Accelerometers

 
 
 

   An accelerometer is an apparatus, either mechanical or electromechanical, for measuring acceleration or deceleration - that is, the rate of increase or decrease in the velocity of a moving object. Accelerometers are used to measure the efficiency of the braking systems on road and rail vehicles; those used in aircraft and spacecraft can determine accelerations in several directions simultaneously. There are also accelerometers for detecting vibrations in machinery.
    The measurement of acceleration or one of its derivative properties such as vibration, shock, or tilt has become very commonplace in a wide range of products. At first you might think of seismic activity or machinery performance monitoring, but would automotive airbags, sports training products, or computer peripherals come to mind as well? The technology behind certain accelerometers has advanced to such a degree that many are now very cost effective and user friendly for the consumer market (joysticks, for example, and running shoes)

    The types of sensor used to measure acceleration, shock, or tilt include piezo film, electromechanical servo, piezoelectric, liquid tilt, bulk micro-machined piezo-resistive, capacitive, and surface micro-machined capacitive. Each has distinct characteristics in output signal, development cost, and type of operating environment in which it best functions.


The Piezoelectric Accelerometer
    Among the desirable features of the piezoelectric (PE) accelerometer are accuracy, durability, large dynamic range, ease of installation, and long life span. Although these devices cost more than other types, in many situations their benefits outweigh the higher price. To provide useful data, PE accelerometers require proper signal conditioning circuitry. We will briefly review the important characteristics of a PE accelerometer and circuit techniques for signal conditioning. In particular, we will examine an interface that will allow the accelerometer output's magnitude and frequency to be measured by a microcontroller unit (MCU).
          
    The PE accelerometer uses an internal PE element coupled with a loading mass to form a single-degree-of-freedom "mass-spring" system. The accelerometer is a charge-sensitive device; an instantaneous change in stress on the internal PE element produces a charge at the accelerometer's output terminals that is proportional to the applied acceleration. For interfacing purposes, the PE accelerometer can be modelled as a voltage generator, Eg, in series with an internal capacitance, Ci. The internal capacitance is an important characteristic because it can have a significant effect on overall system sensitivity. A typical PE accelerometer's sensitivity is specified in Pico coulombs per g (pC/g). Typical sensitivities are 0.51000 pC/g.

PE accelerometers can be applied to measure vibration levels ranging from 4 g to >104 g. The useful measurement range of a given unit is often limited only by its signal conditioning and measurement systems.

    The accelerometers can be used to measure very low frequencies. In practice, the low-frequency response is usually limited by the signal conditioning electronics in order to eliminate noise from sources such as thermal effects, strain on the accelerometer base, and tribo-electric noise generated in the connecting cable. The low-frequency cut-off is typically set around 2 Hz, but may be set higher if the lowest frequencies are not of interest to the user.

The accelerometer's useful upper frequency limit is dependent on its resonance frequency. The device will exhibit a sharp peak in its electrical output at the resonance frequency that must be compensated for. The upper resonance frequency is a function of the unit's mechanical characteristics and the way it is attached to the test object. As a general rule, the output sensitivity and upper resonance frequency of a PE accelerometer are dependent on the size (mass) of the accelerometer. For example, a larger accelerometer will have increased output sensitivity but a lower resonance frequency.

Surface Micro-machined Accelerometers
    In recent years, silicon micro-machined sensors have made tremendous advances in terms of cost and level of on-chip integration for measurements such as acceleration and/or vibration. These products provide the sensor and the signal conditioning circuitry on chip, and require only a few external components. Some manufacturers have taken this approach one step further by converting the analogue output of the analogue signal conditioning to a digital format such as duty cycle. This method not only lifts the burden of designing fairly complex analogue circuitry for the sensor, but also reduces cost and board area. Micro-machined accelerometers are now being incorporated into products such as joysticks and airbags, applications that were previously impossible due to sensor price and/or size.

A surface micro-machined device consists of springs, masses, and motion-sensing components. These sensors are made with the standard IC processing techniques used in wafer fabrication facilities. After layers of oxide and poly-silicon, IC photolithography and selective etching are used to create the sensor as a 3D structure suspended above the substrate to allow free movement in all directions. The core of the sensor is a surface micro-machined poly-silicon structure or mass suspended above the substrate with "springs." These springs hold the mass and provide resistance to movement due to acceleration forces.

Both the mass and the wafer have fixed plates that form a differential capacitor in which the fixed plates on the wafer are driven 180 out of phase. Any movement of the mass unbalances the capacitor, resulting in a square wave output with the amplitude proportional to the acceleration. Each axis has a demodulator that rectifies the signal and determines the direction of acceleration. This output is fed to a duty cycle modulator (DCM) that incorporates external capacitors to set the bandwidth of each axis. The DCM filters the analogue signal and converts it to a duty cycle output whose period is set by an external resistor. A 0 g acceleration produces a 50% duty cycle output. A low-cost microcontroller can be used to measure acceleration by timing both the duty cycle and the period of each axis.

   
                   
   

 

   
                   
 

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