About homing and switches...

[JW1977]

Hi!

Not much real experience of running a CNC machine here I’ve just been reading and building the thing for some time now.
So, got some questions about homing.

A. How and when do i set that zeropoint for all axes (home)?

B. There’s an expression “homing on index”. What’s index and how does that work?
I guess “index” gives the signal to the motion controller to home all axes but what is it?

C. I will be running CSMIO IP-M with hybrid servo drivers.
It seems I can use this homing on index function with these drivers, correct?

D. Does a closed loop system need homing switches?
Cant see why but still, i might have missed something.

E. Thinking about hall sensors for limit switches.
Can anybody point something out?

Thanks, JW

[AndyUK]

*ahem* Wheres your build thread? *stamps foot* We want build threads!

I'll try to answer some things...

A) I'd do it every time I start the machine up. You can either run a homing routine from the software which will use your homing switches, or you can just tell the software "Hey, you're at zero now".

B) My understanding is (and I'm sure someone will correct me if i'm way out) that homing on index just means setting the zero point based on the index (ie reported potentiometer position) of the servo, rather than using an external switch. To understand this, you have to know how the close loop servos work - they have a feedback potentiometer which moves only when the motor actually turns, so if something stops the motor rotating it won't change. This position report is called the index. I realise thats two circular ways of saying the same thing, but hey.

C) ?? I know its on IP-S ?? Surely it will say in the IP-M manual.

D) I don't think it does need them, but they could still be helpful. e.g. when you disconnect your drive motors from the ballscrew, the motor and pot might rotate but the CNC wouldn't move, this would change your index for the home position.

E) I believe hall sensors will require a magnet to be mounted as the sensing object. Take a look at induction switches too.

[JW1977]

Yes, the buildthread... I will get to that. :-)

A. When i think about it i begin to wonder if home and zero is the same thing?
I dont really know how to ask without making things complicated but what i would need to understand is the basics.
For example, you make a part in Fusion 360 and you have origo or 0. Will that also be the 0 for the machine or how does it know?
Further more is home for the machine just going to the end of all axes or is home that zero point?

B. It makes sense, Zero would be my indexpoint. It knows where it is and it can return beacuse it counts the steps all the time.

C. The function is there but they are talking servos, not hybrid steppers but it makes sense that it would work.

D. Will do, thanks!

By the way, had a look at your thread and i like your design. Its about the same type as mine.
Really like the "Epoxy project". I have been thinking about that myself in another project.

[Kitwn]

JW,
Not sure on the 'index' thing but even closed loop servos do not indicate absolute position so you do need homing switches. Whereas stepper motors merely assume each step pulse sent to the driver represents a real movement at the motor, the feedback sensors on servo motors produce pulses confirming that a certain amount of movement really has taken place. For both types of motor your controller must have a known starting position from which to count pulses and calculate current position.
It is possible to have multi-section sensor discs which can tell the absolute rotational position of the motor/disc but that still doesn't tell you where you are along the axis. The only way to do that is to fit a linear optical track (the best ones are apparently made of glass) alongside the rails and sense absolute position from that. A combination of linear track and rotating disc can provide absolute position with high accuracy but not within my pocket-money budget!

Kit
PS Andy beat me to an answer while I was typing. The type of servo motors I'm thinking of have an optical disc attached to provide a train of pulses as the shaft rotates rather than a potentiometer. Counting pulses tells the controller how far the shaft has turned.

[JW1977]

*Kitwn*
I actually thought the accuracy where that good. Ive got a Prusa Mk3 3D printer and if im not mistaken it works that way without switches.
I have to look into this some more but i guess it cant hurt using the switches anyway.
Your machine looks nice on the profile picture there!

[Robin Hewitt]

Bouncing off home switches is for setups where the material position is known and invariable.

For everything else you set the material then position the tool so it agrees with the G code.

[JW1977]

Are you meaning that bouncing of home switches is for open loop systems?

Would be happy for a brief explanantion of how you set the tool so it agrees with the G-code, real basic...

[cropwell]

Are you meaning that bouncing of home switches is for open loop systems?

Would be happy for a brief explanantion of how you set the tool so it agrees with the G-code, real basic...

First of all the Prusa Homes by detecting motor stall current, so it has a reference point for a start.

You need a reference point, otherwise your machine does not know where it is. So Home switches provide that. and set machine 0,0,0. If you have a fixture (jig) for the workpiece, you can then use work offsets to relativise your Gcode to the work. This gives you repeatability.

BUT you don't have to do it that way, especially for one-off machining. You can set x and Y zeroes manually above the workpiece and then touch-off against the material top (manually or with a touch plate and macro!). So your Gcode then operates as is relative to the 0,0,0 you have set on the machine.

Whether your system is open or closed loop is irrelevant, as you still need your start reference.

[Kitwn]

*Kitwn*
Your machine looks nice on the profile picture there!

The trick is not to look to closely, the shiny blue paint hides a multitude of sins. The Z axis is still the 18mm plywood mock-up. I really will get round to buying some 160mm wide aluminium bar to build the proper version soon, honest.
Kit

[paulus.v]

Hi JW,

Let me try to clarify what the homing on index function on a servo motor is.

The incremental encoder on a servo motor generates pulses when the motor moves. The motor driver is capable of counting the pulses and store them on power off but if, for example, you move the motor with the power off or by a power failure where the axis moves from inertia, that axis position is lost. So the absolute position of a cnc machine cannot be determined reliable from the servo incremental encoders data and homing sensors are needed.

Most incremental encoders have an additional "index" signal that provides a reference position of the rotor. It is output at each motor revolution. This signal can be used to increase the accuracy of the homing to an almost perfect and repeatable position. The homing on index routine moves the axis until the home sensor is triggered then rotates the motor to the index position. In this case the sensors accuracy need only to be grater than the axis movement of half a motor revolution. And the sensor be mounted so that when triggers the motor shaft is at about 180 degrees from the index position. Depending on the actual setup, the amount of axis movement per motor revolution, switches could have enough accuracy for homing on index.

Why is homing important?

First of all because if you have homing and set each axis maximum movement, the machine knows its limits! And will never hit a limit switch. If you load a g-code that exceeds the machine working area you get a warning. Otherwise you will only find out that you made a mistake when the axis hits the limit and ruins your work because...

Secondly, if something bad happens and you have to cut power to the machine you loose the workpiece position. If you have accurate homing, the workpiece zero position is referenced to the machine zero (home). If those numbers are stored you can easily resume the work after a power failure or if you want to pause and continue the next morning... just home the machine and go to the stored workpiece coordinates. And continue cutting where you left off.

Paul.