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Curiosity always drives us to explore the essence of things. Want to know what breakthroughs and innovations exist in smart ultrasonic water meters? What key role does TDC-GP22 play in this, and how does the ARM CortexM3 MCU further assist in ultra-low power breakthroughs? This article will introduce the key role of TDC-GP22 in smart ultrasonic water meters and how the ARM CortexM3 MCU further assists in ultra-low power breakthroughs.

“As smart electric meters develop vigorously in power systems, water meters are also rapidly evolving towards intelligence and full electronics. This is partly due to the global water resource shortage forcing governments to focus on water conservation and quantity control, and partly due to the maturity of modern industrial technology making the realization of smart water meters possible.” Mr. Chen Gang, Technical Director of South China Region at Sygent, emphasized, “Smart water meters that can meet tiered pricing will become a major tool for building a water-saving society!”

The innovation and development of water meter technology is a vital tool for achieving intelligent water measurement. Ultrasonic water meters are fully electronic water meters manufactured using the principle of ultrasonic time difference and industrial-grade electronic components. By utilizing a pair of ultrasonic transducers to alternately (or simultaneously) transmit and receive ultrasonic waves, the time difference of the ultrasonic waves’ propagation in the medium is detected to indirectly measure the fluid’s flow rate, and then calculate the flow rate based on the flow rate—this indirect measurement method—ultrasonic measurement, makes smart water meters a reality.

Figure 1. Schematic diagram of the ultrasonic time difference measurement principle

Compared with traditional mechanical water meters, high accuracy, good reliability, wide range ratio, long lifespan, no moving parts, arbitrary angle installation, and very flexible replacement are all significant advantages of ultrasonic water meters:

● High accuracy: High measurement resolution;

● Good reliability: Stable and reliable ultrasonic echo measurement;

● Wide range ratio: Very low starting flow rate (< 2 L/h), minor changes can be detected;

● Long lifespan: Despite the fast measurement speed, it can have a very long service life;

● No moving parts: Does not affect fluid characteristics, measurement performance is superior;

● Very flexible replacement: Very simple to replace, no need to shut off the pipeline.

Research data shows that in Europe and some regions of the world: large-caliber industrial water meters and small-caliber civil water meters have begun to develop towards ultrasonic technology. Some well-known meter companies in Europe have begun or are already in mass production in their project development work, indicating the enormous potential of ultrasonic water meters (market potential: >10Mio./year).

In China, ultrasonic measurement methods have been widely applied and validated in heat meters and flow meters. Due to the advantages of ultrasonic methods and the continuous development and improvement of ultrasonic measurement circuits, smart water meters will also develop towards this method. The national standard for “ultrasonic water meters” drafted by leading industry enterprises commissioned by the Ministry of Housing and Urban-Rural Development has been basically completed.

The measuring chip from Germany’s Acam company has been widely recognized and adopted in ultrasonic measurements. Returning to the Time Digital Converter TDC-GP22 mentioned at the beginning of the article, it is a time digital converter produced by Acam using pure digital CMOS technology, capable of quantifying time interval measurements with an accuracy of 22ps, which can be said to inherently bear the mission of promoting the technological revolution of smart ultrasonic technology.

Figure 2. TDC-GP22—High integration measurement chip for smart ultrasonic water meter applications

In fact, Acam’s first TDC chip, TDC-GP1, was already launched in 1996 and has been widely used in ultrasonic flow meters. The company launched the cost-effective TDC-GP2 chip in 2005, establishing a good foundation in the ultrasonic heat meter market. By early 2011, the TDC-GP21 chip was introduced, specifically designed for ultrasonic heat circuit applications. With the technical validation and market accumulation from the previous stages, at the end of 2011, Acam officially launched the more powerful chip TDC-GP22 tailored for water meters.

Figure 3. Internal structure principle diagram of TDC-GP22

As shown in Figure 3, the Time Digital Converter TDC is the technical core of the TDC-GP22 chip, which uses the absolute time delay of signals through logic gates to accurately quantify time intervals. Moreover, this high-precision time measurement unit TDC has a resolution of 22ps, providing a fundamental measurement guarantee for the application of time difference flow meters, thus achieving a design with high precision and a wide range ratio.

Important features of TDC-GP22 also include:

● Temperature measurement accuracy (2mk rms);

● Temperature drift of TOF flight time difference (<0.3ps/k);

● Provides a complete analog front end required for ultrasonic water meters: internally integrates a chopping stable low-noise comparator (comparator trigger offset range at ±35 mV) and low crosstalk analog switches, which solves the customer’s analog design issues and improves the system measurement quality.

The pulse generator of TDC-GP22 can directly drive ultrasonic transducers in small diameter flow measurements without the need for additional driving chips, simplifying design and reducing costs; high precision time measurement, simple external circuits, and integrated internal signal processing algorithms, along with ultra-low overall power measurement characteristics make it very suitable for ultrasonic water meter applications.

Since TDC-GP22 is developed based on TDC-GP21, its functions, pins, and registers are 100% compatible with TDC-GP21 (can be replaced 1:1). In addition to possessing all the features of TDC-GP21, TDC-GP22 also adds three important functions. The reason why TDC-GP22 is regarded as the revolution of ultrasonic water meter circuits is closely related to these three newly added important functions.

1. Intelligent first echo detection function, which makes the time window setting no longer affected by time difference changes, thus achieving precise pulse interval measurement, as well as backflow, empty pipe identification, and alarm.

2. First pulse width measurement function (currently only TDC-GP22 can achieve): The pulse width measurement of GP22 can help detect whether there are bubbles affecting the measurement in the water meter application, as well as detect long-term cover in the pipe section and provide alarm signals.

3. Simplified multi-pulse result calculation function, the TDC-GP22 chip will automatically process and calculate three pulse results and provide an average value. In this way, the entire measurement process is simplified, and the measurement results are completely handled by TDC-GP22, allowing the MCU to read the results directly, saving microcontroller resources and meeting the measurement speed requirements of the water meter.

Through the above discussion, we have gained a deeper understanding of the performance advantages of TDC-GP22 suitable for ultrasonic water meters, but we have not yet mentioned the critical power consumption issue that is vital in electronic smart water meter applications. The standard requirements for water meters require that the battery must not be replaced for at least 6 years, which imposes stringent requirements on the overall system’s power consumption and has always restricted the development of ultrasonic water meters.

Figure 4 is the system block diagram of the low-power ultrasonic dripping meter solution developed by Sygent, mainly consisting of three parts: transducer, TDC-GP22, and the control circuit part centered on Silicon Labs EFM32TG840Fxx. Sygent’s solution has the following features and advantages: 1. Range ratio: 1:125; 2. Average power consumption: < 30 uA; 3. Starting flow: < 2 L/h; 4. A single 3.6V lithium battery can work for 6+1 years; 5. Interface output: infrared, M-BUS.

Figure 4. Physical and system block diagram of the low-power ultrasonic water meter solution launched by Sygent

In fact, TDC-GP22 already possesses very low power consumption characteristics (static current: <0.1uA@85℃; sleep current: 1uA under 32K continuous operation), but when TDC-GP22 meets the ultra-low power Silicon Labs EFM32TG840Fxx series MCU based on ARM CortexM3, its power consumption performance becomes even more perfect, which is precisely the initial consideration for device selection in Sygent’s ultrasonic water meter solution shown in Figure 4.

Since controllers in the system are usually high power consumers, to reduce the overall power consumption of the system, ultra-low power MCUs are essential. The EFM32 series MCUs are ultra-low power ARM products launched by Silicon Labs, with only one-fourth the power consumption of existing 8-bit, 16-bit, and 32-bit MCUs, while possessing rich peripheral interfaces. EFM32TG840 is a product from the Tiny Gecko series within the EFM32 series MCUs.

Figure 5. Summary of features of Silicon Labs EFM32TG840Fxx series MCUs

The EFM32 series MCUs consume 150μA when executing actual code from Flash in active mode, 900nA in deep sleep mode, and 20nA in shutoff mode. The chip’s sleep mode wake-up time is less than 2μs, and the supply voltage range can reach 1.8 to 3.8V.

In terms of peripherals, EFM32 not only provides basic AD modules, DA modules, analog comparators, UART/SPI/IIC interfaces, and external bus interfaces, but also offers unique low-power peripherals, including low-power UART and timers. The unique “peripheral reflex system (PRS)” of EFM32 can operate in parallel with the standard 32-bit ARM bus, allowing EFM32 peripherals to operate and communicate autonomously without CPU intervention, which can extend CPU sleep time and save a lot of energy. In addition, EFM32 series products also integrate an LCD controller, RTC, LESENCE interface, AES module, etc.

Based on the Cortex-M3 core, the EFM32 series MCU products have powerful computing capabilities that reduce the working state time of the MCU, with an ultra-low operating power of 150μA/MHz. Coupled with the unique low-power peripherals of EMF32, it will be possible for ultrasonic water meters to operate for 8 years without battery replacement. At the same time, the high integration of the EFM32 series MCUs further reduces the system costs for customers.

The high integration of the TDC-GP22 ultrasonic chip simplifies the design of peripheral circuits, and the low-power high-performance MCU greatly extends battery life. Moreover, Sygent can provide a one-stop solution for ultrasonic water meters (including complete feasible software, schematics, PCB examples, and technical support), undoubtedly paving the way for water meter companies to seize innovative opportunities more quickly for development and market launch.

Source: ofweek

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