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.
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.
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:
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 :-)
I did wonder what that knob was for! Thanks, Liz.
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?
AC synchronous. Old clocks use them.
Or a stepper. Or a "torque motor"
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.
On a sunny day (Tue, 20 Feb 2024 08:11:44 -0800) it happened John Larkin snipped-for-privacy@997PotHill.com wrote in snipped-for-privacy@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
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.
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).
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.
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.
The most important difference between the two systems, is starting torque.
Since the sensorless systems use the moving magnets to determine commutation timing, the motor must be spinning in order to commutate. Of course, it needs commutation in order to spin, so you essentially have a catch-22 situation.
There are different strategies to overcome the startup problem. The simplest one is to simply commutate "blindly" at low current and see if any timing signals show up. There are more sophisticated methods, but common to them all is that they provide very low torque at zero speed. For propellers or helicopter rotors, this is not a problem, so sensorless systems are used. For cars, however, starting torque is important, so sensored systems are used.
The sensors are simply a few Hall effect sensors. There is no need for any shaft encoders. In hobby products, the sensors are built in to the motor at the factory, so the end user simply sees a few extra wires that need to be connected to the controller.
Stepper motors always provide the same torque when they step slowly at any speed - as long as the current through coil can get up to the tolerable peak, you will get the same torque.
If the magnetic field lines up with position of the rotor, you won't get any torque, so the strategy is to start by stepping the magnetic field slowly enough that rotor can follow the rotating magnetic field, which gets rid of any initial stiction. At low step rates the rotor can oscillate around the zero torque position, and you have to avoid steps rates that match that oscillation frequency. Once you have got the rotor moving slowly, you know where it is and you can start your acceleration sequence.
The Hall sensors are shaft encoders - the rotor is bonded to the shaft, and magnets in the rotor are what you are detecting.
The end user may see them as a few extra wires, but sophisticated users will see them for what they are.
Cursitor Doom isn't a sophisticated user, but if he is posting here we need to treat him as if he could acquire some sophistication.
Up until that last paragraph I was just about to commend you on being more like the old Bill Sloman who posted helpful advice here back in the day. You just can't resist throwing barbs, can you? Sigh...
His only supporter here was 3rdWit, who it transpired was just a sock-puppet. I'm afraid Bill's become something of a sad and rather tragic figure here in recent years.
It's a moral obligation, in your case. Your undiscriminating enthusiasm for pro-Putin, pro-Trump and pro-climate change denial propaganda means that I do need to remind people that you shouldn't be taken seriously.
I do try to educate you, but you don't seem to want to learn.
John Larkin divides the world into people who praise him as he feels he deserves to be praised, and the rest, who are his enemies.
He'd be a lot more successful if he had a more realistic idea of the limits of his capabilities, so we aren't actually his enemies, even if he likes to think we are.
An enemy is somebody who tries to damage you. Vanity is a vice, and feeding somebody's vanity is what an enemy would do.
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