealed environment to make a mechanical contact when subjected to vibration.
It is a very transient signal output, so you need to use a 'pulse extender' or interrupts to detect it and take action.
In the right environment, mercury filled tilt switches can also be used as vibration sensors. They obviously need to be correctly orientated and will provide very transient signals.
When choosing a vibration sensor for each application it is important to look at:
There a number of smart home specific sensors available on the market, which are essentially one of the above 'dumb' sensors packaged along with electronics to enable easy connection. Typically these will use Z-Wave or ZigBee networking technologies. Often, a vibration sensor will be included as part of a multi-sensor.
We prefer to use very cheap, dumb, wired sensors and these would typically be installed inside external doors. The advantage of wired sensors are optimum performance, very low latency and no batteries.
The basic vibration sensor generate very transient events (very short duration peaks) and it is therefore best to use interrupts to detect them being triggered and some form of rate limiting, to avoid mutiple triggers in quick succession. This is easily achieved using an Arduino.
We would interface wired 12V dc sensors like these to our contextual smart home using our Smartisant 8-Channel Fused Input Board IP1, which also provides the required 5V power. This would be connected to an Arduino processor which uses our unified communications protocol to securely send events to our Home Control System, updating it with changes of state. Because the signals from these sensors are very short duration pulses, we would typically use interrupts.
Vibrations sensors can often be activated during normal activity in the home, e.g. someone slamming a door. This means our smart home's alarm will only trigger if the security alarm is armed, resulting in a whole home voice announcement and high priority notifications.
We use common models for all the sensors connected to our Home Control System but and model vibration and shock sensors using a generic object type called 'Momentary'. These types of objects only send 'On' state changes. This means that any added sensor inherits all of the capabilities supported by our models and all of the common capabilities of our smart home. Essentially, every sensor connected becomes instantly very smart.
Exactly how vibration sensors are used depend on each installation and purpose. All of the vibration sensors in our contextual smart home will generate a notification and some will also trigger the alarm.
