Openers DO YOU WORK WITH ONE OF THESE? SLIDING GATE SWING GATE BARRIER GATE SLIDING DOOR ROLL-UP DOOR DRIVE-THRU'S Corporate Headquarters 9603 John Street Santa Fe Springs, CA 90670 Tel: (562) 923-9600, (800) 733-7872 • Fax: (562) 923-7555 Then you need this handy Reference Guide. It will answer the questions you have about using an inductive vehicle loop detector for automatic control of the devices illustrated. Rev. 8, 8/27/02 Written by: Ray Hoag
Transcript
Openers
DO YOU WORK WITH ONE OF THESE?SLIDING GATE
SWING GATE
BARRIER GATE
SLIDING DOOR
ROLL-UP DOOR
DRIVE-THRU'S
Corporate Headquarters9603 John Street
Santa Fe Springs, CA 90670Tel: (562) 923-9600, (800) 733-7872 • Fax: (562) 923-7555
Then you need this handy Reference Guide. It will answer thequestions you have about using an inductive vehicle loop detectorfor automatic control of the devices illustrated.
Rev. 8, 8/27/02Written by: Ray Hoag
INDUCTIVE LOOP VEHICLE DETECTORSI. INTRODUCTION
Q. What is an inductive loop vehicle detector and how does it work?
A. The complete detector consists of a loop of wire and an electronic detection unit. The operationis based on the principle of metal detection. With a metal detector, a movable “search coil” (orloop) is moved until it is over a buried object, which disturbs the electrical field generated by theloop and causes the detector to respond with an output, usually audible (it can be heard). In thevehicle detector application, the loop is buried in the pavement and the object to be detected(the vehicle) moves over the loop. When the loop detects a metallic object over it, the detectorprovides a switch closure (relay) output.
Q. What is the coil?
A. The coil consists of two or more turns of wire buried in the pavement and wound in a square,rectangular, or round form. See Figure 1. There are two types of commonly used loops, thesaw cut loop and the preformed loop. More about this later.
Figure 1
Q. How is this system applied?
A. When a prescribed pattern of loops is buried on either side of a gate or door and connected to adetector, the system will detect the presence of a vehicle over the loops. This signal can beconnected to operate the gate or door actuator. When properly programmed with the operator,the gate or door will open and remain open as long as the loop detects the presence of thevehicle. When the vehicle leaves the loop, the gate or door is allowed to automatically close.
U.S. Traffic Corporation loop detectors use extensively tested and proven digital design techniques.These techniques are used in conjunction with a high speed microprocessor. The result is adetection system offering high performance, high reliability, compact size, and low cost.
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II. DESIGN FACTS
1. Proper installation of the loops is essential for reliable functioning of the detector system. Mostdetector problems are caused by improper loop installation!
2. The geometry (size and shape) of the loop defines the detection zone characteristics. SeeFigure 2 for a few examples of loop geometries. Loop size may vary and will depend on lanewidth, traffic patterns, and types of vehicles to be detected. Loop sizes may vary from 18 inchesby 54 inches for Drive-Thru applications, to 3 feet by 6 feet to parking lots and parking structuresaccepting normal passenger vehicles and motorcycles, to 6 feet by 6 feet or larger toadditionally handle higher clearance vehicles (4 x 4’s and trucks) and wider roadways andgates. The short leg of any loop should be no less than 18 inches or, usually, greater than 6feet.
3. The number of turns required in a loop depends on the perimeter (the sum of the four sides) ofthe loop, as shown in Table 1 for English measurement or Table 2 for metric measurement.
Figure 2
4. When connecting more than one loop to a detector, always connect the loops in series .Never connect more then four loops to one detector. The total area of multiple loops in seriesshould not exceed 200 sq. ft. Always construct each loop as though it were the only loop beinginstalled (i.e., bring the lead-in pair from each loop back separately to the pull-box-box oroperator cabinet). If the loops are close together, the direction of the windings of each loopshould be considered. Loops located physically near each other and wound in the samedirection electrically (i.e., both CW or both CCW) will cause field cancellation effects (a deadzone) between the loops. This is desirable when two loops are placed on either side of a slidingmetal gate or door (see Figure 3). This makes the gate or door transparent to the loops andallows it to pass between the loops without creating the effect of a vehicle on the loops.
If the loops are wound in electrically opposite directions (i.e., one CW and the other CCW), fieldenhancement will occur between the loops (see Figure 4). If the loops were connected in thismanner on either side of a gate or door and a vehicle had cleared the loop, the controller wouldcommand the gate to close. The gate or door, moving between the loops, would disturb the fields ofthe loops and cause the detector to think that another vehicle had entered the loop, causing thegate or door to open. Once the gate or door opened, the detector would sense clear loops andsignal a closure. This process would continue to repeat itself until power was removed.
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II. DESIGN FACTS (Cont.)
NOTES:
1. Loop inductances shown above include no lead-in/home-run length. The additional inductance for the lead-in/home-run can becalculated at 0.22 µh/ft. & added to the loop inductance for the total inductance of the loop network.
2. Ideally, loop inductance should be equal to or greater than 100 µh.
3. Inductance values shown are for rectangular loops. Sawcut loops with relieved corners are slightly less.
4. Loop inductance calculations are based on the formula: L = PK,
1. Loop inductances shown above include no lead-in/home-run length. The additional inductance for the lead-in/home-run can becalculated at 0.22 µh/ft. & added to the loop inductance for the total inductance of the loop network.
2. Ideally, loop inductance should be equal to or greater than 100 µh.
3. Inductance values shown are for rectangular loops. Sawcut loops with relieved corners are slightly less.
4. Loop inductance calculations are based on the formula: L = PK,
5. A single loop located on one side of a gate or door and connected its own detector must be setback from the gate or door a minimum distance equal to 25% of the length of the side of the loopfacing the gate or door or 3’, whichever is greater, to prevent detection of the moving gate ordoor..
6. Loops connected in series as shown in Figure 2 do not require the minimum setback describedin the previous step. As long as the two loops are identical, they can be located within 6" - 12"of the gate or door, as long as they are equally spaced about the door. This makes the gate ordoor transparent to the loops and permits safety loops to be spaced apart a distance shorterthan the shortest wheelbase expected, particularly useful in forklift and small vehicleapplications.
7. The detection height of a loop equals 2/3 of the shorter dimension of a loop. This is particularlycritical where tractor trailers, four wheel drive vehicles, and other high clearance vehicles areanticipated.
8. To calculate the minimum dimension of the loop facing the gate or door, subtract the width of thenarrowest vehicle from the width of the opening. For a fork truck, use 4’; for a car, use 6’; for atruck, use 8’. When the loop is centered in the roadway, there will be 1/2 a vehicle width oneither end of the loop and assuring that at least 1/2 of the width of the vehicle will be over theloop even if the vehicle scrapes the curb, door jamb, or gate post.
9. During the construction of new installations (i.e., new concrete or asphalt), a preformed loopmay be used as an alternate to the saw cut type. U.S. Traffic Corporation provides two types ofpreformed loops with the following specifications. Table 3 shows part numbers for some of thesizes available in either type. For other sizes, contact the factory.
a) 700 Series PVC Preformed Loops:
• Loops are encased in Schedule 40 pipes to provide high reliability and long life.
• A stub is provided for attaching additional PVC pipe to protect the lead-in cable.
• Twenty feet of shielded lead-in cable rated for direct burial is standard. (Additionallength available on request).
• Needs at least a 1" cover of sand or dirt to act as a thermal blanket when installed innew asphalt.
b) 1700 Series High Temperature Preformed Loops:
• Can withstand hot tar or asphalt being poured directly over the preformed loop (up toan approximate temperature of 450° F).
• Outer tubing is constructed from durable cross-linked polyethylene (XLPE) - verywaterproof and abrasion resistant.
• T-fitting is constructed from glass-filled polyester.
• May also be used in concrete.
• Ideal for bridges, highways, roadways, and parking structures.
• Lightweight and flexible for easy shipping and handling.
• Lead-in wire and outer protective tubing over the lead-in wire made to customerlength. Eliminates need for customer furnished PVC or conduit.
• May be attached to existing base with 1/2" EMT clamps and paved over.
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II. DESIGN FACTS (Cont.)
10. The loop installation area must be chosen to avoid locating the loop wires directly on reinforcingsteel, electrical cables, conduits, or water pipes. Loop wires should be installed a minimum of 2inches above, NEVER BELOW, reinforcing bars or other metallic objects. Bars and other metallicobjects running at right angles to the loop wires have less effect than when parallel. Electricalcables near the loop can possibly cause false impulses to the magnetic field generated by the loop,causing erratic operation of the detector. Never install loops within 10 feet of underground electricaldeicing systems.
11. Physically adjacent loops operating on separate detector modules may interfere (cross talk)with each other. This interference can be eliminated by changing the operating frequency on one ofthe detector modules.
12. The detector module must operate from a stable A.C. power source. If the detector issubjected to excessive line voltage variations, the detector may cause either false outputs or maydrop an output when a vehicle is over the loop. An example would be a gate operator which islocated some distance from the power distribution panel. If the power feeder wires are undersize,the voltage at the operator will momentarily drop, due to the starting inrush current of the motor.The detector senses a voltage drop of more than 25% of nominal line voltage as a power failure.When the voltage returns to the proper level, the detector automatically resets. If a vehicle is overthe loop, it will be lost when the detector resets.
III. LOOP INSTALLATION TECHNIQUES
A. Saw cut loop installations.
In order to have a detector system operate as a reliable, high performance system, it is necessary topay careful attention to the loop installation. The use of proper installation techniques can reducefrustration, aggravation, and unnecessary service calls, while increasing reliability and the numberof happy customer/user faces. The following guidelines should be observed.
1. The saw slots must be the proper depth (1-1/2 to 3 inches), clean, and with no sharp cornerswhich could damage the wire insulation during installation. The greater depth should be used insofter pavement materials, such as asphalt, to protect the loop installation for a longer period of timeagainst damage from surface erosion and wear. The minimum depth required may be calculatedmultiplying the width of the saw cut, as shown in Table 4, by the number of turns of wire requiredand adding the diameter of the backer, as shown in Table 4, and at least 1/2" of sealant. Loopshould not be installed more than 5" below the surface.
2. Figure 5 illustrates a typical saw cut loop installation using the chamfer cut corner treatment.The corners are cut at a 45° angle to eliminate sharp edges, which could wear through the wire andcause it to break. Figure 6 illustrates a point of caution when cutting chamfered corners. Cut onesaw cut across another saw cut only enough to insure an even depth at the intersection. Do notallow the square sides to intersect at the corners. The triangular shaped piece of pavement canbreak away, causing it to “float “ and damage the loop. Another form of corner treatment is shown inFigure 7. In this method, the saw cuts intersect at right angles and the points of intersection arecore drilled with a 1" to 1 1/4" bit to the same depth as the saw cuts. These holes eliminate thesharp corners at the points of intersection.
3. The wire used in the loop should be 16 AWG stranded with insulation rated for direct burial. 20AWG wire may be used in monolithic concrete slabs where the presence of rebar or post tensioncables too close to the surface make it impossible to install the larger wire. The sides of a loopusing 20 AWG wire should not cross expansion joints, as movement between slabs can tear theloop apart. Remember that the different types of wire available from wholesale outlets are designedfor power distribution, NOT FOR LOOPS. Since moisture can cause significant changes in thedielectric constant of the insulation, which results in excessive loop (frequency) drift, choose aninsulation which is most impervious to moisture. Polyvinyl chloride (PVC) insulation should beavoided, since it tends to absorb moisture. Cross linked polyethylene (XLPE) insulated wire is veryresistant to moisture absorption and provides good abrasion resistance. U.S. Traffic Corporation’sP/N DSI-116, DSI-116S, and DSI-120 loop wires meet these requirements (see Table 4). The loopmust be wound with one continuous length of wire. No splices are allowed in the slots!! The wiremust be carefully installed in the slot. U.S. Traffic Corporation offers a useful roller tool (see Table4) that can be used to safely install the wire (and the “backer rod” mentioned in the next paragraph)firmly in the saw cut slot. Sharp objects, such as screwdrivers, putty knives, and thin sheets ofmetal should not be used.
4. The wire must be held tightly in the bottom of the slot by means of a plastic foam type materialcalled ‘backer rod ‘ (see Table 4). This approach shall be used for the entire perimeter of the loop,as well as the saw cut between the corner of the loop and the point where the slot exits thepavement. When this approach is used to completely cover the wire, it forms a barrier between thewire and the sealant, which holds the wire firmly down in the slot but allows it to shift laterally withany pavement shift, thereby increasing the life expectancy of the loop. Loose wires, on the otherhand, can cause false calls when subjected to vibration or sudden movement. If the wire iscompletely covered, it is ESSENTIAL that no voids exist, which would allow water to collect. Thewater will expand during freezing conditions. Freeze/thaw cycling will push the wire through thesealant up and out of the slot, causing a loop failure.
5. Choose a sealant carefully to match the application and the pavement. Hard setting epoxiesshould not be used with asphalt. Caution should be observed when using hot sealants, as hightemperatures can damage or destroy some types of wire insulation.
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III - LOOP INSTALLATION TECHNIQUES (Cont.)
Figure 5
Figure 6
9
III - LOOP INSTALLATION TECHNIQUES (Cont.)
Figure 7
6. Where the loop wires leave the lead-in saw cut at the edge of the pavement, they shall be tightlytwisted with a minimum of five turns per foot. Use tape on the twisted portion to hold the wirestightly together. This will prevent false calls from movement between the wires.
7. A loop feeder (home run) cable shall be used if the distance between the end of the saw cut andthe detector exceeds approximately 10 feet. The feeder cable shall be a shielded, twisted pair witha high density polyethylene insulation. U.S. Traffic Corporation’s P/N DSI 1602 loop lead-in wiremeets these requirements. The shield shall be floated (left unconnected and insulated) at the spliceend and shall be grounded to earth ground at the cabinet end only. Any other groundingarrangement can lead to ground loops and cause erratic system operation.
8. All splices (at the end of the saw cut or loop feeder cable) shall be soldered, even when initiallydone with crimp type splices. WIRE NUTS ARE NOT ALLOWED. Each splice point shall beprotected with a moisture proof seal. Failure to observe these two precautions are two of the mostcommon causes of future problems in the system. CAUTION: when soldering, use only enoughlocalized heat to adequately flow the solder through the connection without burning the surroundinginsulation. Soldering shall be done with a copper tip. DO NOT USE DIRECT FLAME!!
9. Another common problem is caused by loose connections at the terminal strip (where a terminalstrip is used) in the cabinet. Crimp type terminals should be soldered for additional security and thescrews on the terminal strip securely tightened down. Adding lockwashers is a further deterrent tothe screws loosening up with time and vibration.
10. It is strongly advisable to perform a loop inductance and leakage test. This can beaccomplished using the U.S. Traffic Corporation Inductive Loop Analyzer P/N ILA-550 or the DigitalLoop Testing System (comprised of DSI P/N 501 Frequency Tester Module, DSI P/N 503 LoopExcitation Module, and DSI P/N 507 Loop Insulation Tester) or equivalent. Refer to appropriateproduct manuals for test instructions. Loop inductance measurement shall be between 20 and2,500 microhenries. Leakage resistance shall be equal to or greater than 100 megohms.
10
III - LOOP INSTALLATION TECHNIQUES (Cont.)
B. Preformed loop installation.
The same careful attention to proper installation techniques described above for saw cut loopsapply to the installation of preformed-formed loops. As previously described in Section II-10,preformed loops are recommended for installations in new concrete or asphalt. These loops shouldbe installed 2" - 3" and never more than 5" below the surface. Also note that the 1700 Series isespecially recommended for asphalt, as the asphalt can be poured directly on the loop, eliminatingthe loss of capping created by the need for the blanket of sand or dirt required over the 700 Seriesloops. Installation of the 700 Series loop is illustrated in Figure 8. A 1700 Series loop would beinstalled in a similar manner. The installer should plumb in his own 1/2" PVC from the operatorcabinet, menu board, or speaker post to the stub on the 700 Series loop and splice to the stub onthe loop, after pulling the loop lead-in thru to its final destination. The 1700 Series loop will notrequire the extra PVC to be installed.
C. Loop Electrical Wiring Diagrams.
It is beyond the scope of this publication to show all configurations of loop installations and how theyare wired. Figure 9 illustrates two basic situations, centered around a single loop. In Figure 9A, theloop lead-in is run directly to the detector, while Figure 9B illustrates the use of a feeder (home run)cable to extend the loop lead-in a longer distance back to the detector through conduit or PVC pipefrom a pull box, hand hold, or other interconnect box.
IMPORTANT! When installing multiple loops, DO NOT construct them out of one long continuouspiece of wire. This will make it impossible to change the phasing or isolate the loops for futuretroubleshooting and will necessitate replacing the entire loop system, if any part of the systemshould become damaged. Construct each loop as if it were the only loop being installed. Make theseries connections in the pull box or at the detector and follow the pattern shown in Figure 10.
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III - LOOP INSTALLATION TECHNIQUES (Cont.)
Figure 8
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III - LOOP INSTALLATION TECHNIQUES (Cont.)
Figure 9
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IV - DETERMINING LOOP PHASING
It was mentioned previously that, when two loops are used on the same detector, they should behooked in series and the direction in which each loop is wound electrically is important for propergate or door operation. It is also very important that the loop connections be accessible formaintenance and repair. In new installations, where the loops are already installed and covered, wecan determine the phasing (the direction in which each loop was wound) and connect them in thedesired rotation, as previously discussed. The required tools are simple: a 6V lantern battery and apocket compass. Follow these simple steps to determine phasing (see Figure 10).
1. Identify the wire pairs with the loops in the street. This can be accomplished by temporarilyconnecting each wire pair to the detector and using the U.S. Traffic Corporation Loop FinderModule, DS P/N 502, to locate the active loop.
2. Temporarily connect all loops in series.
3. Connect the free wire from the first loop to one terminal of the battery. Leave it connected untilall loops have been checked for proper phasing.
4. Place the compass directly over the wire of the first loop on the side nearest the second loop.
5. Momentarily connect the free wire from the last loop to the battery and note the movement of thecompass needle. Warning: Do not hold the connection very long, as the low resistance of theloop acts as a short on the battery and can overheat the loop wire and discharge the battery!
6. Place the compass directly over the side of the second loop on the side nearest the first loop andrepeat step 5
7. If the needle pointed in the same direction in steps 4 and 6, the connections are correct forattracting fields (noise reduction). If the needle pointed in opposite directions in steps 4 and 6,the connections are correct for opposing fields (allows a gate or door to pass between the loopswithout being detected). To reverse the phasing, reverse the connections for the second loopand repeat steps 4 through 6 to verify the desired phasing.
8. Upon completion, disconnect the wires from the battery and make permanent (soldered andwaterproofed) connections on all wires in the loop circuit, including the wires connection to thelead-in wires to the cabinet.
Figure 10
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V - EQUIPMENT INSTALLATION
It is not practical to show every model of detector in inventory. However, this section will attempt todescribe some of the more common configurations, which can be adapted by the installer to meeteach specific installation. There are three different types of product packing. The first is the shelfmount type, completely enclosed in a metal case with a connector, either front or back, and requir-ing a harness with a mating connector, to allow for easy installation and removal. The connectormay be one of several types. Table 5 shows three common harnesses, used in parking and accesscontrol applications, with mating connectors, pin-outs, and wire colors. Please note that the pinnumbers and functions are also shown on the detector cover. One exception to this type is thedetector used for drive thru installations. The Model 917-2 unit features a power cord and individualwires from the front panel to make the remaining connections (see Figure 12 for wire colors).Please note that the wire colors and functions are also shown on the detector cover.
The second type is a small unit in a plug-in plastic relay enclosure. This type of unit plugs into arelay socket supplied. The relay socket has screw terminals to which the installer can connect hisown wiring directly. No harness is required. Refer to documentation furnished with the unit for theactual connections.
The third type is an open printed circuit board, usually incorporated in the operator, with a specialconnector on the board. A special harness is required. See Table 5 for the pin-outs and wire colorsfor the harnesses for the Models 927 and 929.
1. Unpack the detector and mating harness. Use the following resources, as needed, to con-nect the harness: Table 5, pin-outs and color code sheet included with the harness, pin-outsand function list shown on the cover of shelf mount detector, applicable product manual,Figure 11 for parking and access control detectors, and Figure 12 for drive thru detectors.
NOTE: Figure 11 does not show the wire colors because of the variation in connector types andpin-outs for the wide number of models covered. It does, however, demonstrate the concept.
2. Connect the loop lead-in wires from the buried loop to the appropriate wires of the matingharness. Refer to Figure 10.
NOTE: These connections MUST be soldered. All connections exposed to the atmosphere MUSTbe properly sealed with a weatherproof seal. The loop wires in the harness MUST remain twisted.
3. Connect the output relay wires of the mating harness, as required, to the external equipment.
4. Be certain that the power to be used for the detector is OFF. Connect the power lines from themating harness to the power source lines.
5. Check all connections and verify that the source power matches the detector to be installed(12V for a 12V unit and 120V for a 120V unit, AC for an AC unit and DC for a DC unit).
6. Plug the mating harness into the detector unit.
VI - EQUIPMENT OPERATION
Refer to the applicable Product Manual for the detector being installed. NOTE: Most U.S. TrafficCorporation Detector Systems loop detectors (except for those used in drive thru applications) maybe ordered in either the failsafe or failsecure mode of output configuration. Refer to Section VIII foran explanation of the differences. See Table 6 for failsafe configuration. See Table 7 for failsecureconfiguration. See Table 8 for drive thru detector configuration; remember that these detectors canhave either presence or pulse outputs, as selected.
Single leaf and biparting double leaf swing gates provide a real challenge in providing adequatesafety loop protection. Figure 13 shows a single leaf loop layout and Figure 14 shows a double leafloop layout. No loop or roadway dimensions are shown, as this is a generalized drawing suitable formany applications. Let us use Figure 13 first. For this discussion, the gateway is 14’ wide. LoopsA1 and A2 are 3’ x 8’, although the 3’ dimension could be increased to 6’ to increase the detectionheight for higher clearance vehicles, if necessary. Remember that the detection height of a loop isequal to 2/3 of the shorter dimension of the loop. If the 8’ dimension parallel to the gates were to beincreased beyond 12’, the 3’ setback dimension would need to be increased to 25% of the length ofthe side of the loop parallel to (facing) the gate. The setback is required to prevent loops A1 or A2from detecting the gates in motion. This leaves a 20’ long zone with no protection between thesetwo loops. To protect this zone, loop B is installed. Its dimensions are 6’ x 8’ to divide the zone ofvulnerability into two 7’ zones, which should be less than the wheelbase of most small cars. It isimportant to remember that, with all the plastic in the bumpers of many modern cars, the detectionzone may be shortened.
Figure 13
Now let us examine Figure 14. Again assume a gate width of 14’ and 3’ x 8’ for loops A1 and A2.The same setback rules as used above apply here. This leaves a 13’ long zone with no protectionbetween these two loops, somewhat long for some modern small cars. If we installed a 3’ x 8’ loopfor loop B midway between loops A1 and A2, it would divide the zone of vulnerability into two 5’zones. That would be great, except for one minor problem. Loop B would be only 2’ from the gatein the closed position, less than the minimum 3’ setback required. The gate would never clear thefield of the loop when it closed. To solve this problem, locate loop B 3’ inside the closed gate. It willnow be 6’ from loop A1 and 4’ from loop A2. This will guarantee continuous detection of any vehiclepassing through the gate.
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VII - SWING GATES (Cont.)
Figure 14
In Figure 15 , we see that the output of detector B is run through the normally closed contacts of theclose limit switch. The only time these contacts are open is during the gate closing cycle. It isunimportant if detector B picks up the moving gate while it is opening, as it only reinforces the opencommand. However, the output of this detector must be disconnected from the operator during theclosing cycle to keep the gate from continuously cycling open and shut during the time the gate issupposed to be closing. Should a second vehicle try to sneak through the gate while it is closing,loop A1 will cause the gate to reopen and will reactivate loop B when the gate returns to the fullyopen position. The gate will be allowed to close when the vehicle clears loop A2.
Figure 15
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VIII - FAILSAFE? FAILSECURE? WHICH DO I WANT?
Are you confused about the meaning of failsafe and failsecure configuration for the inductive loopvehicle detector? Maybe this will help.
The detector output consists of a single pole, double throw relay, providing both a normally open(N.O.) and a normally closed (N.C.) output. See Figure 16. The relaycontacts are “dry”; they provide no output voltage. They only switch anexternally applied voltage, much like a light switch or a doorbell button.The doorbell button, the light switch, and the relay contacts have one thingin common. They touch the ends of two wires together to close the circuitor separate them to open the circuit. Please don’t try this at home or atwork to turn the lights on and off. Use a light switch. It is safer.
Where a detector has two outputs, this discussion applies only to theprimary output, generally called Output A. To simplify things, we willdiscuss the operation of the N.O. contacts, the most common usage. Imagine the output to be likethe doorbell button. As long as the button is not pressed, the circuit is open and nothing getsthrough to ring the doorbell. If the button is pressed, the circuit is closed and current gets through toring the doorbell. The same logic, only inverted, applies to the N.C. contacts. In the failsafeconfiguration, N.O. and N.C. output conditions exist only with detector input power applied, loop(s)connected, and no vehicle on the loop(s).
If the closure of the N.O. contacts is considered to be a detect output, the failsafe will produce thisoutput:
• when a vehicle is detected on the loop(s),• when there is a loop failure, or• when input power to the detector is lost.
This will explain why a loop failure or loss of input power will cause the free exit detector to open agate or door or the safety detector to hold the gate or door open, once opened and the vehicle hasleft the loop(s). The advantage of this mode is that the user is immediately aware of a problem andwill report it. Also, vehicles are not trapped within the perimeter of the gate area in the event of aloop failure or loss of input power. The disadvantage is that security may be compromised.
A failsecure detector, on the other hand, will produce a detect output only when a vehicle is on theloop(s). This is great for high security applications when used as a free exit detector. In the eventof a loop failure or loss of power to the detector, the gate or door will not open and you are sure tohear about it, probably not in a nice tone of voice. Don’t use a failsecure detector for a safetyapplication. If the loop fails or input power is lost, there is no obvious indication of a problem andNO PROTECTION. This problem can be partially compensated in our newer detectors featuring asecond output, generally called Output B, with a switch selectable Loop Fail mode. This output maybe used to control a local warning strobe or to turn on a remote visual or audible warning indicationin the event of a loop failure. Or the Loop Fail output can be paralleled with Output A. This setupwill produce an output
• when a vehicle is detected or• when a loop fault occurs.
IT WILL NOT PRODUCE AN OUTPUT IF POWER TO THE DETECTOR IS LOST.
Unless a fuse in the detector fails (a very remote possibility) or someone inadvertently turns off thecircuit breaker for the detector, somewhere back up the line, the possibility of losing power to thedetector alone is pretty slim. In the event of a complete loss of commercial power, there would beno adverse effects using either configuration. No power - no gate or door operation, unless, ofcourse, the operator contains a backup source of power.
Output B, where provided, is always in the failsecure configuration.20
VIII - FAILSAFE? FAILSECURE? WHICH DO I WANT? (Cont.)
Below is a typical output relay logic table (taken from the 326 detector series) The N.C. relaycontacts for Output B are not available on detectors using a 10-pin MS or Molex connector and onolder detectors (416 and prior) using an 11-pin Amphenol connector. Check the manual or thecover of the unit. If you study this table, you may be able to make sense out of the precedingexplanation.
IX - A HELPING HAND
Have you ever wanted to check a detector by yourself by dragging a piece of metal across loop butyou couldn’t run back fast enough to see what was happening to the indicator on the detector? Andyou wished you could find a warm body to help you? You probably already have a helper for whichyou don’t have to pay straight time, overtime, vacation, sick leave, hospitalization, social securityand will never talk back to you. Interested?
Do you have one or more power cords with you? Try doubling it up and throwing it across the loop.Drag the male and female ends back to where you can watch the detector. Plug the male end intothe female end and watch the detect indicator turn on or red. See Figure 17. Pull them apart andwatch the detect indicator turn off or green. See Figure 18. If the distance between the loop andthe detector is more than half the length of the power cord, you may have to add one or moreadditional power cords to the chain. This is an excellent demonstration of why you should neverinstall a new loop over all or any part of an abandoned loop. It could over time, develop shortsbetween the turn or across the lead-in and produce random detection, in the same manner as thepower cord but uncontrolled. Always either remove the abandoned loop or cut all four sides at rightanles to break it up.
