In my previous post, I introduced the LDC calculator tool, which you can download here. It can calculate a number of useful parameters for an Inductive Sensing application.
LDC devices use inductors to sense the movement of conductive targets. At TI, we commonly use spiral traces routed on a printed circuit board (PCB) to form sensing inductors. WEBENCH® Coil Designer is a useful online tool that can help you design a sensor inductor and also generate a layout.
The racetrack inductor designer tool is another useful designer in the LDC tools spreadsheet. While the racetrack inductor designer only calculates sensor parameters and does not generate a layout, it is quick to use. Simply click Racetrack Inductor Designer on the Contents tab of the LDC calculator tool, or click the Racetrack_Inductor_Designer tab, as shown in Figure 1. You’ll wind up on the tab shown in Figure 2.
Figure 1: Accessing the racetrack inductor designer
Figure 2: Racetrack coil designer tool
The LDC calculator tool is pretty accurate – typically a physical sensor will be within 10% of the calculations. For racetrack-shaped sensors, however, the accuracy may degrade when the ratio of the long side to the short side is greater than 4.
The first step in the process to design a sensor is to determine the PCB manufacturing limitations. Table 1 shows an example of limits from one PCB manufacturer.
PCB minimum trace width/space
0.125 mm (5 mil)
Via minimum pad size
0.6 mm (24 mil)
Via minimum hole size
0.25 mm (10 mil)
Sensor minimum inner diameter
0.825 mm (21 mil)
0.6 mm + 2 × 0.125 mm
One via pad + two trace spaces
Stack-up thickness between layers
0.80 mm (32 mil)
Desired PCB thickness
Table 1: Sensor fabrication parameters
You now need to know a few system limitations – what is the maximum possible size of the sensor, and how close the target can be to the sensor. I’ve summarized the values for an example system in Table 2.
Maximum sensor diameter
Target closest distance
Based on system mechanicals
Table 2: Sensor parameters
For this example, I will use the LDC1612 and comply with the limitations from Tables 1 and 2. Figure 3 shows the calculating region of the racetrack coil designer. I placed a number to the left of each parameter so you can follow me as I walk through each setting below.
Figure 3: Sequence to enter parameters
Next, follow these steps to calculate a basic sensor design:
Figure 4: Example error warning generated by the calculator tool
After entering your numbers, you may need to adjust the number of turns or the sensor capacitance. After trying several settings, I wound up with the values shown in Table 3. I chose 130 pF for the sensor capacitor so that I could safely use a 10% tolerance part.
Ratio of long edge to short edge
Spacing between traces
Width of trace
Table 3: Resulting parameters
Because the sensor’s electrical parameters change when the target is close, you need to verify that the sensor is still within the valid operating range of the LDC when this occurs. With the closest target distance of 1.8 mm, my sensor has the electrical specifications shown in Table 4, which are within the LDC1614’s operating region.
Sensor Inductance from Target Interaction
Sensor Frequency with Target Interaction
Rp with Target Interaction
Q Factor with target
Table 4: Sensor parameters due to Target Interaction
The LDC tool spreadsheet has a lot more uses. In my next post, I’ll review the LDC0851 calculator tab.
Do you have any questions or feedback about the racetrack inductor designer tool? Log in to comment below.
in LDC 1101 how can we calculate sensing distance in racetrack coil design ? please suggest
In case of circular coil its half of the coil diameter .
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