Limit switches, inertia and controllers

**Voicecoil** · 29-01-2019

So, I'm trying to decide exactly where to put the limit switches exactly, however it seems to me that there's quite a few variable involved. The big unknown is how quickly the gantry/carriage (gantry will be about 22Kg all in, carriage 7Kg less) will slow down, and does the controller (UC400ETH) actively brake the steppers or just shut them off. Obviously I don't want to lose cutting area by setting the limits too conservatively, but neither do I want to risk breaking stuff

Anyone got any ideas please?

**Neale** · 29-01-2019

It's a good question, but in practice I don't think that you need to worry too much. I'm using Mach3 and the CSMIO/IP-M motion controller, but I'm assuming that the UC300 is similar in its functionality. I home at 40% of rapid speed (Mach3 configuration parameter); I believe that Mach3 respects the acceleration parameters you have specified for each axis so that it will never try to slow the axis faster than the steppers can handle - assuming that you have set appropriate acceleration/deceleration parameters, anyway! So, the motion controller does not need to brake the steppers as such, it just reduces the pulse rate sent to the stepper as it would for any change of speed. One very useful trick that is not obvious is to arrange the proximity switches (if that's what you are using) so that the trigger passes by it, not heads straight for it. This means that if the axis does overrun for any reason, it doesn't smash the switch! You might think that this will give less accurate trigger points but that doesn't seem to be the case in practice. You probably only need a few millimetres between the limit/home switch position and the physical end of travel, to only for the slight overrun as the first phase of homing completes. Again, I assume that the UC300 is similar, but the Mach3/CSMIO homing process involves driving the axis towards the home switch until it triggers. There will be a little overrun as the axis is decelerated. The axis is then stopped and reversed more slowly until the switch triggers again. The second trigger point is where the home position is registered.

Oh, and my gantry weighs around 40Kg and I have absolutely no problems.

**cropwell** · 30-01-2019

Originally Posted by Neale

One very useful trick that is not obvious is to arrange the proximity switches (if that's what you are using) so that the trigger passes by it, not heads straight for it. This means that if the axis does overrun for any reason, it doesn't smash the switch!

Yerss - My MD machine was designed with axial approach to the homing switch. It is too easy to jog onto it and crush the coil. It was an expensive mistake buying a replacement switch from MD as they charged £17 for the replacement (£2 on AliExpress).

The inductive switches such as the LJ12A-4-Z/BX trigger OK on the aluminium of the gantry, but, although they give a supply voltage range of 6 - 36V they need 12v min to operate reliably.

The -4- is the trigger distance

**Neale** · 30-01-2019

I agree with comments about voltage - when I was setting up my machine I tested the proximity switches on a variable-voltage power supply and they were getting a bit flaky once down to about 9V. 12V gives solid performance. I run mine on 24V, partly to allow for any voltage drops where I have a pair wired in series (where an axis has + and - limits, I pair these together to share an input). In testing, I wired 4 in series, just to see what happened, and that worked OK as well. Not surprising if you look at how they work, but it's comforting to know that it's also true in practice as well as in theory!

**routercnc** · 30-01-2019

If you haven’t built a machine before it’s easy to assume that the steppers are like regular motors and will spin on and coast down to a stop allowing the axis to sail on a bit. But as Neale says they only move if they get a series of step pulse signals. If the pulses stop then the motor pretty much stops dead and (someone jump in here!) is held in position by the coils being energised in one place. In fact Estops make a bit of a mild bump/bang as the stop is so quick so you only do it when called for. I’d be surprised if they overshoot more than an 1/8 of a turn and probably less.
The estop is so fast that under normal conditions it is kinder to the machine to use a software accel/decel ramp. What looks like a soft ramp up to speed and soft ramp down to a stop is actually the motor following the ramp parameters you asked for in the setup, not an uncontrolled overrun In MACH3 this is easy to do and other software no doubt has this as it is required for nice machine control.
So to cover all bases (homing and fast jog) maybe allow 10mm between home and limit. Make the home adjustable and you can claw some travel back once built and tuned.

**cropwell** · 31-01-2019

I always thought that the drivers went to half current hold when not actually stepping (if that feature selected). Maybe half current only comes in after a time delay allowing the motor to overcome any overrun due to rotational momentum? Anybody know?

**Voicecoil** · 31-01-2019

Thanks for the advice lads, sounds like it's less of a concern than I thought it might be. Incidentally I tested some LJ12A-4-Z series switches (of Chinese origin) and the actual sensing distance with 8mm thick material was 3.5mm for steel and 1.5mm for ali.

**Neale** · 31-01-2019

Sensing distance is one thing, but the other thing to watch is the hysteresis - difference between switch-on and switch-off points. I had problems with my machine where I homed Z, then X and Y together. However, because of the tiny difference between on/off switch points, the vibration during X/Y homing was enough to trigger the Z home switch which by this time had become a limit switch. Fortunately, there is a Mach3 setting which offsets the home position slightly after homing which avoided the problem, but it's something to watch for. Spec sheets for more expensive switches seem to suggest something like 10% of the switching distance but mine was rather less than that.

**routercnc** · 02-02-2019

On this general subject there are at least 4 ‘end of axis’ type features:
Home (physical sensor to home and zero the axis)
Soft limits (software detection for end of travel)
Limits (physical sensor for end of travel and invoke stop or estop depending on your setup)
Buffer (mechanical buffer for physical hardstop- not often seen on DIY machines but VMCs often have a rubber stop on the ballscrew as a last resort as these machines are heavy and move fast!)

I mention this as the other day I was jogging my machine around and managed to almost hit the home switch ( proximity type). I don’t have soft limits on and the home switch also feeds into my end of travel pin and does not invoke an estop (by design). I haven’t had a chance to investigate further but it looked like when I approached the home at full jog speed it treated it like I had released the jog key and decelerated to a gentle stop. But because it was moving quickly, unlike during homing, it used up all of the 4mm travel. If this is the case I will have to tune the accel / decel to be quicker or set up soft limits. My sensors are head on as I couldn’t get them side on. Anyone else seen this or do you estop if the end of travel is detected?