光开关矩阵的诞生：从设计到美国专利

Tu Xiang Zheng (涂向真)   

我的一项用于全光通信的MOEMS光开关矩阵设计获得美国发明专利，专利名称：Method for making optical switch array，专利号：6,773,942，授权日期：August 10, 2004。

这项设计的过程是漫长的，艰辛的，充满挑战，有时还富有戏剧性。事情的起因，要从我在美国宾州大学电子工程系当访问教授谈起。我承担的研究课题涉及到光学测量，不少时间要在暗室中度过。光学实验室设在宾大Moore大楼的一层，与世界上第一台全数字电子计算机“ENIAC”的展室相对。我进出实验室都要经过一条L型的通道，实际上，通道已经改装成博士生候选人的办公室。到宾州大学读博士的学生要试读一年，准备资格考试，考试合格的学生被正式录取为博士生，录取的学生由博导教授另外安排办公室，未被录取的学生就要离开宾大，另找出路。 

有一次，我在实验室做测量，把室内灯全关了，正用红外相机拍摄光波相干的干涉条纹。突然，实验室的门被打开了，射进一道刺眼的亮光。我一瞥，一位学生走进来，连连向我道歉，说他不小心把书架撞倒了，倒在实验室门上，把门撞开了。

一次生，二次熟，此后这位学生每次见到我都要跟打招呼，还主动和我聊天。他说，他来自南斯拉夫，父亲是工程师，母亲是教师，来美国学习主要靠他的一位在德国工作的叔叔资助。他叫Smiweiqe，学的是光学工程，对光学实验感兴趣，想要我给一个机会，看我做光学实验。

我满足了Smiweiqe的要求，让他看了我做实验的全过程，还让他看我拍的干涉条纹照片。他看后说，这是一组等厚干涉条纹，是由两块干净的镜片紧紧压叠，两镜片间的空气层就形成空气薄膜，用水银灯或纳灯作为光源，就可以观察到薄膜干涉现象。如果镜片表面不很平，所夹空气层厚度不均匀，观察到的将是一些不规则的等厚干涉条纹，通常是一些大大小小的同心环．若用很平的镜片，则会出现一些平行条纹。他还说，条纹很清晰，很光滑，镜片一定很光亮。然后，就在实验台上找镜片，找了半天也没找到，就问我，我说那就是，指着一根单模光纤和一块微机械加工的硅片，他显得很茫然，睁大眼睛盯着我。我就给Smiweiqe解释，这不是一块普通的镜片，而是硅片内部的一个<111>晶面，是通过各向异性腐蚀（110）硅片形成的。Smiweiqe点了点头，似乎有所领悟，其实，他并不完全知道其中的奥妙。

这块<111>晶面，高50微米，宽100微米，只有针尖那么大。这么微小的镜子在日常生活中没有任何用处的，但用它构建高度密集的微型反射镜矩阵，并配合直径8微米的单模光纤矩阵，就成了以光速传送信息的通信网络，把整个地球缩小成客厅，会议室，电影院，体育馆，赛马场，人们之间没有空间距离，相会交流没有任何阻隔。于是，又有一个发明的梦想在我头脑中生成，尽管这只是梦想，与现实还有很大差距，也正是还有差距，才会产生发挥自我的决心，完成理想的欲望和永不放弃的坚持。

我带着这个梦想进了新泽西州的Rutges大学，在那里我读到一篇论文，是新泽西理工学院的两位教授发表的，他们做了一面直立在硅片表面的（111）晶面微型反射镜。新泽西理工学院在纽瓦克，离我所在的Rutges大学的Busch校园很近，我总想找个机会去参观一下。委托我开光纤传感器检漏产品的公司的汤和羌得知我的想法后，专门为我安排了一次学术交流活动。

羌开车送我到新泽西理工学院，经他介绍，与两位教授见了面。这两位教授也研究微光机电系统（MOEMS），主要开发微镜组件。我也作了自我介绍，着重介绍我研发的光纤传感器。交流结束后，两位教授领我们参观他们的实验室。他们的实验室不大，不是做微电子产品的无尘室，只能做一般的化学实验。在我的要求下，他们让我在显微镜下观看了他们制作的（111）晶面微型反射镜。

两位教授的数据进一步证明，各向异性腐蚀硅片产生的（111）晶面具有原子级的光滑度，反射系数也足够大，完全可以用作反射镜，这与我的认知是相同的。但是，要制作成具有开关功能的反射镜矩阵，对他们说来，要解决的问题还很多，路途还很遥远，因为他们还没有考虑反射镜的移动问题。

我开始光开关矩阵的设计，在我面前挡道的是如何让（111）晶面垂直移动。我想了好多个办法，都由于工艺流程过于复杂，设备要求过于苛刻，最后一个个被我放弃。我化了大半年的时间，查遍文献，绞尽脑汁，仍然是无法可想，无计可施，“移动”的问题就像是一座大山，把我的前进之路堵得严严实实，我痛苦不堪，不得不暂时在梦中不断探索。

足足有两年的时间，光开关矩阵就像幽灵，在我脑海中时隐时现，时时刻刻提醒我，不要停留原地，要继续前进。有时还督促我，鞭策我，要我快马加鞭，只争朝夕，努力攀登科技高峰。

有一次，在我查阅文献时，看到一篇文章报道：在硅片的多孔硅表层上，可以进行外延生长，形成硅单晶层。这真的是“踏破铁鞋无处寻，得来全不费功夫”，我终于找到梦寐以求的锐利武器，攻克光开关矩阵的难关几乎不成问题。我对多孔硅研究得比较透，而硅外延生长是初进研究所所做的工作，只是一直没有在多孔硅上生长外延层的经验。

解决了（111）晶面垂直移动的难题后，光开关矩阵的设计就算是大功告成了。由于设计方案新颖，技术路线独特，实施工艺流程成熟，我专利申请顺利通过美国专利局的审查，授于发明专利权。

我设计的MOEMS光开关矩阵属于光路遮挡型，矩阵由N×N的微镜组成，用来连接N条输入光纤和N条输出光纤。微镜和光纤在同一个平面上，微镜有原位和上移两种位置，相应于开和关两种状态。微镜的上移是在静电力的作用下产生的。当某一微镜上移处于开状态时，由其对应的输入光纤射出的光束经其反射而进入其对应的输出光纤，从而实现任意输入光束耦合为输出信号。

MOEMS光开关的应用，可以从小型光交叉连接直到大型光交叉连接。在光通信网络中，光开关矩阵具有光路选择，多条光纤线路的交叉互连，上下光路及对故障光纤线路进行旁路等重要功能，是光通信网络中许多设备的关键光器件。

MOEMS光开关既有机械式光开关的低插损，低串扰，低偏振敏感性和高消光比的优点，又有波导开关的高开关速度，小体积，易于大规模集成等优点。MOEMS光开关与光信号的格式，波长，协议，调制方式，偏振，传输方向等均无关，与未来光网络发展所要求的透明性和可扩展等趋势相符合。

 

Utilizing the patented technology described in US Patent 11,601,111 B2

To fabricate MEMS AlScN film microphones

 

Tu Xiang Zheng 

 2/19/2025

Piezoelectric MEMS microphones offer several advantages over traditional capacitive MEMS microphones:

·        Self-Powered Operation: They do not require a DC bias voltage, simplifying circuit design and reducing power consumption.

·        High Sensitivity with Low Noise: Efficient charge generation enhances the signal-to-noise ratio (SNR), improving audio clarity.

·        Robust and Reliable: They exhibit excellent resistance to environmental factors such as humidity, temperature fluctuations, and mechanical shocks, making them suitable for harsh environments.

·        Broad Frequency Response: Capable of detecting a wide range of frequencies, from audible sounds to ultrasonic signals, enabling diverse applications.

These advantages make piezoelectric MEMS microphones suitable for various applications:

·        Consumer Electronics: Integrated into smartphones, laptops, and smart voice assistants for superior audio capture.

·        Industrial Monitoring: Used in acoustic leak detection, structural health monitoring, and predictive maintenance.

·        Medical Devices: Essential for hearing aids, digital stethoscopes, and ultrasonic diagnostic equipment.

·        IoT & Smart Systems: Enable voice-activated controls, smart home security, and environmental sensing in connected devices.

Low Power Piezoelectric MEMS Microphone Market Size

The global Low Power Piezoelectric MEMS Microphone market was valued at US$ 8285 million in 2023 and is anticipated to reach US$ 17530 million by 2030, witnessing a CAGR of 11.3% during the forecast period 2024-2030.

A MEMS piezoelectric film microphone, as illustrated in the above figure, can be fabricated using the pattern-built-in porous silicon wafer technology described in US Patent 11,601,111 B2. This advanced fabrication method enables the integration of high-performance materials while optimizing structural integrity and acoustic performance.

自由女神像之旅：沉思与感悟

Tu Xiang Zheng (涂向真)

我从费城的宾州大学搬到新泽西州的Rutgers大学，就住进了人们常说的“纽约的后花园”。我住在Piscataway，距纽约不远，说是“一衣带水”或“近在咫尺”，都是真实的写照。不少纽约人，成天挤在人群里，夹在楼缝里，做梦都想有一套别墅式的住房，让生活过得尽可能舒适些。在纽约，这种房子的价格近似天文数字，工薪阶层难以问津，而在一河之隔的新泽西州，却是大多数人都可以接受的范围。于是，迎着朝霞进闹市，披着晚霞归家，车水马龙，浩浩荡荡，形成了纽约的独特景观。

我虽然也在新泽西州这座大花园里，但住的是租来的公寓，没有专供种花草树木的前后院，比别墅差远了。但对我这个海外游子来说，有个栖身之地，也就知足了。纽约人看好新泽西州是后花园，我这个新泽西州的过客却看好纽约门前的大都会。只要有空闲，随时都可以过去看看。不是说“百闻不如一见”吗？我就是想见识一下纽约的“庐山真面目”。

很早以前，我就听说纽约的自由女神像是自由和民主的普世象征，自然要首先拜访它。自由女神像位于纽约赫德森河口的“自由岛”上，是海轮进入纽约的必经之地。这座雕像是法国人民送给美国的礼物，由法国著名雕塑家弗雷德里克·奥古斯特·巴托尔迪在巴黎设计并制作，历时十年，于1884年5月完成，1885年6月装箱运至纽约，1886年10月由当时的美国总统克利夫兰主持揭幕仪式。雕像内部是一座大楼，共22层，乘电梯可达第10层，再攀登螺旋形阶梯，可到达雕像的皇冠处。皇冠设有四面小窗，凭窗俯瞰，纽约的壮丽景色尽收眼底。底层是美国移民博物馆，馆内的每一件展品，都仿佛在向观众讲述美国先民移民的艰辛历程。

我仔细推敲神像基石上铭刻的犹太女诗人Emma Lazarus（爱玛·拉扎鲁斯）诗作《The New Colossus》（新巨人）的诗句：“Give me your tired, your poor, Your huddled masses yearning to breathe free, The wretched refuse of your teeming shore. Send these, the homeless, tempest-tost to me， I lift my lamp beside the golden door！” 这实际上是女神向漂洋过海投奔美国的移民致送的欢迎词。我认为其大意是：“给我！那些困乏贫苦、结伴而来、渴望自由呼吸的人们，还有那些遭人遗弃、饱受苦难的人们。都来吧，那些背井离乡、颠沛流离的人们。我高举自由的灯火，照亮前方的道路！”

追求自由是全世界人民的共同理想和愿望。不过，移民美国只是奔向自由的一种方式，而我国人民百余年来，前赴后继，奋斗不息，为的是争取自身的自由。记得我上中学时就学过一首诗，至今仍然清晰记得：“生命诚可贵，爱情价更高；若为自由故，两者皆可抛。” 这是我国革命作家殷夫译自匈牙利诗人裴多菲的诗作。我查阅了英文版的译文：“Liberty, love! These two I need. For my love I will sacrifice life, for liberty I will sacrifice my love.” 如果直译，我认为应是：“自由与爱情，这两样我都需要。为了爱情，我愿牺牲生命；为了自由，我愿牺牲爱情。” 相比之下，殷夫的译文更具中国文化气息，甚至可以说是重新创作，使得这首诗更加震撼人心。

我还记得，中学时代唱过音乐家冼星海谱曲的《在太行山上》，歌词中有：“红日照遍了东方，自由之神在纵情歌唱”，听起来震撼、激昂，让人热血沸腾，心潮澎湃。这既是艺术的魅力，更是自由之神的力量。其实，自由之神不只是美国有，中国也有。中国的观世音就是自由之神，她大慈大悲，当人们遇到灾难时，只要念她的名号，她便前往救度。抗日战争中，自由之神与抗日军民在一起，高唱着“敌人从哪里进攻，我们就要他在哪里灭亡”的歌声，奋不顾身，英勇杀敌，成为中国人民世代流传的最熟悉旋律之一。

据说，雕塑家巴托尔迪设计自由女神像，是依据他小时候在法国资产阶级革命时期的亲身经历。1851年，路易·波拿巴发动政变，推翻法兰西第二共和国。一日，一群共和党人在街头筑起防御工事，与政变者展开巷战。暮色时分，一位忠于共和政权的年轻姑娘，手持燃烧的火炬，跃过障碍物，高呼口号向敌人冲去，不幸中弹牺牲。从此，这位高擎火炬的勇敢姑娘便成了雕塑家心中追求自由的象征。

像这样为自由献身的巾帼英雄，在中国历史长河中层出不穷。古代最著名的要数花木兰。北朝时期，边关告急，朝廷招兵御敌。木兰女扮男装，代父从军，征战疆场数年，屡建功勋。根据这一故事改编的豫剧《花木兰》深受欢迎，豫剧名家常香玉的表演更是栩栩如生，令人赞叹不已。

有不少美国学者认为，自由女神像的建立是为了纪念林肯发布《解放黑奴宣言》，这一观点颇具道理。女神像脚下破碎的手铐、脚镣和锁链，象征着奴隶挣脱枷锁，获得自由。美国曾长期实行奴隶制，1862年9月，林肯发布了著名的《解放黑奴宣言》，宣布废除奴隶制，使被奴役长达两个世纪的黑人获得自由。

参观自由女神像的那一天，我在返回途中浮想联翩。自由对于人类社会的重要性不言而喻，而在物质世界里，自由同样带来变革。我研究半导体，想到半导体中的价带电子需要被激发到导带，成为“自由”电子，才能形成PN结，实现各种半导体器件功能。正如人类社会的自由促进发展，物理世界的自由同样带来科技进步。

最后，我想起孙中山先生的名言：“一个人的自由，以不侵犯他人的自由为范围，才是真自由。” 以及美国前总统罗斯福提出的“四大自由”：表达自由、信仰自由、免于匮乏的自由、免于恐惧的自由。这些应成为世人追求自由的方向。尽管道路漫长，但我们仍应如屈原所言：“路漫漫其修远兮，吾将上下而求索。”

Develop a Vacuum Packaged Infrared Imaging Sensor Array

Utilizing US Patent 11,118,979 B2

Xiang Zheng Tu

 

 

A major advantage of infrared imaging sensors is their ability to detect living objects. They operate effectively in both day and night conditions and provide all-weather capability.

US Patent 11,118,979 B2 describes a wafer-level vacuum packaged infrared image sensor array with outstanding performance. Each pixel within the array includes a thermopile made from polysilicon silicon, which offers low resistance, minimal thermal noise, high integration, and heightened sensitivity. The vacuum enhancement in the packaged array is achieved through low-temperature oxidation of a porous silicon layer formed in the lid silicon substrate, which is then bonded to the infrared sensor array's silicon substrate. This process is driven by the reduction of surface energy in the porous silicon.

Wafer-Level Vacuum Packaging Process

The sensors are produced on a semiconductor wafer using materials such as silicon, indium antimonide, or mercury cadmium telluride, tailored to the application and required wavelength range.

Micro-Electromechanical Systems (MEMS) Integration**: Integration of MEMS technology with the sensors on the same wafer often includes micro-optical components like lenses and mirrors, as well as mechanical components such as actuators. 

Following fabrication, the wafer is encapsulated to create a vacuum seal. This usually involves bonding a cap wafer or a glass lid to the sensor wafer using methods like anodic bonding, fusion bonding, or adhesives compatible with the operational environment. 

The encapsulation process is followed by dicing the wafer into discrete units, each containing one sensor package.

Advantages of Wafer-Level Vacuum Packaging

This technique produces extremely compact sensor packages, ideal for space-constrained applications such as in mobile devices or compact electronics.

Simultaneous packaging of multiple sensors on a wafer scale significantly reduces manufacturing costs compared to packaging sensors individually.

The vacuum environment inside the package minimizes atmospheric gas interference and reduces thermal noise, thus enhancing the sensor's sensitivity and signal-to-noise ratio. This feature is crucial for thermal infrared sensors.

The sensors are well-protected against environmental factors such as humidity, dust, and chemical contaminants, which can degrade the sensor elements over time.

This manufacturing method is highly scalable, facilitating an increase in production without substantial cost increases.

Wafer-level vacuum packaging represents a significant advancement in infrared sensor technology, especially for applications demanding high reliability and precision, such as in aerospace, military, and advanced automotive systems, including autonomous driving.

Heating Temperature Accuracy Control for Unburned Cigarettes Based on POSIFA’s Thermal Flow Sensors

Xiang Zheng Tu

 

Unburned cigarettes are becoming popular because they have been proved to reduce the health risks significantly. It has been shown that when shredded tobacco sample is heated at 10 ⁰C rate in nitrogen its weight loss curve delineates four regions: region I (30-120⁰C), related to the evaporation of water absorbed in the sample; region II (120-250⁰C), related to the emission of acetaldehyde, carbon dioxide, nicotine, and water; region III (250-370⁰C), related to the emission of acetaldehyde, carbon dioxide, nicotine, and more water; and region IV (370-550⁰C), related to the emission of more carbon dioxide and carbon monoxide.

A nicotine emission rate curve for the shredded tobacco sample is shown in the above figure. As can be seen, nicotine vapor is limited to form in the heating temperature range of 175 to 350⁰C. Since in this temperature range the tobacco is only heated but not burned it is impossible for the tobacco to emit any harmful chemicals such as CO, NO and NO_x. At the heart of any unburned cigarette is a sophisticated electronic controller. With such a controller the temperature of the heater is controlled just in the predetermined range.

Again reference to the above figure an electronic controller is designed based on a POSIFA’s thermal flow sensor. The thermal water flow sensor is made up of two thermopiles and operated in conjunction with a resistive heater element for thermoelectric sensing. The mass flow rate of air passing through the thermal flow sensor is calculated on the basis of the measured temperature difference between the hot and cold junctions of the thermopile, and the thermal conductivity coefficient, electric heat rate and specific heat of air.

For air flow rate measurement the house of the unburned cigarette is selected to be a bypass configuration which has a main line and a bypass line. The thermal flow sensor is installed in the bypass line. The flow ration between the main line and the bypass line is determined in advance. Then the flow rate of the main line can be calculated by measuring the flow rate in the bypass line by the thermal flow sensor.

The output of the thermal flow sensor is sent to a microcontroller for digital processing and converted into a PWM signal used to modulate a heating voltage for heating the heater of the cigarette. The microcontroller also processes the output of a temperature sensor which is used to monitor the heated heater. The microcontroller is operated with a program so that the heater is heated up to 175 to 350°C, while monitoring the temperature to ensure a consistent taste experience for user and to avoid burning. It also has an over-heating protection function, which turns itself off if necessary.

In a traditional unburned cigarette a puff at 120 s usually create a sudden and significant temperature drop due to the cooling effect by incoming air. This temperature drop by puffing became less significant with the thermal flow sensor based microcontroller. This is because the longer puff can be detected by the thermal flow sensor and feedback to the microcontroller for providing higher heating voltage. Since any puff can be detected by the thermal flow sensor the switch function for applying electric power can be replaced by the puff itself. And the heating temperature also can be increased according to the strength of the puff so that the used more enjoy the real taste of the unburned cigarettes.

Smart House Water Consumption Systems

Xiang Zheng Tu

 

Our smart house water consumption systems are based on proven thermal water flow sensors. As shown in the above figure, the smart house water consumption system mainly consists of a variable frequency water pump, a house water filter, two POSIFA’s thermal water flow sensors and a smart phone.

A thermal water flow sensor is made up of two thermopiles, which is used as the sensing temperature difference element and operated in conjunction with a resistive heater element for thermoelectric sensing. The water mass flow passing through the thermal sensor is calculated on the basis of the measured temperature difference between the hot and cold junctions of the thermopile, and the thermal conductivity coefficient, electric heat rate and specific heat of water. Compare with other type of water flow sensors the thermal water flow sensors have the advantages as:

·       Thermal water flow sensors have no moving part and no any mechanical failures to take place.

·       Thermal water flow sensors are MEMS devices with small size, higher sensitivity, higher reliability, low power consumption, ease of fabrication, and low cost.

·       Thermal water flow sensors calculate mass flow rather than volumetric flow and do not require temperature or pressure correction, which means there is no additional expense for the purchase and installation of additional equipment.

·       Thermal water flow sensors provide excellent accuracy and repeatability over a wide range of flow rates using bypass flow tube design. The sensor is placed in a bypass around a restriction in the main pipe and is sized to operate in the laminar flow region over its full operating range. 

·       Thermal conductivity water flow sensors are not influenced by the air bubbles entrained in the water. The effect of the bubbles can be added to the series conductivity by using conductivity of the air-water mixture for the water conductivity. The thermal conductivity of continuous water phase with entrapped air bubbles can be calculated.

The variable frequency water pump is equipped with a viable frequency drive. The viable frequency drove is used to adjust the speed of an electric motor by modulating the power being delivered. It provides continuous control, matching motor speed to the specific demands of the water flow. This makes the pump more efficient and also saves the user money by reducing excess energy from being wasted. When a user implements the variable frequency pump benefits are experienced over the life cycle of the pump. On an average 85 percent of a pump’s life cycle cost is attributed to its energy consumption and only 15 percent the actual cost of the pump motor. Motors associated with pumps tend to be sized where the pump may to meet peak loads, but not necessarily for normal continuous operation. Typically, for every 1 percent reduction in the variable frequency drive output the user can save 2.7 percent of energy costs. As energy consumptions continue to rise, it will become more imperative to find ways to cut energy consumption. Variable frequency water pump application is a key aspect to this effort.

The variable frequency drive is controlled to maintain a constant water flow in the output pipe. The down thermal water flow sensor measures the flow rate in the pipe to the water service and as this change sends a signal to the smart phone, which in turn sends a speed demand signal to the drive and this in turn adjust the speed of the motor so that the water flow rate reaches the presetting value. 

The water flow date measured by the down thermal flow sensor can be reviewed and analysis by the smart phone. The screen of the smart phone displays variety of water flow readings including average, minimum, maximum water flow rate, and total water and energy consumption in a month or year. With these data users may understand exactly when, where and how much water they’re consuming in their home on a daily basis.

The water filter takes away impurities from water, like chlorine taste, odor, zinc, copper, cadmium, and mercury. There are several water filters for soft water filtration like activated carbon filters, reverse osmosis, alkaline water ionizers, UV filters, and infrared filters. These filters are inexpensive but they require frequent replacements. Replacing water filter depends on several factors. Water filters typically have an estimated life cycle. However, this is only a guideline based on average water use. This isn't always a good indicator since water use varies per user. Referencing to the figure, the up thermal flow sensor and down thermal flow sensor respectively monitor the input and output water flow rate of the water filter. When the drop in flow rate passes a predetermined value then it’s time to change the filter.

Constant Water Flow System with Thermal Water Flow Sensor and

Variable Frequency Pump

Xiang Zheng Tu

 

It has been reported that electrical energy consumed by pumps, fans and compressors represents a significant proportion of the electricity used around the world. It is estimated that in industrial processes and building utilities, 72 % of electricity is consumed by motors, of which 63 % is used to drive fluid flow in pumps, fans and compressors.

Many heating, cooling and ventilation distribution systems operate at a constant flow rate, even though peak demand may only be required for a few hours. The conventional response to meeting the changing demand for heating and cooling within a building is to restrict flow to individual rooms, while maintaining peak flow in the central system. However, through the use of this approach, considerable energy is used and equipment lifetime is shortened.

For saving energy a better approach is to use a variable speed drive on pumps and fans to vary air or water flow to meet more precisely changing load demands. As shown in the above figure, a constant water flow system comprises a variable frequency pump and a POSIFA’s thermal flow sensor. When the water flow decreases in the user pipe the thermal flow sensor measurement signal decreases. This signal feedback to the variable frequency inverter and results its output frequency decreased. With decreased frequency the operation speed of the variable frequency pump is also decreased and the water flow in the user pipe starts to drop back as to original balance. When the water flow increases the similar process will happen only the direction is opposite.

There is an everyday analogy that can help explain the efficiency advantage of a variable frequency pump.  Imagine you are driving a car. If you are driving on a highway and entering a population area, speed must be reduced so that you do not risk your own and other lives. The best possible way to do that is to reduce motor-rotation speed by taking your foot off the gas pedal and, if necessary, changing to a lower gear. Another possibility would be to use the same gear, keeping your foot on the gas, and at the same time reducing speed simply by braking. This would not only cause wear in the engine and brakes, but also use a lot of fuel and reduce your overall control of the vehicle, which is the case for a "control valve."

In most traditional cases, the variable frequency pumps are controlled to maintain a constant pressure within air ducts or water pipes in which the pressures are measured by pressure differential flow sensors. The differential pressure flow sensor is based on Bernoulli’s Equation, where the pressure drop is a squared function of the fluid velocity.

This relationship can limit the ability of differential pressure sensors to measure large flow ranges. Generally the flow measurement range of 10-100 flow units (10:1 flow turndown) would require a differential pressure flow sensor range of 1-100 differential pressure units (100:1 differential pressure turndown). Therefore, the actual 10:1 flow turndown requires a 100:1 differential pressure flow transmitter turndown.

It would be much better to use the thermal flow sensors replacing the pressure differential flow sensors for controlling the variable frequency pumps. The thermal flow sensor measurement is based on heat transfer from a heated element. The measurement is in mass flow, and additional pressure and temperature correction is not required which is not like the pressure differential flow sensors. They also provide excellent accuracy and repeatability and are easy to install.