Integrated MPU performs complex ultrasonic flow calculations

The design of ultrasonic flow meters can be greatly simplified and shortened by the TDC-GP30, allowing manufacturers to develop cheaper, more compact and energy-efficient ultrasonic meters. The TDC-GP30 is a SoC solution with extremely low power consumption, which undertakes all the measurement tasks up to the calibrated output value.

The TDC-GP30 has been developed to provide a universal solution for ultrasonic heating and water meters that is easy to adapt, and a single-chip solution for different industrial applications for pure flow or volume measurement. All flow, volume and temperature calculations are done directly by TDC-GP30. An external MPU is used only for the interfaces to LCD, wireless and other standard applications, and is no longer a part of the electronics assembly required by the legal metrology.

An integrated Standard Pulse Interface allows a one-to-one coupling in the electronic assembly of mechanical and hybrid measuring instruments, and pulses of mechanical measuring instruments can be generated equivalent to the TDC-GP30. Existing processor solutions for mechanical meter types as well as their software remain mostly unaffected in the process, while changing a mechanical flow measurement (e.g. with a propeller) with an ultrasonic metering tube.

The TDC-GP30 functions as a complete front-end solution with integrated digital signal processing (32-Bit MPU) up to the output signal that can be calibrated, and displays a real Single-Chip-solution up to the digital volume output through UART, Pulse and SPI, so that the subsequent standard MCU can be used exclusively for the management of communication of the measurement device - e.g. for wireless.

The TDC-GP30 measures water flow with the help of high-precision time differences of ultrasonic pulses in the direction of the flow as well as against the flow. When used in cold water meters, the measured transit times of the ultrasonic impulses are additionally used for the necessary measurement of the water temperature, and the temperature dependence of the ultrasonic run time is thus put to a beneficial use.

At a measuring rate of 8Hz, typical for ultrasonic water counters, the complete TDC-GP30 requires only about 8µA, including all calculations and mathematical corrections of the flow. While doing so, approximately 2µA are consumed on permanent features such as 1.8V LDO and 32kHz oscillator, and 6µA for the implementation of the flow measurement and the automatically initiated calculation of the flow. The 32-Bit MPU requires approximately 3.6µA (0.45µA/Measurement) while doing so - a value hitherto unachievable on a low-power standard MCU basis. This shows the effectiveness of a customised computer architecture as well as the OP-code tailor-made for it.

The entire firmware is developed and maintained in assembler to achieve the most compact code in combination with highest performance and absolutely minimum power consumption. The extremely low power consumption of the chip allows the use of low-cost standard 2/3 AA or AA lithium thionyl chloride batteries. At a measurement frequency of 6-8Hz, an operating time of more than 20 years can therefore be reached.

With the TDC-GP30, acam also offers firmware as an option for the integrated processors, which includes all functions up to the calibrated output value. This firmware is optimally matched to the processor, can be parameterised by the customer to a high degree and is thus a tool that can be applied universally in order to be able to carry out quick and secure developments. For cold water meters (up to 60°C), the necessary temperature measurement is carried out via the ultrasonic run time in water, so that no additional temperature sensor is required.

For customers that wish to develop their own firmware or extend the solution offered by acam, a convenient assembler with GUI is available, which is included by default in the development system.

acam strongly supports customers in the necessary characterisation of a meter/measuring tube in development. In the process, the customer accepts (as usual) the characteristic of a measuring tube type via n-flows and m-temperatures, and enters them in the development system of the TDC-GP30. An integrated "calculation engine" calculates the corresponding coefficients of the correction algorithm of the acam firmware from these customer entries and provides them in a downloadable file. These coefficients are finally stored in the corresponding NVRAM area of the TDC-GP30.

This basic parameter set will then be used for the production of a meter type, which is characterised and adapted according to each production measuring tube. Only a 2-point calibration (two known flows in case of a known temperature) is required for the parameter adjustment. This presumes an appropriate production stability of the mechanical system (of the measuring tube). The basic parameter set can be used unchanged, as long as the property (the character) of the measuring tube type is moved in sufficiently close tolerances.

Since the beginning of 2014, the TDC-GP30 has been available in its beta version and has since undergone rigorous testing, in which it has delivered impressive results. In the meantime, the TDC-GP30 has been developed into a production-ready product and is also available in samples. The first customers have already been supplied. Interested customers can therefore get the finished chip with the necessary development and programming environment just in time for Sensor+Test 2015 in Nuremberg. This also supports the characterisation and calibration functions.