Motor Speed Control

[Cursitor Doom]

Gentlemen,

Can motor speed control ever approach the effectiveness of the old
style drive belts and pullys approach? Would simple PWM be enough or
would there be some additional trickery needed?

CD.

[Ralph Mowery]

In article <qak4ti1ncqfkmihf9...@4ax.com>,
c...@notformail.com says...
> ewsgroups: sci.electronics.design

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.

Were you thinking of AC or DC motors ?

The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.

They have probably gotten better in the last 10 years.

[Cursitor Doom]

DC. This will be for an old reel-to-reel tape recorder that's been in
storage for decades. All the rubber drive belts have perished and
replacements are unobtainable. It has 3 speeds: 3 inches per second,
7.5 IPS and 15. I believe it's a 24V motor but will have to check. If
the idea is feasible, I have a couple of other R-Rs I'd like to get
working again as well.

[John Larkin]

On Sun, 18 Feb 2024 20:50:19 +0000, Cursitor Doom <c...@notformail.com>
wrote:

I saw an article somewhere that suggested one can make replacement
drive belts from strips of back-to-back Scotch tape.

The old Ampex and such tape recorders usually used AC motors.

[Cursitor Doom]

On Sun, 18 Feb 2024 13:23:04 -0800, John Larkin <j...@997PotHill.com>

One of the other tape recorders I have used a 240V motor. Not sure
whether AC or DC motors are more suited to PWM control.

[John Larkin]

On Sun, 18 Feb 2024 22:10:09 +0000, Cursitor Doom <c...@notformail.com>

The Ampex r-r recorders, and others, used a synchronous AC motor for
the capstain drive and shaded-pole AC motors for the reels.

The takeup reel motor ran stalled, and the feed reel motor had DC
applied to make a mild drag. Cheaper recorders had a single motor and
a mess of belts and clutches and such.

My first job was as a tech in a language lab, so I learned a lot about
tape recorders. I recall that the pay was about 70 cents per hour.

AC motors don't PWM well. DC motors don't speed control well, without
some sort of feedback loop. A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

[Cursitor Doom]

On Sun, 18 Feb 2024 15:00:19 -0800, John Larkin <j...@997PotHill.com>

Thanks, John; very interesting. What do you mean by takeup reel motor
ran stalled? And what is it that controls the speed of the tape - the
capstan/pinch-wheel motor or the relevant reel motor?

[Bertrand Sindri]

Cursitor Doom <c...@notformail.com> wrote:
> On Sun, 18 Feb 2024 15:00:19 -0800, John Larkin <j...@997PotHill.com>
> wrote:
>
>>On Sun, 18 Feb 2024 22:10:09 +0000, Cursitor Doom <c...@notformail.com>
>>wrote:
>>
>>>On Sun, 18 Feb 2024 13:23:04 -0800, John Larkin <j...@997PotHill.com>
>>>wrote:
>>>
>>>>On Sun, 18 Feb 2024 20:50:19 +0000, Cursitor Doom <c...@notformail.com>
>>>>wrote:
>>>>
>>>>>On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery
>>>>><rmow...@charter.net> wrote:
>>>>>
>>>>>>In article <qak4ti1ncqfkmihf9...@4ax.com>,
>>>>>>c...@notformail.com says...

>>>>>>> Can motor speed control ever approach the effectiveness of the old
>>>>>>> style drive belts and pullys approach? Would simple PWM be enough
>>>>>>> or would there be some additional trickery needed?
>>>>>>
>>>>>>When I retired about 10 years ago we had many motor speed controls
>>>>>>that worked very well ranging from less than 1 HP to 300 HP. Some
>>>>>>were for DC motors and some were for 3 phase AC motors.
>>>>>>
>>>>>>Were you thinking of AC or DC motors ?
>>>>>>
>>>>>>The AC motor speed controlers used a very odd waveform and sometimes
>>>>>>the motors would produce a sound of say 1000 Hz in frequency. The
>>>>>>speed controlers were microprocessed based and we had to set several
>>>>>>parameters depending on the motor and type of service.
>>>>>>
>>>>>>They have probably gotten better in the last 10 years.
>>>>>
>>>>>DC. This will be for an old reel-to-reel tape recorder that's been in
>>>>>storage for decades. All the rubber drive belts have perished and
>>>>>replacements are unobtainable. It has 3 speeds: 3 inches per second,
>>>>>7.5 IPS and 15. I believe it's a 24V motor but will have to check. If
>>>>>the idea is feasible, I have a couple of other R-Rs I'd like to get
>>>>>working again as well.

>>>>[snip]

>>>>The old Ampex and such tape recorders usually used AC motors.
>>>
>>>One of the other tape recorders I have used a 240V motor. Not sure
>>>whether AC or DC motors are more suited to PWM control.
>>
>>The Ampex r-r recorders, and others, used a synchronous AC motor for the
>>capstain drive and shaded-pole AC motors for the reels.
>>
>>The takeup reel motor ran stalled, and the feed reel motor had DC applied
>>to make a mild drag. Cheaper recorders had a single motor and a mess of

>>belts and clutches and such. [snip] AC motors don't PWM well. DC motors

>>don't speed control well, without some sort of feedback loop. A BLDC
>>motor with tach could make a very nice capstain drive, with a lot of
>>electronics.
>
> Thanks, John; very interesting. What do you mean by takeup reel motor ran
> stalled?

The take-up motor is set to a speed (RPM) such that it maintains a tension
on the tape on the capstan/pinch-wheel. This is to assure that the tape
spools onto the take-up reel instead of unspooling on the floor. The result
is that the motor is always being held back from its desired speed (i.e.,
stalled) by the tape.

> And what is it that controls the speed of the tape - the
> capstan/pinch-wheel motor or the relevant reel motor?

The capstan and pinch-wheel controls the linear tape speed across the heads.
The take-up motor maintains tension to keep the tape spooling onto the
take-up reel, and the feed motor maintains a small back tension to prevent
the feed reel from unspooling onto the floor.

[John Larkin]

On Sun, 18 Feb 2024 23:18:44 +0000, Cursitor Doom <c...@notformail.com>

The right reel was usually the takeup. It had 120 vac applied through
a power resistor, to give a gentle takeup torque. The left reel motor,
the feed side, had a little DC applied to make some viscous drag. In
rewind, the roles were reversed, full AC on the left and no drag on
the right.

Stopping, specially during rewind, was tricky. A really good deck
would seldom dump a reel of tape on the floor.

Tape speed was controlled by the capstain, with its synchronous motor.
The takeup reel motor was then forced to run at a tiny fraction is its
native speed, basically stalled.

This was a studio-grade recorder. Cheaper decks had one
non-synchronous motor that did everything.

[Cursitor Doom]

On Sun, 18 Feb 2024 15:46:25 -0800, John Larkin <j...@997PotHill.com>

Very clear explanation, John; many thanks indeed for that.
The two main decks I'd like to get working again are both Ferrograph
ones, so basically top the range of non-studio decks. Built like
tanks. I'm guessing each must weigh 120lbs! They had issues at the
time with rubber components disintegrating. Kind of damaged their
reputation by the end of the 1970s. A real shame, as they were
exceptionally high quality in every other respect.

[John Larkin]

On Sun, 18 Feb 2024 23:18:44 +0000, Cursitor Doom <c...@notformail.com>

Actually, my first job was a summer thing, in a physics lab
researching Stark-effect microwave spectroscopy. I designed and built
a couple of high-voltage square wave generators. That was the first
time I saw a Tektronix scope, which was the most beautiful thing I'd
ever seen, until I discovered girls.

>>
>>AC motors don't PWM well. DC motors don't speed control well, without
>>some sort of feedback loop. A BLDC motor with tach could make a very
>>nice capstain drive, with a lot of electronics.
>
>Thanks, John; very interesting. What do you mean by takeup reel motor
>ran stalled? And what is it that controls the speed of the tape - the
>capstan/pinch-wheel motor or the relevant reel motor?

[Cursitor Doom]

Thanks, Betrand. That pretty much ties in with what I'm seeing when
it's powered up. Looks like all of them will need a pretty thorough
service to get them back into spec. A lot can go wrong in 40 years!

[Anthony William Sloman]

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.

Brushless motors work that way, but hide the details from the customer, Classical DC motors rely on the commutator for phase control. and commutators eventually wear out.

--
Bill Sloman, Sydney

[Jeff Layman]

On 19/02/2024 00:34, Cursitor Doom wrote:

> The two main decks I'd like to get working again are both Ferrograph
> ones, so basically top the range of non-studio decks. Built like
> tanks. I'm guessing each must weigh 120lbs! They had issues at the
> time with rubber components disintegrating. Kind of damaged their
> reputation by the end of the 1970s. A real shame, as they were
> exceptionally high quality in every other respect.

Nothing to do with what you're after, but you might find something of
eventual use here:
<http://ukhhsoc.torrens.org/makers/Ferrograph/TapeRecorders/index.html>

--

Jeff

[Carlos E.R.]

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.

>
> Brushless motors work that way, but hide the details from the customer, Classical DC motors rely on the commutator for phase control. and commutators eventually wear out.
>

--

Cheers, Carlos.

[Anthony William Sloman]

On Monday, February 19, 2024 at 11:25:38 PM UTC+11, Carlos E.R. 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.

You do have to keep pulse width modulation fast, and filter the waveforms to keep the higher frequency components out of the motor (where they can warm up the
magnetic elements).

https://iopscience.iop.org/article/10.1088/0957-0233/7/11/015/meta

does go into that - I was driving a Peltier element rather than a motor, but was really intent on minimising high frequency interference.

At the time 200kHz was a nice chopping frequency to aim for. There are now faster MOSFets around and a couple of MHz is possible without excessive switching losses. At the time we used the same technique on motors for shifting samples around and a cheap transputer (which was fashionable at the time) produced all the fast waveforms needed for that. There are now some very fast simpler processors around

<snip>

--
Bill Sloman, Sydney

[Cursitor Doom]

On Mon, 19 Feb 2024 07:57:50 +0000, Jeff Layman <Je...@invalid.invalid>
wrote:

Thanks, Jeff. Some promo stuff I've not seen before in among that lot.

[Cursitor Doom]

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.

[Robert Roland]

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.

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.
--
RoRo

[Anthony William Sloman]

On Tuesday, February 20, 2024 at 12:41:56 AM UTC+11, Robert Roland 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.

A brushless DC motor has already got built-in shaft position sensing, so a tachometer is redundant.

If you want to control. torque as well as rotational speed, you do need to know where the rotating magnets are with respect to the static coils, and a brushless DC motor won't necessarily let you get at that information.

A tachometer just tells you how faster the motor shaft is spinning, which may not be all that you need to know.

> 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.

And the hard-ware that senses where the rotating magnets are with respect to the frame, which can be pretty simple too.

--
Bill Sloman, Sydney

[John Larkin]

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.

>
>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.

[John Larkin]

On Mon, 19 Feb 2024 13:35:58 +0000, Cursitor Doom <c...@notformail.com>
wrote:

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.

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.

The dynamics of handling tape are non-trivial. Transitioning between
play or rewind or fast foreward, to stop, is tricky and involves state
memory.

[Fred Bloggs]

The only kind of motor control of any importance:

https://www.powerelectronicsnews.com/overview-of-motor-control-in-electric-vehicles/

>
> CD.

[Three Jeeps]

I suggest you try these websites for replacement belts:

fixyouraudio.com
webspareparts.com
https://www.thevoiceofmusic.com/
https://www.vintage-electronics.net/
thakker.eu

On occasion, I got a belt size that was close but slightly oversized and cut it to correct size and used superglue to join the two ends
I've also got small rubber belt material from (I can't remember the place), cut it to size and joined with superglue.

Using the above resources or making my own belts, I've repaired some vintage late 1970s JVC cassette decks (KD85, KD-A7), and two prize R2R's Akai GX77 and Pioneer RT-707.

Still looking for that 'affordable' Nak Dragon....direct drive.....

good luck

[Cursitor Doom]

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 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

[Cursitor Doom]

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

By which I mean the prospect of several hundred feet of tape spewing
out on a sudden stop like a fucking tagliatelle factory on overtime.

[Lasse Langwadt Christensen]

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

[John Larkin]

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

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.

[John Larkin]

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.

[John Larkin]

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

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

[Lasse Langwadt Christensen]

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

[Anthony William Sloman]

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

[Anthony William Sloman]

> >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

[Anthony William Sloman]

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

[Phil Allison]

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

[Anthony William Sloman]

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

[Cursitor Doom]

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.

[Cursitor Doom]

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

>

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

[Jan Panteltje]

On a sunny day (Tue, 20 Feb 2024 12:39:34 +0000) it happened Cursitor Doom
<c...@notformail.com> wrote in <fa79tipemgalpgavd...@4ax.com>:

>On Tue, 20 Feb 2024 12:27:10 +0000, Cursitor Doom <c...@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

[DJ Delorie]

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.

[John Larkin]

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.

[Cursitor Doom]

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

[John Larkin]

On Mon, 19 Feb 2024 16:32:22 -0800 (PST), Lasse Langwadt Christensen

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.

[John Larkin]

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

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.

[Anthony William Sloman]

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

[John Larkin]

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

>

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.

[Anthony William Sloman]

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

[DJ Delorie]

Cursitor Doom <c...@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.

[DJ Delorie]

Anthony William Sloman <bill....@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.

[Liz Tuddenham]

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

> On Tue, 20 Feb 2024 12:27:10 +0000, Cursitor Doom <c...@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

[Robert Roland]

On Tue, 20 Feb 2024 07:39:31 -0800, John Larkin <j...@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

[DJ Delorie]

Robert Roland <fa...@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 :-)

[Cursitor Doom]

On Tue, 20 Feb 2024 17:32:51 +0000, l...@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>John Larkin <j...@997PotHill.com> wrote:
>
>> On Tue, 20 Feb 2024 12:27:10 +0000, Cursitor Doom <c...@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.

I did wonder what that knob was for! Thanks, Liz.

[john larkin]

Small fan-type BLDC motors often have 4 pins: V+, ground, PWM in, and
tach out. The PWM input controls speed, not very accurately, from zero
to max.

[Cursitor Doom]

Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

[john larkin]

On Tue, 20 Feb 2024 23:09:10 +0000, Cursitor Doom <c...@notformail.com>

AC synchronous. Old clocks use them.

Or a stepper. Or a "torque motor"

[Dave Platt]

In article <m6cati52vmtk96c5l...@4ax.com>,

Cursitor Doom <c...@notformail.com> wrote:

>>Small fan-type BLDC motors often have 4 pins: V+, ground, PWM in, and
>>tach out. The PWM input controls speed, not very accurately, from zero
>>to max.
>
>Hence the need for feedback. I wonder if there's a motor that can spin
>at a given speed accurately without f/back?

Yes. Some brushless DC motors have integral controllers which are
designed with this in mind. Since the controller for a BLDC motor has
to be aware of the rotor position (in order to commutate the phases at
the right times) it has the information it needs to control the speed.

There's one such in my LP turntable, for example... it has a couple of
speed-adjust pots. Once set properly, it keeps the platter rotating
at a stable 33 1/3 RPM, despite variations in the torque required to
overcome drag (from the stylus, record brush, etc.).

The simpler BLDC motor controllers simply hard-switch the supplied DC
voltage to the coils at the proper times - for these motors, the speed
depends on the supplied DC voltage.

The more sophisticated controllers will PWM the supplied DC to the
coils. As I understand it, the timing of the switching between coils
depends on the rotor position, while the PWM duty cycle (and thus the
average voltage applied) is altered to control the speed.

[Three Jeeps]

The AC synchronous motor is probably the typical example. However there are some situations that make it hard to control, such as starting and exceeding slip spec. (Not the best choice for use in a R-R IMHO

A stepper motor can do this but there are challenges in other areas as well, they can miss steps, and starting speed vs torque control.
Any book on electric machinery will provide a good understanding of motor types, classifications, and performance curves.
In general, If you want good controllability (speed, position) a DC motor driven by power amplifier will do well....dig out your books on classical control theory and modern (state space) control for understanding on how to design various position, speed controllers/regulators. Central to this is understanding static and dynamic loads the system will experience along with motor mechanics (e.g. inertia, etc). IF you really want to redesign the R-R motor system, you should get matlab and simulink and the control theory tool box and have at it. Sure beats the bad old days of hand drawing bode plots and gain phase plots and overlaying the effects of compensation networks....

If you have never implemented discretized PID, filters, and other control approaches, you will have fun discovering things like integral windup, digital limiting, and correctly implementing digital filters and precise periodic sampling. These issues and solutions have been well documented, but if you never have done this before, or know where to look, you are in for a wonderful learning experience.
You just might want to get new belts and clean up the motors for the R-R deck....

[Jan Panteltje]

On a sunny day (Tue, 20 Feb 2024 08:11:44 -0800) it happened John Larkin
<j...@997PotHill.com> wrote in <mvi9ti90okknq83gn...@4ax.com>:

The video quadruplex AVR1 from Ampex had the air-column buffers
it used photocells to see were the tape was in the air compartment
and a servo on the suppply reel motor to keep just enough tape in that air space
https://www.flickr.com/photos/watvhistory/3365534784/in/photostream/
note the holes in the capstan drive, no rubber wheel! it sucked the tape fixed...
The white thing top left is motor with a wheel with 4 rotating heads....
more pictures in that link, look a the electronics
https://www.flickr.com/photos/watvhistory/3365533536/in/photostream/
we had several of those, often not working when powered on in the morning
finaly it was decided to leave it on all the time,...
Used for video editing, a reel with tape was really heavy...
Had to keep it running, some factory mods were applied over time too IIRC.
vacuum system?
https://www.flickr.com/photos/watvhistory/3365534384/in/photostream/
Expensive machine...

[Robert Roland]

On Tue, 20 Feb 2024 23:09:10 +0000, Cursitor Doom <c...@notformail.com>
wrote:

>Hence the need for feedback. I wonder if there's a motor that can spin
>at a given speed accurately without f/back?

That would be called a synchronous motor. A BLDC motor is actually a
synchronous motor. If it gets blindly commutated at a certain speed,
it will rotate at that speed (but it will be inefficient). It is
possible to abuse a BLDC motor as a stepper motor. If you apply
current to one of its windings, the rotor will snap into one position
and hold that position.

In order to optimize efficiency, the controller needs to know when to
commutate. Hobby controllers are available in two types, sensorless
and sensored. Sensorless systems need almost no additional hardware
for the feedback. They simply measure the EMF produced by the rotating
magnets.
--
RoRo

[Lasse Langwadt Christensen]

onsdag den 21. februar 2024 kl. 14.52.19 UTC+1 skrev Robert Roland:
> On Tue, 20 Feb 2024 23:09:10 +0000, Cursitor Doom <c...@notformail.com>
> wrote:
> >Hence the need for feedback. I wonder if there's a motor that can spin
> >at a given speed accurately without f/back?
> That would be called a synchronous motor. A BLDC motor is actually a
> synchronous motor. If it gets blindly commutated at a certain speed,
> it will rotate at that speed (but it will be inefficient).

only at very slow speeds, else it'll just vibrate

[Anthony William Sloman]

> only at very slow speeds, else it'll just vibrate.

You do need to have some kind of sensor for rotation. If you have got that - no matter how crude - you can feed an acceleration sequence in to the motor coils and spin it up gradually enough that you can be pretty confident is will end up rotating at the synchronous speed.

At EMI Central Research we put together a linear scan driven by a pin on a loop of chain, and a stepper motor drove the gear that moved the chain around its loop.

If the magnet at the pin didn't got past the Hall effect sensor when it should, the motor had been stalled, and the state machine read the 16 step acceleration sequence out of PROM to get the system going again. It was quick and reliable - we did four scans a second - and people would lean on the system to stop it to watch it recover.

--
Bill Sloman, Sydney

[Cursitor Doom]

On Wed, 21 Feb 2024 14:52:19 +0100, Robert Roland <fa...@ddress.no>
wrote:

Thanks. I'm just trying to work out which type would be most suited to
the role of a capstan roller motor to use at 3 fixed speeds (after
gearing down if necessary).

[Bill Sloman]

The obvious answer is a stepper motor (synchronous motor) with a
crystal-controlled frequency drive.

It will work better if the controller can generate acceleration and
deceleration sequences to make slow and smooth changes in rotational
speed - the spools of tape have rotational intertia and you can't change
their speed of rotation all that quickly.

Once you have got it up to speed, the rotational frequency will be as
stable as your crystal clock. There will some phase lag between the
drive waveform and the position of the rotor - it creates the torque
that counteracts the friction losses, but that should be pretty stable.

You do need some kind of stall detector to accelerate the motor up to
speed again after some ham-fisted user has stopped it's rotation.

--
Bill Sloman, Sydney

[Cursitor Doom]

On Thu, 22 Feb 2024 15:03:24 +1100, Bill Sloman <bill....@ieee.org>
wrote:

That *is* something I'm concerned could spoil the party with the
simpler solutions proposed here. Not sure if it'll make much
difference in practice, but we'll find out empirically I guess.