Design Con 2015

In defense of calibration

-August 01, 2003

Electrical Lab stats
Land and air
Partners in test
Estimated Measurements

Baltimore—During the Iraq war, F-16 Falcon fighter planes took to the skies, and Longbow Apache helicopters provided air support for ground troops. These and other air- and ground-based war machines rely on radar systems and other sensors from Northrop Grumman that keep weapons systems on target.

Located a few yards from the runway at Baltimore/Washington International Airport, Northrop Grumman Electronic Systems sector (www.es.northropgrumman.com) manufactures radar electronics and antenna arrays for military aircraft and ground-based defense and civil systems. ("Land and air " describes the products manufactured in Baltimore.)

Radar systems consist of digital, analog, RF, and power electronics that need testing. Throughout the Northrop Grumman facility, engineers and technicians use oscilloscopes, multimeters, spectrum analyzers, network analyzers, frequency counters, RF power meters, and power supplies—from new to downright old—in their work. Every test instrument requires periodic calibration and maintenance. The Electronic Systems sector's Baltimore calibration lab keeps those test instruments on the job.

Headed by manager Gary Jennings, the calibration lab regularly calibrates and repairs over 30,000 instruments each year. Of those instruments, approximately 20,000 perform electronic measurements on lab benches, in production test stations, and in the field. The remaining instruments are used during manufacturing and inspection to measure physical and dimensional properties such as temperature, pressure, torque, mass, and length.


Gary Jennings, Northrup Grumman's calibration lab manager, stands in front of a jet containing a radar array manufactured by Northrup Grumman.

The electronic calibration lab consists of 26 calibration stations, each dedicated to a class of instruments (see "Electrical Lab stats "). Twenty technicians perform the calibrations while eight software/metrology engineers, led by Rich Appel, write and update calibration procedures. Two metrologists, including Bernie McDermott, assist technicians in installing new equipment and processes. An administrative staff handles the logistics of logging equipment and keeping the instruments flowing through the lab.

With so many calibrations to perform on such a wide range of instruments, the calibration lab must run efficiently to supply its 500 internal customers (instrument owners) with calibrated equipment. Often, that equipment remains in service for over 20 years. To minimize turnaround time and ensure repeatable results, the eight engineers have automated the calibration process wherever possible.

Automate, automate

Computers control each calibration station and, where possible, any equipment under test (EUT) that has an IEEE 488, RS-232, or VXI I/O bus. The engineers develop automated calibration procedures using SureCal (www.surecal.com), a metrology software package developed in-house that controls test instruments. They store the procedures on a network, thus ensuring that technicians such as John Keller use the latest procedure every time. Until a few years ago, engineers distributed calibration procedures on floppy disks. By keeping the calibration procedures on the network, engineers save time loading them into the individual stations, and all stations always run the latest version.


Figure 1. Technicians get instructions that show them how to connect instruments for calibration.

Besides writing the automation software, the support engineers develop graphical user interfaces for each calibration. They also develop online instructions that show technicians how to connect each EUT to the equipment in the test rack. Figure 1 shows a typical instruction screen that shows a technician how to connect an HP (Agilent) 83630B synthesized swept-signal generator to instruments such as a microwave receiver, a function generator, and a voltmeter.

Simply automating calibrations covers only part of the lab's success story. The rest comes in how the staff manages the workflow and keeps customers informed about the status of their calibrations. Computer networking and an Intranet provide technicians, engineers, and the internal customers with the information they need. The database contains records on the 30,000 instruments the lab calibrates each year as well as on 15,000 inactive instruments.

To manage workflow and keep accurate calibration records, the lab's engineers use LabMate calibration management software from Norfox Software (www.norfox.com). LabMate stores information such as equipment owner name, instrument type, serial number, calibration history, and next calibration date. Calibration data resides on a network server.

Move through the lab


Figure 2. Over 500 internal customers send test equipment to the lab for calibration. A network server generates status e-mails and makes calibration data available.

The workflow (Figure 2) for each calibration starts 60 days before the instrument's due date at the lab. At the start of each month, equipment owners receive e-mails notifying them of an upcoming calibration. The following month, owners receive a second reminder.

When an owner brings an instrument to the lab for calibration, lab coordinator Fay Wilson enters it into the database. The equipment goes to a shelf for calibration at its assigned station.

When a technician receives an instrument, he or she opens its database record and downloads the instrument's calibration procedure. After connecting the EUT to the test instruments, the technician starts the calibration. If the EUT has a communications bus, the software will take over its control. If the instrument lacks a communications port, the technician will manually adjust the EUT when prompted, then enter the test results into the database. The technician then adheres a calibration sticker on the EUT and places it on the shelf for return to its owner.

If a technician receives an instrument that's out of calibration, he or she notifies the owner. Depending on how the owner uses that equipment, products tested with that instrument may require retesting because the tests could have resulted in a "false positive"—the passing of product that should have failed a test.

From data collected at each calibration, the calibration staff can make recommendations to other engineering groups about an instrument's reliability and repeatability. Lab manager Jennings points out, "From what we learn through calibration, we can support our engineering groups in determining the best instruments for engineering, manufacturing, and test."

Jennings also analyzes workflow data so he can best use technicians and calibration stations. Because each EUT has an assigned calibration station, Jennings can look at each station's workload and decide if a test instrument needs reassigning to balance the lab's workload, thus improving turnaround time. Jennings also uses workload information to provide balance among technicians, often reassigning them to different calibration stations.

Reducing uncertainty

A calibration lab needs more than a smooth workflow to satisfy its customers. The measurements made on equipment calibrated in the lab must confirm to specifications. To keep the calibration stations within specification, metrologists must calibrate the lab's equipment.

Calibrating the lab's test equipment reduces measurement uncertainty and improves product quality. "No measurement has any value without an uncertainty," says metrologist McDermott, so calculating uncertainty and maintaining measurement quality sit atop the challenges that the engineers face.

Part of the uncertainty comes from how users make measurements. "People who use test equipment often don't understand the sources of errors," says Appel, "yet we see an increasing emphasis on reducing measurement uncertainty."

Errors that come from connectors, cables, and probes add to any instrument's uncertainty. To compensate for cable and connector errors, Northrop Grumman engineers use a "system calibration" approach, where technicians calibrate their customers' test equipment using the same types of connectors and cable that an owner uses. By measuring signal losses in connectors and cables, the engineers can use software to compensate for errors during a calibration, thus reducing an instrument's uncertainty. (See "Estimated measurements .")

Even with calibrated equipment in the stations, the lab needs trained technicians to maintain quality. Northrop Grumman's staff works with a local community college to develop training courses for calibration technicians. The training courses consist of 10 modules that cover all aspects of measurements. Electronic technicians can earn a certificate when they complete seven of the 10 modules relevant to their jobs.

The modules, each of which take 40 hours of class time to complete, consist of basic measurement theory, statistics, uncertainty, and risk assessment. All technicians must complete these modules. Electronic technicians must also complete modules covering time and frequency, DC and AC signals, inductance and capacitance, oscilloscopes, and RF/microwave measurements. The training program lets technicians move among calibration stations. That, says Jennings, "gives technicians a wide range of experience, plus it lets us shuffle the staff to adjust to changing workloads."

Uphill battle

Northrop Grumman's calibration staff has found that other departments don't always appreciate their work. McDermott says, "Test engineers and technicians are starting to appreciate that added value that calibration brings to a product. But some people feel that calibration only adds to a products cost without seeing the value that calibration adds. So, test engineers and metrologists must defend the calibration function. Management needs to understand the financial value that calibrated equipment brings to a company's products."

"When you run a successful calibration program," says Jennings," people take it for granted." As a result, some people may think that calibration isn't important—until something goes wrong with product quality. Only then do they realize that calibration adds value to a company's products. To demonstrate the calibration lab's effectiveness, the Northrop Grumman staff takes a proactive approach. For example, the lab staff calculates the optimum calibration intervals, thus ensuring that equipment receives calibration frequently enough to maintain quality yet infrequently enough to minimize costs. They report any changes in intervals, and their financial impact, up the management chain.

In building a successful calibration lab, Jennings and his team of technicians, engineers, and metrologists have demonstrated that calibration adds value to an organization. The value comes in the company producing quality products while minimizing costs. The next time someone tells you to forgo a calibration, point out that calibrated equipment contributes to a company producing a quality product. Without calibration, you don't know what you're producing.

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