Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The TDC1000 is an analog front-end for ultrasonic sensing applications. The device is typically used for the driving and sensing of ultrasonic transducers to perform accurate time-of-flight measurements. Ultrasonic time-of-flight sensing allows for fluid level measurements, fluid identification or concentration, flow measurements, and proximity/distance sensing.
|DESIGN PARAMETER||EXAMPLE VALUE|
|Range||2 – 10 cm|
|Accuracy||0.5% concentration variation|
For level sensing applications, the total time-of-flight (TOF) of the sound wave in the fluid is measured. The pulses transmitted by transducer B travel through the fluid, typically from the bottom, to the surface of the fluid. The discontinuity between the fluid and air generates a reflected wave which returns back to transducer B.
At the beginning of a measurement cycle, the transducer is connected to a transmit channel of the AFE, and the transmit burst excites the transducer to generate an ultrasound wave. Synchronous to the TX burst, a START pulse is generated by the TDC1000 to indicate the start of a measurement. After the transmission is completed, and depending on the device configuration, the transducer is connected to a receive channel of the AFE.
When a valid echo is received, the TDC1000 generates a STOP pulse. Generation of multiple STOP pulses is possible through register configuration of the device. The START and STOP signal times are compared to determine the TOF.
The level of the fluid can be determined using the following equation:
Level measurements have 2 main criteria: resolution and range (maximum height). Resolution accuracies of 1-2 mm are achievable but are impractical due to any environmental disturbances, such as tank vibrations, creating millimeter level surface waves. Ranges of up to 1 m are measurable utilizing VDD level excitation pulses, but surface disturbances and signal loss over longer distances make the reliable echo reception an issue. Greater level measurement reception can be achieved by mechanical means (level guide tube) and/or electronic means (level shifting the TX pulses to greater voltages; see TIDA-00322).
The TDC1000 can be used to measure the time-of-flight for a known distance to calculate the speed of sound (cmedium) in the fluid. This application utilizes the same circuitry as the level example but a transducer in a different configuration connected to the second channel. In this example, the speed of sound in the fluid (cmedium) is measured by using transducer A.
The temperature can also be measured to compensate for the temperature variation of sound. With the known distance, TOF and temperature, the speed of sound in the fluid can be determined and the identity of the medium verified.
After measuring the time-of-flight for the fixed distance, the speed of sound can be calculated as follows:
The measurement process is identical to the level example above. The speed of sound can be used to uniquely identify a variety of fluids. In this example, the concentration of diesel exhaust fluid (DEF) is measured with a desired accuracy resolution of 0.5% of concentration variation. For most fluids, the speed of sound varies over temperature, so every application will be different. In this example, all samples were all at ambient temperature of 23°C.
The data used in the following level and fluid identification graphs was collected using ultrasonic test cells. The test cells are acrylic plastic containers with width of 2.54 cm and ultrasonic transducers attached to the outside using cyanoacrylate glue. The transducers in this experiment are STEMiNC 1MHz piezo electric ceramic discs (SMD10T2R111). Equivalent transducers with the following characteristics could be used:
|Fluid Height in Tank||Time-of-Flight (µs)|
|Full (10 cm)||145|
|Full – 1 (9 cm)||131|
|Full – 2 (8 cm)||118|
|Fluid||Speed of sound (m/s)|
|Fluid||Speed of sound (m/s)|
|Fluid||Speed of sound (m/s)|
The parameters in Table 16 are considered for this example.
|DESIGN PARAMETER||EXAMPLE VALUE|
|Pipe diameter||15 mm|
|Distance between transducers||60 mm|
|Minimum flow rate||0.015 m3/h|
|Accuracy at minimum flow rate||5%|
The design of flow-meters requires a thorough technical assessment of the system where the device will be used. The following is a list of areas to consider:
The appropriate ultrasonic sensor and the proper electronics for interfacing to the sensor are determined based on the system requirements. The following is a list of specifications applicable to the senor/assembly used in the system:
If the flow meter is intended for residential applications, it must be designed to meet the required standards. For example, per the INTERNATIONAL ORGANIZATION OF LEGAL METROLOGY (OIML), the metrological requirements of water meters are defined by the values of Q1, Q2, Q3 and Q4, which are described in Table 17.
|Q1||Lowest flow rate at which the meter is to operate within the maximum permissible errors.|
|Q2||Flow rate between the permanent flow rate and the minimum flow rate that divides the flow rate range into two zones, the upper flow rate zone and the lower flow rate zone, each characterized by its own maximum permissible errors.|
|Q3||Highest flow rate within the rated operating condition at which the meter is to operate within the maximum permissible errors.|
|Q4||Highest flow rate at which the meter is to operate for a short period of time within the maximum permissible errors, while maintaining its metrological performance when it is subsequently operating within the rated operating conditions.|
A water meter is designated by the numerical value of Q3 in m3/h and the ratio Q3/Q1. The value of Q3 and the ratio of Q3/Q1 are selected from the lists provided in the OIML standards.
Water meters have to be designed and manufactured such that their errors do not exceed the maximum permissible errors (MPE) defined in the standards. For example, in OIML standards, water meters need to be designated as either accuracy class 1 or accuracy class 2, according to the requirements.
For class 1 water meters, the maximum permissible error in the upper flow rate zone (Q2 ≤ Q ≤ Q4) is ±1%, for temperatures from 0.1°C to 30°C, and ±2% for temperatures greater than 30°C. The maximum permissible error for the lower flow-rate zone (Q1 ≤ Q < Q2) is ±3%, regardless of the temperature range.
For class 2 water meters, the maximum permissible error for the upper flow rate zone (Q2 ≤ Q ≤ Q4) is ±2%, for temperatures from 0.1°C to 30°C, and ±3% for temperatures greater than 30°C. The maximum permissible error for the lower flow rate zone (Q1 ≤ Q < Q2) is ±5% regardless of the temperature range.
The flow meter accuracy specified in the standards dictates the required accuracy in the electronics used for driving the ultrasonic transducers, circuits in the receiver path, and time measurement sub circuits. The stringent accuracy required at lower flow rates would require a very low noise signal chain in the transmitter and receiver circuits used in ultrasonic flow meters, as well as the ability to measure picosecond time intervals.
Transit-time ultrasonic flow meters works based on the principle that sound waves in a moving fluid travel faster in the direction of flow (downstream), and slower in the opposite direction of flow (upstream).
The system requires at least two transducers. The first transducer operates as a transmitter during the upstream cycle and as a receiver during the downstream cycle, and the second transducer operates as a receiver during the upstream cycle and as a transmitter during the downstream cycle. An ultrasonic flow meter operates by alternating transmit and receive cycles between the pair of transducers and accurately measuring the time-of-flight both directions.
In this example, the upstream TOF is defined as:
In this example, the downstream TOF is defined as:
The difference of TOF is defined as:
After the difference in time-of-flight (ΔTOF) is calculated, the water velocity inside the pipe can be related to the ΔTOF using the following equation:
Finally, the mass flow rate can be calculated as follows:
Based on the minimum mass flow requirement and accuracy requirements in Table 16, the ΔTOF accuracy needed can be calculated as follows:
For this reason, this system requires a high accuracy timer/stopwatch that can measure the lower flow rate state.
The TDC1000 is used to drive the transmitter, amplify and filter the received signal and conditioning the echo for START and STOP pulse generation. The TDC7200 ps-accurate timer is used to measure the time interval between the rising edge of the START pulse and the rising edge of the STOP pulse produced by the TDC1000.
The microcontroller should first configure the TDC7200 and the TDC1000 for the measurement. When the microcontroller issues a start command to the TDC7200 via the SPI interface, the TDC7200 sends a trigger pulse to the TRIGGER pin of the TDC1000. When the TDC1000 drives the transmit transducer, a synchronous START pulse is produced on the START pin, which commands the TDC7200 to start its counters. When a valid echo pulse is received on the receive transducer, the TDC1000 generates a STOP pulse on the STOP pin, which commands the TDC7200 to stop its counters. This procedure is repeated for the upstream and downstream cycles.
A temperature measurement can be performed and the result can be used to correct for temperature dependency of the speed of sound.
The following figures show data and histograms created with data collected under a zero flow condition at room temperature. A simple offset calibration has been applied, where the overall average of the data is subtracted from the data.