Today I used the Amplifier Q-AMP230V to drive a LMS integral shaker. Now I am interested to get information regarding the amplification of this setup. What is the ratio between the output of the shaker in Newton_rms and the input to the amplifier in Volt_rms? Of course this is dependent of the gain setting at the amplifier. But are there tables for this ratio (N/V) for a given gain? Is this ratio frequency dependent? So basically I am looking for a shaker-sensitivity-sheet. With the shaker there only came a sensitivity sheet for the internal force sensor and the acceleration-sensor but not for the force-output of the shaker.
The gain at the amplifier is adjusted by a stepped wheel in dB. What is the reference value for that dB Amplification-value?
Further I wonder if the amplifier changes the phase of the input-signal? I suspect there is a 180° phase change between the input voltage and the measured force at the shaker. Could that be the case?
I am grateful for any help with this issue. Thank you very much in advance!
Solved! Go to Solution.
I asked my colleauge, Murty, to assist and this is his response:
The manual for the Amplifier does not have any specific information such as “shaker sensitivity sheet” and does not answer any of your questions directly. So let’s examine your questions one-by-one in detail.
Before we do that - I do not understand the motivation behind your questions. So, let’s look at things from a broad perspective first. In general, when one uses the amplifier with the integral shaker, they aim to measure FRFs. They put in the force sensor and accelerometer sensitivities in the Channel Setup worksheet in Testlab. They use the force sensor signal as the reference and the accelerometer signals as the responses, then calculate the FRFs. The Testlab software measures the force and acceleration signals in V, then applies the sensitivity factors and converts the signals to Engineering Units (EU) of N or lbf and m/s^2 or g, respectively. All this processing is completely independent of the Amplifier gain setting because the transducer Voltages are being measured and converted into EU (as opposed to the Amp output!).
Now let’s get to your questions.
There is no such thing as sensitivity for “force-output of shaker”. Only sensors/transducers have sensitivity. The shaker is not a sensor or transducer. The force sensor in the shaker measures the force being applied to the structure.
Since, the Amplifier manual only says the Amplification is “-90 to 36 dB relative to input signal”, it is not enough information.
So, I propose the following procedure for you to answer all of your questions yourself!
Use an accurate function generator that can output a random or broadband signal with known RMS amplitude. You can even use the built-in Source of the SCADAS front-end!
Now you have a choice – whether to test the amplifier by itself. In this case, at each amplifier gain setting, you can measure the FRF between the amplifier input and output and clearly see how the FRF amplitude (gain) relates to the gain setting on the amplifier (amp input is the reference, amp output is response). You can also look at phase of the FRF and answer your phase question!
If you want to test the Shaker-force/Amp-input relation, then measure that FRF! Again, all your questions will be answered.
thank you very much for your fast and comprehensive answer. I did exactly like you said. I used a 50 Hz and a 159 Hz voltage sinus signal with a known amplitude to drive the amplifier. With an amplification of +26 dB I could measure a ratio of ~17,5 N/V. That ratio was approximately the same for 50 and 159 Hz. Also I didn't notice any phase changes. The 180° phase change I observed last week occured by another component I was using.
The reasoning behind my questions were that I am trying to use the integral shaker for active vibration control. I have an electrical controler circuit which measures the acceleration on a structure and uses the integral shaker to counteract that vibration. In order for the controller to drive the shaker with the correct forces I needed the ratio between the output voltage of the controller and the produced force amplitude. I played a bit around with my obtained value and so far it seems to work great. I was able to reduce vibration levels of my structure significantly.
Here a picture of the FRF (Shaker-Force / Amplifier Input Voltage) I measured for the shaker with +26 dB amplification setting of the amplifier. I used a pink noise voltage signal as input to the amplifier. As one can see 17,5 N/V are a reasonable estimate for the frequency range between 40 and 300 Hz.
So thank you very much!
Please find below some additional information that you might find useful.
Regarding the amplifier, its stepped wheel allows an amplification step of 2dB. The dB reference is 1 [V]. For instance, when selecting +20dB on the gain dial, the input voltage will be amplified by a factor 10, at 12dB, a factor 4(rounded).
The phase between input and output of the amplifier is practically 0 degrees. Voltage is considered to be a scalar. The current is proportional to the force for most of the operating range. The phase is dependent on the selection of power connections. We have incorporated multiple voice coil connections which are distributed equally around the shaker. This improves the accessibility tremendously which comes in handy especially when installing the shaker in space constraint locations like the knuckles or engine mounts of a full vehicle.
The first picture attached shows the bottom view of the shaker. The four coil connections have been indicated with arrows.
For FRF application one would like to know the phase relationship between the introduced force which is measured by the integrated force sensor and responses like acceleration, strain or sound pressure.
The shaker technical documentation shows the force sensor outputs a positive force outwards of the structure. The arrow in the second attached picture highlights the positive force direction.