Electrostatic Discharge
Electrostatic Discharge in Medical Electronics: It's Not Just Wrist Straps Anymore
Douglas C. Smith
Hidden sources of electrostatic discharge, from hospital-room furniture to pocketed coins, can cause interference to sensitive equipment.
Like other electronic products, medical electronics are not immune to unexplained electrostatic discharge (ESD) problems that are becoming more difficult to identify and reproduce. ESD from personnel or metal objects can be a very potent source of electromagnetic interference (EMI) to electronic equipment both in the field and in the manufacturing process. And medical settings, which often include a large number of metal surgical instruments, can therefore generate a significant number of ESD events, disrupting equipment more often than other environments. The net result is that nuisance EMI as well as equipment malfunctions have been experienced in both the manufacturing process and in field use of medical electronics.
Standard ESD tests such as those in IEC 61000-4-2 were not designed to locate the ESD now frequently found in the medical electronics industry. Additional tests may be needed to reveal hidden sources of ESD. This article details some of the problems that have been encountered recently and discusses how to identify hidden and unusual forms of ESD that are difficult to correlate to equipment problems. Because the connection of ESD to equipment malfunction is often not obvious, the search for the source can sometimes be costly with little or no result.
The Medical Environment
The unique environment in which medical electronics are used leads to some very powerful and interesting ESD events. For example, consider hospital beds. The process of moving a patient from a canvas patient lift onto a bed and then withdrawing the lift back across the sheets generates tremendous static charge built up as a result of the two large surface areas sliding past each other. To complicate matters, the charge is held in the free-space capacitance of the bed's frame. A hospital bed's capacitance is much greater than the capacitance normally used for ESD event simulation.
Figure 1. Simple antenna for ESD event detection.
ESD events resulting from charged bed frames are so powerful that they have induced interference problems in nearby equipment that has otherwise passed normal ESD testing. Such problems are difficult to anticipate and reproduce in a laboratory. It has been reported that discharges from bed frames have even burnt holes in nurses' nylon stockings. Improved standards could help equipment designers avoid problems caused by this type of furniture discharge. Some medical companies have already developed internal standards for discharges from hospital beds to address this unusual problem. These events, however, actually represent the most common ESD events in medical settings, and because they are identifiable, they can be easily addressed. It's the hidden and unusual forms of ESD that present a potentially much greater problem for medical electronics.
Unusual and Hidden Sources of ESD
The primary problem with unusual and hidden sources of ESD arises from the difficulty of identifying ESD as the cause. Compare the following two scenarios:
- A nurse charged to a high voltage throws a 1-cm spark to a computer case as the computer is touched, and the computer locks up. Although the lockup may cause a serious problem, at least the cause of the problem is known, and, therefore, a solution can be implemented quickly.
- A person walks by electronic equipment jingling coins in a pocket. As the person passes by the electronic equipment, it malfunctions. Making any connection between the small, almost invisible ESD events taking place between coins and an equipment malfunction is all but impossible. The result is that considerable resources are often spent trying to identify the cause of the malfunction. Oftentimes the root cause may never be found at all because no evidence exists to readily associate the two actions.
Figure 2. Coins and batteries are two sources of hidden ESD events.
Another source of hidden ESD is the standard office chair, consisting of a central post with star legs at the bottom. Many of these chairs emit a multitude of ESD-induced EMI pulses for as long as a minute after someone rises from a chair. Others have reported on this type of interference, which has caused problems in many types of equipment, including lightwave transmission systems, air traffic control, and computer systems. 1,2
Figure 3. Test setup for ESD event detection.
It is important to note, however, that hidden ESD events not only affect equipment in the field, but can also severely affect the manufacturing environment. ESD can cause a test to fail on a properly functioning piece of equipment if the test equipment experiences upset due to either the ESD event or corrupted test data.
Hidden ESD Event Characteristics
Looking at the characteristics of hidden ESD events can help determine the reasons they are so problematic. It is critical to note that the number, the intensity, and the frequency range of an ESD event contribute to its effect on electronic equipment.
Jingling Coin ESD. Figure 1 shows a simple monopole antenna formed from a 15-cm length of stiff wire extending from the center conductor of a coaxial cable, whose shield forms the other half of the antenna. The small white blocks on the cable are ferrite cores that help define the antenna structure and prevent ESD-induced currents on the outside of the cable shield from reaching the test equipment.
Figure 4. Antenna signal generated by jingling coins.
Figure 2 illustrates two simple but potentially undetectable sources of ESD events. On the left, common pocket coins placed in a plastic sandwich bag simulate coins in a polyester pocket. On the right, two AA batteries in a plastic bag represent two small pieces of metal coming into contact in a pocket or purse. Metal pens could also have been used. The overall test setup is shown in Figure 3. The oscilloscope in Figure 3 is an Agilent 54845A running at a sample rate of 8 GSa/sec with a bandwidth of 1.5 GHz. The antenna can be seen standing vertically in front of the oscilloscope, which is displaying the result of an ESD event.
Figure 5. Antenna signal generated by batteries colliding in a bag.
Figure 4 shows data taken with the test setup of Figure 3 for the coins. Oscilloscope scale settings are 1 V per division and 2 nanoseconds per division. The bag of coins was shaken at a distance of about 15 cm from the antenna. The bandwidth of the signal is well beyond the 1.5-GHz response of the oscilloscope, extending to tens of GHz, but a peak value of 4 V was still recorded! Given the high speed of today's electronic equipment, such interference can produce devastating effects. Manufacturing test equipment is also at risk.
Figure 6. Test setup for measuring coin ESD at 2 m.
The number—as well as the intensity—of ESD events is also important. Jingling coins or other pieces of small metal can generate hundreds of ESD events in just a few seconds. A small AM radio exemplifies this effect. Place the radio, tuned to a clear frequency, near the bag of coins. When the bag is shaken, loud static can be heard in the radio. Each ESD event in the bag causes a "pop" in the radio's frequency. Because there are so many events, it appears to be a nearly continuous static. A large number of ESD events across a wide bandwidth increase the probability of interference with the operating frequency. Measured with a 5-GHz oscilloscope, the rise time of the induced voltage in the radio's antenna was limited by the 80-picosecond rise time of the oscilloscope!
Figure 7. Test setup for chair ESD.
When two pieces of metal—such as two batteries—touch each other, an ESD event is possible. Figure 5 shows the voltage induced into an antenna caused by the two batteries colliding in a plastic bag at a distance of 30 cm. The oscilloscope scale settings, as before, were 1 V per division and 2 nanoseconds per division. The interference caused by this event is similar to two metal pens touching in a purse. Again, the amplitude of the event was about 4 V. The pulse widths in Figure 5 are slightly wider than those in Figure 4 because the size of the batteries allowed them to radiate to lower frequencies than the coins did. These wider pulse widths indicate that the ESD event of the batteries is potentially more dangerous because it affects a wider range of equipment.
Figure 8. ESD event count from chair test.
The energy from jingling coins can propagate significant distances. Figure 6 shows a bag of coins about 2 m from an antenna typically used in electromagnetic compatibility testing.3 A peak reading of about 1 V was recorded by a 500-MHz oscilloscope. Although the actual value was likely 10 times the recorded size, the bandwidth of the oscilloscope limited the visible signals. Observed effects on equipment have occurred at 2 m, the distance used in this test.
Chair ESD. The simple office chair can also be a source of ESD that can cause significant field problems. Figure 7 shows a test setup using a Lucent T-100 ESD event detector with a wire antenna placed near a typical office chair. The test required only that a person sit in the chair and then rise. The single sitting and rising action resulted in 15 events as shown in Figure 8.
Typical amplitudes of such events as received by an antenna are similar to that in Figure 1. Scope scale factors were 500 mV per division and 2 nanoseconds per division. The antenna was held vertically about 40 cm from the chair. The amplitude displayed was over 2 V and was limited by the 500-MHz bandwidth of the oscilloscope used.
As with the ESD generated by jingling coins, the chair's ESD occurs from a combination of the large size of the amplitude of the events and the number of them over a short period. Chairs that exhibit this effect normally produce about a dozen events over the first 10–15 seconds after a person rises from the chair. Some have even produced hundreds of events as long as a minute after a person rose from the chair!
Chair ESD similar to that described here has caused problems with a wide range of electronic equipment, including medical electronics, partly because a chair's size enables it to generate electromagnetic radiation to a much lower frequency than coins or other small pieces of metal. About 33% of chairs tested exhibit ESD effects.
A small AM radio can also be used to test chairs for internal ESD events. Place a radio, tuned to a clear frequency, under the middle of a chair near its central post. ESD events will be heard as "popcorn" noise from the radio when a person rises from the chair. The popping noise indicates that the chair is likely to affect any electronic equipment within a couple of meters.
Conclusion
Can a source of interference that generates volts of signal in short wires cause problems? Of course it can, and multiple events exacerbate the problem because the individual ESD events are not discernible. Unexplained field equipment problems can often be linked to such ESD events. The key to determining the capacity of these ESD events to cause problems is to identify their ability to generate significant signals in nearby conductors. The number of events also contributes to the level of interference. Unfortunately, identifying equipment malfunctions often leads to the expenditure of significant resources to track factory or field problems, which often do not exist, but rather the problems have been caused by hidden or unusual ESD.
As equipment speeds increase, so does the potential for more ESD-related interference problems. Equipment design, especially for safety-critical devices such as medical electronics, will require examination beyond the currently recommended ESD tests performed with simulators such as those specified in IEC 61000-4-2.
References
1. Douglas C Smith, "A New Type of Furniture ESD and Its Implications," in Proceedings of Electrical Overstress/Electrostatic Discharge Symposium (Rome, NY: ESD Association, 1993), 3–7. This paper is available for download.
2. Yasuo Tonoya, Masashi Ono, and Masamitsu Honda, "Impulsive ESD Noise Occurred from an Office Chair," in Proceedings of Electrical Overstress/Electrostatic Discharge Symposium (Rome, NY: ESD Association, 1993), 9–16.
3. DC Smith, "Unusual Forms of ESD and Their Effects," in Proceedings of Electrical Overstress/Electrostatic Discharge Symposium (Rome, NY: ESD Association, 1999), 329–333. This paper is available for download.
Douglas C. Smith is founder and president of D. C. Smith Consultants (Los Gatos, CA). He can be reached at doug@dsmith.org. Smith conducts seminars on a variety of EMC and ESD topics.
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