Back To Basics: Eddy Current Sensors

Posted under Back to Basics, Technology on March 1st, 2026 by

I was doing some research for a client project this week. The solution eventually needed some eddy current displacement sensors. It’s the kind of tool you forget until contact measurement starts to become a problem. Let’s discuss that this week.

An eddy current displacement sensor is basically a coil driven with a high-frequency AC signal. That alternating field induces circulating eddy currents in any nearby conductive metal. By Lenz’s law, the magnetic field produced by those eddy currents opposes the probe’s field. For the same drive current, the probe coil stores less magnetic energy because some of the field is being pushed back. From the coil’s point of view that shows up as a drop in effective inductance, so the inductive reactance term falls. At the same time, energy is dissipated as I²R heating in the target, which the coil feels as added loss, so the effective resistance rises.

The probe electronics drive the coil at a fixed frequency and measure the resulting voltage/current amplitude and phase, infer the complex impedance (R+jωL), and then use a stored calibration curve to convert that impedance change into a distance output.

These sensors are used when you need reliable distance or vibration data in harsh conditions. Like maintaining tip clearance, predictive maintenance on rotating equipment cases, measuring shaft runout etc. They’re popular because they’re non-contact, fast, and can work in all the worse places with oil mist, dust, coolant spray, and even high-temperature scenarios.

The biggest tradeoff is that they only see conductive targets, range is usually a few mm (to low cm), and readings shift with material, geometry, EMI pickup, and temperature, so mounting, shielding, and calibration matter in real machines. Most companies providing the sensors, also do give the matching drivers with temperature calibration as a full set.

If you plan on using multiple of these units, please space them out so that one sensor’s field doesn’t affect the field of the other. Some systems use different drive frequencies for adjacent units.

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Back to Basics: Auto Balancing Bridge Circuits

Posted under Back to Basics, Educational on February 15th, 2026 by

I was reading up for a client project on impedance measurements. While digging through, I ran into the auto-balancing bridge circuit used inside many LCR meters, and it’s worth understanding.

In the simplest form, impedance is just Z = V/I. You drive the DUT with a sine, measure the voltage across it, measure the current through it, and then divide. The problem is that an AC ammeter is never ideal. Its input impedance, wiring, and stray capacitance start affecting what you measure.

The auto-balancing bridge avoids measuring current directly. Check the images. Circuit forces the DUT current Ix to flow through a known range resistor Rr using a feedback amplifier. The Low terminal is driven to a virtual ground, close to 0V. Now the current becomes a voltage Vr across Rr, so Vr = Ix * Rr. You also measure Vx across the DUT. Put those together and you get Zx = Vx / Ix = Rr * (Vx / Vr). The instrument is really doing a vector ratio(Vx / Vr).

This setup would use two vector voltmeters, one for Vx and one for Vr, then take the ratio. The catch is there can be mismatch between meters. So many LCR meters instead switch one voltmeter receiver between Vx and Vr, so the same front end measures both and tracking error largely cancels.

Below about 100 kHz, an op-amp transimpedance stage can hold the Low node near 0V and convert Ix into Vr. Above that, bandwidth limits and parasitics make the balance drift. So instruments add a null detector that senses leftover error current, split it into 0° and 90° parts, and drive a vector modulator to tweak amplitude and phase until the error goes to zero. That closed-loop correction is the auto in auto-balancing. BTW, its called auto-balancing because the instrument doesn’t rely on you to manually “balance a bridge” like old Wheatstone-style bridges. It uses a feedback loop that continuously drives the error toward zero on its own.

You use this circuit for getting your impedance U curves for capacitors, ferrite beads etc to a particular freq limit. For very high freqs (GHz), you often switch to network analysis/VNA methods.

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