Yesterday we hooked up the Quadrant Maglev control to the power supply to test the components in the Input/Output part of the circuit.
The output from the buffer was an unexpected high noise signal which was caused by some circuit components.
The following is a story of how it was done.
To test the components of input/output, we measured the output across TP_PD3(Test Point -Photo Diode 3).
We got a high noise signal with a frequency of several kHz.
We tested the values of various electronic components. The resistances R5 and R6 did not measure as mentioned(each had a value of 50 K in the schematic). The value of R6 was 10 K and we replaced R5 with a 10 K resistor. We still got the noise signal at 5.760 kHz with a Pk-Pk voltage of 2.6 V. The resistors in R-LED measured 1.5 K instead of the marked 2.2 K.
We had three suspects in hand:
BUF634P : The data sheet for the BUF634P instructed a short across the 1-4 terminals in the presence of capacitive load. We followed this to overcome the effect(if any) of the extra-long BNC cables which we were using. The oscilloscope still waved 'Hi!' at a few kHz. We removed the buffer and also the feedback resistor R42 from the circuit, what we were testing boiled down to measuring the output of the Sallen-Key filter. The output still contained the funny yet properly periodic signal at a few kHz.
C24: Removing C24 did not do any good.
C23: As a final step C23 was removed. And ... We got a stable DC at 9.86 V(almost stable DC with a low noise at a few MHz). C24 and the buffer were replaced and output seemed fine. The output was a high frequency sine wave which was riding on a DC of 9.96 V.
We rechecked if the LED was on and the infrared viewer gave a positive signal.
We went ahead obtaining the transfer function of the feedback control for which we used a spectrum analyzer.
The input for feedback system is a photo current whereas the spectrum analyzer tests the circuit with a voltage impulse. Hence the voltage input from the spectrum analyzer needs to be converted into current of suitable amplitude(few microamps) for testing the spectrum analyzer. Similarly the output which is a coil current needs to be changed to a voltage output through a load for feeding into the channel of the spectrum analyzer. We used a suitable resistance box with BNC receiving ends to do this. We obtained a plot for the transfer function which is shown below.
- Check the calculated transfer functions with the plot of the spectrum analyzer
- Model the entire(OSEM, magnet, actuators etc.) system in Simulink and calculate the overall transfer function
- Stable levitation of the 1X1 system