Why human body voltage is important in managing ESD and how to measure it
Our ability to measure the charge accumulated by a human being is a critical part of managing electrostatic discharge (ESD) in industrial environments. Unfortunately, these measurements require that the person involved is connected by wire to a high-impedance or electrostatic voltmeter. This is fine in the laboratory but severely limits our possibilities on the factory floor. In this article, we describe how wireless measurements of human body voltage can be made and why they are important for ESD managers.
It’s been a long time since any of us had to untwist a telephone cord to put a handset back in the cradle. But we still seem to have an awful lot of cables and wires to contend with on the factory floor. Wi-Fi networks and IoT sensors are reducing the clutter, but it seems as though there are some domains, particularly in managing ESD, where we still have to have a physical wire connection to remove charge or make a measurement. So we’re going to question one of the sacred cows of electronics by asking: do we really need wired connections to measure the voltage on a person’s body?
Controlling human body potential and static charge is a key part of any ESD control program, and actually measuring body voltage is how we validate these programs. For example, the ANSI/ESD STM 97.2, Floor Materials And Footwear – Voltage Measurement in Combination With A Person, requires a “walking test” in which an operator walks on the floor while holding an electrode connected to a charge plate monitor (effectively, a very high impedance voltmeter). The operator’s voltage is recorded while they walk, and the limits of body voltage determine whether the floor is compliant or not.
The ANSI/ESD standard itself illustrates why wired measurement is a problem here. The operator can’t actually walk because they’re wired to a fixed instrument. Instead, they perform a statutory set of steps in place (colloquially known as the “ESD Shuffle”) which look exactly like a 1950s dance class, right down to the numbered foot movements (Figure 1). To be fair, the test is also designed to qualify small areas of flooring, but many ESD managers would much rather have a method and hardware that would allow them to properly qualify large areas of floor by walking around.
If we can measure body voltage wirelessly, it opens up a wide range of possibilities, where we can continuously monitor the voltage of all our personnel across the entire factory environment. This enables real-time detection of ESD hazards such as non-conducting floors and static-generating equipment. The data gathered on ESD performance becomes a new and useful input into statistical process improvement methods such as Six Sigma and Lean Manufacturing.
Wireless Voltage Measurement
The first point to make about wireless voltage measurement is that we’ve been using it for years in the form of surface (electrostatic) voltmeters or field meters. These voltmeters measure the electric field generated between the measured object and the voltmeter and convert it to a voltage measurement. However, the voltmeter itself usually requires a ground reference of some kind. But it’s possible to simply turn the voltmeter around – we can mount it on the object to be measured and derive the voltage from there.
The key to this type of wireless voltage measurement lies in understanding the relationship between charge, capacitance, electric fields, and voltage. We remember from basic electrostatics that all conductors have an intrinsic self-capacitance. For example, a conducting sphere has C = 4πrε, where r is the sphere radius and ε the permittivity of the surrounding insulator (usually air, for which we approximate ε0, or the permittivity of free space). Human beings have a self-capacitance. The standards use 100 pF as a generic value, but most people’s self-capacitances are significantly larger (we’ve measured 231 and 205 pF, respectively).
When a conductive object or person acquires charge q (by shuffling feet on a carpet, for example), their voltage changes accordingly: V = q/C. This charge distributes itself across the surface of the conductive object, with some dependency of charge density on geometry. The surface charge induces an electric field that is perpendicular to the surface and proportional in strength to the charge density, as follows:
where E is the DC electric field and A is the area over which the charge is distributed.
So, if we can measure the electric field E at the surface, and we can estimate the area A and self-capacitance C, then we can approximate the voltage as follows:
Note that the area and self-capacitance of a conductor tend to be proportional, so errors in estimation tend to cancel each other out here.
Practical Wireless Body Voltage Measurement
We have implemented this method by using a novel wireless miniature electric field mill to measure the DC electric field mill on a person’s body. This sensor is about half the size of a pack of cards and weighs less than 50 g (see Figure 2).
We have chosen to wear the sensor on a hook-and-loop strap on the upper arm. The upper arm is chosen because it is close to the bulk of the body, and the sensor is characteristically pointing out. If the sensor were mounted on the wrist, say, there could be two problems. The first is that as a body extremity, the wrist is less likely to reflect a charge density representative of the average body charge density. The second is that if the wrist is tilted so that the electric field sensor points towards the chest, the field measured will be that of the interior of the conductive surface, i.e., close to zero.
Figure 3 shows the sensor unit in use on a wearer’s arm. The unit is enclosed in a conductive housing and is connected to the wearer’s skin in one of two ways: either it is worn with a short-sleeved shirt so that the base plate is directly contacting the skin, or it is worn over a conductive smock so that the surface of the smock effectively becomes the conductive surface of the body.
Figure 4 shows a comparison of the body voltage measured using the wireless sensor and a simultaneous measurement made with a gold-standard electrostatic voltmeter (a 3M Model 711 Charge Analyzer). It can be seen that while the dynamics of each signal are somewhat different, owing to different noise filter time constants, the signals are essentially identical.
The differences between the wireless measurement system and any conventional electrostatic voltmeter are typically of the same order as the differences between any two conventional electrostatic voltmeters. This is not surprising given that all of these systems are using the same basic sensors (either rotating electric field mills or vibrating non-contact electric field sensors). In the case of electrostatic voltmeters, the sensor is pointed at a plate that is at approximately the same potential as the user. In the case of our wireless measurement, the sensor is mounted directly on the user and is pointed into free space.
The astute reader will be wondering about the assumptions that are necessary for this method to work. Briefly, they are:
- The field perpendicular to the body at one point should be representative of the average field induced by the average charge density on the body;
- The charge density under the sensor is representative of the charge density of the whole body;
- The field induced at the point of measurement is not significantly affected by external sources of charge; and
- The total capacitance of the body does not increase significantly from the self-capacitance.
All of these assumptions are valid under a wide range of conditions of interest. The most commonplace departure from them is when there is a strongly charged object in close proximity to the user. (This is most often another person, ungrounded, wearing a synthetic fiber garment; or a piece of office furniture, such as a chair, made with insulating plastics.)
In practice, we are finding that wearing appropriate ESD garments is the single biggest factor in achieving accurate measurement. A person clad in an ESD smock and hat can achieve errors of less than 10V when compared to charge plate analyzer voltmeters. Cotton apparel also gives good results, with errors of typically less than 100V. Not surprisingly, if the device is worn over synthetic apparel, the accuracy degrades significantly, with errors up to 1000V being possible.
The second most common source of error is a charged object in the environment. We have used this feature to “sanitize” our workplace for ESD, as it very quickly reveals the location of any charge. Simply hold a ground point in one hand and then rotate until the device shows a non-zero voltage, at which time the device will be pointing at the errant source of charge. However, there are some sources of charge you simply can’t control. For example, we have found that if you use the device while standing next to a picture window with a thunderstorm brewing outside, the strong atmospheric electric field can create a small error in measurement!
Given that the measurement is intrinsically wireless, it makes no sense to interface it to a PC or similar display except by wireless network. The device has a Bluetooth network interface, and we are able to view the data on a mobile phone app or via a browser on a Bluetooth-enabled PC. This makes mobile use of the system within the workplace very simple.
A novel application of this system is that, because we have a real-time measurement of the body voltage, we can detect when a static charge has been discharged from the body into an object (such as a PCB). We do this simply by detecting sudden reductions in body voltage, which allows us to establish a rate of voltage change dV/dt, which is unambiguously a discharge to ground. Figure 5 shows such an event displayed on a mobile phone app. We can measure the actual magnitude of the discharge in voltage terms, allowing an ESD manager or production manager to make an educated guess, on the spot and in real time, as to whether the system which received the discharge should be assessed for ESD damage.
The ability to wirelessly measure and log the voltage of people and objects in the workplace opens a host of possibilities for ESD control. The obvious applications are to monitor the effectiveness of current ESD mitigation strategies and also to enable ESD safety in cases where continuous grounding is difficult or impossible. In addition, the same physics that applies to personal voltage measurement also applies to objects (with some adjustment factor for self-capacitance).
For example, carts and trolleys, which are the bane of many an ESD manager, can be actively monitored, with alarms generated when they exceed a safe voltage level. In the chemical transport industry, the presence of static on vehicles is an ever-present combustion hazard, and the ability to measure vehicle static levels could be a considerable boon to that sector.
We are in the infancy of understanding this measurement technique, but the sky is the limit (literally – airplanes have their static buildup issues too!).
Jonathan Tapson is the Chief Technology Officer at Iona Tech, and can be reached at jon@ ionatech.com.
Daan Stevenson is the Chief Executive Officer of Iona Tech, and can be reached at daan@ ionatech.com.