**The sensor only weighs a couple of ounces. It is not going to cause your robot to be overweight. There are more signifacant things on your robot that will cause it to be overweight. **
OK, all those that have had their robots arrive at inspection at 129.75 pounds, raise your hands!!!
An awful lot of teams spend an amazing amount of time carving out every extra ounce from their robots to allow them to just barely fit within the weight limits of the competition. Just take a look at the number of machines peppered with “lightening holes” that were obviously added in during the last few days of the build phase (or even at the competition site). So, if there is a way to avoid using a redundant sensors and saving four ounces on the robot, there are a lot of folks that will be interested in doing it.
Using one pressure sensor to detect when the system pressure drops below 110 psi, and firing the compressor for “X seconds” (where “X” is the time required for the compressor to increase system pressure from 110 psi to 120 psi) when the pressure drops below the set point is an acceptable algorithm.
The key is to have the compressor deliver a bolus of air for a fixed period of time – the shortest time sufficient to recharge the system under ideal conditions (as determined experimentally under optimal conditions with a fresh battery and no stress on the system), and not to try to extend the recharge period based on perceived system efficiency. Instead, just repeat the cycle as frequently as necessary – once each time the system pressure drops below 110 psi.
If, due to decreased battery output or other efficiency factors, the system is only charged to 115 psi instead of 120, we don’t really care – that is close enough, since what we are really concerned with is that the high pressure side of the system has enough pressure to recharge the low pressure (60 psi) side as air is used by the cylinders (note: 110 is not a magic number – this set point can be anywhere above 60 psi; based on how extensively your robot uses the pneumatics, adjust as necessary to ensure that pressure is maintained in the high pressure portion of the system to provide a constant reserve pressure to the low pressure side while not causing the pump to cycle continuously). If control system/battery/compressor efficiency drops, the recharge will just approach, but not meet or exceed the 120 psi limit. There is no concern about overcharging the accumulators.
For this set up to fail and cause any sort of unexpected energy release, there has to be at least a quadruple failure. The compressor needs to fail-on for an extended period of time to start to build up pressure in the system. The relief valve, set to 120 psi, has to fail to release at the design pressure. The compressor bypass seals need to fail to release at the overpressure level (if I remember the spec sheets correctly, around 140 psi). And finally, some element of the physical system (tubing, cylinder, or accumulator) needs to mechanically fail and release the pressure (assuming the compressor, which has a design output of 120 psi, is even capable of generating anything close to the 250 psi failure ratings of any of these components before it has a pressure lock and stalls out).
Although a constant-use system would probably not be designed this way, this approach is suitable for the low duty cycle, short lifetime applications like our competition robots.
-dave
p.s. If the boiler explosion referenced is the one that occurred at the Cuyahoga County Fair in July 2001, it actually was not caused by a safety valve failure. The boiler in question was in very poor condition, and had apparently been improperly maintained for several years. As a result, the crown sheet above the firebox was severely corroded. The corrosion in several places near the crown staybolts had reduced the crown sheet from a design thickness of 0.375” to less than 0.125”, and in some locations to less than 0.087”. Although it was later determined that the safety valve was inoperable (it was rusted shut, along with the inspection port and fusable plug), this explosion occurred because of a mechanical failure at a pressure well below the designed operating pressure of the boiler.
Letter excerpts from John D. Payton (Director, certified Boiler Engineers for the Commonwealth of Pennsylvania), who conducted an inspection of the damaged boiler at the request of County Sheriff Neil Hassinger and Chief Dean Jagger (Chief Boiler Inspector for the State of Ohio):
“…the crown sheet failure started at the 0.087” thickness area, the weakest point in the crown sheet, and this is where the most bending damage is done to the sheet. The rest of the sheet shows signs of being peeled away much as peeling wallpaper off of a wall. This was caused by the massive expansion of released steam. It is my evaluation that because of the very poor condition of the crown sheet with the reduction of the original thickness from 0.375” to 0.087” leaving only 23% of the original thickness, this was insufficient metal to hold the pressure of the steam resulting in a mechanical failure of the boiler.”
“Computations using formulas ASME 1924 and ASME 1998 computed using a thickness of 0.087” indicated a failure at between 40 psi and 47 psi.”
“The inoperative safety valve had no direct bearing on the explosion as the valve was set for 125 psi and, with the condition of the crown sheet, it is doubtful that 125 psi. was attainable.”
Y = AX^2 + BX + C
