On Mon, 19 Feb 2024 23:18:01 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>On Mon, 19 Feb 2024 09:10:13 -0800, John Larkin <jl@997PotHill.com>
>wrote:
>
>>On Mon, 19 Feb 2024 13:35:58 +0000, Cursitor Doom <cd@notformail.com>
>>wrote:
>>
>>>On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."
>>><robin_listas@es.invalid> wrote:
>>>
>>>>On 2024-02-19 01:44, Anthony William Sloman wrote:
>>>>> On Monday, February 19, 2024 at 5:52:19?AM UTC+11, Cursitor Doom wrote:
>>>>>> Gentlemen,
>>>>>>
>>>>>> Can motor speed control ever approach the effectiveness of the old
>>>>>> style drive belts and pulleys approach? Would simple PWM be enough or
>>>>>> would there be some additional trickery needed?
>>>>>
>>>>> If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
>>>>>
>>>>> It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.
>>>>
>>>>There might be a problem if this causes mechanical vibration in the
>>>>motor (maybe be audible). This vibration might affect the tape speed and
>>>>be audible in the result.
>>>
>>>In these machines they use a heavy flywheel on the end of the capstan
>>>roller, so that shouldn't be an issue.
>>
>>If there's a belt, that will further lowpass filter angular vibration.
>
>Very true.
>
>>
>>A microstepper would be a great capstain driver, but needs drive
>>logic, a uP with PWM blocks maybe. And a bunch of code.
>
>It's another option - albeit perhaps a last one on grounds of
>complexity.
>
>>That could be a product, if there's enough market for fixing up old
>>tape decks. A Pi Pico could be the compute engine. Micro Python would
>>be fast enough.
>
>Or maybe an Arduino.
>
>>The dynamics of handling tape are non-trivial. Transitioning between
>>play or rewind or fast foreward, to stop, is tricky and involves state
>>memory.
>
>I dunno what "state memory" is, but the rest of that paragraph had
>already unhappily occurred to me.
>
>That

It means that something has to remember what was going on before it
was told to stop. In old tape decks, the memory was something
mechanical, or part of the play/stop/foreward/rewind switch.

Pulling the line cord during rewind *would* throw tape all over the
place.

The student decks in the language lab got used all day, and abused, so
the switches failed a lot. Funny that I hadn't thought about that for
maybe 50 years but I remember it pretty well now.

A uP could do the motion control really well, better than mechanical
logic.

On Mon, 19 Feb 2024 15:31:47 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>mandag den 19. februar 2024 kl. 18.11.59 UTC+1 skrev John Larkin:
>> On Mon, 19 Feb 2024 13:35:58 +0000, Cursitor Doom <c...@notformail.com>
>> wrote:
>> >On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."
>> ><robin_...@es.invalid> wrote:
>> >
>> >>On 2024-02-19 01:44, Anthony William Sloman wrote:
>> >>> On Monday, February 19, 2024 at 5:52:19?AM UTC+11, Cursitor Doom wrote:
>> >>>> Gentlemen,
>> >>>>
>> >>>> Can motor speed control ever approach the effectiveness of the old
>> >>>> style drive belts and pulleys approach? Would simple PWM be enough or
>> >>>> would there be some additional trickery needed?
>> >>>
>> >>> If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
>> >>>
>> >>> It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.
>> >>
>> >>There might be a problem if this causes mechanical vibration in the
>> >>motor (maybe be audible). This vibration might affect the tape speed and
>> >>be audible in the result.
>> >
>> >In these machines they use a heavy flywheel on the end of the capstan
>> >roller, so that shouldn't be an issue.
>> If there's a belt, that will further lowpass filter angular vibration.
>>
>> A microstepper would be a great capstain driver, but needs drive
>> logic, a uP with PWM blocks maybe. And a bunch of code.
>
>I doubt it, a regular stepper will be noisy

Most any stepper motor can be microstepped, and make smooth quiet
motion. Drive the windings with sine/cosine waves instead of brutal
steps. You can do that in uP code: software DDS and a sin/cos lookup
table, into a couple of PWM blocks.

I did that ages ago with a 68332, with maybe 1% of the compute power
of a Raspberry Pi Pico.

On Mon, 19 Feb 2024 16:13:51 -0800, John Larkin <jl@997PotHill.com>
wrote:

>On Mon, 19 Feb 2024 15:31:47 -0800 (PST), Lasse Langwadt Christensen
><langwadt@fonz.dk> wrote:
>
>>mandag den 19. februar 2024 kl. 18.11.59 UTC+1 skrev John Larkin:
>>> On Mon, 19 Feb 2024 13:35:58 +0000, Cursitor Doom <c...@notformail.com>
>>> wrote:
>>> >On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."
>>> ><robin_...@es.invalid> wrote:
>>> >
>>> >>On 2024-02-19 01:44, Anthony William Sloman wrote:
>>> >>> On Monday, February 19, 2024 at 5:52:19?AM UTC+11, Cursitor Doom wrote:
>>> >>>> Gentlemen,
>>> >>>>
>>> >>>> Can motor speed control ever approach the effectiveness of the old
>>> >>>> style drive belts and pulleys approach? Would simple PWM be enough or
>>> >>>> would there be some additional trickery needed?
>>> >>>
>>> >>> If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
>>> >>>
>>> >>> It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.
>>> >>
>>> >>There might be a problem if this causes mechanical vibration in the
>>> >>motor (maybe be audible). This vibration might affect the tape speed and
>>> >>be audible in the result.
>>> >
>>> >In these machines they use a heavy flywheel on the end of the capstan
>>> >roller, so that shouldn't be an issue.
>>> If there's a belt, that will further lowpass filter angular vibration.
>>>
>>> A microstepper would be a great capstain driver, but needs drive
>>> logic, a uP with PWM blocks maybe. And a bunch of code.
>>
>>I doubt it, a regular stepper will be noisy
>
>Most any stepper motor can be microstepped, and make smooth quiet
>motion. Drive the windings with sine/cosine waves instead of brutal
>steps. You can do that in uP code: software DDS and a sin/cos lookup
>table, into a couple of PWM blocks.
>
>I did that ages ago with a 68332, with maybe 1% of the compute power
>of a Raspberry Pi Pico.
>
>
>
>

No, even worse, my quad microstepper used a 6803 cpu!

tirsdag den 20. februar 2024 kl. 01.15.30 UTC+1 skrev John Larkin:
> On Mon, 19 Feb 2024 15:31:47 -0800 (PST), Lasse Langwadt Christensen
> <lang...@fonz.dk> wrote:
>
> >mandag den 19. februar 2024 kl. 18.11.59 UTC+1 skrev John Larkin:
> >> On Mon, 19 Feb 2024 13:35:58 +0000, Cursitor Doom <c...@notformail.com>
> >> wrote:
> >> >On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."
> >> ><robin_...@es.invalid> wrote:
> >> >
> >> >>On 2024-02-19 01:44, Anthony William Sloman wrote:
> >> >>> On Monday, February 19, 2024 at 5:52:19?AM UTC+11, Cursitor Doom wrote:
> >> >>>> Gentlemen,
> >> >>>>
> >> >>>> Can motor speed control ever approach the effectiveness of the old
> >> >>>> style drive belts and pulleys approach? Would simple PWM be enough or
> >> >>>> would there be some additional trickery needed?
> >> >>>
> >> >>> If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
> >> >>>
> >> >>> It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.
> >> >>
> >> >>There might be a problem if this causes mechanical vibration in the
> >> >>motor (maybe be audible). This vibration might affect the tape speed and
> >> >>be audible in the result.
> >> >
> >> >In these machines they use a heavy flywheel on the end of the capstan
> >> >roller, so that shouldn't be an issue.
> >> If there's a belt, that will further lowpass filter angular vibration.
> >>
> >> A microstepper would be a great capstain driver, but needs drive
> >> logic, a uP with PWM blocks maybe. And a bunch of code.
> >
> >I doubt it, a regular stepper will be noisy
> Most any stepper motor can be microstepped, and make smooth quiet
> motion. Drive the windings with sine/cosine waves instead of brutal
> steps. You can do that in uP code: software DDS and a sin/cos lookup
> table, into a couple of PWM blocks.

sure, and almost all steppers are now driven like that and there a plenty of
cheap integrated IC that does it all, but unless you run very slow they still make noise

On Tuesday, February 20, 2024 at 4:02:22 AM UTC+11, John Larkin wrote:
> On Mon, 19 Feb 2024 14:41:50 +0100, Robert Roland <fa...@ddress.no>
> wrote:
> >On Sun, 18 Feb 2024 15:00:19 -0800, John Larkin <j...@997PotHill.com>
> >wrote:
> >
> >>A BLDC motor with tach could make a very
> >>nice capstain drive, with a lot of electronics.
> >
> >A BLDC motor does not really need a tach feedback. The speed
> >controller performs the commutation, so it already knows how fast the
> >motor is spinning.
>
> Most have speed that's about linearly dependent on supply voltage.
> Speed droops with load. Ears are pretty sensitive to wow and flutter.

Brushless DC motors are AC motors with build-in sensors and electronics that replace the commutator in a DC motor.

The electronics can be as complicated as you are prepared to pay for, and if you throw in a stable crystal oscillator that can be used to determine the motor speed, independent of the supply voltage (if the supply voltage is high enough).

> >Some R/C hobby BLDC controllers have a governor mode, where they keep
> >the motor speed constant regardless of torque.
> >
> >The electronics is not complicated at all. It is essentially a
> >microcontroller and six MOSFETs.
>
> Yeah, microcontrollers are simple.

From a printed circuit layout or a bill of materials point of view.

--
Bill Sloman, Sydney

On Tuesday, February 20, 2024 at 10:18:10 AM UTC+11, Cursitor Doom wrote:
> On Mon, 19 Feb 2024 09:10:13 -0800, John Larkin <j...@997PotHill.com>
> wrote:
>
> >On Mon, 19 Feb 2024 13:35:58 +0000, Cursitor Doom <c...@notformail.com>
> >wrote:
> >
> >>On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."
> >><robin_...@es.invalid> wrote:
> >>
> >>>On 2024-02-19 01:44, Anthony William Sloman wrote:
> >>>> On Monday, February 19, 2024 at 5:52:19?AM UTC+11, Cursitor Doom wrote:
> >>>>> Gentlemen,
> >>>>>
> >>>>> Can motor speed control ever approach the effectiveness of the old
> >>>>> style drive belts and pulleys approach? Would simple PWM be enough or
> >>>>> would there be some additional trickery needed?
> >>>>
> >>>> If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
> >>>>
> >>>> It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.
> >>>
> >>>There might be a problem if this causes mechanical vibration in the
> >>>motor (maybe be audible). This vibration might affect the tape speed and
> >>>be audible in the result.
> >>
> >>In these machines they use a heavy flywheel on the end of the capstan
> >>roller, so that shouldn't be an issue.
> >
> >If there's a belt, that will further lowpass filter angular vibration.
> Very true.
> >
> >A microstepper would be a great capstain driver, but needs drive
> >logic, a uP with PWM blocks maybe. And a bunch of code.
> It's another option - albeit perhaps a last one on grounds of
> complexity.
> >That could be a product, if there's enough market for fixing up old
> >tape decks. A Pi Pico could be the compute engine. Micro Python would
> >be fast enough.
> Or maybe an Arduino.
> >The dynamics of handling tape are non-trivial. Transitioning between
> >play or rewind or fast forward, to stop, is tricky and involves state
> >memory.
> I dunno what "state memory" is, but the rest of that paragraph had
> already unhappily occurred to me.

What John Larkin presumably meant was that tap reels have intertia, and the software needs to keep track of the rotational energy stored in the reel and drive the motor in a way that removes that stored energy in a way that minimises the length of tape that might form a loose loop while it is going on.

--
Bill Sloman, Sydney

On Tuesday, February 20, 2024 at 11:32:27 AM UTC+11, Lasse Langwadt Christensen wrote:
> tirsdag den 20. februar 2024 kl. 01.15.30 UTC+1 skrev John Larkin:
> > On Mon, 19 Feb 2024 15:31:47 -0800 (PST), Lasse Langwadt Christensen <lang...@fonz.dk> wrote:
> > >mandag den 19. februar 2024 kl. 18.11.59 UTC+1 skrev John Larkin:
> > >> On Mon, 19 Feb 2024 13:35:58 +0000, Cursitor Doom <c...@notformail.com> wrote:
> > >> >On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R." <robin_...@es.invalid> wrote:
> > >> >>On 2024-02-19 01:44, Anthony William Sloman wrote:
> > >> >>> On Monday, February 19, 2024 at 5:52:19?AM UTC+11, Cursitor Doom wrote:

<snip>

> > >> A microstepper would be a great capstain driver, but needs drive
> > >> logic, a uP with PWM blocks maybe. And a bunch of code.
> > >
> > >I doubt it, a regular stepper will be noisy.
> >
> > Most any stepper motor can be microstepped, and make smooth quiet
> > motion. Drive the windings with sine/cosine waves instead of brutal
> > steps. You can do that in uP code: software DDS and a sin/cos lookup
> > table, into a couple of PWM blocks.
>
> sure, and almost all steppers are now driven like that and there a plenty of
> cheap integrated IC that does it all, but unless you run very slow they still make noise.

How? Magnetostriction in the magnetic path? The static coils in the frame are applying force to the moving magnets in the rotor, so they will move around a bit - more if the mechanical design didn't spend enough money on keeping them still and keeping any structural resonances well damped.

--
Bill Sloman, Sydney

Anthony William Sloman wrote:
----------------------------------------------
>
> Brushless DC motors are AC motors with build-in sensors and electronics that replace the commutator in a DC motor.
>

** Some BLDC motors have no electronic sensors - particularly those used for RC hobby applications.
Just 3, high current wires supplying 3 drive coils with one taking turns to act as a position detector.

See vid, skip first 11 minutes.

https://www.youtube.com/watch?v=yiD5nCfmbV0

Many advantages including interchangeability of motors and controllers.

....... Phil

On Tuesday, February 20, 2024 at 3:49:38 PM UTC+11, Phil Allison wrote:
> Anthony William Sloman wrote:
> ----------------------------------------------
> >
> > Brushless DC motors are AC motors with build-in sensors and electronics that replace the commutator in a DC motor.
> >
> ** Some BLDC motors have no electronic sensors - particularly those used for RC hobby applications.
> Just 3, high current wires supplying 3 drive coils with one taking turns to act as a position detector.
>
> See vid, skip first 11 minutes.
>
> https://www.youtube.com/watch?v=yiD5nCfmbV0
>
> Many advantages including interchangeability of motors and controllers.

But the only one that matters is that it is cheap. It is cheap and nasty, but when cheap is all-important, nasty doesn't matter.

Cursitor Doom is the kind of customer who would like that kind of solution.

--
Bill Sloman, Sydney

Can I just get some clarification on one point here. The two spools
are not speed controlled as such and just spin or drag (as the case
may be) at the same speed regardless of the tape speed selected? So
it's only the capstan motor that needs precise control speed? That
seems to be implication of what's been posted here so far and it would
make things much simpler if there was only one motor's speed to worry
about.

On Tue, 20 Feb 2024 12:27:10 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>
>Can I just get some clarification on one point here. The two spools
>are not speed controlled as such and just spin or drag (as the case
>may be) at the same speed regardless of the tape speed selected? So
>it's only the capstan motor that needs precise control speed? That
>seems to be implication of what's been posted here so far and it would
>make things much simpler if there was only one motor's speed to worry
>about.

When the machine is in "play" or "record" mode, I mean; not during FF
or rewind.

On a sunny day (Tue, 20 Feb 2024 12:39:34 +0000) it happened Cursitor Doom
<cd@notformail.com> wrote in <fa79tipemgalpgavdkla66du9ml41q21tc@4ax.com>:

>On Tue, 20 Feb 2024 12:27:10 +0000, Cursitor Doom <cd@notformail.com>
>wrote:
>
>>
>>Can I just get some clarification on one point here. The two spools
>>are not speed controlled as such and just spin or drag (as the case
>>may be) at the same speed regardless of the tape speed selected? So
>>it's only the capstan motor that needs precise control speed? That
>>seems to be implication of what's been posted here so far and it would
>>make things much simpler if there was only one motor's speed to worry
>>about.

Indeed

>When the machine is in "play" or "record" mode, I mean; not during FF
>or rewind.

Yes

Cursitor Doom <cd@notformail.com> writes:
> Can I just get some clarification on one point here. The two spools
> are not speed controlled as such and just spin or drag (as the case
> may be) at the same speed regardless of the tape speed selected?

The speed of the tape depends not only on the spool RPM but also how
much tape is present, since that changes the effective diameter. If the
spools have different amounts of tape on them (normal) they'll have to
move at different RPMs to have the same linear tape speed.

On Mon, 19 Feb 2024 14:41:50 +0100, Robert Roland <fake@ddress.no>
wrote:

>On Sun, 18 Feb 2024 15:00:19 -0800, John Larkin <jl@997PotHill.com>
>wrote:
>
>>A BLDC motor with tach could make a very
>>nice capstain drive, with a lot of electronics.
>
>A BLDC motor does not really need a tach feedback. The speed
>controller performs the commutation, so it already knows how fast the
>motor is spinning.
>

The usual BLDC doesn't speed regulate. Speed depends on the DC supply
voltage and the loading. Basically, it tries as hard as it can.

On Tue, 20 Feb 2024 09:52:13 -0500, DJ Delorie <dj@delorie.com> wrote:

>Cursitor Doom <cd@notformail.com> writes:
>> Can I just get some clarification on one point here. The two spools
>> are not speed controlled as such and just spin or drag (as the case
>> may be) at the same speed regardless of the tape speed selected?
>
>The speed of the tape depends not only on the spool RPM but also how
>much tape is present, since that changes the effective diameter. If the
>spools have different amounts of tape on them (normal) they'll have to
>move at different RPMs to have the same linear tape speed.

If I understand this correctly, one doesn't need to worry about that
aspect, because it will 'just happen automatically' on play and
record.

On Mon, 19 Feb 2024 16:32:22 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>tirsdag den 20. februar 2024 kl. 01.15.30 UTC+1 skrev John Larkin:
>> On Mon, 19 Feb 2024 15:31:47 -0800 (PST), Lasse Langwadt Christensen
>> <lang...@fonz.dk> wrote:
>>
>> >mandag den 19. februar 2024 kl. 18.11.59 UTC+1 skrev John Larkin:
>> >> On Mon, 19 Feb 2024 13:35:58 +0000, Cursitor Doom <c...@notformail.com>
>> >> wrote:
>> >> >On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."
>> >> ><robin_...@es.invalid> wrote:
>> >> >
>> >> >>On 2024-02-19 01:44, Anthony William Sloman wrote:
>> >> >>> On Monday, February 19, 2024 at 5:52:19?AM UTC+11, Cursitor Doom wrote:
>> >> >>>> Gentlemen,
>> >> >>>>
>> >> >>>> Can motor speed control ever approach the effectiveness of the old
>> >> >>>> style drive belts and pulleys approach? Would simple PWM be enough or
>> >> >>>> would there be some additional trickery needed?
>> >> >>>
>> >> >>> If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
>> >> >>>
>> >> >>> It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.
>> >> >>
>> >> >>There might be a problem if this causes mechanical vibration in the
>> >> >>motor (maybe be audible). This vibration might affect the tape speed and
>> >> >>be audible in the result.
>> >> >
>> >> >In these machines they use a heavy flywheel on the end of the capstan
>> >> >roller, so that shouldn't be an issue.
>> >> If there's a belt, that will further lowpass filter angular vibration.
>> >>
>> >> A microstepper would be a great capstain driver, but needs drive
>> >> logic, a uP with PWM blocks maybe. And a bunch of code.
>> >
>> >I doubt it, a regular stepper will be noisy
>> Most any stepper motor can be microstepped, and make smooth quiet
>> motion. Drive the windings with sine/cosine waves instead of brutal
>> steps. You can do that in uP code: software DDS and a sin/cos lookup
>> table, into a couple of PWM blocks.
>
>sure, and almost all steppers are now driven like that and there a plenty of
>cheap integrated IC that does it all, but unless you run very slow they still make noise

Less than an AC motor and generally inaudible.

With more effort, a non-sinusoidal lookup table can tune the drive
waveforms for even higher angular precision and less noise,
compensating for imperfect tooth profiles. But that's over the top.

On Tue, 20 Feb 2024 09:52:13 -0500, DJ Delorie <dj@delorie.com> wrote:

>Cursitor Doom <cd@notformail.com> writes:
>> Can I just get some clarification on one point here. The two spools
>> are not speed controlled as such and just spin or drag (as the case
>> may be) at the same speed regardless of the tape speed selected?
>
>The speed of the tape depends not only on the spool RPM but also how
>much tape is present, since that changes the effective diameter. If the
>spools have different amounts of tape on them (normal) they'll have to
>move at different RPMs to have the same linear tape speed.

The capstain determines the tape speed. The takeup reel motor just
applies a gentle torque and the feed reel motor a bit of drag. The
reel speeds are controlled by the tape speed, namely the capstain.

The tape could come out of a box on the floor and output to another
heap and it would work about the same.

On Wednesday, February 21, 2024 at 2:41:14 AM UTC+11, John Larkin wrote:
> On Mon, 19 Feb 2024 14:41:50 +0100, Robert Roland <fa...@ddress.no> wrote:
> >On Sun, 18 Feb 2024 15:00:19 -0800, John Larkin <j...@997PotHill.com> wrote:
> >
> >>A BLDC motor with tach could make a very
> >>nice capstain drive, with a lot of electronics.
> >
> >A BLDC motor does not really need a tach feedback. The speed
> >controller performs the commutation, so it already knows how fast the
> >motor is spinning.
> >
> The usual BLDC doesn't speed regulate. Speed depends on the DC supply
> voltage and the loading. Basically, it tries as hard as it can.

The "usual" brushless DC motor might not regulate speed, but it has got all the hardware built-in that would let it do that. If the application doesn't call for speed regulation the customer won't be offered the option, but that's just a commercial choice.

--
Bill Sloman, Sydney

On Tue, 20 Feb 2024 12:27:10 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>
>Can I just get some clarification on one point here. The two spools
>are not speed controlled as such and just spin or drag (as the case
>may be) at the same speed regardless of the tape speed selected? So
>it's only the capstan motor that needs precise control speed? That
>seems to be implication of what's been posted here so far and it would
>make things much simpler if there was only one motor's speed to worry
>about.

I don't recall any decks that changed the reel motor drives at
different capstain speeds. Certainly none that were aware of the
amount of tape on each reel.

Big sci-fi movie type computer tape drives used air columns to buffer
the reels. That reduced the effective inertia of the reels to about
zero. The capstains could start/stop every record, ballpark an inch of
tape, as needed.

I designed a tape controller, Ampex 9-track to PDP-11. I'd forgotten
all about that until this thread.

On Wednesday, February 21, 2024 at 2:58:19 AM UTC+11, John Larkin wrote:
> On Tue, 20 Feb 2024 09:52:13 -0500, DJ Delorie <d...@delorie.com> wrote:
> >Cursitor Doom <c...@notformail.com> writes:
> >> Can I just get some clarification on one point here. The two spools
> >> are not speed controlled as such and just spin or drag (as the case
> >> may be) at the same speed regardless of the tape speed selected?
> >
> >The speed of the tape depends not only on the spool RPM but also how
> >much tape is present, since that changes the effective diameter. If the
> >spools have different amounts of tape on them (normal) they'll have to
> >move at different RPMs to have the same linear tape speed.
> The capstain determines the tape speed. The takeup reel motor just
> applies a gentle torque and the feed reel motor a bit of drag. The
> reel speeds are controlled by the tape speed, namely the capstain.
>
> The tape could come out of a box on the floor and output to another
> heap and it would work about the same.

The IBM 7044 that I used (and operated from time to time) when I was a graduate student relied on magnetic tape for mass storage.

The tape drives fed the tape into a deep trough on one side of the reader before it went into the reading head, and had a second trough on the other side to take up the tape after it had been read. Air was pumped into both troughs to keep two fairly long loops under minimal tension. If you were operating the machine at four in the morning there wasn't a lot other stuff going on to attract your attention.

--
Bill Sloman, Sydney

Cursitor Doom <cd@notformail.com> writes:
>>The speed of the tape depends not only on the spool RPM but also how
>>much tape is present, since that changes the effective diameter. If the
>>spools have different amounts of tape on them (normal) they'll have to
>>move at different RPMs to have the same linear tape speed.
>
> If I understand this correctly, one doesn't need to worry about that
> aspect, because it will 'just happen automatically' on play and
> record.

Ah, I misunderstood the question. The reels can be run at a "constant
speed" which is known to be slower[*]/faster enough than the capstain, but
with sufficiently low torque that the capstain can override them.

[*] or unpowered, using only drag

I would consider this to be run at constant *torque* mode, not constant
*speed* mode, since you don't care how fast the spools are moving, just
how much drag or tension they're creating for the tape.

If you *forced* the reels to run at a set speed, the tape would break.

> The capstain determines the tape speed. The takeup reel motor just
> applies a gentle torque and the feed reel motor a bit of drag. The
> reel speeds are controlled by the tape speed, namely the capstain.

Well yeah, I knew that. I was oversimplifying.

Anthony William Sloman <bill.sloman@ieee.org> writes:
> The tape drives fed the tape into a deep trough on one side of the
> reader before it went into the reading head, and had a second trough
> on the other side to take up the tape after it had been read. Air was
> pumped into both troughs to keep two fairly long loops under minimal
> tension. If you were operating the machine at four in the morning
> there wasn't a lot other stuff going on to attract your attention.

I remember the Data General tape drives like that. Those were vacuum
driven instead - the tape was sucked into the trough, past a column of
tiny holes that measured the vacuum - and thus the tape position - so
that the reels knew when to spin. Or so the operator explained to me.
From my point of view it was just magic.

John Larkin <jl@997PotHill.com> wrote:

> On Tue, 20 Feb 2024 12:27:10 +0000, Cursitor Doom <cd@notformail.com>
> wrote:
>
> >
> >Can I just get some clarification on one point here. The two spools
> >are not speed controlled as such and just spin or drag (as the case
> >may be) at the same speed regardless of the tape speed selected? So
> >it's only the capstan motor that needs precise control speed? That
> >seems to be implication of what's been posted here so far and it would
> >make things much simpler if there was only one motor's speed to worry
> >about.
>
> I don't recall any decks that changed the reel motor drives at
> different capstain speeds. Certainly none that were aware of the
> amount of tape on each reel.

I have a vague recollection that there was a machine that boosted the
takeup motor temporarily at high tape speeds to reduce 'billowing'
during startup -- but I can't remember which machine it was (EMI BTR2 or
Marconi-Stille ??). The Ferrograph Series 7 had two torque settings to
deal with small and large-hub reels - but it didn't seem to make much
difference.

The Collaro 'pushmi-pullyu' deck had a constant speed spool motor with
variable friction drive to the spool hubs controlled by tension arms.
The spool motor and the capstan motors were identical (apart from
diection of rotations) and swapped functions when the deck went into
reverse.

Grundig 'Stenorette' dictating machines had a constant rotational speed
spool permanently built into the machine, and no capstan. The tape
cassette had a loose end hanging out with a loop which you hooked around
a pillar on the drive spool and the tape gradually sped up as it built
up on the spool hub. As the recordings had also been made on the same
type of machine, the pitch didn't vary on playback.

Wire recorders almost all used spool drive, but there was one which used
a capstan with the wire wrapped around it in a single turn with no
pressure roller.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

On Tue, 20 Feb 2024 07:39:31 -0800, John Larkin <jl@997PotHill.com>
wrote:

>The usual BLDC doesn't speed regulate. Speed depends on the DC supply
>voltage and the loading. Basically, it tries as hard as it can.

That's correct. But some speed controllers also support a constant
speed mode, also called governor mode. Some modellers prefer this with
helicopters, as it helps keep the rotor speed constant.

Here's one example:

https://hobbyking.com/en_us/yep-120a-hv-4-14s-brushless-speed-controller-opto.html
--
RoRo

Robert Roland <fake@ddress.no> writes:
>>The usual BLDC doesn't speed regulate. Speed depends on the DC supply
>>voltage and the loading. Basically, it tries as hard as it can.
>
> That's correct. But some speed controllers also support a constant
> speed mode,

And when you get to industrial BLDC motors, you add smarter controllers
and position feedback, and you can control pretty much anything wrt that
motor - speed, torque, position, acceleration, etc. Yup, model
helicopters and big CNC machines use the same type of motors :-)