Gas station without pumps

2012 July 11

Nerf gun analysis, continued

Filed under: Robotics — gasstationwithoutpumps @ 19:04
Tags: , , ,

In Nerf gun analysis, I computed volumes for the reservoirs and made conjectures about why the gun was not working well with the small reservoir and 19 msec pulses.  I had a couple of updates, where I first thought I had gotten the small reservoir working by careful orientation of the solenoid to be pulling with gravity instead of against it.  Then, when my son and I tried together (after he got home from theater rehearsal), that stopped working.  We tried switching to 25 msec pulses, which worked fine with the solenoid horizontal, but again had trouble with the solenoid vertical.

We did do a series of pressure measurements for the system with the 25 msec pulses:

Pressure drop per pulse for the system with small reservoir connected to the large reservoir by an air hose, firing with no dart in the barrel. Once again we see a linear pressure drop, indicating that a constant mass of air is moved in each pulse, independent of pressure.  This series is cleaner than the previous one, because the gauge on the pump was read close up always from the same position, reducing parallax errors.

Given the volume of the reservoirs, we can again compute the amount of air moved on each pulse:

component length (cm) diameter (cm) volume (mL)
barrel  68.5  1.5  121
reservoir  48.5  4  609.5
mini-reservoir  21  2  66
air hose  762  0.6  215.5

The total volume behind the valve is about 891 mL, and a pressure drop of 2.24 psi is 0.152 atm.  Using a density of 1.225 mg/mL at 1 atm, we have  about 166 mg of air being released on each pulse: a little more than before.

The gun seemed reliable with 25 msec pulses when the solenoid was horizontal, but still had problems when the solenoid was working against gravity, so perhaps we need pulses that are longer still—say 30 msec. We tried 30 msec and it seemed to fire ok even with the solenoid working against gravity.  We did a series of pressure readings  with the 30 msec pulses also:

Series of blank fires with 30 msec pulses with the solenoid horizontal. We are now releasing about 2.49 psi  * 0.06806 atm/psi * 891 mL * 1.225 mg/(mL atm) or 185 mg of air per firing (150 mL at standard pressure).

We should probably do another series with the solenoid working against gravity, so see if we can see a smaller air release in that orientation.

I did get the microphone setup working, and I could see that the air blast out of the barrel was starting about 5–10 msec after a 19 msec solenoid pulse was finished, which is much slower than it would take the air to travel that far.  The delay was smaller at lower pressure, so I suspect that what we are seeing is mechanical delay in the movement of the valve.  I’ll look at it again with the longer pulses, to see if the air blast now comes out while the solenoid is still powered.  It is a bit difficult to see the pulses, since we are looking a  single shot on an old analog scope, not captured by a digital storage scope.  Perhaps tonight when it gets dark I can try taking some long-duration photos and capturing the scan.

1 Comment »

  1. […] Nerf gun analysis and Nerf gun analysis, continued, I looked at the pressure drop in the reservoir as the Nerf gun was blank fired.  (I probably […]

    Pingback by Nerf gun on the oscilloscope « Gas station without pumps — 2012 July 11 @ 23:10 | Reply


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