On 14 June 2011 23:20, Peter Loron <[email protected]> wrote: > On 06/14/2011 08:53 AM, Kirk Wallace wrote: > > On Mon, 2011-06-13 at 13:47 -0700, Peter Loron wrote: > >> Hello, folks. Can anybody comment on using hall effect switches on a 3 > >> axis mill for limit / home switches? Seems like they would have some > >> possible advantages over physical contact switches. > >> > >> Thanks. > >> > >> -Pete > > My Hardinge lathe uses Hall limit and home sensors. The Z axis target > > magnet is small and mounted such that there is minor protection from > > chips. There is a small gap between the magnet and sensors when aligned. > > If chips are in the magnet area, they get wiped off, but not too small > > such that small chips don't bind in the gap. the magnets are weak enough > > to have largesh chips fall off. The trip location accuracy only needs to > > be within about 2 or 3mm because, in my case, the home sensor is used in > > conjunction with the screw encoder index, which should be accurate > > to .002mm or better. What I like about Hall sensors is that they are > > machining fluid proof and will practically never wear out or change the > > trip point or behavior over time. I tend to think using mechanical > > micro-switches is silly, but a lot people use them to good effect. The > > Hardinge Z axis uses a Hall sensor with magnet in a micro-switch > > housing: > > > http://sensing.honeywell.com/index.cfm?ci_id=154286&la_id=1&Ne=7&sid=1308EB555700&ci_id=154286&N=3591+4294966517&la_id=1 > > Short URL: http://alturl.com/v2jzu > > > > These are mounted in a location well protected from chips, but the area > > is not sealed from coolant, which is fine because the sensors don't mind > > fluids. > > > > The Z sensors are in an aluminum threaded barrel similar to this one: > > > http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=480-3256-ND > > Short URL: http://alturl.com/netsd > > > > They tend to be expensive, but it should not be too difficult to machine > > a barrel and pot an IC sensor with leads into it > > Yeah, I'm going to roll my own...I have a pile of Allegro sensors here. > Also, retail on the Honeywell sensors is probably more that I've got > invested into the whole mill... :-) > >
The traditional way for use of these sensors is to trigger on direct approach. The problem is overshoot. If you approach in this manner, in an error condition, you can crush the sensor. However, if you mount the sensor at 90deg to a strip plate, then you approach the plate/leaf with a 'wiping' action. Now overshoot is not a problem. Also, you can have a foam or other strip, to wipe the sensor first on approach. One would have to experiment to see if the trigger hysteresis is OK using this method. For better homing, there really should be two sensors interlinked, a simple switch at end of travel, coarse, and one using an opto slot on the feedscrew, fine, to give a repeat homing within one or two encoder counts/steps. Regards Roland ------------------------------------------------------------------------------ EditLive Enterprise is the world's most technically advanced content authoring tool. Experience the power of Track Changes, Inline Image Editing and ensure content is compliant with Accessibility Checking. http://p.sf.net/sfu/ephox-dev2dev _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
