On Mon, 2008-10-20 at 13:06 -0400, Stephen Wille Padnos wrote: > Mark Cason wrote: > > > I am new to this list, so this just may be a rant, but the purpose of > >a E-Stop system, (and I do mean system, as this is how it should be > >looked at) is NOT to remove power from a machine, but to FREEZE the > >machine in place, at the expense of damaging the machine. (and possibly > >saving somebody's life) > > > > > It depends on the machine and the industry regulations that you need to > comply with. In the semiconductor industry, for example, the Semi-S2 > specs require that an E-Stop remove "all hazardous energies" from the > system as quickly as possible. Other parts of the spec talk about how > fast the atmosphere in the machine must be evacuated (they use caustic > and poisonous gases). > > Of course, many people on this list are using EMC in a home shop > environment, where there are no hard and fast rules, only rules of > thumb. I agree that E-Stop is a system, and that many people don't > fully grasp what it means (most notably anyone who thinks that a button > on a GUI or a hotkey in the software constitutes E-Stop).
The notion of "Home Shop" gets real murky, because a lot of people have "Friends" come over and "Help". I can tell you that with my OSHA experience, that if you ever give one of your friends ANYTHING, they become employees, and as such you are required to be 100% OSHA compliant. And by ANYTHING, I mean money, spare parts, even something as innocuous as a bumper sticker makes you an employer. There are exceptions, but not many. > > I have been seeing info being given out on this list, where people are > >giving out advice to just hook up a E-Stop button to the power supply to > >stop a machine. This info MAY work, but it is not the recommended way > >to do things. I look at that big red button on most machines as nothing > >more as a power off button. > > > > > For a small machine, particularly a stepper-based one, disconnecting > power is a viable means of stopping the machine. It's not the fastest, > but it probably is the most reliable. Decelerating the motor at the > fastest rate possible would be better, but you still run the risk of > decelerating too quickly, which will cause a loss of sync, which might > as well be a power-off in some situations. True, I was not implying that all systems should be treated the same, just that telling people to turn off all power to a machine, is NOT the best thing to do. Decelerating a machine as fast as possible is exactly what should happen. Just think of a shirt hanging up in a bit. Also, SYNC is not something that I would be worried about, if it was my hand. A scrapped part is much cheaper than a ride in a ambulance. > >Here is an example that I can give you: > > Say somebody builds a hydraulic, or worse, pneumatic Z-Axis. If > >someone gets there hand stuck in a part directly under the Z-Axis, and > >they hit a E-Stop that removes power from the machine, then that > >person's hand will will sustain serious injury. the spindle will still > >be turning for a couple seconds after power is lost, and the Z-Axis, > >with no power holding it up, will start to fall. A small diameter end > >mill can cut a nice round hole quickly through flesh. > > > > > A Z axis of these types should have a brake. Brakes usually engage when > power is removed, specifically for the purposes of E-Stop. Of course, > we still have the question of whether the motors would stop faster if > power is removed or if the drive uses all possible effort to stop the > motors, but that's for the individual machine integrator to decide based > on their situation. > > You do want to turn the motor power off at some point, but deciding when > is difficult. Consider a person whose hand is caight between a tool > (not spinning - no need to be gross here) and a vise. What you want is > for the pressure on the hand to go away - ie, stop powering the motor > into the hand. But, you want to be able to drive the axis back a little > manually, which can't be done if the drive is still commanding a stop > (or hold). Yes, in that situation, some sort of limiter should be placed in the path of the axis when it is disengaged. However, ideally, the spindle should be powered down, and power applied to a braking element. While the hydraulics/pneumatics should remain powered up, and the program stopped. effectively stopping movement. The point is not to release pressure on a persons hand (or any other body part) but, not to put further pressure on it. Then, someone like myself who maintained the machines, would manually move individual components of a machine, to effect a rescue. This is just theoretical, my only concern is that someone does not get seriously injured, or more the point, not get injured any more than they should have been. Getting hung up in a machine is one thing, having something fall on you and getting hurt far worse than you were before, is something else entirely. <snip> > Any blanket statement could cause someone to do something that will get > someone hurt eventually, including one that says that removing power is > never the answer. I think you're bringing up a very good point though - > *THINK* about what you need to do to make *YOUR* machine safe, and then > do it. > > Thanks for listening > - Steve My point exactly. Thank you. I have a story to tell that might explain where I am coming from, and why. When I was young, I grew up on a farm in the deep south. One day I was hooking up a hitch to a dirt scoop that we had. The hitch was 6" square tubing 1/2" thick, with two 1" thick steel plates welded to the sides with a hole in them for a pin to go through. There was a corresponding set of steel plates welded to the scoop which made a hinge. I could not get the pin to align with the far set of holes, so I stuck my finger through them to move the end of the pin so I could get it through. Something I have done many times before. Only this time when I stuck my finger in the holes, the hitch shifted, and I was stuck there with my finger in what was in effect a large pair of scissors. My finger was cut to the bone on two sides. and all my tools were just outside my reach. I was stuck there for over half an hour, nobody in sight, trying to figure out how I was going to get out of yet another mess. I finally got my shoulder wedged under the hitch, pushed up as hard as I could, then pulled my finger out of the hole. I have also had surgery on both eyes for "metallic objects". Two times, they pierced completely to the center of my eye. I am very lucky that I can still see as well as I can. One other thing, I was pulled out of the back of a fast moving truck while driving down a gravel road. I skidded over 200 feet before I came to a stop. Luckily (if you could call it luck) the only injury I sustained was a large hole in my side on my right hip bone. My guts were full of rocks, and the bone was protruding a little, but I could still walk. It took several hours in the hospital for the doctor to clean the wound, remove the ground pork surrounding the hole, and sew me up. My dad did not have any insurance, and I did not receive the best care. I got what was available at the time. These are by far not the only things that have happened to me, nor are they the worst things that have happened. I have had many stitches and staples in my youth, and most of them could have been avoided. We were never taught about safety when we were growing up. All the years of growing up on a farm took a big toll on me. Although I was fine all through my twenties, and my early thirties, A disk finally gave out in my back, and now I am disabled with nerve damage. I learned real quickly how important safety is when I was in college, and ever since then, I have aspired to make things as safe as I can make it. My short years working maintenance for some "BIG NAME" companies taught me a lot. Not too long before I became disabled, I was also OSHA certified. I did not work for OSHA, but I went around and made sure that other companies were OSHA compliant, BEFORE OSHA came a knockin. I am just trying to let everyone know just where I stand on safety. ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
