Wednesday, September 05, 2007

Types of Sensors

segnoThis is a second article in a series, following my introduction to the continuum of instruments, devices and sensors. Here I'll outline the scope and properties of readily obtainable sensors. I hope that the simplicity of this summary will serve to demystify what can be a very jargon-filled field. I also hope that you'll start thinking of innovative ways to use some of this environmental data for music.

To start with, sensors are available in two operational classes, analog or digital, depending on their output. When choosing a sensor one must consider:
* the physical property measured
* the response time (how long it takes from physical change to electrical signal)
* the response range (low to high values) for analog units
* the sensitivity, measured on a response curve (linear, logarithmic)
* whether it is continuous (provides data in a constant stream) or triggered (provides data only on an action)

I will now describe briefly some common types of sensor by physical property. All of these may be obtained in standard kit parts. There are hundreds of other more specialised sensors, as some of the references in the previous article illustrated.

Movement is easily sensed with a switch, such as your bog standard light switch. That's so commonplace as to seem most useless! When switches require minimal force to operate they may be called contacts. Simple switches have half of a circuit on either side; when the switch is thrown the circuit completes. Magnetic switches have the circuit on one side only, connected to an electromagnet. A simple magnet can then complete the circuit.

Photoelectric switches are momentarily detect when a beam of light is broken. Motion detectors are react to changes in infrared light and generally give continuous varying output. Piezoelectric switches are small contact disks that are handy for placing under carpets, etc.

Magnetic motion trackers are used in VR research to track all six degrees of freedom of movement about a person's body.

Position can be determined using infrared sensors. These send out an IR beam and measure how much comes back, and are good for up to 2 meters or so. Ultrasonic sensors do the same with a sound pulse, but can reach ranges of 10 meters.

Stretch sensors, which come in lengths up to about 40cm, change their resistance when longitudinally stretched. Flex or bend sensors do likewise when they deviate from a straight line. Together these are useful when making VR gloves and other responsive clothing.

Rotation or tilt is digitally measured with a mercury switch; these trigger at a certain orientation. An array of them can be used for a more accurate value determination. Continuous rotation potentiometers provide analog readings in cases where it is reasonable to fit a sensor to the axis of rotation. Tilt sensors that use more sophisticated methods are also known as inclinometers.

Electronic compasses determine orientation relative to magnetic North. Accelerometers measure acceleration and also provide knowledge of tilt relative to gravity. Gyroscopes may also be used to measure orientation.

Vibration can also be detected by piezoelectrics, since these are basically audio sensors with high sensitivity at low frequencies.

Light can be sensed using a photoresistor (AKA photocell, photodiode, or LDR). Similarly, one can use a phototransistor, which has greater sensitivity but has a slower response time. Different materials in a phototransistor will make it sensitive to different wavelengths of light. Solar cells come in two basic types: one has current output rated in microamps (indoor use), the other milliamps (outdoor use). Neither are responsive in the least, but can be used to measure cumulative light radiation.

Temperature is measured by a thermistor. These are responsive to different temperatures (commonly available at 25 and 100 degrees C) and have quite different response curves depending on the particular model.

Force sensors can be used to measure either static or dynamic forces, like human touch or the wind.

Humidity can be measured in air or soil by a hygrometer. One type works by comparing the temperature of a dry thermometer to one that is wet. Another (intriguingly) works by stretching a hair (sometimes human), and measuring its length at different humidities.

Radiation can be measured with so-called Geiger counters, which detect alpha, beta, and gamma radiation.

Toxic gas sensors are available in dozens of types, many very expensive.

And finally, let us not forget that sound can be measured by a little device we call a microphone. It too is an environmental sensor!

So where to buy these?

Electronics wholesalers are listed by Ezio. Others that specialise in sensors are SparkFun and Images Scientific Instruments.

Laetitia Sonami has a list of sensor suppliers. So too does the ITP course at NYU.

The commercial amalgamator sensorland gathers data from numerous suppliers with explanatory articles and tutorials. Cooler yet is the dedicated parts search engine, Octopart.

Since most of these references are for the USA, what do you do if you're in Europe? Well, the larger distributors are all used to shipping around the world. Some even have dedicated telephone numbers for contact from overseas. Still, if you prefer a store closer to home, here are some I've found, most of which should offer sensors: Conrad in Germany, Lawicel, Lextronic and Jelu in Sweden, Farnell and Radiospares in France.

In my next article I'll look at inexpensive micro-processor boards which collect the data from your sensors and forward the results to a computer or other device.

I thank the Arts Council for their support in this research.


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