Description

Learn how to measure stamped sheet metal thickness with MTI’s Digital Accumeasure, a high-precision capacitance measurement system with nanometer accuracy and stability.

Problem

High silicon steels are alloys of iron and silicon that are easily magnetized and demagnetized. They can be supplied as sheet metal with a very thin dielectric coating. As this application note explains, precise measurements of stamped samples with a dielectric coating of ~ 10µi were needed.

Solution

MTI’s Digital Accumeasure uses capacitance to measure thickness. This non-contact measurement system converts a capacitive electric field measurement (displacement) directly into a highly precise 24-bit digital reading. MTI’s Digital Accumeasure features a digital amplifier and probes with capacitance sensors.

There are two ways to use capacitance to measure the thickness of stamped sheet metal samples. The first method uses a single element probe and a grounded plate. The second uses two differential probes.

Method 1: Measure Stamped Sheet Metal Thickness with Single-Element Probe

The first method places the sample on a grounded plate and calibrates the probe to a subsequent sample in the gap, as shown in Figure 2. The sample must have a known thickness that includes the dielectric coating. All subsequent measurements are then traceable to this sample’s accuracy. The disadvantage of this method is that the sample must be held firmly against the grounded plate to get accurate readings.

Illustration of a Single Element Probe

Single element probe

Method 2: Measure Stamped Sheet Metal Thickness with Differential Probes

The second method uses two differential probes with a probe on each side of the sample. This method tends to produce the most accurate results. MTI suggests use of push-pull probes to achieve 1 µm accuracy.

Differential Probes

Differential probes

Method 1 Testing: Measure Stamped Sheet Metal Thickness with Single-Element Probe

The equipment used is a Digital Accumeasure with ASP-250M_CTA probe (250 µm basic range). The amplifier has a 6X range extension so that the maximum permissible gap with this probe is 6X 250 µm or 1.5 mm. The probe diameter is 6.4 mm. The accuracy is < 1µm, assuming that the sample is held firmly against the grounded plate. See Figures 1 to 5 below.

Figure 1 - Measure Stamped Sheet Metal Thickness

Figure 1: Measuring sample thickness on a grounded surface

Figure 2 - Measure Stamped Sheet Metal Thickness

Figure 2: Calibrating the probe to a sample in the gap. Note that the rubber band prevents the V-block from rocking. The stator pole sample is shown.

Figure 3 - Measure Stamped Sheet Metal Thickness

Figure 3: Gap calibration routine (menu-guided)

Place a sample of known thickness in the gap and perform the gap calibration.

Figures 4 and 5 - Measure Stamped Sheet Metal Thickness

Figure 4 (Left): Another sample (505 µm read with a caliper)

Figure 5 (Right): System reading (503 µm indicated)

Method 2 Testing: Measure Stamped Sheet Metal Thickness with Differential Probes

Two push-pull probes were installed in an application jig where the probes are clamped opposite to each other and with a flat deck in-between for sample-holding. See Figure 6 and 7 below.

Next, a sample of known thickness is slid between the probes. The calibration route is then run. The sample does not have to be grounded but can be floating or grounded. No difference in thickness will be observed. The push-pull probes require four channels for operation.

Digital Amplifer

In the image above, note that the digital amplifier is configured for two push-pull probes. The probe faces have an active sensing area that is 10 mm round. In the chart below, see Area “E”. These push-pull probes can work a gap up to 2 mm. Larger probe diameters are need for a larger gap.

Chart

Figure 6 - Measure Stamped Sheet Metal Thickness

Figure 6: The thickness jig holds opposing probes and samples between the probes. The cantilevered top probe holder allows for sliding samples between the probes.

Figure 7 - Measure Stamped Sheet Metal Thickness

Figure 7: Opposing push-pull probes. The gap between the probes is about 2 mm.

Figure 8 - Measure Stamped Sheet Metal Thickness

Figure 8: The 0.505 mm sample is inserted for calibration between the probes. The marked reading is from a contact caliper and used to set the initial capacitance calibration. Unless the probes are moved, there is no need to redo the calibration.

Figure 9 - Measure Stamped Sheet Metal Thickness

Figure 9: Stator stamping was inserted and 0.357 mm thickness was read on the pole piece. The caliper had read 0.358 mm. The difference between contact and capacitance was 1 µm.

Figure 10 - Measure Stamped Sheet Metal Thickness

Figure 10: Dynamic plot

By sliding the stamped sheet shown in Figure 4, we can see that the thickness variation is ~ 5 um.

Conclusion

As this application note explains, both capacitance-based methods for measuring thickness worked to the required accuracy (1 µm). The MTI Digital Accumeasure could read the total thickness of all four samples to a resolution of 0.1 µm. Method 1 uses a single probe and is the least expensive method; however, it requires that the stamped sample be held down against a grounded metal plate.

Method 2 lets the stamping float in between the probes. The stamped part does not have to be grounded. For the most accurate results, however, the stamped part should not tilt or twist since this causes a small tangent error of a few microns. The simultaneous sampling of the gap capacitance with probes 1 and 2 cancels common mode error even with the part moving up and down in the gap so that true thickness is read and there is no need to push the part against a grounded plate. Figure 10 shows a typical plot where the thickness varies. This was made using the differential push-pull probes. The variation can be plotted with the software program provided by MTI Instruments and the data can be logged to a file.

Would you like to contact MTI Instruments directly to learn more about the products used in this application note?