I usually use 24 VDC PNP NO proxs for general use for a number of reasons.
If you have voltage at the output of the sensor, then the prox is sensing metal. Most logic is positive which seems to help the debugging and troubleshooting process. However if the input card requires a sinking sensor then that won't work. The 7i77 (I haven't used that one yet) has sinking inputs, which means that you need a sensor that is sourcing. So a 24 VDC PNP type NO prox will work fine with that. Don't waste your time with two wire proxs.. Go with three wire devices. The wiring is usually - 24 volts DC - Brn, 0 volts - Blue, NO output Black, NC output White (If the device has a NC output also). But sometimes manufacturers deviate from that. If you want to use a NPN prox switch with a sinking input, it can often be done, but it gets ugly as it requires a pull up resistor and that will also invert the logic to the input. See page 17 http://www.mesanet.com/pdf/parallel/7i77man.pdf You can't predict which way a prox switch is going to fail. I've seen them fail both off and on. However, the ones I have used rarely fail unless they are fried via overvoltage or mechanically damaged. The mention about using a shielded prox is an excellent suggestion. You need to know the difference. I have seen unshielded proxs cause a lot of problems. Use a shielded prox whenever possible. If you have a higher powered machine that will do damage if it hits the hard stops, then you need to use quality mechanical limit switches for the machine limits and those are normally wired as NC. A hardwired safety circuit is normally used with the hard limits so when a hard limit is made, the machine is brought to an abrupt stop, as fast as possible to prevent damage to the machine and or nearby people before the hard machine stops are made. Usually a "sealed in" relay that is used as a "power on" or Estop relay is "unsealed" via the NC limit switch contacts so that the circuit drops out and the motors are braked. This circuit may be tied into, or part of the Estop circuit, but that depends on the machine. Generally, the bigger and more powerful the machine the more elaborate the safety circuits. Many large machines will simply self destruct if they are allowed to get out of control. Some large machines will use a dozen or more true "safety relays" to implement the safety circuits. Safety rated PLCs are also becoming popular as they can be used to replace expensive "safety relays" on elaborate machines. I've had good luck with Automation Direct prox switches although Turck, Banner, Omron, Eaton and others also make some quality devices. http://www.automationdirect.com/adc/Shopping/Catalog/Sensors_-z-_Encoders/Inductive_Proximity_Sensors_-z-_Proximity_Switches/12mm_Round_%28AM_-z-_VM1_-z-_PMW_-z-_PBM_Series%29/Standard_Sensing_Distance_%28AM1_-z-_PBM_Series%29 Dave On 2/22/2014 4:12 AM, Sven Wesley wrote: > I am going to shopaholic a bunch of proximity sensors to replace mechanical > homing and limit switches for two machines with Mesa cards (5i25/7i77) and > then it struck me, what is the best option? NPN or PNP? > How do they behave if there's no power to them or if they fail? For example > a PNP NO, is it open all the time or only open if the drive circuit has > power? What I am trying to grasp is what type of switch I should use to > trust the machine even if the switch fails (broken switch no operation). > > /Sven > ------------------------------------------------------------------------------ > Managing the Performance of Cloud-Based Applications > Take advantage of what the Cloud has to offer - Avoid Common Pitfalls. > Read the Whitepaper. > http://pubads.g.doubleclick.net/gampad/clk?id=121054471&iu=/4140/ostg.clktrk > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ Managing the Performance of Cloud-Based Applications Take advantage of what the Cloud has to offer - Avoid Common Pitfalls. Read the Whitepaper. http://pubads.g.doubleclick.net/gampad/clk?id=121054471&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
