Have you ever wondered exactly how loops work? Well, it’s something you should consider. Understanding the fundamental technology behind these products can help you install more accurately and quickly, avoid installation problems and troubleshoot faster. Here’s your guide to how loops work.
When direct current (DC) passes through a wire, an electric magnetic field (EMF) or flux is created around the wire. For example, when a wire is coiled around a metal rod energized with a battery, current flows through the wire and causes the rod to act like a magnet. Therefore, the more turns in the coil or increases in current flow, the greater the magnetic field or pull.
If the current is removed, the magnet field collapses back into the wire. In the case of alternating current (AC), the current changes direction and creates a magnetic field opposite what it was when the current was moving in the original direction. In an AC circuit, the field that is collapsing is pushing against the developing field. This “pushing back” is a form of resistance known as inductance. Any time you have AC passing through a wire, you will have inductance. All detection loops will have AC applied to them; that is why detection loops are sometimes called inductance loops. Iinductance is measured in units called henrys (a henry is a large unit; inductances in practical circuits are measured in millihenrys or microhenrys). A common range of inductance for a detection loop is 40 to 300 microhenrys.
When a detector energizes a loop with AC, the size of the loop, number of windings in the loop, length of lead-in wire and wire size will all determine the total inductance of the loop circuit. The detector will determine how much current is flowing through the loop and set that amount as the standard. When a metal object enters the EMF created by the loop current, the metal object absorbs some of the collapsing EMFs. Because some of the collapsing EMF is now absorbed, it lowers the resistance in the loop circuit. This causes an increase in current flow through the wire that is detected by the detector. When this happens, the detector will either open or close a relay switch to activate a command in the gate operator such as open for exit, reverse for safety, or hold open or closed for a swing gate with a center or shadow function.
Loop phasing comes into play when two loops are used with the same detector. A common application would be when two reverse loops are used with one on each side of a sliding or vertical gate. Proper phasing is accomplished when the loops are connected in series with each loop current flowing in the same clockwise or counterclockwise direction.
With the current flowing in the same direction in each loop, you will notice that the two loop legs nearest the gate will have currents moving in the opposite directions. This will cause what is referred to as “field cancellation effect.” That is, the fields of sensitivity will both have the same magnetic polarity (north or south) toward each other. Since like fields repel, the fields of sensitivity are pushed up and away from each other, causing a dead or null field between the loops. This cancellation effect will allow the loops to be placed closer to the gate without being detected. Because the fields are pushed up, they now are more sensitive next to the gate path, making for a safer reversing loop feature.
On the other hand, if the loops are connected with the current flowing in the opposite directions, the poles of each loop will have opposite polarity, causing the fields to attract each other. This will cause what is referred to as “field enhancement effect.” This will cause the area between the two loops (the gate path) to attract each other, increasing the sensitivity in the zone where the gate travels, and therefore increasing the chance of the gate being detected as it closes. If detected, the reverse detector will keep reversing the operator as the gate tries to close, setting up the requirement to send out a service technician.
By arming yourself with this knowledge, you will have less repeat service calls and happier customers.
Brian Dickson is the production manager of BD Loops, a manufacturer that has made preformed direct burial and sawcut inductance loops for the gate industry for more than eight years. BD Loops has more than 180,000 loops installed with only one loop failure, and is available through 130 distributors nationally. The company has several letters of recommendation testifying to its professionalism and design, and is a member of the following associations: AFA, IDA, NOMMA and IMSA. Visit BD Loops at www.bdloops.com and use the distributor locator tool to find a distributor near you. Call BD Loops at 714.890.1604.
MORE INFO:
Easy Reference of Terms
Henry (H) —The SI unit of electric inductance. A changing magnetic field induces an electric current in a loop of wire (or in a coil of many loops) located in the field. The henry is a large unit; inductances in practical circuits are measured in millihenrys (mH) or microhenrys (µH). The unit is named for the American physicist Joseph Henry.
Alternating Current (AC) —In alternating current, the movement (or flow) of electric charge periodically reverses direction. An electric charge would, for instance, move forward, then backward, then forward, then backward, over and over again.
Direct Current (DC) —The unidirectional flow of electric charge. It may flow in a conductor such as a wire, but can also be through semiconductors, insulators or even through a vacuum as in electron or ion beams. With direct current, the electric charge flows in a constant direction, distinguishing it from alternating current (AC).
Inductance — 1. The property of an electric circuit by which an electromotive force is induced in it as the result of a changing magnetic flux; 2. A circuit element, typically a conducting coil, in which electromotive force is generated by electromagnetic induction.
Related Content:
AC vs. DC : The Ups and Downs in Operator Technology