Hi folks,
I wish to attempt a recovery from a misaligned head on a set of Rodime RO202E drives by applying the procedure described here:
https://www.pdp8online.com/mfm/microstep/index.shtml . I have the stepper controller called for on that page, and I believe it's appropriate to the drive. To increase odds of success, I'd hoped to ask a few questions ahead of time. Bear with me --- I wish to ask in full detail :-)
1. Stepping.
[Microstep] says, "The Miniscribe was using a half step technique", while [Manual] says "The two phase stepper motor operates in a fractional-step mode under microprocessor control (0.45° per track)."
Does it sound like [Manual] is describing a similar configuration to the Miniscribe? 0.45° per track is 800 steps per rotation, which is 2*400 steps, where 400 is apparently a standard configuration for a stepper and a value also mentioned in drive_read_stepper.c.
See also Figure 18 describing the patterns of current going through the stepper wiring on PDF page 34 of [Manual].
2. Track zero.
[Manual] indicates that the track 0 sensor may really be a "track 3-4 sensor" --- it sounds like the Rodime controller detects this position and then counts steps backwards to track 0:
Track zero is defined from a combination of a particular stepper motor phase (one of eight) and a flag mounted on the drive arm. This flag is set up, using a special assembly fixture to switch on opto-interrupter (track zero transducer) mounted on the base casting. The flag is set to switch between tracks 3 and 4. Track zero requires the flag to be true (transducer interrupted) and the correct stepper phase to be selected.
Will it be necessary to alter drive_read_stepper.c or some other code to account for this way of detecting track zero?
3. Stepper wiring.
I'm unfamiliar with wiring conventions for stepper motors. While drive_read_stepper.c talks about A+, A-, B+, and B-, Rodime used different nomenclature: ACa, ACb, BCa, and BCb. These are associated with wires that are yellow, white, red, and blue respectively, and I see that this matches the wire ordering for the Miniscribe in drive_read_stepper.c. Does this sound like a likely correspondence?
There is no label of any kind on the Rodime's stepper motor. Which brings me to:
4. Stepper current.
- PBL 3717 output currents top out at 1.2A
- The way the 3717 is used in the circuit would require the drive to use a control voltage of 14.5V to command the 3717 to deliver the full 1.2A. I don't think the control signal could easily exceed 10.5V or so (it's driven by an LM324 with 12V as Vcc) which means 870mA as wired, although I could be wrong about the LM324.
Anyway, how would you approach driving a stepper you knew nothing about? Start low and ramp up?
(Stepper voltage will have to be 12V per the schematic --- or a little less since the voltage passes through diodes before going to the 3717. Meanwhile, it's amazing that the wee chip on the Sparkfun board can deliver 2A...)
5. Track zero again.
Do I understand correctly from [Microstep] that I'll have to convert the track zero signal down to 3.3V?
Thanks for reading this far --- with your answers I'll be able to give it a try as soon as I finish sourcing all of the connectors etc.
Cheers,
--Tom