Pardon, but I cannot resist: > OUT = previous integrated error + gain * (error + current > error/integral time constant + delta error * derivative time > constant)
> Then previous integrated error = new output. May a formula that contains - an "error" - a "current error" - a "delta error" - a "previous integrated error" be considered to contain a lot of errors? If the formula is to be taken serious I would call it an I-Regulator with a strange integration rule. Surely NOT what a PID looks like. For a really nice introduction into control theory have a look at http://dpm1480.pbworks.com/f/PID%20without%20a%20PhD.pdf Tim Wescott is a very experienced engineer and one of the protagonists in the newsgroup on DSP topics. Regards Ulrich Bangert > -----Ursprungliche Nachricht----- > Von: time-nuts-boun...@febo.com > [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bill Hawkins > Gesendet: Dienstag, 11. Mai 2010 09:29 > An: 'Discussion of precise time and frequency measurement' > Betreff: Re: [time-nuts] Lying to Lady Heather > > > Bob, > > I'd expect the PID output to change instantly with error. The > equation is > roughly: > > OUT = previous integrated error + gain * (error + current > error/integral time constant + delta error * derivative time > constant) > where multiply or divide occurs before addition, as > controlled by parentheses. > > Then previous integrated error = new output. > > The time constants are relative to the sampling time of the > PID algorithm. > > Like all integrals, something has to set the initial value. > > Damping is a function of gain and time constants. Either a > high gain or a short integral time will cause the output to > oscillate, as will a long derivative time. > > How are you calculating damping? > > Bill Hawkins > > > > -----Original Message----- > From: time-nuts-boun...@febo.com > [mailto:time-nuts-boun...@febo.com] On Behalf Of Bob Camp > Sent: Monday, May 10, 2010 7:22 PM > To: Discussion of precise time and frequency measurement > Subject: [time-nuts] Lying to Lady Heather > > Hi > > I've spent some time lying to Lady Heather, with some > interesting results: > > 1) Classical control loop theory would suggest that damping > should be fairly close to 1 for reasonable operation. Greater > than 10 should be highly damped. Less than 0.1 should ring > quite a bit. The TBolt doesn't seem to work this way. You can > go to << 0.1 and still have a stable response to a step. You > can go out to > 100 and not get a "lazy" response to a step. > You can get to a point that it will ring, but it's down < > 0.001. Obviously the TBolt and I read different books. > > 2) In a PID setup, you would have control on each > coefficient. With the TBolt setup the "gain" seems to be the > only way to impact the D part of the PID. You can watch the > DAC output as you increase the gain. The swing of the DAC > responding to the GPS pps jumping will decrease as you > increase the gain number. It sounds backwards, but it makes > sense. With "correct" gain, each time there is a step in the > GPS PPS, the DAC immediately changes, no matter what the > damping or time constant. Again, seems strange, but that's > the way it works. > > 3) Time Constant does seem to slow down the "integrator" in the PID. > > Why lie to Lady Heather? > > On a very stable unit - watch the DAC voltage. It's climbing > up and down like crazy on a second to second basis. It's > reasonable to believe that the OCXO is more stable than GPS > at one second. The DAC should be fairly quiet second to > second. DAC LSB's are around 1 ppt. That's around (like a > factor or 3 or 5) the stability of the OCXO at 1 second. One > or two LSB per second might make sense. Anything 5 or 10X > than that is mostly noise that you simply don't need. > > Tell the unit enough lies (like gain = -60) and sure enough > the DAC slows down and hops 1 LSB every so often. When GPS is > stable it will stay in one state for 10's of seconds. Even > with 10 ns hops in the GPS, it still stays down in the 1 to 2 > LSB range. That's *got* to be more stable. > > Why is this good - nice as a frequency standard. > > Why this is bad - TBolt pps does not track GPS PPS very > closely. Not good for E911 service. > > Bottom line - there's lots of ways to optimize a TBolt. > > Bob > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.