What Is a Capacitive Accelerometer?

An acceleration sensor is a sensor capable of measuring acceleration. It usually consists of mass, damper, elastic element, sensitive element, and adaptive circuit. During acceleration, the sensor uses the Newton's second law to obtain the acceleration value by measuring the inertial force on the mass. Depending on the sensor's sensitive components, common acceleration sensors include capacitive, inductive, strain, piezoresistive, and piezoelectric.

Accelerometer

An acceleration sensor is a sensor capable of measuring acceleration. It usually consists of mass, damper, elastic element, sensitive element, and adaptive circuit. During acceleration, the sensor uses the Newton's second law to obtain the acceleration value by measuring the inertial force on the mass. Depending on the sensor's sensitive components, common acceleration sensors include capacitive, inductive, strain, piezoresistive, and piezoelectric.

Chinese name

Accelerometer

Foreign name

acceleration transducer

Function

Measuring acceleration

Definition

Sensors that convert acceleration into signals

On April 4, 2014, the latest patent application issued by the U.S. Trademark and Patent Office shows that Apple is developing a new type of headset device. They are trying to add multiple sensors and microphones to the headset to make the noise reduction function stronger. Make headphones smarter. ^[1]

The principle of a linear accelerometer is the principle of inertia, which is the balance of forces. A (acceleration) = F (inertial force) / M (mass). We only need to measure F. How to measure F? Just use electromagnetic force to balance this force. Then we can get the relationship between F and current. It is only necessary to use experiments to calibrate this scale factor. Of course, the intermediate signal transmission, amplification, and filtering are the matter of the circuit.

Most acceleration sensors are based on

1. Technical specifications in terms of sensitivity: For an instrument, generally the higher the sensitivity, the better, because the more sensitive it is, the easier it is to feel the changes in acceleration that occur in the surrounding environment. The acceleration changes are large, and naturally, The output voltage changes correspondingly, so the measurement is easier and more convenient, and the measured data will be more accurate.

2. Technical specifications in terms of bandwidth: Bandwidth refers to the effective frequency band that a sensor can measure. For example, a sensor with a bandwidth of hundreds of HZ can measure vibration; a sensor with a bandwidth of 50 HZ can effectively measure the inclination Already.

3. Technical indicators in measuring range: The measuring ranges required to measure the movement of different things are not the same, and should be measured according to the actual situation.

Parse sensors on your phone

An acceleration sensor is an electronic device that can measure acceleration forces. Acceleration force is the force that acts on an object during acceleration, just like the gravity of the earth, which is gravity. The acceleration force can be a constant, such as g, or a variable. Therefore, its range is larger than the gravity sensor, but when the acceleration sensor mentioned in the mobile phone is actually the gravity sensor, the two can be regarded as equivalent.

Direction sensor

The mobile phone orientation sensor refers to a component installed on a mobile phone to detect the orientation state of the mobile phone itself, rather than the function of a compass as commonly understood.

The mobile phone orientation detection function can detect that the mobile phone is in the upright, inverted, left horizontal, right horizontal, up, and down states. The mobile phone with direction detection function is more convenient and more user-friendly. For example, after the phone is rotated, the screen image can automatically rotate and switch the aspect ratio, and the text or menu can also be rotated at the same time, making it easy for you to read; when listening to MP3. Someone may say: This is the same as that gravity sensor?

The two are not the same. The direction sensor or the angular velocity sensor is more suitable. Generally, the direction sensor on the mobile phone senses the azimuth, rotation and tilt angle on the horizontal plane. If you might think this is a bit theoretical, give an example. Directional sensors can play urban racing games very well. Only the gravity sensor can play, but the results are confusing.

In order to obtain highly realistic test data, users should fully understand the working characteristics of the instrument used, how these characteristics affect each other, how the entire environment affects these characteristics, and how the accelerometer affects the measured motion.

The accelerometer is a key measurement element and is available in a variety of designs. Each design type is designed for a specific purpose, in order to obtain high-fidelity measurement data.

Engineers should carefully analyze the measurement requirements, choose the most suitable accelerometer, and usually compare the three among sensitivity, weight and frequency response range to make the most suitable choice.

The main working characteristics of sensors are divided into two types: effective response and spurious response.

Effective response

In the direction of the sensor's sensitive axis, the response of the sensor caused by the input mechanical vibration or shock. This response is expected from the correct use of sensors for measurement and reliable data.

spurious response

When using a sensor to measure mechanical vibration or shock, the response of the sensor caused by other physical factors that exist at the same time. This response is disturbing the correct measurement and is not expected. (See National Standard GB / T 13823.1-93)

The effective responses are:

Sensitivity; amplitude frequency response and phase frequency response; nonlinearity.

The spurious response mainly includes:

Temperature response; transient temperature sensitivity; lateral sensitivity; rotational motion sensitivity; base strain sensitivity; magnetic sensitivity; mounting torque sensitivity; response to special environments. (See National Standard GB / T 13823.1-93)

Sensitivity: (Sensitivity)

The ratio of the specified output amount to the specified input amount.

Reference sensitivity: (Reference Sensitivity)

Sensitivity of the sensor at a given reference frequency and reference amplitude.

The higher the sensor sensitivity, the greater the signal-to-noise ratio of the measurement system, and the system is less susceptible to electrostatic interference or electromagnetic fields. For a specific type of accelerometer design, the higher the sensitivity, the heavier the sensor and the lower the resonance frequency. So the choice of sensitivity is limited by its weight and frequency response.

In general, the sensitivity of a sensor includes two information, amplitude and phase, which are complex quantities that change with frequency.

Amplitude frequency response and phase frequency response

When the magnitude of the input mechanical vibration is constant, the change in the amplitude of the sensor output power with the vibration frequency is called the amplitude-frequency response. The change of the output power phase with the vibration frequency is called phase frequency response.

The vibration frequency is continuously changed in the working frequency band, and the amplitude of the input mechanical vibration is maintained. At the same time, the amplitude-frequency response can be measured by observing the output of the sensor. If the phase difference between the output power of the sensor and the input mechanical vibration is measured at the same time, the phase frequency response can be measured again.

In general, only the amplitude-frequency response is required. When using the sensor at the upper and lower limit frequencies of the proximity sensor, or when required, the phase frequency response must be known.

Nonlinearity

In a given frequency and amplitude range, the output is proportional to the input, which is called a linear change. The degree to which the calibration result of an actual sensor deviates from the linear change is called the non-linearity of the sensor.

Within the dynamic range of the sensor from the minimum to the maximum, gradually increase the amount of input mechanical vibration and measure the change in the output amplitude of the sensor at the same time, you can determine the deviation between the output value of the sensor and the linear output value. When using a sine vibration generator for measurement, several frequencies can be selected within the working frequency range of the sensor to cover the entire dynamic range of the sensor.

Generally, the maximum deviation between the output value and the linear value of the sensor is near the upper limit of the sensor's dynamic range. The amount of deviation allowed depends on the requirements of the specific measurement.

For a piezoelectric accelerometer, the non-linearity is generally expressed as a percentage of its sensitivity increase within a certain acceleration range. Piezoresistive, variable capacitance accelerometers have good linearity in their dynamic range, and they represent the combined values of non-linearity, hysteresis, and non-repetition.

Effect of mass load

If the dynamic mass of the accelerometer is close to the dynamic mass of the structure being measured, it will cause significant attenuation of the vibration. For this reason, when measuring vibration on thin and light sheet-like members such as printed circuit boards, a lightweight accelerometer must be used in order to obtain accurate data. If the measured object exhibits a single degree of freedom response, the accelerometer will reduce its resonance frequency. Mini accelerometers must be used in all modal tests.

Low frequency response

Using piezoelectric acceleration timing, the low frequency cut-off frequency of the amplifier used is mostly 2-5Hz, the purpose is to eliminate the pyroelectric output of many piezoelectric sensors. Good isolation designs such as isolated shear designs can be used at lower frequencies. Piezoresistive and variable capacitance accelerometers have zero frequency response.

High frequency response

The high frequency response of an accelerometer varies with the mechanical properties and mounting method of the accelerometer. When installed securely, most accelerometers exhibit the frequency response characteristics of an undamped single-degree-of-freedom system. With ± 5% as the requirement, its frequency response is approximately flat to one fifth of the resonance frequency of the installation. If appropriate correction factors are added, useful data can be obtained at higher frequencies.

Temperature response

The change in sensor sensitivity with temperature is called the temperature response of the sensor. It is expressed as a percentage of the difference between the sensitivity at the test temperature and the sensitivity at room temperature relative to the sensitivity at room temperature.

The temperature range of common piezoelectric accelerometers is below zero to + 177 ° C or + 260 ° C. For some specific models, the low temperature can reach absolute zero and the high temperature can reach 760 ° C. Many types of piezoelectric accelerometer designs have flat temperature response over a wide temperature range. The typical temperature range of piezoresistive and variable capacitance accelerometers is -18 ° C ~ + 93 ° C.

Transient temperature sensitivity of piezoelectric sensors

A sensor with a pyroelectric effect will produce an electrical output under the influence of transient temperature. The ratio of the maximum value of the output to the product of the sensor sensitivity and the temperature change is called transient temperature sensitivity.

When the temperature changes, the piezoelectric element generates an output signal, which is called the pyroelectric effect. Sudden changes in specimen or air temperature can cause this temperature change. In most cases this effect is very low frequency and can only be detected if the response of the signal conditioner is below 1 Hz. If the signal conditioner has an interstage high-pass filter, special attention should be paid to the fact that the pyroelectric signal may saturate the amplifier and make it inoperative for a short time.

Pyroelectric effect of base isolation type, shear type, isolation shear type is small. This effect of piezoresistive and variable capacitance is negligible.

Lateral sensitivity

For unidirectional measurement, it is necessary to require the accelerometer not to have any response to the lateral movement of the measured object. But an accelerometer cannot be perfect, and always has a certain lateral sensitivity, which is related to the direction of lateral vibration, and its lateral sensitivity is generally 1 to 5% of the axial sensitivity. Endefor calibrates the lateral sensitivity of each accelerometer and gives its maximum value.

Lateral sensitivity ratio

The sensitivity of the sensor when excited in a direction perpendicular to the sensor's sensitive axis is called lateral sensitivity. The ratio of the lateral sensitivity to the sensitivity along the sensitivity axis is called the lateral sensitivity ratio.

Spin sensitivity

Some linear vibration sensors are sensitive to rotational motion. Care must be taken when conducting tests. So as not to cause measurement errors.

Base strain sensitivity

When the sensor base is strained, it will cause an unwanted signal output. The ratio of the output value to the product of the sensor sensitivity and the strain value is called the base strain sensitivity.

In some tests, there may be dynamic bending, twisting, stretching, etc. where the accelerometer is installed. Due to the close contact with the strain zone, the accelerometer base will also be strained. Part of the strain is transmitted to the sensitive element, which produces an output signal that is independent of vibrational motion.

The shear-type accelerometer is an order of magnitude less sensitive than the compression type to base strain. Applying insulation mounting screws or adhesive adapters can reduce this effect.

Magnetic sensitivity

The undesired signal output produced by the sensor when placed in a magnetic field, the ratio of the output value to the product of the sensor sensitivity and the magnetic induction intensity of the magnetic field is called the sensor's magnetic sensitivity.

Moment sensitivity

With a thread-mounted sensor, changes in mounting torque can cause sensitivity changes. The maximum difference between the sensitivity when 1/2 times the specified installation torque or 2 times the specified installation torque and the sensitivity when the specified installation torque is applied, the percentage of the ratio of the sensitivity to the sensitivity when the specified installation torque is applied is called the installation torque sensitivity.

Special environment response

In the special environment of strong electrostatic field, alternating magnetic field, radio frequency field, sound field, cable influence, nuclear radiation, etc., some sensors will be severely affected. These physical factors will cause the sensor to produce a spurious response.

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.

The natural frequency of the acceleration sensor is determined by the degree of coupling of the bond, and choosing the right adhesive will be an important step. Some important issues must be considered: the weight of the acceleration sensor, the frequency and bandwidth during the test, and the amplitude and temperature during the test. There are also some issues that will arise during the test: the limitation of the sine curve and the random vibrations that occur during the test. Generally, engineers and technicians will choose the appropriate adhesive to bond the acceleration sensor according to the different needs of the test.
Adhesives used for the acceleration installation of acceleration sensors are generally cyanoacrylate, magnet, double-sided tape, paraffin, and thermal adhesive. The key issue is how to effectively use these adhesives. In the adhesion process of the acceleration sensor, the amount of adhesive used plays a key role in whether the acceleration sensor can achieve a good frequency response. Bonding the acceleration sensor with the least amount of adhesive on a small film will make the frequency response of the acceleration sensor the best.

Before installing the sensor, use a hydrocarbon solution to clean the surface to be installed. When installing the sensor, usually use cyanoacrylate, magnet, double-sided tape, paraffin, and apply it evenly to the adhesion accelerometer The surface to be adhered must not be too thick or too thin, a suitable thickness will have a good bonding effect. There are many considerations for the use of thermal adhesives. The most important thing is to pay attention to the setting time of thermal adhesives during installation.
Of course, we need to pay attention when using adhesive installation. It is best not to use the adhesive when it is close to the maximum limit temperature. It should be delayed for a period of time, otherwise the adhesive itself will be damaged and the tensile strength of the adhesive will be reduced. . In any case, all the limiting factors must be taken into account when we want to install the acceleration sensor by the adhesive method. Similarly, the above acceleration sensor installation is only for most cases. The most suitable sensor installation method is needed to measure acceleration on some special equipment.

Consulting company INTECHNOCONSULTING's sensor market report shows that the global sensor market capacity in 2008 was 50.6 billion US dollars, and the global sensor market is expected to reach more than 60 billion US dollars in 2010. The survey shows that Eastern Europe, Asia Pacific, and Canada have become the fastest growing regions of the sensor market, while the United States, Germany, and Japan are still the regions with the largest sensor market distribution. As far as the world is concerned, the fastest growing sensor market is still the automotive market, and the second is the process control market, which is optimistic about the prospect of the communication market.

Some sensor markets such as

With the popularity of smart phones, etc., devices are required to have higher functions and designability. In this case, the demand for highly integrated and miniaturized components is strong. In addition, higher performance results in increased battery consumption, and therefore, lower power consumption is required for various components mounted on the device.

The industry's smallest size acceleration sensor has a maximum resolution of 14bit, has low power consumption, high impact resistance, and programmable standby wake-up function, and can perform tilt detection, motion detection, etc .; another high performance, low power Cost-effective, low-noise acceleration sensor with high stability, the highest resolution of 4bit, can be used for high-precision tilt detection, motion detection, etc. These two devices are mainly used in smartphones, tablets / laptops, digital cameras, and game consoles And other small livelihood equipment.

The demand for devices with more sensory motion, such as smartphones and game consoles, is increasing, and new demands such as motion remote control for smart TVs have emerged. In these motion detections, a conventional acceleration sensor is used, and a gyroscope is added to improve the operation experience.

The small package gyroscope uses a FIFO buffer, which can reduce the access frequency from the microcontroller, and has a rotation motion detection function. With the further popularization of acceleration sensors and gyroscopes, the use cases in small devices are increasing. The demand for a single chip and the use of two sensors together in one system's communication interface is increasing.

The small-scale packaged 3-axis acceleration sensor and 3-axis gyroscope are gradually emerging. They not only have various features and functions of the above-mentioned small-package gyroscope, but also have industry-leading low power consumption, only 4mA. They are mostly used in smartphones, tablets, game consoles, remote controls, PNDs and other small livelihood devices.