Procrastinatus

Why talk about it now?

Rogowski Coil Redeux

Posted by aonomus on August 27, 2010

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
Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

 
%d bloggers like this: