[time-nuts] Lying to Lady Heather

Ulrich Bangert df6jb at ulrich-bangert.de
Tue May 11 08:32:32 UTC 2010


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-bounces at febo.com 
> [mailto:time-nuts-bounces at 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-bounces at febo.com 
> [mailto:time-nuts-bounces at 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 at 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 at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.




More information about the time-nuts mailing list