Machinedesign 2519 Adxl202 Surface Micromachined Accelerometer 0 0

Silicon in Motion

April 1, 2000
More micromachined devices are emerging from research laboratories and many of them will change the way you design motion systems

Almost every new car sold in North America today has micromachined sensors on board. One of the first applications was an accelerometer used in an air bag deployment system. Because of its accuracy and low price, which cut air bag system costs by a factor of three in just a few years, engineers in all disciplines are now investigating ways to use these accelerometers. Now you'll find them operating in almost any application that must measure gravity, shock, and vibration. And for motion designers, they provide fresh sensor options and solutions to some common problems.

Following the circuit

Manufacturers use various micromachining techniques to create these sensors, each offering different properties. In one version of surface micromachining, the sensing element is a differential capacitor made of polysilicon. It consists of several fixed plates and a movable beam with extended fingers. The beam is suspended less than 2 μm above the surface of the silicon substrate by a set of polysilicon springs or tethers.

The beam moves in response to acceleration like any mass-spring system. Motion roughly corresponds to Newton's second law where force equals mass times acceleration. It also obeys Hooke's law where even a static acceleration like gravity causes the beam to move from its center position with a displacement proportional to the applied acceleration.

Out-of-phase square waves excite the fixed plates. The signal seen on the beam is demodulated and appears as a voltage that varies linearly with acceleration.

All sensor components and signal conditioning circuitry are built using a standard semiconductor BiMOS process, and all are located on the same piece of silicon. Thus, these sensors look more like an integrated circuit than an accelerometer. This development helps keep their costs low plus makes installation on a printed circuit board easy.

The latest micromachined accelerometers can sense motion in two planes, using either one or two beams. Yet, their space requirements remain low. And their signal conditioning requirements are on par with those of today's solid-state temperature and pressure sensors. Some accelerometers will even provide a digital output.

Expanding options

Industrial applications for micromachined accelerometers abound. While some of these sensors simply replace older, costlier technology, more enable previously impractical as well as new functions. Many of the most intriguing and novel applications have designers looking at accelerometers in a new way.

Shock sensing. Most micromachined accelerometers are used to sense shock. In industrial applications, they often function as "bump" sensors and shipping container monitors.

These accelerometers add intelligence to bump sensors, which classically have been made from weighted pendulums and magnetic switches. The newer versions differentiate between low and high amplitude shocks as well as between short and long duration shocks.

In addition, they can elicit different responses to distinct types of shocks. For example, low amplitude, short duration shocks caused by humans bumping into a conveyor system may elicit an audible warning. But a lift truck running into a conveyor would be considered a high amplitude shock and send a "stop the process" alarm.

Bearing monitoring. Accelerometers are the sensor of choice for monitoring bearings in large, costly, or mission- critical equipment. Even though these devices and their associated signal conditioning equipment are costly, it's justifiable because it's more important to minimize downtime.

For less critical applications, however, standard practice is to wait until the bearings become "noisy" from wear. In some cases, this noise level is high enough for people to hear. But by this time, of course, it's too late to schedule a replacement at the next regular maintenance period. So, some unplanned down-time is likely. To avoid this situation, operators change out perfectly good bearings at routine intervals.

Until now, that is. Surface micromachined accelerometers that incorporate their own signal conditioning are truly "g to Volts" transducers, and can cost under $10.

In a typical bearing monitoring system, a micromachined accelerometer connects to an off-the-shelf data acquisition card in a PC. Just as with solid state temperature sensors, the output level is adequate for most PC data acquisition cards so no further signal conditioning is required. The PC performs a simple Fast Fourier Transform (FFT) algorithm to convert the bearing's acceleration response into the frequency domain. If the energy in the frequency spectrum exceeds a pre-programmed level indicating bearing wear, the PC warns of impending bearing failure.

Larger distributed systems may use sensors that are amplified and fed into a low cost 8-bit microcontroller. The microcontroller then performs a similar FFT calculation that actuates a warning system through an LED, contact closure, or digital communication, such as RS-232. These systems can cost less than $20 per point.

Rotating equipment. Many types of rotating equipment operate on unbalanced loads. But when the imbalance is too great, it may damage the machine or process.

Most systems use limit switches or similar devices to stop the motor if the imbalance becomes too great. More sophisticated systems monitor motor current to estimate load variation. But dynamic current sensors (usually current transformers) are fairly expensive, and cannot sense high frequency load changes.

Mounting a surface-machined accelerometer to the housing of the rotating equipment lets engineers read the true vibration of the system over a range from dc to 10 kHz.

Motion control. Many closedloop motion control systems use only positional feedback, which may be insufficient for applications that require accurate, high-speed positioning. One solution is to add an additional feedback loop that uses acceleration information.

Today's micromachined accelerometers usually connect directly to the motion control system's analog input without additional signal conditioning circuitry.

Human interface. Most human interface devices, such as the mouse and joy-stick, are clunky at best. None are truly intuitive. For example, can you imagine driving a car with a mouse? Signing your name with a joy-stick? Natural and intuitive human interfaces are crucial for many types of industrial equipment, but the input device is often the limiting factor in using the machine.

Accelerometers can add a more natural "feel" to input devices. Some manufacturers currently use the tilt measuring capabilities of these sensors in PC game-pads like the Microsoft Freestyle Pro. As you tilt the controller, performing pitch and roll movements that mimic one's natural body movement, the controller translates the changes in static acceleration to cursor or machine motion. The result is faster and more precise machine control.

Safety enhancement. When picking up a load on an inclined surface, there is always a risk of it toppling over. Tilt sensing in an industrial environment is challenging because conventional accelerometers do not measure static accelerations like gravity. So the most common method for detecting tilt uses mercury tilt switches or electrolytic liquid tilt sensors. These are generally glass or plastic vials with electrodes and a fluid. While inexpensive, they require a fair amount of signal conditioning, and they are unreliable in industrial environments. The electrolytic fluid may freeze in cold temperatures and the vials are fragile.

Several fork-lift truck manufacturers now incorporate micromachined accelerometers for tilt sensing in their products to inhibit lifting or limit the height of lift. Some accelerometers can sense incline variations of less than 5 deg, the minimum normally required for most applications. Plus, a ceramic package with no moving parts enhances reliability.

Auto-leveling. Many types of machinery operate best when level or on a uniform tilt, such as that found in some conveyor systems. Leveling a large piece of machinery can be a daunting task, particularly if the floor has a tendency to settle. Periodic checks and adjustments usually must be done when a machine isn't operating, which means scheduling down-time.

Micromachined accelerometers can be incorporated into machines to function as tilt sensors. Here, they help to dynamically level a machine.

The bandwidth is limited to a low frequency, less than 1 Hz, and the sensor must handle copious amounts of averaging to separate the machine's vibration (movement) from the inclination signal. A microprocessor then determines which locations to raise or lower and motordriven screw-drive feet do the rest.

Weight compensation. A weigh scale is another device that must be level for accurate measurements. Using a micromachined accelerometer to measure the tilt of the scale, the true weight may be computed regardless of the inclination.

Harvey Weinberg is an applications engineer at the Micromachined Products Div., Analog Devices Inc., Cambridge, Mass.

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