the the Technology Interface / Summer97



Derek R. Barker
Director of Sales and Marketing
Ranger Security Detectors, Inc., El Paso, Texas

Presented: Workshop on “Access Control and Accountability
of Nuclear Materials”, Lawrence Livermore Laboratory,
Livermore, California May 21, 1997


This paper describes a multiple-zone walk-through metal detection technology that uses a Continuous Wave Multiple Sensor Measurement Technique rather than the widely used Pulse Induction Method. Multiple-zone measurement techniques increase screening efficiency and cut operating costs. This new technology is competitively priced and suitable for both weapon screening and loss prevention applications.


A new walk-through metal detection technology developed in El Paso, Texas, is proving an attractive alternative to the commonly used twenty year old Pulse Induction method. It is based on a unique Continuous Wave Multiple Sensor Measurement Technique that increases screening efficiency and cuts operating costs.

The design objective was to develop a realistically priced family of high performance detectors. This multiple zone technology, called Intelliscan, is suitable for high security and high traffic weapons screening and loss prevention applications. The family of detectors includes eighteen, six and two zone models.

Eighteen zones of detection allow the exact location of concealed weapons to be pinpointed. A display shows if a weapon is located on the left, right or center of the person being screened and if it is at ankle, knee, waist, chest or head height. If more than one weapon is being carried each is detected and its location shown. During an ensuing manual search, security personnel can immediately target the object, or objects, that created the alarm(s.) The six zone model functions identically to the eighteen zone equipment but simply indicates the height of the object above ground level. The two zone unit has an upper detection region and a floor zone with fully adjustable sensitivity. The display indicates if an alarm occurred at foot level or in the upper regions of the detector. The importance of adjustable floor zone sensitivity is discussed latter.

An important feature of horizontal multiple zone technology is its ability to efficiently discriminate between miniature handguns and harmless objects. People being screened can pass through the detector without the inconvenience of having to empty their pockets of normal amounts of keys and coins, etc. Weapons manufactured from steel, various grades of stainless steel, aluminum, zinc alloys and mixed alloys are detected using a single operating mode.

The technology offers excellent electrical interference rejection, thus eradicating false alarms that halt traffic flows. Uniform detection exists throughout the entire detector even when it is installed on a floor containing reinforcing steel. The equipment is also continuously active, which means that weapons or contraband cannot be passed, slid or tossed through the detector undetected.


FIGURE 1.     
    Transmitter elements and receiver sensors scan the person being screened from both sides. Low intensity magnetic fields are used that are safe for wearers of heart pacemakers, pregnant women, film and magnetic storage media. Metal objects are analyzed 33,000 times a second. This contrasts with Pulse Induction detectors that typically scan at around 400 pulses per second. The continuous wave technique’s multiple dual-balanced sensor array and 80 fold higher analysis rate, enables metal objects to be measured with great precision and over a wide range of metal transit speeds.

An embedded computer is used for Digital Signal Processing (DSP) and a self-monitoring diagnostic program ensures consistent, highly stable drift free operation. The detection circuitry is continuously active at all times. Highly efficient electromagnetic interference rejection algorithms eradicate the high false alarm rates common to many detectors. The technology can be used in environments where many detectors are rendered inoperative.


High quality detectors produce less false and unwanted alarms. False alarms result from electrical interference, whereas, unwanted alarms arise due to the inability of the technology to discriminate efficiently between harmless objects and the targeted weapon. Detectors with good discrimination, therefore, increase traffic flow rates, cut operating costs and minimize inconvenience. The false and unwanted alarm rate for a high quality multiple-zone detector is typically less than 5%. Conventional lower quality detectors often have false alarm rates that exceed 25%. This means that at least one in four people create an alarm and must be physically searched unnecessarily. Typically, less than one in twenty people have to be searched when a high discrimination detector is used. Traffic congestion and operating costs are immediately reduced. Consequently, less detectors are needed for a given number of people. Using less detectors means a lower real estate requirement and fewer security personnel.


      FIGURE 2. 
 Good discrimination is the key to high traffic throughput. In weapon screening applications, discrimination can be classified as the ability of a detector to differentiate between a weapon and objects such as keys, coins, belt buckles, money clips, etc. Also, false alarms arising from electrical interference are virtually eradicated. Figure 1 shows an example of items that can be carried though a multiple-zone detector without it generating an alarm; Its sensitivity is adjusted to detect consistently an FAA test piece and mini-revolver.

Figure 2 shows how signals from harmless objects are processed by a conventional single zone detector. Pulsed magnetic fields produced by a transmitter (left panel) pass through the person being screened to the receiver panel. As a pulse passes through a metal object it causes eddy currents to be induced in its surface. Once the pulse passes beyond the object its field strength decreases and the eddy currents in the metal start to decay. As the currents decay they generate a low intensity magnetic field. This field is detected by the receiver coils and processed into an electronic signal. The size of the signal from a particular object is determined by its size, shape, surface area, its orientation with respect to the primary magnetic flux, the type of metal and the speed it passes through the detector.

       FIGURE 3. 
Most people carry metal in the form of jewelry, belt buckles, metal frame glasses, pocket change, money clips, watches and metal within their shoes. As these various items pass through the detector they generate discrete signals that are integrated into a composite signal. A cumulative effect takes place; the more items being carried the larger the signal becomes. When this cumulative signal grows in magnitude and is larger than the signal generated by a weapon, the detection threshold is exceeded and an unwanted alarm occurs.

This cumulative effect occurs to a much lesser extent in a multi-zone detector. True multi-zone detectors house a number of horizontal zones (independent detection channels.) These horizontal zones can be considered as independent detectors housed in a single frame assembly. The sensitivity of each zone can be individually adjusted over a wide span of sensitivities. The detection channel associated with a specific zone only processes the signals from objects that pass through the zone. This typically reduces the size of the cumulative signal, as unlike a conventional detector, the signal from a belt buckle is not processed in the same channel as the signals from shoes or eyeglasses.

Figure 3 depicts how the Intelliscan’s eighteen zones of detection are arranged, while figure 4 shows the display panel. The six adjustable horizontal zones are subdivided into three vertical zones. The vertical zones are used to identify if a weapon is being carried on the left, right, or in the center of the body. The display panel for the six zone model is shown in figure 5.

FIGURE 4.       


Multi-zone detectors can compensate for the detection losses that occur when a support floor contains structural metal and a portion of the magnetic flux deviates through the steel. The sensitivity of the unit’s lower zones can be independently adjusted to offset the resultant reduction in signal strength at ground level. To compensate for these losses, it is often necessary to increase the sensitivity of the floor zone by 40 to 60%.

When a conventional detector is installed on a floor containing reinforcing metal, the overall sensitivity of the entire detector must be increased. For example, with no metal in the floor the optimum sensitivity for a specific weapon, or test object, might be 30%. When a floor contains metal the overall sensitivity may have to be raised from 30 to 45% in order to detect the weapon at floor level. Raising the overall sensitivity means that the central and upper regions of the detector become overly sensitive. Objects smaller than the weapon that pass through these regions will produce unwanted alarms and halt traffic unnecessarily.

FIGURE 5.       

By way of contrast, the upper zones of a multi-zone detector can be operated at normal sensitivity. In the above case, the lowest zone’s sensitivity would be adjusted to consistently detect the weapon at around 45% while the other zones would be operated at an optimal 30%. The ability to make independent adjustments allows a consistent detection response to be achieved throughout the entire gate assembly. This prevents the upper region of the detector from becoming overly sensitive.


When comparing sales literature it is easy to draw the conclusion that the performance of all detectors are similar. This if far from the truth. Before purchasing a detector test it at the location where you intend to use it. It is possible that it may not be capable of functioning satisfactorily in your environment. The following are characteristics you should look for when evaluating a detector. If you already have a detector check that it complies:


Select a detector that gives uniform detection throughout the entire interrogation region.

When testing the performance of a detector carry the weapon or test object about four feet above the ground and along the center line of the detector. Adjust its sensitivity to just consistently detect the object. If a handgun is used point the barrel downward. Make several passes and check that an alarm is generated each time. Now check the uniformity of detection by passing the object through other regions of the detector e.g. the zones shown in figure 3.

The detection response from a poorly designed conventional detector will not be linear. High sensitivity hot spots and low sensitivity regions known as dead spots may be present within different regions of the detection field. The detection response from a particular metal object will vary as it is carried through different sectors of the gate. It is imperative to know where dead spots are situated. Always set up the detection sensitivity (the threshold level at which an alarm occurs for a specific weapon or test object) while passing the weapon through the worst dead spot. Failure to adopt this procedure could result in a weapon passing through the unit undetected. When the overall sensitivity has to be raised to detect a weapon in the region with the lowest sensitivity, the other regions of the detector become overly sensitive and unwanted alarms from harmless objects increase.


When screening for small metal components high sensitivities must be used and high quality interference rejection is essential. Without it, the level of false alarms could be intolerable. Interference is also referred to as noise.

Electrical and electromagnetic interference are the Achilles heel of metal detectors, they can play havoc with their performance and often render them unusable. Interference rejection is what separates quality detectors from others. Quality detectors utilize highly efficient noise inhibition software algorithms. Noise originates from a variety of sources; flickering fluorescent light tubes, computer monitors, photocopiers, two-way radios,etc. The level of ambient noise will vary from location to location. If practical, prior to purchasing a detector it is advisable to test it in the location where you wish to eventually install it.

Even some well known manufacturers have never mastered the problem of inhibiting noise. They resort to deactivating their detectors while no traffic is passing through them. This prevents false alarms occurring when no one is near the detector. This type of unit only functions as a metal detector when the person passing through the unit breaks an invisible IR sensor beam. In reality, it does not solve the interference rejection problem, it simply masks it. When the detection circuitry is activated by someone passing through the detector, false alarms will occur if interference is present. In noisy environments these detectors are inefficient or virtually inoperable.


Always select a detector that is continuously active. At no time should the detection circuitry be deactivated.

When a detector utilizes deactivation sensors of the type mentioned above (they are sometimes called traffic counters) security can be easily breached. A sizable percentage of detectors fall under this category and their manufacturers never mention this weakness. A large handgun, assault weapon or grenades can be passed, thrown, kicked or dragged through them without generating an alarm. If you currently utilize a detector that is not continuously active, never leave it unsupervised. Under no circumstances incorporate this type of unit in an unmanned automatic entry door system. If you are unsure about your detector try sliding or tossing a test object through it.


For both weapons screening and pilferage prevention the detector must be capable of efficiently detecting a variety of metals in a single operating program.

Although detectors are usually equipped with multiple operating programs only one program can be selected at any time. Generally, most people only ever use one program. Having a large number of programs is not necessarily an asset as a high percentage are probably irrelevant to your requirement. It should not be considered an important factor when selecting a detector. Do, however, ensure that at least one program functions well on all the metals you want to detect. When screening for weapons, a single program must give consistent results on handguns and knives manufactured from all grades of stainless steel, steel, aluminum, zinc alloys and mixed alloys. Similar sized objects made from different metals or alloys should be detected within a relatively narrow band of sensitivity settings.


Irrespective of whether the target object is carried very slowly, quickly or tossed through the detector it should always be detected.

Object speed is one of the factors that impact upon the magnitude of the detection signal. Quality detectors provide consistent detectability over a wide dynamic range of object speeds. A range greater than 0.33 to 16 feet per seconds is desirable. A useful test is to strap a weapon to the inside of your ankle and walk down the center line of the detector at a very brisk pace. Alternatively, toss the object through the center of the detector. In both cases it should be detected.


A detector must reliably detect a weapon irrespective of its position on the body, or the orientation in which it is carried, e.g. at any angle or any plane.

As the orientation of an object changes the signal response also changes. This is particularly noticeable as rod or knife shaped objects are rotated in a longitudinal direction. If you observe that changes in a test object’s orientation results in inconsistent performance, e.g. alarms occur in certain orientations but not in others, increase sensitivity to a level that produces consistent alarms in all orientations. These variations should be relatively small.


A detector should be easy to install and setup.

If you need to move the detector, or the local environmental conditions change because of a new interference source, it is important that you are able to easily reset operating parameters without outside assistance. Be wary of complicated detectors that need to be optimized by a factory technician. Reliance on outside assistance is likely to lead to future down time and an increase in operating costs.


Check that rapid temperature changes do not adversely affect the stability of the detector.

This can be important if it is used at the entrance of a building where large changes in temperature are experienced every time the doors open.


In a large manufacturing plant the challenge is to efficiently screen large numbers of employees within a short period. The magnitude of the screening task increases as the number of employees increase and the quantity of metal in the targeted component decreases. In applications such as computer manufacturing, detectors must operate reliably at very high sensitivities.

Fast, efficient screening is reliant on the cooperation of the work force and good quality detectors. Some clothing and structural metal in footwear often contain significantly more metal than the components being targeted. To prevent traffic bottlenecks, employees must be educated and encouraged to enter their workplace without metal on their person, albeit, carried or within clothing. Employees may initially resent being screened, however, if the introduction of detectors is planned carefully and handled with tact, acceptance will follow. Most employees are willing to accept that in today’s competitive environment companies must protect themselves against the theft of high priced components. Failure to do so, lowers profitability, jeopardizes jobs and a company’s survival.

An important consideration is the deterrence factor. Employees need to be aware that if they carry contraband through the detector that it will always be detected. When a unit is first installed it is advisable to operate it at very high sensitivity and allow employees to observe that it can detect very small items. A detector that cannot do the job adequately is a liability rather than an asset.


It is advisable to check a detector daily. Carry a test object through it several times and check that it alarms. A good detector should operate consistently and rarely need adjustment.

Pre- and after-sales support are key considerations. Ensure that installation and training by a local manufacturer’s representative is an integral part of the delivery.

Availability of a factory customer 1-800 telephone support line and local consultation is important.

If required for continuous outdoor use, purchase a fully weatherproof unit capable of operating under all local climatic conditions. The unit should also tolerate harsh U.V. radiation without serious degradation.

Be aware that not all multi-zone detectors can identify the location of more than one object simultaneously. In some devices only the location of the largest object is displayed.


The new Continuous Wave Multiple-Zone detectors are realistically priced and set a new standard in metal detection. Their low false and unwanted alarm rate and ability to accurately pinpoint the location of weapons, make them very attractive for screening large numbers of people. They are ideal for busy airports, sporting events, entertainment complexes and any other high traffic location. They are equally suited to high security applications and loss prevention.

First time purchasers of metal detectors are advised to buy a continuously active detector. If you have to work with a non-continuously active detector ensure that it is closely supervised and under no circumstances should it be used in an unmanned automatic entry system.