NI 9505 current sense spike in reading

[Hendrick]

I am using a cRIO 9505 module to control and get readings from a voice coil.

I'm using the current sense function to record the current of the voice coil when it is pressed down.

As you can see from the attached screenshot (the red line is the current), whenever I press the voice coil down, there is a spike in the reading.

Would anyone know what is causing this and more importantly how to solve this?

Thanks

[Matt_McLaughlin]

Hi Hendrick,

Could you please provide a few more details? How and why are you pressing down this voice coil? As I understand it, voice coils contain magnets and movement would therefore cause a change in current. Could you perhaps add some details on your application and what behavior you are expecting when this coil is pressed down?

Thanks,

[Hendrick]

Hi Matt,

my aim is to get the voice coil to measure a vibrating surface. I am currently pressing down the voice coil with my foot. The coil acts in the vertical direction. When it is pressed down I expect the current to increase and when I pull it up I expect the current to decrease. The problem is that the current is rather erratic when the voice coil is pressed down, it goes through a kind of sigmoidal shape before reaching a more consistent reading.

Thanks

[Matt_McLaughlin]

Hi Hendrick,

I believe that the current change you are seeing is likely responding to the movement of the coil from compressed to uncompressed rather than if it is in a compressed or uncompressed state. In the screenshot you posted above, if I am interpreting the picture correctly, it appears that your results correspond to this idea. There is a sloped increase in current and then a drop back to the typical level as the coil is compressed and then has reached its fully compressed position. Then the opposite is seen as the coil is released and then reaches the released position. Does that make sense or am I misinterpreting your data?

Also, if it does seem that I'm on the wrong track could you explain more about how your setup is configured and what your are trying to measure. A screenshot with additional labels as to what is happening when may also be helpful.

Thanks!

[Hendrick]

Hi Matt,

perhaps I should have explained in more detail what was happening in the screenshot. I was shaking my leg with the ball of my foot on the voice coil. The shaking of my foot is an oscillation which compresses and decompresses the voice coil, hence the sinusoid waveform of the current.

Zero current is always the setpoint at which the coil is neither compressed for uncompressed. The current does increase when the coil is compressed and decreases when the coil is decompressed.

I've re-highlighted the screenshot so that the issue may be more clear. The yellow box indicates where I completely compressed the voice coil down with my foot (and did not shake my leg). This behaviour is undesired as the current should not have the sigmoidal shape when I'm completely compressing the coil, this may be an issue of current sense or how I am reading the current sense.

The green box indicates where I start to shake my foot on the voice coil (compression and decompression) and the resulting sinusoidal current waveform is ideal (no erratic current behaviour).

The orange box is from the same foot shaking of the green box, but for some reason or another, the waveform has the erratic behaviour as shown in the yellow box.

I'm trying to measure the oscillation of a shaking leg (from the ball of the foot) through current measurements from the voice coil.

Hopefully I'm making more sense now but please do alert me if I am not.

Thanks

[Matt_McLaughlin]

Hey Hendrick,

Some of these spikes and jumps in the signal, especially those in that final orange section, look like they may just be high frequency noise. Would you be able to try implementing a low-pass filter on the system?