Electronic Design

Low-G Sensors Mold A New MEMS Market

Configurable in any configuration up to three axes, these accelerometers and E-field sensors target consumer, automotive, industrial, and medical uses.

Three new highly sensitive low-g acceleration sensors, along with an electric-field (E-field) sensor, will look to deepen the market penetration of microelectromechanical-system (MEMS) devices. The sensors, developed by Freescale Semiconductor, have set their sights on applications throughout the consumer, medical, automotive, and industrial fields. They span a wide range of sensitivity levels and are low cost. (See the end of this article for more pricing details.)

Using the three accelerometers, designers can select any combination of X-, Y-, and Z-axis sensing within an acceleration range of 1.5 to 10 g. The sensors—the MMA6270Q (X and Y axes), the MMA6280Q (X and Z axes), and the MMA7261Q (X, Y, and Z axes)—target low-cost consumer electronics that require detection of small changes in force resulting from tilt, motion, positioning, shock, or vibration (Fig. 1). Each sensor allows designers to choose a sensitivity range through pin selection. The MC34940 E-field sensor is intended for cost-sensitive applications that require non-contact sensing of objects.

An innovative feature of the accelerometers is their "g-select" capability, which makes it possible to select one of four sensitivity levels at any time during the sensor's operation. This feature is handy for applications requiring different sensitivities for optimum performance. Selectable sensitivities include 1.5, 2, 4, or 6 g for the MMA6270Q and MMA6280Q, and 2.5, 3.3, 6.7, or 10 g for the MMA7261Q. Selection is determined by how the logic input is placed on each sensor's pins 1 and 2, which sets the sensor's internal gain. In the first two models, the two g-select pins can be left unconnected for applications requiring only a 2.5-g sensitivity level or 480 mV/g (a 1.5 g level or 800 mV/g for the MMA6280Q).

"The low 1.5-g sensitivity level of the MMA6280Q XZ sensor is the first of its type on the market," says Dan Slocum, product marketing manager. "Being able to sense in the lateral and perpendicular directions eliminates the need for daughtercards and thus reduces the costs of assembly. It's ideal for consumer-electronics applications like washing machines and pedometers."

All three accelerometers feature a low current consumption of 500 µA, a 3-µA sleep mode, and a fast response time of 1 ms. They contain integral signal conditioning with a low-pass filter. All three operate from a supply of 2.2 to 3.6 V and come in a 6- by 6- by 1.45-mm quad flat no-lead (QFN) package. And, they operate from -20°C to 85°C.

Freescale envisions a wide range of applications for these accelerometers (see the table). These include sensing fall, tilt, movement, positioning, shock, and vibration functions.

"Driving remote electrodes through coaxial cables is a unique feature," explains Philip Sieh, applications engineer. Between capacitive loads of 10 and 500 pF, designers can obtain maximum harmonic levels below the fundamental frequency of about -20 dB. The shield driver's gain bandwidth product is rated at 4.5 MHz measured at 120 kHz.

The MC34940 E-field sensor IC generates a low-frequency sinewave. The frequency is adjustable by using an external resistor. The sensor also is optimized for a frequency of 120 kHz.

The other part of Freescale's sensor introduction, the MC34940 E-field touch and noncontact sensor, works by detecting an electric field's strength as the capacitance changes between an object being sensed and ground (Fig. 2). Two examples of a touch sensing application are shown for single and multiple electrodes (Fig. 3).

The sensor is the fruit of a collaborative effort between Freescale, the Massachusetts Institute of Technology's Media Lab, and Elesys North America. Elesys is currently using the MC33794 in its Seat-Sentry occupant sensing system in an automotive application.

Intended targets for the MC34940 are appliances like refrigerators (ice-cube detection, icing/defrosting cycle, interactive displays), kitchen extractor hoods (hand detection), ovens (control panel), cooking tops (liquid boil-over load detection, control panel), and washing machines (unbalanced load detection, water-level detection, control panel). Other applications include industrial controls, desk lamps, point-of-sale (POS) terminals, and keyboards and keypads. It also can be used for near-field image and liquid-level sensing in medical and automotive applications.

The E-field sensor can support up to 28 touch pads and seven electrodes. It allows for three-dimensional position determination and detects a wide range of objects and materials, including people and metallic and non-metallic materials. A handy feature is its shield driver, which lets the sensor drive remote electrodes through coaxial cables.

Freescale will provide development and evaluation kits for all sensor products. The RD3152MMA7260Q ZigBee sensing triple-axis reference (ZSTAR) design demonstrates the latest innovations from Freescale using 2.4-GHz wireless connectivity, a three-axis sensor, and embedded flash microcontrollers.

The development tool contains the MC9S08QG8 MCU, combined with the MMA7260Q XYZ-axis accelerometer, which can be directly replaced with the MMA6270Q, MMA6280Q, or MMA7261Q. A 2.4-GHz wireless transceiver and an interface for USB 2.0 full-speed are included as well.

Evaluation kits are also available for the three MEMS-based sensors. Small circuit boards can be used to evaluate the devices to quickly develop prototypes. In addition, the E-field sensor evaluation kit contains the evaluation board, a driver CD-ROM, connectors, and program software.

The sensors are available in production quantities. Suggested price in 25,000-piece quantities starts at $3.58 each for the MMA6270Q, $3.85 each for the MMA6280Q, and $4.43 each for the MMA7261Q. In high volumes, suggested product pricing is less than $2 each. The 24-pin wide small-outline IC MC34940 E-field sensor costs $1.85 each in 10,000-unit lots. Evaluation kits are priced at $35 each.

Freescale Semiconductor Inc.

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