using encoder index for homing and a axes position

[rokag3]

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

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

[Zig]

I am using linear scale with an index pulse equivalent to the resolution of the scale.
The pulse is used to implement the homing signal. The homing signal is then used by EMC2 to home the machine.
The index pulse is very precise and repeatable but it does not last long enough ( in time ) for the homing function to be implemented where by the axis seeks the hone position, finds the index, returns back a small distance and seeks the index at a slower rate.
IF this werea conventional mechanical switch it would allow the axis to travel back a bit before the sigal was removed and then ot wuld seek at a lower speed.
I have therefore simply fed the pulse into a monostable which simulates the action ofa mechanical springloaded contac. This allows for some time for the axis to return to a pre homeindex position and repeat seek for home at a slower speed.

While latches and one bit memory logic is nice.. this ( Pulse stretcher) is a bit more intuitive and works.

[rokag3]

yes Zig this is the purpose of the d flipflop to detect this very brief signal my problem is how to implement it "in a clean way" in my system? I mean having something that will last and be reliable ?
in fact i need a good schematic

[rokag3]

in the thread "how to use index pulse for homing purpose we have this obsolete link.
http://www.cncbuildingblocks.com/homeindex.html
__________________
Gerry
Zig do you have a copy of it?

[ger21]

That link was to a commercially manufactured board that worked with mach3 for homing to an encoder count. It was made by Ed Gilbert, and you may be able to get a hold of him through the Mach3 yahoo group. He's no longer making the boards, and I don't think he'd give up the schematics.

[rokag3]

thank you gerry,
in fact the solution of tom hubbin work and it's cheap the only problem it's my mediocrity in electronic i will like to have a supervision from an electronic guru.
this solution is not new but hard to find schematic.I think that this was due to the price of the encoder with index. The very cheap amt 102/103 baked up by Mariss make this solution very interisting

[Zig]

Yassou Rokag3,

From the exchanges i think You wish to use mach3. Please list the homing sequence You intend to implement.

In a way of rationalising the question I use EMC2 and the homing sequence during which the software seeks the index at high speed and having found the index it steps back and seeks home index at a slower speed to optimise position accuracy.

[rokag3]

In a way of rationalising the question I use EMC2 and the homing sequence during which the software seeks the index at high speed and having found the index it steps back and seeks home index at a slower speed to optimise position accuracy.

efaristo for the yassou
this is the way to go and i am going to use mach3 but actually i am in the hardware and i want to do it clean it's for a mill lagun ftv2

[Zig]

Here is a part of my interface circuit.

I use the comparator chip with a 120 ohm load for the differential index pulse driver from the linear scales.

The pulse is condition and earth refferenced by the comparator before it goes to the CMOS timer configured as a pulse stretcher. The signal then goes to the header on the breakout board. the header also supplies +5V to the electronics.

Hope this is of some use to You..


EDIT.. you may not need the comparator front end.. depends on the encoder You use. You may have to adjust the duration of the extended pulse to suit your seek speed.

[rokag3]

thank you i am going to look at that carefully
Lucien

[rokag3]

hello,
in red the index signal
in blue the switch signal
in green what i want
in black what i have
how to configure my 7474 to have the green result ?
Lucien

[rokag3]

ok i find a solution at least on the simulator
the first switch top left represent the cheap switch optique fork or mechanical it is monitored on the logic analiser by the first line.
the second switch represent the encoder index it is the second line on the logic analiser
the output is represented by the green led NormallyClose
this simulator is free work with java and very intuitive
now what ckind of resistances etc.... this is my limit