What is an Accelerometer?

An accelerometer is a meter that measures the linear acceleration of a carrier.

The accelerometer consists of a detection mass (also called a sensitive mass), a support, a potentiometer, a spring, a damper, and a housing.

Among them, the accelerometer that measures aircraft overload is one of the earliest aircraft instruments to be applied.

Chinese name: Accelerometer
Explanation: Instrument for measuring linear acceleration of a carrier
Constituent elements: Testing quality, support, potentiometer, spring, etc.

Nature: A one-degree-of-freedom oscillating system
Classification: Linear accelerometer, pendulum accelerometer, etc.
Advantages and characteristics: Excellent deviation stability and low cost

Basic introduction of accelerometer

Accelerometer An instrument that measures acceleration. Acceleration measurement is an important subject raised by engineering technology. When the object has a large acceleration, the object, the instruments and other objects and other objects that have no relative acceleration are subject to a force that can generate the same acceleration, that is, a dynamic load. To know the dynamic load, measure the acceleration. Secondly, to know the space position of each instantaneous aircraft, rocket, and ship, you can continuously measure its acceleration through inertial navigation (see the gyro platform inertial navigation system), then obtain the velocity component through the integral operation, and integrate again to obtain the position in one direction. Coordinate signals, and the measurement results of the instruments in the three coordinate directions will synthesize the motion curve and give the space position where the aircraft is located at each instant. For another example, in some control systems, acceleration signals are often required as part of the information required to generate control effects, and the problem of continuously measuring acceleration also arises here. A device capable of continuously giving an acceleration signal is called an acceleration sensor.

The components of a common accelerometer are as follows: housing (fixed to the measured object), reference mass, sensitive components, signal output device, etc. Accelerometers require a certain range, accuracy, sensitivity, etc. These requirements are often contradictory to some extent. Accelerometers based on different principles have different ranges (from several g to hundreds of thousands of g ), and their sensitivities to abrupt acceleration frequencies are also different. Common accelerometers are based on the following principles: The reference mass is connected to the housing by a spring (see figure). The relative displacement of the accelerometer and the housing reflects the magnitude of the acceleration component. This signal is output as a voltage through a potentiometer; The mass is fixedly connected to the shell by the elastic thin rod, and the rod is deformed by the dynamic load caused by acceleration. The amount of deformation is induced by the strain resistance wire, and its output is an electrical signal proportional to the size of the acceleration plate; It is firmly connected to the case, and the dynamic load of the mass generates pressure on the piezoelectric element. The piezoelectric element outputs an electric signal proportional to the pressure, ie, the acceleration component: The reference mass is connected to the case by a spring, and is placed inside the coil to reflect the acceleration component. The magnitude of the displacement changes the inductance of the coil, thereby outputting an electrical signal proportional to the acceleration. In addition, there are servo-type accelerometers in which a feedback loop is introduced to improve the accuracy of the measurement. In order to measure the acceleration vector in a plane or space, two or three accelerometers are needed, each measuring an acceleration component. ^[1]

The principle of an angular accelerometer is similar to an accelerometer. Its outer box is mounted on a rotating object. Due to the angular acceleration, a tangential dynamic load is generated on the reference mass, and a signal proportional to the magnitude of the tangential acceleration or angular acceleration can be output. With different moving objects and measurement requirements, accelerometers have various principles and implementations. For example, on the aircraft, there is a gyro accelerometer designed according to the gyro principle. ^[2]

Instrument for measuring linear acceleration of a carrier. Accelerometers that measure aircraft overload are one of the first aircraft instruments to be used. Accelerometers are also commonly used on aircraft to monitor engine failures and fatigue damage to aircraft structures. In flight tests of various types of aircraft, accelerometers are important tools for studying aircraft flutter and fatigue life. In flight control systems, accelerometers are important dynamic characteristic correction elements. In the inertial navigation system, a high-accuracy accelerometer is one of the most basic sensitive components. The performance of accelerometers varies greatly in different applications. High-precision inertial navigation systems require accelerometer resolution of up to 0.001 g , but the range is not large. Accelerometers that measure aircraft overload may require a range of 10 g . The accuracy requirements are not high.

Accelerometer (8 photos)

Basic model of accelerometer

The accelerometer consists of a detection mass (also called a sensitive mass), a support, a potentiometer, a spring, a damper, and a housing. Constrained by the support of the test mass can only move along one axis, this axis is often referred to as the input axis or sensitive axis. When the instrument housing is accelerated along the direction of the sensitive axis with the carrier, according to Newton's law, the detection mass with a certain inertia tries to keep its original motion state unchanged. A relative movement between it and the housing will cause the spring to deform, so the mass will be accelerated by the force of the spring. When the spring force is in balance with the inertial force generated when the detected mass accelerates, there is no longer relative movement between the detected mass and the housing, and the deformation of the spring reflects the magnitude of the measured acceleration. The potentiometer acts as a displacement sensing element and converts acceleration signals into electrical signals for output. The accelerometer is essentially a one-degree-of-freedom oscillating system. A damper must be used to improve the dynamic quality of the system.

How accelerometers work

Accelerometer closed-loop liquid floating pendulum

Its working principle is: when the instrument housing is accelerating along the input axis, the detection mass rotates around the output shaft due to inertia, and the sensor element converts this rotation angle into an electrical signal, which is amplified and fed to the torque device to form a closed loop. The feedback torque produced by the torque device is balanced with the inertia moment experienced by the test mass. The electric signal (the magnitude of the current or the number of pulses per unit time) sent to the torque device is used to measure the magnitude and direction of the acceleration. The pendulum assembly is placed in a float. The buoyancy generated by the floating liquid can remove the load of the gemstone bearing from the float pendulum assembly, reduce the supporting friction torque, and improve the accuracy of the instrument. The floating liquid cannot play the role of fixed axis. Therefore, in the high-precision pendulum accelerometer, the magnetically suspended method is also used to suspend the unloaded float pendulum assembly at the center position, so that it is out of contact with the support, and further eliminates the friction torque. The viscosity of the floating liquid has a damping effect on the pendulum component, which can reduce dynamic errors and improve the ability to resist vibration and shock. The bellows are used to compensate the volume change of the floating liquid due to temperature. In order to keep the specific gravity and viscosity of the floating liquid basically unchanged to ensure the stable performance of the instrument, a strict temperature control device is generally required.

Accelerometer Flexible Pendulum

Adopt a flexible support pendulum accelerometer. The pendulum assembly is connected to the meter housing with two flexible rods. The bending stiffness of the flexible rod around the output shaft is low, while the stiffness in other directions is high. Its basic working principle is similar to the liquid floating pendulum accelerometer. This system has a high-gain servo amplifier that keeps the pendulum assembly working near zero. In this way, the bending of the flexible rod is very small, and the elastic moment introduced is also small, so the instrument can achieve high accuracy. There are two types of accelerometers, oil-filled and dry. The oil-filled interior is filled with a highly viscous liquid as a damping liquid, which can improve the dynamic characteristics of the instrument and improve its resistance to vibration and shock. The dry accelerometer adopts electromagnetic damping or air film damping, which is convenient for miniaturization, reducing costs and shortening the startup time, but the accuracy is lower than that of the oil-filled type.

Accelerometer vibrating wire

A linear accelerometer supported by two identical strings. The two string wires vibrate in a constant amplitude in the air gap magnetic field of the permanent magnet. The vibration frequency of the string is proportional to the square root of the tension of the string. When there is no acceleration, the tension of the two strings is equal, the vibration frequency is also equal, and the frequency difference is equal to zero. When there is acceleration along the input axis, the inertial force acting on the detection mass increases the tension of one string wire and increases the vibration frequency; while the tension of the other string wire decreases and the vibration frequency decreases. There is a frequency control device in the instrument to keep the sum of the vibration frequencies of the two string wires unchanged. In this way, the difference between the vibration frequencies of the two strings is proportional to the input acceleration. This difference frequency is converted into a pulse signal by the detection circuit. The pulse frequency is proportional to the acceleration, and the total number of pulses is proportional to the speed. Therefore, this instrument is also an integral accelerometer. String tension is greatly affected by material characteristics and temperature, so a precise temperature control device and string tension adjustment mechanism are needed.

Accelerometer Pendulum Integral Gyro

A meter that uses a two-degree-of-freedom gyroscope with a certain pendulum on the rotation axis to measure acceleration. The center of mass of the gyro rotor deviates from the inner ring axis and forms a pendulum. If the rotor does not rotate, the gyro component part is basically a pendulum accelerometer. When there is acceleration along the input axis (that is, the outer ring axis of the gyro), the pendulum rotates around the output axis (that is, the inner ring axis), so that the signal from the angle sensor on the axis is amplified and fed to the outer ring torque motor to force the gyroscope. The component moves around the outer ring axis and generates a gyro moment on the inner ring axis. It is balanced with the moment of inertia to keep the angle sensor near zero. The angular velocity of the gyro component around the outer ring axis is proportional to the input acceleration, and the magnitude of the rotation angle is the integral of the input acceleration, that is, the speed value. A pulse output device is usually installed on the outer ring shaft to obtain the acceleration and velocity information measured by the accelerometer: the pulse frequency represents the acceleration; the total number of pulses represents the speed. This type of accelerometer relies on the gyro moment to balance the moment of inertia. It can maintain high measurement accuracy over a large range, but it has a complicated structure, a large volume and a relatively expensive price.

Accelerometer classification introduction

There are many types of accelerometers:

According to the displacement method of the detection mass, there are linear accelerometers (the detection mass is linear displacement) and pendulum accelerometers (the detection mass rotates around the support axis);

According to the support method, there are gem support, flexible support, air float, liquid float, magnetic levitation and electrostatic levitation.

According to the composition of the measurement system, there are open-loop and closed-loop types.

According to the working principle, there are vibrating wire type, vibrating beam type and pendulum integral gyro accelerometer.

Classified by the number of input axes, there are single-axis, dual-axis and tri-axis accelerometers;

Classified by sensing element, there are piezoelectric type, piezoresistive type and potentiometer type.

The characteristics of several different classifications are usually used to name an accelerometer.

Accelerometer advantages and features

Excellent deviation stability, good environmental performance (shock, vibration and temperature)
Low cost low voltage analog output over-current protection LCC48
Integrated temperature sensor

Accelerometer performance index

[1]: According to IEEE 1293-1998 12.3.8, measure for 48 hours at 20 ° C; stabilize for 1 hour before measurement.
[2]: 1-year stability according to IEEE 528-2001 definition: power on / power on, stored at -55 ° C and 85 ° C, T cycle between -40 ° C to 125 ° C, -55 ° C to 85 ° C Electrical interference, vibration and shock.
[3]: Temperature coefficient refers to the range of -40 ~ 20 , in which the typical output value is linear.
[4]: The bandwidth is defined as the frequency range where the sensitivity is reduced by less than 3dB.