Question about Limit switches

[watsonstudios]

Is it common or even nessesary to have limit switches on your axis'?

Do most people put limit switches on their machines?

Are they for safety - so the axis doesn't slam into the end of it's travel? and if it did, would that burn out the motors if they are trying to move the axis but is at the end of it's travel?

If the axis hits a limit switch, does it screw up the step count, or does the motor (or driver) still know where it is? Or should the software (i.e.Mach 3) know not to go far enough to hit a limit switch.

If someone could explain in detail the purpose and workings of how limit switches are incorporated, that would be wonderful.

Jon

[ger21]

Limit switches are safety devices, to prevent the machine from crashing and causing damage or destroying itself. If you have servos, then the motors could overheat if the machine crashed against the stops. Steppers probably wouldn't be damaged, but if they are strong enough, they could tear apart the machine.

If you hit a limit switch, the machine will usually lose position and need to be homed or referenced. Mach3 incorporates "soft" limits (software limits) which you can set up to stop the machine before it gets to the limits, which I think will avoid losing steps.

Basically, the purpose of the limit switches is to prevent machine crashes. They are usually installed at both ends of each axis. One switch on each axis can also act as a home switch in addition to a limit switch, which will save you a few switches.

[HuFlungDung]

A stepper system will likely be out of position if it hits a limit, because the control issued the move and has no feedback to tell it how far the machine moved before the power was cut.

A servo system should not lose position when hitting a limit. The limits should be positioned before the dead stop so that the machine has time to decelerate under control of the system. Even then, the encoder feedback on the system cannot be fooled, and the position is always accessible from anywhere along the slideways.

Many servo systems will permit motion of the axis off the limit switch in the one free direction.

Needless to say, the interruption caused by hitting the limit may require rerunning the program from the beginning, but midprogram starts are possible under certain circumstances.

[watsonstudios]

Another question I have now that I'm getting close to hooking up the motors and electronics. The reason I'm asking about home switches is because someone told me you need them to use the soft limits feature in Mach3.

I assume Home switches are the same type of switch as a limit switch(a little black box with a plunger usually depressed by a flat metal arm with or without a roller on the end) My question is, if I'm using 8 limit switches(2 per axis on a four axis system) Do I need seperate home switches?
Do I add more switches in addition to the limit switches? Can I use the same switches for home & limits? Do I only put home switches at one end of each axis where I want the machine to zero out?

Can someone please explain what, where and how many switches I will need in a typical setup? I know where to put the limit switches on each axis but I dont know how & where to incorporate the home switches.

[mphel]

As I understand it, you only need two switches per axis, if using Mach software it will treat the limit as a home when you tell it to "reference all" if you set it to reference in a negative direction it will treat the limit switch in that direction as a home switch until it has been triggered and reset. With a single parallel port you have limited inputs so its best to configure home and limits to share an input so you dont run out.
As far as soft limits goes, if you set them in configs to the usable size of your table they will still work if you dont have switches. jog the axis to their zeros and then "reference all" and the machine coordinates will be set to zero in this position, then when soft limits are enabled the machine should stop movement when it reaches the values defined in configs. The only problem will arrise if steps are missed and the machine returns to zero, it will foul before the DROs think it has reached home! Hardware switches will obviously overcome this problem. hope this helps a bit!!

[Greolt]

Originally Posted by watsonstudios ......................if I'm using 8 limit switches(2 per axis on a four axis system) Do I need seperate home switches? Do I add more switches in addition to the limit switches? Can I use the same switches for home & limits? Do I only put home switches at one end of each axis where I want the machine to zero out?

If using Mach then you can use the limit as a home switch. Mach knows when to see it as a home.

Also you can have the switch move with the axis and have actuators each end for limits. So that means only one switch per axis.

Don't know about your fourth axis. Is it rotary? Then probably only one switch too.

Another thing is depending on how many inputs you have available you can wire multiple switches to one input.

Greg

[watsonstudios]

Originally Posted by Greolt Don't know about your fourth axis. Is it rotary? Then probably only one switch too. Another thing is depending on how many inputs you have available you can wire multiple switches to one input. Greg

I actually only have 3 axis' the fourth is really having a dual X axis drive so I will still need 4 motors and drivers. So I guess I will only need 6 switches to cover all my axis' and if I can use the same switches as home switches, that would be great.

Now, are the home switches only at the end of the machine where you want zero to be? In other words, only using 1 switch per axis for home? Say at the top of Z, the back of X and to the right of Y?

[rustamd]

Originally Posted by watsonstudios I actually only have 3 axis' the fourth is really having a dual X axis drive so I will still need 4 motors and drivers. So I guess I will only need 6 switches to cover all my axis' and if I can use the same switches as home switches, that would be great.

For limit switches you should be ok with 1 per axis.

Originally Posted by watsonstudios Now, are the home switches only at the end of the machine where you want zero to be? In other words, only using 1 switch per axis for home? Say at the top of Z, the back of X and to the right of Y?

You are correct about Home switches, only on one end of axis. However, you would be much better of with having two on your X axis(if this is the one you have two motors on) so it will square itself when it homes.

So that would mean you need 7 switches, 3 for X, 2 for Y and 2 for Z.

[watsonstudios]

Originally Posted by rustamd However, you would be much better of with having two on your X axis(if this is the one you have two motors on) so it will square itself when it homes.

I don't know if that statement makes sense to me because of the rigidity of the Y axis along the X rails. The Y axis is fixed & square along the X so the motors would have little or no effect on how square it is unless one motor pushes further than the other in which case one motor will give out before it twists the Y off square. I could be wrong but these are 495oz motors and I don't know how that translates into pounds of pressure.

(Let's call the two X axis motors 1 and 2)
When having both X axis motors, one master(1) and one slave(2), I would assume that the master would control the slave (in Mach3) so one could not work independently of the other - which would prevent any twisting or racking.

On the X, What if motor(1) fails or the wiring fails, will the other motor(2) fight until it fails or burns-up? Or will Mach know that motor(1) is not working and shut motor(2) down or stop the system? It's an extreme scenario but it could happen.

Hope I didn't stray off topic too much.

[rustamd]

It still home's independently.

[rustamd]

What i mean, it can share one switch, but if you have two home switches, i will home indepdently. If it homes exactly at same time you are lucky, that means your gantry is not racking. But pretty much 90% of the time you will have some racking(might be very small, but still). This will help it have a good start for next operation. Of course everything is just mine opinion.

[shashank ayyar]

Originally Posted by HuFlungDung A stepper system will likely be out of position if it hits a limit, because the control issued the move and has no feedback to tell it how far the machine moved before the power was cut.

is there a way to hook up the switches so they issue the feed back?