Sensors

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Revision as of 08:49, 23 October 2010 by 68.0.124.33 (talk) (→‎Magnetic Field Sensor: "Planar Hall sensor for influenza immunoassay")
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Accelerometers, Gyroscopes, Temperature Sensors, Pressure Sensors, ...

General

A sensor is a device that converts some physical measurement into an electrical signal. This allows remote sensing, display of variables, and recording of values. There are a wide variety of sensors, we touch on them only fairly briefly here. Note that some standard components can be used to make or as sensors by themselves. A normal signal diode, for example, has a forward voltage drop that depends on temperature. Thus it can be used as a temperature sensor. In "Further Reading" we have some good references for more information on sensors and interfacing to sensors.

Accelerometer

These measure ( gasp ) acceleration. May be caused by gravity or the second derivative of position ( relative to an inertial frame ). Since acceleration is a vector to go the whole distace you need a 3 axis sensor, but often 1 or 2 D will do. Often the output is a voltage(s). Feed it to a microcontroller with an a to d converter, perhaps through an operational amplifier. Op amp

Capacitive Sensor

These measure capacitance, but normally this is the means to measuring something else: for example moving your hand close to a metal plate increases its capacitance to ground. Measuring the capacitance thus measures the the touch ( or near touch ). Two insulated electrodes on either side of a glass tube can detect the presence of water in the tube by the change capacitance.

Some kinds of capacitive sensors can be used as sensors#multi-touch sensors.

Current Sensor

Main Article: current sense

Perhaps the most basic current sensor is the resistor. Current is converted to voltage via ohm's law. Typically small values of resistance are used, ranging from say 1 ohm down. Processing of this signal may then be done with op amps to change the ground reference or scale the values. Some integrated chips are available for this type of processing.

Another current sensor is the Hall Effect sensor, this actually senses magnetic field, but currents produce fields. Signals from Hall Effect sensors are typically small so again op amps may be used.

See current sense for many examples.

Gas sensor

IR Infra Red Sensor Reciever Detector

Knock Sensor


Light Sensor

Linear Encoder

Like rotary encoders but for linear motion. Modern digital calipers are based on this technology ( i think ). Positioning of print heads in computers may also uses this to position the print head.


Magnetic Field Sensor

A magnetic field sensor is used inside some kinds of current sensor: current sense#magnetic field sense

  • Hall effect sensor
  • "Planar Hall sensor for influenza immunoassay" by Louise Wellendorph Ejsing 2006

Pressure and Barometric Sensor

Solid state units have become available for this. We need more material here. They can be used for weather stations, measuring altitude, or measuring pressure.


Rotary Encoder

A rotary encoder is a device that measure ( or encodes ) rotational motion. The old fashioned mice with a ball used dual rotary encoders one to measure motion in the x direction one to measure motion in the y direction. This may be done using switching technology or by using optical methods ( so some encoders may be considered optical electronics, optoelectronics ). Even stepper motors may be used as rotary encoders. It normally requires a microcontroller ( or specialty chip ) to decode the signals. In addition to mice rotary encoders are used for the user control on volume controls ( finally coupled to say digital potentiaometrs ), measure the position of joints in robots, or measuring the speed and/or position of motors ( as in some servo motors ).

Stress Sensor

One type is know as a strain gage. Often based on materials where the resistance varies when elongated. Typically have very small signal output that are measured in bridge configurations.

Temperature Sensor

Some are analog some are digital. All can use microcontroller interfaces again digital or analog. Two analog devices are LM34 and LM35, they are very easy to use because the voltage output is directly proportional to the temperature. Dallas semiconductor makes a line of sensors based on their one wire networking, cool. Temperature may also be sensed by thermistors ( temperature sensitive resistors ), the forward voltage on a diode ( or transistor ) or by thermocouples ( junctions of two different conductors ). Thermocouples can work up to almost the melting temperature of the conductors. You can also sense temperature based on the light emitted ( usually ir ) from the object. Many of the temperature sensors can be highly non-linear in their response, programming in a microcontroller can be used to linearize the readings.

multi-touch sensor

A few kinds of sensors can be used to detect and locate multiple simultaneous finger-presses. Such sensors are necessary for a multi-touch display.

Further Reading

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