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南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位 論文 外文翻譯 Capacitive Sensor Operation Part 1 The Basics Part 1 of this two part article reviews the concepts and theory of capacitive sensing to help to optimize capacitive sensor performance Part 2 of this article will discuss how to put these concepts to work Noncontact capacitive sensors measure the changes in an electrical property called capacitance Capacitance describes how two conductive objects with a space between them respond to a voltage difference applied to them A voltage applied to the conductors creates an electric field between them causing positive and negative charges to collect on each object Capacitive sensors use an alternating voltage that causes the charges to continually reverse their positions The movement of the charges creates an alternating electric current that is detected by the sensor The amount of current flow is determined by the capacitance and the capacitance is determined by the surface area and proximity of the conductive objects Larger and closer objects cause greater current than smaller and more distant objects Capacitance is also affected by the type of nonconductive material in the gap between the objects Technically speaking the capacitance is directly proportional to the surface area of the objects and the dielectric constant of the material between them and inversely proportional to the distance between them as shown In typical capacitive sensing applications the probe or sensor is one of the conductive objects and the target object is the other Using capacitive sensors to sense plastics and other insulators will be discussed in the second part of this article The sizes of the sensor and the target are assumed to be constant as is the material between them Therefore any change in capacitance is a result of a change in the distance between the probe and the target The electronics are calibrated to generate specific voltage changes for corresponding changes in capacitance These voltages are scaled to represent specific changes in distance The amount of voltage change for a given amount of distance change is called the sensitivity A common sensitivity setting is 1 0 V 100 m That means that for every 100 m change in distance the output voltage changes exactly 1 0 V With this calibration a 2 V change in the output means that the target has moved 200 m relative to the probe Focusing the Electric Field When a voltage is applied to a conductor the electric field emanates from every surface In 南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位 論文 外文翻譯 a capacitive sensor the sensing voltage is applied to the sensing area of the probe For accurate measurements the electric field from the sensing area needs to be contained within the space between the probe and the target If the electric field is allowed to spread to other items or other areas on the target then a change in the position of the other item will be measured as a change in the position of the target A technique called guarding is used to prevent this from happening To create a guard the back and sides of the sensing area are surrounded by another conductor that is kept at the same voltage as the sensing area itself When the voltage is applied to the sensing area a separate circuit applies the exact same voltage to the guard Because there is no difference in voltage between the sensing area and the guard there is no electric field between them Any other conductors beside or behind the probe form an electric field with the guard instead of with the sensing area Only the unguarded front of the sensing area is allowed to form an electric field with the target Definitions Sensitivity indicates how much the output voltage changes as a result of a change in the gap between the target and the probe A common sensitivity is 1 V 0 1 mm This means that for every 0 1 mm of change in the gap the output voltage will change 1 V When the output voltage is plotted against the gap size the slope of the line is the sensitivity A system s sensitivity is set during calibration When sensitivity deviates from the ideal value this is called sensitivity error gain error or scaling error Since sensitivity is the slope of a line sensitivity error is usually presented as a percentage of slope a comparison of the ideal slope with the actual slope Offset error occurs when a constant value is added to the output voltage of the system Capacitive gauging systems are usually zeroed during setup eliminating any offset deviations from the original calibration However should the offset error change after the system is zeroed error will be introduced into the measurement Temperature change is the primary factor in offset error Sensitivity can vary slightly between any two points of data The accumulated effect of this variation is called linearity erro The linearity specification is the measurement of how far the output varies from a straight line To calculate the linearity error calibration data are compared to the straight line that would best fit the points This straight reference line is calculated from the calibration data using least squares fitting The amount of error at the point on the calibration line furthest away from this ideal line is the linearity error Linearity error is usually expressed in terms 南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位 論文 外文翻譯 of percent of full scale F S If the error at the worst point is 0 001 mm and the full scale range of the calibration is 1 mm the linearity error will be 0 1 Note that linearity error does not account for errors in sensitivity It is only a measure of the straightness of the line rather than the slope of the line A system with gross sensitivity errors can still be very linear Error band accounts for the combination of linearity and sensitivity errors It is the measurement of the worst case absolute error in the calibrated range The error band is calculated by comparing the output voltages at specific gaps to their expected value The worst case error from this comparison is listed as the system s error band In Figure 7 the worst case error occurs for a 0 50 mm gap and the error band in bold is 0 010 Gap mm Expected Value VDC Actual Value VDC Error mm 0 50 10 000 9 800 0 010 0 75 5 000 4 900 0 005 1 00 0 000 0 000 0 000 1 25 5 000 5 000 0 000 1 50 10 000 10 100 0 005 Figure 7 Error values Bandwidth is defined as the frequency at which the output falls to 3 dB a frequency that is also called the cutoff frequency A 3 dB drop in the signal level is an approximately 30 decrease With a 15 kHz bandwidth a change of 1 V at low frequency will only produce a 0 7 V change at 15 kHz Wide bandwidth sensors can sense high frequency motion and provide fast responding outputs to maximize the phase margin when used in servo control feedback systems however lower bandwidth sensors will have reduced output noise which means higher resolution Some sensors provide selectable bandwidth to maximize either resolution or response time Resolution is defined as the smallest reliable measurement that a system can make The resolution of a measurement system must be better than the final accuracy the measurement requires If you need to know a measurement within 0 02 m then the resolution of the measurement system must be better than 0 02 m The primary determining factor of resolution is electrical noise Electrical noise appears in the output voltage causing small instantaneous errors in the output Even when the probe target gap is perfectly constant the output voltage of the driver has some small 南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位 論文 外文翻譯 but measurable amount of noise that would seem to indicate that the gap is changing This noise is inherent in electronic components and can be minimized but never eliminated If a driver has an output noise of 0 002 V with a sensitivity of 10 V 1 mm then it has an output noise of 0 000 2 mm 0 2 m This means that at any instant in time the output could have an error of 0 2 m The amount of noise in the output is directly related to bandwidth Generally speaking noise is distributed over a wide range of frequencies If the higher frequencies are filtered before the output the result is less noise and better resolution Figures 8 9 When examining resolution specifications it is critical to know at what bandwidth the specifications apply Capacitive Sensor Operation Part 2 System Optimization Part 2 of this two part article focuses on how to optimize the performance of your capacitive sensor and to understand how target material shape and size will affect the sensor s response Effects of Target Size The target size is a primary consideration when selecting a probe for a specific application When the sensing electric field is focused by guarding it creates a slightly conical field that is a projection of the sensing area The minimum target diameter is usually 130 of the diameter of the sensing area The further the probe is from the target the larger the minimum target size Range of Measurement The range in which a probe is useful is a function of the size of the sensing area The greater the area the larger the range Because the driver electronics are designed for a certain amount of capacitance at the probe a smaller probe must be considerably closer to the target to achieve the desired amount of capacitance In general the maximum gap at which a probe is useful is approximately 40 of the sensing area diameter Typical calibrations usually keep the gap to a value considerably less than this Although the electronics are adjustable during calibration there is a limit to the range of adjustment Multiple Channel Sensing Frequently a target is measured simultaneously by multiple probes Because the system measures a changing electric field the excitation voltagefor each probe must be synchronized or the probes will interfere with each other If they were not synchronized one probe would be trying to increase the electric field while another was trying to 南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位 論文 外文翻譯 decrease it the result would be a false reading Driver electronics can be configured as masters or slaves the master sets the synchronization for the slaves in multichannel systems Effects of Target Material The sensing electric field is seeking a conductive surface Provided that the target is a conductor capacitive sensors are not affected by the specific target material they will measure all conductors brass steel aluminum or salt water as the same Because the sensing electric field stops at the surface of the conductor target thickness does not affect the measurement 南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位 論文 外文翻譯 中文翻譯 電容式傳感器操作第一部分 基礎(chǔ) 這篇文章的第一部分回顧了電容式傳感器的概念和理論來幫助我們優(yōu)化電容式 傳感器的性能 第二部分討論了怎樣使這些概念去工作 非接觸式電容傳感器測量的電特性變化稱為電容 電容描述了有一定距離的 兩個導(dǎo)電物體怎樣產(chǎn)生一個電壓差 電壓施加到導(dǎo)體上并產(chǎn)生電場 造成正負(fù)電荷 聚集到每個導(dǎo)體上 如果電壓的極性是相反的 那么電荷也是相反的 電容式傳感器使用交流電壓就會引起電子不斷反轉(zhuǎn)他們的位置 傳感器就能檢 測出電子移動所產(chǎn)生的交流電流 電流的流量是由電容決定的 而電容是有導(dǎo)體的 表面積和導(dǎo)體之間的距離決定的 表面積更大 距離更近的導(dǎo)體比小面積遠(yuǎn)距離導(dǎo) 體能夠引起更大的電流 導(dǎo)體之間介質(zhì)的材料也影響電容 從技術(shù)上講 電容是與 導(dǎo)體的表面積和在導(dǎo)體之間介質(zhì)的介電常數(shù)成正比的 與導(dǎo)體之間的距離成反比 公式如下 距 離介 電 常 數(shù)面 積電 容 在典型的電容式傳感應(yīng)用 探針或傳感器是導(dǎo)體中的一個 另一個則是測量對 象 利用電容式傳感器來感應(yīng)塑料和其他絕緣體將在本文的第二部分討論 傳 感器和被測對象的大小假定不變 這是由他們之間的材料確定 因此 電容的任何 改變都是探針和目標(biāo)之間的距離變化產(chǎn)生的 被校準(zhǔn)的電子產(chǎn)生特定的電壓變化電 容也產(chǎn)生相應(yīng)變化 這些電壓變化是與距離變化成比例的 在給定距離上產(chǎn)生的電 壓變化叫做靈敏度 一個常見的靈敏度設(shè)置時1 0 V 100 m 這就意味著每改變 100 m的距離 輸出就會變化1V 有了這個校準(zhǔn) 一個2V的輸出變化就意味著目標(biāo)距 離探測器發(fā)生了200 m的變化 關(guān)于電場 當(dāng)電壓應(yīng)用于導(dǎo)體 電場從每個表面產(chǎn)生 在電容傳感器中 感應(yīng)電壓應(yīng)用到 探頭的感應(yīng)區(qū)為了準(zhǔn)確測量 感應(yīng)區(qū)的電場需包含在探針與目標(biāo)的空間內(nèi) 如果電 場可以傳播到其他項(xiàng)目 或者目標(biāo)的其他地區(qū) 在其他項(xiàng)目上這個位置的改變作為衡 量在目標(biāo)的這個位置上測量的變化 一種名為 守衛(wèi) 的技術(shù)是用來防止這種情況 發(fā)生 要創(chuàng)建一個守衛(wèi) 感應(yīng)區(qū)背部和四周都是被另一個導(dǎo)體包圍 以使這個感應(yīng) 區(qū)本身為同一電壓 當(dāng)電壓施加到感應(yīng)區(qū) 一個單獨(dú)的電路應(yīng)用于完全相同的電壓 給守衛(wèi) 因?yàn)樵诟袘?yīng)區(qū)和守衛(wèi)之間沒有電壓差 所以在他們之間就沒有電場 在探 針周圍或后面的導(dǎo)體能與守衛(wèi)形成電場 而不是和感應(yīng)區(qū) 只有無守衛(wèi)的感應(yīng)區(qū)允 南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位 論文 外文翻譯 許和目標(biāo)形成電場 定義 靈敏度表示在目標(biāo)和探頭之間的差距變化時 輸出電壓的變化 一個常用靈敏 度單位是1 V 0 1 mm 這意味著距離每改變0 1mm 輸出電壓改變1V 以距離為行坐 標(biāo)輸出電壓為縱坐標(biāo)描點(diǎn) 這條線的斜率就是靈敏度 在校準(zhǔn)時 就設(shè)置系統(tǒng)的靈敏度 當(dāng)靈敏度偏離理想值 這是所謂的靈敏度誤 差 增益誤差 縮放錯誤 由于靈敏度是一個直線的斜率 靈敏度錯誤通常是表現(xiàn) 為一個百分比的斜坡 一對理想與實(shí)際斜率的比較 偏移誤差發(fā)生時 常值被添加到系統(tǒng)的輸出電壓 在設(shè)置期間電容測量系統(tǒng)通 常是 零 從原來的校準(zhǔn)中解決了偏移誤差 但是在系統(tǒng)清零后 偏移誤差應(yīng)當(dāng)改 變 誤差將被引入到測量 溫度的變化是偏移誤差的主要因素 靈敏度能夠在數(shù)據(jù)的任何兩點(diǎn)之間變化 這一變化的累積效應(yīng)被稱為線性誤差 線性度規(guī)范是測量輸出結(jié)果偏離直線多遠(yuǎn) 為了計算線性誤差 標(biāo)定數(shù)據(jù)與最適合這些點(diǎn)的直線相比 這參考線是采用最 小二乘擬合數(shù)據(jù)計算出的 校準(zhǔn)線上的誤差點(diǎn)中離基準(zhǔn)線最遠(yuǎn)的點(diǎn)是線性誤差 線 性誤差通常在百分之方面表示滿量程 FS 的 如果在最低點(diǎn)誤差為0 001毫米 全面的校準(zhǔn)范圍為 1毫米 線性誤差為0 1 請注意 線性誤差不算到靈敏度誤差中 這僅僅是該行的直線度測量 而不是 直線的斜率 一個有著嚴(yán)重靈敏度錯誤的系統(tǒng)仍然可以非常好的線性的 誤差帶是線性和靈敏度誤差的組合 這是在校準(zhǔn)測量范圍內(nèi)最壞的情況下測量 的絕對誤差 該誤差帶的計算方法是比較在輸出電壓和他們的預(yù)期值的具體差距 從這個比較最壞情況的錯誤被列為該系統(tǒng)的誤差帶 在圖7中 最壞的情況下誤差為 0 50毫米的差距和誤差帶 粗體 是 0 010 間隔 mm 預(yù)期值 VDC 實(shí)際指標(biāo) VDC 誤差 mm 0 50 10 000 9 800 0 010 0 75 5 000 4 900 0 005 1 00 0 000 0 000 0 000 1 25 5 000 5 000 0 000 1 50 10 000 10 100 0 005 圖7 誤差值 帶寬的定義是 當(dāng)輸出頻率下降至 3分貝的頻率 這也是所謂的截止頻率 一 南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位 論文 外文翻譯 個在信號水平 3分貝下降 是近30 的跌幅 與15 kHz的帶寬 為 1V的低頻率的 變化 只會在15千赫 0 7V的變化 寬的帶寬傳感器可以感知高頻移動 并提供快 速響應(yīng) 在使用反饋的伺服控制系統(tǒng)中以最大限度地輸出相位裕度 但是 低帶寬 的傳感器會減少輸出噪聲 這意味著更高的分辨率 有些傳感器提供可選擇的帶寬 以最大限度地提高或分辨率或響應(yīng)時間 分辨率是定義為一個系統(tǒng)可以做到最小的可靠的測量 一個測量系統(tǒng)的分辨率 必須大于最終精確度的測量要求 如果您需要知道在0 02微米內(nèi)的尺寸 則該測量 系統(tǒng)的分辨率必須比0 02微米好 分辨率的主要決定因素是電氣噪聲 電噪聲出現(xiàn)在輸出電壓引起很小的輸出誤 差 即使當(dāng)探針 目標(biāo)距離是完全不變 驅(qū)動器的輸出電壓具有小但可測量的噪音 似乎就表明 這一距離在改變 這種噪聲是電子元器件固有的 可以最小化 但從 來沒有消除 如果一個驅(qū)動程序有一個為10V 1毫米的靈敏度為0 002 V的輸出噪聲 那么它 的輸出噪聲0 000 2毫米 0 2微米 這意味著 在經(jīng)過一段時間后的任何瞬間 輸 出能有0 2微米的誤差 對噪聲的輸出量對帶寬有直接關(guān)系 一般來說 噪聲的頻率分布廣泛 如果更 高頻率的輸出前過濾 其結(jié)果是減少噪音和高分辨率 圖8 9 在檢查分辨率時 關(guān)鍵是知道規(guī)格適用在什么帶寬 電容式傳感器操作第二部分 系統(tǒng)優(yōu)化 這部分分為這篇文章的第二部分著重就如何優(yōu)化您的電容式傳感器的性能 并 了解靶材料 形狀和大小如何影響傳感器的響應(yīng) 目標(biāo)大小的影響 當(dāng)選擇一個探測器進(jìn)行特定的應(yīng)用時 目標(biāo)的大小是一個主要的考慮因素 當(dāng) 守衛(wèi)關(guān)注感應(yīng)電場時 它創(chuàng)建一個輕微的錐形場這是一個敏感領(lǐng)域的投影 最低目 標(biāo)的直徑通常是感應(yīng)區(qū)直徑130 探頭離目標(biāo)越遠(yuǎn) 最小目標(biāo)的大小越大 測量范圍 該范圍是在其中一個探測器是一種有用的感應(yīng)區(qū)大小的函數(shù) 面積越大 范圍 越大 由于電子產(chǎn)品的驅(qū)動程序在探頭中被設(shè)計成有固定的電容 探頭越小越應(yīng)當(dāng) 靠近目標(biāo) 來獲得設(shè)計的電容量 一般來說 在其中一個有用的探測器中最大的距 離大約是感應(yīng)區(qū)域面積直徑的40 典型的校準(zhǔn)通常保持對一個值大大低于這一標(biāo)準(zhǔn) 的間距 雖然電子產(chǎn)品在校準(zhǔn)時可調(diào)節(jié)的 但是有一個對調(diào)整范圍的限制 多通道遙感 南昌航空大學(xué)科技學(xué)院學(xué)士學(xué)位 論文 外文翻譯 通常情況下 目標(biāo)是同時被多個探頭測量 由于系統(tǒng)測量不斷變化的電場 每 個探頭激勵電壓必須同步或探針會互相干擾 如果他們不同步 一個探頭將努力增 加電場 另一個則試圖減少它 其結(jié)果將是一個錯誤的讀數(shù) 電子驅(qū)動器可以被配 置為主或副 主系統(tǒng)為副系統(tǒng)設(shè)置了多通道同步系統(tǒng) 目標(biāo)材料的影響 該感應(yīng)電場正在尋求一個導(dǎo)電表面 只要目標(biāo)是一個導(dǎo)體 電容傳感器不會受 到目標(biāo)材料影響 他們會衡量所有導(dǎo)線 如黃銅 鋼 鋁 或咸水作為相同 由于 感應(yīng)電場在導(dǎo)體表面停止 目標(biāo)厚度不影響測量 測量非導(dǎo)體 電容式傳感器是最經(jīng)常被用來衡量在導(dǎo)電目標(biāo)位置的變化 但電容式傳感器可 以有效測量存在 密度 厚度以及非導(dǎo)體的位置 非導(dǎo)電材料 如塑料比空氣有不 同的電介質(zhì)常數(shù) 介電常數(shù)決定兩個導(dǎo)體之間不導(dǎo)電材料如何影響電容 當(dāng)一個非 導(dǎo)體插入探頭和一個固定的參考指標(biāo)之間 感應(yīng)場穿過材料到接地目標(biāo) 該非導(dǎo)電 材料的出現(xiàn)改變介電常數(shù) 因此改變電容 電容會鑒于材料的密度或厚度而改變