Completely wireless measurement of the voltage of objects

Imagine a satellite, floating in space. How would you measure its voltage? Yes, it probably has a non-zero voltage, a consequence of geospatial electric fields and ion and electron currents.

The question above is discomfiting for several reasons. We’re used to the idea that a voltage is the potential difference between points A and B; if the satellite is point A, where’s point B? We can possibly imagine a tremendously long wire attaching the satellite to a voltmeter, but where would you put the other terminal of the voltmeter? What would the measurement even mean? Is there an absolute voltage, independent of a reference point?

It turns out that there are some simple answers to these questions, rooted in basic electrostatics. In this article, we’ll unpack the physics, and then show some surprisingly pragmatic sensors and circuits for measuring the voltages of objects, without attaching any wires at all.

Physics of wireless voltage measurement

All conductive objects have some capacitance, which we can separate into self-capacitance, and capacitance with respect to other conductors. For isolated objects, the self-capacitance dominates; for a conductive sphere this is the textbook expression C = 4πεε0R, where ε0 is the permittivity of free space, ε the relative permittivity, and R the radius of the sphere. Human beings have a self-capacitance in the range 100-300 pF, which gives us enough energy storage to blow up CMOS chips, or ignite chemical fires, if we are electrostatically charged enough.

When conductive objects acquire charge q, their voltage changes: V = q/C. This answers one of the questions above—the voltage of the satellite is its total charge divided by its capacitance.

How can we measure the charge? Gauss’ Law tells us that for a given charge density on a conductive surface, there is a corresponding electric field perpendicular to the surface:

E = σ/ε n ̂

Where n ̂ is the unit vector normal to the surface; hereafter we’ll call the field magnitude E. The charge density depends on the total charge, which is spread over the effective surface area A of the object:

Σ = q/A

Our process for measuring the voltage emerges from the following:

Measure the field magnitude E perpendicular to the surface
This gives us the charge density
Extrapolate over the whole surface A to get q
Divide q by the (measured or estimated) self-capacitance C to get V

So, our satellite measurement problem can be solved by making an electric field measurement E perpendicular to the surface of the satellite and calculating V from that.

But where is the reference for this voltage measurement? A clue is in the relationship V = q/C. For a satellite in space, C is effectively the self-capacitance, which is often described as capacitance relative to a hypothetical hollow conducting sphere of infinite radius; that sphere is effectively our voltage reference.

In the case where the object’s capacitance is increased by a mutual capacitance to a nearby conductor, that object will affect both the local charge density , the E field magnitude, and the V reference accordingly.

Pragmatic voltage measurements on floating objects

Making good DC electric field measurements is surprisingly difficult. Notable 19th Century scientists such as Kelvin, Coulomb and Peltier all developed “electrometers” with varying degrees of success. Kelvin also invented an electrostatic generator—the “Kelvin water dropper”—which is worth a few minutes reading on Wikipedia for its sheer ingenuity.

Some reasonably good electromechanical sensors emerged in the 20th Century, but progress stalled there. We have fantastic MEMS sensors for acceleration, magnetic fields, pressure—almost every physical variable; but there are no commercial silicon sensors for precise DC electric field measurement.

The reason for this is a fundamental problem of packaging. Silicon sensors must be packaged to protect the chip from contamination, oxidation, and mechanical damage. If we package a DC E-field sensor in the usual plastic material, the material cover acquires and holds static charge from everyday contamination such as dust and airborne moisture, which usually carry ionic charge. And the contaminant charge affects the local E-field. If we package the sensor in a conductive material, the conductor shorts the field we are trying to measure.

This is an unsolved problem. The best sensors we have right now are all conventional electromechanical devices, in which an electrode is alternately covered or exposed to the field by a conductive shutter. This turns the DC field into an AC signal, and eliminates a lot of offset, drift and contamination sources because most of these create a DC signal that is not modulated by the shutter movement. The AC signal is then demodulated in phase with the shutter movement.

The two standard sensor types are the tuning fork, in which the electrode is mounted behind shutters mounted on the tines of a vibrating tuning fork; and the electric field mill, which has a rotating shutter on an electric motor, which alternately exposes or covers the sense electrode.

Figure 1 shows a miniature electric field mill, integrated onto the PCB of a sensing unit.

Figure 1 The shutter can clearly be seen above differentially connected cloverleaf-shaped sense electrodes. The object at right of the arrangement is a photodetector, which provides the position of the shutter to the demodulation circuits. The four leaves of the clover are connected into two pairs diagonally, so that each pair is alternately exposed or covered by the shutter. Source: Iona Tech

The actual signal detected is the movement of charge on and off the sensor plates, as the inducing field comes and goes. The plates are connected to an electrometer-grade op-amp for differential amplification, and then to a phase-sensitive demodulator (the demodulation can be done in a microcontroller).

The field mill sensor requires careful packaging, as nearby conductors will distort the electric field, and nearby insulators will collect static charge and ionic contamination, also distorting the electric field. In practice, we have found that covering the sensor with a carefully designed conductive grid will protect it, without affecting measurements more than can be accounted for in calibration.

Wireless control of ESD

The satellite example is fun, but what could we actually use this method for? At Iona Tech, we’re focused on the problem of preventing electrostatic discharge (ESD) damage in electronics manufacturing. ESD has been a problem since the adoption of CMOS technology in the 1980’s, and the method for tackling it mostly date back to that era—essentially, ground everything in sight. Ground the workers, ground the workstations, ground the machinery, and ground the components.

Because those methods occasionally fail—wrist tethers break, conductive floors lose their conductivity, and brush contacts fail—grounding is backed up with humidifiers and ionizers to neutralize static buildup at source. The biggest technical advance in the last few decades has been constant monitors for wrist straps, which measure the impedance load on the strap, and infer whether it’s being worn and operating correctly, or not.

These methods don’t work for everybody. They don’t work in situations where mobility is important—and for some reason, engineers in particular can simply not sit still. They are always moving from one workstation to another, or fetching more coffee, and if required to tether before and after each movement, they get annoyed. As one ESD manager told us, “The laws of physics apparently don’t apply to engineers.”

Other problem situations are aerospace manufacturing, where integrating a large satellite becomes a tether spaghetti nightmare. Then there is automotive industry, which assembles highly computerized cars on moving conveyor belts, around which tethers are a major occupational safety hazard.

Iona Tech’s CEO Daan Stevenson learned this the hard way. He was developing prototype autonomous UAV ground stations for a now-defunct UAV startup in Denver, Colorado. Working outdoors in the high and dry air of the Rocky Mountain foothills, he was continuously frying circuit boards; and he had only bad options for grounding, and none at all for humification or ionization.

He asked the question: what if we just measured whether people were becoming charged up, and set off an alarm if they were? With a wearable body voltmeter, we could have complete mobility and assurance of ESD safety, without tethers or any of the other paraphernalia.

This idle question led to a five-year quest for truly wireless control of ESD, which included a grant to study the problem from the National Science Foundation. The solution came in the form of the miniature wearable electric field mill shown below. These have been integrated into an Industrial Internet of Things (IIoT) module, called a StatIQ Band, that is worn on a strap on the upper arm, as shown in the image below.

Figure 2 The technician is wearing an ESD smock, which provides a conductive plane for measurement, similar to direct skin contact. Source: Iona Tech

Figure 2 shows a wearable body voltage monitor (an Iona Tech StatIQ Band) being worn on the arm of an electronics assembly technician. The device would not work well over a static charging garment material such as polyester. The electric field mill can be seen, protected by a minimal gold-plated cover.

Once you can measure your body voltage in a simple and convenient way, all kinds of applications become possible. You can evaluate everything in your working environment for the effect it has on charge generation—carpets, apparel, chairs, literally everything you come into contact with.

You can evaluate the effects of flooring and ionization. Having an alarm go off before you touch the electronics you are working on is a great way to avoid damage, but also creates a Pavlovian response, so that you automatically touch a grounded point before picking up the soldering iron or scope probe.

Figure 3 shows an example of measurements comparing a floating on-body voltage measurement with a conventional wired measurement using a charge plate monitor (a 3M 711 Electrostatic Voltmeter).

Figure 3 The subject was walking on a conductive floor, and then on a regular carpeted floor. The measurements indicate the difference in triboelectric charging in the two cases as well as illustrate the accuracy of the floating voltage measurement.

A further possibility with this type of device is to detect ESD events. When your body discharges into a circuit, its voltage drops dramatically. Detecting this high dv/dt event, and measuring its magnitude, gives us a direct measure of the energy absorbed by the discharge sink. This can be useful in making pass/fail/rework decisions for any electronics being handled when an ESD discharge occurs.

Other applications

Outside of electronics ESD, there are many domains in which detecting static voltage buildup is of real value. Fuel and chemical fires are often triggered by ESD, and the consequence are much higher than just a failed PCB. And then there is lightning, the biggest ESD event of all. We have often sat in our laboratory with a StatIQ Band propped in the window, watching the Rocky Mountain thunderstorms rolling in, and betting on the timing of next blast, based on measured field strength.

This technique is not only useful for human body voltage measurement. There are many cases where knowing the static buildup on a vehicle or aircraft is of importance. For example, fuel trucks have to pay careful attention to grounding and bonding when transferring fuel; the opportunity to generate voltage alarms or interlock the fuel pump if dangerous voltages are detected is significant.

In addition, being able to create a record of traceability of the charge or voltage history of an electronic package has great value in industries like aerospace manufacturing, where a single PCB can be worth $100,000 and may pass through the hands of several contractors while being produced and installed.

Measuring the voltage of a floating object seems like an impossibility, until the electrostatic theory is unpacked, and the necessary sensor is made available. When implemented, it opens up a world of interesting possibilities in instrumentation.

Jonathan Tapson is the chief technology officer (CTO) at Iona Tech.

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Completely wireless measurement of the voltage of objects

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