Types of Sensors and Their Applications

There are a number of ways in which sensing devices may be classified:

  1. By their type of operation - analog or digital.
  2. Whether the quantity is sensed directly or indirectly.
  3. By the medium by which they operate - optical, electrical etc.
  4. By their application.

  1. Proximity Sensing

    Proximity sensing normally means detecting:
    1. Presence or absence of an object.
    2. The size or simple shape of an object.

    Proximity sensors can be further classified as contact or non - contact, and as analog or digital in operation. The choice of sensor is determined by the physical, environmental and control conditions. They include the following:

    Mechanical -
    Any suitable mechanical / electrical switch may be adopted but because a certain amount of force is required to operate a mechanical switch it is common to use microswitches.

    Pneumatic -
    These proximity sensors operate by breaking or disturbing an air flow. The pneumatic proximity sensor is an example of a contact type sensor. These cannot be used where light components may be blown away.

    Optical -
    In there simplest form, optical proximity sensors operate by breaking a light beam which falls onto a light sensitive device such as a photocell. These are examples of non contact sensors. Care must be exercised with the lighting environment of these sensors for example optical sensors can be blinded by flashes from arc welding processes, airborne dust and smoke clouds may impede light transmission etc.

    Electrical -
    Electrical proximity sensors may be contact or non - contact. Simple contact sensors operate by making the sensor and the component complete an electrical circuit. Non - contact electrical proximity sensors rely on the electrical principles of either induction for detecting metals or capacitance for detecting non metals as well.

    Range Sensing -
    Range sensing concerns detecting how near or far a component is from the sensing position, although they can also be used as proximity sensors. Distance or range sensors use non - contact analog techniques. Short range sensing, between a few millimetres and a few hundred millimetres is carried out using electrical capacitance, inductance and magnetic technique. Longer range sensing is carried out using transmitted energy waves of various types eg radio waves, sound waves and lasers.

  2. Force Sensing

    There are six types of force that may require sensing. In each case the application of the force may be static ( Stationary ) or dynamic t Moving ). Force is a vector quantity in that it must be specified in both magnitude and direction. Force sensors are therefore analog an operation and sensitive to the direction in which they act. The six types of forces are:-

    1. Tensile Force
    2. Compressive Force
    3. Shear force
    4. Torsional Force
    5. Bending Force
    6. Frictional Force

    A number of techniques exist for sensing force, some direct and some indirect.

    Tensile Forces: -
    Can be determined by Strain Gauges, these show a change in their" electrical resistance when their length is increased. These gauges measure change in electrical resistance which can be translated into force and are therefore indirect devices.

    Compressive Forces: -
    Can be determined by devices known as Load Cells, these operate" by detecting either a change in dimension of the cell under compressive load or by detecting an increase in the pressure within the cell under load or by exhibiting a change in electrical resistance under a compressive load.

    Torsional and Bending Forces: -
    Can be regarded as a combination of tensile and compressive forces so a combination of the above technique are employed.

    Frictional Forces: -
    These relate to situations where movement is to be restrained, so" friction force is detected indirectly using a combination of force and movement sensors. eg:


  3. Tactile Sensing

    Tactile sensing means sensing through touch. The simplest types of tactile sensors use an array of simple touch sensors arranged in rows and columns. These are commonly called matrix sensors. Each individual sensor is activated when brought into contact with the object. By detecting which sensors are active ( digital ) or the magnitude of the output signal ( analog ) an imprint of the component can be determined. The imprint is then compared to previously stored imprint information o determine the size or shape of the component. Mechanical, optical and electrical tactile sensors are available.

    A tactile matrix sensor:

  4. Heat Sensing

    Heat sensing may be required as part of process control or as a means of safety control. A number of methods are available the choice of which depends for the most part on the temperature to be sensed. Some of the common methods are, Bi - metallic strips, thermocouples, electrical resistance thermometers, or thermistors. For more sophisticated systems involving low level heat sources an infra red camera can be employed.

  5. Acoustic Sensing ( Hearing )

    Acoustic sensors can detect and sometimes discriminate between different sounds. They can be employed in speech recognition for giving verbal commands or for recognising abnormal sounds such as explosions. The most common acoustic sensor is the microphone . The obvious problem with acoustic sensors in an industrial environment is the large amount of background noise. Acoustic sensors can fairly easily be tuned to respond only to certain frequencies thus enabling them to discriminate between different noises.

  6. Gas Sensing ( Smell )

    Gas or smoke sensors which are sensitive to particular gases rely on chemical changes in materials which are contained in the sensor, chemical changes produce physical expansion or sufficient heat to trigger a switching device.

  7. Robot Vision ( Sight )

    Vision is probably the most active field of current research into Robot sensory feedback. Robot vision means capturing an image in real time via a camera of some sort and converting this image into a form which can be analysed by a computer system. This conversion usually means converting the image into a digital field which can be understood by the computer. The entire process of image capture, digitising and data analysis should be quick enough to enable the Robot system to respond to the analysed image and take appropriate action during the performance of the task set. The perfection of Robot vision will allow the full potential of artificial intelligence to be bestowed on industrial Robots. Its uses include the detection of presence, position and movement, recognition and identification of different components, patterns and features, complex scene analysis and identification applications.
    However even the most simple vision techniques require vast amounts of computer memory and can take quite long process times.

SENSING DEVICES
PROXIMITY SENSORS

Capacitive
A range of proximity detectors with built in amplifiers and solid state output stages making them extremely versatile electronic switches These sense the presence of non-conducting materials such as wood, PVC, glass etc. as well as ferrous and non-ferrous metals. All types have built in potentiometers for sensitivity adjustment and LED indicators. Applications include batch counting alarm systems limit switching etc.

Inductive
A miniature inductive proximity detector housed in a threaded anodised aluminium cylindrical case with integral 3-core PVC sheathed cable. Environmental protection to IP 67. The device incorporates electrical protection against reverse polarity, supply line and load transients and has a current limiting PTC resistor in the load output.

Optical
A high quality proximity switch operating on the principle of the emission detection of modulated infra-red light. This mechanically and electrically rugged unit is housed in a threaded aluminium case with a glass end-window and has many applications where any material is required to be detected at a range far exceeding that associated with conventional proximity devices.
 

GAS SENSOR
A platinum wire (pellistor) flammable gas sensor that is designed to sense such gases as propane, butane, methane, isobutane, liquefied petroleum gas, natural gas, and town gas. The performance of the sensor is stable against change of ambient temperature and humidity. Bridge supply voltage 3V dc 10%.

ULTRASONIC TRANSDUCER
An ultrasonic transmitter and high sensitivity receiver designed for sending and receiving continuous or modulated waves in the 40kHz region through air. Applications include remote-control systems, batch counting data transmissions etc. Typical operating distance 5m. Note: These units are not watertight.

STRAIN GAUGE
General Purpose foil type polyester-backed strain gauges. Available with temperature compensation for steel or aluminium.
 

SLOTTED OPTO SWITCH
A slotted opto switch comprising an infra-red source and integrated photo-detector The i.c photo-detector consists of a photo-diode, amplifier, voltage regulator, schmitt trigger and output stage. An important feature is its' wide supply voltage range 4.5V to 16V, which allows the output to interface directly to TTL, LS/TTL, and C-MOS. The detector can sink 10 TTL loads when the infra-red beam is interrupted. The outtput rise and fall times are independant of object speed and the integrated voltage regulator ensures high noise immunity. Applications include position detection, paper sensing, counting, optical encoding and level sensing. Operating temperature range: -40 C to +100 C.

REFLECTIVE OPTO SWITCH
An infra-red emitting LED and photo-transistor sensor housed in a moulded package. The photo-transistor responds to radiation from the diode when a reflective object is placed within the field of view. The device is ideal for counting shaft revolutions, line counting on paper tapes etc.. The moulded case incorporates a dust cover and infra-red filter to prevent the ingress of dust and ambient illumination problems. A slotted mounting hole alllows for adjustment to the sensing distance.