LDC1314EVM: Would it work to connect a variable reluctance sensor as the "coil"?

Hi Roy,

Could you provide the part number of the VR you plan to use? We would need to look into this more to provide an answer for you.

Thanks,
Jiashow

Thanks for offering help Jiashow!

I have tried to connect directly to you on this fora and in the same time asked to be contacted via email but I am not sure you got it!? Would it be possible for you to handle further discussions via mail (or phone)? If not could you recommend someone within TI? 

Thanks in advance and Happy Easter!

Roy

Hello Roy,

Sorry for the delay in response. We have not tried driving a VR sensor with the LDC, as the sensor design for an LDC is simpler. LDC sensor design and operating principle are different than a VR sensor. Unlike a VR sensor, which generates a voltage signal based on the flux concentration in a ferrite object, the LDC drives an AC signal at the natural resonant frequency of the sensor. You do not need a permanent magnet or a DC bias in the sensor to operate with the LDC.

The characteristics to assess are the sensor Rp and operating frequency. Refer to the application note LDC Sensor Design (http://www.ti.com/lit/snoa930) for more information on sensor construction.

LDC sensors are similar to VR sensors in that they need to be mounted very close to the target. LDC sensors do give a signal even when the target is not moving; but you probably will need to use a second channel as a reference sensor to compensate for environmental shifts.

The VR sensor inductance of 880mH is large for an LDC sensor, which is generally less than 80mH; the LDC161x and LDC131x devices can drive an 880mH sensor, but we usually design systems to operate over 500kHz. THe LDC131x device exhibits very poor resolution with low sensor frequencies or you need to greatly reduce the sample rate. The LDC161x has very high resolution and so it can still provide reasonable signal measurements with very low sensor frequencies.

Use with a VR sensor with an LDC can produce large voltage spikes which exceed the Input ABS max ratings. When using an LDC sensor, we maintain a reasonably consistent AC signal amplitude (typically varying by less than 30%).

Use of the LDC1101, which measures the Rp in conjunction with the L is an interesting idea, however the Rp experiences a 100x larger shift in temperature than the L.

To summarize - you could connect a VR sensor to an LDC, but you probably will not get good results. Optimizing the sensor for the LDC will provide a great improvement in performance.

Regards,

ChrisO

Thank you Chris!

may I please ask what it is that you are implying with your statement "Use of the LDC1101, which measures the Rp in conjunction with the L is an interesting idea, however the Rp experiences a 100x larger shift in temperature than the L" ? The word "however" makes me feel that you do not think it would work because Rp and L are not equally proportional towards temperature. Is that a prerequisite for your technology? 

I understand your view point of preferring to redesign the sensor. Our view point however is the same as in patent US6969987 that "a variable reluctance sensor with an excitation apparatus added to the sensor coil benefits from the tried and tested reliability of a variable reluctance sensor while introducing the accurate low speed measurement of a variable inductance sensor. If the variable inductance circuit fails, the variable reluctance speed sensor remains capable of accurately functioning. Therefore, the capabilities of a variable inductance sensor can be incorporated without the risks associated with a device that has no history"

"Additional benefits of a hybrid sensor include, a signal at speeds approaching zero, a usable signal from smooth targets, greater detection range through metal housings and over moderate gaps and signal amplitude that is not speed dependent. In the case of a hybrid sensor that incorporates a magnet on the target, this extends the additional benefits beyond moderate gaps to large gaps."

The above patent also implies that it should be possible to solve the problem and I thought and hoped that your LDC's could be part of the solution. Maybe I am wrong...(note that potential annually volumes are >1000000 but much more if you considering complete truck market)

Kind Regards

Roy