1. Intr Introd oduc ucti tion on
Among the range of mechanical mechanical sensors (position, (position, speed, acceleration, acceleration, shock), there are two types of the sensors, one passive, the other active, whose measure forces and deformations. Passive sensors are mostly used in mechanics. These sensors can be resistive, capacitive or inductive. Inductive sensors are often used for displacement measurements. On the other hand, resist resistive ive sensor sensorss are often often used used for deform deformati ation on measur measureme ements nts and are someti sometimes mes called called,, somewhat incorrectly, constraint gauges. 2. Stra train Gauge
A strain gauge is a device used to measure the strain of an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, bridge, is related to the strain by the quantity known as the gauge gaug e factor. Strain gauges are used in many instruments that produce mechanical strain because of the affect being measured.In their own right, they are used to measure the strain in a structure being stretched or compressed.The strain gauge element is a very thin wire that is formed into the shape shown. This produces a long wire all in one direction but on a small surface area. The element is often formed by etching a thin foil on a plastic backing. The completed element is then glued to the surface of the material or component that will be strained. The axis of the strain gauge is aligned with the direction of the strain. When the component is stretched or compressed, the length of the resistance wire is changed. This produces a corresponding change in the electrical resistance.
3. Accelerometer
Accelerometers are widely used to measure tilt, inertial forces, shock, and vibration. They find wide usage in automotive, medical, industrial control, and other applications. The use of accelerometers to measure acceleration is a relatively new technique in biomechanics. Their use in biomechanics is not widespread, possibly due to the problems associated with these measurements. There are various methods in biomechanics that are used to measure either linear and/or acceleration.
Some involve the capture of images at known intervals of time (e.g.,
cinematography, videography, and stroboscopic photography) form which the second time derivative of position is determined. These optical methods have inherent problems associated with errors in position data which result in erratic acceleration parameter. Accelerometers are the most recent developments in the electronic measurement of acceleration. There are different types of accelerometers. The type is based on the measurement technique employed within the accelerometer. The types of accelerometer are : a) Piezoelectric:
Piezoelectric devices are sensitive to changes in the acceleration applied to the element during compression. The charge output of the piezo element varies with the applied acceleration. There are several different internal designs. Piezo accelerometers cannot measure to DC (0 Hz), but can measure to very high frequencies (in excess of 50 kHz).
b) Strain gauge (piezoresistive): A mass under load is instrumented using a full-bridge strain gauge. The output of the bridge varies with the applied acceleration. Strain gauge accelerometers can measure to DC, but do not have as high a frequency response as piezo devices (typically under 1
kHz). The transducer may also be fabricated from micro-machined components, often in silicon, rather than tradition mechanical components. These devices usually have a wider frequency response than mechanical transducers and are more rugged.
c) Capacitive: An accelerometer can be built from a capacitive element. In these a mass attached to plates vibrates and the change in capacitance (gap) b etween these plates is proportional to the applied acceleration. These transducers also measure to DC and are generally more rugged than mechanical strain gauge accelerometers.
d) Servo: An older design, not often used these days except in specialized applications. A servo accelerometer contains a mass whose position is controlled by a servo feedback mechanism. The feedback signal is proportional to acceleration. Servo accelerometers can be quite fragile, but can measure very low levels of acceleration. They also respond to DC.
1. Strain Gauge Based Accelerometer
Strain gauge accelerometers, often called "piezoresistive" accelerometers, use strain gauges acting as arms of a Wheatstone bridge to convert mechanical strain to a DC output voltage. The gauges are either mounted to the spring, or between the seismic mass and the stationary frame. In the picture, strain gauge windings, which contribute to the spring action, are
stressed (two in tension, two in compression), and a DC output voltage is generated by the four arms of the bridge that is proportional to the applied acceleration. These accelerometers can be made more sensitive with the use of semiconductor gauges and stiffer springs, yielding a higher frequency response and output signal amplitude. And unlike other types of accelerometers, strain gauge acce lerometers respond to steady-state accelerations.
Figure 1: Piezoresistive accelerometer 4.1 Application
1) Car Alarms 2) Tilt or Inclination 3) Patient Monitors 4) Inertial Forces 5) Laptop Computer Disc Drive Protection 6) Airbag Crash Sensors
7) Car Navigation systems 8) Elevator Controls 9) Shock or Vibration 10) Machine Monitoring 4.2 General Principle
A seismic mass is placed on an elastic return blade equipped with two or four piezoresistive gauges in a Wheatstone Bridge. The blade flexion is translated into gauged deformation. These gauges enable conversion of the acceleration into an electric quantity, since the received signal is proportional to the acceleration of the moving object.
Figure 2: Principle of piezoresistive strain gauge accelerometer The resistivity variation depends on material, resistivity, doping level, type of doping agent and the crystallographic direction in which the material is machined, and the resistivity itself is given by the concentration of the doping agent. The gauge factor of silicon varies as follows: – [+100 to + 175] for type P; – [–100 to –140] for type N.
The important parameters are: – gauge factor K; – temperature coefficient of resistance; – temperature coefficient of gauge factor. Gauge factor K is initially determined by doping level but also depends on temperature. Figure 3 shows that the gauge factor and temperature coefficients are inversely proportional to the level of doping.
Figure 3: Effects of the doping level and temperature on silicon type P 4.3 Assembly of the gauges •
Flat Gauge
These gauges have 2 relatively broad mounting plates, joined together by a narrow central element (Figure 4). In this configuration, strains are concentrated in a miniature element, the surface of which is polished, free from any potential unwanted strain.
Figure 4: Diagram of a flat gauge The strain induced by fixing will be kept to a small fraction of the useful strain at the throttling level. These strain gauges are made of a single silicon crystal with a high degree of purity. The silicon doped with phosphorus gives a negative gauge factor, while doping with boron gives a positive gauge factor. •
Carved gauges We can use the “notched gauge” principle by development of chemical etching. Figure 5
shows a carved monolithic element: the notches releasing the gauges and the seismic masses are the un-etched parts of silicon.
Figure 5: Monolithic sensitive element for accelerometer 7270
The linearity and the sensitivity are optimized because of: – the monolithic structure; – the extremely small size to ensure a very high force/weight ratio; – the freedom of the gauges. The resonance at several MHz and the linear range of more than 100,000g exceeds the performance of former sensors. These sensors are particularly stable because there are no adhesive joints between the mass, the gauges and the substrate.
Figure 6: Expose view of Piezoresistive accelerometer
4.4 Features and limits of these accelerometers 4.4.1 Sensitivity, frequency response
The sensitivity is defined by equation: S = (m/a)= (V m ε / aε )= S1*S2 Vm = output voltage of the Wheatstone Bridge = deformation
a = acceleration S2 is the electric sensitivity of the Wheatstone Bridge formed by the 4 gauges. S2 =
Vm / ε
S1 characterizes the response of the mechanical part of the accelerometer:
S1 = ε /a The sensitivity varies from 1 to 25 mV/g according to the gauge. 4.4.2 Bandwith
The accelerometers for measurement at continuous and low frequencies have critical damping τ = 1. The useful bandwidth extends from 0 to ¼ of the resonance frequency. Piezoresistive accelerometers have a factor of merit defined by equation: β = S.f 0 2 f 0 = natural frequency S = sensitivity β = factor of merit For a given construction and technology, we cannot have, at the same time, an accelerometer having high sensitivity and wide bandwidth. Moreover, the higher the damping of the accelerometer is, the bigger the subsequent phase shift becomes. The typical frequency response range is from 0 to 3,000 Hz. 4.4.3 Influence of temperature
This can take 3 different forms: – Influence on zero. – Influence on sensitivity. Due to thermal variations of the Young’s modulus and the gauge coefficient, the sensitivity decreases as the temperature increases. The order of magnitude is 1 to 2.10-4 of nominal sensitivity/°C. The resulting error can reach a small percentage of the value at 20°C but this error is stable. It can thus be partially corrected b y calculation.
– Variation of the damping coefficient: The piezoresistive accelerometers are damped using silicon oil. When temperature increases, the kinematic viscosity decreases and then the damping coefficient also decreases. The achievable working temperature is –50 to 150°C. Above 150°C doping is no longer effective. 4.5 Technological limitations: connecting cable
Connecting cables bring a deterioration of the transmitted signal at the entry to the signal conditioner, which is related to its length and its frequency. In the case of a significant length of cable, the accelerometer with constant current instead of constant voltage can be supplied in order to eliminate the influence of the resistance of the cable. The accelerometer requires a very stable source of power. 4.5Technological limitations: shocks and vibrations
Accelerometers are sensitive to shocks and vibrations. For inertial navigation it is necessary to equip them with a mechanical filter eliminating high frequencies and to place them in an enclosure protected from shocks. The main advantages and disadvantages of piezoresistive accelerometers are summarized below. Advantages
Disadvantages
high sensitivity
no significant linearity
low cost
high sensitivity to temperature
quite high bandwidth
generally average performance
simple data processing
the lower the sensitivity, the higher the bandwidth
possible miniaturization possibility of obtaining a very high natural frequency (> 30 KHz)
INSTRUMENTATION AND AVIONICS
STRAIN GAUGE BASED ACCELEROMETER
Mohd Nazri B Ismail 0436303
