Only in Cursitor Doom's imaginary universe.
Only in Cursitor Doom's imaginary universe. Cursitor Doom is fond of his fatuous and implausible delusions. John Larkin's delusion that he actually designs his electronic circuits is the same kind of self-serving error.
Only in Cursitor Doom's imaginary universe.
It's individuals interacting, which makes it a social medium.
When the individual are as flawed as Cursitor Doom and John Larkin, some of the interactions can get downright ugly, and they resent being called to account.
you mean like a centrifugal governor?
PWM could. if you sample the back EMF during the off time of the PWM and feed that back to the regulator... (or read the motor speed some other way, you could have an interruptor typse sensor and control speed using a PLL)
Oldschool when they weren't using centrifugal governors they would put a compensating negative resistance in series with the motor and feed the combination from a fixed DC voltage or fake that result.
That's not all that "old school" - Philips got a patent on it around the
1970's. It wasn't remotely good enough for audio work, and neither were centrifugal governors. Synchronous motors with stable frequency drives was what the old school relied on
Philips used the negative resistance approach for speed control in their portable cassette players - so it wasn't too bad. Synchronous AC motors weren't an option in a portable unit.
Other manufacturers did use centrifugal governors.
kw
So did steam engines.
The feedback from a DC motor depends on the strength of the permanent magnets in the motor being regulated, and that is temperature dependent. Philips may have relied on it, but it was still ghastly.
Watches are portable, and electronic watches rely on a 32,768 Hz watch crystal as the frequency reference. Some of them included stepper motors to drive a mechanical display.
Synchronous motors obviously are a practical option in a portable unit, though perhaps not in a really cheap one.
Only at the very cheap and nasty end of the market.
They were. Electronic watches used them, with 32,768Hz watch crystal as the frequency reference.
Which weren't sold on the basis of their frequency accuracy or the absence of wow and flutter.
All the old the tape recorders I pulled apart to see how they worked used centrifugal governors (little leaf switches on the rotor), except the one with high speed dubbing.
The Philips patent was used on record players, somehow that's not audio? they didn't even use an op-amp just 2 transistors and used V_BE as a voltage reference.
To me old-school is analogue speed control.
...
Obviously Philips didn't agree with you. For a consumer product used over a benign temperature range it was fine.
The temperature coefficient was low enough to keep the tape speed within
1% or so.
At the time these devices were first designed (mid-late 60's) synchronous motors weren't a practical option for a consumer item.
...
kw
Back then they were called "stepper motors" and would have been entirely practical. Admittedly, I didn't get to design one into what would have been a cheap product until 1978 (and at EMI Central Research) but they were pretty cheap.
Stepper motors are much too inefficient and have too much torque ripple for capstan drive - not at all suitable for a battery powered device, they also tend to be noisy.
Even implementing the discrete drive electronics would be more costly than necessary at a time where individual transistors were a significant cost; Philips' solution used two transistors - creating a divide by 4 plus driver transistors plus an oscillator would probably require about ten transistors plus numerous other components.
If stepper motors would be such a great solution how come nobody has had your insight and used them in the past sixty years for tape drives?
The permanent magnet DC motor with negative resistance driver worked perfectly well. It was low cost, used available technology, low power, was quiet and met the design requirements.
kw
Twaddle. A stepper motor is a synchronous motor, and if you are careful how you drive it, it doesn't have any torque ripple, and it isn't any less efficient than any other synchronous motor.
ESCAP did do a range of small stepper motors where a sine wave drive did give a uniform rate of rotation - with others you had to massage the waveform a bit to get uniform rotation.
Which you could could buy in an integrated circuit. Most of mine were in a chunk of PROM.
Beats me.
The strength of the permanent magnet depends on the it's temperature, so the velocity feedback you get out of the motor coils does too.
It might have been "adequate" but it wasn't all that good.
A capstan motor sounds more could use a small AC Slosyn synchronous motor rather than a DC Slosyn stepper. They do look quite similar and both are made by the same vendors but they're not the same.
Prepare to be shocked!
...
Stepper motors are invariably of the reluctance type. With simple drivers they have a great deal of cogging, which is undesirable in a capstan drive motor.
Not in 1970. Even after that time they did not possess any advantage over DC motor drive with speed stabilization based on back-emf.
Even for AC powered units where power was not an issue stepper motors were never used. Synchronous motors with synthesized drive were occasionally a feature but many/most used back-emf stabilization with DC motors.
ICs were available to integrate that circuitry:
eg
Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all your power budget.
There is little benefit to being more than adequate if it costs more and will not be perceived by the customer as being better.
I'm afraid history is against you and regardless of your remonstrations stepper motors were never used significantly or at all for capstan motors.
kw
...
Most(all?) portable cassette players used a single motor for capstan and take-up reel; it would definitely consume power and would probably be the largest item in the power budget - probably only 50-100mW allowable determined by battery life from a few C-cells or even two AA cells in later units.
Microstepping is easy now - not so much even at the end of the cassette tape era 30-40 years ago when CDs started to take over
Crutchfield still has a couple of tape decks being sold. I'm sure the market is very small.
The only reason I've used a cassette player in the last 20-30 years is to transcribe tapes I already have into a digital format or to be able to play things in a car that has a cassette player installed.
I wouldn't expect there is any significant new development being done.
kw
Don't be silly. Back-emf depends on the strenght of the magnetic field generating the basck-emf, and that is temperature dependent.
Synchronous motors rotate at a rate that reflects the stability of the frequency source that determines the drive frequency, and reasonably stable frequency source - watch crystals have been around for ages.
It didn't - and it wasn't bipolar.
Tape recorder that didn't play back the recorded frequency weren't perceived to be "good" by their customers. That didn't worry the bottom end of the market.
History doesn't make a cheap and nasty solution anything other than cheap and nasty. The thread is about what Cursitor Doom should do to get his antique tape recorder working again, and getting hold of the original motors used to drive it doesn't seem to be an option.
...
Yes, I was wrong.
Given appropriate driving circuitry that would have been expensive and power consuming in 1970.
...
kw
...
At about 0.2% per deg the magnetic field strength stability was adequate for the speed accuracy required under the required environmental conditions.
MOS EPROMS such as the 1702 were cumbersome to use with multiple supplies required. The logic to drive them would have been TTL consuming significant amounts of power as well as expensive.
The first EPROMS that were easy to use, such as the 2708 weren't widely available till the late 70's.
Few customers had perfect pitch, an error of 1% was much preferable to high cost.
You may call them 'cheap and nasty' but the major portion of the market found this solution acceptable, only the high end went for more exotic approaches.
Wow and flutter performance was much more important and using a DC motor and belt drive with small capstans and a flywheel gave acceptable performance.
I see that tape decks available at Crutchfield currently have a pitch control so the speed can be varied anyway.
kw
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.