Design Feature: May 9, 1996

Last year, industry observers expected the proposed IEEE and National Institute of Standards and Technology (NIST) smart-sensor standard to emerge as a finished standard (Reference 1). Alas, the best-laid plans of mice and men don't always pan out: Instead, the various companies and agencies involved in its drafting are still reviewing the standard. Nevertheless, a variety of recent standardless sensors are incorporating various degrees and kinds of smartness.

Despite the fact that the sensor standard is not yet a reality, it's worthwhile to reiterate the significance of what the standard will mean when it does emerge. Whether you're a sensor manufacturer or user, you must take account of the sensor-network protocol used in the end application. Here's where the Tower of Babel comes in: A plethora of protocols exists, each protocol having its own interface requirements. The requirements stipulate such parameters as headers, data-word length and type, bit rate, cyclic redundancy check, and many others. Table 1 shows 31 distinct smart-sensor network protocols (Reference 2).

The idea of the proposed standard is to standardize the smart features in sensors, so that you can connect the sensors to standard sensor-to-network processors in a plug-and-play manner. The smart features can include onboard diagnostics, a standard communications protocol, and an onboard product data sheet, including, possibly, a calibration-look-up table. The plan for the standard makes considerable sense. It's not economical to design a sensor family that can connect directly to all the network types, but it's easy to envision sensor-to-network processors with common front ends and output interfaces you can program for particular network protocols.

Though the lack of a universal standard has so far impeded the development of fully smart sensors and universal sensor-to-network interface chips, some available interface devices do address some large niche markets. Motorola, for example, produces versions of its 68HC705 µC that provide a J1850 automotive-network interface and variations of the company's 68HC05 µC that incorporate computer-automated-network (CAN) interface functions. In addition, Motorola offers a family of processor chips, dubbed "Neuron," that addresses LonTalk/LonWorks applications.

What constitutes "smart"?

Roger Grace of Roger Grace Associates is a marketing counsel who conducted an information-gathering project to help in defining the new smart-sensor standard. According to Grace, a fully smart sensor incorporates a communications capability, self-diagnostics, and intelligence (decision-making). Each of the criteria for smartness can have a range of degrees. For example, "communications capability" could imply anything from a simple, serial bit stream to a full UART-type handshaking utility. Similarly, "intelligence" could embrace anything from a simple switch closure to a complicated sequence of commands to the host system.

One aspect of self-diagnostics is automatic calibration. A recent pressure transducer from Lucas NovaSensor provides that capability. (According to Reference 2, a transducer is a device, calibrated to minimize the errors in the conversion process, that converts energy from one domain into another. A sensor is a device that provides a useful output to a specified measurand. A sensor is a basic element of a transducer.) The $289 P4100 Series from Lucas NovaSensors mates a 12-bit digital signal-conditioning module to a piezoresistive sensor (Figure 1). The signal-conditioning module provides offset and linearity corrections, as well as temperature-drift compensation.

In terms of "communications," the P4100 provides this utility in its most simplistic form—an analog output according to system requirements: 0 to 5V, 0 to 10V, or 4 to 20 mA. Speaking of analog, the transducer provides a wholly analog through-path to the signal, unlike other digitally compensated devices that perform an A/D and then a D/A conversion in the signal path. Lucas maintains that this all-analog path eliminates the noise, quantization, and linearity errors the dual-conversion process can introduce. In contrast to the analog-output P4100, SenSym's $115 SMRT pressure sensor offers a serial digital output; a $385 evaluation board uses this digital output to provide RS-232C and LCD outputs.

More advanced communications capabilities are available in Honeywell's PPT Series of pressure transducers. These devices use an RS-232C link to receive configuration commands and to transmit pressure information. Like the P4100, the PPT transducers use digital correction and compensation to achieve high accuracy (0.05% of full-scale output). A unique attribute of the PPT units is that they let you choose any degree of "smartness" you want. You can use the device as a simple analog-output transducer. Alternatively, you can use it as an addressable digital sensor on the RS-232C bus, communicating with many other transducers. Or, you can configure it to be somewhere in between these two extremes.

One possible configuration is a user-configurable analog sensor, for which you can use the RS-232C port to issue commands from a PC. The PPT can store all configuration changes in an EEPROM, so that the PPT powers up with the latest parameters you program. A simple set of commands over the RS-232C line tailors the analog-output and pressure-span parameters to your application. The factory-set range of the PPT is 0 to 5V, corresponding to pressures from 0 to 20 psi. One possible configuration has an analog-output range of 1 to 5V for a pressure span of 12 to 16 psi (Figure 2). The beauty of this programmability is that the 0.05% full-scale accuracy still applies for any ranges of voltage and pressure (within specified limits) you set.

If you use the PPT in its completely digital mode, you have several programmable measurement options:

• Selectable pressure units: atmosphere, bar, millibar, inches, or millimeters of mercury; inches or meters of water column; kilograms per square centimeter; pounds per square inch; kilopascals; and megapascals.
• Programmable integration time from 167 msec to 4 sec.
• Tracking input changes: The PPT automatically changes its reading rate as a function of pressure changes.
• Skipping readings to reduce network communications traffic.
• Addressable networking (as many as 89 PPTs) on an RS-232C bus.

Temperature sensing is a natural for silicon ICs. The natural temperature drift inherent in various silicon structures provides a good starting point for building smart temperature sensors. Companies such as Analog Devices, Dallas Semiconductor, Linear Technology, and Telcom Semiconductor offer temperature-sensing ICs with varying degrees of smartness. For example, ICs from Telcom provide very basic decision-making intelligence: outputs that are either on or off, depending on the temperature.

Figure 3 shows the block diagram for Telcom's $3.39 (1000) TC623 temperature sensor. By connecting resistors from the low- and high-set terminals to the positive supply, you can program two independent trip temperatures. The trip points exhibit 2°C hysteresis, to eliminate "chatter" when the temperature is exactly at the trip levels. The $0.95 (10,000) Models TC622 (4.5 to 18V) and TC624 (2.7 to 4.5V) provide the same function but have only one set of terminal and trip temperature.

The two independent trip temperatures of the TC623 can help you design intelligent safety measures into equipment. For example, assume that you design a computer whose normal CPU operating temperature is 65°C and whose maximum allowable CPU temperature is 85°C. You can program the TC623's low-set input for a trip point of 70°C (5°C above normal) and the high-set input for a trip point of 80°C (5°C below maximum). Under normal operating conditions, the CPU's operating temperature never exceeds 70°C, and the TC623's outputs remain off. Now, assume that new circumstances, such as restricted airflow, cause the CPU temperature to rise.

When the CPU temperature reaches 70°C, the TC623's low-limit output switches on, and the system responds by reducing the CPU clock speed, thereby reducing dissipation. If the temperature continues to rise, more decisive steps are in order. When the CPU temperature reaches the high-set limit of 80°C, both the high-limit and control outputs become active. The control output starts a cooling fan, and the high-limit output even further reduces the CPU clock speed. At this point, the system might notify you that it's running hot. If the high-limit output stays on for a long period, the system might respond by powering down all devices but the computer's RAM.

The $2.50 (5000) DS1620 from Dallas Semiconductor also provides low and high temperature setpoints, and it adds a 9-bit digital-thermometer output. The device has nonvolatile storage for the low and high thermostatic settings. A three-wire interface communicates temperature settings and readings to and from the IC. Figure 4 shows a typical application for the DS1620. The three thermal alarm outputs are TH, TL, and TCOM. TH stays high if temperature equals or exceeds a user-defined temperature. TL goes high if temperature is less than or equal to another user-defined temperature. TCOM goes high when temperature exceeds TH and stays high until the temperature falls below TL.

Dallas Semiconductor adds some smarts to its temperature-sensor line by giving multidrop capability to the $2.77 (10,000) DS1820 temperature sensor. This wire is a three-pin, temperature-to-digital thermometer that gives temperature values as nine-bit words. Each DS1820 has a unique serial number etched in silicon (a 64-bit, lasered ROM), so multiple devices can exist on the same one-wire bus. The DS1820 can derive power either from its supply pin or from "parasite power." For parasite power, an internal capacitor stores energy derived from the one-wire communication line.

A three-wire temperature sensor is also available from Analog Devices. The $2.49 (1000) TMP03 (open-collector output) and TMP04 (CMOS/TTL-compatible output) provides a modulated serial digital output with a ratiometric encoding format. This format is immune to clock-drift errors that can arise with other serial modulation techniques, such as voltage-to-frequency conversion. As a final example of temperature-sensor ICs, Linear Technology's $3.94 (1000) LTC1392 provides a 10-bit digital temperature readout.

Microelectromechanical systems (MEMS), involving micromachining of silicon structures, represent a breakthrough development in sensor technology. Analog Devices was one of the first companies to exploit the technique in mass production with its ADXL Series of accelerometers, widely used in automotive air-bag systems. Its latest product, the $9.95 (10,000) ADLX05, provides a ±5g measurement range vs its predecessor ADXL50's ±50g. The devices qualify as smart sensors, because they perform a self-test upon receipt of a digital command.

EG&G IC Sensors also uses micromachining in its $66 (500) Model 3255 accelerometer. This two-chip device provides three acceleration ranges: ±50, ±250, and ±500g. The IC contains the machined sensor chip and a companion ASIC. You can implement a self-test feature by applying a voltage to the test pin. This voltage creates an electrostatic force that attracts the seismic mass of the sensor toward the top cap of the sensor structure, thereby simulating an acceleration.

Motorola, too, takes a two-chip approach with its accelerometers. Its MMAS40G family uses surface micromachining to yield a ±40g full-scale range. The company claims the device has a sealed sensor (g-cell) design, accomplished by using wafer-to-wafer bonding techniques. Its claim to smartness is a self-test feature that allows you to apply a test voltage to the g-cell's capacitor plates to simulate the movement that would result from deceleration. The MMAS40G devices cost less than $5 in automotive OEM quantities.

Another fertile area for smartness in sensors is light detection and measurement. Texas Instruments' TSL Series of light-to-frequency converters provides direct communications with a microcontroller. The $2.76 (1000) TSL230, designed for the UV-to-visible range, is programmable for light sensitivity and for output-frequency scaling. A 2-bit word programs the device for irradiance values of zero (power-down), 130, 13, or 1.3 mW/cm2. Another 2-bit word activates on-chip dividers that divide the output frequency by 1, 2, 10, or 100. The $1.75 (1000) TLS235 (UV-to-visible) and TLS245 (IR) also communicate directly with a microcontroller. e

You can reach Senior Technical Editor Bill Travis at (617) 558-4471, fax (617) 558-4470, e-mail b.travis@cahners.com.

References

1. Travis, Bill, "Smart-sensor standard will ease networking woes," EDN, June 22, 1995, pg 49.
2. Frank, Randy, Understanding Smart Sensors, pg 105, Artech House, Norwood, MA.