Sensors Case

Acceleration Sensors

An accelerometer is a type of sensor that measures acceleration and vibrations of a system. There is a wide variety of accelerometer because of the extensity of their applications. They vary most commonly in the way they convert mechanical accelerations into either a digital or analog signal. The difference in analog and digital accelerometers is the type of signal they output. The analog voltage output is a continuous signal that is proportionate to acceleration. The most common type of digital output is created using pulse width modulation. The amount of time that a voltage is elevated is proportional to the amount of acceleration using a square wave at a desired frequency. A simple accelerometer used in industrial setting is shown in figure 1.

Main Technology Used in Accelerometers:

* Capacitive -Metal beam or micro-machined feature produces capacitance, the deflection in the micro structure caused by acceleration produces a change in capacitance that is converted to a signal. An example can be seen in figure 2.

* Piezoelectric -Piezoelectric crystal mounted to mass voltage output converted to acceleration

* Piezoresistive - micro-machined feature changes electrical resistance with accelerations.

* Hall Effect - acceleration is converted to an electrical signal by sensing the change in a magnetic field that varies

* Magnetoresistive -Material resistivity changes in presence of magnetic field that change with the presence on an acceleration

Figure 2 capacitance accelerometer at rest and under and acceleration

Accelerometer Specifications and Parameters

* Dynamic Range - is the maximum amplitude of acceleration that the accelerometer can measure before the signal is distorted or starts to clip. This is typically specified in g's. One g is equal to the acceleration of gravity at sea level

* Sensitivity - is the output voltage produced a certain force caused by acceleration. The higher the sensitivity the higher the output voltage for a given acceleration. High output accelerometers are used to measure low level vibrations and low output accelerometers are used to measure high vibration levels.

* Noise - Electronic noise is generated when the circuit is amplified. Typically noise decreases as frequency increases.

* Resonance Frequency - is the frequency at which the sensor this will produce an inaccurate signal and possibly damage the sensor.

* Accelerometer Selection - When an accelerometer is selected for an application many parameters must be considered.

* Vibration amplitude to be monitored?

* Frequency range to be monitored?

* Temperature range of the installation?

* Size and shape of the sample to be monitored?

* Are interfering electromagnetic fields?

* Electrical noise level?

* Is the surface where the accelerometer is to be mounted grounded?

* Environmental conditions

* Bandwidth - the amount of time acceleration is can be measured. The higher the bandwidth the higher frequency vibration that can be measured accurately.

Figure 3 Parameters for various accelerometers

For accelerometers that use a spring mass system to detect acceleration the following relationship is derived.

XT(t) = Xoe-µt sin(2pfNt)

where Xr(t) = transient mass position

Xo = peak position, initially

µ = damping coefficient

fN = natural frequency

This equation can be used to process and condition the signal to result in an acceleration measurement.

Strain Gauge Sensors

Strain gauges are fundamentally simple but effective devices. Strain gauges measure the stress in an element and can be used in a variety of applications including: load cells torque sensors, pressure sensors, position sensors, etc. Strains gauges are simply a foil pattern that and electric current is passed through. When the foil slightly deforms from a surface deformation, cause by a load, the resistance through the foil changes. This change in resistance is converted to a signal that can be read and processed to give various types of

The change in resistance acquired from the foil pattern must be converted into a read able signal. This is accomplished by connecting the gauges into a Wheatstone bridge circuit. The output voltage of the circuit is given by:

Four, two, or 1 gauge can be connected into the circuit. When the resistances through the gauges change the circuit becomes unbalance which causes a signal. This signal is very weak so it must be amplified so it can be read and processed into the desired data. Strain gauges are mounted to the element in question by use of high strength adhesives. The surface must be smooth, free of loose particles, and dry for proper mounting. The adhesive, usually, a glue is applied to the strain gage when it is positioned on the material. The placement of the gauge is important for getting the most accurate measurements. There are many different arrangements, sizes, shapes of strain gauges for different applications. Some common arrangements are shown in figure 6. The more gauges connected the circuit the more sensitive the measurement will be. Unfortunately it is not always possible to mount multiple gauges.

Force Transducer

A force transducer works by measuring the applied force from the proportional deformation of a spring element the larger the force, the more deformation of the spring element. The range of frequencies a force transducer can measure is limited by the elastic range of the spring element. Force transducers have many applications including process monitoring, product testing, quality control and material testing to name a few.

The setup