Version 01.01, Nov. 06, 2005 - Additional probe notes.

Magnetic Sensor

This page discusses "lessons learned" trying to make low-cost, easy to install current/voltage probe. A perfect sensor not required to get usable data provided mitigate the risks.

Toroidal Core

The perfect magnetic loop is a toroidal core but the secondary winding would have to be hand threaded through the core as well as the primary, MG1 or MG2 power lead. This is a lot of labor and in the case of the MG1 cables, not easy without disconnecting and reconnecting a lot of cables and tubing.

Fortunately, a perfect core is not a hard requirement since many commercial current probes use a 'clamp on' sensor, a sectioned core. Furthermore, the area of the magnetic material is not critical since we are after voltage levels, not pulling a significant power load. So the following experimental cores have been attempted:

Secondary Winding

The goal is to generate audio line voltage, ~.775 VAC into a 600 ohm load to provide full range, 16-bit audio samples. Using a coat-hanger wire loop, we measured voltage levels of ~5-7 mVAC for MG1. This projected the following turn ratios for the secondary coils:


Currently single pairs of Cat-5 cable are bringing the secondary signal into the cabin and the audio jack. The software suggests we are seeing ~6-7 bits of noise, 128 units, out of the 16-bit sample size, 65,536 units, or two and a half orders of magnitude. We would like to reduce this to 2-3 bits.

First we will investigate quesent noise, the noise when all systems are powered on but the engine, MG1 and MG2 are off. The laptop normally runs off of an inverter and we will disconnect it to see if it is inducing noise.

Other noise risks:

The DC impediance measures 4 and 2 ohms through the coils. This suggests a very low value, 5-50 ohms, would more closely match the secondary impediance. Alternatively, a 600 ohm value would more closely match the standard audio input impediance.

Audio Signal Sampling

We are using the open source, "Audacity", audio recording software. Once saved, the data is exported as a ".WAV" file and a program passes through the data to analyze and extract the signals. But there is still a lot of work that remains.

As we improve the data quality, we will be able to reduce the sample interval, currently 22 kHz, and look at "live" data reduction. Also, we will add USB CAM image capture to look for correlations between traffic conditions and mileage. Adding a magnetic compass combined with wheel turns will provide a dead-reconing navigation capability leading to route-optimized speed management. However, we have found some interesting, software detectable patterns:

MG2 Startup
MG2 Transition Traction to Regeneration

In this sample, we see a transition from traction to regeneration. The sailent characteristics are:
MG2 Traction Waveform
MG2 Regenerative Waveform