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Offline VNR

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Exact homing
« on: May 10, 2006, 07:43:18 PM »
Hi, im currently designing a Drilling machine (only one axis) for production pourposes on a job shop. Im investigating Mach3 and i want to know if is possible to do and exact homing. This means in the another production machines that i have in my job shop that when the home switch is reached in referencing and then the axis motion changes the direction action stops when a "0" is reached in the encoder on the servomotor not when the switch changes the state. This produce the same exact position every time the machine is referenced. Can i do this using Mach3 ? How ?

Thank you in advance, and excuseme for my poor english.
VNR

 
Re: Exact homing
« Reply #1 on: May 10, 2006, 08:12:53 PM »
I don't have Servo motors so maybe I don't understand something you are describing but, if you have Servos with Encoders don't the encoders always tell you exactly where you are?  If that is true then why don't you just set up your home position as X 0.000 Y. 0.000 & Z 0.000 then use a Go To Zero command to get to the home position.  You will be in the same place (home) every time.  Or did I misunderstand your question?

Regards,
Sid

P.s. your English is pretty good!

Offline Graham Waterworth

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Re: Exact homing
« Reply #2 on: May 11, 2006, 03:52:52 AM »
If you have encoders with the index pulse then this can be used to set exact zero point.

Mach will not do this for you, you have to write a visual basic script for the homing. (Or will it Brian)

Graham.
Without engineers the world stops
Re: Exact homing
« Reply #3 on: May 11, 2006, 02:18:18 PM »
You should be able to do it with the Grex at some point and you may be able to do it in VB but I have never tested it...
Fixing problems one post at a time ;)

www.newfangledsolutions.com
www.machsupport.com

Offline VNR

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Re: Exact homing
« Reply #4 on: May 11, 2006, 02:50:53 PM »
SSHNEIDER:  The loop is closed between the driver and the motor, the loop is open between the CNC and the driver. You only send step and direction to the driver form the CNC and the driver/motor dont miss steps because the encoder feedback. But there is NO feedback between the driver and the CNC, the CNC doesnt know if the driver REALLY gets the signals and NEVER knows what exactly position have the encoder.
The problem is that the CNC only knows the starting point, all the movements are based in that point, and that starting position is when the mechanical switch disconects, and it disconects not always in the same exact position because it only have a MECHANICAL precision. The others machines that i have in my job shop (ROMI) seems to stop when the encoder get the ABSOLUTE "0" position, because there is a closed loop between the encoder and the CNC (Siemens).

Regards,
Victor





Offline VNR

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Re: Exact homing
« Reply #5 on: May 11, 2006, 02:57:57 PM »
If you have encoders with the index pulse then this can be used to set exact zero point.

Mach will not do this for you, you have to write a visual basic script for the homing. (Or will it Brian)

Graham.


Graham:

   Do you mean connecting the encoder to the input pins and write a program in VB to get the ABSOLUTE "0" position ?

It sounds good.. I will check if this is possible.

Do you know any Encoder model that have output signals other than the used to connect to the driver ?

Regards,
Victor

Offline Graham Waterworth

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Re: Exact homing
« Reply #6 on: May 12, 2006, 03:26:44 AM »
Hi,

if you look at the USdigital site  ( http://www.usdigital.com/products/ ) there are many encoders listed that will retro fit to motors, some have index pulses that you could connect to a Mach input line.

You would need a micro switch to tell Mach to slow down to a very slow feed while looking for the index mark.

The encoder index mark needs to be set at 180 degrees minimum from the micro switch touch point to give Mach chance to see the index.

I tried this way and it works, the only problem is that the feed rate has to be very very slow.

I have my lathe setup slightly different, its on ball screws and DC servos with indexed encoders but,

I ended up using an optical sensor on the lead screw, it senses a slot in a disc mounted on to the end of the screw, this works better as the motor is geared 5:1 to the leadscrew so the detection time is 5 times longer.  I still use the slow down switch and have it set to trigger at about 270 degrees before the index mark, the Z axis zeros within + / - .005mm each time.

The lathe is not finished yet but its getting there.  I am working on the X axis zeroing at the moment.

That is why you need to use VB as Machs zeroing don't work this way.

Graham
Without engineers the world stops
Re: Exact homing
« Reply #7 on: May 12, 2006, 07:01:11 AM »
Here is a link to a schematic I posted earlier. The ideal is to have a long homing dog. When the table reaches the dog it triggers the home relay (input to Mach on). After decel and reversal when the table leaves the dog in the opposite direction the home relay is latched with the index pulse of the encoder. When the index pulse is encountered the latch is broke (input to Mach off).

http://machsupport.com/forum/index.php?topic=130.0

Darek
Re: Exact homing
« Reply #8 on: May 13, 2006, 09:44:59 AM »
Victor,

To answer your question, no, Mach does not home to an encoder index. It is designed to home to the limit switch. Attempts have been made to adapt circuits into the homing relays to allow homing to an index. The most effective is to adapt a slotted wheel to the motor or leadscrew and use an optical sensor ANDed with the limit switch.

This is a weakness of Mach that will probably not be fixed.

Doug
Re: Exact homing
« Reply #9 on: May 13, 2006, 11:00:31 AM »
This is a weakness of Mach that will probably not be fixed.

Let me re-phrase that:

It is a weakness of all open loop step/direction cnc software.

Doug