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Part Number: LDC1000
Hello TI Team,
We are using LDC1000 for detecting a moving target(ex : moving metal wheel) of heavy mass. Anyhow we are having two consecutively placed inductive coils from which we are reading Rp measurements. We have set the thresholds for with target and without target from our processor. We have managed to detect targets and achieve our application. But my product seems to fail detecting the target sometimes. We have diagnosed that the Rp value changes along with temperature and various others environmental factors. Hence we have designed our application as to change my default value of threshold along with the realistic change. But after our product works for more than 24 hours in a field environment sometimes the LDC is reaching a point where the RP measurement maybe stuck above the target detecting threshold value. Under what circumstances does the Rp change alot like this? How do we control this? Under What circumstances does the Rp value get struck at a particular region? Our target is a moving wheel probably at a speed of 250 km/hr whose detection has been achieved by our R and D team. The problem is the fault which is happening prior to working of more than 24 hours in field open environmental condition. If you prefer we can send the log of Rp change along with temperature which we have measured in house climatic chamber and also in environmental field conditions. We request you to kindly suggest us the resolving method for our design. I would kindly request you to help us by sending a technical team if possible who can help out our design.
Please Note - a) We have tried using LDC1101 which is not suitable for our application since it not able to provide us comparator interrupt output at fast moving target condition.
b) We have tried measuring inductance/ freq count from LDC1000 which does not seem to have much variation for my metal target.
The LDC1000 is currently not recommended for new devices, and we cannot guarantee the part's future availability. I strongly recommend using a different part. We have the LDC Device Selection Guide which may help, but it sounds like the LDC161x could be a good fit. The LDC161x is our highest resolution inductive sensing device, so it may be able to detect shifts in inductance that the LDC1000 cannot. It also has an interrupt pin (INTB) like the LDC1000, which can notify you as soon as a new data conversion is ready. In general we do not recommend Rp sensing, precisely because of its high susceptibility to temperature drift.
If you could describe more about your system requirements I'd be happy to discuss the feasibility of using a different inductive sensing device for your application. It would be helpful to know:
1. The target's material
2. The size of the material
3. The distance between the target and the sensor coil
4. The type of sensor coil -- wire wound or PCB?
5. The required sample rate
6. What type of detection is required? Event counting to determine the wheel's angular velocity? Detection of the presence or absence of the wheel? Something else?
Current and Position Sensing
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In reply to Kristin Jones93:
In reply to venkatesh murthy:
In answer to your questions:
1) No, the other LDC devices do not have the comparator threshold output available. We do have some inductive switch devices that have similar abilities, but they are not as sensitive as the LDC161x and would be less suitable for this application.
2) Yes, both the inductance and the Rp will change when the sensor interacts with the target. I recommend reading this application note for more information about how the coil's inductance will change when interacting with the target. Section 1.2 includes an equation that relates Rp and L to each other, and section 2.4 shows some sample data for how Rp and L change based on target interaction.
Based on the data you provided, it looks like the LDC161x could not meet your sample rate requirement. The LDC1101 would be a better option as it has a higher maximum sample rate, but does not have an interrupt pin like the LDC161x and the LDC1000. Unfortunately, I don't believe we have a device that meets all of your needs. If either the sample rate or the interrupt pin are preferences and not absolute requirements, it may be possible for you to get a working solution with either the LDC161x or the LDC1101.
In general, these things could improve your inductive sensing system:
- The more conductive the target is, the stronger the inductive response will be. If it's possible to add a copper or aluminum coating or layer around the stainless steel wheel, this could improve the system's sensitivity. For more detailed help, please see LDC Target Design.
- We generally recommend PCB sensors instead of wire-wound inductors because they have less part-to-part variation and are more stable across temperature. For guidelines on sensor size and design, please see LDC Sensor Design.
Dear Miss Kristen,
We have done the climatic chamber testing for LDC 1312 using EVM, The acquired plot from GUI is attached for your reference ,we have a drift of 1uH inductance. My target also results in a drift of approximately 1uH . Kindly advice how do i resolve this.
For your reference i have attached the plot with my target moving at 250KMPH.LDC1312_Temp_test details.zip
Could you clarify how long you measured the data? Without knowing several of your register setting I am unable to calculate the time frame from the number of samples provided. Could you also clarify if your target is present during the whole test, part of the test, or none of the test?
The easiest way to remove temperature drift from the system is to use a reference coil that is identical to the sensor coil. This reference coil should be placed such that it cannot sense the target but is still exposed to the same changes in temperature. The output data from the reference coil can be subtracted from the output data from the sensor coil, which removes the temperature drift from the sensor coil data.
Typically, L-measurement shifts about 30ppm/degree C for a PCB sensor coil. Using the data you sent me I estimated your temperature drift to be roughly 1kHz across the full temperature range (if you used a PCB sensor coil). It looks like you are seeing closer to 10kHz drift across your full temperature range. I think this is plausible for a wire-wound coil. I would suggest using a PCB coil to reduce the temperature drift. This application note should help the design process, as well as this LDC Excel Calculator Tool. The "Spiral_Inductor_Designer" tab would be especially helpful for you.
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