Re: Motor Speed Control

On Thu, 7 Mar 2024 12:13:59 -0800, KevinJ93 <kevin_es@whitedigs.com>
wrote:

On 3/7/24 6:07 AM, Bill Sloman wrote:

On 7/03/2024 9:14 pm, KevinJ93 wrote:

...

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.

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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.

 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.

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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.

There are two types, PM and VR. PM steppers use bipolar coil drive and
have a strong unpowered detent. And can act as generators.
Both can microstep nicely, for smooth motion.

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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.

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Not in 1970. Even after that time they did not possess any advantage over DC motor drive with speed stabilization based on back-emf.
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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.

I designed a tape drive system for data storage, using the 3M tape
cartriges. The capstain driver was a stepper motor driven from 60 Hz
AC, with a cap in one leg to get a 90 degree phase shift. Motion was
very smooth.

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ICs were available to integrate that circuitry:
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eg https://www.precisionmicrodrives.com/ab-026
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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.

 Which you could could buy in an integrated circuit. Most of mine were in a chunk of PROM.

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Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all your power budget.
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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?

 Beats me

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.

 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.

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There is little benefit to being more than adequate if it costs more and will not be perceived by the customer as being better.
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I'm afraid history is against you and regardless of your remonstrations stepper motors were never used significantly or at all for capstan motors.

I did it.

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kw
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