Input / Output Devices

Accelerometers

Not long ago, the only accelerometers available employed either a quartz crystal or moving coil. Typical costs of these devices were $500 or more. Now, a new generation of solid-state devices is smaller and lighter than its predecessors and is less expensive as well. Some of the new accelerometers cost only about $50, but cannot handle certain kinds of applications. In fact, choosing an accelerometer sometimes can be harder than using one.

One of the more common accelerometers is called a suspended-mass type. Suspended-mass accelerometers are characterized by their small size (solid-state devices are about as big as a transistor) and ability to respond to dc (static) accelerations. Frequency response ranges from dc to about 5 kHz depending on the particular type. High-g-range units generally have higher frequency response than their low-range equivalents. A simple model of such a device is a mass suspended on the end of a cantilever beam, but actual devices are more complicated.

For instance, the new solid-state versions of this type suspend a silicon mass from four deflection beams. The mass and beams are in a cavity in the silicon structure.

Two features usually protect the beams from overflexing. In addition to protecting the accelerometer, these mechanisms also affect its measuring capabilities. The first safety device is a mechanical stop. It protects the beam from damage during peak accelerations. Second, oil provides viscous damping. Damping also increases the accelerometer's frequency response and prevents damage from frequencies near the beam's resonant frequency.

Suspended-mass devices use two different methods to determine acceleration. The first method is to measure the force (strain) induced in the beam by the mass. Here, strain gages on the beam generate a signal proportional to acceleration. A bridge circuit connects the four strain-sensing elements. The result is a device having performance similar to that of standard piezoresistive strain-sensing elements. Accelerometers of this design are usually called strain-gage bridge accelerometers, regardless of the type of strain sensing element used.

Rather than gauging deflections with strain gages, some suspended-mass devices use capacitive sensing. This produces an electrical signal proportional to displacement. An advantage of some capacitive accelerometers is that they are air damped. This makes their damping coefficient relatively insensitive to temperature, unlike fluid-damped devices.

The most rugged of all accelerometers is the piezoelectric type. A typical unit, rated 10,000 g, is housed in a cylindrical package about 1 in. long by ½ in. in diameter. Piezoelectric transducers also offer a wider temperature range (-195 to 260°C) than any other accelerometers.

The frequency response of these devices is wide because the active sensor is small. Most units have a frequency range of at least 5 kHz. Typical resonant frequencies are over 30 kHz.

Though tough, the devices are not without their problems. One is that they do not respond to static accelerations. For instance, typical units have a minimum response in the range of 0.01 to 5 Hz. But a bigger concern is with the output.

For accurate measurements, the accelerometer and cable must be calibrated as a system. If the cable length is changed, another calibration is required.

The alternative is to measure the charge generated by an acceleration. This techniques negates cable effects, but requires a charge amplifier. Compared with other amplifiers, charge amplifiers are expensive.

Another problem associated with piezoelectric accelerometers is the need to maintain high insulation resistance and low noise in the cabling between the accelerometer and the charge amplifier. Otherwise, measurement errors will be introduced into the system. Generally, the cable must be of special low-noise type (supplied by the accelerometer maker) and must be fastened in place to reduce vibration-induced electrical noise.

As an alternative to an external amplifier, some manufacturers offer piezoelectric accelerometers with built-in amplifiers. These devices overcome the cabling and amplifier problems. They also have low output impedance. This means that connecting them with oscilloscopes or chart recorders is relatively easy. The primary disadvantage of internally amplified units it that they require a separate power supply.

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