Measuring Speed with a Sensing Coil

An important part of building a speed trap is projectile detection. There are several ways to do this, and optical sensors are often used. However, a novel approach is to use a sensing coil. You might think that an inductive kick requires a moving magnet, but a simple trick makes it easy to detect plain moving iron. Here's how...

Sensing Coil Trick

How do we get an inductive kick from unmagnetized iron? The trick is to apply a d.c. bias current to the sensing coil. This will briefly magnetize the plain iron, and therefore generate a voltage spike as it moves into the coil, and another opposite spike as it moves out.

The only remaining question is how much bias, how many turns, what size coil? So let's run a simple experiment.

Sensing Coil Experiment

Let's skip the analysis and mathematics, and simply build a small sensing coil, apply a bias current, and see what happens when we drop some iron through the middle. A digital storage scope will capture results.

I made a small sensing coil using 400 turns of 26 AWG, randomly wound on a plastic firing tube. The resistance was 3.1 ohms, and with a dc bias voltage of 0.106v then Ohm's law says it had 34 mA of current. A series resistor of 220 ohms allowed a voltage spike to develop without being affected by the power supply's regulator. These values are noncritical, and are simply handy parts from my scrap box.

The experiment was to hold the firing tube vertically, and drop a projectile from a height of 260mm. This will reach a speed of about v = sqrt(2gh) = 2.25 m/s as it travels through the sensing coil.

Experimental Results

Setup:

Projectile length = 40.3 mm
Drop height = 260 mm
Vdc = 10.71 v
Vcoil = 0.139 v

Calculation of estimated pulse width:

t = (distance)/(rate) = (0.0403 m)/(2.25 m/s) = 18 msec

Measured results:

+V signal = +60 mv
-V signal = -80 mv
t p-p = 14.4 msec

Proposed Design

The experiment above produced a spike of only 80 mv. This is too small to trigger most electronics. Suppose we wanted a 1-volt spike from a sensing coil for this projectile. How should it be designed?

We must scale up this experimental coil from a 0.08v spike such that it produces a 1.0v spike, or about twelve times greater signal. I would propose a sensing coil:

• 2x turns density = 400 turns/cm (increase diameter by adding more layers)
• 6x bias current = 200 mA

Conclusion

The sensing coil offers simple construction and a sharp and reliable voltage spike. It should be straightforward to capacitively couple this signal into the electronics of a speed trap.

My observations are:

• It remains to be seen how much intererence will come from a nearby firing coil
• The negative pulse was always 30% larger than the positive spike
• The sensing coil did not detect projectils that were shorter than the coil
• I suggest the coil length be 1/2 the projectile length, to ensure signal strength
• It appears to be possible to measure speed with a single coil, by using both positive and negative voltage spikes