Engineered security

Noah Fong, InVision Technologies -- Control Engineering, 12/1/2003

The security threat faced by the world's airlines since the terrorist attacks of September 11, 2001, seems only to increase. Reports continue to surface about Al Qaeda's intent to again use commercial airplanes as terrorist weapons.

Such threats point to an increased need for ever-tighter airport security, coupled with the ability to better detect potentially dangerous items in checked and carry-on luggage. Responsibility for creating devices capable of fulfilling this demand falls largely on the shoulders of engineers.

One company putting the skills of engineers to use in this area is InVision Technologies, a manufacturer of explosives detection systems (EDS). In 2001, the Transportation Security Administration (TSA) awarded InVision contracts totaling more than $500 million to provide equipment for screening 100% of all checked baggage by the TSA's deadline of December 31, 2002. Since then, the TSA has ordered more than $50 million worth of InVision's CTX 9000 DSi—the company's newest EDS designed to integrate into the baggage handling systems of certain U.S. airports.

The CTX 9000 DSi system is FAA-certified to screen 542 bags per hour. As the conveyor moves each bag through the machine, the system creates a scan projection X-ray image. The onboard computer determines areas that require more detailed images, which are then taken by the rotating X-ray source supported on a gantry that rotates at 120 RPM, enabling a precise section image to be generated within half a second. Using computer algorithms, the CTX 9000 DSi analyzes these images and compares their properties with those of known explosives. If a match is found, the system alarms and displays the object on the screen.

Control Engineering's editorial director, David Greenfield, spoke with one of InVision's senior electronics design engineers, Noah Fong, to inquire about engineering decisions that go into the design of such systems, with particular focus on current and voltage management issues.

Q With so many machine components in such a small space in the EDS—conveyors, air conditioning, computers, a rotating gantry, and RF links—what areas are of greatest concern in current and voltage management?

There are definitely some areas of CTX9000 DSi where voltage and current are more important. Different components in the system range from simple logic-level computing and integrated circuits that don't require much power, to motors, X-ray sources, and power distribution. We hook up to a high power grid infrastructure, which means we typically take in, at the lowest level, 480 ac 3-phase. This has to be transformed, converted, and distributed throughout the machine subsystems [a power distribution section handles that task]. Our system is primarily dc, so that power also has to be rectified appropriately to the various subsystems. Roughly 60% of the machine is wired for high voltage, high current. The rest is low voltage, logic-level types of power.

Q With a machine designed for such heavy use in a populated environment with a wide variety of end-users, what do you see as the most important control issue in the design of CTX 9000 DSi?

Modularity of the control is the most important control aspect. All components are on separate control systems. That allows us to use the computer to control the sequence of how things are going. We have conveyors on one control system, the gantry rotation speed control on a separate control system, and the mechanism that turns the X-ray on and off is a separate control system. The X-ray mechanism doesn't have motors, like the gantry and conveyors, but it does have feedback and control to detect if the X-ray is on and operating properly. We also have active curtains—once luggage goes in, a lead curtain closes behind it so that operators are not exposed to X-rays. These lead curtains are a flexible membrane on a powered roller, which unrolls the curtain into place, shielding the ends of the tunnel—motors are used here, not for speed control, but limit of travel. We have a motion control box for the conveyor motors, which is complete in itself, representing the servo amplifiers, the motor speed control, and encoder. A similar setup is used for the gantry as well. We designed it this way so that during the development and production of the machine, we could quickly make software changes to ensure correct sequencing and the timing.

Q Do your concerns with current and voltage management change depending on how and where the end product will be used?

The environment for these machines is controlled to a large extent, so we're not concerned about adjusting the machines to a particular environment. The machines are designed to operate over a wide temperature and humidity range. Although the environments these machines are used in do vary widely, they are usually not extreme.

Q Beyond basic operation of the unit, what are your concerns as an engineer regarding products for managing current and voltage?

My first concern is reliability, then life span of the product. We can't have these machines make a mistake or have a fault that causes them to make a mistake. The components have to work. High current and high voltage stress components, so we want to make sure anything we use is sturdy and rugged enough to last thousands of hours.Transducers and suppressors affect more than half of the CTX 9000 DSi, especially the transition from high voltage to low voltage. That's a sensitive area. Because of the rotation of the gantry, high voltage enters the gantry through slip rings and brushes, similar to a motor commutator. That's a noisy interface, so we have protection on it to keep it clean and not interfere with the gantry electronics.

Q What kind of products does InVision use in these areas and why?

One product supplier is Phoenix Contact. Their components are modular and DIN-rail mounted, which means that we can easily prototype a system when needed. Wherever we can put a DIN rail on a chassis, we can mount their stuff. That also gives us the ability to rearrange and reconfigure easily. Once configured and the prototype works, we can use the same products directly in manufacturing.

For more information from Phoenix Contact, visit www.phoenixcontact.com/ . Learn more about InVision Technologies at www.invision-tech.com.