Re: [PSUBS-MAILIST] life support method?

["Karl Fuller" <fullerk@voyager.co.nz>]

Sean,
Nice explanation, easy to follow, you could be a teacher !
It's curious, when I imagine control systems, they are usually hydraulic or
pneumatically based. Put me to sea on a tuna boat, and I'm at home, as long
as the genset doesn't break down. And that is exactly what happened when I
was 2nd engineer, on a 250ft American tuna purseiner, in the middle of the
Pacific. One genset to run the all the chilled brine holds, hydraulics etc.
The million dollar net tore up next, we had to give up and go to Guam for
repairs.
Thanks,
Karl.
----- Original Message -----
From: Sean T. Stevenson <ststev@uniserve.com>
To: <personal_submersibles@psubs.org>
Sent: Saturday, August 05, 2000 7:10 PM
Subject: Re: [PSUBS-MAILIST] life support method?


> On Sat, 5 Aug 2000 12:55:06 -0400, Karl Fuller wrote:
>
> >Hi Sean,
> >Can you please explain how PID works, obviously not my field of
expertise.
> >Thanks.
> >Karl.
>
> Sure thing.  PID stands for Proportional Integral Differential.
> Essentially, it is a PLC (programmable logic controller) program that
> effects three distinct actions on a given output variable, using
> negative feedback.  Let's start with the P, or proportional action.
> The proportional action changes the magnitude of the output according
> to the magnitude of the input error (difference between actual input
> and setpoint).  Thus it is proportional to that error.  To make this a
> bit easier to understand, imagine a tank full of water, with a drain at
> the bottom.  The water level drops as the water drains out, and our
> controller is opening a valve to let water in the top of the tank.  If
> the system is stable at our setpoint (desired water level), then water
> coming in is equal to water leaving.  Now, suppose we open the drain
> valve a little further so that the water level starts to drop.  Our
> proportional controller detects the error and opens the fill valve
> until the water level is again stable.  Water in equals water out,
> except now the flow rate is higher, and the stable equilibrium point is
> lower, since the level dropped before the new equilibrium was
> established.  This is proportional control.  The problem now is that
> although we are keeping things moving and the level stable, the water
> level is still lower than our intended setpoint.  The limitation of
> proportional only control is that the equilibrium point is only at the
> setpoint for a single specific flow rate.  To correct this error, we
> now introduce the I, or integral control.  What integral control does
> is measure the error between the level and the setpoint, and integrate
> it over a specified time interval, and use the resulting value to
> control the output.  In our water tank example, a new equilibrium has
> been established at a lower level by the proportional controller.  If
> we now introduce integral control, the consistent error over time would
> prompt the output to the fill valve to increase.  Then the water flow
> into the tank would increase, the level would rise, and during the next
> integral period (time interval), the integral controller would detect a
> lesser error over time and decrease the output, until eventually the
> setpoint was reached, and there would be no integral output at all.
> Only the proportional output would be still acting to maintain the
> equilibrium between fill flow and drain flow, and producing exactly the
> same output as it did before we turned on the integral control.  Having
> integral control is great, because not you can maintain your setpoint,
> but what if keeping at the setpoint is critical?  You don't want to be
> off of it for very long, so you set your gains really high and use fast
> acting valves to make correction take as little time as possible.
> You've now introduced a new problem, because you return to the zero
> error condition very quickly, and will tend to overshoot.  Now we get
> to the D, for differential control.  Essentially, what the differential
> control does is detect the rate of change of error, so that if
> correction is too slow, it increases the output, or if correction is
> too fast, output is reduced to avoid overshooting (or undershooting)
> the setpoint.  Ideally, you will end up with a system which is
> critically damped, which corrects offset error quickly, but does so
> without wild fluctuations (underdamped) or excessive time lag to
> correction (overdamped).  Combining all three of these output actions
> into one controller gives us a PID.
>
> Hope this helps.
>
> -Sean
>
>
>
>