Biased Hall effect sensor and NI 9201 module. How can I calculate the velocity of an object?

[JCJones]

For my final year project at university I have designed and built a drop tower for testing the energy dissipation performance of test pieces.

In the system I specified having a biased Hall effect sensor (gear tooth sensor) to calculate the speed of the impactor by having a serrated target (ferritic) moving past it.

The idea behind having a serrated target was to enable an average of several readings of time period to be taken, then to use this to divide the known length of the serrations to calculate speed.

The problem I am having is when using the time period measuring block in LV it only wants to measure the period of the first wave, is there any way I could get LV to measure all four periods (there are four serrations) and average them?

Here is an image of the target incase what I have said was a little confusing: http://tinypic.com/r/2a0b8ck/6

Any help will be greatly appreciated!

[johnsold]

One alternative would be to record the signal from the Hall sensor on an analog input channel rather than a counter.  You would get a signal with 4 peaks which could be analyzed to get the speed.  If the output of the sensor is a continuous signal as opposed to a logic signal the analog method may be more reliable.  Counters sometimes do not respond well to signals which vary slowly (compared to a standard logic signal).

Lynn

[JCJones]

Hi Lynn, thanks for your reply.

Could you possibly repeat your answer in more laymans terms? I'm a mechanical engineering student so my depth of knowledge on anything electronic is very shallow!

As far as I have got, it is being sampled as an analogue input, although I think it has some internal electronics which give it a digital output (it has been producing square waveforms).

Thanks again!

[johnsold]

1. The NI 9201 has only analog inputs, so you have no other choices with that devices.  I did not look it up before my earlier response.  Something about the wording of your first post made me think you were measuring the period with a counter.

2. I am not sure which "time period measuring block" you are using.  Please post your VI.  Before you post it, run it with your Hall sensor so that you have some data recorded.  Go to the Edit menu and choose Make Current Values Default."  Save the VI.  The indicators on your VI will then have your data saved.  Post that VI.

3. I am assuming that you will get a signal which looks something like this, although not necessarily starting exactl at zero time or having exactly uniform spacing between the pulses.

Using some simple logic you can determine the time of the transitions between high and low.  The differences between the times of successive transitions are your periods.  For the image above the falling edges (going from +1 to -1) occur at about 64, 192, 320, and 448.  The differences are 128 in each case.  For your impact data the differences are likely to not all be the same as yout target decelerates.

After I see you VI, I can give some suggestions about how to proceed to calculate the differences.

Lynn