Sensors SENSORS THE LINK BETWEEN CONTROL INTELLIGENCE by mohamedssakr

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									SENSORS - THE LINK BETWEEN
CONTROL INTELLIGENCE AND
THE CONTROLLED VARIABLE
                                                         by D Mavrokefalos, Festo


Today's industries, whether packaging, assembly or process, all utilise sensors of various types. This article will describe the more
regularly used sensors available to a plant or machine designer. Examples of each type of sensor along with a description of their
specific benefits and problems, will be included. No attempt is made here to describe scientifically the actual working of the various
sensors, the accent being on the application and benefits to the user.



                             Sensors used both in the process control industry as well as the automated machine scenario will be
                             discussed in this article. Special emphasis will be placed on non-contact sensors commonly used today.
        INSTRUMENTATION



                             Technology past
                             The most widely used type of sensor in the past has undoubtedly been the limit switch.
                             Limit switches have moved with modern trends and are available today with plug-in features such as micro
                             switch replacement with solid state inductive units.
                             Modern limit switches are highly reliable, repeatable and rugged. Limit switches however, do have certain
                             drawbacks as will be seen later.
                             Mechanical pressure switches have been and are presently used to monitor various conditions and variables
                             and will remain popular in the future.


                             Technology present
                                                                                                                     Inductive sensors
                             Current technology offers the designer an easily understood device
                                                                                                                 Opto-electronic sensors
                             which is both flexible, rugged and inexpensive to install. They
                                                                                                                     Magnetic sensors
                             include those listed in Table 1.
                                                                                                                    Pneumatic sensors
                                                                                                                    Capacitive sensors
                             Technology present and future                                                          Ultra sonic sensors
                             This category of sensor should offer the designer a solution for the                     Pressure sensors
                             future. Plans to have a single two-wire loop on to which many                         Temperature sensors
                             different sensors may be coupled, are underway. The advantage of                          Flow sensors
                             these new generation sensors is that they will be based on well-developed                 Load sensors
                             and tried technologies.                                                              Linear variable sensors
                                                                                                                 Passive infra-red sensors
                             Examples include those listed in Table 2.
                                                                                                                Table l. A list of current sensors.

                                 Laser identification systems              General overview
                                   Visual imaging systems
                                 Acoustic emission systems                 The trend towards solid state control systems such as micro-processors
                                  Multi sensor bus systems                 and programmable logic controllers demands electronic
                                       Tailored sensors                    methods of providing information on process conditions at all
                                                                           operational stages.
                              Table 2. A list of new generation sensors.
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                            To produce a product which meets the manufacturers' quality standards and to eliminate scrap, problems
                            occurring in the production process component or material jams or absences, faulty location, broken or
                            damaged tools - need to he identified quickly. In these applications proximity sensing devices have a valuable


      INSTRUMENTATION       and increasing role to play.
                            Proximity sensors require no physical contact between the sensor and its target to operate. This virtually
                            eliminates wear as a problem. Since they are mainly solid state devices, they also provide fast operation,
                            have the potential for miniaturisation and perhaps most importantly, will readily integrate with the
                            electronics of a modern, automated production process or manufacturing system.
                            Some of the most popularly used non-contact sensing devices in use today are:
                            • inductive sensors
                            • opto electronic sensors
                            • magnetic sensors
                            • capacitive sensors
                            Although inductive sensors are synonymous with the term proximity switch, it is clear that the above sensors
                            may all be described loosely as proximity switches as no physical contact needs to be made to obtain a
                            useable response. These sensors have no moving parts - thus no wear is possible.
                            They are 'contactless' in both senses of the word, so firstly, no actuation force by cams or targets are
                            required which avoids wear and secondly, as they are of a solid state output design, no output contact
                            'bounce' is possible, as sometimes occurs with limit switches.
                            The four popularly used non-contact sensing devices in use today as well as pneumatic sensors, will now
                            be discussed.


                            Inductive sensors
                            These devices are especially suited for use in hostile or
                            outdoor environments, where shock, vibration and
                            contaminants are present. They operate quite simply by
                            moving a metal target towards the active surface of the sensor.
                            A typical inductive sensor is shown in Figure 1.
                            The nominal sensing gap (Sn) is obtained by using a mild
                            steel target equal to the full active surface of the sensor.
                            Being highly directional, the sensor responds only to targets
                                                                                               Figure 1. Inductive sensors are controlling devices
                            passing in front of the sensor wall and will ignore surrounding    which detect the functional motions of processing
                            materials and objects. Inductive sensors will also largely        machines, robots, production lines, conveyor facilities
                                                                                                and generate an electrical signal by contactless
                            ignore material swarf and surrounding magnetic field.                  means. The signal can be used to switch
                            Because of its high speed response - a typical sensor will             solenoid valves, counters, interface cards
                                                                                                     and programmable logic controllers.
                            trigger in 0.2 ms - the inductive sensor will consistently
                            trigger at the same target distance from the sensing face
                            with a repeatability of about 1% of the sensing range.
                            A number of the modern inductive sensors have an adjustable sensing range which allows them to be
                            individually tuned to the specific application. Maximum sensing distance is proportional to the size of the
                            coil producing the sensing field and therefore the bigger the coil, the greater the sensing range.
                            The inductive sensors fall into two categories:
                            • 'self-contained’ - where the control and output circuitry is contained within the sensor housing.
                            • 'Remote' - in which the control and output circuits are housed in a separate enclosure and connected
                               by cable to the sensor.
                            The disadvantage of the self-contained type is mainly that of size. For a given sensing range, the sensor
                            body will need to be larger than the remote type to contain the circuitry that is necessary. The advantages
                            are simpler mounting and lower cost.
                            Applications for those sensors vary enormously. Typical examples include detection of parts in feed chutes
    Contact IDC             for automatic assembly machines, detection of broken tools in automatic machining processes, position
http://www.idc-online.com   sensing of tooling plates in coining presses, length measurement on continuous material production
                            processes, parts counting for automatic dialling lines and so on. All these applications take advantage of
                            the operating speed capability of the sensor. A gap in the flow of the components to the automatic assembly
                            machine will cause the sensor to stop the assembly process before a scrap part is produced. Tool breakage


      INSTRUMENTATION       in an automatic machining process would also cause the sensor to stop the process.
                            Length measurement applications require the sensor to provide pulses
                            from that part of the machine which can be related to the length of
                                                                                                             Mild steel
                                                                                                             Stainless steel
                                                                                                                                            1.0
                                                                                                                                            0.75
                            the part being produced, for example, counting teeth on a gear
                                                                                                             Copper                         0.40
                            wheel which is part of the through-feed mechanism. High degrees of
                                                                                                             Aluminium                      0.40
                            accuracy can be obtained in this way. Correction factors for nonferrous
                                                                                                             Brass                          0.40
                            metals are listed in Table 3 and indicate to what extent the sensing
                            gap will be reduced.                                                                   Table 3: Correction factors for
                                                                                                                     Sn for non-ferrous metals
                            Through their various designs they can be used
                            for numerous applications:
                            • switching gaps from 0.8 to 15 mm
                            • fast switching, practically inertia free
                            • bounce-free operation through elec–tronic output circuit
                            • wear-free operation - no moving parts
                            • unlimited number of switching cycles
                            • contactless actuation without physical change in the actuating element
                            • sensing possible in confined areas through extremely small sizes available


                            Opto-electronic sensors
                            The early opto-electric sensors were crude photoelectric devices which used an incandescent lamp as a
                            light source. In a more sophisticated form such controls have extensive use in conveyor systems detecting
                            the movement of products and parts in a wide variety of industries and applications.
                            However, incandescent light sources are highly intolerant to vibration and this type of device is also prone
                            to false triggering induced by changes in ambient light levels. This, therefore, imposes restrictions on their
                            use. The advent of the light emitting diode (LED) as a light source has changed all this. LEDs have two
                            main advantages over incandescent lamps; they are much less susceptible to damage from vibration or
                            shock and their light output can be modulated. The latest photoelectric controls rely on synchronised
                            detectors which compare the modulated signal controlling the LED in the transmitter with the light pulses
                            received by the sensor. A number of consecutive pulses have to coincide before the sensor will trigger, thus
                            eliminating ambient light interference.
                            They are now, therefore, available to a much wider field of application than was hitherto possible.
                            Continual development of solid state technology has also meant rapid improvement in target detection and
                            resolution and photoelectric controls are found in ever more sophisticated applications - complex pattern
                            recognition, labels and characters.
                            Typically, these devices are used where non-metallic components or processes need to be monitored. They
                            offer the solution for the application requiring sensing distances greater than 20-25 mm. Optical systems
                            may be used as through beam (see Figure 2) or barrier type where distances of up to 10 m may be attained.




                                                      Figure 2. Optoelectronic sensors often open up new application fields.
                                                        Capacity for contactless detection of objects of differing material
    Contact IDC
                                                                   and at varying distances renders them ideal
http://www.idc-online.com                                                     for use in all industry.
                            Other types require a reflector and offer distances of up to 4.5 m. A third type requires only a target to be
                            in place, as it utilises the targets reflectivity alone.
                            Various designs are available which makes it possible to use them in many other switching applications:

      INSTRUMENTATION       •
                            •
                            •
                                switching gaps from 1 mm to over 10 mm possible
                                fast switching, almost without inertia
                                bounce-free switching through electronic output circuit
                            •   wear-free operation through absence of moving parts
                            •   unlimited number of switching cycles
                            •   contactless activation without physical changes in the activating element
                            •   sensing by fibre optics in badly accessible positions


                            Magnetic sensors
                            Magnetic proximity sensors can be divided
                            into three groups, and are all triggered by
                            an external magnet, as suggested in Figure 3.



                                        Figure 3. Magnetic sensors are used for
                                 contactless detection, for example the detection
                                     of permanent magnets on cylinder pistons.



                            Electrical reed switch
                            Reed switch type sensors have the advantage of needing no external power source to drive them. They will
                            readily integrate with a wide range of control systems and, depending on the switch rating, handle relatively
                            large voltages and currents. They can be fully sealed and there are versions available which will accept
                            total immersion.
                            This type of sensor is used mainly for position sensing and has wide usage in security applications, and as
                            an actuator end stop limit switch. Due to their operating characteristics, they tend not to be used for direct
                            component or part sensing. For this reason they have applications more in the process control industry.
                            There are many examples of their use in signal circuits to indicate limits of movement in applications as
                            diverse as machine guards in food processing, conveyor filling systems on commercial food freezers, and
                            as valve and actuator limit sensors.

                            Solid state magnetic sensor (transistorised output)
                            These devices require a power source of between 10-30 VDC. Some devices are 'weld field immune' and
                            may be used in close proximity to welding operations.

                            Pneumatic magnetic sensor
                            These devices require a 600 kPa air supply and will 'snappily' switch an output when in the proximity of
                            an external magnet. They are particularly well-suited for use in 'Class 1 Div 1 or 2' hazardous locations.

                            Capacitive sensors
                            Capacitive sensors will sense any material which will produce a capacitive effect when brought into close
                            proximity with the sensor face. This applies to most materials including liquids. The sensor detects the
                            change in capacitive value as the target approaches and triggers when the value reaches a predetermined level.
                            Capacitive sensors are self-contained and have adjustable sensing ranges. They can be used in a variety of
                            applications including liquid level sensing where they can 'see through' certain types of container walls to
                            the liquid inside the vessel. The ability to sense through the container wails in this way is a major
                            advantage; the proviso is that the substance being sensed must be more detectable than the container wall.
                            It is, however, practical to sense liquid or granular substances through non-metallic screens.
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                            This capability is also of value on conveyor lines where the capacitive sensor can be used to detect the
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                            presence of products inside boxes, bottle wrappings and packages.
                            Pneumatic sensors
                            Contactless sensing using air jets can report the presence or the distance of an object in the form of a


      INSTRUMENTATION       change in the signal pressure.
                            These valves are distinguished by the following advantages:
                            • functional reliability, even under extremely dirty conditions
                            • reliable at high ambient temperatures
                            • suitable for use in explosion-proof areas
                            • insensitive to magnetic influences and sound-waves
                            • dependable, even in complete darkness and when sensing
                               transparent objects
                            The valves cover a sensing distance range of 0 to 10 mm; using the
                            reflex sensors, extremely small sensing distance changes of as little
                            as 0.1 mm can be covered.
                            Pneumatic sensors are classified into three functional groups:
                            • Group 1 - back pressure sensors. When the air jet is closed by
                               the object to be sensed, this leads to a signal pressure build-up
                               at the control port up to the level of the supply pressure
                            • Group 2 - reflex sensors. The reflection of an air jet from the
                               object to be sensed leads to a signal pressure build-up in the
                               control port which is dependent on the sensing distance and
                               supply pressure
                            • Group 3 - air barriers. When the air pressure between the
                               sender and the receiver nozzles is interrupted by the object to
                               be sensed, this leads to a signal pressure drop in the
                               receiver nozzle
                            Pneumatic sensors offer significant advantages, as seen in Figure 4.


                            General causes of unsatisfactory                                                       Figure 4.
                                                                                                        Advantages of pneumatic sensors.
                            sensor performance
                            Sensor manufacturers' field engineers estimate that approximately 60% of the cases of unsatisfactory position
                            sensor performance result from selecting the wrong product for the application, improper installation, or
                            improper maintenance.
                            Ironically, improper selection or maintenance is often based on past satisfaction. A sensor that has
                            performed admirably over a long period in the past, is often arbitrarily specified for applications that might
                            differ markedly from the former application. And conversely, selection is often based on satisfactory, but
                            limited experience in a similar operating environment.


                            Matching the sensor to the application
                            Commonly violated selection/application criteria are operating environment, sensing distance, target size,
                            type of material sensed, and operating speed.


                            Operating environment
                            Humidity, dust, dirt, metal chips, corrosive liquids or mists, vibration, and temperature are just a few of
                            the environmental factors that can affect the operation of a sensor. Even stray radio frequencies can trigger
                            false operations.
                            As a rule, inductive proximity sensors are relatively impervious to dirty, gritty environments, but they can
                            be affected by metal chips or metallic dusts. Photoelectric sensors can be blinded by dust and mists, and
                            can be affected by ambient light or spurious light beams directed toward the sensor detector. When
                            photoelectric sensors are applied in dirty environments, thru-beams perform better than reflective types,
    Contact IDC
                            because their emitted light is not attenuated by passage through a reflective medium.
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                            The combined use of opto-electronic and inductive sensors is
                            shown in Figure 5.



      INSTRUMENTATION             Figure 5. To ensure that the aluminium sealing foil is properly attached
                                inside the bottle cap, a proximity sensor is installed above a conveyor. If a
                                     bottle passes along a through-beam, the sensor gives a synchronising
                                                signal to the proximity sensor to detect the aluminium foil.
                                                      If a fault is detected, the sensor will transmit a signal
                                                              to the air cylinder to reject the faulty product.



                            Sensing distance
                            Photoelectric sensors have much longer operating ranges than inductive proximity sensors. The reflective
                            types have the advantage of simpler installation than thrubeam types, but distance factors limit their
                            application. They are also ideally suited to detecting very small objects at close distances, especially in
                            applications requiring background suppression.
                            Operating range of inductive proximity sensors is limited by the effective range of their RF field, which is
                            only about 15 mm in the largest modules. As a result, sensing distances longer than 15 mm usually require
                            a photoelectric sensor.


                            Target size
                            For inductive proximity sensors, the target should be at least
                            as large as the face of the sensor. For photoelectric sensors, the
                            target should be at least as large as the effective light beam. A
                            typical application of a diffuse optoelectronic sensor is shown
                            in Figure 6.




                                                    Figure 6. The purpose is to detect the marking that
                                            indicates the place where to cut the film, at a fixed length.
                                              A diffuse sensor detects the marking on the film with the
                                            response time of 3 ms. The sensor signal actuates a cutter.




                            Type of material sensed
                            Inductive proximity sensors detect only metals (ferrous and non-ferrous), and range factors must be
                            applied for the specific type of metal.
                            Photoelectric sensors detect the presence of any solid material. Retrore–flective types, however, might
                            require a polariser if the target's surface is reflective. On the other hand, diffuse reflective types cannot be
                            used in applications where the target is not sufficiently reflective.


                            Electrical parameter
                            •   Various options such as (DC-pnp/DC-npn/AC two wire or NAMUR types)
                            •   available supply voltage
                            •   maximum load current
                            •   output configuration (normally open or closed)
                            •   failsafe operation
                            The above are important questions that need to be addressed.


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                            Operating speed
                            Where high system operating speeds are required - as might be the case where the sensor must co-ordinate


      INSTRUMENTATION       with a programmable controller - sensors designed for DC input should be specified. Proximity switches
                            operating on DC inputs are capable of operations as fast as 5000 cycles per second.


                            Installation criteria
                            All of the care given to selecting the proper sensor for the application is futile if care is not exercised in
                            its installation.
                            Proper mounting is essential to satisfactory operation, but a proper location is essential to continued
                            successful operation. Effort should be made to mount the sensor in a location where it is least susceptible
                            to damage from environmental and physical abuse. Inductive proximity sensors will operate in a head-on
                            target-approach configuration, but a transverse, slide-by approach is recommended. The slide-by approach
                            is safer because any unexpected overtravel does not damage the sensor.
                            Shielded inductive proximity sensors can be flush-mounted in metal without provision for additional spacing
                            beyond their plastic range cap. Unshielded types, however, must be provided with additional spacing equal
                            to the range cap diameter. Recurring false detections can be expected if this spacing criterion is violated.
                            Care must be taken not to over-torque the mounting hex nuts when installing threaded-barrel proximity
                            sensors because overtorquing can damage the electronics. Recommended values of torque for sensors of
                            12, l8 and 30 mm diameter are 11, 20 and 41 Nm respectively. In areas of high vibration, a lock washer,
                            and possibly a locking compound, might be necessary.
                            Photoelectric sensors should be applied in a slide-by configuration, but the transverse motion need not be
                            at a right angle to the light beam.
                            With photoelectric sensors, proper alignment is the most critical installation factor. For optimum performance,
                            the light beam must be focused precisely on the target. Retroreflective types also require precise alignment
                            of the reflector.


                            Sensor maintenance
                            Maintenance involves adherence to the basic precept that applies to all electrical and electronic equipment
                            - keep it clean, cool and dry. Ambient temperature that the sensor is exposed to is a function of its location,
                            and should be considered at the time of installation. If, however, the unit is located where it might be
                            exposed to above-normal temperature - for example, adjacent to a component that must undergo welding
                            or shrink-fit heating - the sensor should be removed prior to undertaking the work.
                            For photoelectric sensors, it is especially important to keep the light source, detector lenses, and reflectors
                            clean. Dirt can be compensated for to some extent by adjusting the unit's gain setting. At some point, however,
                            the gain adjustment limit is reached, and such adjustment, at best, is a poor substitute for cleanliness.


                            Conclusion
                            It is hoped that this article has given the reader a broad appreciation of the most commonly used sensors.


                            Acknowledgments
                            The author would like to acknowledge information received from Barrie Grant of Sigma Ltd, Dave
                            Smallwood of Eaton Corporation and the Festo Training Handbooks.


                            Bibliography
                            [1] Paper MID 90-3, approved by the Machine Tools, Robotics, and Factory Automation Committee of the
                            IEEE Industry Applications Society for presentation of the 1989 Industry Applications Society Annual
                            Meeting, San Diego, CA, October 1-5,

    Contact IDC             Manuscript released for publication September 24, 1990.
http://www.idc-online.com   © Electricity + Control 1994

								
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