Sensors for medical applications must work at extreme temperatures, fit into tight spaces, and act quickly. The following looks at a few recent sensors and also reviews a new standard for analog sensors in digital networks.

Is that sensor lying? Calibration system tells

A portable sensor-verification system, Futek's VCal, is aimed at on-site verification, quick check, and calibration of load cells, torque, force, and pressure sensors. The system, from Futek Advanced Sensor Technology Inc., Irvine, Calif., (futek.com) provides 2 channels of ±20 bits resolution with ±4.5mV/V, ±15Vdc, or 4 to 20mA measuring range for sensors with mV/V, Vdc, or mA output. It also supports sensors with RS232 output. It uses a USB connection and is equipped with an upgradable 128Mbyte internal storage capability for test data, drivers, user manual, and software.

Verifying sensors at regular intervals helps prevent costly recalls of products that were out of spec because the sensor “lied.” Users can measure the severity of abused, mishandled, or overloaded sensors after verification and signature analysis. Easily analyzed and shared verification data minimizes or eliminates unknown factors between calibration periods. Users can determine calibration periods required by ISO, QS, A2LA or other programs with a valid verification data-history record. Having an on-site verification system for everyday checking rather than an annual event can free up time and cost by not having to purchase multiple instruments, and continuously sending sensors out for verification.

VCal also has 3 built-in internal shunt cal resistors and capability for an external shunt. The sensor detective also sports internal circuitry for onboard sensor impedance measurements. The unit supports sensors with built in auto-recognition chips for sensor identification or TEDS storage for 1451.4 compliance.

Sensors provide precise pressure management

A recent series of pressure sensors are aimed at applications that require precise pressure management. ASDX and ASDX DO sensors from Honeywell Sensing and Control, Freeport, Ill., (content.honeywell.com/sensing) cover pressures from 0 to 1 psi up to 0 to 100 psi. They are intended for use with non-corrosive, non-ionic working fluids, such as air and dry gases. Typical applications include medical equipment, HVAC controls, barometry, and pneumatic controls.

The pressure sensors are calibrated and temperature compensated with on-board ASIC. “The ASDX DO features a digital output, which translates into higher clarity, better response times, and ease of interface,” says Lynn Koch, senior product specialist. “The sensors digitally correct for offset, sensitivity, temperature coefficients, and non-linearity.”

ASDX DO sensors use I²C compatible protocol, which works with most commonly used microcontrollers and microprocessors without additional components or electronic circuitry. The two-wire I2C interface has a Serial Clock Line input and serial digital output data line. The device output is a corrected pressure value in hexadecimal format with 12-bit accuracy.

The ASDX series is amplified with an analog output span of 4.0 Vdc. ASDX and ASDX DO devices measure absolute, differential, and gage pressures. The absolute devices have an internal vacuum reference and an output proportional to absolute pressure. Differential devices allow application of pressure to either side of the sensing diaphragm and can be used for gage or differential measurements.

All devices in the series are accurate to within 2% full scale and are intended for operation from a single 5.0 Vdc supply. The sensors have standard DIP packaging with compact footprints and are designed and manufactured to ISO 9001.

One of four outputs from photoelectric sensors

A series of photoelectric sensors feature 4-in-1 output for simplified sensor specification and stocking, flexible side-mount or nose-mount options, and visible 360° LED indicators for status indication in all directions. Rather than stocking multiple sensors with different outputs, Tru-Vue sensors' 4-in-1 output options (NPN light on, NPN dark on, PNP light on, PNP dark on) let engineers specify and stock a single part for several applications. The sensors, from Pepperl+Fuchs, Twinsburg, Ohio, (am.pepperl-fuchs.com) are suited for material handling, conveyor, and assembly applications. Devices can be surface mounted, or as 18mm-cylindrical units are, by a threaded nose. A compact housing and barrel length makes them flexible enough to be mounted where conventional 18mm cylindrical sensors cannot.

Available sensing modes include background suppression, diffused, retro-reflective, thru-beam, and fiber optic. Multiple electrical connection options are available. Tamper-proof models are also available to eliminate adjustments by unauthorized personnel.

High-reliability sensors for harsh environments

Hall-effect sensors detect magnetic fields in harsh environments. The temperature-compensated units are suited for sensing motion such as identification of objects or counting rotations of gears. The basic Hall element relies on a magnetic field to sense motion.

OMH3075B and OMH3075S series from TT electronics Optek Technology, Carrollton, Tex., (optekinc.com) contain a Hallogic monolithic integrated circuit, which incorporates a Hall element, a linear amplifier, and Schmitt trigger on a single silicon chip. “The Hall-effect sensor is almost immune to environmental contaminants,” says Steve Coble, Product Line Manager at Optek. “These rugged devices are well suited for brushless dc motors, medical applications, robotic and heavy machinery sensing, or any application in a harsh environment where optoelectronic devices are not suitable.”

Supply voltage for the OMH3075 series is 4.5 to 24V, with up to 25mA of sink current in the “on” state. Output characteristics are constant at switching speeds from dc to more than 100 kHz. Operating temperature range is -55°C to 125°C. Custom designs and specifications are available to meet specific individual requirements. The OMH3075 sensors can be screened to test method per MIL-STD-883, Class B, and Class S.

The sensors have a gold-lead finish (a lead finish in hot solder dip can be provided) and are contained in a hermetic ceramic package. They are compatible with high-temperature (260°C) soldering processes.

Standard for analog sensors tells how to plug and play in digital networks

A recent standard from the IEEE, (standards.ieee.org) adds plug-and-play capability to analog transducers for use in networks intended for digital instruments and measurement systems. The standard, IEEE 1451.4, promises to accelerate the use of networked sensors by simplifying transducer installation, network creation, system maintenance, and upgrade.

IEEE 1451.4, “Standard for a Smart Transducer Interface for Sensors and Actuators - Mixed-Mode Communication Protocols and Transducer Electronic Data Sheet (TEDS) Formats,” creates a universal system for information digital networks need to identify, characterize, interface with and use signals from analog sensors.

“The standard helps tie the huge number of installed analog transducers into digital systems,” says Torben Licht, 1451.4 working group chair and product manager at Bruel & Kjaer. “The protocols it contains replace the diverse transducer solutions manufacturers had introduced with limited success. It will have a big effect by expanding the pool of network-compatible transducers and the use of control networks. It also gives sensor and actuator manufacturers the interfaces they need to deliver products for multiple instruments and networks without having to redesign for compatibility.”

David Potter, working group vice chair and platform manager at National Instruments, says the standard makes the TEDS concept a key innovation in IEEE 1451 standards, compatible with analog measurement. “Adding self-identification at the transducer-analog interface potentially makes any measurement system, analog or digital, easier to set up, configure, and maintain.”

The standard takes advantage of Eeprom chips embedded in sensors to house and communicate details needed for plug-and-play capability. Manufacturers find identifiers for chips from the Internet. A complete TEDS may contain sections for identification and properties for a type of sensor, such as accelerometer, microphone, strain gage, thermocouple, thermistor, and others. The TEDS may also contain a calibration template for the sensor.

For specialized sensor requirements, template-description files can be written by companies or users and published on Web sites. The standard also allows for virtual TEDS files that reside on the Internet for use when embedded memory is unavailable.

“Many companies are beginning to adopt the standard.” says Potter, “Eeproms are available for existing and new sensors, and several companies have begun shipping IEEE 1451.4-compliant products.”

The IEEE 1451 family offers common interfaces among sensors, actuators, instruments and networks that allow for interoperable and interchangeable transducers. IEEE 1451.1 addresses the overall network and how to link transducers in systems and networks. IEEE 1451.2 looks at how to place digital transducers on a network. IEEE 1451.3 is a multidrop standard that allows for placing many transducers on the same cable. The working group is now developing IEEE 1451.5, which is similar to 1415.3, but allows for wireless transducer communications.