Most people think inductive sensing is a method for measuring the distance between a coil and a conductive target, but there are many other use cases for the technology. For example, did you know that you can use a spiral PCB coil and a piece of copper tape to measure linear position?
An inductance-to-digital converter (LDC), like the LDC1000, senses inductance changes of an inductor that comes into proximity with a conductive target, such as a piece of metal. The LDC measures this inductance shift to provide information about the position of the target.
For my linear position slider, I altered the usual approach of changing the distance between the target and the coil. Instead, I kept the target-to-coil distance fixed and changed metal exposure over the coil as I slid the target linearly. I achieved this by using a 100mm long triangular target, which I cut out of a piece of copper tape. The copper tape extends past the widest end of the triangle to ensure maximum metal exposure at that position.
For my sensor coil, I chose a 2-layer PCB coil with 29mm diameter and 70 turns per layer. I chose this coil, because its diameter exceeds the widest part of the shaped target. Figure 1 shows the coil and shaped copper tape target I used for this experiment.
Figure 1: PCB coil and shaped copper target
I then placed the target at a 4mm distance to the PCB coil. Placing the target close to the coil increases the inductance shift as the coil moves from the widest to the narrowest part of the target. For high precision linear position sensors, it is important to minimize target distance to achieve the best resolution.
I moved the target from position 0 (widest part of the target) to position 100 (narrowest part) in 0.5mm steps and measured the inductance at each position. Figure 2 plots the measured data.
Figure 2: Linear slider position vs. measured inductance
Sliding the target from its widest to its narrowest position increases the sensor inductance from 175.2μH to 251.4μH. Since the inductance change is low at the ends, I recommend discarding the top and bottom 5% of the travel range. Therefore, you should use a target that is at least 10% longer than the required travel range. The data samples collected along the remaining 90mm are monotonic and quite linear, and can be used to accurately determine the position of the copper tape target.
To achieve perfect linearity, it is possible to modify the target shape from a triangle to a different shape that results in a linear output. However, it‘s usually easier to linearize the data output in software.
Be sure to check back next Wednesday, when I’ll explain how to design a linear position sensor using an asymmetric coil rather than a shaped target. In the meantime, if you have a question about designing with an inductance-to-digital converter, leave a comment below or search for answers and get help in our Inductive Sensing forum.
Additional resources:
- Read more posts in our Inductive Sensing blog series, including how Ahmad Geethan used the LDC1000 in his award-winning antenna design.
- Check out our infographic, “A Guide to Inductive Sensing.”
- Download the LDC1000 datasheet or request samples.
- Buy the easy-to-use LDC1000 evaluation module.