A slightly less yellow submarine

So, because of the sheer expense of 6″ sch80 pipe and pipe fittings (read:  several hundred dollars for a few feet of pipe and 6 fittings!), there has been a redesign!

A yellow submarine

So besides the obvious song reference, I’m starting on a new long term project (as if I don’t have enough going on?). I’m going to be building a remotely operated vehicle (ROV) with a friend for fun and potential science. This has been on my list of things to do for ages, though prohibited mostly by cost. Being gainfully employed has its benefits (ie: money).

Stay tuned for more details, as the goal is to complete construction in time for summer 2011.

Key specs:

Rated depth: 100m
Rated pressure: 1.081 MPa or 156psi

Construction:
sch80 PVC pipe for electronics pods, 1″ CPVC pipe for frame, and aluminum for cross-bars

Power source:
Internal sealed lead acid batteries

Propulsion:
Modified bilge pumps

Instrumentation:
Pressure, temperature, compass, internal monitors (temperature, voltage, current consumption, [humidity/water sensor]). Video over IP camera

Control system:
Surface control computer
Onboard microcontroller with ethernet connectivity, control via tether.

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

Anderson Powerpoles and DC Backup Power

You may recall from my last post the lovely photo of a large number of sealed lead acid (SLA) batteries gifted to me. I have finally completed the battery bank wiring with a few features in mind.

Originally I had 2x 2s2p 24VDC packs, and a 2s1p 24VDC pack, which I reconfigured to 1s10p.

I started out with this set of 7.2Ah, 12VDC SLA batteries and made up spade crimp terminal to powerpole adapters to allow for easy daisy-chaining of the batteries. I did this for a three reasons.

• Versatility – I can split off 1, 2, 3, 4 batteries to go power some other device without worrying about disassembling a static installation. Additionally I can change the physical arrangement of the batteries themselves (rectangle, single long string, over/under, etc.
• Compatibility – all my gear is fit with Powerpoles already
• Safety – I can make all my connections with a small capacity, then safely series them with connectors that will never allow a short (versus attempting to crimp a long dangly cable wtih 10 crimp connectors on it and connecting it carefully to ensure no shorts occur).

Related to the above 2 photos, I filmed a 2 part video on Anderson Powerpole connectors after I received my shipment from http://www.powerwerx.com