6 years, 9 months ago.

Vibrating Wire Sensor

Good Evening,

I'm after a bit of advice trying to read the output of a vibrating sensor (http://www.mgs.co.uk/data/pdf/65.pdf)

The idea of this type of sensor is that is it firmly attached to a material and changes in strain are measured by means of a vibrating wire. As the strain changes the resonant frequency also changes and gives an accurate representation of the change in strain.

My circuit is as follows:

Mbed PWM pin and digital out connected to a H bridge input (digital out held low). The coil is then subjected to a frequency sweep and then the outputs are switched off. Across the coil is also an instrument amp configured with a high gain.

The problem....

The coil is excited correctly as we can hear the harmonics of the vibrating wire within the sensor, however, we don't get any signal from the sensor when the pulses are removed.

We can see the pulses are ok, and we also see a good output on the amp! But nothing that could be used determine the resonant frequency, and therefore the strain. I currently have a NI data acquisition unit attached to the output of both the PWM and opamp for diagnostic of the analog signals.

Any suggestions would be greatly received!

Anybody have any experience with this type of sensor?

posted by Anthony Cole 01 Jul 2014

Nop, but from how you describe it you might be able to place a resistor between your PWM output and the wire, with a value roughly the same as the impedance of the wire (not a clue how much it is). Then you should see an amplitude which peaks/falls at the resonant frequency.

posted by Erik - 01 Jul 2014

Anthony,

you may well be dampening the response of the the sensor with your methology input/output impedance. I don't have this sensor so can't really help. I don't understand why you are frequency sweeping, as the spec uses the term pluck so you can consider using a ping ie a short pulse.

Dave.

posted by David Fletcher 01 Jul 2014

Hi David, was originally " plucking" the sensor but advice from the manufacturer was to sweep. I suspect the fact I don't see a respond using either method signifies my electronics aren't great!

I have just received a SPDT IC to use as a multiplexer so the sensor can be physically disconnected from the drive circuit and connected to the measurement one.

Will try this and see what happens.

posted by Anthony Cole 01 Jul 2014

Anthony,

Why are you disconnecting then connecting your measurement circuit, when this link http://www.geosense.co.uk/technical/sensor-outputs shows 2 coils ?

Dave.

posted by David Fletcher 01 Jul 2014

Its 1 coil, 1 pair for the coil and 1 pair for the thermistor.

posted by Anthony Cole 01 Jul 2014
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