Temperature Problem Solved for You (V) Efficient Cold Chain Management through Scalable Temperature Sensors

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Temperature Problem Solved for You (V) Efficient Cold Chain Management through Scalable Temperature Sensors [Copy link]

 

In previous articles, we have introduced the basic principles of temperature sensors. In order to preserve nutrients and ensure quality and product safety, manufacturers specify the transportation and storage temperatures of packaged and perishable goods, especially food and medicine. But before reaching consumers at local grocery stores, fruits, vegetables, and frozen foods spend a lot of time during transportation and on the shelves of large refrigeration equipment, as shown in the figure below. Keeping these items at the proper temperature is critical.

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 if they discover a possible deviation from the storage temperature range during transportation or storage.

Cold chain topology

Temperature sensors with measuring instruments have long been popular. However, with advances in semiconductor technology and the fact that most cold chain management occurs in the temperature range of -40°C to +10°C, integrated temperature sensors are 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. Point-to-point topology is useful when managing pallets of goods during transportation.

Point-to-point topology

When sensing a group of refrigerated containers, such as a refrigerator, a single MCU is too expensive to implement multiple times throughout the system. In such cases, the most common topologies are star, shared bus, or daisy chain topologies:

Temperature Sensor Daisy Chain

Star topology allows for easy fault isolation when a fault occurs in one branch. Star topology can use both analog and digital output temperature sensors, but the implementation cost is high due to the high number of controller peripherals and the system cannot be fully expanded.

In a shared bus topology, one MCU acts as the master controller for multiple sensors. This is easily scalable with digital temperature sensors. A shared bus topology shares the wires, but can still be individually addressed using either in-band addressing (like the I2C bus) or out-of-band signaling based on chip selects (as is the case with the Serial Peripheral Interface). However, issues with I2C can be reliable power delivery and signal integrity over long chains.

Daisy chaining does not require out-of-band signaling, but instead uses an in-band addressing scheme. Each level of a chain acts as a buffer for the next, thus improving signal integrity over longer distances.

Regardless of the monitoring stage of cold chain management, electronic systems offer unique advantages because they can not only record the temperature of the pallet or refrigeration equipment, but also provide thresholds to sound an alarm when a certain threshold is exceeded. Such events can be intuitively communicated to operators in the form of audible or visual alarms (such as buzzers or flashing LED indicators), and can also be integrated into cloud services using wired or wireless MCUs to enable 24/7 monitoring and data logging.

Device Recommendations

The TMP107 digital output temperature sensor supports a total of 32 daisy-chained devices and can replace NTC thermistors in cold chain management applications that require high accuracy and system-wide scalability. The TMP107 has a maximum accuracy specification of ±0.4°C over the -20°C to +70°C temperature range and ±0.55°C over the -40°C to +100°C range, with a temperature resolution of 0.015625°C.

With automatic address assignment, the TMP107 allows system developers to write software without having to assign addresses to each sensor node as the system expands with the addition of additional sensor nodes. At the same time, by using push-pull communication inputs/outputs, the system is better resistant to noise, preventing it from affecting temperature values on long cables. This resilience makes data transmission possible over spans of 1,000 feet between adjacent devices in a chain.

The following figure shows the signal integrity of the communication interface at 9,600bps. The SMAART wire digital interface uses the Universal Asynchronous Receiver/Transmitter bus, which is a standard peripheral on almost all MCUs, making software development easier. At the same time, the daisy chain implementation makes it easier to identify the location of a cable break, making maintenance easier and improving overall system reliability.

Eye diagram of TMP107

The TMP107 typically consumes 300μA of current when performing temperature conversions using active bus communication. The shutdown current in low-power mode is 3.8μA. The device has a wide operating voltage range of 1.7V to 5.5V, and due to its low current consumption, it is ideal for battery-powered systems in the transportation stage of cold chain management. Increasing the baud rate allows real-time updates, which helps store frozen foods.

In addition, the TMP107 stores the configuration and temperature limits in internal nonvolatile memory. Therefore, the device can be automatically configured at power-up, eliminating the need for separate device configuration and increasing system operation speed. The device also has eight electrically erasable programmable read-only memory (EEPROM) locations, providing up to 128 bits of EEPROM to store user information or calibration information.

Daisy-chain topology is the best way to achieve efficient cold chain temperature monitoring. The TMP107 provides an ideal combination of accuracy, power consumption, and functionality to support battery-based cold chain management systems.

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Temperature problems solved for you (I) Basic principles of temperature sensing

Temperature Problems Solved for You (II) System Temperature Monitoring

Temperature Problems Solved for You (III) High-Performance Processor Mold Temperature Monitoring

Temperature problems solved for you (IV) Ambient temperature monitoring