• State Resolved
  • Locked Locked
  • Replies 9 replies
  • Subscribers 33 subscribers
  • Views 564 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Instability of Proximity Data

AVINASH KOUSHIK
Prodigy 180 points
Other Parts Discussed in Thread: LDC1000EVM

Hello All,

I wanted to use my LDC1000EVM to evaluate the distance from a metallic target. I am interested mainly in the measurement of extremely small distances (my main target is nm range). Hence LDC1000EVM is a good option to progress in this direction. In order to do this I mounted the Sensor & had a target upon a Linear Stage. The target was made of Aluminium & it had a square geometry with dimensions 1.6 cm*1.6 cm & was 1 mm thick. The area dimensions of this target are nearly the same as that of the coil. I made sure that the target was exactly parallel to the coil.Initially the coil was touching the target & the proximity data was at its peak (32767). Then I started to move the target away from the coil. At about 30 microns, the data change occurred . I was recording the distance(mm) & the corresponding proximity data. The proximity data which I recorded was the average of 5000 samples i.e., I recorded the data which was available in the graph .The sensor sampling frequency was the default value of 1 ms. However the strange thing was that after some time, instead of the proximity data being stabilized & remaining at 1 value, it was continuously rising. Always when I wanted to obtain my reading, the proximity data would reach a minimum & then keep increasing continuously . Hence I was forced to record the least reading which occurred & then proceed with the experiment. Due to this I feel that the recordings have become quite inaccurate & the plot of the Proximity Data v/s the Distance is highly non-linear. Also I had got the Linear Stage for a very short time & plan to conduct the experiment again. I have 2 questions in this regard:

1. When I redo the experiment, how should I obtain the proximity data? i.e., should I reduce the number of samples or should I not take the average at all & instead just the minimum value ?

2.Why is it that the proximity data, instead of being stabilized, just keeps increasing ? I am sure that the target is stable & remains at a fixed position upon movement.

Thanks a lot for your time.

With Regards,

Avinash Ravi Koushik

  • 0
    TI__Expert 4890 points

    Hi,

     proximity data is not stable, because it change with temperature, so please use inductance, that will be good.

    Regards, Sunny.

  • 0 in reply to Sunny Qin
    Prodigy 180 points

    Yes. However should not the proximity data be put to some use when it is available? I thought we could obtain a direct relationship between the distance & the proximity Data. I do not think the relationship between Inductance & the distance will be easy to obtain. Also please tell me if I am taking the samples correctly.

    With regards,

    Avinash

  • 0 in reply to AVINASH KOUSHIK
    TI__Genius 9300 points

    Hello Avinash,

    Yes, taking averaged measurement is a correct way.

    You will easily establish direct relationship between inductance and the distance as well.

    Rp value is very useful when you need to characterize properties of your target, such as resistance - which is temperature dependent.

  • 0 in reply to Evgeny Fomin
    Prodigy 180 points

    Hello Evgeny,

    Thanks a lot for the reply. Now that I know that I will be able to establish a direct relationship b/w Inductance & distance, I can obtain the relationship more easily as Inductance is more stable. But now my earlier concern: Why is it that the proximity data varies so much. Is it just temperature dependence or is it also because of the target ? I have not changed any other register value (Including that of Rp) & it remains the default one as I am using the coil which came along with the EVM. Are the RpMin & RpMax used in calculation of the proximity Data or are they just used in the setting of the Threshold value ?

    With Regards,

    Avinash

  • 0 in reply to AVINASH KOUSHIK
    TI__Genius 9300 points

    Hello Avinash,


    I recommend that you change Rp min value to 798Ω

    The saturation of Rp which you observe is not good.

  • 0 in reply to Evgeny Fomin
    Prodigy 180 points

    Hello Evgeny,

    Is the saturation you are referring to , the saturation of the proximity data? If so, I think, in spite of changing the Rp min, & the target touches the coil, the proximity data still saturates . But I am using the coil which came along with the sensor & hence it has the default value of around 3KOhm. Wouldn't changing this affect the accuracy of the data ? I am now trying to obtain the Inductance values. Won't these be affected as well ? Are ther no other ways to stabilize the proximity data value?

    With regards,

    Avinash

  • 0 in reply to AVINASH KOUSHIK
    TI__Genius 9300 points

    Hello Avinash,

    If proximity data saturates with RpMIN = 798Ω, then the coil is not adequate for the proximity measurements. Indeed, if you place a piece of metal directly on the EVM coil, Rp (proximity measurements) will saturate.

    On the good side - inductance measurements will not be affected. Please set Rp MIN = Rp MAX = 798Ω if you intend to use L-only measurements.

  • 0 in reply to Evgeny Fomin
    Prodigy 180 points

    Hello Evgeny,

    Evgeny Fomin said:

    If proximity data saturates with RpMIN = 798Ω, then the coil is not adequate for the proximity measurements. 

    Hence I either need to change my target or the coil, I guess the former would also be fine since I need to just measure really small distances. So I will reduce the target size which should apparently fix the saturation problem!! Also for the temperature fluctuations, I need to use constant temperature conditions like you said.

    As of now I will just use the L value since it is a lot better in many ways!! . Thanks a lot for your time Evgeny. I really appreciate it.

    With regards,

    Avinash

  • 0 in reply to AVINASH KOUSHIK
    TI__Genius 9300 points

    Hello Avinash,

    You are absolutely correct that by reducing the target size you will be able to avoid saturation. As long as you are still able to meet your resolution requirements, this is an excellent approach.

    You are most welcome!