Fingerprint sensors give consumers a hand

For many years, sciencefiction writers have employed futuristic scanners that compared fingerprints, faces, or retinas. Unfortunately, engineers have had to wait until now to successfully and economically implement these types of “biometric” devices in real systems. Fingerprint sensors available from suppliers such as Atmel, AuthenTec, Fujitsu and other companies now let engineers add biometric capabilities to consumer products such as door locks, ATMs and laptop computers (Figure 1).



Datastrip, of Exton, PA, for example, manufactures handheld security devices that can read information from “credentialed” documents such as passports and government-issued ID cards. The company’s DVSII line of mobile readers scans fingerprints and matches them with information encoded in a smart card, swipe card, internal database or remote database (Figure 2). “Often, you must verify the authenticity of a document and then determine that the document bearer is the proper person,” explained Stuart Tucker, manager of the Technical Sales Support Division at Datastrip. “People must prove who they are.”



“An airport in Europe uses a security system that puts encrypted fingerprint templates on each employee’s smart card,” said Tucker. To enter a secure airport area, employees present their access card. The security system compares features extracted from a new fingerprint scan against the encrypted template saved in the employee’s card. If someone not entitled to a card obtains one, he or she cannot get through the system because the fingerprint information will not match. Most biometric security systems combine “what you know” and “what you have” information. They do not rely solely on biometric information. Although engineers can buy off-the-shelf biometric equipment, they may choose a biometric sensor and add their own processor and algorithms. Vendors offer two basic sensor types — a swipe sensor that requires a user to move a finger across an active linear detector or an area sensor that simply requires momentary placement of a fingertip on a small “window.”



When they start a design, engineers must decide whether their system will validate or identify people. Validation, as described in the airport example, simply matches a scanned fingerprint of an ID-card holder with the two or three fingerprint templates stored during the enrollment steps. Identifying a specific person by comparing a scanned fingerprint with dozens or hundreds of stored templates requires more processing speed or time. (A system may need from 6 to 8 kbytes to store one fingerprint template.) As a rule of thumb (pun intended), most fingerprint systems should accept or reject a fingerprint in under half a second. So, to reduce processor demands, many embedded systems first ask “Who are you?” answered with a smart card or PIN, and then, “Is that really you?” answered by a fingerprint scan.



VERIFICATION SYSTEM

Whether engineers design a validation or a verification system, they must set thresholds for a false-accept rate (FAR) and a false reject rate (FRR). A false accept means the system may accept someone who does not belong. That action may cause a minor loss in a public library, but you do not want false accepts at a nitroglycerine plant. False rejects occur when a system rejects someone it should accept. To reduce false reads, people often enroll more than one finger.



Atmel offers a family of FingerChip sensor that measures the temperatures of a fingertip’s ridges and valleys. The company’s latest CMOS sensor, AT77C105A, also serves as a “navigation” device, much like a touchpad on a laptop (Figure 3). This swipe sensor provides eight lines with 280 pyro-electric detectors per line, and it produces a 4-bit value per detector. The sensor can keep up with the fastest finger motion.



Because the Atmel sensor passively measures small temperature differences, it can operate at very low power. According to Bruno Charrat, Atmel’s biometrics product line director, average current consumption during use comes to approximately 5 to 6mA, an attribute that makes the sensor attractive in portable batterypowered devices.



A complete biometric-based security system requires a sensor, a processor, driver software, biometric software and application code. Additionally, Atmel provides developers with individual components as well as powermanagement technologies and development packages for biometric and security applications.



The development packages include reference designs for products such as smart phones and for products based on the Windows CE operating system. When engineers approach Atmel for components for or assistance with an application, the company can give them an example of a product similar to the one they want to design. Although Atmel relies on third-party suppliers for featureextraction and fingerprintmatching software, it can provide this software to developers under license.



Instead of creating a fingerprint template from a surface image, AuthenTec’s sensors (Figure 4) conduct a modulated RF field into a finger and detect the signal that comes out the fingertip’s live cells, explained Art Stewart, VP of Business Development at AuthenTec. Because that technique does not examine surface features, contamination, dry fingers, sweaty fingers, and so on do not affect a scan.



ALGORITHM CONTROL

The sensor’s RF field operates under control of an algorithm that periodically looks for the presence of a fingertip. If the sensor does not detect a finger, it reverts to a “sleep” mode. When a finger starts to move across the sensor, a processor scans in an image. “When you lift your finger, the RF field turns off,” said Stewart. “Although engineers worried about interference from another signal in a product, we have experienced no problems.”



Although AuthenTec offers area sensors, developers now favor the company’s swipe sensors. The latest device, the 1510, offers a 128 x 8-pixel array, and it produces a 4- bit value for each pixel. Equipment designers can use the array as a navigation device so users can make menu choices and move a display cursor.



“On a sophisticated cell phone, different fingers can launch various Web sites and provide a user name and password,” said Stewart. To assist developers, AuthenTec created proprietary software drivers that manage sensor control, matching algorithms, and a mouse-type interface. Application code uses AuthenTec’s API to “connect” to the sensor.



OEMs who have decided to include fingerprint matching in a product can start with the company’s reference design kit and software developer’s kit. The former include schematics, board layouts and software binary files (driver, algorithms and applications) Software kits come with a sensor module, DLLs, object code, algorithms, drivers and other software tools.



Sensor companies often align themselves with independent consultants and design houses that can offer hardware and software help. “An entrepreneur came to us with a prototype for an accesscontrol system that combined an RFID-based card and a fingerprint sensor,” said Ruffino, sales engineer at Vanteon, a consulting and design firm. “The prototype did not work well, so we redesigned the system and crafted a ‘package’ the entrepreneur could reuse in other applications. And he could take the new design right to a manufacturer.” (Vanteon retains none of the client’s IP.)



Multifinger enrollments, mentioned earlier, can serve as more than a backup technique if users have a problem scanning an enrolled finger. “You can use different fingers to imply different behaviors,” noted Chuck Ruffino. “An index-finger scan could mean normal access. But if you scan another finger, it means, ‘Someone has forced me to access this system.’ The system notifies people you will come into the building, but against your will. So, you can design in a subtle communication mode that a criminal might not detect.”

DSP chips mate sensors and applications DSP chips can play a key role in fingerprint security systems. The Web version of this article www.ecnmag.com contains more editorial information about where DSP chips fit in the signal chain and the tools suppliers offer to simplify signal-processing developments.