Can I measure Vibrating Wire Strain Gauges using Field Point Inputs?

Question

Is wish to connect a Vibrating Wire Strain Gauge using Field Point.
The sensor requires a swept frequency excitation (typically 400 to 1500 Hz) and then generates a frequency output proportional to the strain applied on the vibrating element. The frequency (roughly upto 1kHz) needs to be measured.

Answer 1

What sort of resolution is needed on the frequency sweeping? There are currently four FieldPoint modules that can be used for generating pulsed outputs; the FP-CTR-500, FP-CTR-502, FP-PWM-520 and FP-PG-522. For the remained of this posting, I will use FP-CTR-50X to represent both the FP-CTR-500 and FP-CTR-502 as they are identical in functionality, different in their input/output stages.
The FP-PWM-520 is a Pulse Width Modulation output with 12 bit resolution on duty cycle and 16 bit resolution on the pulse period. The period is adjustable from 1 to 655535 milliseconds, which makes the module slightly less than ideal for frequency sweeping as it's highest frequency is 1 kHz and it's second highest frequency is 500 Hz.
The FP-PG-522 is a Pulse Generator module that can operate in an continuous pulse mode. The On Time, Off Time and resolution (time base) are adjustable. The smallest time base is 100 ms, for a maximum output frequency of 5 kHz. Like the FP-PWM-520, the FP-PG-522 adjusts it's output rates by changing the time rather than the frequncy, making it difficult to smoothly sweep across frequencies.
Finally, the FP-CTR-50X modules may be used. The FP-CTR-50X modules have 8 16 bit counters, 4 Gates and 4 outputs. The module has two internal clocks, 1 kHz ad 32 kHz that can be used to feed the counters. The outputs can be configured to toggle each time an adjustable terminal count is reached, allowing output pulse generation of up to 16 kHz (32 kHz internal clock divided by 2). The FP-CTR-50X modules have the advantage over the above modules in that the counters can be cascaded and set for differing terminal counts allowing multiple pre-scalers which may allow better resolution of generated frequncies.
For measuring the output frequency generated by the gauge, the only current modules that will allow you to directly measure the output are the FP-CTR-50X modules. However, as they were primarily designed for event counting, it takes a little bit of extra programming to allow them to be used to measure frequencies. The effectiveness of frequency measurement on the FP-CTR-50X modules varies on the sampling period, the programming and the network module. There are several KnowledgeBase and Developer Exchange articles talking about how to use a FP-CTR-50X module to measure frequencies, so I will not duplicate the information in this post.
Regards,
Aaron