Precision labs series: Inductive sensing

Hello, and welcome to TI Precision Lab Series on Inductive Sensing. My name is Justin Beigel and I'm an applications engineer in the Current and Position Sensing product line. In this video, we will cover the basic principles of inductive technology.

Inductive sensors detect the proximity of metal targets towards an inductive sense element. The LDC, or inductance-to-digital converter, is connected to a fixed capacitor in parallel with an inductive coil, typically on a PCB. The inductive coil forms magnetic field lines that are proportional to the coil diameter.

This capacitor and inductor network forms an LC tank outside of the LDC device. We typically recommend using a capacitor that is NP0 or C0G grade as they are much more stable over temperature and voltage, providing a better oscillation on the LC tank.

When no metal target is present in the vicinity, the LC tank has an inherent resonant frequency that can be obtained by equation 1. The LDC device must drive the LC tank for it to achieve its resonant frequency, and also converts the fundamental frequency of the LC tank to a high-resolution digital value.

When a conductive target is brought close to the LC tank, eddy currents form on the surface of the conductor. These eddy currents create an opposing magnetic field which couples onto the LC tank and reduces the inductance of the inductive sensor. Therefore, the inductance seen on the LC tank changes as a function of distance of the conductive target.

This circuit shows the electrical model of the LDC sensor. The primary side represents the LC tank, or a sense element, and the secondary side represents the conductive target. Note that both the target and coil system have a resistance associated to it.

This electrical model can be simplified into a series electrical model and parallel electrical model. Some of the TI's LDC devices allow a measurement of both the inductance and parallel resistance also known as RP, whereas others only allow a measurement of the inductance, or L. We recommend measuring the inductance as opposed to the parallel resistance since these measurements have less temperature drift. However, when there is no clock available in the system, RP measurements are often favored.

As can be seen by this FEM simulation, as the conductive target gets closer to the LC tank, eddy currents on the surface of the conductor increase. The closer the conductive target gets, the lower the inductance of the LC tank gets. As denoted by the f sensor formula, the resonant frequency of the sensor then increases as can be seen in figure 1.

This graph shows the relative inductance as a factor of the relative distance of the metal target as a function of coil diameter. As can be seen from the graph, different sizes of coils all exhibit the same general behavior as a function of percentage target distance from the coil. The left quarter of this graph is the most sensitive. Smaller shifts in distance equate to a higher shift in inductance, thus, the closer the target is to the coil, the better sensitivity and resolution.

The relative inductance shift is determined by the outer diameter of the inductive coil or shortest diameter for an elliptical coil. Note that more inductance does not mean more sensing range. The coil diameter is the biggest factor when it comes to the sensing range and sensitivity.

Stainless steel, bronze, aluminum, and copper can all be examples for your LDC sensor. However, copper and aluminum perform the best since highly conductive materials produce a larger inductance shift. For non-conductive materials like plastic and glass, metal film or copper tape can be added to the surface to allow the LDC to sense it.

Skin depth is all about the effectiveness of a metal target. Skin depth specifies how deep into the conductive surface the eddy currents will form. Eddy currents that form closest to the surface produce a more concentrated opposing magnetic field to our sensor, and therefore, provide a stronger frequency shift detected by the LDC.

We recommend at least three skin depths of metal thickness for the best response since anything below that would provide a weak response. Since skin depth is a function of the sensor frequency, one way to increase the skin depth is by raising the inductance of the LC tank, which, in turn, reduces the sensor frequency.

Shallower skin depths also increase RS or series resistance losses, which equates to a higher power consumption. The LDC calculator tool located on every LDC product page has a Skin Depth tab to help determine if a conductor is good enough for an LDC sensor.

Common inductive sensing applications include button, encoders and knobs, and metal proximity sensing. Button configurations typically include a sensor PCB, a spacer, and a flat metal surface that deflects when a user presses onto the surface. The benefits of an inductive button approach include no cutouts or holes needed in manufacturing, the ability to detect different levels of force, being robust against debris, liquid, magnets, and being operable with gloves.

For automotive buttons, check for devices that are AEC-Q100-qualified. Other things to consider when designing for an inductive button are the power requirements of the system, the resolution required of the sensor, and the mechanical limits of the design like coil size, and how the mechanical stack up will be implemented.

The second common application is an encoder or knob for incremental position detection or rotational detection. The target design is a bit more complex for this application as it needs to account for the rotational motion so the LDC can detect incremental position as it senses the absence or presence of the conductive target.

For event counting, sensing both increments and decrements, two sensors must be used. The benefits behind using an LDC approach for encoder applications is not requiring calibration, not needing magnets, and not being affected by them. The ability to measure over 300 events per second and requiring minimal microcontroller memory. The design considerations for this application are the same as button application, but with more focus on the target design.

The last common application seen in inductive technology is metal proximity sensing. This follows the same principle as button application for an axial target design, but a lateral target design requires some attention to the target design similar to the rotational sensing application. For axial designs, one thing to consider is the minimum and maximum distances for the target.

That concludes this video. Thank you for watching. For more information and videos on inductive sensing, please visit our TI product page.