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Vibration and shock are present in all areas of our daily lives. They may be generated and transmitted by motors, turbines, machine-tools, bridges, towers or by the human body.
While some vibrations are desirable, others may be disturbing or even destructive. Consequently, there is often a need to understand the causes of vibrations and to develop methods to measure and prevent them.
The sensors we manufacture serve as a link between vibrating structures and electronic measurement equipment.
2.1.2 What is measured?
The commonly used quantity for the measurement of vibration is acceleration. It has the Standard International unit m/sÂ² (meters per second squared). Sometimes also the non-SI unit gravitational acceleration (g) is used for acceleration (1g is about 9.81 m/sÂ²).
For some applications, for example in machine monitoring, vibration velocity (mm/s) or vibration displacement (Âµm, mm) are measured. Velocity can be derived from acceleration by single integration, displacement by double integration. Integrators can be implemented by an analog circuit or a software routine.
While we have an idea of the order of magnitudes of displacement and velocity, it may be difficult to imagine acceleration:
Accelerations below 0.001 m/sÂ² are measured in seismic surveys.
A racing car driver can experience 50 m/sÂ². Most humans lose consciousness at around 60 m/sÂ².
A car accident of 100 m/sÂ² will break human bones while 300 m/sÂ² are sufficient for a seatbelt to break ribs.
A laptop dropping onto a concrete floor from a height of 1 m may endure as much as 20,000 m/sÂ².
Accelerations beyond 100,000 m/sÂ² are found in ballistics and explosion tests.
2.1.3 Micro Electro Mechanical Systems (MEMS)
MEMSï¼ˆMicro Electro Mechanical Systemsï¼‰is essentially a micro-mechanical devices (such as sensors, brakes, etc.) with electronic circuit integrated in a chip of semiconductor technology. General chip only uses the electrical characteristics of silicon, while the MEMS chip is use both two kinds of electrical and mechanical properties.
Three-dimensional micro electro mechanical systems (3D-MEMS), is processed silicon into three-dimensional structure, its packaging and contact assembly is easy to install and assemble, use this technique produced sensor which have excellent accuracy, small size and low power consumption. The sensor can be produced only by a small piece of silicon, and can measure the acceleration of three mutually perpendicular directions. Such as provide suitable mechanical damping for withstand strong vibration acceleration sensors and high-resolution altimeter. These sensors have very low power consumption, which makes battery-powered devices in their incomparable superiority.
2.1.4 Principle of MEMS Sensor
In the MEMS sensor chip, the movement or tilt of three-axis (X, Y, Z) will cause a small amount of displacement of silicon structures (figure 2.1), and resulting in activity and the fixed component changes between activity and the capacitance. The interface chip in the same package transfers the small change of capacitance into an analog voltage which is proportional to the movement. To ensure that the performance can be reused, the interface chip can be allowed fine-tuning appear in the market, therefore, don't need to adjust the production for final products. The usual analog sampled in two ways: static capacitance and resistance-type piezoelectric more advantages, while the static capacitance is more dominant in the low power consumption, lower current consumption. Silicon capacitive acceleration sensor element is made of silicon and glass. This design ensures the time and temperature relative to the exceptional reliability, accuracy and stability. Usually a 1g component can withstand more than the acceleration of 40 000g (1g=gravitational force acceleration).
Figure 2.1 motions on 3 Axis.
The reliability of wiring in chip is higher than the wiring on printed circuit board soldering, and semiconductor manufacturing processes and packaging technologies provide security for the protection of MEMS devices. Their inherent high reliability for automotive systems and appliances, electronic systems is particularly reliable in a mechanical vibration of the work environment. Meanwhile, the batch manufacturing makes the MEMS component manufacturing costs are usually lower than the large precision mechanical components, mechanical components are usually the most expensive system components. Therefore, MEMS technology allows many applications have the opportunity to achieve higher cost and higher reliability.
2.1.5 Accelerometer Sensor Products
Freescale Semiconductor based on the three-axis low-gravity accelerometers of MEMS, MMA7260Q. MMA7260Q optional sensitivity allows the different context of design in 1.5 g, 2 g, 4 g and 6 g. with 3Î¼A sleep mode, 500Î¼A low operating current, 1.0 ms quick response time and 6 mins 6 mm Ã- 1.45 mm packaging in QFN, and other characteristics
Three-axis accelerometer LIS3L02, integrated MEMS-based sensors and an interface chip of three-axis linear accelerometer. There are three versions for LIS3L02:
LIS3L02A provide an analog output;
LIS3L02P to provide an analog voltage output and pulse width modulation output (PWM);
LIS3L02D provides a serial digital I2C output
Various types of products work in a 2.7-3.6V supply voltage, the equivalent noise acceleration better than 500 millionths of one g (g = acceleration of gravity). It can withstand the maximum acceleration of 1500g in the work without damage, this characteristic sufficient to resist vibration of mobile phone applications.
The basic characteristics of LIS3L02 are as follows:
Operating voltage 2.4V-5.25V, the sensitivity of the total accuracy can be adjusted within Â± 10%, the maximum bandwidth of X axis and Y axis is 4.0 kHz, the use of technology can convert the capacitance change to analog output and the SPU and I2C serial output, the equivalent noise acceleration in the 100Hz bandwidth better than 500 Ã- 10-6 g, it can withstand vibration up to 3000g, operating temperature from -40 degrees to -85 degrees, SMD packaging.
In this project, we should use LIS3L02DQ chip.
3D-MEMS accelerometer SCA610 from Finland VTI, using single crystal silicon structure, no plastic deformation, the affordability of 70000 g, capacitive sensing principle, based on the great changes of capacitor plate distance, the direct measurement of excursion, the capacitance and charge storage capacity of capacitors depends to the distance between the two plates and plate area A: C = e0 * A / d, you can get high accuracy and stability just using a limited number of capacitors, and easy to judge. Temperature coefficient less than 0.2 mg / oC, non-linearity less than 1%, transverse sensitivity less than 3%. SCA610 is formed by an 8-pin plastic DIP SMD permanent template. Lead frame material is nickel plated and gold CuFe2P.
2.1.6 The Application of Accelerometer Sensor
Accelerometer is widely used in portable devices, home appliances, automobile electronics and consumer products, handheld devices for hard drive protection, map / file browser navigation / picture position automatic adjustment functions.
The most common application is the image location of the automatic adjustment, turn the handheld device, the picture shows also with spinning as to ensure the best viewing angle. In the picture / map view can be as a viewfinder to the screen, the user can move the screen to access the map, or zoom in or out. When rotating the terminal, the map remains level, and ensures the best visual effects. Iphone from Apple is a good example.
Somewhere in the path of iPhone software development, Apple opens its door and invite programmers to create applications for iPhone. People began to create applications (mostly games) using the accelerometer feature. User then can control the movements of the game character just by slightly tilting the position of the phone
Besides, Aspects of the game user interface can be used for the direction of rotation of the direction of the maze game control, the directional control of racing games, speed brake and control the size of the screen, 3D game characters move through user-defined interface realise different actions in different functions. You can recur to the accelerometer sensor to find out the position data, then point out the user location.
2.1.7 The application need to use 3D accelerator:
1. Hard disc drop and vibration protection (laptop, with a hard drive PDA, cell phone)
2. Pedometer, use the 3-axis accelerator has better accuracy
3. GPS secondary navigation
4. 3D game
5. Need to use trajectory recognition applications such as robotics, MEMS mouse, etc.
2.2 Embedded Systems
An embedded system is a combination of software and hardware which creates a computer system that performs specific, pre-defined tasks and which is encapsulated within the device it controls (if it is part of a larger device).
Embedded systems can be found in a huge range of electrical items ranging from simple, cheap products such as digital watches to expensive, complex products.
2.2.1 Simplification, Miniaturisation and Cost Reduction
Embedded systems are designed to perform simple, repeatable tasks - often with no or little input from the user. Since the first microprocessor was introduced into calculators there has been a concerted drive to reduce the complexity and size of computerised systems in electronic devices.
Embedded systems are based on the concept of the microcontroller, a single integrated circuit that contains all the technology required to run an application. Microcontrollers make integrated systems possible by combining several features together into what is effectively a complete computer on a chip, including:
* Central Processing Unit
* Input/Output interfaces (such as serial ports)
* Peripherals (such as timers)
* ROM, EEPROM or Flash memory for program storage
* RAM for data storage
* Clock generator
By integrating all of these features into a single chip it is possible to reduce the number of chips and wiring necessary to control an electronic device, dramatically reducing its complexity, size and cost.
2.2.2 Uses of Embedded Systems
There are endless uses for embedded systems in consumer products, with new methods of exploiting them presented every year at such industry events as the MEDC and the Embedded Systems Conference. The most beneficiaries are those enterprises concerned with the sale and manufacture of electrical devices, as the inclusion of microcontrollers to replace general purpose microprocessors can drive down end user prices and unit manufacturing costs dramatically, resulting in increased sales and an improved profit margin.
The use of embedded systems in electrical products can also solve many problems of product size and complexity. For example, a modern product may contain some of embedded systems to control a range of processes within the car, ranging from brake balance control to air conditioning to the ignition system. Without embedded systems it would be impossible to computerise all of these features without the inclusion of a mass of fault prone electronics, complicated.
The only realistic alternative to using embedded systems in a modern automobile would be to install a functional PC within the product to control all of the functions currently managed by microcontrollers. While this may be feasible it would raise several issues:
* Size & Weight: design Microcontrollers to deliver maximum performance for minimum weight and size. A centralised on-board computer system would greatly outweigh a collection of microcontrollers.
* Efficiency: design Microcontrollers to perform repeated functions for long periods of time without requiring or failing service. Other computer systems are prone to hardware and software failure as well as a whole host of other problems recognisable to the users of any computer. Above all other considerations, computer systems must be 100% reliable when trusted to control such functions as braking in an automobile.