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RF modules enable wireless DAQ

Posted: 22 Aug 2005 ?? ?Print Version ?Bookmark and Share

Keywords:sri pmd? rf sensor? data-acquisition? daq? rf module?


SRI PMD has been providing RF sensor telemetry for test-and-measurement and data-acquisition (DAQ) applications for quite some time. The company's Wireless Link line of products provide realtime connectivity between sensors or transducers and instrumentation that can be separated by distances of about 500 feet.

The company's new FCC license-free product is especially useful in applications where conventional wiring is difficult or even impossible. Using SRI PMD's wireless system, you can monitor temperature (using Type J or K thermocouples). You can also monitor signals from strain gauges, using 1-arm, 2-arm or 4-arm bridges. Up to 5V of DC excitation is provided.

You can also use the system with pressure sensors, load cells, and other types of transducers. The new Series 500 looks like it could be just the ticket for locations where hard-wired interconnects or slip rings can't be used.

Mixed sensors
Significantly, SRI PMD's single general-purpose design lets this equipment be applied to a variety of sensor configurations, including those using multiple sensors and mixed inputs. Also, the systems ST-500 transmitter module, measuring 2-by-1.5-by-0.8-inch in size, can be battery powered.

Typical current demand is less than 35mA, and either 3V or 9V battery power can be used. You should be able to site these transmitters quite easily on an as-needed basis.

The ST-500 transmitter will readily accommodate both static and dynamic signals, too. In use, the transmitter's inputs will also accept single-ended as well as differential signals that can be uni-polar or bi-polar. Full-scale ranges are selectable from 2mV to as high as 5V.

The transmitter hardware also has a bandwidth response to at least 4KHz, so even relatively fast sensors can be accommodated. Data is transmitted using FSK (frequency shift keyed) modulation.

Smart transmitter modules
Encapsulated in thermally conductive epoxy, the ST-500 transmitter includes an on-board microcontroller that can crank through 30-million operations/s. Firmware supports the various sensor types, and programming of gain and offset values.

An EEPROM is used to store sensor configurations. The non-volatile and re-programmable EEPROM lets the controller call configuration settings, and change input-offset and gain parameters. The EEPROM also controls the sampling frequency for each sensor input. You can therefore trade off transmission distance and noise immunity with sensor sampling rates.

Sensor channels can also be selectively de-activated to increase the sampling rates for more critical measurements, for example. Finally, an on-board temperature sensor, combined with primary power voltage detection logic, also lets the ST-500 compensate for measurement errors associated with temperature or voltage drift.

Receiver operation
On the receiver side, the system delivers analog voltage outputs, as well as digital outputs that communicate across the system's 12Mbps USB link, as referred to in the company's release notes.

The SR-500 receiver automatically acquires ST-500 signals, supporting reception from as many as eight transmitters. Its analog outputs are range-selectable for 0 to 5V, 0 to 10V, 5V and 10V.

The SR-500 also has the smarts for some data processing. It can perform signal compensation, can do signal averaging, and can run IIR-type (infinite impulse response) filtering.

A digital architecture
The digital architecture of the Series 500 Wireless Link Sensor Telemetry System lets it automatically compensate for steady-state or dynamic errors that might be introduced by external or internal sources. The system also lets you emphasize either frequency response or resolution, using switchable 8-bit or 12-bit modes.

In its 8-bit mode, the system can sample at the maximum sampling rate. In 12-bit mode, it will sample at aggregate to 9.5kS/s and higher.

The system can also be programmed to operate in a number of acquisition modes. These can include a high-speed continuous mode, a sequential mode, and a Sleep/Burst mode. Moreover, the RF deck is digitally programmable for transmit power.

Software support
As the company's press release mentions, SRI PMD supplies software support that runs under Windows 95, Windows 98, Windows 2000, and Windows XP PCs. The software supports storage and retrieval of acquired sensor data and helps during set-up and calibration.

Significantly, in many applications it's possible that the software can eliminate the need for any additional data recorders or stripchart recorders, and the like. Using the software, you can capture measurement data, alter operational parameters of a system-under-test, and do immediate analyses.

- Alex Mendelsohn

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