Accelerometer Selection Guide

加速计选型指导

Overview 概述

Many technologies have been used to construct accelerometers.  The most common include Piezoelectric (PE), Integral Electronics Piezoelectric (IEPE), Piezoresitive (PR) and Variable Capacitance (VC). Each technology offers numerous benefits and is used in different applications.

工程振动量值的物理参数常用位移、速度和加速度来表示。由于在通常的频率范围内振动位移幅值量很小，且位移、速度和加速度之间都可互相转换，所以在实际使用中振动量的大小一般用加速度的值来度量。常用单位为：米/秒2 (m/s2)，或重力加速度(g)。
　　描述振动信号的另一重要参数是信号的频率。绝大多数的工程振动信号均可分解成一系列特定频率和幅值的正弦信号，因此，对某一振动信号的测量，实际上是对组成该振动信号的正弦频率分量的测量。对传感器主要性能指标的考核也是根据传感器在其规定的频率范围内测量幅值精度的高低来评定。

现有加速度计应用多种技术原理。最常用的振动测量传感器按各自的工作原理可分为压电式、压电放大式，压阻式、电容式。每种技术都有相应的优点用于不同测量领域。

Piezoelectric accelerometers (PE)

PE accelerometers incorporate piezoelectric crystals such as quartz or tourmaline or more often with ferroelectric ceramic materials.  These accelerometers are self-charging devices, meaning that they create an electric signal when a force is applied.  No power supply is required however an external charge amplifier is required to convert the high impedance output to a usable low impedance voltage signal. Due to the high impedance output of PE accelerometers noise treated cables are required.

压电式(PE)- 原理和特点
压电式传感器是利用弹簧质量系统原理。敏感芯体质量受振动加速度作用后产生一个与加速度成正比的力，压电材料受此力作用后沿其表面形成与这一力成正比的电荷信号。压电式加速度传感器具有动态范围大、频率范围宽、坚固耐用、受外界干扰小以及压电材料受力自产生电荷信号不需要任何外界电源等特点，是被最为广泛使用的振动测量传感器。虽然压电式加速度传感器的结构简单，商业化使用历史也很长，但因其性能指标与材料特性、设计和加工工艺密切相关，因此在市场上销售的同类传感器性能的实际参数以及其稳定性和一致性差别非常大。与压阻和电容式相比，其最大的缺点是压电式加速度传感器不能测量零频率的信号。压电式加速度传感器虽然能自己产生电荷信号，但其必需要一个外部的电荷放大器将信号放大成电压信号方便低外阻的测量仪器采集。由于压电加速计是高内阻的，所以必须采用低噪声电缆采集电荷信号（非常昂贵）。

Integral Electronics Piezoelectric (IEPE)

Piezoelectric accelerometers are often packaged with on-board integral electronics that provide an amplified voltage signal output. The use of an internal amplifier eliminates the need to use noise treated cables but does require an external constant current power supply to power the electronics.

压电放大式(IEPE)（ICP输出）

压电传感器经常在传感器内部集成一个电荷放大器将电荷转换成电压输出，这种设计可以避免使用低噪声电缆，但需要用用外部的电流源驱动电路。

Piezoresistive (PR)

Piezoresistive accelerometers incorporate an active Wheatstone bridge to generate an output signal when stressed. The Wheatstone bridge configuration is typically etched on a MEMS (Micro-Electro Mechanical Systems) sensing element. PR accelerometers require an external DC power supply and internal signal conditioning can be incorporated to provide an amplified output. They are ideal for both steady-state as well as dynamic measurements.

压阻式

应变压阻式加速度传感器的敏感芯体为半导体材料制成电阻测量电桥，其结构动态模型仍然是弹簧质量系统。现代微加工制造技术MEMS (Micro-Electro Mechanical Systems)的发展使压阻形式敏感芯体的设计具有很大的灵活性以适合各种不同的测量要求。在灵敏度和量程方面，从低灵敏度高量程的冲击测量，到直流高灵敏度的低频测量都有压阻形式的加速度传感器。同时压阻式加速度传感器测量频率范围也可从直流信号到具有刚度高，测量频率范围到几十千赫兹的高频测量。超小型化的设计也是压阻式传感器的一个亮点。需要指出的是尽管压阻敏感芯体的设计和应用具有很大灵活性，但对某个特定设计的压阻式芯体而言其使用范围一般要小于压电型传感器。压阻式加速度传感器的另一缺点是受温度的影响较大，实用的传感器一般都需要进行温度补偿。在价格方面，大批量使用的压阻式传感器成本价具有很大的市场竞争力，但对特殊使用的敏感芯体制造成本将远高于压电型加速度传感器。压阻加速计需要外部驱动电压，内部可以合成信号放大器输出放大信号。此类加速度计的同时适用于静态和动态测试测量。

Variable Capacitance (VC)

Variable capacitance accelerometers incorporate a seismic mass that moves between two parallel capacitor plates. The change in capacitance in directly proportional to the applied acceleration. VC accelerometers require an IC to be closely coupled to the sensing element in order to convert the very small capacitance changes into a voltage output. This conversion process often adds noise to the signal. VC accelerometers are ideal for both steady-state as well as dynamic measurements however the bandwidth is typically limited.

电容式
　　电容型加速度传感器的结构形式一般也采用弹簧质量系统。当质量受加速度作用运动而改变质量块与固定电极之间的间隙进而使电容值变化。电容式加速度计与其它类型的加速度传感器相比具有灵敏度高、零频响应、环境适应性好等特点，尤其是受温度的影响比较小；但不足之处表现在信号的输入与输出为非线性，量程有限，频响窄，受电缆的电容影响，以及电容传感器本身是高阻抗信号源，因此电容传感器的输出信号往往需通过后继电路（IC）给于改善。需要后续电路将微小的电容变化转换成电压信号，信号转换过程容易引进噪声到信号里面啊。在实际应用中电容式加速度传感器较多地用于低频测量，其通用性不如压电式加速度传感器，且成本也比压电式加速度传感器高得多。

Which Technology to Use?如何选择传感器？

No one technology can effectively meet the application requirements of all customers.  Each technology has its own unique strengths and weaknesses.  Below is a chart (Figure 1) that lists Key Measurement Criteria and accelerometer technologies.  The black circle under each technology indicates a unique strength of that technology.  For example, you will see that PE has a black circle beside High Temperature Capability and Broad Temperature Range.

没有任何一种技术能满足所有用户要求，每种技术都尤其独特优点和缺点，下面是一个产品的性能优缺对照表，打黑点意味着产品有此长处。

Figure 1:  Strengths of Accelerometer Technologies

PE vs IEPE Type Accelerometers重点分析压电(PE)和压电放大类(IEPE)传感器优缺点

The advantages and disadvantages of PE and IEPE type accelerometers are listed in the chart below (Figure 2). Some typical applications are also listed below the chart.

以下是两种传感器的优缺比较。

Figure 2:  Strengths of Accelerometer Technologies

· Measurements in contaminated environments (high humidity, moisture, dirty/dusty, high electromagnetic interference) favor the use of low output impedance IEPE transducers.

· 在脏污环境下（高湿度，潮气，灰尘，高电磁干扰）测量时，建议用地内阻的IEPE加速度传感器。

· Measurements in extreme thermal environments favor the selection of PE transducers. General purpose IEPE sensors are limited to a relatively limited operating range (-55^oC to +125^oC) compared to PE type sensors.

· 在极端环境温度下测量，建议用PE加速度传感器。

· Long cable runs favor the use of IEPE transducers due to noise floor and per-channel costs (PE required low noise cables and in-line remote charge converters increase costs).

· 远距离测量时使用IEPE加速度传感器可以减少噪声和电缆成本。

· Shock measurements where the likely measurement range is unknown favors the use of PE type sensors.

· 在测量冲击（量程）大小不清楚的情况下，建议用PE型加速度传感器。

· Measurements on small and lightweight structures favors the selection of low output impedance IEPE transducers which allow the use of very flexible ribbon wires as the output cable.

· 在轻小型机构测量中，因为运动幅度较大，建议用IEPE加速度传感器 ，电缆柔韧度好。

· If use of TEDS technology sensors is desired, use of IEPE transducers is required

· 如果有要求用TEDS身份识别技术，就必须要IEPE加速度传感器了。