Originally Published MPMN July/August 2001

Product Update

Sensors Shrink in Size

Products benefit from increased sensitivity and resistance to interference.

Many medical devices must be able to accurately assess the conditions of their surrounding environments to function properly. To accomplish this important task, design engineers often incorporate any of a number of different sensors. In this section, several recent developments in sensor technology, including ultrasensitive superconducting magnetic sensors and a miniature motion tracker, are discussed that can help to ensure that your products function as accurately as possible.

SQUIDs provide detailed detection in magnetic monitoring systems

According to a report from Frost & Sullivan, SQUIDs may be what are needed to build better magnetocardiography (MCG) and magnetoencephalography (MEG) systems. Not multilimbed aquatic creatures, SQUIDs are the extremely sensitive magnetic sensors designated by an acronym that stands for superconducting quantum interference device. These sensors, also referred to as cryogenic magnetometers, consist of a superconducting ring with one or two weak switching points. The versatile devices can measure most physical parameters that produce a form of magnetic flux as small as 10^-10G including magnetic fields, magnetic susceptibility, and electric current or voltage. A typical anistropic magnetoresistive sensor can only detect fields as small as 10^-5 G. SQUID sensors work at cryogenic temperatures and come in two varieties: low temperature and high temperature. Low-temperature SQUIDs become superconductive at temperatures of –459°F, high-temperature units at –280°F.

While many applications could potentially benefit from this advanced technology, MCG and MEG systems have already started to profit from SQUID sensors. These magnetic monitoring systems use the sensors' high sensitivity to detect the small biomagnetic fields that accompany brain or heart diseases and muscle activity. Unlike CT, MRI, or other imaging systems that can perform similar tasks, SQUID systems offer the benefit of real-time functional imaging. Due to a variety of special concerns, fetal magnetic monitoring particularly stand to gain from SQUID technology. Superconducting magnetometers are noninvasive and thus allow the examination of fetal development at an earlier stage than other systems. "PET and MRI systems require radioactive isotopes or generate high energy fields, both of which can damage developing fetuses," says research analyst Daniela Carrillo. "SQUID systems, on the other hand, are passive devices that allow for safe monitoring at even early stages of development." According to the Frost & Sullivan report, the high temporal resolution of a SQUID system allows for a more accurate beat-to-beat representation of fetal heart rate than cardiotopography units. Though still in development, CTF Systems Inc. has constructed the first fetal MEG unit to use SQUID technology. The machine incorporates 151 sensors and has rendered promising preliminary results.

Magnetic motion tracker uses small sensor

A real-time magnetic motion tracker for medical and simulation applications uses a miniature sensor that allows precise navigation and guidance within the human body. The miniBIRD Model 500 tracking system from Ascension Technology Corp. includes an encapsulated sensor that measures just 10 x 5 x 5 mm. Designed for inclusion in laparoscopes, endoscopes, and probes, the sensor tracks position and orientation with six degrees of freedom. The sensor's signals pass through the body without attenuation and do not experience line-of-sight interference whether the unit is used internally or externally. "Because it uses pulsed dc technology, the magnetic sensor can be used in the presence of metals commonly found in clinical settings," says medical marketing manager Chris Perrera. Clinicians can use the sensor to perform computer-aided surgery, medical training simulations, biomechanical studies, and 3-D reconstruction of 2-D ultrasound images. Multiple sensors can work in combination to track multiple instruments and to provide differential readings of anatomical structures. An interface allows the sensor to connect to most host computers. A larger model, the miniBIRD Model 800, is also available with sensor dimensions of 18 x 8 x 8 mm.

Pressure transmitter links to process valves to form control loop

A programmable pressure sensor connects directly to process valves from most manufacturers to create a local intelligent control loop that reduces total system costs. With built-in intelligence and power systems, the Type 8326 pressure transmitter from Burkert-USA works independently of a PLC to complete a loop that the manufacturer estimates can reduce costs by as much as 35%. The transducer converts pressure into a proportional 4–20-mA output signal that is transmitted to the control valve, which adjusts its position accordingly. But this sensor has several other unique features. "The Type 8326 can simulate pressure outputs using real electrical signals to make sure the rest of the system is working correctly," says VP of marketing Andy Harris. "It can also be calibrated locally to reduce maintenance costs."

Accurate to ±0.15%, the Type 8326 sensor is suited for monitoring fluid pressure, fluid level, and pump and filter functions in pipes, technical equipment, and tanks. Standard pressure measurement spans 0–580 psi, but a turndown ratio of up to 1:20 allows the unit to measure a maximum pressure of 14,500 psi. The component is made of polybutylene terephthalate with stainless-steel wetted parts, making it suitable for use with fluids ranging in temperature from –22° to 221°F. Capable of storing up to 32 hydrostatic points via an optional LCD display, the sensor also has the capability to perform tank linearization. A range of fitting options allow the component to installed in new and existing systems. The sensor is available pretested and ready to install.

Cylindrical proximity sensors detect metallic particles

Suited for detecting metallic particles, miniature cylindrical proximity sensors from Altech Corp. are available with diameters of 3, 4, 5, or 6.5 mm. According to product manager Gil Yanishevsky, these products "are much smaller than other cylindrical sensors on the market, which typically have diameters of 12 to 18 mm." The sturdy sensors operate on a current of 10–30 V and have a maximum output of 200 mA. Frequency response attains 5000 Hz and acceptable operating temperatures span –25° to 70°C. The 3- and 4-mm units are 22 mm long, while the 5- and 6.5-mm units are available in a variety of lengths. The IP 67–protected sensors are constructed of stainless steel or nickel-plated brass. Most models are available with a choice of a fixed-cable or quick-disconnect outputs. All components feature an LED target indicator and can connect to a PLC. Typical applications include automated assembly and packaging lines. The sensors are CE marked and manufactured at an ISO 9001–registered facility.

Hall effect sensor works as switch

A Hall effect sensor from Micropac Industries Inc. functions as a switch that is activated by magnetic fields. In the presence of a sufficiently strong south pole field, the Mii P/N 65016 sensor turns to its On position. A corresponding north pole field resets the sensor to its Off configuration. The component is packaged in a hermetically sealed three-pin ceramic housing, making it suitable for harsh environments. An internal chopper-stabilized amplifier eliminates the input offset voltages normally associated with bipolar devices. This unique construction significantly improves operating point stability. Available manufactured to a variety of standards, the sensor can be included on motor controls, industrial switches, power supplies, and test equipment.

Level switch works in shallow tanks

A liquid level switch for high-volume applications can be used in shallow tanks and restricted areas. Constructed with polypropylene wetted materials rated for potable water systems, the LS-2 switch from Gems Sensors Inc. is 2 in. long and 1 in. in diameter. The device withstands pressures up to 150 PSIG and temperatures of –40° to 210°F. Available with M12 or 1/8-in. NPT mounting options, the switch has a fluted stem that prevents the formation of mineral deposits. Other features include 6-in. lead wires and a 10-VA switch. The unit's solid polypropylene float can be used in both water and chemicals with specific gravities as low as 0.85.

Zachary Turke

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