the idea is simple: why not using the index of your linear scale or cheap amt102/103 in conjunction with a mechanical or optical switch to obtain a very precise and cheap homing function ?
some guy put the index of the encoder directly on the home line.problem the signal of index of the encoder is very brief.
one solution is to use a mechanical switch to the line data of a d type flip-flop
7474 and the signal index of the encoder to the clock. This has been explain very well here
http://groups.yahoo.com/group/CAD_CA...9?xm=1&m=e&l=1
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Hello Carl,
Here is one way to do it with half of a LS74 dual D flip flop, two 1
kohm resistors and a normally open limit switch.
The normally open limit switch is wired with one contact to ground and
the other in series with a 1000ohm resistor to +5v. The junction of the
switch and resistor is the switch output and goes to the flip flop Reset
line and D input.
The encoder index output goes to the flip flop clock input.
The second resistor is used to pull up the unused Set input. One end of
the resistor to +5v and the other end to flip flop Set input. Some folks
don't use a resistor and just wire +5v to the Set input. My early days
of EE was designing to meet mil spec and we tied NOTHING directly to
power except power pins. Unusued TTL inputs were pulled up via
resistors.
Be sure to wire the flip flop power and ground pins to the power supply
also. There should be a 0.1ufd, a 0.001ufd and a 10ufd electrolytic in
parallel with the flip flop power and ground pins. These capacitors
should be located very close to the flip flop.
How it works: Travel toward the limit switch until the limit switch
closes. The flip flop Q output will immediately go low when the limit
switch closes. Then travel away from the limit switch. On the first
rising edge of the encoder index after the limit switch opens, the flip
flop Q output will go high as the encoder index rising edge clocks the
flip flop.
When you first power up you should home twice since the flip flop may
power up either high or low. This will guarantee a valid home. You
should NOT be within one index pulse of the end of the travel range that
is opposite the limit switch.
Now this is not the ideal solution since you would probably prefer a
normally closed switch. That way the default for a disconnected limit
switch is safer. I did this for somebody several months ago. I'll have
to dig up my notes to see how I did that with a N/C limit switch. I may
just have used the other half of the dual flip flop as an inverter.
Tom Hubin
I test it and it work but now i am a litle lost in space with the isolation problems.
I use the power supply of my DRO to power up my encoders the gecko 320 encoder+encoder- is bridged with a 470ohms. the 0V of the DRO is linked with the encoder-
I use the 5volts of my pc-2-route model m2 sound logic break board for the switch.
1st problem: I will prefer to use 12 volts for the switch and use an entry of an optocoupler(tlp521/4) to go to the data line of the 7474 what resistance should i put at the entry of the tlp521?
2nd problem:I will use a second entry of the tlp521 for the index signal of the encoder. the 7474 will be powered by the break board is this ok?
3rd problem the notQ line of the 7474 should be attach to the signal entry of the break board how?
4th Mr Hubin rise some interisting point using normally close switch and make a little reset circuit when power up unfortunately i can wire and understand digital electronic but i am not able to make a "clean" circuit if someone can help ?
in case of rotary encoder a target can be fixed to the main axe an optical fork can play the role of the switch