Fiber coupled fluorometer

One of my more recent projects has been tinkering with fiber optics and sensitive detectors to make a fiber coupled fluorometer. Details to come (eventually).

Rogowski Coil Redeux

Way back when I was building the capacitor bank, I built a inductive current sensor called a rogowski coil so that I could have some instrumentation to see just what kind of voltage and current I was dealing with when I used my capacitor bank. Sadly I wasn’t able to get it to work.

But first a little bit of introduction, the rogowski coil is essentially an inductive current sensor similar to a current transformer, but different in that it is wound on a flexible coilform that allows temporary installation. Additionally the can be tailored to suit a wide range of currents and are ideal for short high intensity transient pulses (such as capacitive discharge). The coil itself when placed around a conductor has a voltage proportional to dI/dt, however this by itself is fairly useless since anyone carrying out an experiment wants to know I over time.

Figure 1 – Graph of data captured from Rogowski coil integrator using a Tektronix 2012B, arbitrary scaling used. Calibration unknown on current data. dV/dt smoothing by moving average over 4 data points.

In order to extract current over time from dI/dt, you must integrate the voltage. This is done using an opamp as an integrator. By itself not terribly challenging; the hard part is data capture/acquisition, and calibration. Note that because the coil itself gives dI/dt, the peak rate of change of the circuit voltage should coincide with the peak current after integration, which is a pretty good indicator of whether you have the right configuration for your integrator.

Originally I had tried to use my sound card with an integrator + buffered circuit, however with the level of AC decoupling present, terribly ineffective with a very slow response. The trick is to avoid extra AC decoupling, and simply correct for voltage offset, which in itself is a good idea, however the challenging part was compensating for the voltage offset. If you don’t correct for voltage offset using opamps, and simply AC decouple, you introduce a RC network at every junction, and blur your current waveform over a longer period of time.

Last week I set up my old original coilgun with the rogowski coil, and a DSO that I have access to and began capturing data to test a circuit, with some success. I finalized the circuit topology and laid out the remaining tasks:

• Prototype rogowski coil integrator with attenuator, integrator, gain stage, and line driver
• Build current shunt reference for DSO
• Build DC amplifier to provide current waveform
• Calibrate stepped attenuator, gain stage to match current reference
• Build final version, calibrate, package in metal enclosure

DIY Syringe Pump Version 3!

So after a completely failed iteration of the syringe pump (constant jamming of the syringe plunger, hair pulling, etc) that followed the first pump, I’ve come up with a much more compact syringe pump that fits onto a standard project case.

The main difference between this version, and the previous version is the orientation of the mechanisms. The original had the stepper motor behind the plunger, while this version has ‘folded’ in on itself, and has the stepper motor underneath the syringe. The effect is a compact syringe pump that does not take up the entire desk.

The next step is to bolt the pump assembly down onto the case, design the software for a PIC based controller for standalone operation with optional external control, and actually use this for microfluidic devices.

Data acquisition & sensors, Copper Corrosion, Failures, and ‘Name that device’

So my new recent project has been to develop a bunch of sensors and a data acquisition unit (DAQ), and I have already finished a few tiny boards with some more work to be done. Just as some eye candy, a voltage sensor (divider for a ADC on a microcontroller, selectable to 1:2, 1:3, or 1:5), and a current sensor (resistive 0.001ohm shunt with a current sense amplifier, MAX4372).

Another thing I thought I would bring up is the effect of silicone caulk on electronics. While on the ASV project, despite my protest that caulk would corrode circuit boards due to the release of acetic acid fumes, we ended up moving ahead with it anyways. Regardless, here are some photos of a circuit board exposed to some fumes from caulk, even at a far away distance.

Everyone has had some failure when trying to develop a new device, and as part of my line of sensors, I was going to use a strain gauge taken out of an electronic bathroom scale. Unfortunately I’ve noticed that the resultant sensitivity is far too low with a differential opamp, and that even 1% resistors are not good enough to get the desired value of 0V difference between the two ends of the wheatstone bridge. I’ll have to revisit it at some point, mainly because a strain gauge would be a useful piece of kit. I’m sure that if I get it trimmed out properly and maybe use a higher Vcc for more resolution that the strain gauge will be useful, but until then its a bit of a failure.

And now for a new thing, ‘name that device’. I don’t have anything to give away or award to whoever gets it right, but regardless here are some photos. If you have any idea of what this device is, leave a comment to this post. I’ll reveal its identity after a week or so.

2.5 kJ Capacitor Bank Complete!

After plenty of delays due to more important projects, I have finally completed the capacitor bank control system and construction.

The capacitor bank is a 22,500uF, 450V capacitor bank with a 10kA surge current capacity using solid state switching. A custom built charger was created so that I could use 12VDC (ie: car batteries) to charge the capacitor bank outdoors for more energetic tests. Along with the charger, a control system to monitor bank voltage, and provide safety interlocks to prevent discharge during firing, automatic voltage control, and discharge load control to abort high-current tests were key aspects of the charge controller.