Efficient cold chain management through scalable temperature sensors
“In the previous articles, we have introduced the basic principles of temperature sensors. In order to maintain nutrition and ensure quality and product safety, manufacturers regulate the transportation and storage temperature of packaging and perishable goods (especially food and medicine). However, fruits, vegetables, and frozen food stayed on the shelves of large refrigeration equipment for a long time during transportation and on the shelves of large refrigeration equipment before they reached consumers through local grocery stores, as shown in the figure below. It is important to keep these commodities at the proper temperature.
In the previous articles, we have introduced the basic principles of temperature sensors. In order to maintain nutrition and ensure quality and product safety, manufacturers regulate the transportation and storage temperature of packaging and perishable goods (especially food and medicine). However, fruits, vegetables, and frozen food stayed on the shelves of large refrigeration equipment for a long time during transportation and on the shelves of large refrigeration equipment before they reached consumers through local grocery stores, as shown in the figure below. It is important to keep these commodities at the proper temperature.
Typical grocery store aisle
Cold chain management will ensure that appropriate conditions are met at every stage of the life cycle of packaging and perishable goods. Cold chain management also ensures that operators can take appropriate measures when they discover that a deviation from the storage temperature range may occur during transportation or storage.
Cold chain topology
Temperature sensors with measuring instruments have long been popular. However, with the advancement of semiconductor technology, and most cold chain management occurs in the temperature range of -40°C to +10°C, integrated temperature sensors have become the best choice for cold chain management.
Depending on the application, there may be different topologies. As shown in the figure below, in a point-to-point topology, a single microcontroller (MCU) is connected to a temperature sensor, which can be an analog or digital output sensor. The point-to-point topology is very useful when managing cargo pallets during transportation.
When sensing a group of refrigeration containers (such as refrigerators), the cost of a single MCU is too high to be implemented multiple times in the entire system. In such cases, the most common topologies are star, shared bus, or daisy chain topologies:
Temperature sensor daisy chain
• The star topology can easily achieve fault isolation when a branch fails. The star topology can use both analog and digital output temperature sensors, but due to the high number of controller peripherals and the system cannot be fully expanded, the implementation cost is high.
• In a shared bus topology, one MCU acts as the main controller for multiple sensors. Expansion can be easily achieved by using a digital temperature sensor. The shared bus topology will share lines, but you can still use in-band addressing (such as the I2C bus) or chip-select-based out-of-band signaling (this is the case with serial peripheral interfaces) for individual addressing. However, the problem facing I2C may be reliable power delivery and signal integrity over long chains.
• Daisy chain does not require out-of-band signaling, but uses an in-band addressing scheme. Each stage of one chain acts as a buffer for the next chain, so it can improve signal integrity over longer distances.
Regardless of the monitoring stage of the cold chain management, Electronic systems have unique advantages because they can not only record the temperature of trays or refrigeration equipment, but also provide thresholds to issue alarms when certain thresholds are exceeded. Such events can be intuitively communicated to the operator in the form of sound or visual alarms (such as buzzers or flashing LED indicators), and can also be integrated into cloud services using wired or wireless MCUs to achieve round-the-clock monitoring and data recording .
The TMP107 digital output temperature sensor supports a total of 32 daisy chain devices, which can replace NTC thermistors in cold chain management applications that require high accuracy and system range scalability. The highest accuracy specification of TMP107 in the temperature range of -20°C to +70°C is ±0.4°C, within the range of -40°C to +100°C, it is ±0.55°C, and the temperature resolution is 0.015625°C .
With the automatic address assignment function, TMP107 allows system developers to write software without assigning an address to each sensor node, because the system will be expanded by adding additional sensor nodes. At the same time, by using push-pull communication input/output, the system can better resist noise and prevent noise from affecting the temperature value on long cables. This flexibility makes it possible to transmit data with a span of 1,000 feet between adjacent devices in the chain.
The figure below shows the signal integrity of the 9,600bps communication interface. The SMAART wire™ digital interface uses a universal asynchronous receiver/transmitter bus, which is a standard peripheral on almost all MCUs, so software development can be easier. At the same time, the daisy chain implementation scheme is easier to identify the location of the cable break, so it is easier to maintain and improve the overall system reliability.
Eye diagram of TMP107
When using active bus communication to perform temperature conversion, the current consumption of TMP107 is usually 300μA. The shutdown current in low power consumption mode is 3.8μA. The device has a wide operating voltage range of 1.7V to 5.5V. Due to its low current consumption, it is very suitable for battery-powered systems in the cold chain management and transportation phase. Increasing the baud rate can realize real-time updates, which is helpful for storing frozen food.
In addition, TMP107 stores configuration and temperature limits in internal non-volatile memory. Therefore, the device can be automatically configured at power-up, eliminating the need for separate device configuration and improving system operating speed. The device also has eight electrically erasable programmable read-only memory (EEPROM) locations, providing up to 128-bit EEPROM to store user information or calibration information.
The daisy chain topology is the best way to achieve efficient cold chain temperature monitoring. TMP107 achieves a perfect combination of accuracy, power consumption and functions, and can support a battery-based cold chain management system.