Overview
Accelerometer is an important sensitive element of inertial navigation and inertial sensors. The inertial navigation system (INS) determines the position and velocity of the vehicle and generates tracking signals by measuring the specific force.
Requirements for accelerometer
As one of the core sensor of the inertial navigation system, the accelerometer should meet the following requirements.
- Minimum acceleration value. Inertial navigation systems require the acceleration value to be as small as possible, which is usually below \({10}^{-5}\) \(g\).
- Low friction interference. The friction interference torque on the rotating axis directly affects the sensitivity limit of the accelerometer. Because only when the inertia torque \({M}_a\) is bigger than the friction interference torque, the rotating axis can be deflected to produce the angle \(\theta\). In general, the friction interference torque on the axis of the accelerometer used in the inertial navigation system is generally required to be less than \({9.8}\)X\({10}^{-9}\) N•m. This requirement is very demanding, and special support technology is necessary to meet the requirement, which is the key technology of the accelerometer.
- Large range. The range of refers to the maximum and minimum range values. Different vehicles have different range requirements for accelerometers. For example, airborne inertial navigation systems require their accelerometers to measure from \({10}^{-5}\) ~ \(6g\), with a maximum of \(12g\) or even \(20g\). In order to measure such a wide range of acceleration, and requires the small rotation angle, and to ensure that the input and output are linear, the stiffness of the spring must be very large by using “electric spring” instead of mechanical spring.
Different Types
According to the composition of the measurement system, it is divided into open-loop and closed-loop types.
According to the support type, it is divided into the types of ball bearing, gem bearing, liquid suspension, air suspension, magnetic levitation, flexible and electrostatic, etc.
According to the working principle, it is divided into pendulum and non-pendulum types.
Liquid suspension pendulum accelerometer
Because the bearing has a large friction torque, supporting the pendulum or mass calibration of the accelerometer limits the sensitivity. Only when the input acceleration is bigger than a certain amount, the inertia torque acting on the pendulum can overcome the friction torque of the bearing and make the pendulum start to rotate. In order to improve the precision of pendulum accelerometers, various support technologies have been developed, such as static gas suspension, electrostatic suspension, and liquid suspension and flexible support technologies.
Working principle of the liquid suspension pendulum accelerometer
In order to reduce the frictional torque on the supporting axis of the pendulum and obtain the required damping, the pendulum is suspended in liquid. The center of gravity \(C_M\) and the supspension center \(C_B\) are located on both sides of the support axis of the pendulum, that is, the output axis OA. The connection of \(C_M\),\(C_B\) is perpendicular to the support axis of the pendulum, which is called the pendulum axis PA. The axis perpendicular to the pendulum axis PA and the output axis OA is called the input axis IA. The three axes IA,OA and PA intersect at one point O to form a pendulum coordinate system.
Composition of liquid suspension pendulum accelerometer
In order to ensure sufficient buoyancy and stable pendulum in the liquid to prevent the drift of the center of gravity and the suspension center, the pendulum structure is generally designed as a cylinder or cuboid with unbalanced mass, and a special center fine-tuning mechanism inside. It should also ensure sufficient structural stability and tightness to prevent liquid from leaking into the pendulum or leaking gas from the pendulum to the liquid. Therefore, the pendulum is usually made of aluminum-magnesium alloy or beryllium, and good bonding and sealing technology is used in the assembly process. Before it is filled with oil, the pendulum must be carefully inspected for leakage.
The support and centering of the pendulum relative to the shell can be made of magnetic suspension support or gem bearing. The magnetic suspension support centering accuracy is high, and can completely eliminate any contact between the pendulum and the shell. However, it increases the complexity of the structure. At present, gem bearings are the most widely used.
Flexible accelerometer
Principle
The flexible accelerometer is also a pendulum type. Its main difference from the liquid suspension type is that its pendulum is not suspended in the liquid, but elastically attached to some type of flexible support. The flexible support eliminates the friction torque of the bearing. When the deflection angle of the pendulum is small, the tiny elastic torque is often negligible.
There are different structure types of flexible accelerometers. One of which is shown in the figure below. One end of the pendulum is fixed to the housing by flexible support, and the other end can be rotated relative to the output axis. The moving coil of the signal and the coil of the torque are fixed on the pendulum, and the stator of the signal and the magnetic steel of the torque are firmly connected with the housing.
Composition of the flexible accelerometer
Flexible support is essentially an elastic support made of elastic material. It has a small stiffness on the sensitive axis, and a large stiffness in others.
The pendulum is composed of bracket, moving coil of the signal and the coil of the torque. It is elastically connected to the housing by a flexible support. In order to improve the amplification factor and resolution of the signal, its moving coil is usually glued to the top of the pendulum. A pair of force coils are also fixed at the top or middle of the pendulum to obtain a larger torque coefficient.
The torquers and signalers used in flexible accelerometer are the same as those used in liquid suspension types. In order to make the scale factor not affected by ambient temperature changes, it must accurately control temperature to minimize the impact of temperature changes on the density, viscosity, the position of the center of gravity of the pendulum and the magnetic field of the torquer.
Micromachined Silicon Accelerometer
There are many types of micromachined silicon accelerometers. According to the sensitive signal classification, it can be divided into types of micromachined silicon capacitive and micromachined silicon resonant.
Micromachined Silicon Capacitive Accelerometer
Micromachined silicon capacitive accelerometer is a kind of commonly used sensor. According to the principle of capacitance effect, the change of the distance between the mass block and the fixed electrode is used to detect the change of acceleration, which has the advantages of high resolution, large dynamic range and good temperature characteristics.
Comb drive micromachined silicon capacitive accelerometer, as the name suggests, its movable electrode is comb shaped, also known as a finger capacitive accelerometer. The comb structure is the most mature structure in MEMS process and is relatively simple to realize.
Micromachined Silicon Resonant Accelerometer
Micromachined silicon resonant accelerometer is a kind of forcing-sensing micromechanical accelerometer. Its basic working principle can be explained by the following figure. The acceleration of mass m along the sensing direction is transformed into inertia force P applied to the axial direction of the vibrating beam, resulting in the change of the vibration frequency of the vibrating beam. The change can be detected by means of comb(finger) capacitance, so as to indirectly measure the acceleration.
From the performance and application, it is clear that the development trend is inevitably high precision, miniaturization, integration and digitalization. MEMS accelerometers, due to their potential advantages in cost, size and quality, will be developed rapidly. Especially in the application fields of low and medium performance, they will replace the traditional types. Some high-performance fields will be gradually replaced by MEMS types in the future.
