Today, I started working on the replacement of the broken LVDT driver in PRM.
- I took self-transfer functions of the BF-LVDTs with the old (broken) driver.
- Found that the broken channel is V2.
- I took single frequency transfer function at 2Hz to record the calibration of each LVDT with the old driver.
- I took note of the resistance values of the gain setting resistors in the broken LVDT driver.
- I replicated these values in the new LVDT driver.
- I installed the new LVDT driver and confirmed some signal comes out of V2.
- Took a single frequency transfer function of H1 and adjusted the demodulation phase to maximize the signal, which yields a slightly larger gain than with the old driver.
- I will continue the phase adjustment tomorrow.
Characterization of the old LVDT driver
- The problem of the old LVDT driver is not only the broken V2 channel, but also a design mistake in the wiring of the demodulator chip (AD630).
- Since pin 18 of the chip is not grounded, the demodulation is performed asymmetrically.
- That is, the incoming signal is switched between the gain of +2 and -1 at the demodulation frequency.
- This should be +2 and -2.
- The gain imbalance allows DC signal to go through the demodulation process.
- This is especially problematic for BF-LVDTs where the primary coil is used both for readout and actuation.
- The low frequency actuation signal inevitably couples to the readback signal of the LVDT even if there is a DC cut capacitor.
- The demodulation process is supposed to provide extra isolation of this actuation signal.
- However, because of the asymmetry, the low frequency signal goes through the demodulation stage without much attenuation.
- This is the source of the spurious coupling we observed in those LVDts.
- The new LVDT driver I brought in from Mozumi has this problem fixed.
- In order to see the improvement by the symmetric demodulation, I first measured the self-transfer functions of the LVDTs with the old driver boards.
- What is self-TF ?
- For example, we send actuation to the H1 coil, then measure the TF from this excitation to the output of H1 LVDT.
- The results are shown in the attached plot.
- As can be seen, the self-TFs are dominated by the spurious coupling at frequencies above 10Hz.
- V2 has a flat TF at low frequencies, because it is broken and not sensing the suspension motion.
- However, V2 has the same sprious coupling at high frequencies.
- This indicates that the actuation signal is going to the primary coil and indeed couples to the LVDT readout.
- Probably, the 10kHz reference signal is not sent to the secondary coil for this channel.
- What is self-TF ?
Record the calibration of the old LVDT driver
- Before switching the driver to the new one, we need to make sure that the LVDT calibration will not change, or change by a known amount.
- Therefore, I took a self-TF of each channel at 2Hz to measure the gain and phase at that frequency.
- These values are used to recalibrate the LVDT signals with the new driver.
|V2||no coherence||no coherence|
Record the resistance of the old LVDT driver
The LVDT driver board has 4 channels. Each channel has two variable resistors for setting the gains for the excitation signal (10kHz) and the instrumentation amplifier to receive the signal coming back from the primary coil. I measured the values of those resistors to keep the same gains in the new LVDT driver. There is another variable resistor to set the demodulation phase of the LVDT. I will adjust this to maximize the gain of the LVDT.
Later, I changed my mind that preserving the resistance value is not necessary, as I will measure the new gain of the LVDTs using the self-TF and update the calibration factors accordingly.
Replicagtion of the resistance values in the new LVDT driver
This is indeed not really necessary, but I copied the resistance values of the gain setting resistors to the new LVDT driver.
Measure the gain of the new LVDTs
- After installing the new LVDT driver to the rack, I measured the self-TF of the H1 LVDT at 2Hz.
- Then I tuned the variable resistor to set the demodulation phase to maximize the signal gain.
- The obtained gain is -65.28dB, which is 4.5dB higher than the old one.
- The phase is -1.64deg, which is almost the same as the old one.
- It should be noted that because the gain asymmetry was removed, the LVDT demodulation gain should be increased by a factor of 2/1.5=1.25dB.
- Time was up today at this moment. I will continue the work tomorrow.