# [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
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