Feature Article

By: S. Oxley, R. Cronan, D. Winkler, M. Torres, TT electronics plc, TT electronics Components Group

Enabling technologies

The medical technology industry thrives on innovation. Not only are medtech manufacturers on a constant quest for next-generation devices that push the boundaries of what technology can do, but they are equally driven to develop products that give physicians the means to provide precise, reliable diagnostics and reduce the need for invasive exploratory surgery. Developments in sensing technology, such as high-voltage, optical, position and current sense devices, help to enable these advances, especially in the diagnostic, imaging, laboratory, patient care, and medical instrument areas.

 

Medical systems present a range of high-voltage sensing requirements from the 5 kV level used for defibrillation to 70 kV for X-ray generation. It is important to control these voltage levels to a high degree of accuracy and stability. The accuracy and stability depends on the characteristics of a high-ratio resistive voltage divider, which can sample the high voltage and feed back a proportional control signal at a voltage level compatible with ADC inputs. But there are other characteristics beyond the voltage rating that are critical for medical applications.

 

For example, portable defibrillators require a physically small part because of the compact size of the finished unit. Also, the device deploys in a wide range of environments with uncontrolled climates, so resistance to high humidity is essential. Since defibrillation dose control is subject to a tight error budget, resistive sensors must have excellent linearity, expressed by voltage coefficient (VCR) and temperature coefficient (TCR), and long-term stability under voltage stress. Finally, the quality and reliability levels associated with emergency medical equipment are essential. The use of high-voltage chip and planar resistors has enabled several types of automated external defibrillators to meet this combination of requirements.

 

At the upper end of the voltage range for an X-ray head, extreme voltage levels must be withstood without discharge or leakage. Low-outgassing coatings should be used where compatibility with oil-filled assemblies is required, and special termination options such as screw contacts can be used to avoid the high electric field concentration associated with the small-radius surfaces of conventional wire terminations. An ideal product for this application is the T48 resistor, which has a 100-kV rating in oil.

 

Optical sensors and LEDs offer reliable and cost-effective sensing technologies for increasingly complex medical devices. Along with the ability to sense position or object placement at very high resolution, optical switches are a “touchless sensing” device and have a reliability advantage over other technologies because the device’s lifetime is independent of its mechanical usage. Self-calibration modules are available to enhance sensor capabilities or to help the user design an effective interface solution for the device.

 

Examples of medical equipment using optical sensors include defibrillators; modular infusion and monitoring systems; electronic ambulatory devices; infusion pumps for hospitals; fluid management pumps; angiographic monitors; urinalysis devices; automated specimen handlers; secure drug dispensing systems; anaesthesia dispensing stations; blood, gas and electrolyte analysis systems; patient oximetry probes; glucose monitors; electronic scalpels; light therapy lamps and LED arrays; and workstation illumination systems.

 

Optical device functions in these medical products include object in place verification; material dosage measurement; liquid level detection; blood oximetry measurement; bubble detection in fluid tubing; high and low voltage circuit isolation; fluid flow rates; skin-care therapy by controlled infrared and visible light exposure; backlighting for X-ray and MRI images; and workstation lighting.

 

Current sensing resistors use a simple application of Ohm’s Law (Voltage = Current × Resistance) to determine current draw. Resistors provide a predictable and temperature-stable linear relationship between voltage and current in a package that manufacturers are accustomed to specifying, purchasing and assembling. The voltage drop across the resistor usually provides a voltage signal to an integrated circuit (IC). Manufacturers of medical electronics use current sense resistors to ensure safe and efficient operation of the battery packs for portable diagnostic and treatment equipment. Current sense resistors in the battery pack management circuits of professional-grade portable defibrillators are one example of this application. These battery packs are capable of high current output, and must be monitored to ensure safe operation and a reliable charge state.

 

Current sensing ensures safe operation by monitoring the current level during the charge cycle of the battery. Excessive current levels during battery charging may indicate a battery malfunction or a short circuit in the charging circuit. If the IC senses a high voltage in excess of pre-determined limits, the module may be shut down to protect the safety of the users and avoid destruction of the equipment.

 

The battery charge state may be monitored by tracking the net amp-hours of battery use. During discharge of the battery, the IC may monitor the current levels (amps) and time of discharge, and therefore calculate discharge amp-hours. A similar calculation is available during charging periods. By tracking the net amp-hours of use (the difference between discharge amp-hours and charge amp-hours) and comparing this figure to the capacity of the battery, a simple measure of the remaining charge of the battery is provided.

 

In the field of medical diagnostic equipment, proper control and analysis requires input from sensors that are near the action. One example of sensor functions that potentiometers serve is that of position control and sensing. Potentiometers are used in a range of patient care equipment from table position controls for X-ray, CAT and MRI machines as well as position controls for dental chairs and hospital beds.

 

One potentiometer design includes a spring-return, linear actuated, conductive plastic position sensor, which combines a rugged housing with a proven ceramic substrate to provide a workhorse miniature position sensor for medical automation applications. The sensor exhibits infinite resolution and a long life of five million actuations. This robust design eliminates the need for direct coupling in medical applications, making the sensor ideal for space-limited applications.

 

Other potentiometer designs suited for medical equipment are Hall-effect, noncontact, high-accuracy, precision devices with extended life cycles. Such position sensors often feature extremely low mechanical torque with a programmable electrical angle up to 360°. The versatility and rotational life coupled with high accuracy and resolution as well as multiple factory-programmable output profiles ranging from linear to nonlinear make Hall effect sensors ideally suited for three-wire voltage divider applications.

 

The aforementioned assemblies are but a sampling of the range of sensing technologies that are increasingly deployed in medical devices. Because of their stability, accuracy, precision and reliability, these technologies provide an optimal solution for portable, hospital and at-home medical equipment. Engineers are continually searching for ways to improve products while reducing cost and time-to-market. Working with a manufacturer capable of offering both standard and custom high reliability, technologically advanced sensing solutions ensures that all application-specific needs are met. 

 

M. Eng., C. Eng., MIET, is Senior Applications Engineer

is Market Development Engineering Manager

is Business Unit Manager

and

is Fixed Film Product Manager

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