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Author Topic: Twiggy track 0 calibration sensor experiment  (Read 6298 times)

sigma7

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Twiggy track 0 calibration sensor experiment
« on: February 21, 2025, 03:18:24 am »

in progress... will be edited

Recently I had a few disks that had some tracks that would not read at all.

Speculating that the problem may be that they were written on a drive with a differently calibrated track 0 calibration sensor, I wanted to try adjusting the sensor position.

However, I didn't want to disturb the factory setting unless I had a way to put it back. I spent weeks pondering how to accurately measure the current position so that I could restore it.

At 62.5 Tracks Per Inch and 8 steps per track, the resolution of the head carriage is 0.002" (0.0508 mm), so I figured I needed to be able to restore the position within 0.001" to safely consider moving it.

A complication is that the optical sensor is not mounted in a way that tightly restricts its rotation, so I would need to measure the carriage location when the sensor triggers rather than the combined location and orientation of the sensor itself.

Then the light bulb went off, and I had a nice nap. Having slept on the problem I realized another approach was available, which is to switch in a separate adjustable sensor so that the factory setting can be left undisturbed.

With a switch I could then toggle back and forth between the factory sensor and the adjustable one, making a particular recal operation lock on either position in a repeatable way.

Reviewing various revisions of the Twiggy digital board schematic, one sees that the track 0 cal sensor circuit had been redesigned a few times. The changes appear (to me) designed to improve the precision and consistency of the trigger operation. I presume this is to make the Twiggy tolerant of swapping digital boards after the sensor position is set. The final design uses 1% resistors for the opto-emitter current and opto-transistor load, and a comparator to minimize hysteresis.

To add a sensor in parallel without affecting the current when the original sensor is in use, I opted to use a DPDT switch to switch both the power input to the opto-interrupters as well as the outputs.

I 3d printed a holder for an opto-interrupter, wired it into the DPDT switch on a small perfboard with connectors to interpose it between the circuit board header and the factory sensor socket. I made a clip-on fin out of the thin SS from a 3.5" floppy disk shutter to add to the carriage.

The final result did seem to work out for the particular disks... when trying to read the problem tracks/sectors, I used the "read varying speed" option which gives one the ability to recal more-or-less on demand and with some trial and error, was able to read all of the sectors.

However, you probably don't need to make one:

There is some speculation that BLU, at some point, will have the ability to do off-track micro-steps during the read-varying-speed option, making the adjustable cal sensor unnecessary.

... and ...

Since working on the adjustable cal sensor, I think I have discovered that the FDC uses the track 0 calibration as a fixed datum only when formatting.

When reading (or reading address fields in preparation to write data), it does some off-track micro-steps to hunt for the best position to read a particular track. I don't know any details yet, perhaps this happens only at the speed check stage for a track, and it may be very limited or not very smart/reliable.

Since the speed bytes and address fields are written only once when the disk is formatted, it isn't clear how the data fields of these particular disks could have a small portion that doesn't align in the same way as most other tracks on the same disk. Perhaps a mechanical glitch such as the disk not clamping correctly when these portions were written, or some electronic glitch causing the carriage to be out of position.

A potential use of a (mal) adjustable cal sensor would be to format disks that are intentionally out of alignment; eg. for testing the ability of other drives to compensate. If someone needs such a disk, let's discuss.

Some pics https://www.flickr.com/photos/114907644@N04/albums/72177720323983848

« Last Edit: February 22, 2025, 04:59:59 pm by sigma7 »
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sigma7

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Re: Twiggy track 0 calibration sensor experiment
« Reply #1 on: February 21, 2025, 03:18:37 am »

This will print easily upside down without supports.

To assemble the sensor bracket, I used M3 screws and heat-set inserts.

Common M3 screws have a pitch of 0.5mm, so moving the sensor by a full track is 4/5 of a turn. When reading with BLU, the TrX error counter is non-zero when the head is over the wrong track as determined by what the address fields say... ie. you've gone too far.

The rubber band is to keep the jackscrew seated against the chassis and provides some friction to maintain its position.

The flat-head mounting screws are 6-32 about 3/8" long. I prefer to not use countersunk screws on plastic due to the splitting that often occurs as the plastic creeps over long periods, but there is little clearance when inserting the floppy.

I used an opto-interrupter I had on hand (which is obsolete) OPB384SB.

The sight-hole beside the interrupter is to simplify aligning the fin with the slot.

The design is in FreeCAD, if you want the FCStd file, send me a DM.
« Last Edit: February 22, 2025, 05:01:02 pm by sigma7 »
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