Always wondered how disk drives worked so reliably... and fast... and so accurate- a real feat of engineering.
Tell me, have you had any experience of Hitachi HDDs. I am advised that they are one of the best types and have always used them.
UPDATE
I now remember there was two types of servo loops, but can't recall more at the moment, One of the guys published a book about it all. I had a copy of the book but it got lost along the way.
HDD's have many servo loops,
My guess about a dozen servo loops or more. for servo acceleration, velocity, position, clock recovery, spindle speed
Most sophisticated electro-mechanical, magnetic mature reliable products in the world in a small package at low cost. without exception.
My first exposure to Hitachi was Nippon Peripheral Limited (NPL) with their (dont laugh) 6MB 5.25" HDD in early 80's. They were a subsidiary of Hitachi and the design used stepper drive motors.
Unlike many newbie questions today about BLDC and stepper motor design, they already were using steppers with variable speed to accelerate to max speed and used a viscous ring damper around the motor shaft to eliminate resonance. If anyone wants to learn about how to design the best stepper-servo design, go back 30 yrs and reverse engineer these drives. It outperforms what they do today with 3 axis milling machines, 3D printers etc in terms of optimum stepper speed control and accuracy.
When we got into 100MB 5.25" HDD qualifications, Hitachi was one of the potential sources. They flew from Japan to do a show and tell. Very professional and advanced in HDD contact start stop reliability. For example at that time when 14" HDA's lasted maybe 5k to 10kh MTBF, their products were >50kh. They were insulted when I asked how did they verify their quality and reliability.
Toshiba and Fujitsu on the other-hand were also good but more in tune with western communication. I knew all the companies and contacts in sales and Engineering and the learned the designs by reverse engineering and DVT test plan I created, in order to qualify the best for purchase in large qty.
Today the best rank in any order depending on specific models ; price, performance and application. My rank today is: Hitachi, WD, Seagate.
The newer the technology, the higher the density and more reliance on error correction, on-the-fly (ECC) and the less warning to the user... before "click of death" ( loss of servo signal)
Great advances in lower surface roughness on glass vs burnished alum. in old days since vertical recording is std. now resulted in flying height which can be measured in atoms instead of u" It still relies on an aerodynamic no-contact flying over the surface 30~100kph.