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Acceleration detectors.pptx

Contents

Introduction to acceleration sensors

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Physically, acceleration is a vector quantity having both direction and magnitude that is defined as the rate at which an object changes its velocity with respect to time. It is a measure of how fast speed changes . An object is accelerating when its velocity is changing. [1. ] In order to measure acceleration, an acceleration sensor called accelerometer is used. Accelerometer measures in units of g. A g is the acceleration measurement for gravity which is equal to 9.81 m/s². However, depending on altitude, this measurement can be 10 m/s²  in some place.

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where f is natural frequency in Hz
k is spring constant in N/m
m is seismic mass in kg

If there was no friction in the spring-mass system, the mass would oscillate forever. This is, however, not the case in reality. The friction that causes the system to rest is defined by a damping coefficient that has a unit of s^-1. The effect of oscillation is described by periodic damped signal which has the equation as follows. [2,2.]

where XT(t) is transient mass position
μ is damping coefficient
f is natural frequency in Hz

Now two parameters that affect the accelerometer have been described. If the spring-mass system is exposed to a vibration, then the resultant acceleration is given by

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                            Figure 2 a. Response of spring-mass system to vibration compared to w^2 prediction b. Effect of various peak motion [2,3]
                                                                          Reprinted from Process Control Instrumentation Technology book

Define f as the applied frequency, and fN is the natural frequency of the accelerometer. When f < fN , the natural frequency has little effect on the operation of the accelerometer. when f > fN, the accelerometer output is independent of the applied frequency. An important point worth noting is that with the applied frequency much larger than the natural frequency, the accelerometer becomes a measurement of vibration displacement . At near the resonance of accelerometer's natural frequency, the output of the accelerometer becomes high non-linear.
A rule of thumb is that with f < fN, the safe maximum applied frequency should be f < 2/5 fN.With f > fN, the minimum applied frequency should be f > 5/2 fN.[2,4.] So to best avoid the severe effect of resonance, the accelerometer should not be used near the resonance of their frequency because the output will become non-linear. Furthermore, the accelerometer should not be used with the applied frequency larger than the natural frequency if the measurement system is meant to measure acceleration.This has an important implication in the selection of the accelerometer for an application.

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Until now, all the basic characteristics of an accelerometer have been explained. One important characteristic is the sensitivity and the question is how to improve the sensitivity of the accelerometer. For analog-output sensors, sensitivity is ratiometric to supply voltage. Therefore, the sensitivity can be improved by increasing the supply voltage for the sensor as long as it does not exceed the maximum rating power supply specified by the datasheet. So this means that doubling the supply, for example, doubles the sensitivity. [9.]

Interface electronic circuits

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The circuit shown in figure 5 itself possesses errors including offset error and sensitivity mismatch error. In order to compensate those errors, two techniques are used to calibrate the circuit. The first technique is called no-turn calibration and the second one is called multiple turn calibration.

Types of accelerometers

So far, I have discussed mainly spring-mass based accelerometers since it is the focus of this wiki page. In addition to that, there are many other accelerometers that are designed based on other physical phenomenon. In fact, what makes a difference between the types is the sensing element and their operating principles. There are many types of accelerometers in real life applications such as capacitive, piezoelectric, piezoresistive, Hall effect, magnetoresistive, heat transfer, MEMS-based accelerometers, to name just a few. Commonly used accelerometers, however, will be represented in this wiki page.

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This is a type of an accelerometer which bases its working principles on the spring-mass system. The potentiometric accelerometer employs a mass (seismic mass), a spring, a dashpot, and a resistive element. The seismic mass is connected between a spring and a dashpot. The wiper of the potentiometer is connected to the mass.The following figure illustrates the structure of the potentiometric accelerometer.




                                                                                    Figure 56. Structure of a potentiometric accelerometer [3]

The way it works is simple. It measures the motion of the seismic mass by attaching the wiper arm to the spring-mass system. When the mass is moving, the position of the wiper changes according, thus changing the resistance of the resistive element. Since the natural frequency fN of the potentiometer accelerometer is generally less then 30Hz, this type of accelerometer should be used in low frequency vibration measurements. 

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Hall effect accelerometer is based the working principles of on spring-mass system. The output voltage varies according to a change in magnetic field from the magnet which is attached on a seismic mass. The mass deflects because of the forces due to acceleration. The output Hall voltage is calibrated in terms of acceleration.


                                                                           Figure 67. Simplified structure of a Hall effect accelerometer [4,3]

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Capacitive accelerometer operates based on spring-mass system working principles. It differs from Hall effect accelerometer and potentiometric accelerometer in its sensing element. Figure 7 8 shows the structure of a capacitive accelerometer, The sensing electrodes are in stationary state, and the diaphragm which is attached to the seismic mass is sandwiched in between the two sensing electrodes creating two capacitors.

                                                                                   Figure 78. Structure of capacitive accelerometer [5]

The vibration because of the forces due to acceleration causes the seismic or proof mass to move. The motion of the mass leads to the capacitance change of the sensing electrodes so as to determine the acceleration. The movement of the diaphragm causes a capacitance shift by altering the distance between the two parallel plates, with the diaphragm itself being one of the plates.  

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Unlike the three types of accelerometers mentioned above, Piezoresistive accelerometers do not use a spring. Instead of that, the mass is attached to cantilever beam which in turn is sandwiched in between strain gages.



                                                                                          Figure 89. Piezoresistive accelerometer [4,2]

Piezoresistive accelerometer's working principle is based on piezoresistive effect. As far as the piezoresistive effect is concerned, applied mechanical stress changes the resistivity of a semiconductor. The force exerted by the seismic mass changes the resistance of the strain gages. Piezoresistive accelerometers are used in high shock applications, and they can also measure accelerations down to zero Hz or up to ±1000g. But, the disadvantage is that they have limited high frequency response. [6.]

MEMS-Based Accelerometers

MEMS stands for Micro-Electro Mechanical System. It is the technology which is based advanced technologies used to form small structures with dimensions in micrometer scale. MEMS technology is now being employed to manufacture state-of-the-art MEMS-based accelerometers.
Initially, MEMS accelerometers was designed using piezoresistors. Since piezoresistors are less sensitive than capacitive detection, the majority of MEMS accelerometers nowadays use capacitive sensing principle. MEMS-based accelerometer typically consists of a proof mass with plates attached through a mechanical suspension system to a reference frame. Movable plates (part of the seismic mass) and the outer plates in stationary state form differential capacitor. Because of the forces due to acceleration, the seismic mass deflects; the deflection is measured in terms of capacitance change. [4,3.]



                                                                            Figure 910. MEMS-based accelerometer structure [4,3]

Selection of an accelerometer

Materials and manufacturing technologies

The datasheet of the LIS3L02AL accelerometer gives very little information about the materials used for manufacturing the sensor as well as how it is manufactured since the information about these is proprietary. What is known publicly is that the suspended structures which are attached to the substrate in anchors are made of silicon. There is no information about the manufacturing process of the sensor from STMicroelectronics. Fortunately, some other manufactures are generous enough to give some information about the manufacturing process of their sensor. The following gives steps in manufacturing accelerometers from IMV corporation.

• Step 1 - Prescription: The main ingredients are lead, zirconium and titanium with 4-5 kinds of additives to have better caking properties by combustion and electric characteristics.
• Step 2 - Mixture: The prescribed ingredients are put into a pot with zirconium boulders and water. The ingredients are mixed and crushed in this pot.
• Step 3 - Drying: The slurry of ingredients is poured into vat and dried in a drying room. Then, it sets as a plate with cracks.
• Step 4 - Crushing into pieces: The set plate is put into a mortar and crushed. To make finer ingredients, it is grinded down to powder.
• Step 5 - Mixture: The grinded ingredients are put into a pot with zirconium boulders and water, and mixed and crushed again. Before proceeding to the next step, the binder (PVA series) for shaping is added as a thickener.
• Step 6 - Making powder of ingredients: The slurry of ingredients is sucked in a tube by using the roller pump. The slurry is then dropped on a heated, spinning plate. The dripped ingredients become powder and are collected during this procedure. The powder made the spray dryer is a fine powder with a consistent grade and good fluidity for shaping.
  
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• Step 7 - Shaping: The ingredients are put into a cylinder, and pressed with 1000-3000kg/cm² of pressure.
  The ingredients are set as a round plate.
  
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• Step 8 - Removing grease: The set ingredients are put into the chamber which is heated to 400-600℃. It is for removing the grease of added binder (PVA series).
  
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• Step 9 - Hardening: The ingredients are stuffed into a case (a box made of alumina). It is calcinated at 1000-1300℃. The ceramics (crystal) is obtained from the ingredients.
  
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• Step 10 - Grinding: The hardened ceramics are ground on both sides to the specified thickness.
  
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• Step 11 - Silver painting: Silver paste is printed on the ground surface of the ceramic by a screen printing press.
  
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• Step 12 - Silver hardening: Silver paste is printed on the ground surface of the ceramic by a screen printing press.
  
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• Step 13 - Polarization:

  The ceramic is put into the silicon oil heated approx.200℃ and high voltage 1-2kV/mm is added between the electrodes.Then, the random polarities are arranged in field direction are arranged to be in the same field direction. Thea piezoelectricity becomes apparent.

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• Step 14 - Checks: Polarized ceramics are checked. Check points include

  
  
  1. Coupling coefficient (machine-electricity conversion coefficient)
  2. Resonance frequency
  3. Electric capacity
  4. Insulation resistance
  5. Curie point temperature
  6. Specific gravity
     
     
• Step 15 - Processing: Ceramics are processed into the shape required for a sensor. These shapes are the compressed type, shear type, round plate type, and square plate.
  
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• Step 16 - Plating: The ceramic for the shear type is plated by non-electrolytic nickel on the processed side (polarization axis and horizontal direction) after processing, and is changed the electrode.
• Step 17 - Construction: Piezoelectric ceramics, machine processing parts and connectors are composed and connected with wires to construct a piezoelectric sensor. 
• Step 18 - Checks: The constructed sensor is checked. Check points include
  
  
  1. Charge sensitivity
  2. Capacitance
  3. Insulation resistance
  4. Frequency characteristics
  5. Temperature characteristics  etc.
     
     

The whole manufacturing process is completed after checking is done. [12.]

Selection of an accelerometer

In order to make a decision on which accelerometer can be used based on the requirements, it is necessary to In order to make a decision on which accelerometer can be used based on the requirements, it is necessary to understand the specifications of the accelerometer. These specifications can be found from the datasheet of an accelerometer. They include dynamic specifications, electrical specifications and mechanical specifications. Some of the important ones are as follows:

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• Size and mass: Since the size and mass of an accelerometer have an effect on the object being tested, the mass of the accelerometers should be greatly smaller than that of the system to be monitored. [4,4.]

In addition to understanding some of the important specifications just mentioned, it should be noted that if the measurement is meant to measure acceleration, then the applied frequency f should be less than the natural frequency fN of the accelerometer.

As a rule of thumb, in low-frequency applications (having a bandwidth on orders from 0 to 10 Hz), position and displacement measurements generally provide good accuracy. In the intermediate-frequency applications (less than 1 kHz), velocity measurement is usually favored. In measuring high-frequency motions with appreciable noise levels, acceleration measurement is preferred. [7,327.]

Applications of accelerometers

Accelerometers have many different applications ranging from industry, entertainment, sports to education. These applications can be, for example, triggering airbag deployments or monitoring of nuclear reactors. Accelerometers can also be used to measure static acceleration (gravity), tilt of an object, dynamic acceleration in an aircraft, shock to an object in a car, orientation or vibration of an object. Cell phones, washing machines or computers nowadays also have accelerometers. 

References