Simple DC motor control

[sq225917]

Are we thinking perhaps this would account for the choice of an optical tacho in several Dc implementations? Give it a stiff supply and adjust very gently only when speed gets past certain bounds.

[martin clark]

Oh, quite, proper feedback of actual speed is going should achieve better repeatability and accuracy. But motors with tachos are considerably pricier than ones without, and even then its down to implementation

[Werner]

Tacho control isn't trivial either. Further the motor assembly is more complex,
prone to defects (tachos tend to be add-ons to standard motors), and when
the tacho fails the system goes open-loop.

Despite Michell using it in the Orbe, to good effect, I heartily dislike the concept.

[Paul R]

FWIW I found a couple of jpg images of stroboscope discs at http://www.zensunni.org/?page_id=9 one covers 50-300rpm, the other 316-1500.

Useful for getting a direct feel for what is happening at the motor shaft.

Paul

[john3]

I previously enquired "How does it 'sound' especially ran against the OL power supply?"

Well I have tried/tested my upgraded/modified OL power supply against (not quite right -yet) Simple DC motor control P/S-I am in no doubt the (Simply turned out to be more complex than first imagined) DC motor control wins by quite a margin. Now if I could only get it to spin clockwise

[Mark Packer]

Originally Posted by john3

...Now if I could only get it to spin clockwise

try turning the record over...

[clivem2]

I can report the my controller is now performing very well indeed. I had speed stability issues, it turns out these were bearing related, the circuit was blameless. So if anyone picked up vibes that all was not well with the circuit please appreciate that getting a turntable to perform well involves every part working to spec. If something is out of kilter you'll know it but tracking down the problem can be quite a journey.

[sq225917]

What was the fix Clive, just a lube change or did it require something more?

[martin clark]

Clive is totally understating how much effort he's put in trying a range of values and other suggestions, some mad, in every part of the circuit to get to this point... with detailed feedback via email, too. Thanks for your patience, Clive

[Briatic]

Just to lower the tone a bit, I've just started assembling my board, but either I've got some old stock op-amps or TI's data sheets are out of date - the packages don't have a notch to enable me to locate pin 1, but there is a small circle pressed into the package - does this identify pin 1 (if it does, why is it betwen pin1 and pin2)? If not, how do I tell?

TIA

Duncan

[martin clark]

Yes, the small circle marks the pin 1 end. It drops into the pcb closest to the square pcb pad locating pin 1 on U10 (there's a white semicircle silk-screened on the pcb to match)

[clivem2]

Originally Posted by sq225917

What was the fix Clive, just a lube change or did it require something more?

I swapped the bearing. The TT has been used for a lot of lube experiments, one of the more recent ones may have been too thin or contaminated (more likely). The bearing's not totally shot and can be re-polished. The platter was spinning down from 33 rpm in 25% less time than it does with a good bearing.

[clivem2]

Originally Posted by martin clark

Clive is totally understating how much effort he's put in trying a range of values and other suggestions, some mad, in every part of the circuit to get to this point... with detailed feedback via email, too. Thanks for your patience, Clive

Thank you for you patience too Martin! Fortunately we've got to the bottom of it.

[sq225917]

Ah, no doubt the increased drag was uneven as well, it alwasy is...

[clivem2]

Originally Posted by sq225917

Ah, no doubt the increased drag was uneven as well, it alwasy is...

For sure, there's a mark (not all the way around) at the base of the spiral lube groove, this will cause asymmetric drag. This also leads me think is this was a contamination issue.