Automotive gauge sensors: methods, pros and cons

The sensor emits an ultrasonic pulse that travels through the material, reflects off the far side and returns; the thickness is derived from the return time. The method measures the thickness of the part itself, not the thin paint layer on it, so it isn’t used in automotive thickness gauges.

The method is well suited to accurately measuring thin galvanic coatings — zinc, nickel, chrome plating and the like — in a range from a few microns to about 150 µm. For automotive thickness gauges it isn’t justified because of its cost and small range.

Well suited to measuring dielectric coatings (paint, clear-coat, primer, filler) over a non-ferrous substrate. It can be used on steel too, but accuracy is low and strongly depends on surface roughness and metal grade — realistically ±50–100 µm. That’s why on steel it’s only used by the cheapest gauges (priced under $40). The transducer frequency sets how deep the currents penetrate the metal. Combined with other methods, this type can work out magnetic filler and zinc. CARSYS gauges use it for measurement on non-ferrous metals and for detecting zinc and magnetic filler — together with other methods and advanced algorithms that substantially improve it.

Works by the change in magnetic induction in the sensor as metal approaches. It measures the thickness of coatings (non-magnetic dielectric and conductive) on steel well, with adequate linearity and a good measurement range. The downsides are relatively low protection from electromagnetic interference and temperature, and a large coil that makes a combined sensor difficult to build. Such sensors usually equip industrial gauges with a remote probe; in automotive devices they’re generally not used.

The principle is similar to parametric magneto-induction, but the sensor has two measuring coils, which raises accuracy and sensitivity. The first coil measures the thickness, the second is used for comparison and provides automatic adjustment to operating conditions, shielding against electromagnetic interference and magnetic fields. The sensor has good temperature stability and very high sensitivity, especially at small thickness, and doesn’t react to the direction of the Earth’s magnetic field (up-down, north-south). The downside is strong nonlinearity, but signal processing and advanced algorithms make the linearity of gauges on this sensor good. In automotive gauges, accuracy is what matters — and this is exactly the sensor type used in CARSYS gauges.

The sensor consists of a permanent magnet and a Hall element: as metal approaches, the magnetic field strengthens in proportion to the distance to the metal. The pros are small size and good linearity. The cons are strong temperature dependence of both the Hall element and the magnet, and the magnet can weaken over time. The Earth’s magnetic field has a strong influence: the sensor essentially works like a compass, adding to the reading toward north and subtracting toward south; in mid-latitudes this is more noticeable when measuring vertically or horizontally. Such sensors were once rarely used in gauges because of these downsides, but manufacturers have recently learned to handle the temperature drift and such devices have appeared. Spotting one is easy: metal and iron shavings stick to it, and readings change with orientation (north — south — up — down), especially at large thickness.