Sensors - October 2002 - Wireless Temperature Monitoring in Remote Systems

Analog Devices has designed a system to meet the demanding requirements of food transporation applications.

The incidence of food poisoning is on the increase. At the end of 2000, more than 250 food-borne diseases were described, but in most cases, the causal agent is unknown. According to a report published in 1999, food-borne diseases cause approximately 76 million illnesses, 325,000 hospitalizations, and 5000 deaths in the U.S. each year. Salmonella caused nearly 1.5 million illnesses in 1999, with more than 95% of them being food borne. In Europe, the regulatory bodies governing chilled- and frozen-food transport have responded by introducing new regulations and standards.

A key parameter of concern is temperature. Monitoring temperature can improve food quality, reduce spoilage, and significantly reduce the risk of food poisoning. Often the most effective way of proving that a third-party transport company has done its job correctly is for the food company to monitor the temperature of the air or food in the vehicle independent of the transport company. For these reasons, customers are requesting temperature monitors, even where legal requirements don’t stipulate their use.

The electronic temperature monitoring system is now a standard specification for temperature-controlled transport. Virtually all transport operators carrying chilled or frozen foods request that all new vehicles be fitted with temperature monitors. These devices keep track of the air temperature inside the refrigerated container and are often viewed as extensions of the refrigeration system. But it’s fundamentally important that the monitor operate independent of the refrigeration system itself.

Most modern refrigeration systems incorporate microprocessor controls, and although you could integrate a recording system in the package, this philosophy is rarely adopted and definitely not recommended. The temperature recorder is there to monitor the performance of the refrigeration system. If the system’s electronics fail or encounter a problem, the recorder could fail to operate reliably (or even fail to operate at all) just when it’s most needed.

What’s Required in a Temperature Monitor?
Remote temperature monitoring systems should allow easy interrogation at any time without interfering with the cargo or its environment. The system should provide accurate measurements and consume as little power as possible to ensure long operating life.

A wireless communications link would allow easy interrogation of the monitor. With this capability, you could upload temperature data from food containers without having to manually remove every individual monitor.

An RTD would accurately measure temperature. These sensors can achieve accuracy in excess of 0.1°, but they typically require a current source, a differential amplifier, and an A/D converter with a true accuracy of 12 bits or better.

The low-power requirement means that the microcontroller must operate in power-down mode most of the time and include a circuit to wake the part up periodically. The wakeup circuit must maintain the same timeout over temperature because every temperature measurement must be accurately time stamped. This is especially true where traceability is a concern in proving responsibility in out-of-control situations. Ideally, a microcontroller with integrated flash/EE program memory and SPI and UART ports is desirable.

Other features worth noting would be to keep the monitor small, reliable, and compact so that it can be easily placed with the food product without causing contamination. Keeping the unit inexpensive would make the system much sought after.

With the above requirements in mind, Figure 1 shows a temperature monitoring system that’s offered by Analog Devices.

Figure 1. A wireless temperature monitor allows easy interrogation without having to manually collect the monitor or interfere with the controlled environment. Analog Devices' AduC834 monitor delivers accurate temperature measurement and low-power operation in a compact and integrated system.

The remote RF temperature monitor consists of an RTD temperature sensor and an RF transceiver. All other functions—including temperature measurement (using the on-chip 24-bit A/D converter) and temperature data storage (using flash/EE memory)—are provided by the ADuC834 MicroConverter.

Interrogating the Monitor
The base unit consists of an RF transceiver, which uses the RS-232 protocol for data transmission. It is the master, enabling communications with any temperature monitor.

Each monitor has a unique 24-bit identification code, so the base unit can communicate with any specific monitor within its range. This not only lets the base unit communicate with multiple monitors, it also adds security to the system because the code must be known before any information is accessed.

The RTD Implementation
A PT100 RTD has a resistance of 100

at 0°. This varies almost linearly with temperature. Typically the resistance will change by 0.385

/°C. You can use optimized linearization techniques to obtain even higher accuracy.

Some implementations force a constant current through the RTD, and the voltage dropped across the RTD (which is proportional to temperature) is measured. This approach is rarely used because of the difficulties of maintaining a precise constant current source over temperature. A more accurate way of measuring the voltage is to use a ratiometric approach. In this method, you place a resistor in series with the RTD. The resistor generates a reference voltage that is fed directly to the A/D converter. Fluctuations in the current can be neglected because they are dropped proportionally over both the reference and the RTD.

You can use the circuit shown in Figure 2 to measure the voltage generated by the RTD.

Figure 2. In this ratiometric RTD temperature measurement, a resistor provides the reference voltage for the A/D converter, ensuring that current source output fluctuations caused by supply or temperature changes will be proportionally dropped across the RTD and the reference resistor. Hence, the A/D converter will not measure them. Direct connection to the RTD is possible via the buffer and the PGA frond-end to the 24-bit ADC.

The differential A/D converter in the circuit includes a programmable gain amplifier (PGA), which allows the direct connection of low-level signals. The use of this A/D converter eliminates the necessity for a separate differential amplifier.

The current source level should be large enough to cause a measurable voltage drop over the RTD but small enough that the power dissipated in the RTD (I²R) causes only a negligible heating effect. Typically, 200 mA is recommended for a 100 ž RTD, resulting in a negligible 4 mW heating effect.

Low Power Consumption
As suggested already, a key requirement of any temperature monitor is that it consume low power to ensure long battery life. Today, 10 years is often quoted as the typical lifetime of an instrument. In choosing a battery for a temperature monitor, there are six key specifications:

Examining these criteria, Analog Devices decided to use a lithium ion battery. Typically, a AA battery supplying 3.6 V will give a battery capacity of 2300 mAh, with a peak current of 20 mA over the operating temperature range –55° to 85°. A battery capacity of 2300 mAh will give a lifetime of more than 10 years if the average current is kept below 26 µA. Hence, in its design, Analog Devices had to keep the peak current below 20 mA and the average current below 26 µA.

Examining the power-down current of the system in Figure 1, you see that the only components consuming noticeable current are the RF transceiver and the MicroConverter. The transceiver can be shut down using <1 µA. At 3.6 V, the MicroConverter typically consumes 12 µA with the wakeup circuit still running. Hence, the complete power-down current is typically 13 µA.

The MicroConverter wakes up every 10 s to see if it’s time to take a temperature reading. If so, it will take the reading and store the result in its own nonvolatile memory (60 KB are available). After taking a temperature measurement, or when no temperature measurement is required, it checks to see if the host is trying to communicate with it. If it is, the monitor will stay awake and transmit/receive information as required by the host. Once finished, or if the host did not initiate communications with the monitor within 10 ms of the monitor’s waking up, the temperature monitor powers back down again.

During wakeup, the transceiver is in receive mode and consumes 6 mA. The MicroConverter, operating at 1.57 MHz, consumes 1.5 mA, resulting in 7.5 mA being consumed for 10 ms. The power consumed during a temperature measurement every minute has a negligible effect on the average current. Hence, for 9.990 s out of every 10 s (99.9%), the temperature monitor consumes 13 µA. For the other 10 ms out of every 10 s (0.1%), the monitor consumes 7.5 mA, resulting in an average current of just under 21 µA. This is within the 26 µA specification.

The other key specification is that the peak current is kept below 20 mA. The maximum current is consumed in transmit mode, when the transceiver consumes 10 mA. The data are transmitted in packets at a rate of 57,600 bps. At this baud rate, the full 60 KB of data can be transmitted in just under 11 s. The MicroConverter operates at 3.1 MHz, in this case consuming about 3 mA. Hence, the total peak current is kept well below the 20 mA specification on the battery.

An added advantage of using the MicroConverter instead of discrete components is the spare auxiliary A/D converter. The converter can be used with the internal 1.25 V reference to measure the battery voltage. The voltage varies with both battery life and temperature. The temperature is known, so an accurate estimate of the remaining battery life can be transmitted back to the host with every communication.

Conclusion
The temperature monitor implemented in hardware using Analog Devices’ ADuC834 MicroConverter was evaluated in trials carried out in the transportation of frozen foods. During the trials, the temperature was kept below –18°C at all times. The temperature of the frozen food was monitored over an 11 hr. period. Figure 3 shows the inadequate refrigeration system in use and emphasizes the value of temperature monitoring in bringing these concerns to light.

Figure 3. This temperature monitor highlights the inadequate cooling system used during the transportation of frozen meats. The frozen food in question should have been stored below -18ºC at all times.

Russell Williamson is a Design Evaluation Engineer, Analog Devices, Limerick, Ireland; 353 61 495269, fax 353 61 304094, russell.williamson@adbvdesign.analog.com.