EE Times - August 6, 2007 - (Page 36) planet <<35 In addition, the second axis enables a full 360° of measurement capability. By itself, a single accelerometer cannot distinguish which half of the rotation cycle it is in. If a motor system rotates to an incline angle of 45°, then another 135°, the accelerometer output will be the same. Another important factor is that the plane that encompasses both accelerometers must be orthogonal to the horizon. As the plane tilts away from this position, additional error terms are introduced. Hardware implementation The hardware realization of this type of tilt system starts with sensor selection. There are a number of factors that will play a role in this selection process. The range of the sensor should theoretically be at least ± g. From a practical perspec1 tive, ± g would provide some margin 1.5 for sensor error and a small amount of vibration—which can sometimes be filtered if it doesn’t saturate the sensor. As the range of the sensor increases, the resolution will decrease, presenting a potential threat to performance goals. Another important consideration will www.eetimes.com feature be the output-signal configuration, which will dictate the level of complexity in the processor interface. The three most common interfaces are analog output, pulsewidth modulation (PWM) and digital. The analog output configuration requires an analog-digital interface. The two other configurations can feed directly into a processor platform, which will perform the acceleration-to-incline-angle math described in Figure 1. While the two direct-interface options (PWM and SPI) present less complexity, the performance of the sensor will likely have substantial influence as well. Thermal, power supply and life-stability influences on the accelerometer’s sensitivity and bias are key parameters to consider in this process. In many cases, these factors will make an analog-output sensor the most attractive option. The digital platform can be from any number of technologies, including microcontrollers, digital signal processors, field-programmable gate arrays and programmable logic devices. The selection will depend greatly on the processing needs at the system level. The analog/digital converter selection will depend on a number of important parameters, including resolution, power forms, reference configuration (or ratiometric parameters), input impedance (buffering may be required), accuracy and stability. Many microcontroller platforms provide A/D converters, but they may not have adequate resolution or an interface that is suitable for ratiometric sensor outputs. Achieving valuable accuracy Successful development of an orientationsensing system must start with a clear performance goal. For example purposes, let’s explore a system that can measure ± away from horizon, on two different 45° axes, at an accuracy of ± 0.5°. Using the relationship between acceleration and incline angle in Figure 1, these design goals translate into acceleration accuracy requirements of approximately 6 mg. Assuming that the accelerometer selection is constrained to mainstream, high-volume offerings, one of the best performing options has a sensitivity variation of ± percent and bias specification of ± 4 25 mg. The bias variation alone is four times the allowable requirement, assuming that every other influencing factor is perfect. By itself, the sensitivity of 4 percent would introduce an additional error of 2.3° at an incline angle of 45°. The amplifier, multiplexer and A/D errors will contribute as well. The bottom line is that this level of accuracy will require calibration. One of the more practical approaches to calibrating a dual-axis accelerometer is known as the “four-point tumble” method. In this arrangement, the accelerometers are vertically mounted to a motor or other apparatus that moves the accelerometers in 90° steps. (See the full version of this article online at www.planetanalog.com, article ID: 200900046, for Figure 3, illustrating the positions at which the accelerometer is characterized, and Figure 4, illustrating the response each accelerometer would have over the entire 360° rotation range. Figure 4 shows ideal curves for both axes, along with an exaggerated measured response for illustration purposes.) This characterization information is used to develop scale and offset factors, which are loaded into correction tables for the accelerometers. Once the accelerometer accuracy is established, incline angle accuracy follows. Ideally, the total change in accelerometer output should be 2 g, and the curves should be centered around zero. The following equations generate the appropriate scale and offset correction factors for each accelerometer: In order to correct for other factors, additional characterization steps may be required. For example, power-supply variation may require doing the same calibration sequence for multiple power supply levels. Conclusion Microelectromechanical-system technology offers a number of advantages to industrial system developers who are looking to integrate analysis of motion or orientation into their systems. Integrating MEMS into such systems will require wise sensor selection, development of the appropriate interface circuit, and assurance of the appropriate level of accuracy through calibration. This integration process will evolve as developers continue to face short development cycles and pressure to improve value through performance increase or cost reduction. Also, the introduction of single-package options, such as Analog Devices’ ADIS16201 family of products, will provide faster solution paths for achieving sub-1° accuracy levels in applications that value incline sensing. The bottom line is that inertial MEMS sensors provide a valuable function to motion control system developers, but in many cases require extra processing to meet all of the performance criteria. The primary question these developers face is whether to develop the required circuit and processes internally, or to purchase the capability and focus on other issues in the design. Ultimately, there is a cost associated with achieving performance improvements. Each developer will have to weigh specific goals in order to choose between the potential for incremental component cost savings and the faster time-to-market and lower development costs associated with internal development. ■ ■ elementals “Using Op Amps as Comparators” Analog Devices Inc. Application Note-849, November 2006 www.analog.com/Uploaded Files/Application_Notes/468752 82066493AN_849.pdf “High-Side Current-Sense Measurement: Circuits and Principles” Maxim Integrated Products AN-746, March 26, 2001 www.maxim-ic.com/ appnotes.cfm/appnote_ number/746/ Once the scale/sensitivity errors have been compensated, then the offset can be calculated: For the full article, go to www.planetanalog.com and search article ID: 200900046 36 Electronic Engineering Times | August 6, 2007 http://www.eetimes.com http://www.planetanalog.com http://www.analog.com/UploadedFiles/Application_Notes/46875282066493AN_849.pdf http://www.maxim-ic.com/appnotes.cfm/appnote_number/746/ http://www.planetanalog.com http://www.rabbitwireless.com
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