01 July 2004
Data acquisition getting smaller
By Gregg S. Gustafson and Linda J. Chapman
Improved monitoring, collection through miniaturization, standardization.
Advancements in miniaturization, standardization, and low-power electronics are creating a new breed of smart sensors in the monitoring industry.
Miniaturization of powerful computer chips and electronic parts has allowed the combining of data loggers and sensing equipment into small, self-contained units. These units contain sophisticated sensors, data loggers with nonvolatile memory, and long-lasting power packs. The internal software runs multiphased, variable-interval test sequences that collect highly accurate data. Digital output circuits control pumps and valves, raise alarms, and dial modems. Sophisticated communication software allows users real-time monitoring and control either from laptops and personal digital assistants (PDAs) in the field or from desktop computers at the home office via modems and the Internet.
Standardization of communication interfaces and protocols makes these devices easy to integrate into new or existing installations. The use of industry standard communication interfaces allows extensive sensor networking. These can easily connect to computers, handheld data collection devices, and a multitude of control and test equipment.
Low-power computer chips and electronic parts allow sensor and data logger units to easily run for a year or more on small, internal batteries. For long-term installations, they can run virtually forever on small, inexpensive solar panels.
Due to today's advances in miniaturization, one can easily enclose powerful smart sensors within housings less than an inch in diameter and only a few inches in length. A typical smart sensor includes a sensing element, an analog signal conditioner, an analog-to-digital (A/D) converter, a digital temperature sensor, a real-time clock, a controller logic unit, nonvolatile storage, and an internal power source.
The sensing element is the "eyes and ears" of the smart sensor. This sensing element might be for pressure, pH, conductivity, turbidity, or any number of other parameters. The output from the sensing element is an analog signal, which feeds into an analog signal conditioner. The signal conditioner cleans and filters the incoming signal and passes it on to an A/D converter. The A/D converter converts the analog signal into a digital signal for input to the central controller logic unit.
A digital temperature sensor acts as an additional touch point to the outside world, feeding current temperature data to the controller unit. Also, feeding into the controller unit is a real-time clock, enabling all readings to be date and time stamped.
The controller logic unit is the heart of the smart sensor. It is typically a tiny, but powerful, computer on a low-power chip. This unit performs four important functions. First of all, it is self-identifying, providing such information as the sensor type and location, as well as calibration factors. It also provides dynamic information such as recording status, memory capacity, and battery level.
Second, the controller logic unit executes specific application algorithms. For example, digital temperature information can combine with pressure sensor input to produce highly accurate pressure data. Programming within the control unit can be as simple or as sophisticated as needed to meet the needs of the system. For example, raw data can simply pass through for the software to use on the target computer or controller. At the other extreme, the data can run through elaborate algorithms, producing complex data output.
Third, the controller logic unit can also run complex, multiphased test sequences, which can alter themselves in response to the incoming data. Finally, the logic unit can trigger digital output circuits to control pumps and valves, raise alarms, and dial modems.
Nonvolatile memory serves as data storage for the smart sensor. This nonvolatile memory protects the collected data in two important ways. First, in the event of a power failure in the sensor unit, no collected data is lost. Once power comes back, the data is once again available. Second, the onboard data storage is a source of data redundancy for real-time monitoring systems. A typical monitoring system might receive and record sensor data at a central computer facility. Should that computer facility lose power or communication with the sensor for any reason, the smart sensor will continue to record the data. Once the power and/or communication with the central system returns, a user can retrieve the information.
The final component of the smart sensor is an internal, long-life power source, typically alkaline or lithium batteries. These batteries allow the smart sensor to be a truly self-contained unit. Alternately, they can serve as backup power if the sensor is using an external power source, such as a generator, battery pack, or solar panel.
A Frost & Sullivan study said "a lack of approved standard interfaces for smart sensor networking is a serious challenge."
Although there is no smart sensor standard, per se, in the market today, well-proven communication interfaces and protocols can easily integrate smart sensors into new or existing installations.
Industry standard RS485 serial communication interfaces allow the networking of up to 32 sensors on cables up to 4,000 feet. These networks can easily extend using repeaters, each repeater allowing the connection of another 31 units and another 4,000 feet of cable. The networks can further extend with wireless modems.
Just as the RS485 interface addresses the issue of a physical communication interface, a standard communication protocol such as Modbus can address software communication issues. Modbus, originally developed by Modicon in 1979, has become an industry standard for communication between intelligent devices. Its small packet size and cyclical redundancy check error detection make this protocol extremely reliable, even in rugged field conditions. The protocol is easy to program and interfaces smoothly with remote terminal units, programmable logic controllers, supervisory control and data acquisition systems, as well as computer networks, laptops, PDAs, and wireless modems.
Using such existing standards as RS485 and Modbus gives smart sensors reliable, time-tested methods of connecting to and communicating with the world.
No power drain
Today's electronics run with very small power draws. Furthermore, with the onboard microcontroller, smart sensors can go into a "sleep" mode, walking only to take readings or communicate with the outside world. Using low-power components and power saving strategies, a smart sensor can run for a year or more on small, internal batteries. For long-term installations, they can run on small solar panels.
Paul Saffo, director of the Institute for the Future in Menlo Park, Calif., said in his essay, "Smart Sensor Focus on the Future" that "the future is not going to be people talking to people; it's not going to be people accessing information. It's going to be about using machines to talk to other machines on behalf of people."
Smart sensors incorporating miniaturized electronics, standard communication interfaces and protocols, and low-power components already reach into that future in a realistic and powerful way. IC
Behind the byline
Gregg S. Gustafson is chief executive and Linda J. Chapman is in new product development at Kirkland, Wash.–based Instrumentation Northwest, Inc.