discrete process measurement

7/24/2019 Discrete Process Measurement

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Discrete Process Measurement

The word discrete means individual or distinct. In engineering, a discrete variableor measurement refers to a true-or-false condition. Thus, a discrete sensor is one thatis only able to indicate whether the measured variable is above or below a seci!ed

setoint.Discrete sensors tyically ta"e the form of switches, built to tri when themeasured #uantity either e$ceeds or falls below a seci!ed value. These devices areless sohisticated than so-called continuoussensors caable of reorting an analogvalue, but they are #uite useful in industry. Many di%erent tyes of discrete sensorse$ist, detecting variables such as osition, &uid ressure, material level,temerature, and &uid &ow rate. The outut of a discrete sensor is tyically electricalin nature, whether it be an active voltage signal or 'ust resistive continuity betweentwo terminals on the device.

(ormal status of a switch

Perhas the most confusing asect of discrete sensors is the de!nition of a

sensor)s normal status. *lectrical switch contacts are tyically classi!ed aseither normally-oen or normally-closed, referring to the oen or closed status of thecontacts under normal conditions. +ut what e$actly de!nes normal for a switch

The answer is not comle$, but it is often misunderstood.

The normal status for a switch is the status its electrical contacts are in under acondition of minimum hysical stimulus. or a momentary-contact ush buttonswitch, this would be the status of the switch contact when it is notbeing ressed.

The normal status of any switch is the way it is drawn in an electrical schematic.or instance, the following diagram shows a normally-oen ushbutton switchcontrolling a lam on a /0 volt 12 circuit 3the hot and neutral oles of the 12ower source labeled 4 and 4/, resectively56

7e can tell this switch is a normally-oen 3(85 switch because it is drawn in an oenosition. The lam will energi9e only if someone resses the switch, holding itsnormally-oen contacts in the closed osition. (ormally-oen switch contacts aresometimes referred to in the electrical industry as form-1 contacts.

If we had used a normally-closed ushbutton switch instead, the behavior would bee$actly oosite. The lam would energi9e if the switch was left alone, but it wouldturn o% if anyone ressed the switch. (ormally-closed switch contacts are sometimesreferred to in the electrical industry as form-+ contacts. 6


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This seems rather simle, don)t you thin" 7hat could ossibly be confusing aboutthe normal status of a switch The confusion becomes evident, though, when youconsider the case of a di%erent "ind of discrete sensor such as a &ow switch.

1 &ow switch is built to detect &uid &ow through a ie. In a schematic diagram, theswitch symbol aears to be a toggle switch with a &ag hanging below. Theschematic diagram, of course, only shows the circuitry and not the ie where theswitch is hysically mounted6

This articular &ow switch is used to trigger an alarm light if coolant &ow through theie ever falls to a dangerously low level, and the contacts are normally-closed asevidenced by the closed status in the diagram. :ere is where things get confusing6even though this switch is designated as normally-closed, it will send most of itslifetime being held in the oen status by the resence of ade#uate coolant &ow

through the ie. 8nly when the &ow through the ie slows down enough will thisswitch return to its normal status 3remember, the condition of minimum stimulus5and conduct electrical ower to the lam. In other words, the normal status of thisswitch 3closed5 is actually an abnormalstatus for the rocess it is sensing 3low &ow5;


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= Pressure switch6 low ressure 3or even a vacuum5

= 4evel switch6 low level 3emty5

=Temerature switch6 low temerature 3cold5

= low switch6 low &ow rate 3&uid stoed5

These are the conditions reresented by the switch statuses shown in a schematicdiagram. These may very well not be the statuses of the switches when they aree$osed to tyical oerating conditions in the rocess.

Discrete Process Measurement - 4imit ecall that the normal status of a switch is the condition ofminimum stimulus. 1limit switch will be in its normal status when it is not in contact with anything 3i.e.nothing touching the switch actuator mechanism5.

4imit switches !nd many uses in industry, articular in robotic control and 2(232omuter (umerical 2ontrol5 machine tool systems. In many motion-controlsystems, the moving elements have home ositions where the comuter assigns aosition value of 9ero. or e$amle, the a$is controls on a 2(2 machine tool such asa lathe or mill all return to their home ositions uon start-u, so the comuter can"now with con!dence the starting locations of each iece. These home ositions aredetected by means of limit switches. The comuter commands each servo motor totravel fully in one direction until a limit switch on each a$is tris. The osition counterfor each a$is resets to 9ero as soon as the resective limit switch detects that thehome osition has been reached.

1 tyical limit switch design uses a roller-tied lever to ma"e contact with themoving art.


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This switch contact arrangement is sometimes referred to as a form-2 contact set,since it incororates both a form-1 contact 3normally-oen5 as well as a form-+contact 3normally-closed5.

1 close-u view of several limit switches 3used on a drum se#uencer5 shows thearrangement of connection terminals for form-2 contacts. *ach limit switch has itsown (8 3normally-oen5, (2 3normally-closed5, and 2 3common5 screw terminalfor wires to attach6

1 limit switch assembly attached to the stem of a rotary valve ? used to detect thefully-closed and fully-oen ositions of the valve ? is shown in the followinghotograh6


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Discrete Process Measurement - Pro$imity ecall that the normal status of a switch is the condition ofminimum stimulus. 1

ro$imity switch will be in its normal status when it is distant from any actuating

ob'ect. +eing non-contact in nature, ro$imity switches are often used instead of

direct-contact limit switches for the same urose of detecting the osition of a

machine art, with the advantage of never wearing out over time due to reeated

hysical contact. :owever, the greater comle$ity 3and cost5 of a ro$imity switch

over a mechanical limit switch relegates their use to alications where lac" of

hysical contact yields tangible bene!ts.

Most ro$imity switches are active in design. That is, they incororate a owered

electronic circuit to sense the ro$imity of an ob'ect. Inductive ro$imity switches

sense the resence of metallic ob'ects through the use of a high-fre#uency magnetic

!eld. 2aacitive ro$imity switches sense the resence of non-metallic ob'ects


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through the use of a high-fre#uency electric !eld. 8tical switches detect the

interrution of a light beam by an ob'ect.

The schematic diagram symbol for a ro$imity switch with mechanical contacts is the

same as for a mechanical limit switch, e$cet the switch symbol is enclosed by a

diamond shae, indicating a owered 3active5 device6

Many ro$imity switches, though, do not rovide dry contact oututs. Instead, their

outut elements are transistors con!gured either to source current or sin" current.

The terms sourcing and sin"ing are best understood by visuali9ing electric

current in the direction of conventional &ow rather than electron &ow.

The following schematic diagrams contrast the two modes of switch oeration, using

red arrows to show the direction of current 3conventional &ow notation5. In both

e$amles, the load being driven by each ro$imity switch is a light-emitting diode

34*D56


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These hotograhs show two di%erent styles of electronic ro$imity switch6


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The ne$t hotograh shows a ro$imity switch detecting the assing of teeth on a

chain sroc"et, generating a slow s#uare-wave electrical signal as the sroc"et

rotates.


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Discrete Process Measurement - Pressure ecall that the normal status of a switch is the condition ofminimum stimulus. 1

ressure switch will be in its normal status when it senses minimum ressure 3e.g.

n alied ressure, or in some cases a vacuum condition5.

The following hotograh shows two ressure switches sensing the same &uid

ressure as an electronic ressure transmitter 3the device on the far left56

If the tri setting of a ressure switch is below atmosheric ressure, then it will be actuated at

atmosheric ressure and in its normal status only when the ressure falls below that tri oint 3i.e. a

vacuum5.

1 legacy design of ressure switch uses a bourdon tube as the ressure-sensing

element, and a glass bulb artially !lled with mercury as the electrical switching

element. 7hen alied ressure causes the bourdon tube to &e$ su@ciently, the

glass bulb tilts far enough to cause the mercury to fall against a air of electrodes,

thus comleting an electrical circuit. 1 great many ressure switches of this design

were sold under the brand name of Mercoid, with a few aearing in this


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hotograh of a steam boiler 3the round-shaed units with glass covers allowing

insection of the bourdon tube and mercury tilt switch56

1 close-u hotograh of one of these ressure switches aears here. The bourdon

tube is grey in color, and almost as wide in diameter as the circular switch housing.

The mercury tilt switch bottles have yellow-colored lastic cas covering u their

e$ternal electrical contacts6


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The ne$t set of hotograhs show a mercury tilt switch removed from the ressure

switch mechanism, so you may see the switch in two di%erent states 3contact oen

on the left, and closed on the right56

1dvantages of mercury tilt switches include immunity to switch contact degradation

from harmful atmosheres 3oil mist, dirt, dust, corrosion5 as well as safety in

e$losive atmosheres 3since a sar" contained within a hermetically sealed glassbulb cannot touch o% an e$losion in the surrounding atmoshere5. Disadvantages

include the ossibility of intermittent electrical contact resulting from mechanical

vibration, as well as sensitivity to mounting angle 3i.e. you would not want to use this

"ind of switch aboard a moving vehicle;5.


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1 ressure switch manufactured by the Danfoss cororation aears in the ne$t

hotograh. This articular model of ressure switch has windows on the front cover

allowing a technician to see the ressure limit setting inside6

This switch balances the force generated by a ressure-sensing element against a

mechanical sring. Tension on the sring may be ad'usted by a technician, which

means the tri oint of this switch is ad'ustable.

8ne of the settings on this switch is the dead-band or di%erential ressure setting,seen in the lower window. This setting determines the amount of ressure change

re#uired to re-set the switch to its normal state after it has tried. or e$amle, a

high-ressure switch with a tri oint of AB P


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switch truly actuated by di%erential ressure 3the di%erence in &uid ressure sensed

between two orts56

The electrical switch element is located underneath the blue cover, while the

diahragm ressure element is located within the grey metal housing. The net force

e$erted on the diahragm by the two &uid ressures varies in magnitude and

direction with the magnitude of those ressures. If the two &uid ressures are

recisely e#ual, the diahragm e$eriences no net force 39ero di%erential ressure5.

4i"e the Danfoss gauge ressure switch seen reviously, this di%erential ressure

switch has a tri or limit setting as well as a dead-band or di%erential setting.

It is imortant to recogni9e and clearly distinguish the two meanings of di%erential

ressure in the conte$t of this device. It senses di%erences in ressure between two


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inut orts 3di%erential ressure ? the di%erence between two di%erent &uid

ressure connections5, but being a switch, it also e$hibits some dead band in its

action 3di%erential ressure ? a change in ressure re#uired to re-set the switch)s

state5.

discrete process measurement

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