Accelerometers

Acceleration is defined as the rate of change of velocity; and velocity is defined as the rate of change of position. Velocity is commonly called speed, so the definition of acceleration could also be the rate of change of speed. Accelerometers are sensors that convert the motion that represents the aspect of acceleration into an electrical signal. These devices are typically used to measure vibration on machines and structures and other related acceleration detectors and motion detectors for mines, highways, and bridges that are located in areas subject to earthquakes. The most recent application for accelerometers is to measure vibration on large machines with pumps or motors with bearings that tend to wear out. When these devices wear out unexpectedly, they cause downtime that is very expensive. If the vibration is monitored, the amount of wear on bearings and gears can be calculated and the parts can be changed during scheduled maintenance so that failure will not cause downtime.

Acceleration is commonly measured in dv/dt (delta velocity over delta time) or by Newton's law of motion that states F= ma. Since force is equal to mass times acceleration, it's possible to determine acceleration by measuring the force and dividing it by the mass. Most accelerometers use quartz or ceramic crystals to generate a piezoelectric effect that is converted into an electrical output.

When force is exerted on a crystal, it's stressed and through the piezoelectric effect, the crystal generates an electrostatic charge output which is expressed in terms of "PicoCoulombs." A signal conditioner, either internal to the sensor or external, converts the high-impedance charge signal from the crystal to a high-voltage low impedance output which is compatible with meter, data collectors, and other readout instruments.

Salient characteristics of piezoelectric sensors include high output, high frequency response, wide dynamic operating range, ruggedness, and durability.

ill. 1 shows a diagram of a basic accelerometer consisting of a crystal, seismic mass, and housing. From this diagram notice that the sensor has a specified amount of mass, called the "seismic" mass. Vibratory forces applied to the base of the accelerometer cause the seismic mass to stress the crystal which generates an electrostatic charge output proportional to acceleration.

Above: ill. 1: A cut-away diagram of a basic accelerometer.

Piezoelectric crystals have both a positive and negative polarity output. In figure 1 (above) the positive electrode (Output) is connected to the mass and the negative electrode (Ground) is connected to the housing. A signal conditioner converts the high impedance charge output into a usable voltage signal.

There are several different piezo accelerometer design configurations including shear, upright compression, inverted compression, and flexural. Although each design has advantages and disadvantages, the shear design is considered the most accurate since it's least sensitive to temperature and base strain inputs. Shear is also the most widely used design.

For industrial machine vibration monitoring applications, shear structured accelerometers with integral electronics are packaged in robust hermetic sealed housings with durable electrical connectors to withstand tough factory environments.

Piezoelectric sensors are further classified by the way they condition the signal. If the sensor contains internal signal conditioning, it's called the voltage mode or Integrated Circuit Piezoelectric (ICP) sensor, and if the signal is conditioned externally by a charge amplifier, it's called a charge mode sensor.

ill. 2 shows a picture and diagram of an ICP shear structured accelerometer. Notice that this type of vibration sensor couples securely to a machine or structure by means of a mounting stud. Vibration sensors can also he magnetically or adhesively mounted. Vibratory forces transmitted into the base of the sensor cause the seismic mass to stress the crystal which generates an output proportional to acceleration.

More info on accelerometer here.