Newbie Home switch and Limit, how it works ?

[Calico]

does anyone can explain to me how Home switch works ?
where to put it in an axis ?
How accurate if I use mach2 ?
how accurate if I use Proximity switch ? any other choice ?

If I turn of the machine, does the machine remember the previus coordinate ?

do I have to calibrate everytime I turn on ?

mach2 can remember that ?

rud

[cadum61]

I have the same question about the Home Switch location. Is there a traditional location for one? Should it be in different locations on different axis or is it a personal preference thing?

[Bubba]

OK, I will take a stab at answering all the questions
Location on axis:
IF you are using a seperate switch, it can be anywhere on the axis as your have to give it position information when you set it up. In my case, I use combination home/limits. Therefore, they are located at the extreme end of the travel. In some cases, this is at the "0" end and in others, it is at "max travel". Your choice.
ACCURACY: As with all things, this will depend on the type of switch and the care used in setting it up.
PROXIMITY SWITCH ACCURACY: I am not sure on this one, as I have never used one. This would take some research on the data sheets, and would most likely depend on the switch used. As to other types, there are the common micro switch (in various kinds of cases or not) and optical switches. For maximum repeatability I prefer the optical units.
SETUP: Yes, the homing routine should be run at each power on. Even though most programs will remember last location, often there is a slight movement of an axis when you power down or up. Exact location cannot be guaranteed on power up. I think this is a good habit to get into. Home as a routine part of setting up for a job.

[JRoque]

Hello,

In my case, I positioned the home (which is also my limit..) switch .5" from the hard stop of the axis. When I tell Mach to home the machine, it turns the motors in reverse until it sees the home switch trigger. It then advances forward slowly until the switch is off again. That's home.

There are several types of switches you can use. They vary in price, precision and application. Start with the mechanical switch that opens/closes a switch as the carriage pushes it. There are magnetic switches that use a magnet on the moving carriage and a magnetically activate switch on the frame. There's the optical switch which uses a tab on the carriage to block a light source (LED) from reaching a photosensor. And there's also a proximity switch that detect changes in capacitance/inductance from an approaching metal (carriage).

All of these have Pros and Cons. Some are:

Mechanical= Pro: low cost, simple. Con: relatively inaccurate (but work fine in most applications), relative short life

Magnetic= Pro: low cost, simple, accurate. Con: may be effected by magnetically charged chips, etc. Longer life

Optical. Pro: accurate, mid-priced. Con: may be effected by chips or other materials that block the light source, relatively fragile

Proximity. Pro: accurate. Con: expensive, usually carries onboard electronics which might make it less reliable.

I chose magnetic switches and have had excellent results with them.

JR

[cadum61]

Bubba, JR,

Thanks for the advice. Sounds like using one of the limit switches as a home switch is the answer. I think I will try the magnetic switch as well.

Chris

[Geof]

JR wrote "... turns the motors in reverse until it sees the home switch trigger. It then advances forward slowly until the switch is off again. That's home." This is a good homing system but it relies on the switch being very consistent because it is travel along the axis that operates the switch and if the switch operation fluctuates several thousandths your home position changes each power up. Commercial CNC equipment homing has an added feature: the motor drives past home until the home switch is triggered, then drives forward until the switch resets and continues until an absolute position signal is sent by the rotary encoder on the servo drive. This means its stops with the feedscrew in an particular angular position which translates into very good accuracy and repeatability in linear position of the axis. With steppers there is no rotary encoder but it is possible to simulate the absolute angular position using a disc with a small slit mounted on the screw, an optical detector and a bit of latching circuitry. When the home switch is triggered this latches the 'home' signal on, the motor reverses and the switch resets but the home signal stays latched and the latch is not released until the optical detector sees the slit. This is the home position repeatable to maybe less than 0.001". It is a good idea to have a direct wired back-up switch behind the home switch so that if the latching circuit fails for some reason this switch triggers before a hard stop is reached.

P.S. Don't ask me about the latching circuit; I made all the mechanical components for a system like this many years ago but is was an associate who did the electronics.

[cadum61]

Thanks Geof,

The electronics is the easy part for me. That is a great idea to use the optical detector to get a more accurate home position. My concern will be how to mechanically connect all of these switches and devices and keep them free from the coolant and metal chips.

Chris

[Geof]

Chris;
Don't get trapped into thinking you have to put the slotted disc and optical detectors at the same end of the feed screw as the home switch or motor. Put an extension on the opposite end into a small enclosure and have the disc and detectors in there away from stuff. You also don't have to worry about the disc running particularly true; just firmly attached and repeatable.

[binary_d]

Are the Home switches patched directly into the parralell port?
Do you need an interface board?

I am useing Mach2

Side question: Is the E-stop switch patched directly to the printer port ( pin 10)? What would I then set mach2 at if it is...active low?


Would a isolated conductive pad adheared to the way and a return probe on an isolated mount work for a contact switch for home and limit?

Larry