DEWBOT X Hatboro-Horsham Shooter Failure

At Hatboro-Horsham, DEWBOT X was generally unable to fire balls with the shooter, especially during autonomous. This problem was never resolved during the course of the competition. Actions taken towards resolution included:

• Modifying shooter code to extend the fire period
• Adding a third and then a fourth air storage tank
• Replacing the shooter solenoid

During match 11, a GoPro camera was set up to watch the pressure gauges. This video shows that the pneumatic system lost pressure at an extraordinary rate during the first 20 seconds of teleoperated play, consuming the entire storage reserve (two 35 in³ tanks at the time) during those 20 seconds. Thereafter, both storage and regulated pressure were identical, ranging between 30 and 45 psig. This pressure is too low for the shooter cylinder to retract the winch's dog gear. The compressor never turned off during the match.

So the proximate cause of the shooter failure was low air pressure.

Air Demand

There are three users of compressed air, the Roller Frame, Shot Angle and Shooter. Air requirements for each of these are:

The Roller Frame is the big air user, requiring over 20 in³ of air (at regulated working pressure) for each cycle. The Roller Frame is also the most frequently deployed air user on the robot. Data from H-H videos reveal the following use frequencies:

Proposed Specifications

Starting with the Hatboro-Horsham frequencies, and assuming that this will increase as the season progresses, a proposal pneumatic specification should allow proper robot function (ability to fire the shooter) during matches having up to:

• 20 cycles of Roller Frame actuation
• 10 cycles of Shot Angle actuation
• 10 cycles of Shooter actuation

Leading to the following compressed air requirements:

Air Storage

DEWBOT X started H-H with two (2) 35 in³ Clippard air storage tanks and finished the competition with four (4) such tanks. The amount of utilizable air stored in these tanks depends upon the difference between storage and working (regulated) pressures. The exact relationship is:

So, if working pressure is 60 psig and initial storage pressure is 110 psig, then four (4) 35 in³ air storage tanks (V₀ = 140 in³) contain a utilizable air volume, or working volume (V_w) of 117 in³ (at working pressure). This is enough air for 5.7 cycles of the Roller Frame. Note that the working volume increases of the working pressure decreases.

Compressors

DEWBOT X was equipped with a VAIR (Kit of Parts) compressor at Hatboro Horsham, with a nominal capacity of 0.88 scfm. If this were the actual feed rate, we would not have had an air problem at H-H.

The team also has a number of Thomas (2010 & earlier Kit of Parts).

The Thomas compressor is considerably larger and heavier than the VAIR. It is also considerably more expensive and as a not-a-kit-of-parts compressor must have its value included in-full in the BoM. This year, we can afford the weight and the BoM cost if the performance warrants it.

Compressor Trials

We measured actual performance of two (2) Thomas and one (1) VAIR compressor (from Deux).

Experimental

A plastic SMC storage tank having ¼” NPTF ports at both ends was plugged at one end with a brass plug. A second brass plug was prepared with Teflon tape but not inserted. The SMC storage tank with both brass plugs was weighed (321 g). The tank was then filled with tap water, sealed with the 2nd brass plug, dried and reweighed (933 g). The difference (612 g) indicates a tank volume (assuming 20°C water) of 613 ml (37.41 in³).

The brass plugs were removed and water thoroughly drained from the SMC storage tank. A pressure gauge was threaded into one end of the tank (using a ¼" x ⅛" brass NPT bushing) and a ¼" x ¼" Male Connector into the other end of the tank. A short length of ¼" tubing connects the Male Connector to a tubing tee. The tubing tee also connects (via short runs of ¼" tubing and via Male Connectors) to a manual vent valve via the branch and to the test compressor via the street connection.

A GoPro® Hero3 Black camera is set to record the pressure gauge at approximately the minimum focus distance. Recording was 1080 60P. For each test, the vent valve is closed, the GoPro is started and the compressor is turned on, in that sequence. There is no pressure switch in the circuit (but there is a pressure relief valve). The compressor is shut down manually upon reaching 120 psig on the gauge. Three compressors were tested: (2) Thomas and (1) VAIR. Each compressor was tested three times.

Analysis

Analysis was performed from the GoPro video footage. Each clip was loaded into Adobe Premiere Elements 11 and: 1) zoomed in to view the gauge; and 2) trimmed so that the start of the analyzed clip was moment when the compressor turned on. The footage was viewed on a frame-by-frame basis with pressure recorded every second. Pressure was recorded with 1 psi precision.

The Thomas compressors were observed to reach 120 psig faster than the VAIR compressor.

Calculation

Pressures from each trial were first converted to standard absolute air volumes (std in³), assuming isothermal compression. Supporting the isothermal assumption was: 1) no temperature increase was noted on the tank exterior surface (by touch); and 2) the system is small and uninsulated, facilitating rapid loss of compression heat. Excluding terminal points (0 & 120 psig), the derivative of standard volume in relation to time (dV/dt, std in³/s) was determined numerically.

Standard conditions are 1 atm (14.696 psia) and 20°C.

Finally, the dV/dt values were regressed versus P (psig) and P². All Thomas compressor data was regressed as a single data set (150 observations - r² = 0.95); all VAIR compressor data was likewise regressed as a data set (100 observations - r² = 0.87).