Re: [SeattleRobotics] testing robot
- Compressor duty cycles are often times specified in 10 minute intervals or
in 1 hour intervals. You need to check with the specific model you are
looking to use. A common value is 30% / 10 minutes, which means you can run
it for about 3 minutes then it needs to cool down for the next 7 minutes.
There are a lot of variables in this. You are also going to find compressors
which claim to have a 100% duty cycle. Be very wary of that claim, it isn't
actually true in most cases and especially at the pressure you are going to
need for your device. (We have 100% duty cycle compressors for use on FIRST
robots, but if you attempt to run them for too long they heat up and fail.
You have to let them cool down, meaning they are not really 100% duty cycle!
It is marketing BS.)
I see you have some of the math below. I don't really understand the details
for your design so I am not able to verify your numbers at the moment, but
you seem to get the idea. If you are generating 2200lbs of force with a 10cm
stroke, my guess is that you are going to use much more air than what you
have stated. I base this on a gut feeling from a guy who does a lot of
pneumatics on FIRST robots and watches the compressors heat up very quickly.
However, if you do the math, you get to tell me I am wrong and I won't feel
bad at all.
Pneumatics are a full of little variances. If you need more precision, you
should consider hydraulic. Air acts like a spring, and is full of all sorts
of surprises. It is not instant power, nor is it linear. It is, however,
easy to work with and pretty reliable once you get it setup. The regulators
do a good job at not allowing the pressure to exceed a certain amount, so
they are usually pretty accurate at limiting the pressure.
Friction will be an issue in any device you make. The fewer moving parts,
the fewer sources of friction.
From: Peter Balch
Sent: Friday, May 17, 2013 11:43 AM
Subject: Re: [SeattleRobotics] testing robot
> Compressors have aI hadn't considered that.
> series of different ratings you need to consider, most important is the
> expected duty cycle. They will get very hot if you exceed their duty
At 700kPa that's a piston area of 14sq cm which at one operation every 2sec
is 4 litres/min. (I think my math is right.) Even if I assume a 10% duty
cycle that's only 40 L/min. A small compressor you'd use to paint your fence
is 150 to 200 L/min. Even a desktop compressor can give 70 L/min.
Would that be enough?
> I would expect this to take a larger (60 or 80 gallon) unit to keep theThat's 200 to 200 L. Seems quite large if I'm using 4 L/min.
> thing from running every couple of minutes.
> You could use the same large cylinder and have two regulators and twoInteresting idea. I was assuming that the cylinder lifted a lever which
> solenoids. One regulator might run at 10psi to generate 220lbs of force,
> other would run at 100psi to generate 2200 lbs of force.
passively dropped to provide the force. The force would depend on what
weight we hung on the lever.
You're suggesting that the cylinder provides the force directly (or via a
lever) and the force is determined by the air pressure.
How accurately can you control the force that way? Doesn't friction and
stiction affect the force? And how accurate is the air pressure?
It would be nice if we could use the same rig for calibration as well as
fatigue testing so a force accurate to a couple of percent would be good.
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