CN1268219A - 容性液面传感器 - Google Patents

容性液面传感器 Download PDF

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CN1268219A
CN1268219A CN98808605A CN98808605A CN1268219A CN 1268219 A CN1268219 A CN 1268219A CN 98808605 A CN98808605 A CN 98808605A CN 98808605 A CN98808605 A CN 98808605A CN 1268219 A CN1268219 A CN 1268219A
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level sensor
output
capacitance level
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couples
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劳尔·布伦纳
亚历克斯·J·埃辛
劳伦斯·J·约内斯
鲍里斯·罗塞尔森
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Kay Rey Sen Saarland Ltd By Share Ltd
Kay Ray Sensall Inc
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
    • G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
    • G01F23/26—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields
    • G01F23/263—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields by measuring variations in capacitance of capacitors
    • G01F23/268—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields by measuring variations in capacitance of capacitors mounting arrangements of probes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
    • G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
    • G01F23/26—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields
    • G01F23/263—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields by measuring variations in capacitance of capacitors
    • G01F23/265—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields by measuring variations in capacitance of capacitors for discrete levels
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
    • G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
    • G01F23/26—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields
    • G01F23/263—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields by measuring variations in capacitance of capacitors
    • G01F23/266—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields by measuring variations in capacitance of capacitors measuring circuits therefor

Abstract

一种用于检测容器(100)内液体(102)表面的容性液面传感器,包括:与具有屏蔽层(32)和内部探测杆(34)的容器(100)耦接的传感元件(14)。振荡器(44)对屏蔽层(32)和探测杆(34)提供RF输出。与内部探测杆(34)耦接的比较电路(48),响应内部探测杆(34)和容器(100)之间的电容提供潮湿/干燥输出,所述电容与容器(100)内液体(102)的表面有关。

Description

容性液面传感器
本发明涉及过程控制行业中所用液面传感器。具体地说，本发明涉及一种容性液面传感器，对同样的两个过程控制环路线路提供输出(也可以是延迟输出)，并从它接收能量。
过程控制环路被广泛用于工业过程，用以控制过程。一种可与这种环路耦接的传感器是液面传感器，特别是以电容为基础的液面传感器。这种传感器通过测试探头(或探测杆)与接地电极之间的电容值检测潮湿或干燥的情况。当测试探头为液体所覆盖时，很容易对接地电极产生一个RF信号。测得这一信号，并给出一个指示潮湿情况的输出。
然而公知的是，在传感器的整个寿命期间，探测杆上形成包层，特别是导电包层，导致读数不准确。一种公知的溶液用于提供一层屏蔽，特别是围绕着探测杆，给探测杆产生相同的电位。这样就在屏蔽层与探测杆之间产生一个电中性区，从而减少了传感器上导电包层的影响。在1982.9.7授予Geiger的美国专利US 4,347,741，1985.5.7授予Kuhlman的美国专利US 4,515,015，以及1991.9.17授予Marsh等人的美国专利US 5,048,335给出多种容性液面传感器。
一种用于检测容器内液体或固体表面的电容表面传感器，包括与具有屏蔽层和内部探测杆的容器耦接的传感元件。一个振荡器对所述屏蔽层和内部探测杆提供RF输出。与所述内部探测杆耦接的比较电路响应所述内部探测杆和容器之间与液体有关的电容提供潮湿/干燥输出。按照本发明的一个方面，所述振荡器包括一个控制产生RF输出的启动输入端和一个对振荡器启动输入端周期性地提供启动信号的脉冲源。按照本发明的另一方面，第一变压器具有与振荡器的RF输出端连接的初级线圈，和连接在传感器的外管与屏蔽层之间的次级线圈。第二变压器包括连接在所述屏蔽层与内部探测杆之间的初级线圈，和提供与比较电路耦接的输出端的次级线圈。本发明的再一方面包括可调节校准信号发生电路和补偿发生电路。本发明的又一方面是利用膨胀垫圈保持容性液面传感器中长型检测探头的密封整体性。
图1是本发明与两个过程控制环路耦接的容性液面传感器的简化方框示意图；
图2是表示按照本发明与一液面传感器耦接的检测电路的简化示意图；
图3是表示因传感器上形成导电物质所致电阻的简化示意图；
图4是图1容性液面传感器的侧视图；
图5是沿图4中5-5线所取的图4容性液面传感器探头的断面图。
本发明提供一种适用于低功率环境，如两个过程控制环路的容性液面传感器。现有技术存在的一个问题是如果传感器和相关联的屏蔽层上形成的导电物质过多，将需要较为大量的能量去引起所述屏蔽层，以维持其间的电中性区域层。结果，现有技术中的电路不能引起屏蔽层，而且为了维修，就要使整个传感器断电或者需要停止运行。
按照本发明，利用脉冲RF信号驱动所述传感器，其中脉冲的占空因数小于100％。另外，由于该脉冲信号受到选通，并且只是周期性地加给传感器，从而进一步降低了功率。附带地，本发明采用外管或“卷边管”围绕被引起的屏蔽层，并且通过耦接技术引起，该耦接技术会降低建立导电部分的灵敏度。
图1是本发明容性液面传感器10的简化方框图。液面传感器10与两个4-20mA的过程控制环路12耦接，整个电路的电流为I。电流I反映被检测液面的特征，为液面传感器10提供工作能量。液面传感器10包括与检测电路16耦接的传感元件14，检测电路16与转换电路18相联系。转换电路18与输入/输出电路(I/O)24耦接，以便与环路12相联系。I/O电路24控制流过环路12的电流I的值，并响应这个电流产生能量输出(PWR)，以推动液面传感器10。能量输出(PWR)可以全部推动液面传感器10。图1的例子是一种具体实施例。例如，本发明工作时不需要微处理器。按照一种具体的实施例，流过环路12的电流值4mA指示传感元件14处为干燥的情况，而20mA的电流值指示为潮湿的情况。要说明的是，其它联系方案和方法以及输出形式均可被本发明所采用。
图2是检测电路16与传感元件14耦接的简化示意图。传感元件14包括卷边管或外管30、所引起的屏蔽层32和探测杆34。检测电路16通过变压器40和42与传感元件14耦合。通常，检测电路16还包括RF振荡器44、放大器46、比较器48和断路电阻网络50。
振荡器44具有启动单元52，它与比如电源输入端54连接，通过电阻58耦接到电源电压(+V)56。电源输入端54还与连到电路接地端62的电容器60耦接。振荡器44设有平衡输出端68，输出最好是基频为200kHz的RF信号。输出端68连到变压器40的初级侧，并与电位器75并联连接。变压器40的初级线圈还与断路电阻网络50连接，该网络包含串联连接的电阻72，74，76和78，以及在四个位置的抽头，这些抽头连到四位开关82。开关82通过电容器82与放大器46耦接，并通过电容器84耦接到控制输出的电位器75。变压器40的次级线圈一端通过外管30与物理接地端90耦接。变压器40次级线圈的另一端通过阻尼电阻92与所引起的屏蔽层32连接，并接到变压器42的初级线圈一端。变压器42初级线圈的另一端连到探测杆34。变压器42的次级线圈与放大器46的微分低阻抗输入端连接，放大器46提供一个输出，给比较器48的同相输入端。比较器48的反相输入端与电压参考值VTH耦接，并对I/O电路24给出比较器的输出。
在工作期间，振荡器44在20ms占空因数下启动0.4ms，使变压器40的初级线圈每20ms受到频率为200kHz的0.4ms信号的驱动。可由转换电路18或者通过任何设有脉冲发生电路的适宜手段产生所述启动信号。另外，可将所述启动信号的频率及占空因数设置为适宜的值。按照一种优选的实施例，所述占空因数小于100％。在另一种优选实施例中，所述占空因数小于50％。本实施例中，此后设置的占空因数只有大约2％。
变压器40的次级线圈接到接地端90，并通过阻尼电阻92接到所引起的屏蔽层32，还通过变压器42的初级线圈接到检测杆34。变压器42的次级线圈连到放大器46，放大器46对比较器(或数字转换器)48提供输出。来自放大器46的输出信号正比于流过变压器42初级线圈的电流。流过变压器42初级线圈的电流与探测杆34与接地端90之间的电容值有关(或者如果测得导电液体，则与电导值有关)。当变压器42的初级线圈具有变化的低阻抗时(因其以低阻抗放大器为负载)，所引起的屏蔽层32和探测杆34是实际上等电位的。
假设将传感元件14装入容器100内，还假设液体102的表面上升，将杆34盖住，从而增大了杆34与接地端90之间的电容，从变压器42的次级线圈到放大器46的输出为高电平。于是，放大器46的输出大于由阈值电压VTH所设定的阈值，同时使比较器48的输出端处于高电平，指示“潮湿”情况。
以下转到一个在所述装置上形成导电包层的例子，图3给出简化了的示意图110。图110中的电阻112表示在所引起的屏蔽层32与接地端90之间形成的导电包层的电阻。电阻114表示探测杆34与所引起的屏蔽层32之间形成的导电包层的电阻。另外，电阻116表示探测杆34与接地端90(即容器100的壁)之间处于潮湿情况下的液体102的电阻。传感元件14包层的导电性越好，所引起的屏蔽层32与接地端90之间电阻112的阻值越低。
对于高导电性的包层，这最终可能会引起振荡器44的负载过重。不过，振荡器44是一个受到选通的振荡器，而且是按脉冲方式工作的，其中把能量储存在电容器60中(图2)，并将这种能量提供给振荡器44。这就大大提高了该装置在产生包层时不致增加为使本装置工作所需能量的能力。如前所述，当液面传感器10只通过环路12接收能量而工作时，所述能量要特别受到限制。但如果所述导电包层保持增加(即电阻112的阻值远小于电阻92的阻值)，则因为电阻92和112之间电压重新分配，会使送到所引起的屏蔽层32的信号开始下降。与此同时，流过变压器42初级线圈的电流(IWET)也将开始减小。然而，探测杆34与接地端90之间处于潮湿情况下的液体102的电阻(电阻116)会总是小于所述导电包层(电阻112)的阻值。由于这样，所述电流IWET将永不会降至有如由VTH确定的“干燥”电流以下。另外，由于电阻114(因产生包层所致杆34与屏蔽层32之间的电阻)与阻抗很低的变压器42初级线圈并联，就可以忽略电阻114，从而使A点和B点实际上同电位。
本发明的另一方面是电路16的校准。利用放大器46输入端的两个反相信号的代数相加可实现这种校准。按解释的方式，由变压器42给出第一信号，表示探测杆34与容器100(即接地端90)之间的电容。第二信号经电位器75直接由振荡器44产生。在校准过程中，调节电位器70，以消除因所述杆34与容器100所致的电容，从而比较器18给出一个适当的“干燥”输出。另外，设置开关80，使得能够从输出信号扣除按所检测物质选择的固定电容值的等效值。这种补偿电容表示当电平从干燥的情况变到潮湿的情况时探测杆34与容器100之间电容的近似增加。
按照本发明的另一方面，可将探测杆34的裸露部分做得实际上短于现有技术中的设计。这是可能的，因为电子线路的灵敏度高，而对于所产生的包层灵敏度降低。
图4是容性液面传感器10的侧视图，表示本发明的另一种情况。传感器10与容器外壳150耦接，包括电子设备的外壳152和在接头156处耦接的纵长的传感元件或探头14。探头14在接头160处与外壳150耦接。探头14的下部162伸入到检测液面的容器中。本发明的一种情况包括一个比现有技术的设计短的下部162。
图5是沿图4的5-5线所取探头14的断面图。如图5所示，探测杆34为绝缘管170所包围，所述绝缘管170最好由特氟隆(聚四氟乙烯)或任何其它绝缘材料制成，它在杆34与所引起的屏蔽层32之间通过密封和电绝缘。类似地，最好由特氟隆制成的绝缘管172将所引起的屏蔽层32与外管30分开。回到图4，使屏蔽层32与绝缘管170在折曲部分180处密封。类似地，外管30与绝缘管172在折曲部分166处密封。再使膨胀垫圈164被卷曲在所述折曲部分166上。虽然密封延伸一个较短的长度，所述密封垫圈164仍在一个较宽的压力和温度范围内保持密封的整体性。具体地说，即使每个管的极小变形也能明显地降低密封的效果。不过，膨胀垫圈164对整个组合件加压，以防止各管，特别是外管30作为部件的在组合件收缩或膨胀的压力或温度情况下隆起。这对于减小因特氟隆温度系数(约为7.6×10-5in/in/f)与不锈钢温度系数(约为9.6×10-6in/in/f)之间的不同所致的变形是特别有效的.按照本发明的这一方面，膨胀垫圈164可为任何适于保持密封整体性，以便能减小下部162长度的结构。
本发明相对于现有技术给出许多优点，包括降低对液面传感器上导电包层的产生的敏感性，而保持低能工作的稳定性。另外，对传感器电路提供新的校准和补偿方法。振荡器能提供具有能量水平提高了的信号，因为振荡器是被选通的，并且采用提供相加能量的较大储存电容器，在工作期间只是周期性地提供信号。
虽然已参照优选实施例描述了本发明，但那些熟悉本领域的人员将能认识到，可从形式和细节上进行多种变化，不致偏离本发明的精髓和范围。例如，按照本发明的一种实施例，输出信号只是一种“高”或“低”的输出。所述输出不限于过程控制行业中标准类型的输出。

Claims (16)

1.一种用于检测容器内液体表面的容性液面传感器，包括：
与具有屏蔽层和内部探测杆的容器耦接的传感元件；
一个振荡器具有启动输入端并对所述屏蔽层和探测杆相应地提供RF输出；
一个对振荡器启动输入端周期性地提供启动信号的脉冲源；
与所述内部探测杆耦接的比较电路，响应所述内部探测杆和容器之间与液体有关的电容提供潮湿/干燥输出。
2.如权利要求1所述的容性液面传感器，其中RF输出端通过变压器与所述探测杆耦接。
3.如权利要求1所述的容性液面传感器，其中，还包括与所述比较电路连接补偿信号，对所述探测杆的电容信号提供补偿，根据被检测的液体选择补偿，以控制干燥/潮湿的输出。
4.如权利要求1所述的容性液面传感器，其中，还包括包围所述屏蔽层并与所述RF输出端耦接的外管。
5.如权利要求4所述的容性液面传感器，其中，还包括第一变压器，它具有与所述RF输出端耦接的初级线圈和连接在所述外管与屏蔽层之间的次级线圈，还包括第二变压器，它具有连接在所述屏蔽层与内部探测杆之间的初级线圈和与所述比较电路耦接的次级线圈。
6.如权利要求1所述的容性液面传感器，其中，还包括与所述比较电路耦接的输入/输出电路用以使容性液面传感器耦接到过程控制环路，并相应地对所述控制环路提供与干燥/潮湿有关的输出。
7.如权利要求1所述的容性液面传感器，其中，还包括所述屏蔽层与内部探测杆之间的密封，还包括使所述屏蔽层压抵所述内部探测杆的膨胀垫圈，以保持密封的整体性。
8.一种用于检测容器内液体表面容性液面传感器，包括：
与具有围绕一屏蔽层之外管的容器耦接的传感元件，所述屏蔽层围绕一内部探测杆；
一个给出RF输出的振荡器；
第一变压器，它具有与RF输出端耦接的初级线圈和连接于所述外管和屏蔽层之间的次级线圈；
第二变压器，它具有连接在所述屏蔽层与内部探测杆之间的初级线圈和给出次级线圈输出的次级线圈；
比较电路，它使第二变压器的次级线圈输出与一阈值电压比较，并相应地给出潮湿/干燥输出，作为所述内部探测杆与容器之间与液体有关之电容的函数。
9.如权利要求8所述的容性液面传感器，其中所述振荡器包括启动输入端，所述容性液面传感器还包括一个对振荡器启动输入端周期性地提供启动信号的脉冲源，以便控制所述RF输出的发生。
10.如权利要求8所述的容性液面传感器，其中还包括一个与所比较电路耦接的可调节校准信号，用以校准所述所述容性液面传感器。
11.如权利要求8所述的容性液面传感器，其中还包括与所述比较电路耦接的补偿电路，用以响应第二变压器次级线圈的输出控制潮湿/干燥输出的发生。
12.如权利要求8所述的容性液面传感器，其中还包括与两个过程控制环路耦接的输出电路。
13.如权利要求1或12所述的容性液面传感器，其中容性液面传感器完全由所述过程控制环路产生的能量驱动。
14.如权利要求1或8所述的容性液面传感器，其中所述比较电路使为相应给出放大的输出而与第二变压器的次级线圈输出耦接的放大器与比较器比较，所述比较器还与一阈值耦接，并根据所述阈值与被放大的输出信号的比较相应给出潮湿/干燥输出。
15.如权利要求8所述的容性液面传感器，其中还包括密封所述外管、屏蔽层和内部探测杆的密封，还包括使所述外管压抵屏蔽层和使所述屏蔽层压抵所述内部探测杆的膨胀垫圈，以保持密封的整体性。
16.如权利要求8所述的容性液面传感器，其中所述第一和第二变压器保持所述内部探测杆与所述屏蔽层关于导电包层的产生实际上同电位，所述导电包层的产生造成所产生的屏蔽层与电接地端之间的低电阻路径。
CN98808605A 1997-08-28 1998-08-26 容性液面传感器 Pending CN1268219A (zh)

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