Sensors - May 2001 - One Chip, Many Functions

TEMPERATURE

One Chip,
Many Functions

To reduce the size and increase the performance of your transformer temperature monitor, consider integrating several features on a single chip.

Don Hollands, Qualitrol Corp.

A typical transformer temperature monitor for the North American market incorporates multiple thermocouple or RTD inputs, relay outputs to control a fan or alarm, a display to indicate the current highest temperature, and output signal connectivity, typically via a 4–20 mA loop or an RS-232/RS-485 interface. European and Asian markets, however, demand a much smaller package size, higher performance, and lower cost.

To meet this need, Qualitrol developed a new version of its North American temperature monitor. The basic functional requirements for the new product were:

Measurement of four temperatures using thermocouples and RTDs from different windings of the transformer, with 10-bit accuracy or better

Photo 1. The original Qualitrol microprocessor-controlled temperature monitor, the Model 118P, used a standard 8052 microprocessor. Separate components are used for the measuring and storage functions.

Display of the temperatures
Output relay to turn cooling fans on and off as a function of the highest measured temperature
Output relays to indicate alarm and trip conditions when the temperature approaches dangerous values
A 0–1 mA or a 4–20 mA current loop output to transmit the current highest temperature information to the customer’s data collection system
Nonvolatile memory to store the highest temperature readings seen by the unit since the last reset
Serial data communications to carry the unit’s values and status to the customer’s data collection computer

One-Chip Solution
Qualitrol’s larger North American product (see Photo 1) uses a standard 8052 microcontroller with an external 8-bit A/D converter, a D/A converter, and nonvolatile memory connected to the microcontroller via an I²C bus (see Figure 1).

Figure 1. The block diagram shows the components that make up Qualitrol's Model 118P temperature monitor, including the A/D converter, D/A converter, EEROM, display driver, and ambient temperature sensor connected to the microprocessor via an I²C bus.

To minimize size and cost, the company looked for a chip to integrate as many of the functions as possible in one package, including a high-resolution A/D converter. It found the 52-pin PQFP AduC812 from Analog Devices. Some additional advantages that the ADuC812 offers over the larger design include:

The A/D converter has 12-bit resolution and a more stable output.
The processor core is 8051-/8052-compatible, allowing the reuse of much of Qualitrol’s code.
The program memory (Flash/EEPROM) can be programmed through the serial port on the production line just prior to testing the product. This lets the manufacturer produce the unit in volume to take advantage of surface-mount automation but still have the flexibility to modify or customize the program as orders are shipped. It can also reduce the time to market because orders can be placed for assembled boards as soon as the hardware design is firm, even if the firmware is still being tested.
The two D/A outputs on the ADuC812 make it possible to set the output at 0-1 mA or 4–20 mA via firmware control.
The built-in I²C port allows easy communication with I²C devices without using the general-purpose I/O pins.

The significant reduction in parts count afforded by the ADuC812 (see Figure 2) helped Qualitrol meet its overall package size reduction goals.

Figure 2. Because many functions were integrated in the AduC812, the Model 118L temperature monitor required fewer components external to the microprocessor. Eliminating the bus connection for these functions by including them in the microprocessor simplifies the microprocessor code, reduces the response time, and improves the noise immunity of the product.

Photo 2. Qualitrol' Model 118L microprocessor-controlled temperature monitor uses an Analog Devices AduC812 microprocessor with an A/D converter, a D/A converter, and EEROM included in the microprocessor.

Photo 2 shows the smaller unit.

With all the functions built into the ADuC812, the hardware design of the circuit was straightforward. The big design task was the firmware.

The ADuC812 QuickStart development kit includes a printed circuit board with an ADuC812 chip. The kit’s software has an assembler, a debugger, a serial downloader, and simulation tools. For Qualitrol’s purposes, the most useful tool in the ADuC812 kit proved to be the simulator program, which allows the assembled or compiled code to run on a Windows PC, showing the activity of all RAM, Flash/ EEROM, registers, and ports. The simulator can perform all the peripheral functions (e.g., A/D conversion, D/A conversion, user Flash/EEROM), as well as the core code. For Qualitrol, this was a great help when testing the code that read the A/D input, accessed calibration data stored in flash memory/EEROM, and calculated the temperature in C, enabling on-the-fly input value changes to check for proper operation under all conditions, including out-of-range and negative values.

Once the simulation was working well, Qualitrol used the evaluation board to verify that the real part worked like the simulation. It also allowed testing of the serial data functions and verification of the timing of the different functions. The evaluation board also provides space to add additional parts, enabling tests of the mA output loop.

Application Details
Figure 2 shows the arrangement of the product hardware. Note that the only required parts external to the ADuC812 are the input conditioning, the RS-232 converter, relay drivers, mA loop drivers, and temperature sensor (the ADuC812 has a built-in temperature sensor, but for thermocouple inputs, Qualitrol needed the sensor near the terminal blocks).

The cost-sensitive nature of the product demanded that the unit be capable of being tested and calibrated without human intervention. A factory test mode implemented through the RS-232 port lets the test fixture exercise the functions of the unit, as well as perform an automatic calibration. The manufacturer calibrates the unit by applying known inputs and reading the output of the A/D converter. This lets it calculate calibration constants and then store them in nonvolatile memory. Firmware uses these constants to convert the A/D count to C for the display.

The D/A converter outputs are calibrated in a similar fashion. One output is calibrated to give 0–1 mA in the current loop, and the other is calibrated to give 4–20 mA. The user can then select one or the other via the firmware—no jumpers or components need be changed to or switched from one to the other.

Because there are no manual trim options on the analog input channels, the design had to accommodate the worst-case tolerance conditions on the input circuits so that the worst-case low output and the worst-case high output fall within the active range of the A/D converter. This means that for any given set of input circuit component values the full range of the A/D converter is not available. With the 12-bit A/D converter, Qualitrol achieved 9 A/D counts/°C. If a 10-bit A/D converter had been used, there would be only 2 A/D counts/°C. This lower count would have made it more difficult to achieve the desired accuracy and display stability.

Dan Hollands is Senior Electronics Engineer, Qualitrol Corp., 1385 Fairport Rd., Fairport, NY 14450; 716-586-1515, x-222, fax 716-377-6235, dan.hollands@qualitrolcorp.com.