INTRODUCTION TO SENSORS

How Engineers Use Sensors to Measure Object Properties and Behaviors:

Electrical and mechanical sensors are widely used to characterize the performance and properties of components and systems, but are also found in household objects.
Sensors are electrical or mechanical components that are used to measure a property or behavior of an object or system. Some sensors measure properties directly, other sensors measure properties indirectly, using conversions or calculations to determine results. Sensors are used by scientists and engineers during research and testing activities, but they can also be found in many household objects, such as temperature sensors in an oven to accelerometers in an automobile airbag system. Sensors are generally categorized by the type of phenomenon that they measure, rather than the functionality of the sensor itself.

MECHANICAL SENSORS:
Mechanical sensors measure a property through mechanical means, although the measurement itself may be collected electronically. An example of a mechanical sensor is a strain gauge. The strain gauge measures the physical deformation of a component by experiencing the same strain as the component, yet the change in resistance of the strain gauge is measured electrically. Other types of mechanical sensors include:
· Pressure sensors
· Accelerometers
· Potentiometers
· Gas and fluid flow meters
· Humidity sensors

ELECTRICAL SENSORS:
Electrical sensors measure electric and magnetic properties. An example of an electrical sensor is an ohmmeter, which is used to measure electrical resistance between two points in a circuit. An ohmmeter sends a fixed voltage through one probe, and measures the returning voltage through a second probe. The drop in voltage is proportional to the resistance, as dictated by Ohm's Law. Other electrical sensors include:
· Voltmeter/Ammeter
· Metal detector
· RADAR
· Magnetometer

THERMAL SENSOR:
Although all thermal sensors measure changes in temperature, there are a variety of types of thermal sensors, each with specific uses, temperature ranges, and accuracies. Some types of thermal sensors include:
· Thermometers
· Thermocouples
· Thermistors
· Bi-metal thermometers

CHEMICAL SENSORS:
Chemical sensors generally detect the concentration of a substance in the air or in a liquid. Some chemical sensors, such as pH glass electrodes are designed to be sensitive to a certain ion. Some other types of chemical sensors include:
· Oxygen sensors
· Carbon monoxide detectors
· Redox electrodes

OPTICAL SENSOR:
Optical sensors detect the presence of light waves. This could include light in the visible spectrum, or outside the visible spectrum, in the case of infrared sensors. Some types of optical sensors include:
· Photodetectors
· Infrared sensors
· Fiberoptic sensors
· Interferometers

OTHER TYPE OF SENSOR:
There are many other types of sensors that don't fall into one of the broad categories described here. Some of these sensors include:
· Radiation sensors, including Geiger counters and dosimeters
· Motion sensors, including radar guns and speedometers
· Acoustic, including sonar and seismometers
· Gyroscopes
While sensors are used frequently by engineers and scientists in their studies, sensors are also use in many household products. Sensors can be found in many everyday objects, including touch-sensitive buttons and screens, infrared remote controls, motion-sensitive lighting, and home thermostats.


Proximity Sensors:
Proximity sensors may be of the contact or non-contact type. Contact proximity sensors are the least expensive. Proximity sensors can have one of many technology types. These include capacitive, eddy current, inductive, photoelectric, ultrasonic, and Hall effect. Capacitive proximity sensors utilize the face or surface of the sensor as one plate of a capacitor, and the surface of a conductive or dielectric target object as the other. The capacitance varies inversely with the distance between capacitor plates in this arrangement, and a certain value can be set to trigger target detection. In an eddy current proximity sensor electrical currents are generated in a conductive material by an induced magnetic field. Interruptions in the flow of the electric currents (eddy currents), which are caused by imperfections or changes in a material's conductive properties, will cause changes in the induced magnetic field. These changes, when detected, indicate the presence of change in the test object. Magnetic inductive devices are identical in configuration to the variable reluctance type and generate the same type of signal. However, inductive pickoff coils have no internal permanent magnet and rely on external magnetic field fluctuations, such as a rotating permanent magnet, in order to generate signal pulse. Photoelectric devices are used to detect various materials at long range, using a beam of light. They detect either the presence or absence of light and use this information to read the data from the output transistor. An ultrasonic proximity sensor emits an ultrasonic pulse, which is reflected by surface and returned to sensor. Speed can be determined by measuring frequency difference (Doppler Effect). The basic "Hall Effect" sensing element is a semiconductor device which, when electrical current is sent through it, will generate an electrical voltage proportional to the magnitude of a magnetic field flowing perpendicular to the surface of the semiconductor.
The most important parameter to consider when specifying proximity sensors is the operating distance. This is the rated operating distance is the distance at which switching takes place. Common body styles for proximity sensors are barrel, limit switch, rectangular, slot style, and ring. Important dimensions to consider when specifying proximity sensors include barrel diameter, length, width, and height.
Proximity sensors can be a sensor element or chip, a sensor or transducer, an instrument or meter, a gauge or indicator, a recorder or totalizer, and a controller. A sensor element or chip denotes a "raw" device such as a strain gage, or one with no integral signal conditioning or packaging. A sensor or transducer is a more complex device with packaging and/or signal conditioning that is powered and provides an output such a dc voltage, a 4-20mA current loop, etc. An instrument or meter is a self-contained unit that provides an output such as a display locally at or near the device. Typically also includes signal processing and/or conditioning. A gauge or indicator is a device that has a (usually analog) display and no electronic output such as a tension gage. A recorder or totalizer is an instrument that records, totalizes, or tracks force measurement over time. Includes simple datalogging capability or advanced features such as mathematical functions, graphing, etc.