Proximity Sensing
Proximity sensing normally means detecting:- Presence or absence of an object.
- 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.
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:-- Tensile Force
- Compressive Force
- Shear force
- Torsional Force
- Bending Force
- 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:
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:
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.
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.
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.
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.
