What to consider when choosing an acceleration sensor
Published time: 2019-11-28
Today, everyone is very concerned about health. Whether it is going out to wear a bracelet, a pedometer, or taking a mobile phone to record the number of walking steps, it has become a lifestyle of many people. So how does the pedometer work? In mobile phone bracelets, a very small chip is usually used-a three-axis acceleration sensor. This three-axis acceleration sensor is a key component of the pedometer. The principle and application of the acceleration sensor are introduced below.
The principle of the acceleration sensor:
Through this acceleration sensor, you can measure the acceleration of the mobile phone or the bracelet in three different directions. By calculating the acceleration value, you can roughly measure the number of steps you walk. Smaller power consumption but lower accuracy
There is a special material called piezoelectric ceramic material. The acceleration sensor prepared by this material can produce different deformations according to the force acting on it, and can produce different voltage changes. The acceleration is measured by the force acting on it, and then the acceleration is used to determine in which direction a person is moving when walking, or the frequency.
Application of various types of structural acceleration sensors
Let's understand the application of these four acceleration sensors in vibration and shock measurement.
Piezoelectric acceleration sensor
A piezoelectric acceleration sensor is a spontaneous sensor whose output charge is proportional to the acceleration it senses. It has the characteristics of high accuracy, wide frequency response, large dynamic range, small size, light weight, long life, easy installation, and good stability. Natural quartz can be used, and sensitive components can be formed by appropriate cutting, but the sensitivity is low and the cost is high. At present, ferroelectric materials are commonly used. This is an artificial ceramic with piezoelectric properties after artificial polarization treatment. A good preparation and sintering process can obtain high piezoelectric sensitivity and working temperature, which can be guaranteed after aging treatment. Long-term temperature stability, it is easy to make sensitive elements of various shapes, and acceleration sensors of various structural forms have been made.
The single-ended compression type has the characteristics of high sensitivity and high resonance frequency, and is suitable for general measurement. The base isolation compression type can minimize the influence of the base coupling, which is more suitable for low vibration level measurement, and also suitable for places with strain or unstable temperature on the mounting surface.
The ring-shear type has the characteristics of small size and light weight, which is suitable for measuring impact or vibration of small and light structural parts. Because the sensitive components are well isolated from the base, it can effectively avoid the influence of the base bending and noise. The element is only subjected to shear, which reduces the pyroelectric effect. Center hole installation ring-shear type can choose the wiring direction arbitrarily. The isolated shear type uses multiple crystals and passive compensation plates to improve sensitivity, widen the temperature range, ensure stability, and have the highest signal-to-noise ratio.
Integrated circuit piezoelectric acceleration sensor
The integrated circuit piezoelectric acceleration sensor is made as a result of the development of microelectronics technology. The microelectronic signal conditioning circuit is installed in the housing of this sensor, so it achieves low impedance output, large output signal, and suitable for cables and connectors. The interference signal is very insensitive and insensitive to various environmental factors. A two-wire cable or two plastic sheathed wires can play the role of power supply and signal transmission at the same time, and the use of long cables will not reduce the sensitivity. No noise, simple structure, low cost, and improved performance. It is especially suitable for various engineering sites and places that require long-distance measurement. For high sensitivity, you can even directly connect to recording instruments, which is convenient to use. Endevco calls this sensor ISOTRON.
Piezoresistive acceleration sensor
The structure of the new piezoresistive acceleration sensor is made of a single piece of silicon crystal. It uses a silicon piezoresistive strain gauge as a sensitive element. This is actually a fixed silicon resistor whose change in resistance is directly proportional to the mechanical stress it is subjected to. This can be made of a small solid silicon wafer through microfabrication, so it is also called an integrated sensor, thereby avoiding the inaccurate mechanical connection caused by the old structure of attaching silicon strain gauges to the cantilever arm. The piezoresistive acceleration sensor is constructed as a Wheatstone bridge in the circuit and generates an electrical signal proportional to the vibration acceleration. Because it is used in pairs, the output stability is guaranteed under different temperature conditions.
This sensor is characterized by being able to measure signals down to DC without phase distortion. Its low output impedance, high output level, low internal noise, and low sensitivity to electromagnetic and electrostatic interference make it easy to adjust the signal. The sensitivity of some piezoresistive acceleration sensors is high enough to directly drive the recorder. It is not sensitive to base strain and thermal transients, and has no zero drift when subjected to large impact accelerations. It can be calibrated by the flip method, so it is widely used in low-frequency vibration and long-lasting shock measurement, such as vibration during transportation And shock measurement, packaging test, shock wave research, automobile crash test, modal analysis, flutter research, research of biomedical phenomena.
Variable capacitance type acceleration sensor
The variable capacitance type acceleration sensor is a silicon acceleration sensor made by another physical principle. Compared with the piezoresistive type, it has higher sensitivity and resistance to environmental vibration and shock. It is not sensitive to temperature and has good stability. , High linearity. The parameters describing the dynamic characteristics of various acceleration sensors are: sensitivity, amplitude-frequency response, phase-frequency response, installation resonance frequency, lateral sensitivity, amplitude non-linearity, etc.
Application field of acceleration sensor
The step counting function is only part of the application of the acceleration sensor. Acceleration sensors all play a very important role in vehicle safety, bridge health, etc.
Liao Yong, an engineer of China Academy of Electronics Science: Why does the airbag on the car pop up? If the car is driven above a certain speed, if it stops suddenly, the airbag will pop up. This is actually an acceleration sensor measuring the car's forward Acceleration in direction. A change in quantity determines whether the airbag should be ejected now.
The above is the introduction of the principles and application knowledge of acceleration sensors. Acceleration sensors can also be used in bridge health monitoring, automotive fields, and hard disk impact protection experiments. Taking the car as an example, it can be used in car crash tests, airbags, ABS anti-lock braking systems, electronic stability programs (ESP), electronically controlled suspension systems, etc.
The above mainly introduces the principle and application analysis of the acceleration sensor, do you understand? Then let's understand what issues to consider when selecting an acceleration sensor.
Analog output vs digital output:
This is the first thing to consider. It depends on the interface between your system and the acceleration sensor. Generally, the voltage and acceleration of the analog output are proportional. For example, 2.5V corresponds to 0g acceleration, and 2.6V corresponds to 0.5g acceleration. Digital outputs typically use pulse-width modulation (PWM) signals.
If the microcontroller you use has only digital inputs, such as BASICStamp, you can only choose digital output acceleration sensors, but the problem is that you must occupy an additional clock unit to process the PWM signals, and it is also a processor for the Not a small burden.
If the microcontroller you are using has an analog input port, such as PIC / AVR / OOPIC, you can use the acceleration sensor of the analog interface very easily. All you need is to add a sentence like "acceleration = read_adc ()" to the program And it only takes a few microseconds to process this instruction.
Number of measuring axes:
For most projects, two-axis acceleration sensors are sufficient for most applications. For some special applications, such as UAV, ROV control, a three-axis acceleration sensor may be suitable.
Maximum measurement value:
If you only need to measure the inclination of the robot relative to the ground, a ± 1.5g acceleration sensor is sufficient. But if you need to measure the dynamic performance of the robot, ± 2g should be enough. If your robot will start or stop suddenly, you need a sensor of ± 5g.
In general, the more sensitive the better. The more sensitive the sensor is, the more sensitive it is to acceleration changes within a certain range, and the larger the output voltage changes, so it is easier to measure and thus obtain more accurate measurement values.
The bandwidth here actually refers to the refresh rate. In other words, how many readings the sensor produces every second. For applications that only measure inclination, a bandwidth of 50 Hz should be sufficient, but for dynamic performance, such as vibration, you will need a sensor with a bandwidth of hundreds of Hz.
Resistance / buffer mechanism:
For some microcontrollers, the resistance of the connected sensor must be less than 10kΩ for A / D conversion. For example, the Analog Devices ’s analog acceleration sensor has a resistance of 32kΩ and cannot work normally on PIC and AVR control boards. Therefore, it is recommended to read the controller manual carefully before purchasing the sensor to ensure that the sensor can work normally.
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