Thursday, January 1, 2009

Hall Effect Sensors

Since my X & Y stages already have endstop sensors that are already mounted and take in account the space needed for the folding shields I most definately want to reuse them. Also since the opto sensors are the wrong physical layout to insert in their place, it makes even more sense to reuse them.

After my shopping trip to get more Allen wrench sizes I start taking off the old end stop sensors and what the heck they are magnetic, not optical!

Hmmm at least they have part numbers on them, time to Google them to figure out what the heck they are. The only hit is for a store for obsolete parts and they want $80.00US apiece for them.

No Matches for technical data so I keep shorting the part number until I finally get some hits as a family of parts.

Turns out they are Honeywell Hall Effect limit sensors, that takes another half a day goggling before I find a circuit to wire them up, which turns out to be simplicity in it self.

From ExMrClean

After a bit of experimenting it is determined that the devices I have are active low. Until the metal vane on the movement stage moves into the sensor zone it is conducting current and illuminating the LED. When it kicks over it stops conducting, which in this case if you monitor signal of the output lead from the sensor it goes from a low to high value.

In examining the standard RJ-45 color code/pin out and how it is mapped into Opto Endstop RJ-45 connector I determined that one of the four pairs of wires is unused.

Since one of my stages has a DB-9 connector for the end stops. I decide to use a single RJ-45 cable for both signals back to the stepper motor board.

In the color code the blue, white-blue is the positive connection. Brown, white-brown is ground. The Green wire is used for the end stop signal and the Orange wire is un-used.

I make a quick mod to the stepper boards to add a jumper from the max optical input pad to pin 2 of the minimum RJ-45 jack. Then at the end of my single Ethernet cable that I have hacked the end off of I wire the minimum output pin of the DB-9 to green and the maximum output pin to the orange wire.

The other stage (Y) used to have a connector but it had obviously failed at some point and to repair it they had fished out the wires and solder new ones directly to it. So for that stage I put the parts onto a small chuck of prototype board and hot glued it in place.

I now had all my end stops working and created a quick exerciser program that cycled all three axis’s between the end stops, counting the steps it took to go from end stop to end stop and let it run for 24 hours.

After looking at the G-code program sketch it has support for active or active low signals from the end stops, but not having mixed values on the same machine!

Since my hall effect sensors are backwards from the normal signal from an end stop, I add an inverter section out of a 7404 chip so it gives the same output as the standard Optical end sensors that we use.

Where to put the need inverter chip? I could have dead bug glued it next to the Sanguino, but elected to install them inline with the cables using plenty of hot glue to encapsulate them and to electrically insulate them.

Rethinking the solution:

I should have simply used the end stop kit from the store and had the LED side of the optical sensor powered via the hall effect sensor.

No comments:

Post a Comment