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 Post subject: Fly height tweaking -- barometric pressure sensors
PostPosted: July 15th, 2019, 2:18 
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Recently MRT introduced a feature enabling manual fly height adjustments for Seagate F3 architecture.

I notice that some drives, especially portable models, have a barometric pressure sensor. In WD's models this is usually located at U7.

http://www.hddoracle.com/viewtopic.php?f=100&t=2483&p=18107&hilit=barometric#p18107

I'm proposing that the fly height could be manipulated by disconnecting the output of the sensor's op-amp and injecting an external adjustable DC voltage. This would make the firmware believe that the air pressure was higher than normal, thereby causing it to reduce the flyheight.

https://www.edn.com/electronics-news/4083247/MEMS-barometers-boost-hard-drives-GPS

Quote:
Freescale had already sold millions of MEMS accelerometers as airbag sensors in high-reliability automotive applications before it entered the consumer electronics market a few years ago with accelerometers for hard drives that can lock up their heads when dropped. Now the company is hawking a companion digital barometric pressure sensor that it says will let hard drive makers optimize head flying heights for maximum information density. The digital pressure sensor debuted today (June 2, 2009 ) at the Computex expo in Taipei, Taiwan.

"A hard disk can increase its storage capacity by 200 percent by lowering the slider's flight height from 10 to 7 nanometers," said Wayne Chavez, consumer and industrial operations manager of Freescale's sensor and actuator division. "Using our barometric pressure sensor to measure air density, hard disk drive makers can reliably lower slider flight height to 5 nanometers, which increases storage capacity by four times."

Without a measure of air density, hard drive heads can't run at minimum flight height without failing when barometric pressure changes dramatically, such as onboard an airborne plane.

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 Post subject: Re: Fly height tweaking -- barometric pressure sensors
PostPosted: July 16th, 2019, 4:53 
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Joined: September 17th, 2016, 16:06
Posts: 103
Location: India
Exciting. Can test stuff for you as I am good with analog elec and ECE grad now :) Was not active here since quite a few months as I was getting done with college.

I agree, I can help you with a precision opamp ckt that can generate DC reference as a log function.
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 Post subject: Re: Fly height tweaking -- barometric pressure sensors
PostPosted: July 16th, 2019, 4:56 
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Joined: September 17th, 2016, 16:06
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It also makes me think. If the fly height is going so so low! Wouldn't class 1000 get obsolete with new drives?
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 Post subject: Re: Fly height tweaking -- barometric pressure sensors
PostPosted: July 16th, 2019, 5:12 
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I would start by calibrating the pressure sensor. That is, measure the output voltage of the final op-amp at various air pressure settings. This should give you some kind of graph, hopefully linear. (You might need a few days or weeks for there to be useful variations in the weather.)

Now disconnect the op-amp and inject an external DC voltage corresponding to the desired air pressure. You could use a trimpot at the centre of a potential divider.

Code:
+5V o-- R1 -- VR -- R2 --o Ground
               ^
               |
            to op-amp

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 Post subject: Re: Fly height tweaking -- barometric pressure sensors
PostPosted: July 16th, 2019, 11:52 
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Hi I was just Googling and came across the fact that most of these sensors use I2C and /or SPI. I still can not logically think if this is a closed loop system or an open loop system. If closed loop, what can possibly increase or decrease the pressure inside. Mostly the RPM fine tuning. If it is open loop then it would be something like this as a bounded function. Eg: If SPI data value = XX then normal operation. If below XX then do ZZ else do something different.

SPI being 3 wire spi may be tappable because it fails requirement of MOSI AND MISO both at once. 4 wire SPI would have conventional hookup which is hard to tap. The voltage divider that you displayed may not really work because these sensors have ADC within and relay values over the com busses fabricated inside them. These busses hook up directly to uC.

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 Post subject: Re: Fly height tweaking -- barometric pressure sensors
PostPosted: July 16th, 2019, 11:56 
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Joined: September 17th, 2016, 16:06
Posts: 103
Location: India
So possibly what can be done is. One should be able to power up the HDD ie SPIN for ETERNITY at a constant RPM that it always spins at under normal working conditions. Find the SPI /I2C bus coming fro these sensors and try to fetch data from there.. That would be reading possibly mimicing of how the HDD behaves when its actually functioning.
To inject any value into the system as if the value is generated by sensor has to be a programmed dummy value as these senors do not relay analog values and only digital data, they go digital direct.

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 Post subject: Re: Fly height tweaking -- barometric pressure sensors
PostPosted: July 16th, 2019, 17:13 
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If you follow the link to the HDD Oracle, you will find that @Severance and I have worked out the circuit diagram for the barometric sensor. The sensor itself appears to be a purely passive device consisting of 4 piezoresistors in a Wheatstone bridge configuration. All the signal conditioning electronics is external to the sensor. There is no digital circuitry, only two op-amps.

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 Post subject: Re: Fly height tweaking -- barometric pressure sensors
PostPosted: July 17th, 2019, 2:06 
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Joined: September 17th, 2016, 16:06
Posts: 103
Location: India
Great, sorry i missed that link and didnt read. However MPL115A is all digital which is mentioned in second link.


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