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Accelerometer Selection Guide

加速計選型指導

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.

工程振動量值的物理參數常用位移、速度和加速度來表示。由于在通常的頻率范圍內振動位移幅值量很小，且位移、速度和加速度之間都可互相轉換，所以在實際使用中振動量的大小一般用加速度的值來度量。常用單位為：米/秒2 (m/s2)，或重力加速度(g)。
　　描述振動信號的另一重要參數是信號的頻率。絕大多數的工程振動信號均可分解成一系列特定頻率和幅值的正弦信號，因此，對某一振動信號的測量，實際上是對組成該振動信號的正弦頻率分量的測量。對傳感器主要性能指標的考核也是根據傳感器在其規定的頻率范圍內測量幅值精度的高低來評定。

現有加速度計應用多種技術原理。最常用的振動測量傳感器按各自的工作原理可分為壓電式、壓電放大式，壓阻式、電容式。每種技術都有相應的優點用于不同測量領域。

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.

壓電式(PE)- 原理和特點
壓電式傳感器是利用彈簧質量系統原理。敏感芯體質量受振動加速度作用后產生一個與加速度成正比的力，壓電材料受此力作用后沿其表面形成與這一力成正比的電荷信號。壓電式加速度傳感器具有動態范圍大、頻率范圍寬、堅固耐用、受外界干擾小以及壓電材料受力自產生電荷信號不需要任何外界電源等特點，是被最為廣泛使用的振動測量傳感器。雖然壓電式加速度傳感器的結構簡單，商業化使用歷史也很長，但因其性能指標與材料特性、設計和加工工藝密切相關，因此在市場上銷售的同類傳感器性能的實際參數以及其穩定性和一致性差別非常大。與壓阻和電容式相比，其最大的缺點是壓電式加速度傳感器不能測量零頻率的信號。壓電式加速度傳感器雖然能自己產生電荷信號，但其必需要一個外部的電荷放大器將信號放大成電壓信號方便低外阻的測量儀器采集。由于壓電加速計是高內阻的，所以必須采用低噪聲電纜采集電荷信號（非常昂貴）。

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.

壓電放大式(IEPE)（ICP輸出）

壓電傳感器經常在傳感器內部集成一個電荷放大器將電荷轉換成電壓輸出，這種設計可以避免使用低噪聲電纜，但需要用用外部的電流源驅動電路。

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.

壓阻式

應變壓阻式加速度傳感器的敏感芯體為半導體材料制成電阻測量電橋，其結構動態模型仍然是彈簧質量系統。現代微加工制造技術MEMS (Micro-Electro Mechanical Systems)的發展使壓阻形式敏感芯體的設計具有很大的靈活性以適合各種不同的測量要求。在靈敏度和量程方面，從低靈敏度高量程的沖擊測量，到直流高靈敏度的低頻測量都有壓阻形式的加速度傳感器。同時壓阻式加速度傳感器測量頻率范圍也可從直流信號到具有剛度高，測量頻率范圍到幾十千赫茲的高頻測量。超小型化的設計也是壓阻式傳感器的一個亮點。需要指出的是盡管壓阻敏感芯體的設計和應用具有很大靈活性，但對某個特定設計的壓阻式芯體而言其使用范圍一般要小于壓電型傳感器。壓阻式加速度傳感器的另一缺點是受溫度的影響較大，實用的傳感器一般都需要進行溫度補償。在價格方面，大批量使用的壓阻式傳感器成本價具有很大的市場競爭力，但對特殊使用的敏感芯體制造成本將遠高于壓電型加速度傳感器。壓阻加速計需要外部驅動電壓，內部可以合成信號放大器輸出放大信號。此類加速度計的同時適用于靜態和動態測試測量。

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.

電容式
　　電容型加速度傳感器的結構形式一般也采用彈簧質量系統。當質量受加速度作用運動而改變質量塊與固定電極之間的間隙進而使電容值變化。電容式加速度計與其它類型的加速度傳感器相比具有靈敏度高、零頻響應、環境適應性好等特點，尤其是受溫度的影響比較小；但不足之處表現在信號的輸入與輸出為非線性，量程有限，頻響窄，受電纜的電容影響，以及電容傳感器本身是高阻抗信號源，因此電容傳感器的輸出信號往往需通過后繼電路（IC）給于改善。需要后續電路將微小的電容變化轉換成電壓信號，信號轉換過程容易引進噪聲到信號里面啊。在實際應用中電容式加速度傳感器較多地用于低頻測量，其通用性不如壓電式加速度傳感器，且成本也比壓電式加速度傳感器高得多。

Which Technology to Use?如何選擇傳感器？

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.

沒有任何一種技術能滿足所有用戶要求，每種技術都尤其獨特優點和缺點，下面是一個產品的性能優缺對照表，打黑點意味著產品有此長處。

Figure 1:  Strengths of Accelerometer Technologies

PE vs IEPE Type Accelerometers重點分析壓電(PE)和壓電放大類(IEPE)傳感器優缺點

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.

以下是兩種傳感器的優缺比較。

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.

· 在臟污環境下（高濕度，潮氣，灰塵，高電磁干擾）測量時，建議用地內阻的IEPE加速度傳感器。

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

· 在極端環境溫度下測量，建議用PE加速度傳感器。

· 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).

· 遠距離測量時使用IEPE加速度傳感器可以減少噪聲和電纜成本。

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

· 在測量沖擊（量程）大小不清楚的情況下，建議用PE型加速度傳感器。

· 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.

· 在輕小型機構測量中，因為運動幅度較大，建議用IEPE加速度傳感器 ，電纜柔韌度好。

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

· 如果有要求用TEDS身份識別技術，就必須要IEPE加速度傳感器了。