MEMS series, Part 1: for sensors, MEMS the word.
Although most engineers buy MEMS devices, some products require the design of custom devices, which ECN will cover in Part 2 of this series.
Apply Some Pressure
Obviously MEMS-based pressure sensors measure pressure, but engineers also use them to "convert" physical quantities, such as air flow or liquid level, into pressure values that an electronic system can measure. Those sorts of versatile applications help explain the widespread and growing use of MEMS pressure sensors.
When engineers plan to use a MEMS pressure sensor, they must carefully review its specifications which include the pressure range and the type of pressure--absolute, differential or gauge--the sensor will measure. Like any sensor, MEMS pressure sensors are not perfect. They have a minimum sensitivity (the change in voltage output per change in pressure input) and a small offset voltage (the voltage produced for the no-pressure-applied condition).
"Engineers also must realize these MEMS sensors exhibit temperature sensitivity," said Dave Monk, Sensor Development Engineering Manager in the Sensor Products Division at Freescale Semiconductor (Tempe, AZ). "Many, but not all devices have on-board temperature compensation that nulls out thermal effects. But in any sensor, engineers must understand the accuracy limits of the device as detailed in the specifications. Engineers will find small remaining errors due to nonlinearities, minor temperature artifacts or hysteresis. Often, they can use software in their microcontroller to reduce those errors, but they may not eliminate them all," he added.
Designs must include provisions to connect a sensor to a pressure source and engineers must remember a pressure sensor gets exposed to whatever atmospheric conditions it will measure. A washing machine, for example, sounds like a benign environment. "But detergents produce an alkaline solution, the same sort of thing we use to etch silicon," said Freescale's Monk. So a system's "plumbing" may need to isolate the sensor from the measured environment but still let the sensor make accurate pressure measurements.
Freescale Semiconductor specifies standard pressure sensors only for dry-air measurements. If engineers use a MEMS sensor rated for dry air in an environment that involves liquids, they may get a corrosion failure. Many engineers do not understand the need for 'media compatibility' when they apply pressure sensors because such problems do not arise for standard ICs and components.
Freescale employs a fluorocarbon or fluorosilicon gel on sensors to isolate them from harsh environments. But, the company will develop a package and tailor gels to produce a customized sensor that can resist acids, bases or fuels. New MEMS sensors that operate inside a vehicle tire to measure air pressure must include a special filter that repels water and oil.
Inertial MEMS sensors--accelerometers and gyroscopes (gyros)--come in the same types of packages as ICs, so designers need not worry about exposure to environmental conditions. But, unlike pressure sensors that measure one physical quantity, a MEMS accelerometer can measure acceleration, position, shock, vibration and incline (tilt).
Those attributes explain why MEMS accelerometers find use in vehicle navigation, roll-over detection and dynamic-control systems. In a roll-over detection system, sensors determine when a vehicle is about to roll and then deploy side airbags to protect vehicle occupants. (Continental Teves of Auburn Hills, MI supplies active rollover protection to the auto industry as part of its MEMS-based electronic stability control system.)
[FIGURE 1 OMITTED]
In a vehicle dynamic-control system, sensors measure a vehicle's trajectory and tell the system where the vehicle is going, while the steering wheel tells it where the driver wants the vehicle to go. The difference between the measurements causes the system to take corrective action to get the vehicle back on course. (The new Volvo XC90 SUV comes with a rollover protection system and a dynamic roll-stability controller.)
[FIGURE 2 OMITTED]
Engineers will not get good performance from a MEMS accelerometer or gyro if they simply plunk one into a circuit. "They must understand the dynamics and the mechanics of their system and know the interplay of the measurements, the critical angles of motion and the rates of acceleration," said Richard Mannherz, Customer Marketing Manager at Analog Devices (Norwood, MA) "Then they can select a sensor based on its dynamic-range and bandwidth specifications."
Here is an example of how mechanics enters into design criteria: In a given piece of equipment, the physical apparatus and the MEMS sensor will each have a resonant frequency. As you might guess, an inertial sensor such as a gyro or accelerometer will not respond well when the frequency of the apparatus comes close to the resonant frequency of the sensor. According to David Krakauer, Product Manager for Gyroscopes at Analog Devices, some sensors with a low resonant frequency may experience more problems than those with higher resonant frequencies because low frequencies tend to predominate in mechanical equipment. Krakauer cautions designers to ensure a module or system mechanically filters out frequencies that occur near a sensor's resonant frequency.
[FIGURE 3 OMITTED]
"MEMS accelerometers are dynamic devices," noted Freescale's Dave Monk, "so they have a certain bandwidth and frequency response. Designers should understand where the frequency roll-off occurs and how the bandwidth will affect their proposed use of a MEMS accelerometer." A shock rating also applies to these sensors. It may seem as though a MEMS sensor in a cell phone operates in a benign environment, but invariably, it gets dropped and experiences a high drop shock. So sensor manufacturers specify a drop-shock rating and engineers should design with that rating in mind.
Designing a MEMS accelerometer into a system also involves electronic considerations. Accelerometers, just like MEMS pressure sensors, provide a fairly linear output, but many applications do not demand high-accuracy measurements. MEMS-accelerometer manufacturers usually describe accuracy in terms of percent full-scale (FS), and low-cost accelerometers used in large quantities by auto manufacturers offer an accuracy of 4 to 5 percent FS. But designers can buy MEMS sensors with an accuracy of 0.1 percent FS, although they are not manufactured for high-volume, low-cost markets.
Some MEMS accelerometers, such as those manufactured by Analog Devices, include several extra components, such as resistors, capacitors and a temperature sensor. Designers can use the components to build filter circuits and to add temperature compensation to measurements. The temperature sensor does not automatically correct measurements, but the overall system can acquire temperature data and use it in an algorithm to adjust raw data. (The company provides application information that explains this technique.)
Plan for Power Needs
In an application such as the tire-pressure sensor mentioned earlier, low power consumption becomes a key design goal in both sensor and circuit design. Consumers will get mad if they need to change a tire because an internal pressure-sensor unit ran out of power. (The pressure monitor comes as part of the valve-stem assembly and operates entirely within a tire. A competing scheme measures tire characteristics by way of a vehicle's existing ABS circuitry.)
"But when you reduce the power, you get proportionally more noise in the system," noted Monk of Freescale Semiconductor. "Designers have to come up with clever ways to manager sensor power without losing accuracy." Freescale's tire-pressure sensors use a capacitive sensor rather than a piezo-resistive device that would consume more power. The sensor's controller remains in a sleep mode most of the time. It wakes up every few seconds, takes measurements, turns off circuitry and then sends pressure data to a vehicle's computer.
"Overall, MEMS sensors are not that different from any complex IC; they just add a mechanical dimension, or 'layer,' to a design," noted Analog Device's Mannherz. "When someone uses a new processor chip, they often need some help. That holds true for MEMS sensors, too, but engineers do not have to go to extraordinary lengths to apply MEMS devices." Data sheets provide key information and specifications, and application notes provide supplemental information about the mechanical aspects of using MEMS accelerometers and gyros.
"Recently, a customer wanted to increase the dynamic range of a MEMS gyro beyond what we specified," said David Krakauer of Analog Devices. "Most customers do not need that capability, so we did not cover it in detail in the data sheet. But we have an application note that explains how to increase dynamic range well beyond the data sheet limits." When customers cannot find information in data sheets and application notes, they can use vendor's application hotlines, Web sites and application engineers as resources.
Get A Board
To get started experimenting with a MEMS sensor, suppliers recommend that engineers obtain an evaluation board for each sensor they expect to use. The board supplies a ready-to-use MEMS device so users can test a sensor in a prototype or known environment and understand the sensor's dynamics before they put it in a design. In addition to a sensor, evaluation boards often include other circuit components needed to filter a sensor's output, increase its gain, and so on. Some sensors come in tiny packages, so an evaluation board provides a convenient format for testing without having to work with fine-pitch SMT or BGA packages.
For Further Reading
Clifford, Michelle A., "Accelerometers Jump into the Consumer Goods Market," Sensors, August 2004. pp. 36-39.
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|Title Annotation:||Special Report|
|Publication:||ECN-Electronic Component News|
|Date:||Oct 1, 2004|
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