pic: 6 speed?? Great in theory - lets see how it works!

[kmcclary]

I just ran across this old thread... Very cool idea.

OOC, Did it ever get made? If so, how did you solve the shifter problem?

My $0.02... Several methods of shifting immediately come to mind:

Method 1 - Threaded rod, Globe Motor and 10-turn pot feedback:

Use a high pitch threaded rod to drive the indexer, such that the entire run from one end to the other is less than 10 rotations.

On one end, attach a Globe gearmotor, with a "slippery" fitting (ex: use a short section of vinyl tubing as a "coupler"). This prevents damage in case of a software error, as the drive will slip if it hits end of travel.

On the other end, attach a 10-turn potentiometer. Wiring: one end to +5V, other end to Ground, wiper to an analog input. Attach it with a hard coupling, but set it so that the pot never over-runs. This gives you an analog voltage into the RC, proportional to the position of the indexer.

Calibration: Manually set the indexer into the center of each gear position. Record that voltage value into a table. Jog it around to record the RANGE of values that still legitimately set a solid gear ratio for each gear setting as well. Taking these error values (epsilons), look them over and decide upon a good maximum error band value you'll allow that still guarantees you are In Gear no matter which gear you select. Call this your Epsilon.

(I'm assuming here all of your drive motor Victors are set to "coast", to protect the gearbox from meshing damage.)

Now, to shift: Your software simply "hunts" for that value, within "+/- Epsilon". Look at the current pot value. Idle the drive motor, Turn on a "Shifting" light on the Operator's Panel. Move the Globe motor in the right direction to increase or decrease the value. Once within Epsilon, STOP, turn off the Shifting light, and re-enable the drive motor. This helps keep you from tearing up the gearbox while shifting, no matter what the operator is requesting of the drive.

Method 2 - Threaded rod, Geneva Wheel idea for indexing, Globe Motor, 10-turn Potentiometer, Home Switch for Geneva drive wheel:

Use a threaded rod for the indexer motion, and add a 10-turn pot to it for feedback, as before. This is now your "gear selected indicator"

On the other end, design and add a Geneva Wheel as an indexer.

See: http://en.wikipedia.org/wiki/Geneva_mechanism
Mechanism: http://upload.wikimedia.org/wikipedi..._animation.gif

Drive it with a Globe Motor. On the drive wheel, add a notch for the green wheel's "the pin is away from the Geneva Wheel" position, and monitor it with either a micro switch or an opto interrupter, connected to one bit of Digital Input. This forms the Home position of the drive wheel for each gear position, where you will stop the motor. (I can show you how to wire this, if you need more information.)

Software: Again, record the pot's analog value for each gear position, and calculate the overall Epsilon.

This time though, the analog value will be used as an INDICATOR of which gear you are in. You still need the Epsilon, as the analog input values WILL drift with slightly with temperature shifts (a known effect of A/D conversion systems). That is easily implemented by a CASE statement, that says if whenever the values is BETWEEN (value-epsilon) and (value+epsilon), AND the HOME is hit (IOW the globe shifter motor is OFF), then it's in THIS gear...

Shifting: As before, coast the drive motor, turn on the SHIFTING indicator, and use the current value to determine which way to kick on the shifting motor. Once the HOME switch is off, use that signal to keep the motor on until the HOME switch is hit again (one gear step). If you're still not in range, keep going. Once the POT is in range, keep going until the HOME switch is off as well, then stop. Turn off the SHIFTING indicator, and re-enable the drive motors.

Method 3 - Two Indexer Cylinders (Up, Down), ratchets, and a Potentiometer or switches for feedback

Two tiny cylinders: 3/4" bore, and 1/2" or 1" stroke (whatever the design calls for). One cylinder is INDEX UP, the other INDEX Down.

Mechanically set it up such that each full cycle of one of the cylinders runs a ratchet step system, that either increases or decreased the gear by one step. ("The design of which shall be left as an exercise for the reader"... Don't you just HATE that when your teachers do that???)

Current position is indicated by either switches, or a potentiometer. One way to connect the potentiometer is via an inverse rack and pinion system, watching the throw of the shifter. An alternative is to use micro switches at each position.

If you use switches, but become short of digital input lines (you need them for other reasons), wiring lines can be saved by:
a) attaching the switches to a Priority Encoder chip, which converts single lines into a binary value. (This is roughly a $1 IC in most logic chip families.) or:
b) attach the switches to an R2R resistor ladder to encode them as an analog voltage, which is then connected to one channel of Analog Input. Decode like a pot as with method 2 (bands of values equal each gear position), as again the A/D converters do drift, and values are rarely EXACTLY the same from one sample to the next.

For safety to keep the mechanism from tearing itself apart if an indicator switch fails, I'd also make sure you either have some End of Travel switches, and/or set up the mechanics such that attempting to index beyond the end of travel will cause something to slip, vs tear up.

Does this make sense?

- Keith