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Accelerometer

Summary

 

Introduction

One of the most common inertial sensors is the accelerometer, a dynamic sensor capable of a vast range of sensing. Accelerometers are available that can measure acceleration in one, two, or three orthogonal axes. They are typically used in one of three modes:

  • As an inertial measurement of velocity and position;
  • As a sensor of inclination, tilt, or orientation in 2 or 3 dimensions, as referenced from the acceleration of gravity (1 g = 9.8m/s2);
  • As a vibration or impact (shock) sensor.

There are considerable advantages to using an analog accelerometer as opposed to an inclinometer such as a liquid tilt sensor – inclinometers tend to output binary information (indicating a state of on or off), thus it is only possible to detect when the tilt has exceeded some thresholding angle.

Principles of Operation

Most accelerometers are Micro-Electro-Mechanical Sensors (MEMS). The basic principle of operation behind the MEMS accelerometer is the displacement of a small proof mass etched into the silicon surface of the integrated circuit and suspended by small beams. Consistent with Newton's second law of motion (F = ma), as an acceleration is applied to the device, a force develops which displaces the mass. The support beams act as a spring, and the fluid (usually air) trapped inside the IC acts as a damper, resulting in a second order lumped physical system. This is the source of the limited operational bandwidth and non-uniform frequency response of accelerometers. For more information, see reference to Elwenspoek, 1993.

Types of Accelerometer

There are several different principles upon which an analog accelerometer can be built. Two very common types utilize capacitive sensing and the piezoelectric effect to sense the displacement of the proof mass proportional to the applied acceleration.

Capacitive

Accelerometers that implement capacitive sensing output a voltage dependent on the distance between two planar surfaces. One or both of these “plates” are charged with an electrical current. Changing the gap between the plates changes the electrical capacity of the system, which can be measured as a voltage output. This method of sensing is known for its high accuracy and stability. Capacitive accelerometers are also less prone to noise and variation with temperature, typically dissipate less power, and can have larger bandwidths due to internal feedback circuitry. (Elwenspoek 1993)

Piezoelectric

Piezoelectric sensing of acceleration is natural, as acceleration is directly proportional to force. When certain types of crystal are compressed, charges of opposite polarity accumulate on opposite sides of the crystal. This is known as the piezoelectric effect. In a piezoelectric accelerometer, charge accumulates on the crystal and is translated and amplified into either an output current or voltage.

Piezoelectric accelerometers only respond to AC phenomenon such as vibration or shock. They have a wide dynamic range, but can be expensive depending on their quality (Doscher 2005)

Piezo-film based accelerometers are best used to measure AC phenomenon such as vibration or shock, rather than DC phenomenon such as the acceleration of gravity. They are inexpensive, and respond to other phenomenon such as temperature, sound, and pressure (Doscher 2005)

Overview of other types that are less used in audio applications

Piezoresistive

Piezoresistive accelerometers (also known as Strain gauge accelerometers) work by measuring the electrical resistance of a material when mechanical stress is applied. They are preferred in high shock applications and they can measure acceleration down to 0Hz. However, they have a limited high frequency response.

Hall effect

Hall effect accelerometers work by measuring the voltage variations caused by the change in magnetic field around them.

Heat transfer

Heat transfer accelerometers consist in a single heat source centered in a substrate and suspended accross cavity. They include equally spaced thermoresistors on the four side of the heat source. They measure the internal changes in heat due to an acceleration. When there is zero acceleration, the heat gradient will be symmetrical. Otherwise, under acceleration, the heat gradient will become asymmetrical due to convection heat transfer

Other

There are many other types of accelerometer, including:

  • Null-balance
  • Servo force balance
  • Strain gauge
  • Resonance
  • Optical
  • Surface acoustic wave (SAW)

Specifications

A typical accelerometer has the following basic specifications:

  • Analog/digital
  • Number of axes
  • Output range (maximum swing)
  • Sensitivity (voltage output per g)
  • Dynamic range
  • Bandwidth
  • Amplitude stability
  • Mass

Analog vs. digital: The most important specification of an accelerometer for a given application is its type of output. Analog accelerometers output a constant variable voltage depending on the amount of acceleration applied. Older digital accelerometers output a variable frequency square wave, a method known as pulse-width modulation. A pulse width modulated accelerometer takes readings at a fixed rate, typically 1000 Hz (though this may be user-configurable based on the IC selected). The value of the acceleration is proportional to the pulse width (or duty cycle) of the PWM signal. Newer digital accelerometers are more likely to output their value using multi-wire digital protocols such as I2C or SPI.

For use with ADCs commonly used for music interaction systems, analog accelerometers are usually preferred.

Number of axes: Accelerometers are available that measure in one, two, or three dimensions. The most familiar type of accelerometer measures across two axes. However, three-axis accelerometers are increasingly common and inexpensive.

Output range: To measure the acceleration of gravity for use as a tilt sensor, an output range of ±1.5 g is sufficient. For use as an impact sensor, one of the most common musical applications, ±5 g or more is desired.

Sensitivity: An indicator of the amount of change in output signal for a given change in acceleration. A sensitive accelerometer will be more precise and probably more accurate.

Dynamic range: The range between the smallest acceleration detectable by the accelerometer to the largest before distorting or clipping the output signal.

Bandwidth: The bandwidth of a sensor is usually measured in Hertz and indicates the limit of the near-unity frequency response of the sensor, or how often a reliable reading can be taken. Humans cannot create body motion much beyond the range of 10-12 Hz. For this reason, a bandwidth of 40-60 Hz is adequate for tilt or human motion sensing. For vibration measurement or accurate reading of impact forces, bandwidth should be in the range of hundreds of Hertz. It should also be noted that for some older microcontrollers, the bandwidth of an accelerometer may extend beyond the Nyquist frequency of the A/D converters on the MCU, so for higher bandwidth sensing, the digital signal may be aliased. This can be remedied with simple passive low-pass filtering prior to sampling, or by simply choosing a better microcontroller. It is worth noting that the bandwidth may change by the way the accelerometer is mounted. A stiffer mounting (ex: using studs) will help to keep a higher usable frequency range and the opposite (ex: using a magnet) will reduce it.

Amplitude stability: This is not a specification in itself, but a description of several. Amplitude stability describes a sensor's change in sensitivity depending on its application, for instance over varying temperature or time (see below).

Mass: The mass of the accelerometer should be significantly smaller than the mass of the system to be monitored so that it does not change the characteristic of the object being tested.

Other specifications include:

  • Zero g offset (voltage output at 0 g)
  • Noise (sensor minimum resolution)
  • Temperature range
  • Bias drift with temperature (effect of temperature on voltage output at 0 g)
  • Sensitivity drift with temperature (effect of temperature on voltage output per g)
  • Power consumption

Output

An accelerometer output value is a scalar corresponding to the magnitude of the acceleration vector. The most common acceleration, and one that we are constantly exposed to, is the acceleration that is a result of the earth's gravitational pull. This is a common reference value from which all other accelerations are measured (known as g, which is ~9.8m/s^2).

Digital output

Accelerometers with PWM output can be used in two different ways. For most accurate results, the PWM signal can be input directly to a microcontroller where the duty cycle is read in firmware and translated into a scaled acceleration value. (Check with the datasheet to obtain the scaling factor and required output impedance.) When a microcontroller with PWM input is not available, or when other means of digitizing the signal are being used, a simple RC reconstruction filter can be used to obtain an analog voltage proportional to the acceleration. At rest (50% duty-cycle) the output voltage will represent no acceleration, higher voltage values (resulting from a higher duty cycle) will represent positive acceleration, and lower values (<50% duty cycle) indicate negative acceleration. These voltages can then be scaled and used as one might the output voltage of an analog output accelerometer. One disadvantage of a digital output is that it takes a little more timing resources of the microcontroller to measure the duty cycle of the PWM signal. Communication protocols could use I2C or SPI.

Analog output

When compared to most other industrial sensors, analog accelerometers require little conditioning and the communication is simple by only using an Analog to Digital Converter (ADC) on the microcontroller. Typically, an accelerometer output signal will need an offset, amplification, and filtration. For analog voltage output accelerometers, the signal can be a positive or negative voltage, depending on the direction of the acceleration. Also, the signal is continuous and proportional to the acceleration force. As with any sensor destined for an analog to digital converter, the value must be scaled and/or amplified to maximally span the range of acquisition. Most analog to digital converters used in musical applications acquire signals in the 0-5 V range.

The image at right depicts an amplification and offset circuit, including the on-board operational amplifier in the adxl 105, minimizing the need for additional IC components. The gain applied to the output is set by the ratio R2/R1. The offset is controlled by biasing the voltage with variable resistor R4. Accelerometers output bias will drift according to ambient temperature. The sensors are calibrated for operation at a specific temperature, typically room temperature. However, in most short duration indoor applications the offset is relatively constant and stable, and thus does not need adjustment. If the sensor is intended to be used in multiple environments with differing ambient temperatures, the bias function should be sufficient for analog calibration of the device. If the ambient temperature is subject to drastic changes over the course of a single usage, the temperature output should be summed into the bias circuit. Smart sensors may even take this into consideration.

The resolution of the data acquired is ultimately determined by the analog to digital converter. It is possible, however, that the noise floor is above the minimum resolution of the converter, reducing the resolution of your system. Assuming that the noise is equally distributed across all frequencies, it is possible to filter the signal to only include frequencies within the range of operation. The filter required depends upon both the type of acquisition as well as the location of the sensor. The bandwidth is primarily influenced by the three different modes of operation of the sensor.

Uses

The acceleration measurement has a variety of uses. The sensor can be implemented in a system that detects velocity, position, shock, vibration, or the acceleration of gravity to determine orientation (Doscher 2005)

A system consisting of two orthogonal sensors is capable of sensing pitch and roll. This is useful in capturing head movements. A third orthogonal sensor can be added to the network to obtain orientation in three dimensional space. This is appropriate for the detection of pen angles, etc. The sensing capabilities of this network can be furthered to six degrees of spatial measurement freedom by the addition of three orthogonal gyroscopes.

As a shock detector, an accelerometer is looking for changes in acceleration. This jerk is sensed as an overdamped vibration.

Verplaetse has outlined the bandwidths associated with various implementations of accelerometers as an input device. These are:

LocationUsageFrequencyAcceleration
Head Tilt 0-8 Hz xx
Hand , Wrist, Finger Cont. 8-12 Hz 0.04-1.0 g
Hand, Arm, Upper Body Cont. 0-12 Hz 0.5-9.0 g
Foot, Leg Cont. 0-12 Hz 0.2-6.6 g

Depending on the sensitivity and dynamic range required, the cost of an accelerometer can grow to thousands of dollars. Nonetheless, highly accurate inexpensive sensors are available.

The 4 Types of PCB Solder Mask

The 4 Types of PCB Solder Mask

Solder Mask, also known as solder resist is a strong, permanent layer that protects copper traces and the interfaces between them on printed circuit boards (PCBs). The main function of a solder mask is to prevent conductive solder bridging between different electronic components and causing short circuits. There are many types of PCB soldermask, such as Epoxy Liquid, Liquid Photoimageable, Dry Film Photoimageable, Top- and Bottom-side Masks.

 

Soldermask Types

Soldermask Types

 

Top- and Bottom-side Masks

A topside solder mask allows the electronic engineer to identify the openings in thegreen solder mask layer already added to the PCB by one of the epoxy, ink or film techniques. Component pins can then be soldered onto the board using those identified places. The pattern of conductive traces on the topside of the circuit board is called top traces and the bottom-side mask specifies openings on the lower surface.

 

Epoxy Liquid

Silkscreened onto the PCB pattern, epoxy liquid is the lowest cost type of solder mask. Epoxy is a thermosetting polymer that has many applications. Silkscreening is a printing technique that uses a woven mesh to support ink-blocking stencils or patterns. The mesh creates open areas for ink transfer. Silk is often used in art but synthetic fibers are more common for electronic applications. The final finishing process involves thermal curing.

 

Liquid Ink Photoimageable

Liquid photoimageable solder mask is delivered as an ink formulation. The ink can be silkscreened or sprayed onto the PCB, then exposed to the pattern and developed. One type of process commonly used with liquid ink formulations is hot air surface leveling (HASL). It requires a clean environment, free of particles and contaminants. After the UV light exposure stage, the mask is removed using high-pressure water sprays called developers. Circuit board finishing requires thermal curing and organic coating.

 

Dry Film Photoimageable

Dry film photoimageable solder mask is applied using vacuum lamination, then exposed and developed. After developing, openings are created in the pattern and parts can be soldered to the copper pads. Copper is layered onto the board inside the holes and on the trace areas using electrochemical processing. Tin is applied to protect the copper circuitry. The dry film is then removed and the exposed copper etched. Finishing also involves thermal curing.

Soldermask Types

Soldermask Types

DIY Printed circuit board

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How To Install Proteus Software With Licence Key Step By Step

Proteus combines ease of use with powerful features to help you design, test and layout professional PCBs like never before. With nearly 800 microcontroller variants ready for simulation straight from the schematic, one of the most intuitive professional PCB layout packages on the market and a world class shape based autorouter included as standard, Proteus Design Suite 8 delivers the complete software package for today and tomorrow's engineers.

It is a software suite containing

=>schematic

=>simulation

=>as well as PCB designing.

To install this software follow these steps:

1) First of all download proteus 7(Labcenter Electronics) from any website.

2) Download crack(Licence key).

3) If you have never installed proteus then first you need to open crack folder then install Licence(Grassington North Yorkshire_LICENCE).

4) After installing crack run setup.

5) Now again open crack folder here you will get 'LXK Proteus 7.9 SP1 ENG v1.0.0' File.(If you didn't find this file then stop your antivirus then extract it from crack.rar file).

6) Right click on 'LXK Proteus 7.9 SP1 ENG v1.0.0' then choose 'Run as administrator'.

7) Then click browse and goto-

    My computer>Local Disc C>Program Files86>Labcenter Electronics>Select 'Proteus 7 Professional'> click'Update'>OK.

8) Now type 'ISIS' in your search box. 

 

HOW BATTERIES WORK

HOW BATTERIES WORK

 

Have you ever mixed vinegar with baking soda to create a volcano for a science fair project? The bubbling that you see is the result of a chemical reaction. This reaction is very similar to how batteries work. The reaction, however, occurs inside a battery, hidden from view by the battery case. This reaction is what creates the electrical energy that the battery supplies to circuits.

A typical battery, such as a AA or C battery has a case or container. Molded to the inside of the case is a cathode mix, which is ground manganese dioxide and conductors carrying a naturally-occurring electrical charge. A separator comes next. This paper keeps the cathode from coming into contact with the anode, which carries the negative charge. The anode and the electrolyte (potassium hydroxide) are inside each battery. A pin, typically made of brass, forms the negative current collector and is in the center of the battery case.

Each battery has a cell that contains three components: two electrodes and an electrolyte between them. The electrolyte is a potassium hydroxide solution in water. The electrolyte is the medium for the movement of ions within the cell and carries the iconic current inside the battery.

The positive and negative terminals of a battery are connected to two different types of metal plates, known as electrodes, which are immersed in chemicals inside the battery. The chemicals react with the metals, causing excess electrons to build up on the negative electrode (the metal plate connected to the negative battery terminal) and producing a shortage of electrons on the positive electrode (the metal plate connected to the positive battery terminal).

Flashlight or smaller batteries, usually labeled A, AA, C, or D have the terminals built into the ends of the batteries. That’s why the battery compartment of your flashlight has a + and a – sign, making it easier for you to install your batteries the correct direction. Larger batteries, like those in a car, have terminals that extend out from the battery. (They generally look like large screw tops.)

The difference in the number of electrons between the positive and negative terminals creates the force known as voltage. This force wants to even out the teams, so to speak, by pushing the excess electrons from the negative electrode to the positive electrode. But the chemicals inside the battery act like a roadblock and prevent the electrons from traveling between the electrodes. If there’s an alternate path that allows the electrons to travel freely from the negative electrode to the positive electrode, the force (voltage) will succeed in pushing the electrons along that path.

When you connect a battery to a circuit, you provide that alternate path for the electrons to follow. So the excess electrons flow out of the battery via the negative terminal, through the circuit, and back into the battery via the positive terminal. That flow of electrons is the electric current that delivers energy to your circuit.

When the electrodes are connected via a circuit, for example, the terminals inside a flashlight or those in your vehicle, the chemicals in the electrolyte start reacting.

As electrons flow through a circuit, the chemicals inside the battery continue to react with the metals, excess electrons keep building up on the negative electrode, and electrons keep flowing to try to even things up — as long as there’s a complete path for the current. If you keep the battery connected in a circuit for a long time, eventually all the chemicals inside the battery are used up and the battery dies (it no longer supplies electrical energy).