Lion Precision Variable Gap Capacitive Displacement Sensor Question

[CurtisSeizert]

I recently got an old (1985 date codes) system built around a pair of Lion Precision DMT-1s, and I am curious about what is going on in the probe. On the outside, the probe looks like the sort of thing that you would connect with a triaxial connector - the shell is earthed, and there is a circular sensor perhaps 5 mm in diameter surrounded by a guard ring. However, the probe connects to the module using a four-pin Bendix connector with the shell connected to the grounded chassis.

I attached a schematic because the driver module is so simple that it only took an hour or two to reverse engineer it. Some of the values are best guess if I couldn't get a good reading and the label was random letters. I omitted some of the decoupling for clarity, and based on the lengths of the connections to the decoupling caps, it probably is not doing a whole lot anyways. The layout is uninspired, even considering that they did it on two layers. There are guard traces around the inverting inputs that are covered in soldermask, and on one amp the power traces are routed under the feedback cap for example.

The analog out voltage is scaled such that 1 mil = 1 V. The readout gives three decimal places, so the LSD corresponds to 1 uin or ca. 25 nm. The LSD is reasonably noise free if the mechanics are stable. I took an fft and scaled the result to get nm/sqrt Hz and attached it if anyone is curious. 1 nm = 40 uV, so the mains interference is at least as bad as you'd expect.

Anyways, for pin numbering, I started at a point that seemed sensible and called it pin 1 then went around counterclockwise looking from the outside of the receptable. Pins 1 and 2 are driven by the secondaries of the transformer. I have labeled the shell as Pin 5, and that is connected to chassis ground. Pin 3 is connected to trim pot labeled "TC" through which it feeds an integrator that seems to set the drive levels for the NPN whose collector is tied to the primary of the output transformer. Pin 4 is connected to a trim pot labeled "GAIN" that seems to set a bias level.

The drive for pins 1 and 2 is a 1 MHz sine wave, +/-40V to chassis ground with the target remote, and about +/-5 V when the target is too close. They maintain the same amplitude through the range. Pins 3 and 4 have much smaller AC signals, around 100 mV. Pin 4 is biased to about -750 mV, and Pin 3 is around -250 mV. The drive gets smaller as the target gets closer. The output of U3 +12.5V, and it drops to about -2V when the target is too close. From probing the pins, it seems there is a PN junction between pins 3 and 4 and two junctions between pins 1 and 2.

Does anyone know what's inside one of these probes? It seems like there is a PN junction between pins 3 and 4 and two junctions between pins 1 and 2. There is enough room in the probe for a couple active devices or an IC, but probably not enough for a transformer as it's only about 4 mm high on the outside. It seems as though this sort of probe has higher performance than the triaxial type in the modern versions. If I were to guess, I would guess that these are similar or identical to LP's "first-generation" probes that are currently available. They have five pin LEMO connectors, so perhaps the shield is brought out to a pin. Those, with the modern drivers, have a resolution of about 30 ppm FS at 10 kHz and 5 ppm FS at 100 Hz. This driver probe combo is probably at least 20 times worse than that, but it would be interesting to see if it would be possible to improve upon this with a better driver.

[Poroit]

Maybe they are Capacitive Proximity Sensors.
https://www.rechner-sensors.com/en

[Conrad Hoffman]

We have a Lion system at work and I always assumed there was nothing in the probe except connections to the shield, ring and center. It's extremely sensitive but the working distance is near nothing and it's easy to get debris in the gap.

[branadic]

Capacitive proximity sensors have a center electrode, a driven guard electrode surrounding it and ground at the outer ring. By todays standard I would use some capactive to digital converters and add active guard as well, which is rather easy to do.

-branadic-

[CurtisSeizert]

The thing that was confusing me about this was the five connections, because, like Branadic mentioned, there are three connections for a capacitive proximity sensor. Obviously, the heart of the transducer are these three components, but I was curious about what else was going on in there that would necessitate five connections. A first guess might be that pins 1 and 2 would be the center electrode and the guard, respectively, and the cable shield (labeled pin 5) is earth. Pins 3 and 4 could just be the two terminals to a diode used as a temperature sensor.

I used a sharpened probe tip to pierce the thin layer of epoxy covering the electrodes on one of the sensors, and I updated the schematic to show what I could surmise of the probe structure from doing this. There are at least 3 PN junctions in there, which I have represented as diodes. The guard electrode has a DC connection to Pin 1 of the connector, which seems a bit odd, as that pin drives the rectifier. The center electrode is attached to the common terminal of a pair of series diodes. When I simulated this arrangement in LTSpice, it reproduced the behavior of the circuit in a general sense. The servo wasn't railed, the output went more negative with larger capacitance at the sensor, etc. Pins 3 and 4 don't seem to have any connection (besides parasitic capacitance) to Pins 1 and 2, so they are probably a temperature compensation diode.

I will say, this sensor/probe combo is not great on the TC front, and I could not adjust the TC pot to get a TC on the other side of zero, or, in fact, one that was discernibly different from what it was when I started, which was on the order of 1 um/K. The copper shim I used was about 400 um thick to get the gap right only accounts for about 0.7% of this with a linear CTE of 17 ppm/K.

One of the reasons I am interested in this is that the CDCs I am aware of for such an application don't have the bandwidth to be of much use in spindle metrology. The AD7747 has a maximum ODR of about 45 SPS, and the full scale range of 8 pF is a bit too high to be useful. To be centered in that range with a 3 mm diameter circular electrode, you would need a gap of about 15 um, and there would be a fair amount of current to the target at such a high capacitance anyways, especially considering the guard will add about the same amount of capacitance again. Looking at Lion Precision's catalog, a "fine" resolution 2 mm probe would have a mid range gap of 100 um, or about 278 fF capacitance to ground, and the RMS resolution is about 0.85 nm (2.3 aF near midscale) at 10 kHz. So the white noise density is better than 0.023 aF/sqrt Hz (it's about 0.019 aF/sqrt Hz comparing the 1 kHz and 10 kHz resolution), or about 3 orders of magnitude better than the AD7747. Even this old unit I picked up does better than 0.1 aF/sqrt Hz white noise density.

MicroEpsilon makes the triaxial type, which is the type of transducer Branadic described. These have a much wider temperature range (ca. -273 to +200 C) than LP's sensors, which are specified from 4-50C, as you would expect from something that is just an air dielectric capacitor. These aren't too far off the Lion Precision specs. A transducer with a 50 um range, like the one I described above has a 1 nm RMS resolution at 8.5 kHz. They don't list the near gap, but the sensor diameter there is 1.13 mm, so if the near gap:measurement range ratio is about what LP uses, the white noise density in capacitance units is comparable. The probe drift for these two cases is comparable, at 40 nm/K and 60 nm/K for LP and ME, respectively.