EE Times - August 6, 2007 - (Page 35) www.planetanalog.com planet Design knowledge for the analog and power engineer Monday, August 6, 2007 Making MEMS accelerometers work in motion control Error budgets and tolerances affect a successful design; look at which tactics can overcome those problems By Joseph Bergeron and Mark Looney Microelectromechanical system (MEMS) inertial sensor technology provides a major structural shift in mechanical sensing. When the performance meets their needs, system developers willingly trade complicated mechanical mounting schemes for the simple solder-reflow attachment processes used for integrating MEMS sensors. Automotive safety systems were one of the first consumers of these products. As the MEMS manufacturing infrastructure developed to support their high-volume requirements, additional market segments, such as mobile handsets and game controllers, embraced these functions and eventually became primary drivers of product development themselves. While the applications for inertial sensing differ widely, they all have common requirements for this function: • inertial sensing (linear, angular) in small, pcboard-capable packaging; • reliable operation; • performance levels that were appropriate for the functions they serve; • low power dissipation; Joseph Bergeron is director of engineering for the MultiChip Products Group at Analog Devices Inc. He earned a BSEE from the University of Rhode Island and has 27 years of experience in integrated-circuit and electronic component development. He can be reached at joe.bergeron@analog.com. Mark Looney is the iSensor application engineer for Analog Devices. He earned an MSEE degree from the University of Nevada in 1995 and has 12 years of experience in design and applications engineering. He can be reached at mark.looney@ analog.com. Analog insight By Bill Schweber • cost effectiveness; • simple integration. Other users who value all of the above factors include industrial-system developers. One example is in motion/motor control systems, which sometimes use orientation sens- about the author ing to establish a reference position or to verify proper command execution. In some cases, using an accelerometer as a tilt sensor provides a valuable function that can simplify system designs. This article highlights the process of turning this simple concept into a function that provides value for motion-control systems. The key differentiator in this case will in the “performance level that is appropriate for the purpose it serves.” The reason is that the accuracy required in these systems is often higher than the performance provided by the discrete sensors. Tilt sensing overview Tilt sensing encompasses a wide variety of approaches, which vary in both complexity and performance. In this case, the tilt-sensing system will use gravity as its only stimulus, and a MEMS accelerometer as its sensing element. MEMS accelerometers typically employ a tiny, spring-loaded structure that is interlaced with a fixed pick-off finger structure. The spring constant of the “floating” structure determines how far it will move when subjected to a force. This distance is observed as a change in capacitance by a modulation/ demodulation circuit. Since the structure is responding to a force, it does not matter if the force is due to acceleration (F = ma) or is a static force such as gravity. Figure 1 illustrates the basic, single-axis MEMS accelerometer approach to tilt sensing. “Horizon” is defined as the plane that is orthogonal to the earth’s gravity. “Incline angle” refers to the tilt angle with respect to the horizon. When the accelerometer is parallel with the horizon (incline angle = 0), its measurement will be zero. As the incline angle approaches 90°, the accelerometer’s measurement will approach 1 g, assuming perfect accuracy on the part of the sensor. An important consideration for a singleaxis tilt sensor is that its sensitivity to the incline angle decreases as the incline angle increases, eventually converging to zero when the accelerometer is in the vertical position: Too much information, or too little? W Depending on the orientation system’s measurement range, a two-axis approach may be useful in maintaining accuracy goals, particularly if the measurement range is greater than ± and the accuracy requirements are 30° sub-1°. In this configuration, which is displayed in Figure 1, the two sensors are orthogonal to one another. As the sensitivity of one degrades, >>36 the other increases, and vice versa. e’re generally inundated with too much information, but there’s one source that gives us too little: the “check engine” light. Many check-engine indications are for minor issues, such as a loose gas cap; others are “false positives,” or sensor misreads. The indicator has just two alerts: steady-on for minor problems, blinking for serious ones. When you design a product, it’s a challenge to balance user needs, system self-assessment, ease of use, cost, display area and so on. There is no easy or single answer to these trade-offs. But it would be worth the design effort to offer drivers a check-engine indicator that tells them more initially, and then lets them drill down, if they choose. Perhaps the indicator output could go from yellow to amber to red, depending on the severity of the problem, or blink faster for more serious issues. I’m taking the DIY route and getting a sub$100 test meter that can read the internal trouble codes via the car’s onboard diagnostics interface. Perhaps I’ll try next to decode my PC’s operating system error messages—or should I even have to? ■ ■ August 6, 2007 | Electronic Engineering Times 35 http://www.planetanalog.com
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