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Unread 09-12-2002, 18:35
Animator Animator is offline
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Join Date: Dec 2002
Location: Cleveland
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Why you should use two sensors

1) 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.

2) You need to have two sensors in order to establish a hysteresis. Hysteresis is used to prevent wear and tear on the air pump.

The code is at the bottom of the post

a) You could use one sensor that would turn the air pump on and off whenever the air pressure drops below 120 PSI, but this will cause the battery to drain very quickly and it will steal vital current from your drive motors (the battery can only deliver so much current. Try running the drive motors and the pump at the same time. You should notice a difference) plus your pump motor will eventually burn out.

b) You could use one sensor to indicate a minimum pressure of 80 - 100 psi, then turn the pump on for a set amount of time aka: x number of program scans, and then turn it off after that amount of time. The problem with this is that you need to make sure that the pump turns off before the accumulators (The silver cylinders included in the kit that attach to the pump) reach 120psi. This means that you MUST use a freshly charged battery and time how long it takes for the pump to go from your minimum pressure to 120 psi. As the battery drains down and the amount of voltage at the terminals decreases, the pump will need more time to fill the accumulators with air, but if you are using one sensor you will need to sacrifice that the acculmulators will not fill all the way. You CANNOT depend upon the safety valve to bleed off your excess air. Will it work, yes, but it is a safety hazard. A few years ago, a steam engine at the Cuyahoga County Fair blew up and killed several people. The reason: the safety valve did not release when the pressure reached a critical pressure. The safety valve included on the pump is meant to be a safety precaution, a last resort, a oh my goodness my sensor is not working please save me. It is not meant to be used for general purpose use.

Using two sensors compensates for the degradation of voltage as the battery wears down, while allowing you to maintain your desired pressures and minimizing wear and tear on the pump motor.

FIRST posted code last year for the air pump. It is excellent:
---------------------------------------------------------------------
Pump = (HighPres & ~PumpMan) & (LowPres | Pump)
Pump_rev = 0
---------------------------------------------------------------------
HighPres = High Pressure Sensor
Low Pres = Low Pressure Sensor
PumpMan = An override switch that I can use to disable the pump for any reason (SAFETY)
Pump = The Pump Relay switch
Pump_rev = The relays included in the kit have a forward and a reverse output. The reverse output on the relay must be set to 0 at all times when using the air pump.
 


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