There's little room for error in the high-performance systems designed to help racecars travel at 200-mph speeds, or bring spacecraft safely back to Earth. Faro Technologies (Lake Mary, FL) is one of those behind-the-scenes companies providing engineers with the tools necessary to make such machines perform. The company manufactures FaroArm, a portable, human-assisted 3D-measurement system that combines the accuracy and repeatability of a robot with the adaptability of a human. The measuring system includes the operator, a passive articulated arm with embedded motion control technology, and software designed to simplify acquisition and use of 3D CAD data to solve complex manufacturing or assembly problems.
Falling somewhere in between the simple, one-dimensional touch probe and the complex CMM (Coordinate Measuring Machine), FaroArm brings high-accuracy 3D measurements to environments where it was not possible before, explains Faro CEO Simon Raab. Depending on the model, FaroArm achieves 0.001-0.004-inch accuracy in working spheres that range from 4-12 ft in diameter. While CMMs achieve greater accuracy (in the tenths of thousandths range), they are more expensive, not portable, and don't hold up in harsh manufacturing environments. The lightweight and portable FaroArm, in contrast, costs one-third to one-fifth that of a conventional CMM and is designed to withstand a variety of harsh environments.
Technically, FaroArm is a six-axis electro-goniometer, or instrument for measuring angles. More simply, it is a set of rotational links, or joints called transfer cases, designed to be moved by a human being. The arm has a probe tip at one end that the operator positions on key points of the part being measured. After each probe placement, a simple button click allows the system to determine probe-tip position from the measured-angle data at each joint. The measured data is automatically correlated with nominal data, typically in the form of a CAD model residing in the machine's CPU, and the result is fast, accurate, computer-aided part and assembly measurement for any product where dimensioning and orientation are critical.
Because Faro engineers wanted to improve functionality and reliability without increasing size, weight, or power consumption, they chose to use DigitalDNA technology from Motorola over FPGA (Field Programmable Gate Array) technology in each transfer case. A single embedded DSP56F803 combines enough processing power and flexible peripheral blocks to replace three discrete components at each of the measuring arm's joints. Using encoder feedback from each of the six joints, the DSP resolves probe-tip position accurately and quickly. But speed is just as critical on the factory floor as it is on the racetrack. Especially when a complex multi-part assembly problem has shut a production line down. Quality control and manufacturing engineers have to figure out if the problem is with the tooling or assembly jigs on the down line. Or is it welding trouble on the other end of the plant? Maybe it's a plastic forming problem on yet another line?
Such complexity means that manufacturers require a portable device like the FaroArm to make precise measurements and perform complex model analysis. To comply with increasing customer expectations, design engineers at Faro focused on three areas of improvement:
Availability of working space. The FaroArm is a portable, lightweight product, so design engineers didn't want to increase the size of the joints or compromise rigidity.
Power consumption. FaroArm re-quires inter-processor communication via links running along side the discrete components making up the arm. Adding components to increase functionality would add to the overall power consumption/dissipation and could interfere with communication, negatively affecting reliability.
Reliability. With six joints and numerous discrete components at each joint, the probability of field and manufacturing failures increases with the addition of new components to the design.
To meet these challenges, Faro design engineers first looked for a board-manufacturing subcontractor that could deliver improved functionality that wouldn't negatively impact size, power consumption, or reliability. But engineers decided to make a fundamental change in their methodology when the new design goals were not realized. "It became clear that we needed to integrate functionality into a single device," says Andy Helm, electrical engineering manager at Faro. "Adding discrete components to an already discrete-heavy design was not going to get the job done."
DSP vs. FPGA. Faro engineers studied FPGA and programmable DSP technologies before making their decision. Both technologies would consolidate most of the existing discrete components into a single device and provide for increased functionality. However, the decision to proceed with DSPs was based on the technology's programmability. Digital signal processors can be programmed during manufacturing, allowing full functional testing before system integration. DSPs can also be re-programmed in the field, providing for increased functionality and diagnostics.
With a single device, engineers eliminated a number of discrete components from the design by using the Motorola DSP56F803 (see sidebar). Functionality consolidation in a single device also results in a number of less-tangible benefits. "Our development cycle time went from several weeks to just a few days," Helm comments. With on-chip debugging capabilities, engineers at Faro could program, test, re-program, and re-test their DSP software multiple times a day to obtain the desired behavior/performance. Prior to implementing the DSP solution, changes requiring physical updates to the circuitry consumed several days between successive tests.
Replacing several discrete components with a single digital signal processor reduces power consumption, according to Helm. DSP functionality incorporated communication capabilities, allowing the processor itself to link with the communication channels in the FaroArm. The result was improved noise-immunity and reduced power consumption. Another tangible benefit of incorporating functionality in a single device was increased reliability. "Considering the compounding effect of failure rates, less is best," says Helm.
Choosing the right sub-contractors meant partnering with a company with the right skill set and tools to add value to the end product, not just selecting a board manufacturer that could deliver at the lowest cost. With the added DSP functionality, Faro selected a sub-contractor with the capability of programming and testing the subassembly prior to delivering it to Faro. The boards received at Faro for final assembly need no testing; they are already tested and proven functional at the manufacturer. Any board that does not pass testing can be diagnosed and re-worked at the manufacturer without involving Faro. Both of these items resulted in significant cost and cycle time reductions.
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